From CERN: “A CERN-led international collaboration develops 3D-printed neutrino detectors”

From CERN

22 June, 2020
Thomas Hortala

A 3D-printed “super-cube” scintillator would be the first occurrence of additive manufacturing being used in particle detectors and would allow more precise data collection.

Example of a plastic Scintillator detector (left) and a stage of its 3D-printing process (right)
(Image: CERN)

Plastic scintillators are one of the most used active materials in high-energy physics. Their properties make it possible to track and distinguish between particle topologies. Among other things, scintillators are used in the detectors of neutrino oscillation experiments, where they reconstruct the final state of the neutrino interaction. Measurements of oscillation phenomena are carried out through comparison of observations of neutrinos in near detectors (close to the target) and far detectors (up to several hundred kilometres away).

CERN is strongly involved in the T2K experiment, the current world-leading neutrino oscillation experiment, in Japan, which recently released promising results [Nature].

T2K Experiment, Tokai to Kamioka, Japan

T2K map, T2K Experiment, Tokai to Kamioka, Japan

A future upgrade of the experiment’s near detector will pave the way for more precise results. The novel detector will comprise a two-tonne polystyrene-based plastic scintillator detector segmented into 1 x 1 x 1 cm3 cubes, leading to a total of around two million sensitive elements: the smaller the cubes, the more precise the results. This technology could be adopted for other projects, such as the DUNE near detector.

FNAL DUNE Near Detector

However, more precise measurements would require finer granularity, making the detector assembly harder.

This is where the CERN EP-Neutrino group – led by Albert De Roeck – steps in, developing a new plastic scintillator production technique that involves additive manufacturing. The R&D is carried out in collaboration with the Institute for Scintillation Materials (ISMA) of the National Academy of Science of Ukraine, which has strong expertise in the development of scintillator materials, and the Haute École d’Ingénierie et Gestion du Canton de Vaud (HEIG-VD), which is expert in additive manufacturing. The final goal is to 3D-print a “super-cube”, that is, a single massive block of scintillator containing many optically independent cubes. 3D-printing would solve the issue of assembling the individual cubes, which could thus be produced in any size, including smaller than 1 cm3, and relatively quickly (volumes bigger than 20 x 20 x 20 cm3 can be produced in about a day).

So far, the collaboration has been fruitful. A preliminary test gave the first proof of concept: the scintillation light yield of a polystyrene-based scintillator 3D-printed with fused deposition modelling (see fig. 2) has been found to be comparable to that of a traditional scintillator. But the road towards a ready-to-use super-cube is still long. Further optimisation of the scintillator parameters and tuning of the 3D-printer configuration, followed by a full characterisation of the 3D-printed scintillator, will need to be achieved before the light reflector material for optically isolating the cubes can be developed.

This new technique could also open up new possibilities for the field of particle detection. A successful 3D-printed plastic scintillator detector could pave the way for a broader use of this technology in detector building, which could shake up the field of high-energy physics, as well as that of medicine, where particle detectors are used, for instance, in cancer therapy. Moreover, the greatly cost-effective 3D-printer could be replicated quite easily and used in a vast number of settings. Umut Kose, from the EP-neutrino group and Neutrino Platform at CERN, explains: “Our dream goes beyond the super-cube. We like to think that, in a few years, 3D-printing will allow high-school students to make their own radiation detection systems. The outreach potential of this technology is mind-blowing”.

Davide Sgalaberna, now at ETH Zurich, cannot hide his enthusiasm for this adventure: “This is the first time that 3D-printing could be used for real particle detectors. We are transforming our personal will into a project, and we are hopeful that this could lead to a breakthrough. That is thrilling”. A thrill shared by Davide’s colleagues, who are more than ready to resume work on the 3D-printed detector once the easing of lockdown allows everyone to return to CERN.

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From CERN: “Particle physicists update strategy for the future of the field in Europe”

From CERN

19 June, 2020

The CERN Council today announced that it has updated the strategy that will guide the future of particle physics in Europe.

Following almost two years of discussion and deliberation, the CERN Council today announced that it has updated the strategy that will guide the future of particle physics in Europe within the global particle-physics landscape. Presented during the open part of the Council’s meeting, held remotely due to the ongoing COVID-19 pandemic, the recommendations highlight the scientific impact of particle physics, as well as its technological, societal and human capital.

By probing ever-higher energy and thus smaller distance scales, particle physics has made discoveries that have transformed the scientific understanding of the world. Nevertheless, many of the mysteries about the universe, such as the nature of dark matter, and the preponderance of matter over antimatter, are still to be explored. The 2020 update of the European Strategy for Particle Physics proposes a vision for both the near- and the long-term future of the field, which maintains Europe’s leading role in addressing the outstanding questions in particle physics and in the innovative technologies being developed within the field.

The highest scientific priorities identified in this update are the study of the Higgs boson – a unique particle that raises scientific profound questions about the fundamental laws of nature – and the exploration of the high-energy frontier. These are two crucial and complementary ways to address the open questions in particle physics.

“The Strategy is above all driven by science and thus presents the scientific priorities for the field,” says Ursula Bassler, President of the CERN Council. “The European Strategy Group (ESG) – a special body set up by the Council – successfully led a strategic reflection to which several hundred European physicists contributed.” The scientific vision outlined in the Strategy should serve as a guideline to CERN and facilitate a coherent science policy across Europe.

The successful completion of the High-Luminosity LHC in the coming decade, for which upgrade work is currently in progress at CERN, should remain the focal point of European particle physics. The strategy emphasises the importance of ramping up research and development (R&D) for advanced accelerator, detector and computing technologies, as a necessary prerequisite for all future projects. Delivering the near and long-term future research programme envisaged in this Strategy update requires both focused and transformational R&D, which also has many potential benefits to society.

The document also highlights the need to pursue an electron-positron collider acting as a “Higgs factory” as the highest-priority facility after the Large Hadron Collider (LHC).

China Circular Electron Positron Collider (CEPC) map

The Higgs boson was discovered at CERN in 2012 by scientists working on the LHC, and is expected to be a powerful tool to look for physics beyond the Standard Model. Such a machine would produce copious amounts of Higgs bosons in a very clean environment, would make dramatic progress in mapping the diverse interactions of the Higgs boson with other particles and would form an essential part of a rich research programme, allowing measurements of extremely high precision. Construction of this future collider at CERN could begin within a timescale of less than 10 years after the full exploitation of the High-Luminosity LHC, which is expected to complete operations in 2038.

The exploration of significantly higher energies than the LHC will allow new discoveries to be made and the answers to existing mysteries, such as the nature of dark matter, to potentially be found. In acknowledgement of the fact that the particle physics community is ready to prepare for the next step towards even higher energies and smaller scales, another significant recommendation of the Strategy is that Europe, in collaboration with the worldwide community, should undertake a technical and financial feasibility study for a next-generation hadron collider at the highest achievable energy, with a view to the longer term.

CERN FCC Future Circular Collider details of proposed 100km-diameter successor to LHC

It is further recommended that Europe continue to support neutrino projects in Japan and the US. Cooperation with neighbouring fields is also important, such as astroparticle and nuclear physics, as well as continued collaboration with non-European countries.

“This is a very ambitious strategy, which outlines a bright future for Europe and for CERN with a prudent, step-wise approach. We will continue to invest in strong cooperative programmes between CERN and other research institutes in CERN’s Member States and beyond,” declares CERN Director-General Fabiola Gianotti. “These collaborations are key to sustained scientific and technological progress and bring many societal benefits.”

“The natural next step is to explore the feasibility of the high-priority recommendations, while continuing to pursue a diverse programme of high-impact projects,” explains ESG chair Halina Abramowicz. “Europe should keep the door open to participating in other headline projects that will serve the field as a whole, such as the proposed International Linear Collider project.”

ILC schematic, being planned for the Kitakami highland, in the Iwate prefecture of northern Japan

Beyond the immediate scientific return, major research infrastructures such as CERN have broad societal impact, thanks to their technological, economic and human capital. Advances in accelerators, detectors and computing have a significant impact on areas like medical and biomedical technologies, aerospace applications, cultural heritage, artificial intelligence, energy, big data and robotics. Partnerships with large research infrastructures help drive innovation in industry. In terms of human capital, the training of early-career scientists, engineers, technicians and professionals provides a talent pool for industry and other fields of society.

The Strategy also highlights two other essential aspects: the environment and the importance of Open Science. “The environmental impact of particle physics activities should continue to be carefully studied and minimized. A detailed plan for the minimization of environmental impact and for the saving and reuse of energy should be part of the approval process for any major project,” says the report. The technologies developed in particle physics to minimise the environmental impact of future facilities may also find more general applications in environmental protection.

The update of the European Strategy for Particle Physics announced today got under way in September 2018, when the CERN Council, comprising representatives from CERN’s Member and Associate Member States, established a European Strategy Group (ESG) to coordinate the process. The ESG worked in close consultation with the scientific community. Nearly two hundred submissions were discussed during an Open Symposium in Granada in May 2019 and distilled into the Physics Briefing Book, a scientific summary of the community’s input, prepared by the Physics Preparatory Group. The ESG converged on the final recommendations during a week-long drafting session held in Germany in January 2020. The group’s findings were presented to the CERN Council in March and were scheduled to be announced on 25 May, in Budapest. This was delayed due to the global Covid-19 situation but they have now been made publicly available.

For more information, consult the documents of the Update of the European Strategy for Particle Physics:

2020 Update of the European Strategy for Particles Physics
Deliberation Document on the 2020 Update of the European Strategy for Particle Physics

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From CERN: “Search for new physics through multiboson production”

From CERN

(Left) CMS event display of a candidate event in which two W bosons and one Z boson are produced and decay into three electrons and a muon. (Right) ATLAS event display of a candidate event in which two Z bosons are produced, along with two jets. The Z bosons subsequently decay into two electrons and two muons. (Image: CERN)

This media update is part of a series related to the 2020 Large Hadron Collider Physics conference, which took place from 25 to 30 May 2020. Originally planned to take place in Paris, the conference was held entirely online due to the COVID-19 pandemic.

At the LHCP conference this year, the ATLAS [below] and CMS [below] collaborations presented new results relating to a physics process called vector boson scattering. CMS also reported the first observation of the so-called “massive triboson production”. Studying these processes to test the Standard Model is important as it could shed light on new physics. The results were presented online at the virtual LHCP conference, originally due to be held in Paris.

During proton collisions at the LHC, many particles, including the carriers of the electroweak force – photons and W and Z bosons – are produced. These bosons are often referred to simply as vector bosons, in the Standard Model, and one of the processes that leads to their pair production is called vector boson scattering.

Vector boson processes are an excellent probe to seek deviation from theoretical predictions. Two rare processes that are of particular interest as they probe the self-interactions of four vector bosons are diboson production via vector boson scattering and triboson production”. The observation and measurement of these processes are important as they test the electroweak symmetry breaking mechanism, whereby the unified electroweak force separates into electromagnetic and weak forces in the Standard Model, and are complementary to the measurements of Higgs boson production and decay.

In a vector boson scattering process, a vector boson is radiated from a quark in each proton and these vector bosons scatter off one another to produce a diboson final state. Triboson production refers instead to the production of three massive vector bosons.

At the LHCP conference, physicists from the ATLAS and CMS collaborations presented new searches for the production of a pair of Z bosons via electroweak production including the vector boson scattering mechanism. ATLAS observed this process at 5.5 sigma and CMS reported strong evidence. CMS also reported the first observation of a W boson produced in association with a photon through the vector boson scattering process, as well as more precise measurements of the same-sign WW production, and an observation of the vector boson scattering production of a W and a Z boson, complementing earlier ATLAS observations.

Another way to probe four-boson interaction is to study the very rare production of three massive bosons or tribosons. This April, the CMS experiment released a 5.7 sigma result of the triboson phenomenon, establishing it as a firm observation, following the first evidence of this process seen by the ATLAS experiment last year.

Most physics processes of fundamental particles involve two or more individual particles that interact with each other via an intermediary particle that is emitted or absorbed in the process.

“The more bosons produced, the rarer the event. This new observation of tribosons was very difficult because it is a much rarer process than the one that led to the Higgs boson discovery, and very interesting because it may reveal signs of new particles and anomalous interactions,” says Roberto Carlin, CMS spokesperson.

In the triboson and vector boson scattering processes, W and Z can interact with themselves to create more W and Z particles, producing two or three bosons. W and Z being highly unstable particles, they quickly decay into leptons (electrons, muons, taus and their corresponding neutrinos) or quarks. But such processes are extremely rare and the diboson and triboson events that physicists look for are mimicked by background processes, making them even more difficult for physicists to analyse.

“To separate signal from background, physicists have to be ingenious and employ advanced machine learning algorithms. This is a challenging task for such rare processes, and requires meticulous and thorough studies,” says Karl Jakobs, ATLAS spokesperson.

The measurements of vector boson scattering and triboson production presented at LHCP 2020 are consistent with the predictions made by the Standard Model, which remains our best understanding of fundamental particles and their interactions. The above observations also provide physicists with tools to probe quartic self-interaction between massive electroweak bosons. The current measurements place constraints on the strength at which these quartic interactions take place and increased precision from the use of new datasets could open up horizons for new physics at higher energy scales in the LHC and lead to possible discoveries of new particles.

See the full article here.

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From CERN: “Exploring new ways to see the Higgs boson”

From CERN

4 June, 2020

The ATLAS and CMS collaborations presented their latest results on new signatures for detecting the Higgs boson at CERN’s Large Hadron Collider.

Collision events recorded by ATLAS (left) and CMS (right), used in the search for rare Higgs boson transformations (Image: CERN)

This media update is part of a series related to the 2020 Large Hadron Collider Physics conference, which took place from 25 to 30 May 2020. Originally planned to take place in Paris, the conference was being held entirely online due to the COVID-19 pandemic.

The ATLAS and CMS collaborations presented their latest results on new signatures for detecting the Higgs boson at CERN’s Large Hadron Collider. These include searches for rare transformations of the Higgs boson into a Z boson – which is a carrier of one of the fundamental forces of nature – and a second particle. Observing and studying transformations that are predicted to be rare helps advance our understanding of particle physics and could also point the way to new physics if observations differ from the predictions. The results also included searches for signs of Higgs transformations into “invisible” particles, which could shine light on potential dark-matter particles. The analyses involved nearly 140 inverse femtobarns of data, or around 10 million billion proton–proton collisions, recorded between 2015 and 2018.

The ATLAS and CMS detectors can never see a Higgs boson directly: an ephemeral particle, it transforms (or “decays”) into lighter particles almost immediately after being produced in proton–proton collisions, and the lighter particles leave telltale signatures in the detectors.

CERN ATLAS Higgs Event

CERN CMS Higgs Event May 27, 2012

However, similar signatures may be produced by other Standard-Model processes. Scientists must therefore first identify the individual pieces that match this signature and then build up enough statistical evidence to confirm that the collisions had indeed produced Higgs bosons.

When it was discovered in 2012, the Higgs boson was observed mainly in transformations into pairs of Z bosons and pairs of photons. These so-called “decay channels” have relatively clean signatures making them more easily detectable, and they have been observed at the LHC. Other transformations are predicted to occur only very rarely, or to have a less clear signature, and are therefore challenging to spot.

At LHCP, ATLAS presented the latest results of their searches for one such rare process, in which a Higgs boson transforms into a Z boson and a photon (γ). The Z thus produced, itself being unstable, transforms into pairs of leptons, either electrons or muons, leaving a signature of two leptons and a photon in the detector. Given the low probability of observing a Higgs transformation to Zγ with the data volume analysed, ATLAS was able to rule out the possibility that more than 0.55% of Higgs bosons produced in the LHC would transform into Zγ. “With this analysis,” says Karl Jakobs, spokesperson of the ATLAS collaboration, “we can show that our experimental sensitivity for this signature has now reached close to the Standard Model’s prediction.” The extracted best value for the H→Zγ signal strength, defined as the ratio of the observed to the predicted Standard-Model signal yield, is found to be 2.0+1.0−0.9.

CMS presented the results of the first search for Higgs transformations also involving a Z boson but accompanied by a ρ (rho) or φ (phi) meson. The Z boson once again transforms into pairs of leptons, while the second particle transforms into pairs of pions (ππ) in the case of the ρ and into pairs of kaons (KK) in the case of the φ. “These transformations are extremely rare,” says Roberto Carlin, spokesperson of the CMS collaboration, “and are not expected to be observed at the LHC unless physics from beyond the Standard Model is involved.” The data analysed allowed CMS to rule out that more than approximately 1.9% of Higgs bosons could transform into Zρ and more than 0.6% could transform into Zφ. While these limits are much greater than the predictions from the Standard Model, they demonstrate the ability of the detectors to make inroads in the search for physics beyond the Standard Model.

The so-called “dark sector” includes hypothetical particles that could make up dark matter, the mysterious element that accounts for more than five times the mass of ordinary matter in the universe. Scientists believe that the Higgs boson could hold clues as to the nature of dark-matter particles, as some extensions of the Standard Model propose that a Higgs boson could transform into dark-matter particles. These particles would not interact with the ATLAS and CMS detectors, meaning they remain “invisible” to them. This would allow them to escape direct detection and manifest as “missing energy” in the collision event. At LHCP, ATLAS presented their latest upper limit – of 13% – on the probability that a Higgs boson could transform into invisible particles known as weakly interacting massive particles, or WIMPs, while CMS presented results from a new search into Higgs transformations to four leptons via at least one intermediate “dark photon”, also presenting limits on the probability of such a transformation occurring at the LHC.

The Higgs boson continues to prove invaluable in helping scientists test the Standard Model of particle physics and seek physics that may lie beyond. These are only some of the many results concerning the Higgs boson that were presented at LHCP. You can read more about them on the ATLAS and CMS websites.

Technical note

When data volumes are not high enough to claim a definite observation of a particular process, physicists can predict the limits that they expect to place on the process. In the case of Higgs transformations, these limits are based on the product of two terms: the rate at which a Higgs boson is produced in proton–proton collisions (production cross-section) and the rate at which it will undergo a particular transformation to lighter particles (branching fraction).

ATLAS expected to place an upper limit of 1.7 times the Standard Model expectation for the process involving Higgs transformations to a Z boson and a photon (H→Zγ) if such a transformation were not present; the collaboration was able to place an upper limit of 3.6 times this value, approaching the sensitivity to the Standard Model’s predictions. The CMS searches were for a much rarer process, predicted by the Standard Model to occur only once in every million Higgs transformations, and the collaboration was able to set upper limits of about 1000 times the Standard Model expectations for the H→Zρ and H→Zφ processes.

Links to the papers and notes

ATLAS search for H→Zγ: https://cds.cern.ch/record/2717799
CMS search for H→Zρ or H→Zϕ: https://cds.cern.ch/record/2718949
ATLAS search for “invisible” transformations of the Higgs boson: https://cds.cern.ch/record/2715447
CMS search for Higgs transformations involving a dark photon: https://cds.cern.ch/record/2718976

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From CERN: “Cosmic rays throw up surprises, again”

From CERN

29 May, 2020
Ana Lopes

Cosmic-ray data collected by the AMS detector on the International Space Station again challenge conventional theory of cosmic-ray origin and propagation.

The AMS detector on the International Space Station (Image: NASA)

Ever since astronauts attached the 7.5 tonne AMS detector to the International Space Station in May 2011, the space-based magnetic spectrometer, which was assembled at CERN, has collected data on more than 150 billion cosmic rays – charged particles that travel through space with energies up to trillions of electron volts. It’s an impressive amount of data, which has provided a wealth of information about these cosmic particles, but remarkably, as the spokesperson of the AMS team Sam Ting has previously noted, none of the AMS results were predicted. In a paper just published in Physical Review Letters, the AMS team reports measurements of heavy primary cosmic rays that, again, are unexpected.

Primary cosmic rays are produced in supernovae explosions in our galaxy, the Milky Way, and beyond. The most common are nuclei of hydrogen, that is, protons, but they can also take other forms, such as heavier nuclei and electrons or their antimatter counterparts. AMS and other experiments have previously measured the number, or more precisely the so-called flux, of several of these types of cosmic rays and how the flux varies with particle energy and rigidity – a measure of a charged particle’s momentum in a magnetic field. But until now there have been no measurements of how the fluxes of the heavy nuclei of neon, magnesium and silicon change with rigidity. Such measurements would help shed new light on the exact nature of primary cosmic rays and how they journey through space.

In its latest paper, the AMS team describes flux measurements of these three cosmic nuclei in the rigidity range from 2.15 GV to 3.0 TV. These measurements are based on 1.8 million neon nuclei, 2.2 million magnesium nuclei and 1.6 million silicon nuclei, collected by AMS during its first 7 years of operation (19 May 2011 to 26 May 2018). The neon, magnesium and silicon fluxes display unexpectedly identical rigidity dependence above 86.5 GV, including an also unexpected deviation above 200 GV from the single-power-law dependence predicted by the conventional theory of cosmic-ray origin and propagation. What’s more, the observed rigidity dependence is surprisingly different from that of the lighter primary helium, carbon and oxygen cosmic rays, which has been previously measured by AMS.

The cosmic-ray plot continues to thicken. The AMS researchers have seen deviations from expected cosmic-ray behaviour before, including a rigidity dependence of the primary helium, carbon and oxygen cosmic rays that is distinctly different from that of the secondary lithium, beryllium and boron cosmic rays; secondary cosmic rays are produced by interactions between the primary cosmic rays and the interstellar medium.

“Historically, cosmic rays are classified into two distinct classes – primaries and secondaries. Our new data on heavy primary cosmic rays show that primary cosmic rays have at least two distinct classes.” says Ting. “This is totally unexpected based on our previous knowledge of cosmic rays.”

The new and surprising data is likely to keep theorists busy rethinking and reworking current cosmic-ray models. “Our previous observations have already generated new developments in cosmic-ray models. The new observations will provide additional challenges for the new models,” says Ting. And if the data that the detector is currently taking and sending back to CERN for analysis – after a successful series of spacewalks that has extended its lifetime – throws up more surprises, theorists are likely to become even busier.

Watch the video below and relive the drama of the complex spacewalks that have extended the remaining lifetime of the AMS detector to match that of the International Space Station itself.

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From CERN : “CERN collaborations present new results on particles with charm quarks”

From CERN

The Xc1(3872) hadron, which contains charm quarks, could be a pair of two-quark particles loosely bound together. (Image: CERN)

The ALICE, CMS and LHCb collaborations present new measurements that show how particles containing charm quarks can serve as “messengers” of hadrons and the quark–gluon plasma, carrying information about these forms of matter.

The ALICE, CMS and LHCb collaborations at CERN present new measurements that show how charmed particles – particles containing charm quarks – can serve as “messengers” of two forms of matter made up of quarks and gluons: hadrons, which make up most of the visible matter in the present-day universe; and the quark–gluon plasma, which is thought to have existed in the early universe and can be recreated in heavy-ion collisions at the Large Hadron Collider (LHC). By studying charmed particles, physicists can learn more about hadrons, in which quarks are bound by gluons, as well as the quark–gluon plasma, in which quarks and gluons are not confined within hadrons.

The main results are:

The LHCb team obtained the most precise yet measurements of two properties of a particle known as χc1(3872), a hadron containing charm quarks. The particle was discovered in 2003 and it has remained unclear whether it is a two-quark hadron, a more exotic hadron such as a tetraquark – a system of four quarks tightly bound together – or a pair of two-quark particles weakly bound in a molecule-like structure. Pinning down the nature of this hadron could extend physicists’ understanding of how quarks bind into hadrons.

“Our results are consistent with χc1(3872) being a pair of two-quark particles loosely bound together, but it does not fully rule out the tetraquark hypothesis or other possibilities,” says LHCb spokesperson Giovanni Passaleva.

The CMS collaboration observed for the first time the transformation, or “decay”, of another particle, called B0s, into the same χc1(3872) particle. The researchers compared this decay with the previously observed decay of the B+ meson, which had led to the first detection of the χc1(3872) in 2003. Both types of decay link the behaviour of this hadron to the up and strange quarks.

“Measured differences in the decay rates are intriguing and could provide further insight into the nature of the χc1(3872), which has not yet been fully established,” says CMS spokesperson Roberto Carlin.

The ALICE collaboration measured the so-called elliptic flow of hadrons containing charm quarks, in heavy-ion collisions. The hadrons are created during collisions that also create a quark–gluon plasma. Hadrons containing heavy quarks, like the charm quark, are excellent “messengers” of the quark–gluon plasma, meaning they carry important information about it.

“The pattern observed by ALICE indicates that the heavy charm quarks are dragged by the quark–gluon plasma’s expansion,” says ALICE spokesperson Luciano Musa.

Looking forward, the LHC collaborations aim to make more precise measurements of these messengers of the quark world using data from the next LHC run, which will benefit from largely upgraded experiment set-ups.

Read more below for a comprehensive description of these results.

Charm quark results related to hadrons

The LHCb and CMS collaborations describe results from their studies of a hadron known as χc1(3872). The particle was discovered in 2003 by the Belle experiment in Japan but it has remained unclear whether it is a two-quark hadron, a more exotic hadron such as a tetraquark – a system of four quarks tightly bound together – or a pair of two-quark particles weakly bound in a molecule-like structure.

Pinning down the nature of χc1(3872) could extend physicists’ understanding of how quarks bind into hadrons. The new studies by the CMS and LHCb collaborations shed new light on – but do not yet fully reveal – the nature of this particle.

Using sophisticated analysis techniques and two different datasets, the LHCb team obtained the most precise measurements yet of the particle’s mass and determined for the first time and with a significance of more than five standard deviations the particle’s “width”, a parameter that determines the particle’s lifetime.

Until now researchers had only been able to obtain upper limits on the allowed values of this parameter. The LHCb researchers detected χc1(3872) particles in their datasets using the classic “bump”-hunting technique of searching for an excess (the bump) of collision events over a smooth background. Each dataset led to a measurement of the mass and width, and the results from both datasets agree with each other.

“Our results are not only the most precise yet, they also show that the mass of χc1(3872) is remarkably close to the sum of the masses of the D0 and D*0 charmed mesons,” says LHCb spokesperson Giovanni Passaleva. “This is consistent with χc1(3872) being a pair of two-quark particles loosely bound together, but it does not fully rule out the tetraquark hypothesis or other possibilities.”

Meanwhile, analysing a large dataset recorded over the course of three years, the CMS collaboration observed for the first time the transformation, or “decay”, of the B0s particle into the χc1(3872) and a ϕ meson. This two-quark particle, B0s, is a relative of the B+ meson, in the decay of which the Belle experiment first detected χc1(3872). Like the LHCb team, the CMS team detected χc1(3872) using the bump technique.

“Our result is particularly interesting because we found that the rate at which the B0s decays to the hadron χc1(3872) and the ϕ meson is similar to that of the B0 into χc1(3872) and an anti-K0 meson, whereas it is about twice as low as that for the previously observed B+ decay into χc1(3872) and the K+ meson,” says CMS spokesperson Roberto Carlin. “In these decays, different quarks, other than the bottom quark, play a role,” Carlin explains. “The fact that the decay rates do not follow an obvious pattern may shed light on the nature of χc1(3872).”

Charm quark results related to the quark–gluon plasma

At the other end of the quark-binding spectrum, the ALICE collaboration measured the so-called elliptic flow of hadrons containing a charm quark, either bound to a light quark (forming a D meson) or to an anticharm (making a J/ψ meson) in heavy-ion collisions. Hadrons containing heavy quarks, charm or bottom, are excellent messengers of the quark–gluon plasma formed in these collisions. They are produced in the initial stages of the collisions, before the emergence of the plasma, and thus interact with the plasma constituents throughout its entire evolution, from its rapid expansion to its cooling and its eventual transformation into hadrons.

When heavy nuclei do not collide head on, the plasma is elongated and its expansion leads to a dominant elliptical modulation of the hadrons’ momentum distribution, or flow. The ALICE team found that, at low momentum, the elliptic flow of D mesons is not as large as that of pions, which contain only light quarks, whereas the elliptic flow of J/ψ mesons is lower than both but distinctly observed.

“This pattern indicates that the heavy charm quarks are dragged by the quark–gluon plasma’s expansion,” says ALICE spokesperson Luciano Musa, “but likely to a lesser extent than light quarks, and that both D and J/ψ mesons at low momentum are in part formed by the binding, or recombination, of flowing quarks.”

Another measurement performed by the ALICE team – of the flow of electrons originating from decays of B hadrons, containing a bottom quark – indicates that bottom quarks are also sensitive to the elongated shape of the quark–gluon plasma. Upsilon particles, which are made up of a bottom quark and its antiquark, as opposed to a charm and anticharm like the J/ψ, do not exhibit significant flow, likely because of their much larger mass and the small number of bottom quarks available for recombination.

Read more on the CMS and LHCb websites:

https://cms.cern/news/discreet-charm-x3872
https://lhcb-public.web.cern.ch/Welcome.html#X(3872)2020

Original papers:

ALICE: https://arxiv.org/abs/2005.11131
ALICE: https://arxiv.org/abs/2005.11130
CMS: https://arxiv.org/abs/2005.04764
LHCb: https://arxiv.org/abs/2005.13422
LHCb: https://arxiv.org/abs/2005.13419

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From CERN: “LS2 Report: Proton Synchrotron prepared for higher injection energies”

From CERN

11 February, 2020
Achintya Rao

CERN’s oldest working accelerator has a new injection kicker magnet and will soon receive a new septum as well.

The new kicker for the PS being installed in the accelerator (Image: Julien Ordan/CERN)

CERN Proton Synchrotron

Proton beams entering the Proton Synchrotron (PS) from the PS Booster have to be deflected into a circulating orbit before they can be accelerated. This is done by two specialised beam-line elements: a strong magnetic septum and a fast injection-kicker magnet. The latter is a precisely synchronised electromagnet that can be switched on and off in about 100 ns, providing a stable and uniform kick that only affects the injected beam batches, while leaving the already circulating beam unperturbed.

After the ongoing second long shutdown of CERN’s accelerator complex (LS2), the PS Booster will accelerate particles to 2 GeV, almost 50% higher than the pre-LS2 value of 1.4 GeV. The PS therefore needed a new septum and a new kicker capable of coping with this increased injection energy. On 31 January, as part of the LHC Injectors Upgrade (LIU) project, the new kicker magnet was installed, replacing the kicker that had operated since 1979. The magnet will soon be aligned, connected to the vacuum system and then connected to the power and control cables.

Like the magnet it replaced, the PS’s new kicker is made of four identical modules sitting in a 1-metre-long vacuum tank. Each module receives power from a separate pulse generator that consists of two high-power electrical switches – a main switch and a dump switch to control the pulse length – and around 280 metres of a so-called “pulse-forming line”, wound and stored on gigantic drums. These lines are thick, coaxial cables filled with sulphur hexafluoride (SF6) at a pressure of 10 bars, to provide the necessary insulation for the charging voltage of 80 kV. Since SF6 is a strong greenhouse gas, special care has to be taken to ensure that it is safely manipulated and recuperated, and that the system has no leaks.

In order to reduce the dependence on the SF6-based cables, part of the transmission line between the pulse generator and the magnet was replaced with conventional cables. “Disconnecting the SF6 cables from the magnet to connect the reserves was a two-person job, and required time-consuming gas-handling procedures to be followed,” explains Thomas Kramer from the TE-ABT (Accelerator Beam Transfer) group. “On the other hand, the new conventional cables have quick-release connectors and can be operated by one person fairly quickly.”

Kramer and colleagues also replaced the old analogue control system for the kicker, parts of which had been in place since the system was constructed in the 1970s. “Things made back then still work reliably,” smiles Kramer, while noting that the new digital systems make it possible to monitor the situation remotely.

One element that remains to be installed is the new septum. This is a delicate device used in the injection system, composed of two cavities separated by a thin wall: one cavity allows the beams from the PS Booster to enter the PS while the second is meant for the circulating beams. The new septum, which required construction of a novel power converter, will be installed upstream of the magnet in the coming weeks.

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From Accelerating News: “Power converters specially designed for CERN can now be used by the wider accelerator community”

From Accelerating News

11 Dec 2019
by Daniela Antonio, Quentin King (CERN)

The SOLEIL synchrotron facilities in Paris, France. Jean-Christophe BENOIST

The Electrical Power Converters (EPC) Group at CERN has developed new software layers to allow the broader particle accelerator community to use the CERN-specific power converters controls.

Power converters are a fundamental part of CERN’s accelerator complex, allowing it to function properly. In particle accelerators, the particle beams are guided by powerful magnets and are accelerated in metallic chambers called radiofrequency cavities. More than five thousand power converters electrically power both these structures. Many different types are needed, ranging in power from a few watts to more than one hundred megawatts. Some produce a steady current or voltage, while others must ramp, or pulse synchronously with all the other equipment in the accelerator. Therefore, the effective operation of each power converter depends on high-performance digital controls that regulate the current in the circuit.

Since the creation of the LHC, CERN power converters use specialised control computers called Function Generator/Controllers (FGCs), integrated into the power converters. An associated FGC software framework was developed to integrate the FGC hardware into CERN’s accelerator controls environment, which is unique to CERN. With the new software stack, the FGC hardware can now be integrated in the TANGO and EPICS control environments, which are the most common control frameworks used at other accelerator infrastructures. This update will open the door for FGCs and CERN-designed power converters to be deployed to other accelerator facilities, such as synchrotrons.

The project to integrate FGCs into the EPICS and TANGO frameworks was conceived in 2014 and resulted in the successful transfer of FGC converter controls to a European manufacturer, who supplied the new power converter to the main cyclotron magnet of the TRIUMF laboratory in 2018. In 2019, CERN provided power converter technology and the associated converter controls to the European Synchrotron Radiation Facility (ESRF). TRIUMF uses EPICS while ESRF uses TANGO.

Thanks to its potential for future technology transfer, the FGC update was one of the five projects selected to receive funding from the KT Fund in 2019. The objective is to continue the FGC framework integration with more commonly used control environments in the context of a collaboration agreement between CERN and SOLEIL.

During the first phase of this collaboration, CERN provided training and lent a standalone FGC and a small power converter controlled by an FGC to SOLEIL. “The SOLEIL upgrade builds on previous experience, gradually moving to a unified controls’ environment for which the FGC framework is particularly suitable,” explains Nick Ziogas, Knowledge Transfer Officer at CERN. In 2020, during phase 2 of the collaboration, eleven controllers of existing commercial power converters installed at the SOLEIL synchrotron (Paris, France) will be replaced by FGCs. SOLEIL intends to procure 1,700 power converters with digital controllers by 2025 for its major upgrade to higher brilliance and these could hopefully be CERN designs using FGC for controls.

At a time when the broader high-energy physics community debates the next-generation of accelerator machines, many laboratories have major upgrades in store for the next decades, aiming to achieve a performance that could lead to more complex science. This means higher luminosity in colliders and higher brilliance in light sources and, therefore, an upgrade of the accelerator infrastructure.

“The FGC framework comes with a powerful software stack that allows for monitoring and diagnosis of faults and the automatic configuration of the controllers following a change of hardware components,” explains Quentin King, head of the Converter Controls Software Section in the EPC Group. “Powerful management tools are vital when large numbers of converter controllers are used. This is a major requirement from all light sources: to have good insight and understanding of what is happening in their power converters.”

The integration of CERN’s FGC framework with the most common accelerator control frameworks, including TANGO, results in an important knowledge transfer opportunity, since it allows the power converters specifically developed for CERN to be used by both partner laboratories and industry, in fields from particle physics to medical and biomedical research.

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Accelerating News is a quarterly online publication for the accelerator community.
ISSN: 2296-6536

The publication showcases news and results from the biggest accelerator research and development projects such as ARIES, HL-LHC, TIARA, FCC study, EuroCirCol, EUPRAXIA, EASITrain as well as interesting stories on other accelerator applications. The newsletter also collects upcoming accelerator research conferences and events.

Accelerating News is published 4 times a year, in mid March, mid June, mid September and mid December.

You can read Accelerating News via the homepage http://www.acceleratingnews.eu (link is external) or by email.

To subscribe to Accelerating News, enter your email in the “Subscribe to our newsletter” box in the footer.

History

Accelerating News evolved from the EuCARD quarterly project newsletter (see past issues), which was first published in June 2009 to a subscription list of approximately 200. Initiated by EuCARD and in collaboration with additional FP7 co-funded projects, the first edition of Accelerating News was published in April 2012 to an initial distribution list of about 800 subscribers. Currently more than 1750 members receive the quarterly issues.

From CERN: “NA61/SHINE gives neutrino experiments a helping hand”

From CERN

How particle measurements made by the NA61/SHINE experiment at CERN are helping neutrino experiments in the US and Japan

Inside the NA61/SHINE experiment at CERN (Image: CERN)

Neutrinos are the lightest of all the known particles that have mass. Yet their behaviour as they travel could help answer one of the greatest puzzles in physics: why the present-day universe is made mostly of matter when the Big Bang should have produced equal amounts of matter and antimatter. In two recent papers, the NA61/SHINE collaboration reports particle measurements that are crucial for accelerator-based experiments studying such neutrino behaviour.

Neutrinos come in three types, or “flavours”, and neutrino experiments are measuring with ever increasing detail how they and their antimatter counterparts, antineutrinos, “oscillate” from one flavour to another while they travel. If it turns out that neutrinos and antineutrinos oscillate in a different way from one another, this may partially account for the present-day matter–antimatter imbalance.

Accelerator-based neutrino experiments look for neutrino oscillations by producing a beam of neutrinos of one flavour and measuring the beam after it has travelled a long distance. The neutrino beams are typically produced by firing a beam of high-energy protons into long, thin carbon or beryllium targets. These proton–target interactions produce hadrons, such as pions and kaons, which are focused using magnetic aluminium horns and directed into long tunnels, in which they transform into neutrinos and other particles.

To get a reliable measurement of the neutrino oscillations, the researchers working on these experiments need to estimate the number of neutrinos in the beam before oscillation and how this number varies with the energy of the particles. Estimating this “neutrino flux” is hard, because neutrinos interact very weakly with other particles and cannot be measured easily. To get around this, researchers estimate instead the number of hadrons. But measuring the number of hadrons is also challenging, because there are too many of them to measure precisely.

This is where experiments such as NA61/SHINE at CERN’s Super Proton Synchrotron come in. NA61/SHINE can reproduce the proton–target interactions that generate the hadrons that transform into neutrinos. It can also reproduce subsequent interactions that protons and hadrons undergo in the targets and focusing horns. These subsequent interactions can produce additional neutrino-yielding hadrons.

The NA61/SHINE collaboration has previously measured hadrons generated in experiments at 31 GeV/c proton energy (where c is the speed of light) to help predict the neutrino flux in the Tokai-to-Kamioka (T2K) neutrino-oscillation experiment in Japan. The collaboration has also been gathering data at 60 and 120 GeV/c energies to benefit the MINERνA, NOνA and DUNE experiments at Fermilab in the US. The analysis of these datasets is progressing well and has most recently led to two papers: one describing measurements of interactions of protons with carbon, beryllium and aluminium, and another reporting measurements of interactions of pions with carbon and beryllium.

“These results are crucial for Fermilab’s neutrino experiments,” says Laura Fields, an NA61/SHINE collaboration member and co-spokesperson for MINERνA. “To predict the neutrino fluxes for these experiments, researchers need an extremely detailed simulation of the entire beamline and all of the interactions that happen within it. For that simulation we need to know the probability that each type of interaction will happen, the particles that will be produced, and their properties. So interaction measurements such as the latest ones will be vital to make these simulations much more accurate,” she explains.

“Looking into the future, NA61/SHINE will focus on measurements for the next generation of neutrino-oscillation experiments, including DUNE and T2HK in Japan, to enable these experiments to produce high-precision results in neutrino physics,” Fields concludes.

See also this Experimental Physics newsletter article.

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From CERN: “The plot thickens for a hypothetical “X17” particle”

From CERN

27 November, 2019
Ana Lopes

Additional evidence of an unknown particle from a Hungarian lab gives a new impetus to NA64 searches.

CERN NA64

The NA64 experiment at CERN (Image: CERN)

Fresh evidence of an unknown particle that could carry a fifth force of nature gives the NA64 collaboration at CERN a new incentive to continue searches.

In 2015, a team of scientists spotted [Physical Review Letters] an unexpected glitch, or “anomaly”, in a nuclear transition that could be explained by the production of an unknown particle. About a year later, theorists suggested [Physical Review Letters] that the new particle could be evidence of a new fundamental force of nature, in addition to electromagnetism, gravity and the strong and weak forces. The findings caught worldwide attention and prompted, among other studies, a direct search [Physical Review Letters] for the particle by the NA64 collaboration at CERN.

A new paper from the same team, led by Attila Krasznahorkay at the Atomki institute in Hungary, now reports another anomaly, in a similar nuclear transition, that could also be explained by the same hypothetical particle.

The first anomaly spotted by Krasznahorkay’s team was seen in a transition of beryllium-8 nuclei. This transition emits a high-energy virtual photon that transforms into an electron and its antimatter counterpart, a positron. Examining the number of electron–positron pairs at different angles of separation, the researchers found an unexpected surplus of pairs at a separation angle of about 140º. In contrast, theory predicts that the number of pairs decreases with increasing separation angle, with no excess at a particular angle. Krasznahorkay and colleagues reasoned that the excess could be interpreted by the production of a new particle with a mass of about 17 million electronvolts (MeV), the “X17” particle, which would transform into an electron–positron pair.

The latest anomaly reported by Krasznahorkay’s team, in a paper [.pdf above] that has yet to be peer-reviewed, is also in the form of an excess of electron–positron pairs, but this time the excess is from a transition of helium-4 nuclei. “In this case, the excess occurs at an angle 115º but it can also be interpreted by the production of a particle with a mass of about 17 MeV,” explained Krasznahorkay. “The result lends support to our previous result and the possible existence of a new elementary particle,” he adds.

Sergei Gninenko, spokesperson for the NA64 collaboration at CERN, which has not found signs of X17 in its direct search, says: “The Atomki anomalies could be due to an experimental effect, a nuclear physics effect or something completely new such as a new particle. To test the hypothesis that they are caused by a new particle, both a detailed theoretical analysis of the compatibility between the beryllium-8 and the helium-4 results as well as independent experimental confirmation is crucial.”

The NA64 collaboration searches for X17 by firing a beam of tens of billions of electrons from the Super Proton Synchrotron accelerator onto a fixed target.

The Super Proton Synchrotron (SPS), CERN’s second-largest accelerator

If X17 did exist, the interactions between the electrons and nuclei in the target would sometimes produce this particle, which would then transform into an electron–positron pair. The collaboration has so far found no indication that such events took place, but its datasets allowed them to exclude part of the possible values for the strength of the interaction between X17 and an electron. The team is now upgrading their detector for the next round of searches, which are expected to be more challenging but at the same time more exciting, says Gninenko.

Among other experiments that could also hunt for X17 in direct searches are the LHCb experiment and the recently approved FASER experiment, both at CERN.

CERN/LHCb detector

CERN FASER experiment schematic

Jesse Thaler, a theoretical physicist from the Massachusetts Institute of Technology, says: “By 2023, the LHCb experiment should be able to make a definitive measurement to confirm or refute the interpretation of the Atomki anomalies as arising from a new fundamental force. In the meantime, experiments such as NA64 can continue to chip away at the possible values for the hypothetical particle’s properties, and every new analysis brings with it the possibility (however remote) of discovery.”

See the full article here.

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