From CERN: “Breaking new ground in the search for dark matter”

From CERN

7 AUGUST, 2020 [Just now in social media.]
Ana Lopes

Our fourth story in the LHC Physics at Ten series discusses the LHC’s hunt for the hypothetical particle that may make up dark matter.

The Large Hadron Collider (LHC) is renowned for the hunt for and discovery of the Higgs boson, but in the 10 years since the machine collided protons at an energy higher than previously achieved at a particle accelerator, researchers have been using it to try to hunt down an equally exciting particle: the hypothetical particle that may make up an invisible form of matter called dark matter, which is five times more prevalent than ordinary matter and without which there would be no universe as we know it. The LHC dark-matter searches have so far come up empty handed, as have non-collider searches, but the incredible work and skill put by the LHC researchers into finding it has led them to narrow down many of the regions where the particle may lie hidden – necessary milestones on the path to a discovery.

“The LHC has really broken new ground in the search for dark matter in the form of weakly interacting massive particles, by covering a wide array of potential signals predicted by either production of dark matter, or production of the particles mediating its interactions with ordinary matter. All of the observed results have been consistent with models that don’t include dark matter, and give us important information as to what kinds of particles can no longer explain it. The results have both pointed experimentalists in new directions for how to search for dark matter, and prompted theorists to rethink existing ideas for what dark matter could be – and in some cases to come up with new ones.”

Simulation of the dark-matter distribution in the universe. Credit: V. Springel et al. 2005.

Make it, break it and shake it

To look for dark matter, experiments essentially “make it, break it or shake it”. The LHC has been trying to make it by colliding beams of protons. Some experiments are using telescopes in space and on the ground to look for indirect signals of dark-matter particles as they collide and break themselves out in space. Others still are chasing these elusive particles directly by searching for the kicks, or “shakes”, they give to atomic nuclei in underground detectors.

The make-it approach is complementary to the break-it and shake-it experiments, and if the LHC detects a potential dark-matter particle, it will require confirmation from the other experiments to prove that it is indeed a dark-matter particle. By contrast, if the direct and indirect experiments detect a signal from a dark-matter particle interaction, experiments at the LHC could be designed to study the details of such an interaction.

Missing-momentum signal and bump hunting

An ATLAS detector event with missing transverse momentum. A photon with transverse momentum of 265 GeV (yellow bar) is balanced by 268 GeV of missing transverse momentum (red dashed line on the opposite side of the detector). (Image: ATLAS/CERN)

So how has the LHC been looking for signs of dark-matter production in proton collisions? The main signature of the presence of a dark-matter particle in such collisions is the so-called missing transverse momentum. To look for this signature, researchers add up the momenta of the particles that the LHC detectors can see – more precisely the momenta at right angles to the colliding beams of protons – and identify any missing momentum needed to reach the total momentum before the collision. The total momentum should be zero because the protons travel along the direction of the beams before they collide. But if the total momentum after the collision is not zero, the missing momentum needed to make it zero could have been carried away by an undetected dark-matter particle.

Missing momentum is the basis for two main types of search at the LHC. One type is guided by so-called complete new physics models, such as supersymmetry (SUSY) models. In SUSY models, the known particles described by the Standard Model of particle physics have a supersymmetric partner particle with a quantum property called spin that differs from that of its counterpart by half of a unit. In addition, in many SUSY models, the lightest supersymmetric particle is a weakly interacting massive particle (WIMP). WIMPs are one of the most captivating candidates for a dark-matter particle because they could generate the current abundance of dark matter in the cosmos. Searches targeting SUSY WIMPs look for missing momentum from a pair of dark-matter particles plus a spray, or “jet”, of particles and/or particles called leptons.

Another type of search involving the missing-momentum signature is guided by simplified models that include a WIMP-like dark-matter particle and a mediator particle that would interact with the known ordinary particles. The mediator can be either a known particle, such as the Z boson or the Higgs boson, or an unknown particle. These models have gained significant traction in recent years because they are very simple yet general in nature (complete models are specific and thus narrower in scope) and they can be used as benchmarks for comparisons between results from the LHC and from non-collider dark-matter experiments. In addition to missing momentum from a pair of dark-matter particles, this second type of search looks for at least one highly energetic object such as a jet of particles or a photon.

In the context of simplified models, there’s an alternative to missing-momentum searches, which is to look not for the dark-matter particle but for the mediator particle through its transformation, or “decay”, into ordinary particles. This approach looks for a bump over a smooth background of events in the collision data, such as a bump in the mass distribution of events with two jets or two leptons.

Narrowing down the WIMP territory

What results have the LHC experiments achieved from these WIMP searches? The short answer is that they haven’t yet found signs of WIMP dark matter. The longer answer is that they have ruled out large chunks of the theoretical WIMP territory and put strong limits on the allowed values of the properties of both the dark-matter particle and the mediator particle, such as their masses and interaction strengths with other particles. Summarising the results from the LHC experiments, ATLAS experiment collaboration member Caterina Doglioni says “We have completed a large number of dedicated searches for invisible particles and visible particles that would occur in processes involving dark matter, and we have interpreted the results of these searches in terms of many different WIMP dark-matter scenarios, from simplified models to SUSY models. This work benefitted from the collaboration between experimentalists and theorists, for example on discussion platforms such as the LHC Dark Matter Working Group (LHC DM WG), which includes theorists and representatives from the ATLAS, CMS and LHCb collaborations. Placing the LHC results in the context of the global WIMP search that includes direct- and indirect-detection experiments has also been a focus of discussion in the dark-matter community, and the discussion continues to date on how to best exploit synergies between different experiments that have the same scientific goal of finding dark matter.”

Giving a specific example of a result obtained with data from the ATLAS experiment, Priscilla Pani, ATLAS experiment co-convener of the LHC Dark Matter WG, highlights how the collaboration has recently searched the full LHC dataset from the machine’s second run (Run 2), collected between 2015 and 2018, to look for instances in which the Higgs boson might decay into dark-matter particles. “We found no instances of this decay but we were able to set the strongest limits to date on the likelihood that it occurs,” says Pani.

Phil Harris, CMS experiment co-convener of the LHC Dark Matter Working Group, highlights searches for a dark-matter mediator decaying into two jets, such as a recent CMS search based on Run 2 data.

Xabier Cid Vidal, LHCb experiment co-convener of the LHC Dark Matter WG, in turn notes how data from Run 1 and Run 2 on the decay of a particle known as the Bs meson has allowed the LHCb collaboration to place strong limits on SUSY models that include WIMPs. “The decay of the Bs meson into two muons is very sensitive to SUSY particles, such as SUSY WIMPs, because the frequency with which the decay occurs can be very different from that predicted by the Standard Model if SUSY particles, even if their masses are too high to be directly detected at the LHC, interfere with the decay,” says Cid Vidal.

Casting a wider net

“10 years ago, experiments (at the LHC and beyond) were searching for dark-matter particles with masses above the proton mass (1 GeV) and below a few TeV. That is, they were targeting classical WIMPs such as those predicted by SUSY. Fast forward 10 years and dark-matter experiments are now searching for WIMP-like particles with masses as low as around 1 MeV and as high as 100 TeV,” says Tait. “And the null results from searches, such as at the LHC, have inspired many other possible explanations for the nature of dark matter, from fuzzy dark matter made of particles with masses as low as 10^−22 eV to primordial black holes with masses equivalent to several suns. In light of this, the dark-matter community has begun to cast a wider net to explore a larger landscape of possibilities.”

The possible explanations for the nature of dark matter. Credit: Image: G. Bertone and T. M. P. Tait.

On the collider front, the LHC researchers have begun to investigate some of these new possibilities. For example, they have started looking at the hypothesis that dark matter is part of a larger dark sector with several new types of dark particles. These dark-sector particles could include a dark-matter equivalent of the photon, the dark photon, which would interact with the other dark-sector particles as well as the known particles, and long-lived particles, which are also predicted by SUSY models.

“We are now expanding upon the experimental methods that we are familiar with, so we can try to catch rare and unusual signals buried in large backgrounds. Moreover, many other current and planned experiments are also targeting dark sectors and particles interacting more feebly than WIMPs. Some of these experiments, such as the newly approved FASER experiment [below], are sharing knowledge, technology and even accelerator complex with the main LHC experiments, and they will complement the reach of LHC searches for non-WIMP dark matter, as shown by the CERN Physics Beyond Colliders initiative.”

Finally, the LHC researchers are still working on data from Run 2, and the data gathered so far, from Run 1 and Run 2, is only about 5% of the total that the experiments will record. Given this, as well as the immense knowledge gained from the many LHC analyses thus far conducted, there’s perhaps a fighting chance that the LHC will discover a dark-matter particle in the next 10 years. “It’s the fact we haven’t found it yet and the possibility that we may find it in the not-so-distant future that keeps me excited about my job,” says Harris. “The last 10 years have shown us that dark matter might be different from what we had initially thought, but that doesn’t mean it is not there for us to find,” says Cid Vidal.

See the full article here.

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Meet CERN in a variety of places:

Quantum Diaries

Cern Courier

THE FOUR MAJOR PROJECT COLLABORATIONS

ATLAS

CERN ATLAS Image Claudia Marcelloni CERN/ATLAS

ALICE

CERN/ALICE Detector

CMS

LHCb

LHC

CERN map

CERN LHC underground tunnel and tube.

SixTrack CERN LHC particles.

OTHER PROJECTS AT CERN

CERN AEGIS

CERN AEgIS 1T antimatter trap stack

CERN ALPHA

CERN ALPHA Antimatter Factory.

CERN ALPHA-g Detector

CERN ALPHA-g Detector

CERN AMS

CERN AMS.

CERN ACACUSA

CERN ASACUSA

CERN ATRAP

CERN ATRAP

CERN AWAKE

CERN AWAKE

CERN CAST

CERN CAST Axion Solar Telescope

CERN CLOUD

CERN CLOUD

CERN COMPASS

CERN COMPASS

CERN DIRAC

CERN DIRAC

CERN FASER

CERN FASER experiment schematic

CERN GBAR

CERN GBAR

CERN ISOLDE

CERN ISOLDE Looking down into the ISOLDE experimental hall.

CERN LHCf

CERN LHCf

CERN NA62

CERN NA62

CERN NA64.

CERN NTOF

CERN NTOF

CERN TOTEM

CERN TOTEM.

CERN UA9

CERN UA9

CERN Proto Dune

CERN Proto Dune

From CERN: “CERN meets quantum technology”

From CERN

28 September, 2020
Matthew Chalmers

The CERN Quantum Technology Initiative will explore the potential of devices harnessing perplexing quantum phenomena such as entanglement to enrich and expand its challenging research programme.

CERN AEgIS 1T antimatter trap stack

Today’s information and communication technology grew out of the invention and development of quantum mechanics during the last century. But, nifty as it is that billions of transistors can be packed into your smartphone or that photons are routed around the internet with the help of lasers, the devices underpinning the “first quantum revolution” merely rely on the weird properties of quantum mechanics – they don’t put them to use directly.

The CERN Quantum Technology Initiative (QTI), which was announced by CERN Director-General Fabiola Gianotti in June, sees CERN join a rapidly-growing global effort to bring about a “second quantum revolution” – whereby phenomena such as superposition and entanglement, which enable an object to be in two places at the same time or to influence another instantaneously, are exploited to build new computing, communication, sensing and simulation devices.

It is difficult to predict the impact of such quantum technologies on society, but for high-energy physics and CERN the benefits are clear. They include advanced computing algorithms to cope with future data-analysis challenges, ultrasensitive detectors to search for hidden-sector particles and gravitational waves, and the use of well-controlled quantum systems to simulate or reproduce the behaviour of complex many-body quantum phenomena for theoretical research.

Though relatively new to the quantum technologies scene, CERN is in the unique position of having in one place the diverse set of skills and technologies – including software, computing and data science, theory, sensors, cryogenics, electronics and material science – necessary for such a multidisciplinary endeavour. AEgIS at CERN’s Antiproton Decelerator, which is able to explore the multi-particle entangled nature of photons from positronium annihilation, is one of several examples of existing CERN experiments already working in relevant technology areas. CERN also provides valuable use cases to help compare classical and quantum approaches to certain applications, as demonstrated recently when a team at Caltech used a quantum computer comprising 1098 superconducting qubits to “rediscover” the Higgs boson from LHC data. CERN’s rich network of academic and industry relations working in unique collaborations such as CERN openlab is a further strength.

The path to CERN’s QTI began with a workshop on quantum computing in high-energy physics organised by CERN openlab in November 2018, which was followed by several initiatives, pilot projects and events. During the next three years, the initiative will assess the potential impact of quantum technologies on CERN and high-energy physics on the timescale of the HL-LHC (late 2030s) and beyond. Governance and operational instruments are being finalised and concrete R&D objectives are being defined in the four main quantum technologies areas: computing; sensing and metrology; communication; and simulation and information processing. The CERN QTI will also develop an international education and training programme in collaboration with experts, universities and industry, and identify mechanisms for knowledge sharing within the CERN Member States, the high-energy physics community, other scientific research communities and society at large.

“By taking part in this rapidly growing field, CERN not only has much to offer, but also stands to benefit directly from it,” says Alberto Di Meglio, coordinator of the CERN QTI and head of CERN openlab. “The CERN Quantum Technology Initiative, by helping structure and coordinate activities with our community and the many international public and private initiatives, is a vital step to prepare for this exciting quantum future.”

See the full article here.

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THE FOUR MAJOR PROJECT COLLABORATIONS

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CERN ATLAS Image Claudia Marcelloni CERN/ATLAS

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LHC

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From CERN: “Chasing particles with tiny electric charges”

From CERN

14 July, 2020
Ana Lopes

Researchers have conducted the first search at a hadron collider for elementary particles with electric charges smaller than a tenth of the electron charge.

Computer simulation of the proposed milliQan detector. The light blue represents the flash of light that would be produced in the detector by the passage of a millicharged particle. (Image: The milliQan collaboration)

All known elementary particles have electric charges that are integer multiples of a third of the electron charge. But some theories predict the existence of “millicharged” elementary particles that would have a charge much smaller that the electron charge and could account for the elusive dark matter that fills the universe. An international team of researchers has now reported [ https://arxiv.org/abs/2005.06518 ] the first search at the Large Hadron Collider (LHC) – and more generally at any hadron collider – for elementary particles with charges smaller than a tenth of the electron charge.

Many previous studies have tried and failed to find millicharged particles, both directly, at collider and non-collider experiments, and indirectly, using astronomical observations. But millicharged particles with masses between about 1 billion electron volts (GeV) and 100 GeV remain largely unexplored owing to the lack of sensitivity of current detectors to such particles.

This is where a proposed detector called milliQan could make a difference.

MilliQan detector. CERN

The detector would be sensitive to 1–100 GeV millicharged particles produced in proton–proton collisions at the LHC, through the flash of light created in its interior by the passage of such a particle. The detector has yet to be approved, and if approved then built, but a demonstrator detector that is a mere 1% of the full detector and was installed at the LHC in 2017 and gathered data in 2018 has now delivered promising results.

The data taken by the milliQan demonstrator rule out the existence of millicharged particles with masses between 20 and 4700 MeV for charges varying between 0.006 and 0.3 times the electron charge, depending on the mass. The results are consistent with those previously obtained by other experiments and represent a hadron collider’s first venture into the territory of particles with a charge smaller than 0.1 times the electron charge.

“We are very pleased by these results from the demonstrator. It has certainly achieved the original goal of providing feedback on our design and giving us experience with its operation, but to demonstrate that with only a 1% prototype we were already able to place new constraints on the properties of millicharged particles was a nice bonus. We are now quite confident that the full-scale milliQan detector will perform as expected, and we look forward to securing the funding to make this happen,” says Chris Hill, co-spokesperson of the milliQan collaboration.

See the full article here.

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From CERN: “The first accelerators are back in action”

From CERN

8 July, 2020
Corinne Pralavorio

It’s the end of Long Shutdown 2 for the PS Booster, the first accelerator to be recommissioned, alongside Linac 4.

The area where the injection line to the PS Booster (on the right) and the extraction line for the PS (on the left) intersect. These two transfer lines have been completely refurbished. The ring of the Booster is visible on the left (Image: CERN)

The CERN Control Centre is back in shift work mode, with walls of screens showing the status of the beams, and coffee flowing freely day and night. On Friday, 3 July, the Long Shutdown 2 accelerator coordination team handed over the key of the PS Booster to the accelerator operators. Linac 4 and the PS Booster thus become the first two accelerators to be recommissioned, 18 months after the start of LS2.

However, recommissioning will be far more complex than simply turning a key. When the operators handed the Booster over to the LS2 teams, they were driving a model built in the last century, and now they find themselves at the wheel of a completely transformed supercar. Work has been carried out on the engine (the power supply and power converters), the accelerator (the radiofrequency cavities), the steering (the magnets), the injection, the cooling circuit, the control and safety systems… in fact, a whole host of components have been replaced or upgraded (see below). “Around 40% of the machine has been replaced,” says David Hay, the “chief mechanic”, or engineer in charge of the coordination of LS2 activities at the PS Booster.

The aims of the work on this nearly 50-year-old accelerator, forming part of the LHC Injector Upgrade (LIU) project, were twofold: to accelerate the particles arriving at higher energies from the brand new Linac 4 and to increase the brightness of, or the concentration of particles in, the beam.

David Hay, who is responsible for LS2 coordination at the PS Booster, hands over a symbolic key to Bettina Mikulec, who leads the operations team for the PS Booster and Linac 4 (BE-OP-PSB). On the left, Julie Coupard, who is in charge of LS2 coordination for the injectors, and on the right, Gian Piero Di Giovanni, LIU project leader for the PS Booster, and Rende Steerenberg, Operations group leader (BE-OP) (Image: Maximilien Brice/CERN)

Linac 4, the new first link in the chain, accelerates negative hydrogen ions (protons surrounded by two electrons) up to an energy of 160 MeV (compared to 50 MeV previously for the protons from Linac 2). The higher energy and the new injection system, which converts the H- ions into protons, increase the brightness by a factor of two. This means that a beam with the same dimensions will contain twice as many particles. In order to preserve this brightness in the PS, the next accelerator in the chain, the Booster will increase the energy up to 2 GeV (compared to 1.4 GeV previously), thanks to its all-new acceleration system. The electrical repulsion effect between particles of the same charge (Coulomb repulsion) lessens as the energy increases. To put it another way, higher energy helps keep the particles close together and thus contributes to maintaining the brightness. And with more brightness, comes more luminosity. “The Booster is key to increasing the luminosity of the LHC,” explains Gian Piero Di Giovanni, project leader for LIU at the PS Booster, “because it effectively determines the brightness of the beam.” The new injection mode with H- ions and a higher energy will also considerably reduce the particle loss rate. “We will lose only 1 to 2% at injection, compared to over 30% with the old system,” says Di Giovanni.

The work at the Booster took 20 months above ground and 18 months underground. Despite the large scale of the renovations and the difficulties encountered with certain aspects of the civil-engineering work and of the cooling system for the RF cavities, not to mention lockdown, which froze activities for two months, the project has been completed on time. This achievement is down to the commitment of the teams and meticulous and proactive coordination.

Commissioning of some of the new systems started several weeks ago. The operators are now taking charge with new, cutting-edge control software. “We have spent the past two years developing the integration of these new systems,” emphasises Bettina Mikulec, who supervises the operation of the Booster and Linac 4. “We now need to implement and test all the subsystems from the Control Centre and get them working in harmony.” This complex commissioning process will take several months, initially without any beam. Whereas Linac 4 will resume tests with beam this summer, the first particles should be circulating in the PS Booster right at the end of the year.

The metamorphosis of the Booster

Power supply: A new power supply system, similar to the one that was installed for the PS (POPS), based on power converters and capacitors and known as POPS-B, has been installed in a new building above ground. The power converters will supply the magnets with electrical intensities of 5500 amps, compared with 4000 amps previously. Over 95% of the Booster’s power converters have been replaced since Long Shutdown 1. Some 318 new converters, ranging from 1 kW to several MW, supply all the components of the accelerator.
Cooling: The Booster has a new cooling system, with cooling towers in two renovated buildings.
Injection and ejection: To cope with the increase in energy and the use of negative hydrogen ions at injection, the transfer lines from Linac 4 to the Booster and from the Booster to the PS have all been replaced. This includes new magnets (kickers, septa, dipoles, quadrupoles and correctors), new instrumentation and new beam dumps. Since it comprises four superimposed rings, the Booster requires a particularly sophisticated particle distribution system.
Acceleration: The new acceleration system is composed of three structures, each housing eight cavities built using a magnetic material known as FineMet.
Magnets: In the transfer lines and the Booster ring itself, around 60 magnets have been replaced or renovated.
Safety and instrumentation: A whole host of new sensors, beam position monitors, beam loss monitors, wire scanners, etc. have been installed to monitor and measure the particle beams. Devices to stop the beam or particles that stray from the trajectory have been added to the ring. Among these, a collimation system known as an “absorber/scraper”, is the latest device to be installed in the Booster. The role of these devices is even more crucial now that the beam is denser.

See the full article here.

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Please help promote STEM in your local schools.

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Meet CERN in a variety of places:

Quantum Diaries

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THE FOUR MAJOR PROJECT COLLABORATIONS

ATLAS

CERN ATLAS Image Claudia Marcelloni CERN/ATLAS

ALICE

CERN/ALICE Detector

CMS

LHCb

LHC

CERN map

CERN LHC Tunnel

SixTRack CERN LHC particles

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.

See the full article here.

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Please help promote STEM in your local schools.

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THE FOUR MAJOR PROJECT COLLABORATIONS

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CERN ATLAS Image Claudia Marcelloni CERN/ATLAS

ALICE

CERN/ALICE Detector

CMS

LHCb

LHC

CERN map

CERN LHC Tunnel

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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”

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(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.

See the full article here .

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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

See the full article here.

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