Jobs – Page 28 of 527

A pevatron at the galactic centre

**Below the knee** Best-fit HAWC spectral energy distribution (pink) for HAWC J1746–2856 and (dashed purple) after subtracting the two HESS point-source spectra. The latter, which agrees well with the diffuse gamma-ray emission from the galactic centre measured by HESS in 2016 (green), describes a point source with power-law spectrum extending to energies > 100 TeV, confirming the presence of a galactic pevatron. Credit: arXiv:2407.03682

The measured all-particle energy spectrum for cosmic rays (CRs) is famously described by a steeply falling power law. The spectrum is almost featureless from energies of around 30 GeV to 3 PeV, where a break (also known as the “knee”) is encountered, after which the spectrum becomes steeper. It is believed that CRs with energies below the knee have galactic origins. This is supported by the observation of diffuse gamma rays from the galactic disk in the GeV range (a predominant mechanism for the production of gamma rays is via the decay of neutral pions created when relativistic protons interact with the ambient gas). The knee could be explained by either the maximum energy that galactic sources can accelerate CR particles to, or the escape of CR particles from the galaxy if they are energetic enough to overcome the confinement of galactic magnetic fields. Both scenarios, however, assume the presence of astrophysical sources within the galaxy that could accelerate CR particles up to PeV energies. For decades, scientists have therefore been on the hunt for such sources, reasonably called “pevatrons”.

Recently, researchers at the High-Altitude Water Cherenkov (HAWC) observatory in Mexico reported the observation of ultra-high energy (> 100 TeV) gamma rays from the central region of the galaxy. Using nearly seven years of data, the team found that a point source, HAWC J1746-2856, with a simple power-law spectrum and no signs of a cutoff from 6 to 114 TeV best describes the observed gamma-ray flux. A total of 98 events were observed at energies above 100 TeV.

To analyse the spatial distribution of the observed gamma rays, the researchers plotted a significance map of the galactic centre. On this map, they also plotted the point-like supernova remnant SNR G0.9+0.1 and an unidentified extended source HESS J1745-303, both located 1° away from the galactic centre. While supernova remnants have long been a favoured candidate for galactic pevatrons, HAWC did not observe any excess at either of these source positions. There are, however, two other interesting point sources in this region: Sgr A* (HESS J1745-290), the supermassive black hole in the galactic centre; and HESS J1746-285, an unidentified source that is spatially coincident with the galactic radio arc. Imaging atmospheric Cherenkov telescopes such as HESS, VERITAS and MAGIC have measured the gamma-ray emissions from these sources up to an energy of about 20 TeV, but HAWC has an angular resolution about six times larger at such energies and therefore cannot resolve them.

To eliminate the contamination to the flux from these sources, the authors assumed that their spectra cover the full HAWC energy range and then estimated the event count by convolving the reported best-fit model from HESS with the instrument-response functions of HAWC. The resulting HAWC spectral energy distribution, after subtracting these sources (see figure), seems to be compatible with the diffuse emission data points from HESS while still maintaining a power-law behaviour, with no signs of a cutoff and extending up to at least 114 TeV. This is the first detection of gamma rays at energies > 100 TeV from the galactic centre, thereby providing convincing evidence of the presence of a pevatron.

This is the first detection of gamma rays at energies > 100 TeV from the galactic centre

Furthermore, the diffuse emission is spatially correlated with the morphology of the central molecular zone (CMZ) – a region in the innermost 500 pc of the galaxy consisting of enormous molecular clouds corresponding to around 60 million solar masses. Such a correlation supports a hadronic scenario for the origin of cosmic rays, where gamma rays are produced via the interaction of relativistic protons with the ambient gas. In the leptonic scenario, electrons with energies above 100 TeV produce gamma rays via inverse Compton scattering, but such electrons suffer severe radiative losses; for a magnetic field strength of 100 μG, the maximum distance that such electrons can traverse is much smaller than the CMZ. On the other hand, in the hadronic case the escape time for protons is orders of magnitude shorter than the cooling time (via π⁰ decay). The stronger magnetic field could confine them for a longer period but, as the authors argue, the escape time is also much smaller than the age of the galaxy, thereby pointing to a young source that is quasi-continuously injecting and accelerating protons into the CMZ.

The study also computes the energy density of cosmic-ray protons with energies above 100 TeV to be 8.1 × 10^–3 eV/cm³. This is higher than the 1 × 10^–3 eV/cm³ local measurement from the Alpha Magnetic Spectrometer in 2015, indicating the presence of newly accelerated protons in the energy range 0.1–1 PeV. The capabilities of this study did not extend to the identification of the source, but with better modelling of the CMZ in the future, and improved performances of upcoming observatories such as CTAO and SWGO, candidate sites in the galactic centre are expected to be probed with much higher resolution.

Two charming results of data parking

CMS figure 1 — **Fig. 1.** Invariant mass distributions of D^*+ → D⁰ π⁺ (left) and D^*– → D⁰π^– (right), where the neutral D meson decays into K⁰_s K⁰_s, with the fit projections overlaid. The difference in signal yields is illustrated by the orange horizontal line. Credit: CMS Collab.

The high data rate at the LHC creates challenges as well as opportunities. Great care is required to identify interesting events, as only a tiny fraction can trigger the detector’s readout. With the LHC achieving record-breaking instantaneous luminosity, the CMS collaboration has innovated to protect and expand its flavour-physics programme, which studies rare decays and subtle differences between particles containing beauty and charm quarks. Enhancements in the CMS data-taking strategy such as “data parking” have enabled the detector to surpass its initial performance limits. This has led to notable advances in charm physics, including CMS’s first analysis of CP violation in the charm sector and achieving world-leading sensitivity to the rare decay of the D⁰ meson into a pair of muons.

Data parking stores subsets of unprocessed data that cannot be processed promptly due to computing limitations. By parking events triggered by a single muon, CMS collected an inclusive sample of approximately 10 billion b-hadrons in 2018. This sample allowed CMS to reconstruct D⁰ and D⁰ decays into a pair of long-lived K⁰_s mesons, which are relatively easy to detect in the CMS detector despite the high level of pileup and the large number of low-momentum tracks.

CP violation is necessary to explain the matter–antimatter asymmetry observed in the universe, but the magnitude of CP violation from known sources is insufficient. Charmed meson decays are the only meson decays involving an up-type quark where CP violation can be studied. CP violation would be evident if the decay rates for D⁰ → K⁰_s K⁰_s and D⁰ → K⁰_s K⁰_s were found to differ. In the analysis, the flavour of the initial D⁰ or D⁰ meson is determined from the charge of the pion accompanying its creation in the decay of a D^*⁺ meson (see figure 1). To eliminate systematic effects arising from the charge asymmetry in production and detector response, the CP asymmetry is measured relative to that in D⁰ → K⁰_s π⁺π^–. The resulting asymmetry is found to be A_CP(K_SK_S) = 6.2% ± 3.0% (stat) ± 0.2% (syst) ± 0.8% (PDG), consistent with no CP violation within 2.0 standard deviations. Previous analyses by LHCb and Belle were consistent with no CP violation within 2.7 and 1.8 standard deviations, respectively. Before data parking, searching for direct CP violation in the charm sector with a fully hadronic final state was deemed unattainable for CMS.

The CMS collaboration has expanded its flavour-physics programme

For Run 3 the programme was enhanced by introducing an inclusive dimuon trigger covering the low mass range up to 8.5 GeV. With improvements in the CMS Tier-0 prompt reconstruction workflow, Run-3 parking data is now reconstructed without delay using the former Run-2 high-level trigger farm at LHC Point 5 and European Tier-1 resources. In 2024 CMS is collecting data at rates seven times higher than the nominal rates for Run 2, already reaching approximately 70% of the nominal trigger rate for the HL-LHC.

Using the data collected in 2022 and 2023, CMS performed a search for the rare D⁰-meson decay into a pair of muons, which was presented at the ICHEP conference in Prague. Rare decays of the charm quark, less explored compared to those of the bottom quark, offer an opportunity to probe new physics effects beyond the direct reach of current colliders, thanks to possible quantum interference by unknown heavy virtual particles. In 2023, the LHCb collaboration set an upper limit for the branching ratio at 3.5 × 10^–9 at a 95% confidence using Run-2 data. CMS surpassed the LHCb result, achieving a sensitivity of 2.6 × 10^–9 at a 95% confidence. Given that the Standard Model prediction is four orders of magnitude smaller, there is still considerable territory to explore.

Beginning with the 2024 run, the CMS flavour-physics programme will gain an additional data stream known as data scouting. This stream captures at very high-rate events triggered by new high-purity single muon level-one triggers in a reduced format. This format is suitable for reconstructing decays of heavy hadrons, offering performance comparable to standard data processing.

Lattice calculations start to clarify muon g-2

In 1974, Kenneth G Wilson suggested modelling the continuous spacetime of quantum chromodynamics (QCD) with a discrete lattice – space and time would be represented as a grid of points, with quarks on the lattice points and gluons on the links between them. Lattice QCD has only grown in importance since, with international symposia on lattice field theory taking place annually since 1984. Since then the conference has developed and by now furnishes an important forum for both established experts and early-career researchers alike to report recent progress, and the published proceedings provide a valuable resource. The 41st symposium, Lattice 2024, welcomed 500 participants to the University of Liverpool from 28 July to 3 August.

Hadronic contributions

One of the highest profile topics in lattice QCD is the evaluation of hadronic contributions to the magnetic moment of the muon. For many years, the experimental measurements from Brookhaven and Fermilab have appeared to be in tension with the Standard Model (SM), based on theoretical predictions that rely on data from e⁺e^– annihilation to hadrons. Intense work on the lattice by multiple groups is now maturing rapidly and providing a valuable cross-check for data-driven SM calculations.

At the lowest order in quantum electrodynamics, the Dirac equation accounts for precisely two Bohr magnetons in the muon’s magnetic moment (g = 2) – a contribution arising purely from the muon interacting with a single real external photon representing the magnetic field. At higher orders in QED, virtual Standard Model particles modify that value, leading to a so-called anomalous magnetic moment g–2. The Schwinger term adds a virtual photon and a contribution to g-2 of approximately 0.2%. Adding individual virtual W, Z or Higgs bosons adds a well defined contribution a factor of a million or so smaller. The remaining relevant contributions are from hadronic vacuum polarisation (HVP) and hadronic light-by-light (HLBL) scattering. HVP and HLBL both add hadronic contributions integrated to all orders in the strong coupling constant to interactions between the muon and the external electric field, which also feature additional virtual photons. Though their contributions to g-2 are in the ballpark of the small electroweak contribution, they are more difficult to calculate, and dominate the error budget for the SM prediction of the muon’s g-2.

Christine Davies (University of Glasgow) gave a comprehensive survey of muon g–2 that stressed several high-level points: the small HLBL contribution looks to be settled, and is unlikely to be a key piece to the puzzle; recent tensions among the e⁺e^– experiments for HVP have emerged and need to be better understood; and in the most contentious region, all eight recent lattice–QCD calculations agree with each other and with the very recent e⁺e^– → hadrons experiment CMD 3 (2024 Phys. Rev. Lett. 132 231903), though not so much with earlier experiments. Thus, lattice QCD and CMD 3 suggest there is “almost certainly less new physics in muon g–2 than previously hoped, and perhaps none,” said Davies. We shall see: many groups are preparing results for the full HVP, targeting a new whitepaper from the Muon g–2 Theory Initiative by the end of this year, in anticipation of the final measurement from the Fermilab experiment sometime in 2025.

New directions

While the main focus of Lattice calculations is the study of QCD, lattice methods have been applied beyond that. There is a small but active community investigating systems that could be relevant to physics beyond the Standard Model, including composite Higgs models, supersymmetry and dark matter. These studies often inspire formal “theoretical” developments that are of interest beyond the lattice community. Particularly exciting directions this year were the development on emergent phases, non-invertible symmetries and their possible application to formulate chiral gauge theories, one of the outstanding theoretical issues in lattice gauge theories.

The lattice QCD community is one of the main users of high-performance computing resources

The lattice QCD community is one of the main users of high-performance computing resources, with its simulation efforts generating petabytes of Monte Carlo data. For more than 20 years, a community wide effort, the international lattice data grid (ILDG), has allowed this data to be shared. Since its inception, ILDG implemented the FAIR principles – data should be findable, accessible, interoperable and reusable – almost fully. The lattice QCD community is now discussing Open Science. Ed Bennett (Swansea) led a panel discussion that explored the benefits of ILDG embracing open science, such as higher credibility for published results, and not least the means to fulfill the expectations of funding bodies. Sustainably maintaining the infrastructure and employing the personnel required calls for national or even international community efforts to convince the funding agencies to provide corresponding funding lines, but also the researchers of the benefits of open science.

The Kenneth G. Wilson Award for Excellence in Lattice Field Theory was awarded to Michael Wagman (Fermilab) for his lattice-QCD studies of noise reduction in nuclear systems, the structure of nuclei and transverse-momentum-dependent hadronic structure functions. Fifty years on from Wilson’s seminal paper, two of the field’s earliest contributors, John Kogut (US Department of Energy) and Jan Smit (University of Amsterdam), reminisced about the birth of the lattice in a special session chaired by Liverpool pioneer Chris Michael. Both speakers gave fascinating insights into a time where physics was extracted from a handful of small-volume gauge configurations, compared to hundreds of thousands today.

Lattice 2025 will take place at the Tata Institute of Fundamental Research in Mumbai, India, from 3 to 8 November 2025.

Music city tunes in to accelerators

By some counts, there are more than 300 distinct branches of science, a number that continues to grow. In physics alone, which began with astronomy five millennia ago, there are now at least two dozen subdivisions in most taxonomies. Over the past three decades, the science of beams has evolved into a distinct discipline with its own subjects and methods, dedicated peer-reviewed journals – like Physical Review Accelerators and Beams, which turned 25 last year – and nearly two dozen regular regional and international conferences and workshops.

Today, around 5000 accelerator scientists and engineers work in more than 50 countries, collaborating with a pool of technical experts three to four times that size. While most are deeply involved in operations and upgrades, their careers also include designing and constructing new facilities, beam-physics research, developing critical technical components, and project leadership. Their work often involves technology transfer, industrial applications, education and training of future experts, and public and academic outreach.

A global field

The need for regular meetings of the entire field has long been recognised. Historically, regional conferences like the biannual particle-accelerator conferences (PACs) in the US (1965–2009), the biannual EPACs in Europe (1988–2008) and the triannual APACs in Asia (1998–2007) served this purpose. These gatherings covered all types of accelerators, particles and use-cases. As the field became truly global, leaders established the series of international PACs (IPACs), which rotate through the regions in a three-year cycle, convening about 1500 attendees. The 15th IPAC took place from 19 to 24 May in Nashville, Tennessee, with almost 200 registrants from Asia, more than 400 from Europe and nearly 700 from the US.

The “beef” of the conference was in the reports from facilities, but no one person can summarise all the progress, and I must restrict myself to personal highlights in fields that are close to my heart. Fascinating progress was reported on energy-recovery linacs (ERLs) and associated technologies such as superconducting RF and fixed-field-alternating-gradient accelerators, following the recent success of the CBETA accelerator test facility at Cornell. Another hot topic in my eyes was design work and experimental studies towards strong hadron cooling for the Electron–Ion Collider. This year’s progress in industrial and medical accelerators is also impressive, with noteworthy presentations on radioisotope production and radiotherapy (Oliver Kester, TRIUMF and Michael Galonska, GSI), light sources for semiconductor manufacturing (Bruce Dunham, SLAC), accelerator-driven fusion (Richard Magee, TAE Technologies), and 96 exhibitions from companies and institutions worldwide.

CERN’s FCC-ee project was discussed in several sessions. Nuria Catalan-Lasheras (CERN) gave a memorable talk demonstrating impressive progress on high-power klystrons (RF sources). At present, klystrons have about 55% efficiency – RF power divided by wall-plug power – but she noted that they have the potential to go to as high as about 85% efficiency. The path is clear: increase voltage and decrease current, thereby reducing the “microperveance” of the klystrons. This will be crucial at FCC-ee, which must continuously replenish 100 MW of synchrotron radiation losses with 100 MW of RF power. The klystron efficiency improvement alone can save more than 60 MW – fully a third of the current power consumption of the CERN accelerator complex.

The “beef” of the conference was in the reports from facilities, but no one person can summarise all the progress

Muon colliders were presented as a unique opportunity to achieve a substantial energy increase compared to hadrons (Diktys Stratakis, Fermilab). Due to the point-like nature of the muon, the full centre-of-mass energy is available for probing new physics processes in every collision. Therefore, a 10 TeV muon collider can provide comparable high-energy-physics breakthroughs to a 100 TeV proton–proton collider, where colliding partons only carry a fraction of the proton’s energy. Due to its compactness, the cost of a 10 TeV muon collider compares to that of the FCC-ee and is likely to be many times lower than any other alternative concept that can achieve 10 pCM (parton centre-of-mass) energies (T Roser et al. 2023 JINST 18 P05018). The challenge lies in developing technologies for muon production, cooling and acceleration in the next two decades. In the upcoming 19 to 25 years it should be technically feasible for the accelerator community to demonstrate the technologies of a) high-intensity and short proton bunches; b) high-power proton targets; c) muon cooling; d) fast muon acceleration; e) 10 to 12 T superconducting magnets lined with tungsten inserts to protect coils from the muon decay products, and; f) effective spreading of the narrow cones of ultra-high-energy neutrinos by wiggling the beams, to avoid damage caused by the chargeless neutrinos when the muons decay.

In the conference’s closing talk, I reviewed three dozen future-collider proposals, analysed the ultimate energies potentially attainable in all types of colliding beams and accelerators within reasonable cost and power consumption limits, and laid out arguments that energies beyond a PeV (thousands of TeV) can be achieved, concluding that muons are the particles of the future for high-energy physics.

I can attest to IPAC’s success in fostering real-life interactions in the global accelerator landscape

The prize session was a highlight, with acceptance speeches from KEK’s Kaoru Yokoya (APS Wilson Prize) and SLAC’s Gennady Stupakov (IEEE NPSS PAST Award). Yokoya outlined his participation in various electron–positron machines and proposals such as the TRISTAN e⁺e^– collider and the ILC. Stupakov emphasised the importance of beam-dynamics theory in the age of computer modelling and simulations.

Ever since the first edition in Kyoto in 2010, I can attest to IPAC’s success in fostering real-life interactions in the global accelerator landscape. After the conference, I counted more than a hundred encounters of 5 minutes or more – something that would be difficult to achieve at a smaller or more specialised conference. It was pleasing to see many Chinese colleagues attend this US-based conference, but I did not identify any participants from Russia – a concerning development for our science’s international spirit. I hope political barriers will not interfere with next year’s IPAC’25 in Taiwan.

On a personal note, I would like to thank the organisers for putting together great scientific and social programmes, and the dedicated Joint Accelerator Conferences Website team, whose tireless efforts ensured that virtually all conference proceedings – papers, talks and posters – were available online by the final day, setting a standard that other fields of high-energy physics could greatly benefit from.

ALICE does the double slit

In the famous double-slit experiment, an interference pattern consisting of dark and bright bands emerges when a beam of light hits two narrow slits. The same effect has also been seen with particles such as electrons and protons, demonstrating the wave nature of propagating particles in quantum mechanics. Typically, experiments of this type produce interference patterns at the nanometre scale. In a recent study, the ALICE collaboration measured a similar interference pattern at the femtometre scale using ultra-peripheral collisions between lead nuclei at the LHC.

In ultra-peripheral collisions, two nuclei pass close to each other without colliding. With their impact parameter larger than the sum of their radii, one nucleus emits a photon that transforms into a virtual quark–antiquark pair. This pair interacts strongly with the other nucleus, resulting in the emission of a vector meson and the exchange of two gluons. Such vector-meson photoproduction is a well-established tool for probing the internal structure of colliding nuclei.

In vector-meson photoproduction involving symmetric systems, such as two lead nuclei, it is not possible to determine which of the nuclei emitted the photon and which emitted the two gluons. Crucially, however, due to the short range of the strong force between the virtual quark–antiquark pair and the nucleus, the vector mesons must have been produced within or close to one of the two well-separated nuclei. Because of this and their relatively short lifetime, the vector mesons decay quite rapidly into other particles. These decay products form a quantum-mechanically entangled state and generate an interference pattern akin to that of a double-slit interferometer.

In the photoproduction of the electrically neutral ρ⁰ vector meson, the interference pattern takes the form of a cos(2φ) modulation of the ρ⁰ yield, where φ is the angle between the two vectors formed by the sum and difference of the transverse momenta of the two oppositely charged pions into which the ρ⁰ decays. The strength of the modulation is expected to increase as the impact parameter decreases.

Using a dataset of 57,000 ρ⁰ mesons produced in lead–lead collisions at an energy of 5.02 TeV per nucleon pair during Run 2 of the LHC, the ALICE team measured the cos(2φ) modulation of the ρ⁰ yield for different values of the impact parameter. The measurements showed that the strength of the modulation varies strongly with the impact parameter. Theoretical calculations indicate that this behaviour is indeed the result of a quantum interference effect at the femtometre scale.

In the ongoing Run 3 of the LHC and in the next run, Run 4, ALICE is expected to collect more than 15 million ρ⁰ mesons from lead–lead collisions. This enhanced dataset will allow a more detailed analysis of the interference effect, further testing the validity of quantum mechanics at femtometre scales.

Building on success, planning for the future

From 29 January to 1 February, the Chamonix Workshop 2024 upheld its long tradition of fostering open and collaborative discussions within CERN’s accelerator and physics communities. This year marked a significant shift with more explicit inclusion of the injector complex, acknowledging its crucial role in shaping future research endeavours. Chamonix discussions focused on three main areas: maximising the remaining years of Run 3; the High-Luminosity LHC (HL-LHC), preparations for Long Shutdown 3 and operations in Run 4; and a look to the further future and the proposed Future Circular Collider (FCC).

Immense effort

Analysing the performance of CERN’s accelerator complex, speakers noted the impressive progress to date, examined limitations in the LHC and injectors and discussed improvements for optimal performance in upcoming runs. It’s difficult to do justice to the immense technical effort made by all systems, operations and technical infrastructure teams that underpins the exploitation of the complex. Machine availability emerged as a crucial theme, recognised as critical for both maximising the potential of existing facilities and ensuring the success of the HL-LHC. Fault tracking, dedicated maintenance efforts and targeted infrastructure improvements across the complex were highlighted as key contributors to achieving and maintaining optimal uptime.

As the HL-LHC project moves into full series production, the technical challenges associated with magnets, cold powering and crab cavities are being addressed (CERN Courier January/February 2024 p37). Looking beyond Long Shutdown 3 (LS3), potential limitations are already being targeted now, with, for example, electron-cloud mitigation measures planned to be deployed in LS3. The transition to the high-luminosity era will involve a huge programme of work that requires meticulous preparation and a well-coordinated effort across the complex during LS3, which will see the deployment of the HL-LHC, a widespread consolidation effort, and other upgrades such as that planned for the ECN3 cavern at CERN’s North Area.

The vision for the next decades of these facilities is diverse, imaginative and well-motivated from a physics perspective

The breadth and depth of the physics being performed at CERN facilities is quite remarkable, and the Chamonix workshop reconfirmed the high demand from experimentalists across the board. The unique capabilities of ISOLDE, n_TOF, AD-ELENA, and the East and North Areas were recognised. The North Area, for example, provides protons, hadrons, electrons and ion beams for detector R&D, experiments, the CERN neutrino platform, irradiation facilities and counts more than 2000 users. The vision for the next decades of these facilities is diverse, imaginative and well-motivated from a physics perspective. The potential for long-term exploitation and leveraging fully the capabilities of the LHC and other facilities is considerable, demanding continued support and development.

In the longer term, CERN is exploring the potential construction of the FCC via a dedicated feasibility study that has just delivered a mid-term report – a summary of which was presented at Chamonix. The initiative is accompanied by R&D on key accelerator technologies. The physics case for FCC-ee was well made for an audience of mostly non-particle physicists, concluding that the FCC is the only proposed collider that covers each key area in the field – electroweak, QCD, flavour, Higgs and searches for phenomena beyond the Standard Model – in paradigm-shifting depth.

Environmental consciousness

Sustainability was another focus of the Chamonix workshop. Building and operating future facilities with environmental consciousness is a top priority, and full life-cycle analyses will be performed for any options to help ensure a low-carbon future.

Interesting times, lots to do. To quote former CERN Director-General Herwig Schopper from 1983: “It is therefore clear that, for some time to come, there will be interesting work to do and I doubt whether accelerator experts will find themselves without a job.”

Strange correlations benchmark hadronisation

ALICE figure 1 — **Fig. 1.** Pearson correlation coefficient between the net-Ξ number and net-kaon number, as a function of the charged-particle multiplicity of the collision. Model calculations are shown as coloured bands. Credit: arxiv.2405.19890

In high-energy hadronic and heavy-ion collisions, strange quarks are dominantly produced from gluon fusion. In contrast to u and d quarks, they are not present in the colliding particles. Since strangeness is a conserved quantity in QCD, the net number of strange and anti-strange particles must equal zero, making them prime observable to study the dynamics of these collisions. Various experimental results from high-multiplicity pp collisions at the LHC demonstrate striking similarities to Pb–Pb collision results. Notably, the fraction of hadrons carrying one or more strange quarks smoothly increases as a function of particle multiplicity in pp and p–Pb collisions to values consistent with those measured in peripheral Pb–Pb collisions. Multi-particle correlations in pp collisions also closely resemble those in Pb–Pb collisions.

Explaining such observations requires understanding the hadronisation mechanism, which governs how quarks and gluons rearrange into bound states (hadrons). Since there are no first-principle calculations of the hadronisation process available, phenomenological models are used, based on either the Lund string fragmentation (Pythia 8, HIJING) or a statistical approach assuming a system of hadrons and their resonances (HRG) at thermal and chemical equilibrium. Despite having vastly different approaches, both models successfully describe the enhanced production of strange hadrons. This similarity calls for new observables to decisively discriminate between these two approaches.

The data indicate a weaker opposite-sign strangeness correlation than that predicted by string fragmentation

In a recently published study, the ALICE collaboration measured correlations between particles arising from the conservation of quantum numbers to further distinguish the two models. In the string fragmentation model, the quantum numbers are conserved locally through the creation of quark–antiquark pairs from the breaking of colour strings. This leads to a short-range rapidity correlation between strange and anti-strange hadrons. On the other hand, in the statistical hadronisation approach, quantum numbers are conserved globally over a finite volume, leading to long-range correlations between both strange–strange and strange–anti-strange hadron pairs. Quantum-number conservation leads to correlated particle production that is probed by measuring the yields of charged kaons (with one strange quark) and multistrange baryons (Ξ^– and Ξ⁺) on an event-by-event basis. In ALICE, charged kaons are directly tracked in the detectors, while Ξ baryons are reconstructed via their weak decay to a charged pion and a Λ-baryon, which is itself identified via its weak decay into a proton and a charged pion.

Figure 1 shows the first measurement of the correlation between the “net number” of Ξ baryons and kaons, as a function of the charged-particle multiplicity at midrapidity in pp, p–Pb and Pb–Pb collisions, where the net number is the difference between particle and antiparticle multiplicities. The experimental results deviate from the uncorrelated baseline (dashed line), and string fragmentation models that mainly correlate strange hadrons with opposite strange quark content over a small rapidity range fail to describe both observables. At the same time, the measurements agree with the statistical hadronisation model description that includes opposite-sign and same-sign strangeness correlations over large rapidity intervals. The data indicate a weaker opposite-sign strangeness correlation than that predicted by string fragmentation, suggesting that the correlation volume for strangeness conservation extends to about three units of rapidity.

The present study will be extended using the recently collected data during LHC Run 3. The larger data samples will enable similar measurements for the triply strange Ω baryon, as well as the study of higher cumulants.

German community discusses future collider at CERN

German particle-physics community in Bonn — **Bonn workshop** The German particle-physics community gathered at Bonn to discuss the opportunities of a future collider at CERN. Credit: M Schott

More than 150 German particle physicists gathered at Bonn University for a community event on a future collider at CERN. More precisely, the focus set for this meeting was to discuss the opportunities that the FCC-ee would offer should this collider be built at CERN. The event was organised by the German committee for particle physics, KET, and took place from 22 to 24 May. Representatives from almost all German institutes and groups active in particle physics were present, an attendance that shows the large interest in the collider to be built at CERN after the successful completion of the HL-LHC programme.

The main workshop was preceded by a dedicated session with more than 80 early-career scientists, organised by the Young High Energy Physicists Association, yHEP, to bring the generation that will benefit most from a future collider at CERN up to speed on the workshop topics. It included a presentation by former ECFA chair Karl Jakobs (Freiburg University) “From Strategy Discussions to Decision-Taking for Large Projects”, explaining the mechanisms and bodies involved in setting a project like the FCC-ee on track.

The opening session of the main workshop featured a fresh view on “The physics case for an e⁺e^– collider at CERN” by Margarete Mühlleitner (KIT Karlsruhe), who spread excitement about the strong and comprehensive physics case from super-precise measurements of the properties of the Z boson, the W boson and the top quark to what most people associate with a future e⁺e^– collider: precision measurements of the Higgs boson and insights about its connection to many of the still open questions of particle physics like dark matter or the matter–antimatter asymmetry. Markus Klute (KIT Karlsruhe) gave an in-depth review of the FCC-ee project. The midterm results of the FCC feasibility study indicate that no showstoppers were found in all the aspects studied so far and that the integrated FCC programme offers unparalleled exploration potential through precision measurements and direct searches. The picture was rounded off by a presentation from Jenny List (DESY, Hamburg) who talked about alternative options to realise an e⁺e^– Higgs factory at CERN, and the perspective of the early-career researchers was highlighted by Michael Lupberger (Bonn University). While all these presentations concentrated on the science and technology of the FCC-ee or alternatives, Eckart Lilienthal, representing the German Ministry of Education and Research, BMBF, reminded the audience that a future collider project at CERN needs an affordable financial plan and that – given the large uncertainties at present – this requires the community to prepare for different scenarios including one without the FCC-ee. Lilienthal confirmed that the future of CERN remains of the highest priority to BMBF.

The event was an important step in building consensus in the German community for a future collider project at CERN

The workshop went on to review many aspects of the FCC-ee and possible alternatives in more detail: accelerator R&D, detector concepts and technologies, computing and software, theory challenges as well as sustainability. The workshop witnessed the first meetings of the newly established German detector R&D consortia on silicon detectors, gaseous detectors and calorimetry. They will receive BMBF funding for the next three years and will allow German groups to strongly participate in the recently formed international DRD consortia in the context of the ECFA detector roadmap.

The path ahead

The workshop concluded with discussion sessions on the future collider scenarios for CERN, the engagement of the German community and a path to prepare the German input to the update of the European Strategy for Particle Physics. A series of three additional community workshops will be held in Germany before this input is due in March 2025.

The Bonn event was an important step in building consensus in the German community for a future collider project at CERN. The FCC-ee project generated a lot of interest and many groups plan to embark more strongly on this project. Contributions concerning the physics case, theory challenges, detector design and development, software, computing, and accelerator development were discussed. Alternative options for a future collider project at CERN need to be kept open to address the unanswered fundamental questions of particle physics in case the FCC-ee is not built at CERN. This event was clear evidence that a bright future for CERN remains of highest priority for the German particle-physics community and funding agency.

Exploring the Higgs potential at ATLAS

ATLAS figure 1 — **Fig. 1.** 95% confidence upper limits on the signal strength for inclusive gluon–gluon and vector–boson fusion HH production from each of the five analyses and their statistical combination. Credit: ATLAS Collab. 2024 arXiv:2406.09971

Immediately after the Big Bang, all the particles we know about today were massless and moving at the speed of light. About 10^–12 seconds later, the scalar Higgs field spontaneously broke the symmetry of the electroweak force, separating it into the electromagnetic and weak forces, and giving mass to fundamental particles. Without this process, the universe as we know it would not exist.

Since its discovery in 2012, measurements of the Higgs boson – the particle associated with the new field – have refined our understanding of its properties, but it remains unknown how closely the field’s energy potential resembles the predicted Mexican hat shape. Studying the Higgs potential can provide insights into the dynamics of the early universe, and the stability of the vacuum with respect to potential future changes.

The Higgs boson’s self-coupling strength λ governs the cubic and quartic terms in the equation describing the potential. It can be probed using the pair production of Higgs bosons (HH), though this is experimentally challenging as this process is more than 1000 times less likely than the production of a single Higgs boson. This is partly due to destructive interference between the two leading order diagrams in the dominant gluon–gluon fusion production mode.

The ATLAS collaboration recently compiled a series of results targeting HH decays to bbγγ, bbττ, bbbb, bbll plus missing transverse energy (E_T^miss), and multilepton final states. Each analysis uses the full LHC Run 2 data set. A key parameter is the HH signal strength, μ_HH, which divides the measured HH production rate by the Standard Model (SM) prediction. This combination yields the strongest expected constraints to date on μ_HH, and an observed upper limit of 2.9 times the SM prediction (figure 1). The combination also sets the most stringent constraints to date on the strength of the Higgs boson’s self-coupling of –1.2 < κ_λ < 7.2, where κ_λ = λ/λ_SM, its value relative to the SM prediction.

Each analysis contributes in a complementary way to the global picture of HH interactions and faces its own set of unique challenges.

Despite its tiny branching fraction of just 0.26% of all HH decays, HH → bbγγ provides very good sensitivity to μ_HHthanks to the ATLAS detector’s excellent di-photon mass resolution. It also sets the best constraints on λ due to its sensitivity to HH events with low invariant mass.

The HH → bbττ analysis (7.3% of HH decays) exploits state-of-the-art hadronic–tau identification to control the complex mix of electroweak, multijet and top-quark backgrounds. It yields the strongest limits on μ_HH and the second tightest constraints on λ.

HH → bbbb (34%) has good sensitivity to μ_HH thanks to ATLAS’s excellent b-jet identification, but controlling the multijet background presents a formidable challenge, which is tackled in a fully data-driven fashion.

Studying the Higgs potential can provide insights into the dynamics of the early universe

The decays HH → bbWW and HH → bbττ in fully leptonic final states have very similar characteristics and are thus targeted in a single HH → bbll+E_T^miss analysis. Contributions from the bbZZ decay mode, where one Z decays to charged light leptons and the other to neutrinos, are also considered.

Finally, the HH → multilepton analysis is designed to catch decay modes where the HH system cannot be fully reconstructed due to ambiguity in how the decay products should be assigned to the two Higgs bosons. The analysis uses nine signal regions with different multiplicities of light charged leptons, hadronic taus and photons. It is complementary to all the exclusive channels discussed above.

For the ongoing LHC Run 3, ATLAS designed new triggers to enhance sensitivity to the hadronic HH → bbττ and HH → bbbb channels. Improved b-jet identification algorithms will increase the efficiency in selecting HH signals and distinguishing them from background processes. With these and other improvements, our prospects have never looked brighter for homing in on the Higgs self-coupling.

Estonia becomes 24th Member State

On 30 August CERN welcomed Estonia as its 24th Member State, marking the end of a formal application process that started in 2018 and crowning a period of cooperation that stretches back three decades.

“Estonia is delighted to join CERN as a full member because CERN accelerates more than tiny particles, it also accelerates international scientific collaboration and our economies,” said Estonia president Alar Karis. “We have seen this potential during our time as Associate Member State and are keen to begin our full contribution.”

The bilateral relationship formally began in 1996, when Estonia and CERN signed a first cooperation agreement. Estonia has been part of the CMS collaboration since 1997, participating in data analysis and the Worldwide LHC Computing Grid, for which Estonia operates a Tier 2 centre in Tallinn. Researchers from Estonia also contribute to other experiments including CLOUD, COMPASS, NA66 and TOTEM, and to studies for future colliders, while Estonian theorists are highly involved in collaborations with CERN.

“Estonia and CERN have been collaborating closely for some 30 years, and I am very pleased to welcome Estonia to the ever-growing group of CERN Member States,” said Director-General Fabiola Gianotti. “I am sure the country and its scientific community will benefit from increased opportunities in fundamental research, technology development, and education and training.”

Estonia has held Associate Member State status in the pre-stage to membership of CERN since February 2021. As a full Member State, Estonia will now have voting rights in the CERN Council, enhanced opportunities for Estonian nationals to be recruited by CERN and for Estonian industry to bid for CERN contracts.

“On behalf of the CERN Council, I warmly welcome Estonia as the newest Member State of CERN,” said Council president Eliezer Rabinovici. “I am happy to see the community of CERN Member States enlarging, and I am looking forward to the enhanced participation of Estonia in the CERN Council and to its additional scientific contributions to CERN.”

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