Jobs – Page 2 of 535 – CERN Courier

Highlights from the cosmic census

**Family portrait** The black-hole mergers added to the GWTC-5.0 catalogue in the latest observing run, one signal per panel, with time running rightward and frequency upward. Each sweep rises in pitch as the two black holes spiral together. Brighter traces indicate louder signals. Credit: D Davis/University of Rhode Island/LIGO–Virgo–KAGRA

On 14 September 2015, the gravitational waves emitted by a black-hole merger more than a billion light-years away were observed on Earth for the first time (CERN Courier April 2016 p19). The challenge had long seemed daring – if not impossible. A decade on, precision gravitational-wave astronomy has become an established observational discipline. This maturity is most evident in the new Gravitational-Wave Transient Catalog (GWTC-5.0), released in May by the international LIGO–Virgo–KAGRA (LVK) Collaboration, which adds 161 events recorded between 10 April 2024 and 28 January 2025, and brings the number of gravitational-wave signals observed to date to an astounding 390.

The LVK Collaboration brings together several thousand scientists working with the two Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in the United States, Virgo in Italy and KAGRA in Japan. Thanks to their steadily improving sensitivity, these interferometers detected three to four gravitational-wave signals every week during the most recent observing run. This growing collection of data is transforming our understanding of black-hole populations, providing clues about how black holes form, evolve and merge.

One of the most striking findings is that some of the black holes taking part in the observed mergers appear to be second-generation objects, the products of earlier mergers rather than of stellar collapse. The case rests largely on the GW241011 and GW241110 signals, generated by events at about 700 million and 2.4 billion light-years from Earth and detected in October and November 2024, respectively. Several of their measured properties, above all the spin of each pair’s heavier black hole, are best explained if these objects are themselves merger remnants. Astronomers have long suggested that in dense environments, such as stellar clusters, black holes may be more likely to collide and merge repeatedly, and gravitational waves provide insights into these extreme astrophysical environments that are difficult to obtain by other means.

The new catalogue also offers a wide variety of increasingly precise tests of general relativity, enabling us to study how gravity and spacetime behave in the immediate vicinity of a black hole, where Einstein’s theory might intersect with quantum mechanics. GW250114, a particularly clear signal that reached Earth on 14 January 2025, enabled the first detection of two dominant vibrational tones from a black hole, along with hints of a third. As the merged remnant settled, it rang like a struck bell, each tone characterised by an oscillation frequency and a damping time. Measuring the dominant tone and its first overtone made this the most accurate such test so far, and the data agreed with the theory’s predictions. By comparing the inferred areas of the two initial black holes with that of the remnant, LVK researchers also found the result consistent with the increase predicted by Stephen Hawking’s area theorem, which states that the total black-hole horizon area cannot decrease.

Another signal in the catalogue, GW240615, detected on 15 June 2024 by LIGO and Virgo, is the most precisely localised gravitational-wave source yet, pinpointed to an area of only six square degrees. Accurate localisation is of tremendous importance to the broader astronomical community, as it enables the search for electromagnetic counterparts to the observed events, in particular for mergers involving neutron stars. GW240615 originated more than 3 billion light-years from Earth and was produced by the merger of two black holes with masses of approximately 26 and 30 times that of the Sun. As expected for a binary black-hole merger, no electromagnetic counterpart was identified.

The expanding universe

Gravitational waves can also be used to measure the expansion of the universe. Improvements in the localisation capabilities of the detector network and the larger GWTC-5.0 dataset have yielded a new independent measurement of the Hubble constant, H0 = 71.7 +9.4 −7.5 km s^–1 Mpc^–1, 22% more precise than the previous gravitational-wave estimate. The result bears on one of modern cosmology’s unsolved puzzles, the Hubble tension. Its uncertainties are still broad enough to be consistent with determinations from both the nearby and the early universe, which disagree with each other, but gravitational-wave observations add an independent perspective to the debate and may ultimately help to resolve it.

Taken together, these findings show how, in the years to come, gravitational waves will help explore unresolved questions in fundamental physics and in the origin and evolution of the universe. As the census of cosmic mergers grows, they will continue to reveal phenomena that are difficult or impossible to study through electromagnetic observations alone.

A journey worth sharing

Over the past few decades, relations between Big Science organisations (BSOs) and their industrial suppliers have changed substantially. Global circumstances have forced companies to focus on their regular markets and products, leaving less room for contracts that deviate from that core business. Deviation, however, is often what BSOs need, as the technologies their projects demand grow more complex. In addition, they often require large numbers of custom components that may be just purchased once, and only through formal tender procedures. Companies, therefore, often prefer to do business with other companies, where shared long-term interests and mutual trust are typically enough to strike a deal.

The challenge for BSOs is then to remain, or to become again, interesting to industry, with projects at a price both can afford. In achieving this, laboratories have more to offer than is generally appreciated. Companies that work with BSOs have been shown to benefit in their marketing, to expand and improve their product portfolios, and to attract and retain key personnel, because challenging specifications drive innovation.

Relations in the Big Science domain can take many forms, from the supply of standard services and commodities, through collaborative technical development, to the licensing of technology and know-how. No single model fits every company, and each arrangement must serve both sides. Industry, for its part, is indispensable at every scale, from standard cables and connectors to complete magnet and cryogenic systems, and in some areas it moves faster than the BSOs themselves. Design for manufacturability and design for cost, the arts of shaping a component for efficient and affordable production, are cases in point, and both matter more as systems get bigger. Industry’s experience should be brought into a project’s design phase as early as possible, especially where components are needed in large numbers and for long lifetimes.

On demanding and innovative projects, openness is what keeps industry on board

Given their status, BSOs must deal with industry within specific procurement rules. Still, the relationship in spirit must be closer to a partnership, in which communication is open, risk is shared, and both parties ultimately gain. Communication must flow freely within each organisation, between technical and commercial teams, and between the two partners. As the work proceeds, both must exchange detailed specifications and flag problems as they arise. On demanding and innovative projects, where sharing risk matters most, this openness is what keeps industry on board.

The stakes reach beyond the laboratories, of course. For countries participating in a BSO, the return on membership rests not only on the science but also on wider societal benefits (see The collider dividend). Chief among them at present is industrial growth and development, with new products, skills, partnerships, markets and intellectual property flowing back to the member states.

While the scientific return largely takes care of itself, through the institutes and universities involved in the experiments, national support for companies to connect with BSOs via industrial liaison officers (ILOs) varies significantly across countries. For the role to be effective, we must see challenges coming early, in time to identify potential suppliers or to help existing companies and incubators profit from the technologies being developed. ILOs, whether employed by a BSO or representing a country, are therefore central to building these partnerships.

Across their technical, management and commercial teams, BSOs must never lose sight of how fundamental long-term, mutually beneficial relationships with industry are. Their value exceeds limited, short-term financial gains, and savings won contract by contract can, over time, leave fewer companies willing to bid, to the detriment of science and industry alike.

Whirlpools in the perfect fluid

ALICE figure 1 — **Fig. 1.** Longitudinal polarisation of Λ and anti-Λ hyperons as a function of azimuthal angle relative to the second- (left) and third-order (right) event planes, in 20–60% central Pb–Pb collisions at 5.36 TeV per nucleon pair. Bars and boxes show statistical and systematic uncertainties, and the dash–dotted curves are sine fits. The coloured curves in the left panel are hydrodynamic calculations with different treatments of bulk viscosity. Credit: ALICE collab. arXiv:2606.18070

Over the past two decades, heavy-ion experiments have established that the quark–gluon plasma (QGP) expands collectively and behaves as an almost perfect fluid. Much like water swirling in a whirlpool, the QGP can develop vortices – regions where it circulates around an axis. The ALICE collaboration has now probed this local rotation using Λ and anti-Λ hyperons, weakly decaying particles containing a strange quark or antiquark, respectively.

When two nuclei collide off-centre, their overlap region is, on average, elongated rather than circular. Pressure is then higher along some directions, and the QGP formed in the collision expands unevenly. As a result, neighbouring regions of the fluid flow past one another, seeding many local vortices. Some of these rotate about axes parallel to the beam, in directions that alternate across the plane perpendicular to it. This local rotation tends to align the spins of the medium’s quarks. When the plasma cools and its components bind into hadrons, part of this alignment survives as a spin polarisation. Λ and anti-Λ hyperons are excellent probes of the phenomenon, since the proton emitted in the Λ decay tends to follow the parent’s spin direction, and the antiproton in the anti-Λ decay to oppose it.

In the plane transverse to the beam, the direction of each hyperon’s momentum is specified by its azimuthal angle, measured with respect to reference directions known as “event planes” and inferred in each collision from the pattern of emitted particles. Each order of the collision shape defines its own such direction. The second-order plane follows the average elliptic shape of the overlap region, while the third-order one traces a triangular distortion, arising mainly from collision-by-collision variations. The longitudinal polarisation is predicted to rise and fall as the angle changes, with a distinct pattern relative to each plane.

Λ and anti-Λ hyperons are excellent probes of local QGP vortices

Using the high-statistics Run-3 data sample of Pb–Pb collisions at a centre-of-mass energy per nucleon pair of 5.36 TeV, the collaboration has studied the second-order polarisation with improved precision and, for the first time at the LHC, the third-order component (see figure 1). The improvement allows a more stringent comparison with recent viscous hydrodynamic calculations, which shows that polarisation is sensitive to bulk viscosity – the fluid’s resistance to uniform expansion or compression. Bulk viscosity does not generate the alignment directly, but modifies the space–time evolution of the medium and the velocity gradients that do.

The first measurement of the third-order polarisation gives access to the vortices arising from event-specific fluctuations, beyond those tied to the average collision geometry. An interesting feature of the measurement is that the second- and third-order polarisation signals are of comparable magnitude, although the corresponding anisotropic-flow harmonics – the patterns in the particle emission – exhibit a clear hierarchy. This suggests that hyperon polarisation is not simply a reflection of the system’s momentum anisotropy, but carries additional information about the local vortical structure and how viscosity affects the evolution of the QGP. While theoretical calculations for the third-order component are not yet available, the present measurement provides an important benchmark for future studies.

A new purely baryonic decay

LHCb figure 1 — **Fig. 1.** Invariant-mass distribution of the selected Λ⁰_b → Λpp̅ candidates in the higher-resolution LL category of the full Run-2 dataset. The total fit (solid blue) comprises the Λ⁰_b → Λpp̅ signal (dotted red), a small possible Ξ⁰_b → Λpp̅ contribution (dotted magenta) and the combinatorial background (dotted green). Credit: LHCb Collab. arXiv:2605.04348

Almost every particle decay process recorded to date involves at least a meson or a lepton. Only two feature nothing but baryons and antibaryons, and both include a charmed product. The LHCb collaboration has now observed a third, Λ⁰_b → Λpp , this time with no final-state charm quarks, and the first to emerge from a dedicated search.

It is the intricate internal structure of baryons that makes purely baryonic decay processes so interesting. The two previously known cases, Λ⁰_b → Σ⁰_cpp and Λ⁰_b → Σ_c^*⁰pp , surfaced in 2018 from an LHCb analysis of Λ⁰_b → Λ⁺_c pp π^–. In both these modes, as well as in the new one, the original three-quark baryon transforms into three other baryons and antibaryons containing nine valence quarks and antiquarks altogether. The transition must therefore create several quark–antiquark pairs. Measuring these processes can help clarify how the weak decay of a heavy quark is followed by the strong interactions that bind quarks into observable particles. Their spin structure also gives access to angular observables, including triple-product correlations, that are sensitive to CP-violating and time-reversal-odd effects.

The recent search was conducted using 6.0 fb^–1 of proton–proton collision data recorded by LHCb during Run 2 of the LHC, at a centre-of-mass energy of 13 TeV. The relatively long-lived Λ baryon was reconstructed through its decay into a proton and a pion. Since the Λ can travel an appreciable distance before decaying, the data are divided into two categories based on where the decay takes place inside the detector. In the “long–long” (LL) category, both decay products leave hits in the vertex detector near the collision point and in the tracking stations downstream, providing the best mass and vertex resolution (see figure 1). A complementary “downstream–downstream” (DD) category recovers Λ baryons that decay outside the vertex detector and contributes about half of the signal.

Larger Run-3 datasets will allow the dynamics of this decay to be explored in much greater detail

The candidates were separated from random combinations of particles using the displaced-decay topology, multivariate algorithms and LHCb’s particle-identification detectors. Backgrounds containing charm hadrons were removed, while decays through intermediate charmonium resonances were excluded by requiring the proton–antiproton invariant mass to be below 2.85 GeV. A simultaneous fit to both reconstruction categories yields 39 ± 10 decays with a significance of 5.1 standard deviations, including systematic uncertainties. The Λ⁰_b → Λpp decay rate is measured relative to the similar Λ⁰_b → ΛK⁺K^–, giving a ratio of branching fractions of (5.1 ± 1.3 (stat.) ± 0.3 (syst.)) × 10^–2. Since the measurement is restricted to this charmonium-free region for both decays, while the published branching fraction of the normalisation mode covers the full phase space, the ratio cannot yet be converted directly into an absolute branching fraction for Λ⁰_b → Λpp .

Larger Run-3 datasets from the upgraded LHCb detector will allow the internal dynamics of this decay to be explored in much greater detail, including possible threshold enhancements and searches for CP-violating effects through spin-dependent observables. The related decay Ξ⁰_b → Λpp , for which the Run-2 analysis found a modest 2.3 standard-deviation excess, will also be studied in finer detail.

New limits on a composite Higgs

CMS figure 1 — **Fig. 1.** Simultaneous fit of the electroweak oblique parameters W and Y, combined with a previous CMS measurement of W and using 140 fb^–1 of collision data at 13 TeV and 67 fb^–1 at 13.6 TeV. The green and yellow contours enclose the regions favoured by the data at 68% and 95% confidence level, and the best-fit point lies about 2σ from the SM expectation W = Y = 0. The dotted ellipse shows the much larger region allowed by precision measurements at LEP. Credit: CMS Collab. 2026 CMS-PAS-EXO-25-001

Fourteen years of measurements have shown the Higgs boson to behave exactly as the Standard Model (SM) prescribes. However, the data remain compatible with a bound state of undiscovered constituents, held together by a new strong interaction. A recent CMS analysis places stringent constraints on composite-Higgs scenarios by searching for their signatures in high-mass dilepton production.

The possibility has long been motivated by the hierarchy problem (CERN Courier July/August 2022 p47). Quantum corrections tend to drive the Higgs-boson mass far above the electroweak scale, requiring an unnatural cancellation to recover the measured value of 125 GeV. Composite Higgs bosons evade this fine-tuning, as the scale of the new interaction naturally cuts off quantum corrections. The same dynamics predicts additional heavy resonances and small deviations from SM expectations, with lepton pairs offering one of the cleanest examples. The CMS collaboration searched for these effects in dielectron and dimuon events recorded at centre-of-mass energies of 13 and 13.6 TeV.

Rather than generating dedicated samples for each theoretical model, the collaboration reweighted simulated events using next-to-leading-order QCD predictions, allowing a wide range of scenarios to be tested efficiently. Two complementary signatures were investigated. The first is a broad, mass-dependent distortion of the dilepton spectrum, characterised by the electroweak oblique parameters W and Y, which vanish in the SM and quantify new contributions that can be absorbed as modifications to the propagation of the electroweak bosons. The second is a localised excess from a massive vector-boson resonance coupling to fermions and bosons, among the heavy states that composite-Higgs models predict.

Precision measurements can probe physics at energy scales beyond direct collider reach

No significant deviation from the SM was observed. When fitted separately, the W and Y parameters are consistent with zero, yielding tight constraints on both. A simultaneous fit reveals instead a mild excess at the 2σ level, largely driven by a single dielectron event with an invariant mass of 5.2 TeV recorded in 2022 (CERN Courier July/August 2026 p12). The excess is compatible with a statistical fluctuation, and further data will clarify its origin.

By combining these results with previous CMS constraints on W from a search in final states with one high-energy lepton and missing transverse momentum, the analysis delivered the most stringent limits to date on the oblique parameters (see figure 1). Limits were also set on the Z′ resonances predicted by Y-universal and Little Higgs scenarios with custodial symmetry, within or slightly above the visible mass spectrum.

The result illustrates how precision measurements can probe physics at energy scales beyond direct collider reach. Even without discovering new particles, exclusion limits guide theoretical development and experimental priorities, and the combination of direct resonance searches and indirect oblique-parameter constraints sharpens the picture of the allowed scenarios. As larger datasets accumulate during Run 3 and the High-Luminosity LHC era, increasingly precise measurements of the dilepton spectrum will continue to test the SM at energy scales approaching 10 TeV.

Hyperfine splitting in the B_c system

ATLAS figure 1 — **Fig. 1.** Distribution of the mass difference between the reconstructed B^*+_c and B⁺_c candidates. The fit (solid red) reveals a significant excess above the smoothly rising background (dashed blue), corresponding to the B^*+_c → B⁺_c γ decay (purple area). Credit: ATLAS Collab. arXiv:2605.16228

The B⁺_c mesons are heavy bound states of a charm quark and a beauty antiquark, intermediate in mass between charmonium (cc) and bottomonium (bb). The ATLAS collaboration has now reported the first observation of the B_c^*⁺ meson, the lightest excitation of the system, using the full 140 fb^–1 of 13 TeV proton–proton collisions recorded during Run 2 of the LHC. The spectrum of this system is largely uncharted, and mapping it offers a powerful test of QCD.

The ground-state B⁺_c meson was first observed by the CDF collaboration at the Tevatron in 1998 (CERN Courier Summer 1998 p22), but excited bc states have proved considerably more challenging to identify. In analogy with the hyperfine splitting of the hydrogen atom, the B_c^*⁺ meson’s quark and antiquark spins are aligned in the same direction, whereas they are oppositely oriented in the ground-state B⁺_c. The mass difference between the two states is therefore expected to be only a few tens of MeV, the smallest splitting among all known B_c excitations. The excited B_c^*⁺ meson decays into the ground-state B⁺_c meson by emitting a correspondingly low-energy photon, which is extremely difficult to measure.

The analysis identified the B⁺_c mesons through their semileptonic decays to J/ψ(→μ⁺μ^–)μ⁺ν_μX. Such modes are not typically used in hadron spectroscopy, because the missing neutrino prevents the full kinematics from being reconstructed. However, the branching fraction of this channel is about 20 times larger than that of the hadronic decay to J/ψπ⁺ used in earlier studies of excited bc states. This gain in statistics, along with a relatively low level of background, outweighs the complication of the partial final-state reconstruction.

The low-energy photons were identified using their conversions to electron–positron pairs in the material of the beampipe or of the innermost part of the ATLAS tracking detector. A conversion candidate is marked by a pair of oppositely charged tracks emerging from a common vertex, sufficiently displaced from the collision point. To increase the acceptance for the soft photons, a dedicated track reconstruction procedure was introduced with a transverse momentum threshold as low as 100 MeV, compared with the about 500 MeV typical of ATLAS track reconstruction. While running it on the entire dataset would be computationally impractical, the analysis benefited from a recent development in distributed data management that enabled individual raw events containing a B⁺_c candidate to be “picked” from the tapes. This way, the dedicated tracking was executed only on several hundred thousand preselected events, instead of the billions in the full dataset.

Once the B⁺_c decay and photon candidates are combined, the signal of the B_c^*⁺ decay is identified using the distribution of the mass difference m(J/ψμ⁺γ) – m(J/ψμ⁺). The signal produces a distinct peak in this distribution, despite the missing neutrino from the B⁺_c decay (see figure 1). The statistical significance of the observed signal exceeds eight standard deviations. The mass difference between the new particle and the ground-state B⁺_c meson is measured to be 64.5 ± 1.4 (stat.)^+1.0_–1.4 (syst.) MeV. This value is within the range of the available theoretical expectations, although it lies slightly above the most recent precise lattice-QCD calculations, which predict values below 60 MeV.

This discovery fills a gap in the spectrum of experimentally observed states of the bc system and will help inform the development of theoretical models. It also demonstrates the potential of ATLAS for heavy-flavour hadron spectroscopy, a capability that will be further explored with Run-3 data and at the High-Luminosity LHC.

Precision holds court in Paris

From 18 to 22 May, the 14th Large Hadron Collider Physics conference (LHCP 2026) brought 424 physicists to Sorbonne University’s Pierre and Marie Curie campus, in Paris. Over five days of plenary talks, parallel sessions and poster presentations, the ALICE, ATLAS, CMS and LHCb collaborations, and the theory community, presented a flurry of new results, alongside updates on detector upgrades for the High-Luminosity LHC (HL-LHC) phase. The picture that emerged was of a Standard Model (SM) holding up under ever-sharper scrutiny, with experimental uncertainties increasingly rivalling those from theory.

Direct implications

When ATLAS and CMS discovered the Higgs boson, in 2012, they did so with only about 10 fb^–1 of data. Today, the field they opened is entering the era of precision measurements. The Higgs production rate measured across all channels after Run 2 of the LHC agrees with the SM prediction within 5%, and theory is now the dominant source of uncertainty. A preliminary combination of the four most precise ATLAS and CMS measurements using Run 1 and Run 2 data yields m_H = 125.09 ± 0.07 GeV, a precision of 0.6‰ that places the Higgs mass among the best-known parameters of the SM, with direct implications for the stability of the electroweak vacuum.

Precision measurements of fundamental SM parameters continue to advance on multiple fronts. CMS and LHCb measured the W- and Z-boson masses to be 80360.2 ± 9.9 MeV and 91184.2 ± 9.3 MeV, respectively. Together with the ATLAS + CMS combined top-quark mass of 172.52 ± 0.33 GeV and the strong coupling α_s (see How strong is the strong interaction?), determined by a wide range of experiments and methods, these results provide essential inputs to global electroweak fits. Vector-boson scattering, first observed during Run 2, has now also been established in Run-3 data in the WW, WZ and ZZ channels, and ATLAS and CMS presented first evidence for ZZγ production, the smallest cross-section yet measured at the LHC.

The picture that emerged was of a Standard Model holding up under ever-sharper scrutiny

The Higgs-boson mass tells us how sharply the Higgs potential curves at its minimum, but not its full shape. The self-coupling λ, which provides a first look beyond the minimum, bears on whether the electroweak vacuum is truly stable, and therefore on the ultimate fate of our universe (CERN Courier January/February 2026 p31). Directly accessing λ requires measuring Higgs pair production, a process roughly a thousand times rarer than single Higgs production and one of the most ambitious goals of the LHC and HL-LHC programme. Combining their Run-2 searches, ATLAS and CMS constrained κ_λ, the measured self-coupling divided by its SM value, to between –0.71 and 6.1 at 95% confidence level (CL). Newly presented HH analyses using Run-3 data in the HH → bbbb, HH → bbγγ and HH → bbWW channels significantly improved on the sensitivity of the Run-2 measurements, even with smaller datasets, thanks to refined analysis techniques. ATLAS also presented the first search for ttHH production, excluding cross-sections above 20 times the SM prediction at 95% CL.

Sharpening up

In the flavour sector, LHCb and CMS presented new precision results on the Cabibbo–Kobayashi–Maskawa matrix angles γ and β, with CMS measuring time-dependent CP violation in neutral B decays using the full Run-3 statistics. These measurements provide increasingly stringent tests of the unitarity triangle and sharpen the sensitivity to potential new-physics contributions.

Quantum chromodynamics (QCD) governs both the dominant and some of the least understood processes at the LHC. New results on the dead-cone effect, the suppression of gluon radiation close to the flight direction of a massive quark, confirm simulation predictions and reveal large differences between b and light quarks. The catalogue of bound states keeps growing as well, with new exotic ones from LHCb bringing the number of hadrons discovered at the LHC to 82 and toponium candidates coming from ATLAS and CMS (CERN Courier May/June 2026 p8). The first oxygen–oxygen and neon–neon collisions at the LHC, delivered in 2025, offered several different views of the quark–gluon plasma, from collective flow to nuclear geometry (CERN Courier November/December 2025 p8). Among them, the rates at which J/ψ and Υ mesons melt in the medium and re-form from it differ distinctly from those seen in lead–lead.

The most interesting years may still lie ahead

Some 90 theorists attended, with many young researchers presenting new ideas and precision calculations reaching next-to-next-to- and next-to-next-to-next-to-leading order accuracy for key benchmark processes. The SM effective-field-theory framework has become a central tool for interpreting precision data and probing new physics indirectly, while lattice QCD has matured into an indispensable tool for controlling flavour observables. Achieving consistent precision across perturbative calculations, parton distribution functions, parton showers and hadronisation remains a major challenge, and reliable uncertainty estimates are now as important as the higher perturbative orders themselves.

An extensive beyond-the-SM programme covered supersymmetry, dark matter, exotic Higgs decays and rare processes, with much emphasis on searches for long-lived particles. No discovery has emerged, but exclusion limits now reach multi-TeV scales, driven by new triggers, dedicated reconstruction and dramatic machine-learning improvements deployed at every stage of the analysis chain.

Bright lights

The LHC has delivered luminosity at an extraordinary rate. With more than 1 ab^–1 of collisions provided to ATLAS and CMS, half of which in the last two and a half years, the field has firmly entered the attobarn era. Improved analysis techniques are now providing sensitivity gains that rival integrated luminosity itself, through better efficiencies and signal-to-background discrimination. The most interesting years may still lie ahead, with progress being driven by the ingenuity and creativity of the young community.

Beauty preaches from Maastricht

The 21st International Conference on B-Physics at Frontier Machines, Beauty 2026, took place from 1 to 5 June in the charming setting of St. Jan’s Church in Maastricht, the Netherlands. Maastricht University and Nikhef, the Dutch National Institute for Subatomic Physics, welcomed around 100 participants from all over the world for a programme ranging from CP violation and rare decays to spectroscopy, the production of heavy-flavoured hadrons and the prospects for quark-flavour studies at the High-Luminosity LHC (HL-LHC), Belle II, the Super Tau Charm Facility (STCF) and the Future Circular Collider (FCC).

Powerful probes

Heavy-flavour decays, in particular those of hadrons containing b quarks, offer powerful probes for testing the Standard Model (SM) and searching for new physics at the high-precision frontier. Reviewing the latest results in this field and discussing its future directions has been the central goal of the Beauty conference series since its first edition, held in Prague in 1993. This year, the LHC experiments showed an impressively broad range of new results.

LHCb reported a new study of the highly suppressed flavour-changing neutral-current decay B⁺ → π⁺μ⁺μ^–, based on data from Runs 1 and 2 of the LHC, as well as early results from Run 3. Notably, the other LHC experiments are also contributing a growing breadth of flavour results. For example, CMS presented its first study of CP violation in the “golden mode” B⁰ → J/ψK_S, which is sensitive to the unitarity-triangle angle β. To control hadronic uncertainties in the interpretation of this result, the analysis also determined CP-violating observables in the sister mode B⁰_s → J/ψK_S to record precision.

The collaborations also reported on their achievements in hadron spectroscopy, including through the observation by ATLAS of a low-lying excited B_c meson (see Hyperfine splitting in the B_c system). The conference hosted the exciting announcement of LHCb’s discovery of the doubly charmed baryon Ω⁺_cc (CERN Courier July/August 2026 p12), a result that showcases the improved capabilities of the upgraded Run-3 detector. This particle completes the family of doubly charmed ground-state baryons, extensively studied by theorists since the 1970s. Interesting in themselves, these and other multi-charmed baryons may also serve as important probes of the quark–gluon plasma at ALICE and other experiments in the HL-LHC era. Beyond the LHC, participants heard about the impressive studies of lower-mass hadrons possible at BESIII in Beijing, including the X(2370), a tantalising candidate for the elusive glueball (see A case for pure glue).

The LHC experiments showed an impressively broad range of new results

The event featured more talks from Belle II than any previous edition, reflecting the steadily improving performance of the SuperKEKB accelerator, which has delivered a dataset at the Υ(4S) resonance that now exceeds that collected by the earlier Belle experiment. One of the most striking results was a measurement of the time-dependent CP-violating asymmetry in B⁰ → π⁰π⁰ decays, exploiting a novel technique that circumvents the need to reconstruct the decay vertex of the signal B meson and exploits the quantum-entanglement of the two beauty mesons produced in the Υ(4S) decay.

First observed in 1947, kaons were fundamental in establishing flavour physics as a discipline, and remain of high interest today. A report was given of the ongoing measurement programme of the super-rare decay K⁺ → π⁺νν at CERN’s NA62 (CERN Courier May/June 2026 p9), and on the prospects for observing the even more suppressed neutral-kaon analogue K_L → π⁰νν at the KOTO experiment in Japan, predicted to occur only three times every 100 billion decays.

A flavour of the future

The final day of the conference turned to the future, laying out how the upgrades of the LHC experiments for Run 4 and beyond will enable substantial increases in sensitivity. Complementary studies, targeting decays with neutrals and missing energy, will take place at Belle II, while the STCF in China, if realised, would extend them to lower energies. On a longer timescale, the FCC, proposed to begin operation at CERN in the 2040s, would open up rich flavour opportunities, in particular with a vast sample of b hadrons produced at the Z resonance.

A key aspect of the event was the close interaction between theorists and experimentalists, fostered by 14 invited theory talks ranging from the global flavour-physics landscape to lattice QCD and models with Z′ bosons and invisible particles. Furthermore, the next generation took central stage through new sessions dedicated to early-career researchers, including lightning talks and posters presented by Maastricht University BSc students. The week left no doubt that flavour physics remains a vibrant field and a key player in the search for physics beyond the SM.

Shared goals for colliders and climate

While particle physics accounts for only a small share of global emissions, its laboratories have long worked to reduce their footprint, showing how cutting-edge science can be pursued sustainably. The fifth Sustainable HEP conference, held online from 8 to 10 July, attracted more than 220 participants worldwide to take stock of that effort, and of the ingenious technologies meant to carry it forward.

Environmental impact

Archana Sharma (CERN) highlighted a fundamental shift in the questions asked of gaseous detectors in high-energy physics. In the 1970s, the challenge was to detect particles at all, while the LHC era demanded survival in extreme conditions. Today, minimal environmental impact has joined the list of priorities. Emissions from the LHC detector systems have already fallen over the past decade, thanks to gas recuperation, recirculation and the use of climate-friendly alternatives to traditional gas mixtures. Future facilities, Sharma stressed, should be designed for sustainability from the outset, rather than retrofitted later. Her point was echoed across the sessions. Contributions covered the operation of resistive-plate chambers in CMS and ATLAS, along with the EcoGas@GIF++ collaboration, which tests eco-friendly mixtures under LHC-like irradiation at CERN’s Gamma Irradiation Facility. Others explored material choices for future detectors, from high-strength copper alloys to greener shielding and cryogenics.

Future facilities should be designed for sustainability from the outset

Accelerators and other large-scale projects received similar attention. Ben Shepherd (STFC) reported on the activities of the Laboratory Directors Group’s sustainability working group, comparing the environmental impact of proposed colliders from construction to decommissioning. He described efforts to avoid environmental impacts where possible, reduce those that cannot be avoided, and compensate for the remainder, spanning civil engineering, procurement and energy optimisation. Examples include heat-recovery systems, energy-recovery linacs, and the growing use of high-temperature superconductors and permanent magnets.

Veronique Boisvert (Royal Holloway) briefed participants on the sustainability recommendations of the European Strategy for Particle Physics update, the roadmap defining Europe’s priorities for future particle-physics research and facilities. William van Sprolant (CvS énergies sàrl) presented the ULISSE Project, an exploratory large-scale initiative aiming at storing CERN’s summer waste heat in Lake Geneva for use in winter. Speakers presented initiatives across the field including Nikhef’s laboratory-wide roadmap, the iSAS programme on energy-efficient accelerating technology, sustainability work at the Electron–Ion Collider and life-cycle studies at LHCb. Other presentations covered methane recovery from livestock emissions and updates from the SC4RC computing conference.

Scientific computing came under the same scrutiny. Contributions included a particularly clear introduction to Fast Generative Simulation for HEP from Henry Day-Hall (DESY), alongside discussions of current and future efforts to shrink its carbon footprint by optimising the use of resources, improving simulation efficiency and adopting GPU-based systems, whose parallel architecture delivers more computation per watt. Participants heard of the power-efficient operation of large-scale computing facilities through the Worldwide LHC Computing Grid, which links centres around the world to process LHC data, carbon-aware computing schedules in ATLAS and the GreenPhysECS project, aimed at designing energy-efficient computing architectures for future facilities.

From physics to sustainability

As in previous editions, voices from beyond particle physics widened the view. Juan Pablo Cordero (University of York) presented open-source predictive models for transparent food policy-making. This year’s conference hosted a panel session featuring five speakers, who shared their experiences of transitioning from physics to sustainability. They cited adaptability, continuous learning and collaboration as the skills that helped them move into their new fields.

Lasting change requires ambitious policies and collective solutions

In his keynote speech, climate scientist Jagadish Shukla (George Mason University) discussed the science, politics and ethics of climate change. Those who have contributed least to global warming, he reminded participants, will suffer most and earliest. Yet he argued for hope, grounded in technological progress and shared effort. Lasting change requires ambitious policies and collective solutions, as the antidote to climate anxiety, he concluded, is climate action.

Patience pays at the lifetime frontier

The 16th workshop of the Long-Lived Particle Community (LLP2026) gathered more than 70 theorists, phenomenologists and experimentalists at Cambridge’s Ray Dolby Centre, the new home of the Cavendish Laboratory, from 29 June to 3 July. Discussions centred on particles that live long enough to travel a measurable distance before decaying, and on the searches, algorithms and detectors designed to catch them.

Long-lived particles (LLPs) arise in a wide range of theories beyond the Standard Model (SM), from supersymmetry and extended Higgs sectors to models of neutrino masses or dark matter. Due to their long lifetimes, LLPs could decay far from the collision point and leave signatures that are exotic and hard to identify.

On the hunt

The CMS collaboration presented eight new searches in which displaced muons and dedicated LLP triggers played a central role. Anna Mascellani (ETH Zurich) presented a search for heavy neutral leptons (HNLs). Among the simplest and best-motivated LLP candidates, they could explain the origin of neutrino masses through the so-called “seesaw” mechanism. The new channel targets HNL decays beyond the tracker using standalone displaced muons, reconstructed exclusively in the muon system. The gain in sensitivity at longer lifetimes yields the best expected limits on HNL mixing with muon neutrinos for masses between 2.5 and 4.2 GeV. Celia Fernandez Madrazo (Boston University) presented a search for light, narrow resonances decaying to two displaced muons, recorded with a dedicated scouting trigger stream, in which events are kept at a high rate by recording only a reduced amount of trigger-level information. The saving in bandwidth allows muons with transverse momenta down to 3 GeV to be collected, and relaxed requirements on the number of hits give Run-3 scouting access – for the first time – to muons displaced by more than 11 cm. Together, the two extensions yield the most stringent limits to date on dark-shower models, which predict light, long-lived dark pions decaying after 10 cm or more.

The workshop showed how new triggers and analysis strategies are testing a wide range of theoretical models

ATLAS presented several results with 13 and 13.6 TeV data. Paul Swallow (University of Cambridge) reported a search for displaced vertices and missing transverse momentum built on a novel fuzzy-vertexing algorithm, optimised for displaced decays to heavy quarks. Since b quarks themselves travel a sizeable distance before decaying, an LLP decaying to them need not produce particles pointing back to a single vertex. Rather than forcing one, the algorithm reconstructs several nearby seed vertices and then merges them into a single one, improving efficiency for the high-mass, high-multiplicity displaced vertices expected from such decays.

Models predicting quirks, hypothetical particles charged both under the SM and a new confining force analogous to the strong force, drew particular interest. Produced in pairs and bound to each other by a confining string, quirks would cross a detector slowly, as charged long-lived particles. Arash Jofrehei (University of Geneva) presented the first direct LHC search for quirks with masses above the weak scale, performed with FASER, and Alejandro Novo Cal (IGFAE, University of Santiago de Compostela) proposed a search at LHCb, complementary in reach to those of heavy stable charged particles or monojets at ATLAS and CMS. Thanks to the forward acceptance and lack of magnetic field of the LHCb Vertex Locator (VELO), alongside a flexible trigger design, the approach could probe values of the new confining scale between 0.1 and 10 keV. The stakes reach into cosmology, as Graham Kribs (CERN and University of Oregon) stressed. If macroscopic dark-colour strings were observed, the reheating temperature at which the hot Big Bang began after inflation would have to lie below about 100 GeV, since a hotter start would have left long-lived bound states of dark gluons in conflict with early-universe observations.

Martin Hirsch (IFIC, CSIC and University of Valencia) showed that axion-like particles (ALPs) can couple HNLs to gluons, giving the LHC and proposed LLP detectors high sensitivity to long-lived HNLs for GeV-scale ALP masses. As Oleg Brandt (University of Cambridge) observed, the two candidates are usually treated separately, and their combination offers interesting new benchmarks.

Future facilities

On the facilities side, John Anders (University of Liverpool) reviewed design and detector studies for the proposed Forward Physics Facility, including a full simulation of its cavern. Updates followed from SHiP/NA67, CODEX-b, MATHUSLA and ANUBIS, with sensitivity studies for benchmark models ranging from HNLs and Higgs-portal scalars to dark photons and ALPs. Looking further ahead, Rhitaja Sengupta (BCTP, University of Bonn) presented dedicated LLP detector concepts for FCC-ee and FCC-hh, including DELIGHT, FOREHUNT and DELIGHT-SHIELD, stressing how a future collider’s design phase offers the best opportunity to integrate dedicated LLP detectors within the main detector designs.

Discussions extended beyond CERN. Zeren Simon Wang (Hefei University of Technology) presented prospects for searching for very long-lived HNLs through baryon-number-violating decays of charmed baryons at the Super Tau Charm Facility proposed in Hefei, China. The same symmetry violation motivated Patrick Adolf (TU Dortmund) to propose a new class of nucleon-decay observables involving LLPs, in which a vector mediator produced in the decay travels a macroscopic distance before turning into an electron–positron pair inside a detector such as Super-Kamiokande, leaving temporally correlated “echo” vertices.

LLP2026 opened just as the LHC entered Long Shutdown 3, and the results presented drew on only part of the Run-3 dataset. Even so, the workshop showed how new triggers and analysis strategies are testing a wide range of theoretical models. The detectors and facilities now taking shape promise to go further, so that no new physics remains hidden at the long-lifetime frontier.

Topics

Reset your password

Registration complete