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New tetraquark a whisker away from stability

**Jumbled together** T_cc⁺ could be a compact cc diquark tightly bound by gluons to light up and down antiquarks. Credit: D Dominguez

All the exotic hadrons that have been observed so far decay rapidly via the strong interaction. The ccūd̄ tetraquark (T_cc⁺ ) just discovered by the LHCb collaboration is no exception. However, it is the longest-lived state yet, and reinforces expectations that its beautiful cousin, bbūd , will be stable with respect to the strong interaction when its peak emerges in future data.

“We have discovered a ccūd tetraquark with a mass just below the D^*+D⁰ threshold which, according to most models, indicates that it is a bound state,” says LHCb analyst Ivan Polyakov (Syracuse University). “It still decays to D mesons via the strong interaction, but much less intensively than other exotic hadrons.”

Most of the exotic hadronic states discovered in the past 20 years or so are cc̄qq̄ tetraquarks or cc̄qqq pentaquarks, where q represents an up, down or strange quark. A year ago LHCb also discovered a hidden-double-charm cc̄cc̄ tetraquark, X(6900), and two open-charm csūd tetraquarks, X₀(2900) and X₁(2900). The new ccūd state, presented today at the European Physical Society conference on high-energy physics to have been observed with a significance substantially in excess of five standard deviations, is the first exotic hadronic state with so-called double open heavy flavour — in this case, two charm quarks unaccompanied by antiparticles of the same flavour.

Astoundingly, its observation by LHCb reveals that it is a mere 270 keV below the threshold

Prime Candidate

Tetraquark states with two heavy quarks and two light antiquarks have been the prime candidates for stable exotic hadronic states since the 1980s. LHCb’s discovery, four years ago, of the Ξ_cc⁺⁺ (ccu) baryon allowed QCD phenomenologists to firmly predict the existence of a stable bbūd tetraquark, however the stability of a potential ccūd state remained unclear. Predictions of the mass of the ccūd state varied substantially, from 250 MeV below to 200 MeV above the D^*+D⁰ mass threshold, say the team. Astoundingly, its observation by LHCb reveals that it is a mere 273 ± 61 keV below the threshold — a bound state, then, but with the threshold for strong decays to D^*+D⁰ lying within the observed resonance’s narrow width of 410 ± 165 keV, prescribed by the uncertainty principle. The T_cc⁺ tetraquark can therefore decay via the strong interaction, but strikingly slowly. By contrast, most exotic hadronic states have widths from tens to several hundreds of MeV.

“Such closeness to the threshold is not very common in heavy-hadron spectroscopy,” says analyst Vanya Belyaev (Kurchatov Institute/ITEP). “Until now, the only similar closeness was observed for the enigmatic χ_c1(3872) state, whose mass coincides with the D^*0D⁰ threshold with a precision of about 120 keV.” As it is wider, however, it is not yet known whether the χ_c1(3872) is below or above threshold.

I am fascinated by the idea that a strong coupling to a decay channel might attract the bare mass of the hadron
Mikhail Mikhasenko

“The surprising proximity of T_cc⁺ and χ_c1(3872) to the D^*D thresholds must have deep reasoning,” adds analyst Mikhail Mikhasenko (ORIGINS, Munich). “I am fascinated by the idea that, roughly speaking, a strong coupling to a decay channel might attract the bare mass of the hadron. Tremendous progress in lattice QCD over the past 10 years gives us hope that we will discover the answer soon.”

The cause of this attraction, says Mikhasenko, could be linked to a “quantum admixture” of two models that vie to explain the structure of the new tetraquark: it could be a D^*+ and a D⁰ meson, bound by the exchange of colourneutral objects such as light mesons, or a colour-charged cc “diquark” tightly bound via gluon exchange to up and down antiquarks (see “Jumbled together” figure). Diquarks are a frequently employed mathematical construct in low-energy quantum chromodynamics (QCD): if two heavy quarks are sufficiently close together, QCD becomes perturbative, and they may be shown to attract each other and exhibit effective anticolour charge. For example, a red-green cc diquark would have a wavefunction similar to an anti-blue anti-quark, and could pair up with a blue quark to form a baryon — or, hypothetically, a blue anti-diquark, to form a colour-neutral tetraquark.

“The question is if the D and D^* are more or less separated, jumbled together to such a degree that all quarks are intertwined in a compact object, or something in between,” says Polyakov. “The first scenario resembles a relatively large ~4 fm deuteron, whereas the second can be compared to a relatively compact ~2 fm alpha particle.”

The new T_cc⁺ tetraquark is an enticing target for further study. Its narrow decay into a D⁰D⁰π⁺ final state — the virtual D^*+ decays promptly into D⁰π⁺ — includes no particles that are difficult to detect, leading to a better precision on its mass than for existing measurements of charmed baryons. This, in turn, can provide a stringent test for existing theoretical models and could potentially probe previously unreachable QCD effects, says the team. And, if detected, its beautiful cousin would be an even bigger boon. “Observing a tightly bound exotic hadron that would be stable with respect to the strong interaction would be a cornerstone in understanding QCD at the scale of hadrons,” says Polyakov. “The bbūd , which is believed to satisfy this requirement, is produced rarely and is out of reach of the current luminosity of the LHC. However, it may become accessible in LHC Run 3 or at the High-Luminosity LHC.” In the meantime, there is no shortage of work in hadron spectroscopy, jokes Belyaev. “We definitely have more peaks than researchers!”

Charm breaks fragmentation universality

The study of heavy-flavour hadron production in proton–proton (pp) collisions provides an important test for quantum chromodynamics (QCD) calculations. Heavy-flavour hadron production is usually computed with perturbative–QCD (pQCD) calculations as the convolution of the parton distribution functions (PDFs) of the incoming protons, the partonic cross section and the fragmentation functions that describe the transition from charm quarks into charm hadrons. The latter are typically parametrised from measurements performed in e⁺e^– or ep collisions, under the assumption that the hadronisation of charm quarks into charm hadrons is a universal process that is independent of the colliding systems.

The assumption that charm-to-hadron fragmentation is universal is not valid

The large data samples collected during Run 2 of the LHC at √s = 5.02 TeV allowed the ALICE collaboration measure the vast majority of charm quarks produced in the pp collisions by reconstructing the decays of the ground-state charm hadrons, measuring all the charm-meson species and the most abundant charm baryons (Λ_c⁺, and Ξ_c^0,+) down to very low transverse momenta. The result was presented today at the European Physical Society conference on high-energy physics (EPS-HEP 2021).

Fig. 1. Charm–quark fragmentation fractions into charm hadrons measured in pp collisions at √s = 5.02 TeV (black squares), in e⁺e^–collisions (purple and yellow markers) and in ep collisions (closed and open red circles). The D* meson is depicted separately since its contribution is also included in the ground-state charm mesons.

Charm fragmentation fractions, f(c → H_c), represent the probability for a charm quark to hadronise into a given charm hadron. These have now been measured for the first time at the LHC in pp collisions at midrapidity, and, in the case of the Ξ_c⁰ , for the first time in any collision system (figure 1). The measured f(c → H_c) are observed to be different from those measured in e⁺e^– and ep collisions – evidence that the assumption that charm-to-hadron fragmentation is universal is not valid.

Charm quarks were found to hadronise into baryons almost 40% of the time – four times more often than at colliders with electron beams. Several models have been proposed to explain this “baryon enhancement”. The explanations feature various different assumptions, such as including hadronisation via coalescence, considering a set of as-yet-unobserved higher-mass charm-baryon states, and including string formation beyond the leading-colour approximation.

The cc̄ production cross section per unit of rapidity at midrapidity (dσ^cc̄/dy|_|y|_<0.5) was calculated by summing the cross sections of all measured ground-state charm hadrons (D⁰, D⁺, D_s⁺ , Λ_c⁺ , and Ξ_c⁰). The contribution of the Ξ_c⁰ was multiplied by a factor of two, in order to account for the contribution of the Ξ_c⁺. The resulting cc̄ cross section per unit of rapidity at midrapidity is dσ^cc̄/dy|_|y|_<0.5 = 1165 ± 44(stat) ⁺¹³⁴ _–101 (syst) μb. This measurement was obtained for the first time in hadronic collisions at the LHC including the charm-baryon states. The cc̄ cross section measured at the LHC lies at the upper edge of the theoretical pQCD calculations.

The measurements described above not only provide constraints to pQCD calculations, but also act as important references for investigating the interaction of charm quarks with the medium created in heavy-ion collisions. These measurements could be extended to include rarer baryons and studied as a function of the event multiplicity in pp and heavy-ion systems in future LHC runs.

Steven Weinberg 1933–2021

Steven Weinberg, one of the greatest theoretical physicists of all time, passed away on 23 July, aged 88. He revolutionised particle physics, quantum field theory and cosmology with conceptual breakthroughs that still form the foundation of our understanding of physical reality.

Weinberg is well known for the unified theory of weak and electromagnetic forces, which earned him the Nobel Prize in Physics in 1979, jointly awarded with Sheldon Glashow and Abdus Salam, and led to the prediction of the Z and W vector bosons, later discovered at CERN in 1983. His breakthrough was the realisation that some new theoretical ideas, initially believed to play a role in the description of nuclear strong interactions, could instead explain the nature of the weak force. “Then it suddenly occurred to me that this was a perfectly good sort of theory, but I was applying it to the wrong kind of interaction. The right place to apply these ideas was not to the strong interactions, but to the weak and electromagnetic interactions,” as he later recalled. With his work, Weinberg had made the next step in the unification of physical laws, after Newton understood that the motion of apples on Earth and planets in the sky are governed by the same gravitational force, and Maxwell understood that electric and magnetic phenomena are the expression of a single force.

In my life, I have built only one model
Steven Weinberg

In his research, Weinberg always focused on an overarching vision of physics and not on a model description of any single phenomenon. At a lunch among theorists, when a colleague referred to him as a model builder, he jokingly retorted: “I am not a model builder. In my life, I have built only one model.” Indeed, Weinberg’s greatest legacy is his visionary approach to vast areas of physics, in which he starts from complex theoretical concepts, reinterprets them in original ways, and applies them to the description of the physical world. A good example is his construction of effective field theories, which are still today the basic tool to understand the Standard Model of particle interactions. His inimitable way of thinking has been the inspiration and guidance for generations of physicists, and it will certainly continue to serve future generations.

Steven Weinberg is among the very few individuals who, during the course of the history of civilisation, have radically changed the way we look at the universe.

Cosmic-ray anisotropy probed across 10 decades in energy

Spanning 13 decades in energy and more than 26 decades in intensity, cosmic rays are one of the hottest topics in astroparticle physics today. Spectral features such as a “knee” at a few PeV and an “ankle” at a few EeV give insights into their varying origins, but studies of their arrival direction can also provide valuable information. Though magnetic fields mean we cannot normally trace cosmic rays directly back to their point of origin, angular anisotropies provide important independent evidence towards probable sources at different energies. This week, at the 37th International Cosmic Ray Conference (ICRC), a range of space- and ground-based experiments greatly increased our knowledge of cosmic-ray anisotropies, with new results spanning 10 decades in energy, from GeV to tens of EeV.

Vela Supernova Remnant — **Spectral** Nearby supernova remnants such as Vela could explain sub-TeV spectral features of cosmic rays. Credit: ESO/J Pérez

At sub-TeV energies, spectral features seen by the AMS-02 and CALET detectors on the International Space Station and the Chinese–European DAMPE satellite could potentially be explained by a local galactic source such as a supernova remnant like Vela (see “Spectral” figure). If a nearby source is indeed responsible for a significant fraction of the cosmic rays observed at such energies, it could show up in the arrival direction of these cosmic rays in the form of a dipole feature, despite bending by galactic magnetic fields; however, results from AMS-02 at ICRC showed no evidence of a dipole in the arrival direction of protons or any other light nucleus. This was confirmed by DAMPE, which excluded dipole features with amplitudes above about 0.1% in the 100s of GeV energy range. The search continues, however, with DAMPE, AMS-02 and CALET all set to take further data over the coming years.

Close to the knee, the dipole has a maximum rather than a minimum close to the galactic centre

Moving to higher energies, clear anisotropic dipole excesses have been observed over the last decade by ground-based experiments such as the ARGO-YBJ observatory in China, the HAWC observatory in Mexico and the IceCube observatory at the South Pole – though with different “phases” at different energies. The anisotropy in the TeV to the 100s of TeV energy range could point towards a nearby source, though models proposing the structure of the interstellar magnetic field as the true origin for the anisotropy also exist. This feature was further confirmed this year by the LHAASO experiment in China, using a year of data that was taken while constructing the detector. The results from LHAASO also confirm a switch in the phase of the anisotropy when moving from 100s of TeV to PeV energies, as reported by IceCube and other experiments in recent years: at PeV energies, close to the knee, the dipole has a maximum rather than a minimum close to the galactic centre. This could indicate an excess of “pevatron” sources near the galactic centre.

Antennae Galaxies — **Antennae galaxies** Starburst galaxies such as that resulting from the collision of NGC 4038 and NGC 4039 could explain cosmic-ray anisotropies at energies exceeding EeV. Credit: NASA/ESA

Extragalactic sources

While results up to PeV energies give an insight into sources within our galaxy, it is theorised that the flux starts to be dominated by extragalactic sources somewhere between the knee and the ankle of the cosmic-ray spectrum. Evidence for this was increased by new results from the Pierre Auger Observatory in Argentina and the Telescope Array in the US. These two observatories, which observe different hemispheres, find strong evidence for excesses in the cosmic-ray flux in certain regions of the sky at energies exceeding EeV. At energies as high as these, cosmic rays point more clearly to their origin, and galactic cosmic rays should have very clear point-like sources that are not observed, providing evidence that they originate outside of our galaxy. A prime candidate for such sources are so-called starburst galaxies, wherein star formation happens unusually rapidly, during a short period of the galaxy’s evolution (see “Antennae galaxies” figure). As presented at ICRC 2021, the available data was fitted to models where starburst galaxies are the primary source of EeV cosmic rays. The model fits the anisotropy data with more than 4σ significance relative to the null hypothesis with normal galaxies, indicating starburst galaxies to likely be at least one source of EeV cosmic rays.

While some of the features will likely be fully confirmed within the coming years simply by accumulating statistics, new features are also likely to arise. One example is further constraints on the lack of any observed anisotropy at sub-TeV energies using data from space-based missions, while new data from ground-based experiments will start to bridge the measurement gap between PeV and EeV energies. The latter will be especially important in gaining an understanding of the energy scale at which extragalactic sources start to dominate. To fully exploit the data it will be necessary to compare complex cosmic-ray-propagation simulations with diverse data such as the pevatron sources discovered this year by LHAASO.

Latvia to become Associate Member of CERN

Lativan prime minister Krišjānis Kariņš and Fabiola Gianotti — **Strengthening ties** Lativan prime minister Krišjānis Kariņš and CERN Director-General Fabiola Gianotti on the occasion of the remote signature of the agreement. Credit: CERN-PHOTO-202104-049-17

On 14 April, representatives of CERN and the Republic of Latvia gathered in a virtual ceremony to sign an agreement admitting Latvia as an Associate Member State.

Latvia, which is the third of the Baltic States to join CERN in recent years after Lithuania and Estonia, first became involved with CERN activities in the early 1990s. Latvian researchers have since participated in many CERN projects, including contributions to the CMS hadron calorimeter and, more recently, participation in the Future Circular Collider study.

“As we become CERN’s newest Associate Member State, we look forward to enhancing our contribution to the Organization’s major scientific endeavours, as well as to investing the unparalleled scientific and technological excellence gained by this membership in further building the economy and well-being of our societies,” said Latvian prime minister Krišjānis Kariņš.

As an Associate Member State, Latvia will be entitled to appoint representatives to attend meetings of the CERN Council and Finance Committee. Its nationals will be eligible for staff positions, fellowships and studentships, and its industries will be entitled to bid for CERN contracts, increasing opportunities for collaboration in advanced technologies.

“We are delighted to welcome Latvia as a new Associate Member State,” said CERN Director-General Fabiola Gianotti. “The present agreement contributes to strengthening the ties between CERN and Latvia, thereby offering opportunities for the further growth of particle physics in Latvia through partnership in research, technological development and education.”

Long-lived particles gather interest

From 25 to 28 May, the long-lived particle (LLP) community marked five years of stretching the limits of searches for new physics with its ninth and best-attended workshop yet, with more than 300 registered participants.

LLP9 played host to six new results, three each from ATLAS and CMS. These included a remarkable new ATLAS paper searching for stopped particles – beyond-the-Standard Model (BSM) LLPs that can be produced in a proton–proton collision and then get stuck in the detector before decaying minutes, days or weeks later. Good hypothetical examples are the so-called gluino R-hadrons that occur in supersymmetric models. Also featured was a new CMS search for displaced di-muon resonances using “data scouting” – a unique method of increasing the number of potential signal events kept at the trigger level by reducing the event information that is retained. Both experiments presented new results searching for the Higgs boson decaying to LLPs (see “LLP candidate” figure).

Long-lived particles can also be produced in a collision inside ATLAS, CMS or LHCb and live long enough to drift entirely outside of the detector volume. To ensure that this discovery avenue is also covered for the future of the LHC’s operation, there is a rich set of dedicated LLP detectors either approved or proposed, and LLP9 featured updates from MoEDAL, FASER, MATHUSLA, CODEX-b, MilliQan, FACET and SND@LHC, as well as a presentation about the proposed forward physics facility for the High-Luminosity LHC (HL-LHC).

Reinterpreting machine learning

The liveliest parts of any LLP community workshop are the brainstorming and hands-on working-group sessions. LLP9 included multiple vibrant discussions and working sessions, including on heavy neutral leptons and the ability of physicists who are not members of experimental collaborations to be able to re-interpret LLP searches – a key issue for the LLP community. At LLP9, participants examined the challenges inherent in re-interpreting LLP results that use machine learning techniques, by now a common feature of particle-physics analyses. For example, boosted decision trees (BDTs) and neural networks (NNs) can be quite powerful for either object identification or event-level discrimination in LLP searches, but it’s not entirely clear how best to give theorists access to the full original BDT or NN used internally by the experiments.

LLP searches at the LHC often must also grapple with background sources that are negligible for the majority of searches for prompt objects. These backgrounds – such as cosmic muons, beam-induced backgrounds, beam-halo effects and cavern backgrounds – are reasonably well-understood for Run 2 and Run 3, but little study has been performed for the upcoming HL-LHC, and LLP9 featured a brainstorming session about what such non-standard backgrounds might look like in the future.

Also looking to the future, two very forward-thinking working-group sessions were held on LLPs at a potential future muon collider and at the proposed Future Circular Collider (FCC). Hadron collisions at ~100 TeV in FCC-hh would open up completely unprecedented discovery potential, including for LLPs, but it’s unclear how to optimise detector designs for both LLPs and the full slate of prompt searches.

Simulating dark showers is a longstanding challenge

Finally, LLP9 hosted an in-depth working-group session dedicated to the simulation of “dark showers”, in collaboration with the organisers of the dark-showers study group connected to the Snowmass process, which is currently shaping the future of US particle physics. Dark showers are a generic and poorly understood feature of a potential BSM dark sector with similarities to QCD, which could have its own “dark hadronisation” rules. Simulating dark showers is a longstanding challenge. More than 50 participants joined for a hands-on demonstration of simulation tools and a discussion of the dark-showers Pythia module, highlighting the growing interest in this subject in the LLP community.

LLP9 was raucous and stimulating, and identified multiple new avenues of research. LLPX, the tenth workshop in the series, will be held in November this year.

Sustainable high-energy physics

**Sustainable HEP** Before the pandemic, a typical high-energy physics researcher crossed the globe several times a year for conferences, collaboration meetings and detector shifts. Credit: SustHEP 2021

COVID-19 put the community on a steep learning curve regarding new forms of online communication and collaboration. Before the pandemic, a typical high-energy physics (HEP) researcher was expected to cross the world several times a year for conferences, collaboration meetings and detector shifts, at the cost of thousands of dollars and a sizeable carbon footprint. The online workshop Sustainable HEP — a new initiative this year — attracted more than 300 participants from 45 countries from 28 to 30 June to discuss how the lessons learned in the past two years might help HEP transition to a more sustainable future.

The first day of the workshop focused on how new forms of online interaction could change our professional travel culture. Shaun Hotchkiss (University of Auckland) stressed in a session dedicated to best-practice examples that the purpose of online meetings should not simply be to emulate traditional 20th-century in-person conferences and collaboration meetings. Instead, the community needs to rethink what virtual scientific exchange could look like in the 21st century. This might, for instance, include replacing traditional live presentations by pre-recorded talks that are pre-watched by the audience at their own convenience, leaving more precious conference time for in-depth discussions and interactions among the participants.

Social justice

The second day highlighted social-justice issues, and the potential for greater inclusivity using online formats. Alice Gathoni (British Institute in Eastern Africa) powerfully described the true meaning of online meetings to her: everyone wants to belong. It was only during the first online meetings during the pandemic that she truly felt a real sense of belonging to the global scientific community.

The third day was dedicated to existing sustainability initiatives and new technologies. Mike Seidel (PSI) presented studies on energy-recovery linacs and discussed energy-management concepts for future colliders, including daily “standby modes”. Other options include beam dynamics explicitly designed to maximise the ratio of luminosity to power, more efficient radio-frequency cavities, the use of permanent magnets, and high-temperature superconductor cables and cavities. He concluded his talk by asking thought-provoking questions such as whether the HEP community should engage with its international networks to help establish sustainable energy-supply solutions.

The workshop ended by drafting a closing statement that calls upon the HEP community to align its activities with the Paris Climate Agreement and the goal of limiting global warming to 1.5 degrees. This statement can be signed by members of the HEP community until 20 August.

AI and GPUs take centre stage at vCHEP

vCHEP2021 group photo — **vCHEP 2021** More than 1000 participants took part across 20 time zones, from Brisbane to Honolulu. Credit: vCHEP 2021

The 25th International Conference on Computing in High-Energy and Nuclear Physics (CHEP) gathered more than 1000 participants online from 17 to 21 May. Dubbed “vCHEP”, the event took place virtually after this year’s in-person event in Norfolk, Virginia, had to be cancelled due to the COVID-19 pandemic. Participants tuned in across 20 time zones, from Brisbane to Honolulu, to live talks, recorded sessions, excellent discussions on chat apps (to replace the traditional coffee-break interactions) and special sessions that linked job seekers with recruiters.

Given vCHEP’s virtual nature this year, there was a different focus on the content. Plenary speakers are usually invited, but this time the organisers invited papers of up to 10 pages to be submitted, and chose a plenary programme from the most interesting and innovative. Just 30 had to be selected from more than 200 submissions — twice as many as expected — but the outcome was a diverse programme tackling the huge issues of data rate and event complexity in future experiments in nuclear and high-energy physics (HEP).

Artificial intelligence

So what were the hot topics at vCHEP? One outstanding one was artificial intelligence and machine learning. There were more papers submitted on this theme than any other, showing that the field is continuing to innovate in this domain.

Interest in using graph neural networks for the problem of charged-particle tracking was very high, with three plenary talks. Using a graph to represent the hits in a tracker as nodes and possible connections between hits as edges is a very natural way to represent the data that we get from experiments. The network can be effectively trained to pick out the edges representing the true tracks and reject those that are just spurious connections. The time needed to get to a good solution has improved dramatically in just a few years, and the scaling of the solution to dense environments, such as at the High-Luminosity LHC (HL-LHC), is very promising for this relatively new technique.

ATLAS showed off their new fast-simulation framework

On the simulation side, work was presented showcasing new neural-network architectures that use a “bounded information-bottleneck autoencoder” to improve training stability, providing a solution that replicates important features such as how real minimum-ionising particles interact with calorimeters. ATLAS also showed off their new fast-simulation framework, which combines traditional parametric simulation with generative adversarial networks, to provide better agreement with Geant4 than ever before.

New architectures

Machine learning is very well suited to new computing architectures, such as graphics processing units (GPUs), but many other experimental-physics codes are also being rewritten to take advantage of these new architectures. IceCube are simulating photon transport in the Antarctic ice on GPUs, and presented detailed work on their performance analysis that led to recent significant speed-ups. Meanwhile, LHCb will introduce GPUs to their trigger farm for Run 3, and showed how much this will improve the energy consumption per event of the high-level trigger. This will help to meet the physical constraints of power and cooling close to the detector, and is a first step towards bringing HEP’s overall computing energy consumption to the table as an important parameter.

LHCb will introduce GPUs to their trigger farm for Run 3

Encouraging work on porting event generation to GPUs was also presented — particularly appropriately, given the spiralling costs of higher order generators for HL-LHC physics. Looking at the long-term future of these new code bases, there were investigations of porting calorimeter simulation and liquid-argon time-projection chamber software to different toolkits for heterogeneous programming, a topic that will become even more important as computing centres diversify their offerings.

Keeping up with benchmarking and valuing these heterogeneous resources is an important topic for the Worldwide LHC Computing Grid, and a report from the HEPiX Benchmarking group pointed to the future for evaluating modern CPUs and GPUs for a variety of real-world HEP applications. Staying on the facilities topic, R&D was presented on how to optimise delivering reliable and affordable storage for HEP, based on CephFS and the CERN-developed EOS storage system. This will be critical to providing the massive storage needed in the future. The network between facilities will likely become dynamically configurable in the future, and how best to take advantage of machine learning for traffic prediction is being investigated.

Quantum computing

vCHEP was also the first edition of CHEP with a dedicated parallel session on quantum computing. Meshing very well with CERN’s Quantum Initiative, this showed how seriously investigations of how to use this technology in the future are being taken. Interesting results on using quantum support-vector machines to train networks for signal/background classification for B-meson decays were highlighted.

On a meta note, presentations also explored how to adapt outreach events to a virtual setup, to keep up public engagement during lockdown, and how best to use online software training to equip the future generation of physicists with the advanced software skills they will need.

Was vCHEP a success? So far, the feedback is overwhelmingly positive. It was a showcase for the excellent work going on in the field, and 11 of the best papers will be published in a special edition of Computing and Software for Big Science — another first for CHEP in 2021.

Experiment and theory trade blows at SQM 2021

The 19th international conference on strangeness in quark matter (SQM) was hosted virtually by Brookhaven National Laboratory from 17 to 22 May, attracting more than 300 participants. The series deals with the role of strange and heavy-flavour quarks in high-energy heavy-ion collisions and astrophysical phenomena.

New results on the production of strangeness in heavy-ion collisions were presented for a variety of collision energies and systems. In an experimental highlight, the ALICE collaboration reported that the number of strange baryons depends more on the final-state multiplicity than the initial-state energy. On the theory side, it was shown that several models can explain the suppression of strange particles at low multiplicities. ALICE also presented new measurements of the charm cross section and fragmentation functions in proton–proton (pp) collisions. When compared to e⁺e^– collisions, these results suggest that the universality of parton-to-hadron fragmentation may be broken.

Moving on to heavy flavours, the ATLAS collaboration presented results for the suppression of heavy-flavour production compared to pp collisions and the angular anisotropy of heavy mesons in heavy-ion collisions. These measurements are crucial for constraining models of in-medium energy loss. Interestingly, while charm seems to follow the flow of the quark–gluon plasma, beauty does not seem to flow. Better statistics are needed to constrain theoretical models. On the theory side, extremely interesting new calculations using open quantum systems coupled with potential non-relativistic QCD calculations were used to compute both the suppression and anisotropic flow of bottomonium states.

Hints of extrema

Another important goal of the field is to determine experimentally whether a critical point exists in the phase diagram of strongly interacting matter, and, if so, where it is located. The STAR experiment at the Relativistic Heavy Ion Collider (RHIC) presented results on higher order cumulants of net-proton fluctuations over a range of collision energies. Extrema as a function of beam energy are expected to indicate critical behaviour. New data from the Beam Energy Scan II programme at RHIC is expected to provide much-needed statistics to confirm hints of extrema in the data. On the theory side, new lattice QCD calculations of second-order net-baryon cumulants were presented, as well as new expansion schemes to extend the lattice-QCD equation of state to larger net baryon chemical potentials that are not computable directly, because of the fermion-sign problem. Another study included the lattice-QCD equation of state and susceptibilities in a hydrodynamic calculation to allow for a more direct comparison to experimental measurements of net-proton fluctuations. Significant differences between net-proton and net-baryon fluctuations were quantified.

The study of the quark–gluon plasma’s vorticity via the measurement of the polarisation of hyperons was also a major topic. Theoretical calculations obtain the opposite sign to the data for the angular differential measurement. Attempts to solve this discrepancy presented at SQM 2021 featured shear-dependent terms and a stronger “memory” of the strange-quark spin.

Various new applications of machine learning and artificial intelligence were also discussed, for example, for determining the order of the phase transition and constraining the neutron-star equation of state.

Overall, there were 41 plenary and 96 parallel talks at SQM 2021, poignantly including presentations in memory of Jean Cleymans, Jean Letessier, Dick Majka and Jack Sandweiss, who all made exceptional impacts on the field.

The next SQM conference will be held from 13 to 18 June 2022 in Busan, South Korea.

IPAC thrives online

The annual International Particle Accelerator Conference (IPAC) promotes collaboration among scientists, engineers, technicians, students and industrial partners across the globe. Originally to be hosted this year by the Laboratório Nacional de Luz Síncrotron (LNLS) in Campinas, Brazil, the conference was moved online when it became clear that the global pandemic would prohibit travel. IPAC21 was nevertheless highly successful, attracting more than 1750 participants online from 24 to 28 May. Despite the technical and logistical challenges, the virtual platform provided many advantages, including low or zero registration fees and a larger, younger and more diverse demographic than typical in-person events, which tend to attract about 1000 delegates.

In order to allow worldwide virtual participation, live plenary presentations were limited to two hours daily. Highlights included Harry Westfahl, Jr. (LNLS) on the scientific capabilities of fourth-generation storage-ring light sources; Thomas Glasmacher (FRIB) on the newly commissioned Facility for Rare Isotope Beams at Michigan State University; Norbert Holtkamp (SLAC) on the future of high-power free-electron lasers; Houjun Qian (DESY) on radio-frequency photocathode guns; and Young-Kee Kim (University of Chicago) on future directions in US particle physics. The closing plenary talk was a sobering presentation on climate change and the Brazilian Amazonia region by Paulo Artaxo (University of São Paulo).

The remainder of the talks were pre-recorded with live Q&A sessions, and 400 teleconferencing rooms per day were set up to allow virtual poster sessions. Highlights in topical sessions included “Women in Science: The Inconvenient Truth” by Márcia Barbosa (Universidade Federal do Rio Grande do Sul) and an industrial forum hosted by Raffaella Geometrante (KYMA) on the intersection between government accelerator projects and industry.

IPAC22 is currently planned as an in-person conference in Bangkok, Thailand, from 17 to 22 June next year.

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