Cern News

ATLAS probes uncharted territory with LHC Run 3 data Display of a collision event recorded by the ATLAS detector at an energy of 13.6 TeV, featuring two candidate displaced electrons each represented by a track (blue lines) that points to an energy deposit in the ATLAS calorimeter (green). The inset shows an axial view of the detector, illustrating the electron tracks displaced from the interaction point (red circle) by a few mm. (Image: ATLAS/CERN) (Image: ATLAS/CERN)

Despite its immense success in describing the fundamental building blocks of matter and their interactions, the Standard Model of particle physics is known to be incomplete. Experiments around the globe and in space are therefore searching for signs of new physics phenomena that would guide physicists towards a more comprehensive theory.

At the biannual ICHEP conference that took place in Prague earlier this week, the ATLAS collaboration presented its first results from searches for new physics at record collision energies, targeting magnetic monopoles produced in heavy-ion collisions and long-lived particles created in proton–proton collisions.

Magnetic monopoles are hypothetical particles with only a single north or south pole, making them magnetically charged. Their existence would demonstrate the complete symmetry between electricity and magnetism. It would also confirm aspects of “grand unified theories” beyond the Standard Model that unify the strong, weak and electromagnetic forces at very high energies.

Researchers at the Large Hadron Collider (LHC) are searching for monopoles produced in high-energy collisions. Monopoles would be highly ionising, meaning they would strip electrons from atoms and would leave behind significant energy deposits in particle detectors.

In a new search for magnetic monopoles, the ATLAS collaboration analysed its first heavy-ion (lead–lead) collision data from LHC Run 3, which was collected in the autumn of 2023 at an unprecedentedly high energy of 5.36 TeV per pair of nucleons (protons or neutrons). Specifically, ATLAS researchers looked at ultraperipheral collisions, in which the ions do not collide centrally via the short-ranged strong interaction, but instead pass by close enough to interact through the weaker but long-ranged electromagnetic force. Collisions among lead ions can produce the largest magnetic fields in the Universe, with strength up to 1016 Tesla.

If a pair of magnetic monopoles was produced in such interactions, it would be the sole particle system to be found in an otherwise empty detector, and it would manifest itself as a concentrated cloud of ionisation electrons. Looking for the unique signal features and analysing backgrounds that could mimic them, ATLAS saw no signs of monopoles in their Run 3 heavy-ion data.

Consequently, the result sets the world's best limits on the production rate of monopoles created in ultraperipheral heavy-ion collisions for monopole masses below 120 GeV. Moreover, this analysis introduces a methodology for studying highly ionising particles in heavy-ion data from the LHC and beyond.

Most searches for new physics look for new particles that would decay “promptly” and produce decay products that emanate from the LHC’s proton–proton interaction points. However, beyond-the-Standard-Model physics theories, including supersymmetry, also predict “long-lived particles” that would produce decay products away from the interaction point. Such particles require dedicated techniques to reconstruct particle tracks and may have eluded detection in prior searches.

ATLAS has released the result of a new search for a pair of long-lived particles, each of which decays into an electron, muon or tau lepton, resulting in two particle tracks that are “displaced” from the ATLAS interaction point (see image above) – a rare signature that could be indicative of new physics. In particular, ATLAS looked for a new signature where one of the long-lived particles travels far enough before decaying so that only a single electron is detected.

This is the first ATLAS search of this type using the 13.6 TeV proton–proton collision data from LHC’s Run 3. In preparation of Run 3, ATLAS researchers had enhanced the online collision-event selection – the "trigger" – with the reconstruction of displaced tracks, which enabled the present search for new long-lived particles.

The event yields in all search regions matched Standard-Model expectations. These results set the strictest limits yet on the long-lived supersymmetric partners of electrons, muons, and tau leptons.

With more data from the LHC and its future upgrade, the High-Luminosity LHC, ATLAS physicists will continue their quest to find long-lived particles, magnetic monopoles and other hypothetical particles – all while further refining their search techniques and developing new experimental strategies.

Find out more here and here.

abelchio Fri, 07/26/2024 - 09:55 Byline ATLAS collaboration Publication Date Fri, 07/26/2024 - 09:50

Bike to Work 2024: CERN achieves the best participation rate

This year’s Bike to Work campaign has come to an end and the outcome is very positive for CERN, with a higher rate of participation than ever before: 1037 people took up the challenge and joined one of the 276 CERN teams. Congratulations!

One of the new bicycle shelters at CERN: a good place to park your bicycle and find out more about the benefits of commuting by bicycle. (Image: CERN)

Bike to Work encourages workers in companies all over Switzerland to commute to work by bicycle as often as possible throughout May and June. This year, a total of 28% of CERN’s 3700 eligible population (staff and users) signed up to take part, placing the Organization in first place for the participation rate in its category (companies with 1000-4999 employees).

Unfortunately, the CERN participants did not have much luck in winning team prizes this year, but some entrants did win individual prizes – well done to them!

Road safety is a key consideration in making sure that CERN cyclists can ride safely around the site alongside all other road users. Road incident and accident statistics are monitored by the HSE unit and reported to the SCE department, which works tirelessly to continuously improve the cycling and pedestrian routes both inside and outside CERN.

The challenge now is to continue to cycle to work. The CERN community is encouraged to cycle all year around, but remember: safety first! Don’t forget to check out the safety rules for cycling and the road safety pages and to complete the online course Road Traffic – Bike Riding before getting in the saddle.

anschaef Wed, 07/24/2024 - 11:28 Byline Jens Vigen Publication Date Wed, 07/24/2024 - 11:26

ATLAS announces its 2024 Outstanding Achievement Award winners

The ATLAS collaboration held its seventh biennial Outstanding Achievement Awards ceremony on 20 June 2024. These awards recognise the invaluable technical work performed across the collaboration in various fields. This year, the Awards Committee honoured two individuals and seven groups for their exceptional contributions to detector operation, detector upgrades, software, computing and teamwork during the period from February 2022 to October 2023.

“The work of the award winners demanded great creativity and determination,” said Sarah Demers, co-chair of the Awards Committee. “Not only were their contributions crucial during this recent data-taking period, but they also set the stage for high-quality physics analyses and operations in the years to come.”

Find out about the award winners on the ATLAS website.

ndinmore Wed, 07/24/2024 - 10:13 Byline ATLAS collaboration Publication Date Wed, 07/24/2024 - 10:12

HiLumi News: new large helium tanks

Exceptional machines call for exceptional operations. Two large helium tanks for the High-Luminosity LHC (HL-LHC) were installed at Point 1 in June, and two more at Point 5 in July. They will store the helium for the refrigerators that will cool the HL-LHC’s new focusing magnets on both sides of the ATLAS and CMS experiments.

Each tank weighs over 62 tonnes and is 28 metres long and 3.5 metres in diameter. The tanks will each be able to store 250 cubic metres of gaseous helium at a pressure of 20 bars at room temperature, representing a weight of around 800 kg. As the tanks were manufactured in Portugal, it took more than eight days to transport them to CERN under escort as an abnormal load.

The new refrigerators – one for each point – will be delivered next year. “We have a good year of work ahead of us to install the complete infrastructure and connect the tanks to the helium distribution system,” explains Antonio Suraci from the Cryogenics group.

Around 130 tonnes of helium are needed to cool the superconducting magnets of the LHC and its experiments. When the HL-LHC is up and running, it will consume almost the same amount of helium, but the design of the cryogenic system will have to be modified to supply this new equipment on each side of the ATLAS and CMS experiments.

 

Delivery and installation of helium tanks for the High-Luminosity LHC project. (Video: CERN)

 

cmenard Wed, 07/17/2024 - 14:54 Publication Date Thu, 07/18/2024 - 11:40

CERN and Pro Helvetia announce the artists selected for the Connect India residency

Connect is an art residency programme launched by Arts at CERN and Pro Helvetia in 2021, which serves as a platform to foster experimentation in art and science by bringing artists into contact with fundamental science and cutting-edge research at CERN and other international scientific organisations.

The two awardees, Lou Masduraud and Shailesh BR, selected by a board of cultural experts, will be invited to a three-week residency at the International Centre for Theoretical Sciences (ICTS) in Bengaluru, followed by a three-week stay at CERN in Geneva. They will receive support from the Arts at CERN and ICTS curatorial teams to explore new forms of artistic expression and transform these explorations into art productions.

Shailesh BR is a visual artist based in Delhi NCR, India. His practice explores fundamental aspects of our world by examining existing knowledge, systems, traditions and philosophical thoughts. Utilising a diverse visual vocabulary, he incorporates methods from science and technology into his artistic interventions, creating drawings, object modifications and machines.

Lou Masduraud lives and works in Geneva. Through sculpture and installation, Masduraud proposes alternative narratives to dominant realities. Her body of work explores the intricate network of human activities through formal and material investigations of everyday elements such as fountains, basement windows and pipes, in both public and private spaces.

Connect India offers these artists a unique opportunity to engage with cutting-edge fundamental scientific research in both Geneva and Bengaluru. At CERN, physicists and engineers design and use a wide array of experiments in particle physics, while ICTS focuses on theoretical sciences, encompassing fields like physics, mathematics, astronomy and computational biology.

Connect India marks the second collaboration with ICTS, following successful dual residencies at scientific organisations in Chile and South Africa. Since its launch in 2021, the Connect collaboration between Pro Helvetia and Arts at CERN has hosted seven editions and established itself as a key platform for exchange between communities of artists and scientists around the world.

ldragu Tue, 07/16/2024 - 15:29 Publication Date Thu, 07/18/2024 - 15:27

CMS congratulates its 2023 Award and Thesis Award winners CMS PhD Thesis Award winners 2023 CMS PhD Thesis Award winners 2023 (Image: CMS collaboration)

Each year, the CMS collaboration honours the work of exceptional PhD students with the Thesis Award. This award recognises doctoral research conducted within the collaboration that pushes the boundaries of high-energy physics. Out of a highly competitive pool of 27 nominees, the winners of the 2023 CMS PhD Thesis Award are Jona Motta (LLR, Institut Polytechnique de Paris), Christopher Edward Brown (Imperial College London) and Spandan Mondal (RWTH Aachen University, Germany).

“Doctoral students do a lot of impressive work in CMS. Writing a PhD thesis to document this work is a tremendous effort and achievement. Some of the students decide to invest substantial extra efforts in writing an exceptionally clear, effective and original documentation of their research work. They write for their peers, future students, who will follow in their steps, and for all those in search of in-depth, detailed, accurate but also accessible knowledge related to CMS scientific or technical frontline research,” says Marta Felcini, chair of the CMS PhD Thesis Award Committee.

Read more about the winners on the CMS website.

CMS Award winners 2023 CMS award winners 2023. (Image: CERN)

Each year since 2000, the CMS Awards Committee has recognised outstanding contributions from members of the CMS collaboration, honouring their dedication to the experiment. Nominations can be made by any CMS member for exceptional work in various fields, and the winners are selected by a dedicated committee.

“This year, the CMS Awards celebrate the hard work that was done by our collaborators both on the operations and on the upgrades to ensure the success of the present and future detector,” say the CMS Awards Committee Chairs.

As well as the 50 award winners, the RD53 collaboration was singled out by CMS for special recognition. This dedicated group is responsible for the development of the ATLAS and CMS Phase 2 inner tracker readout chips.

Find out more about the collaboration and each of the 50 awardees on the CMS website.

ndinmore Tue, 07/16/2024 - 09:21 Byline CMS collaboration Publication Date Tue, 07/16/2024 - 09:15

LHCb investigates the properties of one of physics’ most puzzling particles The LHCb experiment. (Image: CERN)

χc1(3872) is an intriguing particle. It was first discovered over 20 years ago in B+ meson decays by the BELLE collaboration, KEK, Japan. Since then, the LHCb collaboration reported it in 2010 and has measured some of its properties. But here’s the catch – physicists still don’t know what it is actually made up of.

In the quark model of particle physics, there are baryons (made up of three quarks), mesons (made up of a quark–antiquark pair) and exotic particles (made up of an unconventional number of quarks). To find out what χc1(3872) consists of, physicists must measure its properties, such as its mass or quantum number. Theories suggest that χc1(3872) could be a conventional charmonium state, made up of charm and anticharm quarks, or an exotic particle composed of four quarks. An exotic particle of this type could be a tightly bound tetraquark, a molecular state, a cc-gluon hybrid state, a vector glueball or a mixture of different possibilities.

Previously, the LHCb collaboration has found its quantum number to be 1++ and, in 2020, made precise measurements of the width (lifetime) and mass of the particle. The collaboration also measured what is known as its low-energy scattering parameters. The results showed that its mass is just a tad smaller than the sum of the masses of the D0 and D*0 mesons.

Following these results, the theoretical community was divided. Some argued that χc1(3872) was a molecular state consisting of spatially separated D0 and D*0 mesons. This molecular state would be much larger than the typical size of particles and more comparable to a heavy nucleus. However, this argument encounters a problem, namely that physicists expect molecular objects to be suppressed in hadron–hadron collisions, and the χc1(3872) is produced abundantly. Other theorists interpreted the results as clear evidence that χc1(3872) has a “compact” component. This would mean it is a particle with much smaller size, containing either a tightly bound charmonium or a tetraquark.

One way to help determine what χc1(3872) contains is to calculate the ratio between probabilities of the decays into different lighter particles (branching fractions). By comparing the rate at which it decays either to an excited charmonium state or to a charmonium state and a photon, physicists can gather clues as to what type of particle it is. There is a clear theoretical signature: if the ratio is non-vanishing, it is evidence for some compact component in χc1(3872), disfavouring the pure molecular model.

Now, using the complete set of LHC Run 1 and Run 2 data, the LHCb collaboration has found these ratios to be non-vanishing, with a significance exceeding six standard deviations.  The large measured value of the ratios is inconsistent with the expectations based on the pure D0D*0 molecular hypothesis for the χc1(3872) particle. Instead, it supports a wide range of predictions based on other hypotheses of the χc1(3872) structure, including conventional (compact) charmonium, a compact tetraquark containing a charm quark, charm antiquark, light quark and light antiquark, or a mixture of molecules with a substantial compact core component. In short, the result provides a strong argument in favour of the χc1(3872) structure containing a compact component.

The χc1(3872) particle continues to fascinate the particle physics community. Find out more in the paper or on the LHCb website.

ndinmore Mon, 07/15/2024 - 12:23 Byline LHCb collaboration Publication Date Tue, 07/16/2024 - 09:40

Digital archaeology: new LEP data now available to all

Unlike letters carved on the Rosetta stone, digital data is not written on a virtually immutable support. Just a few years after it is written, its format becomes obsolete, the readout analysis tools can’t run on computers and the visualisation code no longer works. But data can still contain interesting scientific information that should remain available to future generations of scientists.

A set of data of potentially high interest is that of LEP, CERN’s former flagship accelerator that collided electrons and positrons up until 2000. Like the current LHC, LEP had four collision points, each hosting an experiment – ALEPH, DELPHI, OPAL and L3 – that was operated by hundreds of scientists. LEP holds the record for the world’s highest e+e- energy collisions but the data collected over two decades ago remains available to only a small community of people.

Like archaeologists who unearth the remnants of past civilizations, digital archaeologists are computing experts who retrieve data years after the collaborations have moved on to other experiments. “The first step is to reach agreement within the collaboration as to opening and sharing their data and the software required to exploit it. Then, just like archaeologists, we dig into the documents that the former collaborations have written about the data architecture, and retrieve the software used for actual analysis”, explains Ulrich Schwickerath, a former DELPHI physicist and computing expert working in the IT department. This is no easy task because the information often lies in unpublished documents or in private repositories that might have not even been shared within the collaboration.

The analysis software from LEP times was deposited in CERNLIB, a CERN-developed software library that was discontinued in 2003. “Shortly after the last release of CERNLIB, many external enthusiasts kept it alive and applied quick fixes to the software, known as patches. In a community-based effort, these patches were gathered together in order to create a community version, allowing the old software to be adapted to modern architectures,” Ulrich explains. “Since then, together with a few LEP enthusiasts, we have managed to resurrect the software stacks of the DELPHI and OPAL experiments using the new community-driven version of CERNLIB. We are working towards making the dataset fully available in the original format, as compatible as possible with modern hardware and software tools, and to revise the old visualisation codes so that today's scientists can run proper analyses.”

The data from ALEPH and DELPHI is now available, and the DELPHI data is shared on the CERN Open Data Portal. So whether you're a researcher, teacher, student or just an interested non-physicist, start your discovery of electron-positron annihilation data with the DELPHI detector by visiting this webpage.

 

 

ndinmore Mon, 07/15/2024 - 12:15 Byline Antonella Del Rosso Publication Date Mon, 07/15/2024 - 12:13

Accelerator Report: No summer holidays for the accelerator complex

Since the technical stop in June, Linac4 has been running quite smoothly, delivering beam to the PS Booster with good availability, of 98.7%. Despite a small water leak from the cooling system in one of the PS Booster quadrupole magnets, beam availability remains high, at 94%. The leak, though small, is being carefully managed by diverting the water outside the magnet to prevent further issues. Continuous monitoring is in place, using a camera and data from the water-cooling station. As long as the leak does not worsen, operations will proceed as usual until the end of the 2024 run. If the leak increases significantly, a spare magnet is ready for installation, which would require a beam stop of several days.

As mentioned in a previous Accelerator Report, physics at ISOLDE started on 8 April. Since then, approximately 20 different experimental runs have been conducted at the low-energy experimental stations, using various isotopes produced by impinging the high-intensity PS Booster proton beam on different types of targets. In parallel, the HIE-ISOLDE superconducting linear accelerator started the cooldown and conditioning of its 20 accelerating cavities mounted in four cryomodules and, last week, the first beams were accelerated to set up the experiments downstream. On 12 July, the physics campaign using post-accelerated isotope beams will start and continue until the end of the 2024 ISOLDE run, scheduled for 25 November.

Five superconducting cavities mounted in a cryomodule. The picture was taken in a clean room before the insertion of the cryomodule into its cryostat. (Image: CERN) The HIE-ISOLDE linear accelerator with its four cryomodules. (Image: CERN)

 

 

 

 

 

 

 

 

On the SPS side, following the successful exchange of a magnet on 18 June, the SPS resumed beam delivery for its usual clients: the North Area and the LHC. However, on 25 June, the SPS operators received an alarm indicating that some magnets in the SPS were overheating. The magnets are equipped with a magnet protection system that prevents them from being powered when their temperature rises too high, which also stops beam production.

The experts discovered that a blockage in the water-cooling circuit was causing the overheating. The circuit was unblocked and refilled on 26 June, allowing beam production to resume. The blockage was caused by pieces of rubber that had remained in the circuit following an incident earlier this year.

Since resolving this latest and ­– hopefully – last issue, the SPS has had very good beam delivery, achieving beam availability of 97% for the LHC and 93% for the North Area, making up for some of the previously lost beam time. Additionally, the HiRadMat run, initially scheduled from 1 to 5 July, was already successfully completed by 2 July, allowing additional beam time for the North Area experiments.

The LHC resumed beam production on 28 June, after bringing forward some machine development (MD) activities to give the ATLAS experiment a chance to recover from a cryogenic cooling issue. Since then, the luminosity production has been good, although the beams have often been dumped prematurely due to various unrelated and mostly minor technical issues. Despite this, the LHC had a machine availability of 70% last week, with the time spent in colliding-beams mode (i.e. stable beam ratio) of 51.3%, slightly above our target of 50%.

The integrated luminosity forecast (green line) with the achieved integrated luminosity for ATLAS (blue dots) and CMS (black dots). The difference between the two experiments is mainly due to ATLAS’s cryogenic cooling issue. The fact that both are below the green line is mainly due to the advanced MD activities and the technical issues encountered, resulting in shorter stable beam periods and more frequent filling. Physics time will be recovered later, as the next MD block will be shorter. (Image: CERN) anschaef Wed, 07/10/2024 - 12:11 Byline Rende Steerenberg Publication Date Thu, 07/11/2024 - 10:10

How can I use CERN’s Open Science Office? Why should I care about open science?

The aim of open science is to make scientific research more accessible, transparent and efficient for the benefit of scientists and society. It includes making the products of research openly available – i.e. providing open access to research publications and sharing research data and open-source software and hardware – but also covers effecting cultural change in scientific processes to ensure that the production of knowledge is inclusive, sustainable and equitable.

What does CERN’s Open Science Office do?

The Open Science Office answers questions, provides guidance and connects the CERN community with experts and resources. It organises CERN open science governance meetings and plans to organise training courses and workshops in the future. It was created in 2023, following the release of CERN’s 2022 open science policy ,and will publish CERN’s first open science report in 2025.

Why is open science important to CERN?

We are lucky at CERN that our founding Convention gave us an early mandate for open science, and it has long been the norm within particle physics to share research results openly. With the digital transformation enabled by the World Wide Web (released by CERN in open source), CERN has pioneered a range of open science activities and services, making our Laboratory a world leader in open science. In recent years, government agencies and research funders from Member States and beyond have recognised the value of open science. This recognition is reflected in increased requirements to demonstrate open science practices when submitting funding applications.

How can I contribute to open science?

You are already doing so when you submit your preprints to CDS or arxiv, publish your research papers in open access in line with our open access policy, make your experimental data accessible at HEPdata or through the CERN Open Data Portal or produce open source software. By sharing your work under licenses that let others use it, study its source code, redistribute it and share improvements, you are helping to promote transparent and inclusive research practices at CERN and beyond.

How can the Open Science Office help me?

Contact the Open Science Office with any questions you might have. We offer guidance on open access publishing options for your papers, as well as advice on creating Data Management Plans (DMPs) to comply with funders’ requirements, and much more.

I want my software or hardware to be open source. How can the Open Science Office help?

The newly launched Open Source Programme Office serves the CERN community with answers and insights regarding open source issues.

How can I find out more about open science at CERN?

You can consult the CERN open science website, which includes details of upcoming events and training courses. You can also register for the Open Science Practitioners Forum e-group and join its regular meetings to have a say in CERN’s open science strategy.

For more questions, contact the CERN open science team at os-office@cern.ch

ndinmore Wed, 07/10/2024 - 09:45 Publication Date Wed, 07/10/2024 - 09:43

Computer Security: Don’t print naked

If you’re interested in what’s going on at CERN, in professional projects and plans, or in private problems and parties, hanging out at one of CERN’s printers is a very effective approach (but, please… don’t!). Too many people are still printing confidential documents without caring that they might be read by third parties hanging around – see for example the image below of a document found on one of CERN’s printers a few years ago…

 

This shouldn’t happen. Confidential documents, documents with sensitive content, personal information and private emails should be properly protected – even when they’re converted to paper format.

The CERN printing infrastructure is capable of ensuring the confidentiality of your documents: you can send a print job in such a way that it won’t be printed until you input a PIN code at the machine. So the next time you need to print such a document, go to “Printer Properties” (1) and select the “Secure Print” output method (2). Once you hit the “Print” button, you’ll get another dialog box asking you to provide a PIN number (3). Below, you can see screenshots of the different steps on the Windows platform. Instructions for other operating systems can be found in this ServiceNow knowledge base article.

 

After you’ve provided a PIN, your document will be queued on the printer of your choice. For easy PIN input on the printer, we recommend you use any Canon machine. The printer will hold your document for the next 12 hours (4 hours on some models). Once you’re at the printer, hit the “SCAN/PRINT” button, choose “Secure Print” (4), select the jobs you want to print (5), hit “Print”, and punch in your PIN (6). (Note: this sequence may vary between models.)

 

Your document is only printed after you provide the correct PIN. And as you’re now standing beside the printer, you can be confident that your confidential document is being handled confidentially. And, even better, you can set this as your default printing preference and ensure none of your printouts go walkabout again! See these knowledge base articles for more details – including short video tutorials. In addition, you can download this poster for the corresponding awareness campaign by the Office of Data Privacy (ODP) and pin it up (close to) your printer(s).

So, don’t let confidential information leak… use the “secure print” function on your nearest preferred printer. And: respect the environment. Don’t print unless necessary. Confidential documents that are never printed are less likely to slip out of your control!

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Do you want to learn more about computer security incidents and issues at CERN? Follow our Monthly Report. For further information, questions or help, check our website or contact us at Computer.Security@cern.ch.

 

 

More information on data privacy at CERN can be found on the ODP website or obtained from your Departmental Data Privacy Coordinator.

anschaef Mon, 07/08/2024 - 14:59 Byline Computer Security team Publication Date Mon, 07/08/2024 - 14:56

Less hungry magnets for the experiments of the future

 

How can we advance cutting-edge research but consume less energy? CERN’s scientists are working on innovative solutions, and superconductivity is one of the key ingredients.

A team has recently successfully tested a demonstrator magnet coil that will significantly reduce the power consumption of certain experiments. The coil is made of magnesium diboride (MgB2) superconducting cables, which are used in the high-intensity electrical transfer line that will power the High-Luminosity LHC (HL-LHC), the successor to the LHC. It is mounted in a low-carbon steel magnetic yoke that holds and concentrates the field lines, in a so-called superferric configuration.

This innovative magnet is intended for the SHiP experiment, which is designed to detect very weakly interacting particles and is scheduled to be commissioned in 2031. One of the detector’s two magnets must produce a field of approximately 0.5 tesla. The field is of moderate intensity but must be produced in a huge volume that is 6 metres high and 4 metres wide and deep. A normal-conducting resistive electromagnet would have an electrical power of over one megawatt and, as it would have to operate continuously, its power consumption would be high.

Hence the idea of using a superconductor that conducts electricity without resistance and thus without energy loss from heating. This is the principle behind the LHC magnets. However, they are based on a niobium–titanium alloy, which requires them to be cooled to a very low temperature of -271 °C (2 kelvin) using superfluid helium produced by a complex cryogenic plant.

Magnesium diboride cables have the advantage of being superconducting at -253 °C (20 kelvin). They can be cooled using gaseous helium and therefore require a less complex cryogenic system, thus offering better thermodynamic efficiency. They could not be used for accelerator magnets such as those of the LHC, which generate fields of around 8 tesla. However, they are suitable for a large magnet with a moderate field like that of SHiP.

The coil of the prototype magnet is made up of superconducting magnesium diboride cables. (Image: CERN)

 

Built last September, the 1-metre-long demonstrator coil has just successfully passed operating tests in which it was cooled by gaseous helium to temperatures of 20 to 30 kelvin. Although many steps remain to be completed before the SHiP magnet is ready, these are promising tests that open up prospects for this technology both at CERN and in industry.

 “Such a magnet could consume up to 100 times less electrical power than an ordinary superferric magnet,” says Arnaud Devred, who is carrying out the project with a team from CERN’s Magnets group. “In the longer term, we could, for example, consider retrofitting certain magnets with MgB2 coils in order to reduce their electricity consumption. This project therefore represents a great way of showcasing the technological developments for the HL-LHC.”

The superconducting links of the HL-LHC are attracting a lot of interest because they use high-temperature superconductors, whose large-scale use would allow significant energy savings in many areas, including in our everyday lives. Thanks to this highly innovative development, the scope of this technology can be extended to include electromagnets. The SHiP spectrometer magnet could be one of the first applications.

 

cmenard Fri, 07/05/2024 - 14:57 Byline Corinne Pralavorio Publication Date Fri, 07/05/2024 - 16:00

LHCb investigates the rare Σ+→pμ+μ- decay

The LHCb collaboration reported the observation of the hyperon Σ+→pμ+μ- rare decay at the XV International Conference on Beauty, Charm, Hyperons in Hadronic Interactions (BEACH 2024) in Charleston, South Carolina, USA. A hyperon is a particle containing three quarks, like the proton and neutron, including one or more strange quarks.

Rare decays of known particles are a promising tool for searching for physics beyond the Standard Model (SM) of particle physics. In the SM, the Σ+→pμ+μ- process is only possible through “loop diagrams”: rather than the decay happening directly, intermediate states need to be exchanged in a “loop”, as illustrated in diagrams (a) and (b) below.

In quantum field theory, the probability of such a process occurring is the sum of the probabilities of all possible particles exchanged in this loop, both known and unknown. This is what makes such a process sensitive to new phenomena. If a discrepancy between the experimental measurement and theoretical calculations was observed, it could be caused by a contribution from some unknown particles. These particles could either be exchanged in the loop or mediate this decay directly, interacting with the quarks and then decaying into a pair of muons. In the latter case, shown in diagram (c) below, the new particle would leave a footprint in the properties of the two muons.

Feynman diagrams illustrating the Σ+→pμ+μ- decay in the Standard Model (diagrams a and b) and with a new X0 intermediate particle (diagram c). (Image: LHCb)

Studies of the Σ+→pμ+μ- decay were especially exciting thanks to a hint of a structure that had been observed in the properties of the muon pair in 2005 by the HyperCP (E871) collaboration. With only three events, the structure was far from conclusive, and the LHCb study was expected to shine some light on the situation.

Ultimately, LHCb data does not show any significant peaking structures in the dimuon mass region highlighted by HyperCP, hence disconfirming the hint. The new analysis does, however, observe the decay with high significance, and precise measurement of the decay probability along with other parameters will follow, allowing further searches for discrepancies with SM predictions.

Read more in the LHCb presentations at BEACH as well as in the conference note.

ptraczyk Thu, 06/27/2024 - 16:45 Byline Piotr Traczyk Publication Date Fri, 06/28/2024 - 10:00

Accelerator Report: Technical stops always hold a few surprises

The LHC technical stop started on 10 June, and the injector complex’s technical stop two days later. These stops were scheduled to align with planned work on the Swiss electrical network. The planned work by Swissgrid, the Swiss electricity transmission grid operator, could have caused fluctuations on the network, potentially disrupting or damaging the accelerator subsystems.

Swissgrid scheduled their work from Wednesday, 12 June to Friday, 14 June (Saturday being reserved as a back-up day in case additional time was needed). CERN's technical teams worked hard to complete their activities in the accelerator complex by 2.00 p.m. on the Friday, hoping to restart the complex early if Swissgrid finished ahead of schedule. At around 5.30 p.m., Swissgrid informed CERN that their work was complete. This notification marked the official restart of the accelerator complex.

Restarting the entire accelerator complex after a technical stop is rarely seamless. It can be particularly challenging late on a Friday afternoon… Nevertheless, with excellent support from standby services and experts, the restart went rather well. By Friday evening, beams were circulating in most of the accelerators in the complex, including the LHC.

Following each technical stop, the LHC requires a brief period for revalidation and intensity ramp-up. This time, the revalidation also included corrections to address the collimation hierarchy issue.

Unfortunately, the process was interrupted on Sunday, 16 June, in the morning, when a vacuum leak was discovered in the SPS (see picture). This required the replacement of a magnet, which was scheduled for the following day. Despite the leak, beams could still be provided to the LHC and the SPS North Area experiments during the night.

The vacuum pressure in several parts of the SPS. In royal blue, the vacuum pressure in a magnet with a leaking vacuum chamber. Two things can be observed: 1) the vacuum pressure is increasing over time, and 2) the ripple on the vacuum pressure signal is synchronous with the pulsing of the magnet. This means that, as the magnetic field of the magnet increases, the vacuum leak area opens wider. (Image: CERN)

On 17 June, technical teams replaced the magnet and, by 4.30 p.m., the magnet with the leaking vacuum chamber had been removed from the SPS tunnel. The newly installed spare magnet and vacuum chamber were connected, and vacuum pumping began. By 18 June, at around 10.00 a.m., the vacuum pressure was low enough to resume beam operation. However, a new issue, this time with the accelerating radiofrequency (RF) cavities, prevented beam acceleration in the SPS. After work by the RF experts, beam acceleration was possible by early evening, allowing the LHC revalidation to continue.

The LHC revalidation and intensity ramp-up were completed by the early hours of 20 June, allowing luminosity production to resume. However, when the ATLAS magnets were ramped up to their nominal magnetic field, clogging was discovered in the main refrigerator cold box. This required a warm-up and purging cycle, which began that morning and finished on 25 June.

During the weekend, after completing the intensity ramp-up, luminosity production resumed. However, to prevent a significant difference in integrated luminosity between ATLAS and CMS, it was agreed to bring forward some scheduled machine development (MD) activities and reduce luminosity production until the ATLAS magnet was fully operational again. These MD activities will be deducted from the next MD block, which is scheduled to start on 19 August, and will be replaced by luminosity production.

anschaef Thu, 06/27/2024 - 11:31 Byline Rende Steerenberg Publication Date Thu, 06/27/2024 - 11:29

New beam dumps: cut along the dotted line

When particle beams circulating in the LHC need to be stopped, they are directed towards the beam dumps. In 2020, during Long Shutdown 2, the main LHC beam dumps, which had been in place in the accelerator from the start, were replaced by spare dumps – themselves heavily modified with respect to the original design – because they were showing signs of wear and tear. To withstand the onslaught of the current run, Run 3, these spares had been upgraded before being installed as the main beam dumps. They are still in place today and operating successfully.

The autopsies carried out on the first beam dumps highlighted their strengths and weaknesses. Those findings, coupled with additional studies carried out at HiRadMat, led to a strategy for designing new spare beam dumps and the future HL-LHC beam dumps.

A tried and tested recipe
Originally, the beam dumps were made of three main materials: blocks of high-density graphite and discs of low-density graphite held in place by two discs of extruded graphite. “The autopsies revealed that both of the extruded graphite discs were cracked, while the high- and low-intensity graphites were generally in good condition, thereby validating their use in the beam dumps for Run 3,” recalls Nicola Solieri, project engineer in charge in the SY department’s Targets, Collimators and Dumps section (SY-STI-TCD).

The new spare beam dumps therefore comprise these two components: six blocks of high-intensity graphite and 1700 discs of low-density graphite, now contained between two carbon-fibre-reinforced carbon discs – a material tested and validated at HiRadMat – which replace the old extruded graphite discs.

The composition of the new spare beam dumps. In dark grey, the high-density graphite blocks. In light grey, the 1700 low-density graphite discs. In black, the two new carbon-fibre-reinforced carbon discs. (Image: CERN)

A novel design: easier to “cut”
“The autopsy and disposal of the original beam dumps brought to light a major challenge: how to dismantle them,” says Marco Calviani, SY-STI-TCD section leader. “In 2021, cutting them open was a really tricky operation, in particular because of their radiation levels. In the end we managed to find a solution using CERN’s expertise, but it was clear that we would need to address this issue for future beam dumps.” To do so, the team in charge of the project came up with a “detachable” beam dump design (see images 1–3). “To make it easier to dismantle the beam dumps in the future, as well as making the LHC’s operation more sustainable and improving waste management, we wanted to keep the number of welds to a minimum, and we left a 5-cm space between each of the components to make it easier to take them apart,” explains Nicola Solieri. These measures will greatly facilitate dismantling operations in the event that the spare beam dumps do end up being used and thus become radioactive. And if they are not used in the LHC, their components will be easier to reuse for the HL-LHC beam dumps.

Image 1: Each high-density graphite block is heat-shrunk inside a steel cylinder. The cylinder is heated to 300 °C to expand it, then slipped over the graphite block. The cylinder contracts as it cools, tightening around the graphite block like a skin. This technique was carried out at CERN for the first time on an object of this size. (Image: CERN) Image 2: With the beam dump tube in a vertical position, the teams from the SY-STI-TCD section insert the 1700 low-density graphite discs, which are held in place at either end by rings, instead of being welded. (Image: CERN) Image 3: The 13 “slices” of the beam dump are welded together end to end in CERN’s main workshop, an operation coordinated by the Engineering department’s Mechanical and Materials Engineering group (EN-MME). One of the carbon-fibre-reinforced carbon discs can be seen at the end of the tube. (Image: CERN)

“The whole process of developing these new beam dumps, from design to assembly, was conducted in house with CERN’s competences, which was not the case with the original beam dumps,” underlines Damien Grenier, technical engineer in charge of the assembly of the dumps. “This way, we ensure that CERN has all the know-how and skills required for the key stages of beam dump production and assembly, ready for the HL-LHC.” Indeed, challenges abound for the design of the HL-LHC beam dumps, which will be subject to unprecedented beam intensities. A complex R&D programme is under way with a view to identifying and validating new materials for the beam dump core and container, as well as new assembly techniques.

Meanwhile, the spare beam dumps were finalised in autumn 2023 and installed in their storage cavern at the beginning of 2024. “The two spare beam dumps were produced in record time thanks to the involvement and the tireless efforts of many colleagues from various CERN groups and departments,” continues Marco Calviani. “This is further proof that many challenges can be overcome in house thanks to the team spirit that reigns at CERN and using materials developed by industry.”

anschaef Thu, 06/27/2024 - 11:14 Byline Anaïs Schaeffer Publication Date Thu, 06/27/2024 - 11:06

Computer Security: Dear summer students, welcome!

A warm welcome to the summer-student class of 2024! We’re glad that you made it to CERN! We offer a packed agenda for the next two months: challenging lectures; interesting projects to tackle with your team; and lots of time to take a great big gulp of CERN’s academic freedom, spirit and creativity! In order to make your digital life as comfortable as possible, however, there are a few things you need to know.

When you join CERN, you’re given a CERN computing account. Take care of your account password as any evil-doer might misuse it to spam the world on your behalf, abuse CERN’s computing clusters in your name, download journals in bulk from CERN’s digital library, or simply compromise your CERN PC and extract your photos, documents or personal data, or spy on you using your computer’s microphone or webcam. Worst-case scenario, the whole Organization is at risk! Similarly, take good care of your CERN and personal computers, tablets and smartphones. Give them some freedom to update themselves so you benefit from the latest protective measures. “Auto-update” is a good friend, just make sure that it’s enabled – as it should be by default.

A particularly nasty way to lose your password, at CERN or at home, is to reply to so-called “phishing emails”, i.e. emails asking for your password. No serious person – the CERN Computer Security team, the CERN Service Desk or your CERN supervisor – would send such an email, only dishonest people or fraudsters would. So stay on the lookout and don’t enter your password in weird webpages. Don’t click on links in emails obviously not intended for you, for example, emails not addressed to you, not coming from the real CERN, not written in one of your native languages, or of no relevance to you. Ask us at Computer.Security@cern.ch if you have any doubts. Similarly, don’t randomly click on web links, but stop and think first. Otherwise, you might infect your computer in no time – and the sole remedy will be a full reinstallation of your device (easier if you have backups!).

CERN has awesome network connectivity to the world. But it’s for professional purposes. While private usage is tolerated, please do not abuse this. Keep your bandwidth low. In particular, refrain from bulk downloading movies or software. Remember “copyright”? It also applies at CERN. Any violation of copyright reported to CERN will be followed up and any infringement costs will be passed on to the perpetrator. The same holds true for pirated software. If you have stored pirated licence keys on your device, it’s time to delete them. Companies are monitoring for abuse of their software and infringement costs can quickly reach five to six figures. This one is of particular importance: if you need particular software, have a look at CERN’s central software repositories.

While you’re at CERN, you might be working on a project requiring digital resources – setting up a webpage, writing some code, developing hardware. Please don’t reinvent the wheel if you need a database. Or a webserver. Or some software. The CERN IT department can provide a wide variety of centrally managed and secure services for your digital convenience. Just put yourself on their shoulders and build on top. Free up your time and brain for creativity and let CERN IT provide the tools. Moreover, make sure that all your development work, software, design drawings, documentation and so on are made available to your supervisor when you leave. This will ensure your legacy lives on at CERN. If you keep them to yourself, they’ll get purged and deleted, and your time at CERN will be forgotten.

For those who are curious, here is how the contents of this Bulletin article looked 21 years ago. (Image: CERN)

Finally, like anywhere else, there are some rules to respect. Use of CERN’s computing facilities is governed by the CERN Computing Rules. Basically, be reasonable. Don’t do anything that could be considered immoral, illegal or abusive. Similarly, personal use of CERN’s computing facilities is tolerated, but within the aforementioned limits. For example, browsing pornography is forbidden unless you have a good professional reason to do so (and it might be awkward receiving a corresponding cease-and-desist email from us). In another example, crypto-mining on CERN’s computing clusters is definitely a no-no. Just don’t.

So, make sure that you respect these few ground rules – keep your system up to date – protect your password – STOP-THINK-DON’T CLICK – respect copyright – preserve your work – follow the CERN Computing Rules. We wish you a great and exciting stay at CERN. Have fun and enjoy!

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Do you want to learn more about computer security incidents and issues at CERN? Follow our Monthly Report. For further information, questions or help, check our website or contact us at Computer.Security@cern.ch.

anschaef Wed, 06/26/2024 - 11:22 Byline Computer Security team Publication Date Wed, 06/26/2024 - 11:11

How well do you know the CERN & Society Foundation?

Did you know that the CERN & Society Foundation is celebrating its 10th anniversary this month?

Created in June 2014 to support and promote CERN’s mission and its benefits to the wider public, the CERN & Society Foundation works nationally and internationally across three main areas: education and outreach, innovation and knowledge exchange, and culture and creativity.

• Education and outreach includes projects such as the Beamline for Schools competition, the CERN Non-Member State PhD Studentship Scheme and the Non-Member State Summer Students programme.
• Innovation and knowledge exchange encompasses several projects, including: CERN-MEDICIS, which enables researchers to develop new approaches to fight cancer; TimePix, which is a detector for monitoring radiation in the environment and has an educational component for students; and Zenodo, an open source tool, built and developed at CERN to ensure that everyone can join in with open science.
• Culture and creativity projects include the Arts at CERN programme. 

A key achievement of the Foundation is of course the CERN Science Gateway: as part of the Foundation’s portfolio, the CERN Science Gateway would not have been possible without the generous support of its sponsors.

Thanks to our donors, we have been fostering education, technology and the arts for the last ten years. We have given talented students, teachers and artists from all over the world access to CERN’s groundbreaking facilities and world-class education. The Foundation was created not only to awaken the youngest minds to the beauty of science but also to reinforce the existing dialogue between CERN and society at large.

Pascale Goy, Head of the CERN & Society Foundation

You can find out more about the Foundation via its website and its latest Annual Review. To find out how you can support the Foundation, click here.

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How to enter the CERN & Society Foundation anniversary game:

• You must have a CERN email address to take part in this game.
• Watch the video above and add together all of the impressive numbers that appear in the on-screen text.
• Send your total number to bulletin-editors@cern.ch by Sunday, 7 July at 11.59 p.m. CEST.
• Three winners will be drawn at random from among those who answer correctly. They will each win a CERN & Society Foundation water bottle.
• The three winners will be announced in the next CERN Bulletin.

Good luck!

anschaef Tue, 06/25/2024 - 12:15 Byline Anaïs Schaeffer Publication Date Tue, 06/25/2024 - 12:10

CERN's communications receive European recognition

The European Association of Communication Directors (EACD) has awarded CERN’s Education, Communication and Outreach group the EACD 2024 Communications Excellence Award.

The award is given each year to individuals or groups who demonstrate excellence and best practice in communications and who build bridges through effective international communications, contributing to broader society.

The jury rewarded CERN’s communication in particular for its commitment to transparency, knowledge sharing and transnational communications.

“In its 70th birthday year CERN continues to lead not just in scientific research, but in engaging the world and communicating its discoveries and innovation”, said Kim Larsen, the EACD President.

“This award is well-deserved recognition of the work done by everyone involved in CERN communications and the many scientists we work with every day”, explains Ana Godinho, head of the Education, Communication and Outreach group, who received the award at the EACD Annual Summit on 27 and 28 May in Brussels.

cmenard Mon, 06/24/2024 - 15:21 Publication Date Mon, 06/24/2024 - 15:19

Students from Estonia, Japan and the USA win the 11th edition of Beamline for Schools

Geneva and Hamburg, 25 June 2024.  Beamline for Schools (BL4S) is a physics competition run by CERN, the European laboratory for particle physics, open to secondary school pupils from all around the world. Participants are invited to prepare a proposal for a physics experiment that can be undertaken at the beamline of a particle accelerator, either at CERN or at DESY (Deutsches Elektronen-Synchrotron in Hamburg, Germany). In 2024, three winning teams have been chosen, based on the scientific merit of their proposal and the communication merit of their video.

“Mavericks”, a team from the Secondary School of Sciences in Tallinn and the Hugo Treffner Gymnasium in Tartu, Estonia, and the team “Sakura Particles”, which brings together pupils from Kawawa Senior High School in Kanagawa, Joshigakuin Senior High School and Junten High School in Tokyo, Kawagoe Girls High School in Saitama and Kitano High School in Osaka, Japan, will travel to CERN in September 2024 to perform the experiments that they proposed. The team “SPEEDers” from Andover High School in Andover, USA, will carry out their experiment at a DESY beamline.

A beamline is a facility that provides high-energy fluxes of subatomic particles that can be used to conduct experiments in different fields, including fundamental physics, material science and medicine. 

BL4S started in 2014 in the context of CERN’s 60th anniversary. Over the past 10 years, more than 20 000 pupils from all over the world have taken part in the competition, and 25 teams have been selected as winners. The participation rate has been rising consistently over the years, with a record 461 teams from 78 countries submitting an experiment proposal in 2024. 

“Preparing a proposal for a particle physics experiment is a very challenging task. The success of Beamline for Schools shows that, when provided with the right support, high-school students can design feasible, interesting and imaginative experiments,” says Charlotte Warakaulle, CERN Director for International Relations. “We are continuously impressed by the quality of the proposals, and this year is no exception. The candidates demonstrated impressive creativity and great rigour, two essential qualities for students who might decide to take up scientific careers.”

The fruitful collaboration between CERN and DESY started in 2019 during a long shutdown period of the CERN accelerators. This is the sixth year that the German laboratory has hosted competition winners. 

“Every year I am very impressed by the creativity and determination of the team members,” says Beate Heinemann, Director in charge of Particle Physics at DESY. “I am already looking forward to hosting the team from the USA this year. This programme is so important to me as it advances not only science but also the cultural exchange between young people from different nations.”

“Our experiment will focus on detector development for high-altitude ballooning applications,” says Saskia Põldmaa, one of the “Mavericks” members, from Estonia. “This is by far the biggest opportunity we have had so far in our lifetime so we will hold onto it dearly. We can’t wait to calibrate our homemade muon detector!”

“Our team focuses on detector development for muon tomography applications. We will test and optimise our homemade two-dimensional position-sensitive detector,” says Chiori Matsushita from the Japanese “Sakura Particles” team. “CERN has always been a dream for us. Finally getting to go there, not as a tourist but to do experiments, is amazing!”

“We focus on beam diagnostics: our aim is to measure and analyse the Smith-Purcell (SP) radiation emitted by different diffraction gratings when DESY’s electron or positron beams pass by,” says Niranjan Nair from the US “SPEEDers” team. “We are thrilled to have the opportunity to not just watch scientific advancement passively, but actively contribute to it at DESY: the ultimate goal of our experiment is to research SP radiation as a tool for beam diagnostics.”

The winning proposals were selected by a committee of CERN and DESY scientists from a shortlist of 49 particularly promising experiments. In addition, three teams will be recognised for the most creative video proposals and another 13 teams for the quality of physics outreach activities they are organising in their local communities, taking advantage of the knowledge gained by participating in BL4S.

Beamline for Schools is an education and outreach project funded by the CERN & Society Foundation’s donors. This 11th edition is supported notably by ROLEX through its Perpetual Planet Initiative and by the Wilhelm and Else Heraeus Foundation.

Further information:

• BL4S website: https://beamlineforschools.cern/
• 2024 edition: https://beamline-for-schools.web.cern.ch/2024-edition
• Shortlisted teams and special prizes in 2024: https://beamline-for-schools.web.cern.ch/sites/default/files/BL4S_all-winners_2024_final.pdf  
• Previous winners: https://beamlineforschools.cern/resources/winners
• Countries represented among the shortlisted teams: Bahrain, Bangladesh, Belgium, Brazil, Canada, Chile, Czechia, Denmark, Estonia, France, Germany, Greece, Hong Kong SAR China, India, Indonesia, Italy, Japan, Kazakhstan, Pakistan, Poland, Romania, Singapore, Spain, Thailand, Türkiye, United Arab Emirates, United Kingdom, United States. 
• The prizes awarded for the best outreach project have been kindly provided by the Belgian project “Stars Shine for Everyone”.

ldragu Mon, 06/24/2024 - 13:32 Publication Date Tue, 06/25/2024 - 09:00

Going the extra mile to squeeze supersymmetry out of CMS data

Supersymmetry (SUSY) is an exciting and beautiful theory that answers some of the open questions in particle physics. It predicts that all known particles have a “superpartner” with somewhat different properties. For example, the heaviest quark of the Standard Model, the top quark, would have a superpartner called the top squark, or simply the “stop”. In 2021 the CMS collaboration analysed the entire set of collision data collected from 2016 to 2018 and found features suggesting that it might contain stop particles. In that case, “might” meant that there was less than 5% chance that data containing only known particles could look like what was observed. Instead of waiting many years to collect more data with the hope of reproducing this behaviour, the CMS collaboration decided to reanalyse the same data with upgraded analysis techniques.

The new analysis looks for the simultaneous production of pairs of stops. Each stop decays into a top quark accompanied by several lighter quarks or gluons, which then form bound states known as hadrons, ultimately creating clusters of particles reconstructed in the detector as “jets”. The signal footprint is therefore two top quarks and multiple jets. What makes the analysis challenging is that a very similar footprint is produced by one of the most common Standard Model processes in the LHC: the pair production of top quarks. Top quark production with many accompanying jets is a process that is difficult to accurately simulate, so to have a reliable determination of this background, it must be estimated from observed data.

A commonly used method of estimating backgrounds from data is called the “ABCD method”. It requires two uncorrelated observables that can discriminate between signal and background. The data set can then be divided into four regions (A, B, C and D) depending on the value of each observable being “signal-like” or “background-like”. The subdivision then provides a region dominated by the signal, a region dominated by backgrounds and two intermediate regions. The key feature of the ABCD method is that, following the mathematics of probabilities for independent events, one can estimate the background in the signal-dominated region using only the information from the other regions. The problem with using this method for the stop search is that all simple variables are correlated in this search, making the method invalid. To overcome this issue, CMS physicists have implemented an innovative approach based on advanced machine-learning techniques to determine two variables with a minimal level of correlation. These two variables are then used to divide the data into the four aforementioned regions. The figure below shows the correlation between the two variables for the signal and the background and demonstrates that the signal mostly lies in region “A”. 

Distributions of signal (red) and background (grey) in the four (A, B, C and D) regions, defined based on two uncorrelated variables (SNN1 and SNN2) determined using machine learning. (Credit: CMS collaboration)

Using this novel method, the CMS collaboration was able to accurately predict the dominant background in this analysis from observed data, without relying on simulations with large uncertainties associated with the modelling of the jet multiplicity distribution. This resulted in a large gain in analysis sensitivity. If the signal hinted at by the 2021 analysis was real, it would now have been observed without any doubt. The fact that a signal was not seen in this analysis implies that, in specific SUSY scenarios, a stop decaying ultimately to top quarks and jets must have a mass greater than 700 GeV. With a much more sensitive analysis method in place, the physicists are now eagerly looking forward to analysing the data of the ongoing LHC Run 3 to go even further and to find where Nature hides its answers.

Read more in the CMS Physics Analysis Summary.

ptraczyk Thu, 06/20/2024 - 14:28 Byline CMS collaboration Publication Date Thu, 06/27/2024 - 09:00