It’s tough to be a lone dissenting voice, but the CDF collaboration is sticking to its guns. Ongoing cross-checks at the Tevatron experiment reinforce its 2022 measurement of the mass of the W boson, which stands seven standard deviations above the Standard Model (SM) prediction. All other measurements are statistically compatible with the SM, though slightly higher, including the most recent by the CMS collaboration at the LHC, which almost matched CDF’s stated precision of 9.4 MeV (CERN Courier November/December 2024 p7).
With CMS’s measurement came fresh scrutiny for the CDF collaboration, which had established one of the most interesting anomalies in fundamental science – a higher-than-expected W mass might reveal the presence of undiscovered heavy virtual particles. Particular scrutiny focused on the quoted momentum resolution of the CDF detector, which the collaboration claims exceeds the precision of any other collider detector by more than a factor of two. A new analysis by CDF verifies the stated accuracy of 25 parts per million by constraining possible biases using a large sample of cosmic-ray muons.
“The publication lays out the ‘warts and all’ of the tracking aspect and explains why the CDF measurement should be taken seriously despite being in disagreement with both the SM and silicon-tracker-based LHC measurements,” says spokesperson David Toback of Texas A&M University. “The paper should be seen as required reading for anyone who truly wants to understand, without bias, the path forward for these incredibly difficult analyses.”
The 2022 W-mass measurement exclusively used information from CDF’s drift chamber – a descendant of the multiwire proportional chamber invented at CERN by Georges Charpak in 1968 – and discarded information from its inner silicon vertex detector as it offered only marginal improvements to momentum resolution. The new analysis by CDF collaborator Ashutosh Kotwal of Duke University studies possible geometrical defects in the experiment’s drift chamber that could introduce unsuspected biases in the measured momenta of the electrons and muons emitted in the decays of W bosons.
“Silicon trackers have replaced wire-based technology in many parts of modern particle detectors, but the drift chamber continues to hold its own as the technology of choice when high accuracy is required over large tracking volumes for extended time periods in harsh collider environments,” opines Kotwal. “The new analysis demonstrates the efficiency and stability of the CDF drift chamber and its insensitivity to radiation damage.”
The CDF II detector operated at Fermilab’s Tevatron collider from 1999 to 2011. Its cylindrical drift chamber was coaxial with the colliding proton and antiproton beams, and immersed in an axial 1.4 T magnetic field. A helical fit yielded track parameters.
Fast radio bursts (FRBs) are short but powerful bursts of radio waves that are believed to be emitted by dense astrophysical objects such as neutron stars or black holes. They were discovered by Duncan Lorimer and his student David Narkevic in 2007 while studying archival data from the Parkes radio telescope in Australia. Since then, more than a thousand FRBs have been detected, located both within and without the Milky Way. These bursts usually last only a few milliseconds but can release enormous amounts of energy – an FRB detected in 2022 gave off more energy in a millisecond than the Sun does in 30 years – however, the exact mechanism underlying their creation remains a mystery.
Inhomogeneities caused by the presence of gas and dust in the interstellar medium scatter the radio waves coming from an FRB. This creates a stochastic interference pattern on the signal, called scintillation – a phenomenon akin to the twinkling of stars. In a recent study, astronomer Kenzie Nimmo and her colleagues used scintillation data from FRB 20221022A to constrain the size of its emission region. FRB 20221022A is a 2.5 millisecond burst from a galaxy about 200 million light-years away. It was detected on 22 October 2022 by the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst project (CHIME/FRB).
The CHIME telescope is currently the world’s leading FRB detector, discovering an average of three new FRBs every day. It consists of four stationary 20 m-wide and 100 m-long semi-cylindrical paraboloidal reflectors with a focal length of 5 m (see “Right on CHIME” figure). 256 dual-polarisation feeds suspended along each axis gives it a field of view of more than 200 square degrees. With a wide bandwidth, high sensitivity and a high-performance correlator to pinpoint where in the sky signals are coming from, CHIME is an excellent instrument for the detection of FRBs. The antenna receives radio waves in the frequency range of 400 to 800 MHz.
Two main classes of models compete to explain the emission mechanisms of FRBs. Near-field models hypothesise that emission occurs in close proximity to the turbulent magnetosphere of a central engine, while far-away models hypothesise that emission occurs in relativistic shocks that propagate out to large radial distances. Nimmo and her team measured two distinct scintillation scales in the frequency spectrum of FRB 20221022A: one originating from its host galaxy or local environment, and another from a scattering site within the Milky Way. By using these scattering sites as astrophysical lenses, they were able to constrain the size of the FRB’s emission region to better than 30,000 km. This emission size contradicted expectations from far-away models. It is more consistent with an emission process occurring within or just beyond the magnetosphere of a central compact object – the first clear evidence for the near-field class of models.
Additionally, FRB 20221022A’s detection paper notes a striking change in the burst’s polarisation angle – an “S-shaped” swing covering about 130° – over a mere 2.5 milliseconds. They interpret this as the emission beam physically sweeping across our line of sight, much like a lighthouse beam passing by an observer, and conclude that it hints at a magnetospheric origin of the emission, as highly magnetised regions can twist or shape how radio waves are emitted. The scintillation studies by Nimmo et al. independently support this conclusion, narrowing the possible sources and mechanisms that power FRBs. Moreover, they highlight the potential of the scintillation technique to explore the emission mechanisms in FRBs and understand their environments.
The field of FRB physics looks set to grow by leaps and bounds. CHIME can already identify host galaxies for FRBs, but an “outrigger” programme using similar detectors geographically displaced from the main telescope at the Dominion Radio Astrophysical Observatory near Penticton, British Columbia, aims to strengthen its localisation capabilities to a precision of tens of milliarcsecond. CHIME recently finished deploying its third outrigger telescope in northern California.
Collisions between lead ions at the LHC generate the hottest and densest system ever created in the laboratory. Under these extreme conditions, quarks and gluons are no longer confined inside hadrons but instead form a quark–gluon plasma (QGP). Being heavier than the more abundantly produced light quarks, charm quarks play a special role in probing the plasma since they are created in the collision before the plasma is formed and interact with the plasma as they traverse the collision zone. Charm jets, which are clusters of particles originating from charm quarks, have been investigated for the first time by the ALICE collaboration in Pb–Pb collisions at the LHC using the D0 mesons (that carry a charm quark) as tags.
The primary interest lies in measuring the extent of energy loss experienced by different types of particles as they traverse the plasma, referred to as “in-medium energy loss”. This energy loss specifically depends on the particle type and particle mass, varying between quarks and gluons. Due to their larger mass, charm quarks at low transverse momentum do not reach the speed of light and lose substantially less energy than light quarks through both collisional and radiative processes, as gluon radiation by massive quarks is suppressed: the so-called “dead-cone effect”. Additionally, gluons, which carry a larger colour charge than quarks, experience greater energy loss in the QGP as quantified by the Casimir factors CA = 3 for gluons and CF = 4/3 for quarks. This makes the charm quark an ideal probe for studying the QGP properties. ALICE is well suited to study the in-medium energy loss of charm quarks, which is dependent on the mass of the charm quark and its colour charge.
The production yield of charm jets tagged with fully reconstructed D0 mesons (D0→ K–π+) in central Pb–Pb collisions at a centre-of-mass energy of 5.02 TeV per nucleon pair during LHC Run 2 was measured by ALICE. The results are reported in terms of nuclear modification factor (RAA), which is the ratio of the particle production rate in Pb–Pb collisions to that in proton–proton collisions, scaled by the number of binary nucleon–nucleon collisions. A measured nuclear modification factor of unity would indicate the absence of final-state effects.
The results, shown in figure 1, show a clear suppression (RAA < 1) for both charm jets and inclusive jets (that mainly originate from light quarks and gluons) due to energy loss. Importantly, the charm jets exhibit less suppression than the inclusive jets within the transverse momentum range of 20 to 50 GeV, which is consistent with mass and colour-charge dependence.
The measured results are compared with theoretical model calculations that include mass effects in the in-medium energy loss. Among the different models, LIDO incorporates both the dead-cone effect and the colour-charge effects, which are essential for describing the energy-loss mechanisms. Consequently, it shows reasonable agreement with experimental data, reproducing the observed hierarchy between charm jets and inclusive jets.
The present finding provides a hint of the flavour-dependent energy loss in the QGP, suggesting that charm jets lose less energy than inclusive jets. This highlights the quark-mass and colour-charge dependence of the in-medium energy-loss mechanisms.
A new result from the LHCb collaboration supports the hypothesis that the rare decays B±→ K±e+e– and B±→ K±µ+µ– occur at the same rate, further tightening constraints on the magnitude of lepton flavour universality (LFU) violation in rare B decays. The new measurement is the most precise to date in the high-q2 region and the first of its kind at a hadron collider.
LFU is an accidental symmetry of the Standard Model (SM). Under LFU, each generation of lepton ℓ± (electron, muon and tau lepton) is equally likely to interact with the W boson in decay processes such as B±→ K±ℓ+ℓ–. This symmetry leads to the prediction that the ratio of branching fractions for these decay channels should be unity except for kinematic effects due to the different masses of the charged leptons. The most straightforward ratio to measure is that between the muon and electron decay modes, known as RK. Any significant deviation from RK = 1 could only be explained by the existence of new physics (NP) particles that preferentially couple to one lepton generation over another, violating LFU.
B±→ K±ℓ+ℓ– decays are a powerful probe for virtual NP particles. These decays involve an underlying b–to–s quark transition – an example of a flavour-changing neutral current (FCNC). FCNC transitions are extremely rare in the SM, as they occur only through higher-order Feynman diagrams. This makes them particularly sensitive to contributions from NP particles, which could significantly alter the characteristics of the decays. In this case, the mass of the NP particles could be much larger than can be produced directly at the LHC. “Indirect” searches for NP, such as measuring the precisely predicted ratio RK, can probe mass scales beyond the reach of direct-production searches with current experimental resources.
The new measurement is the most precise to date in the high-q2 region
In the decay process B±→ K±ℓ+ℓ–, the final-state leptons can also originate from an intermediate resonant state, such as a J/ψ or ψ(2S). These resonant channels occur through tree-level Feynman diagrams. Their contributions significantly outnumber the non-resonant FCNC processes and are not expected to be affected by NP. RK is therefore measured in ranges of dilepton invariant mass-squared (q2), which exclude these resonances, to preserve sensitivity to potential NP effects in FCNC processes.
The new result from the LHCb collaboration measures RK in the high-q2 region, above the ψ(2S) resonance. The high-q2 region data has a different composition of backgrounds compared to the low-q2 data, leading to different strategies for their rejection and modelling, and different systematic effects. With RK expected to be unity in all domains in the SM, low-q2 and high-q2 measurements offer powerfully complementary constraints on the magnitude of LFU-violating NP in rare B decays.
The new measurement of RK agrees with the SM prediction of unity and is the most precise to date in the high-q2 region (figure 1). It complements a refined analysis below the J/ψ resonance published by LHCb in 2023, which also reported RK consistent with unity. Both results use the complete proton–proton collision data collected by LHCb from 2011 to 2018. They lay the groundwork for even more precise measurements with data from Run 3 and beyond.
As direct searches for physics beyond the Standard Model continue to push frontiers at the LHC, the b-hadron physics sector remains a crucial source of insight for testing established theoretical models.
The ATLAS collaboration recently published a new measurement of the B0 lifetime using B0→ J/ψK*0 decays from the entire Run-2 dataset it has recorded at 13 TeV. The result improves the precision of previous world-leading measurements by the CMS and LHCb collaborations by a factor of two.
Studies of b-hadron lifetimes probe our understanding of the weak interaction. The lifetimes of b-hadrons can be systematically computed within the heavy-quark expansion (HQE) framework, where b-hadron observables are expressed as a perturbative expansion in inverse powers of the b-quark mass.
ATLAS measures the “effective” B0 lifetime, which represents the average decay time incorporating effects from mixing and CP contributions, as τ(B0) = 1.5053 ± 0.0012 (stat.) ± 0.0035 (syst.) ps. The result is consistent with previous measurements published by ATLAS and other experiments, as summarised in figure 1. It also aligns with theoretical predictions from HQE and lattice QCD, as well as with the experimental world average.
The analysis benefitted from the large Run-2 dataset and a refined trigger selection, enabling the collection of an extensive sample of 2.5 million B0→ J/ψK*0 decays. Events with a J/ψ meson decaying into two muons with sufficient transverse momentum are cleanly identified in the ATLAS Muon Spectrometer by the first-level hardware trigger. In the next-level software trigger, exploiting the full detector information, these muons are then combined with two tracks measured by the Inner Detector, ensuring they originate from the same vertex.
The B0-meson lifetime is determined through a two-dimensional unbinned maximum-likelihood fit, utilising the measured B0-candidate mass and decay time, and accounting for both signal and background components. The limited hadronic particle-identification capability of ATLAS requires careful modelling of the significant backgrounds from other processes that produce J/ψ mesons. The sensitivity of the fit is increased by estimating the uncertainty of the decay-time measurement provided by the ATLAS tracking and vertexing algorithms on a per-candidate basis. The resulting lifetime measurement is limited by systematic uncertainties, with the largest contributions arising from the correlation between B0 mass and lifetime, and ambiguities in modelling the mass distribution.
ATLAS combined its measurement with the average decay width (Γs) of the light and heavy Bs-meson mass eigenstates, also measured by ATLAS, to determine the ratio of decay widths as Γd/Γs = 0.9905 ± 0.0022 (stat.) ± 0.0036 (syst.) ± 0.0057 (ext.). The result is consistent with unity and provides a stringent test of QCD predictions, which also support a value near unity.
In the autumn of 2023, Wojciech Brylinski was analysing data from the NA61/SHINE collaboration at CERN for his thesis, when he noticed an unexpected anomaly – a strikingly large imbalance between charged and neutral kaons in argon–scandium collisions. Instead of producing roughly equal numbers, he found that charged kaons were produced 18.4% more often. This suggested that the “isospin symmetry” between up (u) and down (d) quarks might be broken by more than expected due to the differences in their electric charges and masses – a discrepancy that existing theoretical models would struggle to explain. Known sources of isospin asymmetry only predict deviations of a few percent.
“When Wojciech got started, we thought it would be a trivial verification of the symmetry,” says Marek Gaździcki of Jan Kochanowski University of Kielce, spokesperson of NA61/SHINE at the time of the discovery. “We expected it to be closely obeyed – though we had previously measured discrepancies at NA49, they had large uncertainties and were not significant.”
Isospin symmetry is one facet of flavour symmetry, whereby the strong interaction treats all quark flavours identically, except for kinematic differences arising from their different masses. Strong interactions should therefore generate nearly equal yields of charged K+ (us) and K– (us), and neutral K0 (ds) and K0 (ds), given the similar masses of the two lightest quarks. NA61/SHINE’s data contradict the hypothesis of equal yields with 4.7σ significance.
“I see two options to interpret the results,” says Francesco Giacosa, a theoretical physicist at Jan Kochanowski University working with NA61/SHINE. “First, we substantially underestimate the role of electromagnetic interactions in creating quark–antiquark pairs. Second, strong interactions do not obey flavour symmetry – if so, this would falsify QCD.” Isospin is not a symmetry of the electromagnetic interaction as up and down quarks have different electric charges.
While the experiment routinely measures particle yields in nuclear collisions, finding a discrepancy in isospin symmetry was not something researchers were actively looking for. NA61/SHINE’s primary focus is studying the phase diagram of high-energy nuclear collisions using a range of ion beams. This includes looking at the onset of deconfinement, the formation of a quark-gluon plasma fireball, and the search for the hypothesised QCD critical point where the transition between hadronic matter and quark–gluon plasma changes from a smooth crossover to a first-order phase transition. Data is also shared with neutrino and cosmic-ray experiments to help refine their models.
The collaboration is now planning additional studies using different projectiles, targets and collision energies to determine whether this effect is unique to certain heavy-ion collisions or a more general feature of high-energy interactions. They have also put out a call to theorists to help explain what might have caused such an unexpectedly large asymmetry.
“The observation of the rather large isospin violation stands in sharp contrast to its validity in a wide range of physical systems,” says Rob Pisarski, a theoretical physicist from Brookhaven National Laboratory. “Any explanation must be special to heavy-ion systems at moderate energy. NA61/SHINE’s discrepancy is clearly significant, and shows that QCD still has the power to surprise our naive expectations.”
On 13 February 2023, strings of photodetectors anchored to the seabed off the coast of Sicily detected the most energetic neutrino ever observed, smashing previous records. Embargoed until the publication of a paper in Nature last month, the KM3NeT collaboration believes their observation may have originated in a novel cosmic accelerator, or may even be the first detection of a “cosmogenic” neutrino.
“This event certainly comes as a surprise,” says KM3NeT spokesperson Paul de Jong (Nikhef). “Our measurement converted into a flux exceeds the limits set by IceCube and the Pierre Auger Observatory. If it is a statistical fluctuation, it would correspond to an upward fluctuation at the 2.2σ level. That is unlikely, but not impossible.” With an estimated energy of a remarkable 220 PeV, the neutrino observed by KM3NeT surpasses IceCube’s record by almost a factor of 30.
The existence of ultra-high-energy cosmic neutrinos has been theorised since the 1960s, when astrophysicists began to conceive ways that extreme astrophysical environments could generate particles with very high energies. At about the same time, Arno Penzias and Robert Wilson discovered “cosmic microwave background” (CMB) photons emitted in the era of recombination, when the primordial plasma cooled down and the universe became electrically neutral. Cosmogenic neutrinos were soon hypothesised to result from ultra-high-energy cosmic rays interacting with the CMB. They are expected to have energies above 100 PeV (1017 eV), however, their abundance is uncertain as it depends on cosmic rays, whose sources are still cloaked in intrigue (CERN Courier July/August 2024 p24).
A window to extreme events
But how might they be detected? In this regard, neutrinos present a dichotomy: though outnumbered in the cosmos only by photons, they are notoriously elusive. However, it is precisely their weakly interacting nature that makes them ideal for investigating the most extreme regions of the universe. Cosmic neutrinos travel vast cosmic distances without being scattered or absorbed, providing a direct window into their origins, and enabling scientists to study phenomena such as black-hole jets and neutron-star mergers. Such extreme astrophysical sources test the limits of the Standard Model at energy scales many times higher than is possible in terrestrial particle accelerators.
Because they are so weakly interacting, studying cosmic neutrinos requires giant detectors. Today, three large-scale neutrino telescopes are in operation: IceCube, in Antarctica; KM3NeT, under construction deep in the Mediterranean Sea; and Baikal–GVD, under construction in Lake Baikal in southern Siberia. So far, IceCube, whose construction was completed over 10 years ago, has enabled significant advancements in cosmic-neutrino physics, including the first observation of the Glashow resonance, wherein a 6 PeV electron antineutrino interacts with an electron in the ice sheet to form an on-shell W boson, and the discovery of neutrinos emitted by “active galaxies” powered by a supermassive black hole accreting matter. The previous record-holder for the highest recorded neutrino energy, IceCube has also searched for cosmogenic neutrinos but has not yet observed neutrino candidates above 10 PeV.
Its new northern-hemisphere colleague, KM3NeT, consists of two subdetectors: ORCA, designed to study neutrino properties, and ARCA, which made this detection, designed to detect high-energy cosmic neutrinos and find their astronomical counterparts. Its deep-sea arrays of optical sensors detect Cherenkov light emitted by charged particles created when a neutrino interacts with a quark or electron in the water. At the time of the 2023 event, ARCA comprised 21 vertical detection units, each around 700 m in length. Its location 3.5 km deep under the sea reduces background noise, and its sparse set up over one cubic kilometre optimises the detector for neutrinos of higher energies.
The event that KM3NeT observed in 2023 is thought to be a single muon created by the charged-current interaction of an ultra-high-energy muon neutrino. The muon then crossed horizontally through the entire ARCA detector, emitting Cherenkov light that was picked up by a third of its active sensors. “If it entered the sea as a muon, it would have travelled some 300 km water-equivalent in water or rock, which is impossible,” explains de Jong. “It is most likely the result of a muon neutrino interacting with sea water some distance from the detector.”
The network will improve the chances of detecting new neutrino sources
The best estimate for the neutrino energy of 220 PeV hides substantial uncertainties, given the unknown interaction point and the need to correct for an undetected hadronic shower. The collaboration expects the true value to lie between 110 and 790 PeV with 68% confidence. “The neutrino energy spectrum is steeply falling, so there is a tug-of-war between two effects,” explains de Jong. “Low-energy neutrinos must give a relatively large fraction of their energy to the muon and interact close to the detector, but they are numerous; high-energy neutrinos can interact further away, and give a smaller fraction of their energy to the muon, but they are rare.”
More data is needed to understand the sources of ultra-high-energy neutrinos such as that observed by KM3NeT, where construction has continued in the two years since this remarkable early detection. So far, 33 of 230 ARCA detection units and 24 of 115 ORCA detection units have been installed. Once construction is complete, likely by the end of the decade, KM3NeT will be similar in size to IceCube.
“Once KM3NeT and Baikal–GVD are fully constructed, we will have three large-scale neutrino telescopes of about the same size in operation around the world,” adds Mauricio Bustamante, theoretical astroparticle physicist at the Niels Bohr Institute of the University of Copenhagen. “This expanded network will monitor the full sky with nearly equal sensitivity in any direction, improving the chances of detecting new neutrino sources, including faint ones in new regions of the sky.”
Quarks contribute less than 1% to the mass of protons and neutrons. This provokes an astonishing question: where does the other 99% of the mass of the visible universe come from? The answer lies in the gluon, and how it interacts with itself to bind quarks together inside hadrons.
Much remains to be understood about gluon dynamics. At present, the chief experimental challenge is to observe the onset of gluon saturation – a dynamic equilibrium between gluon splitting and recombination predicted by QCD. The experimental key looks likely to be a rare but intriguing type of LHC interaction known as an ultraperipheral collision (UPC), and the breakthrough may come as soon as the next experimental run.
Gluon saturation is expected to end the rapid growth in gluon density measured at the HERA electron–proton collider at DESY in the 1990s and 2000s. HERA observed this growth as the energy of interactions increased and as the fraction of the proton’s momentum borne by the gluons (Bjorken x) decreased.
So gluons become more numerous in hadrons as their energy decreases – but to what end?
Gluonic hotspots are now being probed with unprecedented precision at the LHC and are central to understanding the high-energy regime of QCD
Nonlinear effects are expected to arise due to processes like gluon recombination, wherein two gluons combine to become one. When gluon recombination becomes a significant factor in QCD dynamics, gluon saturation sets in – an emergent phenomenon whose energy scale is a critical parameter to determine experimentally. At this scale, gluons begin to act like classical fields and gluon density plateaus. A dilute partonic picture transitions to a dense, saturated state. For recombination to take precedence over splitting, gluon momenta must be very small, corresponding to low values of Bjorken x. The saturation scale should also be directly proportional to the colour-charge density, making heavy nuclei like lead ideal for studying nonlinear QCD phenomena.
But despite strong theoretical reasoning and tantalising experimental hints, direct evidence for gluon saturation remains elusive.
Since the conclusion of the HERA programme, the quest to explore gluon saturation has shifted focus to the LHC. But with no point-like electron to probe the hadronic target, LHC physicists had to find a new point-like probe: light itself. UPCs at the LHC exploit the flux of quasi-real high-energy photons generated by ultra-relativistic particles. For heavy ions like lead, this flux of photons is enhanced by the square of the nuclear charge, enabling studies of photon-proton (γp) and photon-nucleus interactions at centre-of-mass energies reaching the TeV scale.
Keeping it clean
What really sets UPCs apart is their clean environment. UPCs occur at large impact parameters well outside the range of the strong nuclear force, allowing the nuclei to remain intact. Unlike hadronic collisions, which can produce thousands of particles, UPCs often involve only a few final-state particles, for example a single J/ψ, providing an ideal laboratory for gluon saturation. J/ψ are produced when a cc pair created by two or more gluons from one nucleus is brought on-shell by interacting with a quasi-real photon from the other nucleus (see “Sensitivity to saturation” figure).
Gluon saturation models predict deviations in the γp → J/ψp cross section from the power-law behaviour observed at HERA. The LHC experiments are placing a significant focus on investigating the energy dependence of this process to identify potential signatures of saturation, with ALICE and LHCb extending studies to higher γp centre-of-mass energies (Wγp) and lower Bjorken x than HERA. The results so far reveal that the cross-section continues to increase with energy, consistent with the power-law trend (see “Approaching the plateau?” figure).
The symmetric nature of pp collisions introduces significant challenges. In pp collisions, either proton can act as the photon source, leading to an intrinsic ambiguity in identifying the photon emitter. In proton–lead (pPb) collisions, the lead nucleus overwhelmingly dominates photon emission, eliminating this ambiguity. This makes pPb collisions an ideal environment for precise studies of the photoproduction of J/ψ by protons.
During LHC Run 1, the ALICE experiment probed Wγp up to 706 GeV in pPb collisions, more than doubling HERA’s maximum reach of 300 GeV. This translates to probing Bjorken-x values as low as 10–5, significantly beyond the regime explored at HERA. LHCb took a different approach. The collaboration inferred the behaviour of pp collisions at high energies (“W+ solutions”) by assuming knowledge of their energy dependence at low energies (“W- solutions”), allowing LHCb to probe gluon energies as small as 10–6 in Bjorken x and Wγp up to 2 TeV.
There is not yet any theoretical consensus on whether LHC data align with gluon-saturation predictions, and the measurements remain statistically limited, leaving room for further exploration. Theoretical challenges include incomplete next-to-leading-order calculations and the reliance of some models on fits to HERA data. Progress will depend on robust and model-independent calculations and high-quality UPC data from pPb collisions in LHC Run 3 and Run 4.
Some models predict a slowing increase in the γp → J/ψp cross section with energy at small Bjorken x. If these models are correct, gluon saturation will likely be discovered in LHC Run 4, where we expect to see a clear observation of whether pPb data deviate from the power law observed so far.
Gluonic hotspots
If a UPC photon interacts with the collective colour field of a nucleus – coherent scattering – it probes its overall distribution of gluons. If a UPC photon interacts with individual nucleons or smaller sub-nucleonic structures – incoherent scattering – it can probe smaller-scale gluon fluctuations.
These fluctuations, known as gluonic hotspots, are theorised to become more numerous and overlap in the regime of gluon saturation (see “Onset of saturation” figure). Now being probed with unprecedented precision at the LHC, they are central to understanding the high-energy regime of QCD.
Gluonic hotspots are used to model the internal transverse structure of colliding protons or nuclei (see “Hotspot snapshots” figure). The saturation scale is inherently impact-parameter dependent, with the densest colour charge densities concentrated at the core of the proton or nucleus, and diminishing toward the periphery, though subject to fluctuations. Researchers are increasingly interested in exploring how these fluctuations depend on the impact parameter of collisions to better characterise the spatial dynamics of colour charge. Future analyses will pinpoint contributions from localised hotspots where saturation effects are most likely to be observed.
The energy dependence of incoherent or dissociative photoproduction promises a clear signature for gluon saturation, independent of the coherent power-law method described above. As saturation sets in, all gluon configurations in the target converge to similar densities, causing the variance of the gluon field to decrease, and with it the dissociative cross section. Detecting a peak and a decline in the incoherent cross-section as a function of energy would represent a clear signature of gluon saturation.
The ALICE collaboration has taken significant steps in exploring this quantum terrain, demonstrating the possibility of studying different geometrical configurations of quantum fluctuations in processes where protons or lead nucleons dissociate. The results highlight a striking correlation between momentum transfer, which is inversely proportional to the impact parameter, and the size of the target structure. The observation that sub-nucleonic structures impart the greatest momentum transfer is compelling evidence for gluonic quantum fluctuations at the sub-nucleon level.
Into the shadows
In 1982 the European Muon Collaboration observed an intriguing phenomenon: nuclei appeared to contain fewer gluons than expected based on the contributions from their individual protons and neutrons. This effect, known as nuclear shadowing, was observed in experiments conducted at CERN at moderate values of Bjorken x. It is now known to occur because the interaction of a probe with one gluon reduces the likelihood of the probe interacting with other gluons within the nucleus – the gluons hiding behind them, in their shadow, so to speak. At smaller values of Bjorken x, saturation further suppresses the number of gluons contributing to the interaction.
The relationship between gluon saturation and nuclear shadowing is poorly understood, and separating their effects remains an open challenge. The situation is further complicated by an experimental reliance on lead–lead (PbPb) collisions, which, like pp collisions, suffer from ambiguity in identifying the interacting nucleus, unless the interaction is accompanied by an ejected neutron.
The ALICE, CMS and LHCb experiments have extensively studied nuclear shadowing via the exclusive production of vector mesons such as J/ψ in ultraperipheral PbPb
collisions. Results span photon–nucleus collision energies from 10 to 1000 GeV. The onset of nuclear shadowing, or another nonlinear QCD phenomenon like saturation, is clearly visible as a function of energy and Bjorken x (see “Nuclear shadowing” figure).
Multidimensional maps
While both saturation-based and gluon shadowing models describe the data reasonably well at high energies, neither framework captures the observed trends across the entire kinematic range. Future efforts must go beyond energy dependence by being differential in momentum transfer and studying a range of vector mesons with complementary sensitivities to the saturation scale.
Soon to be constructed at Brookhaven National Laboratory, the Electron-Ion Collider (EIC) promises to transform our understanding of gluonic matter. Designed specifically for QCD research, the EIC will probe gluon saturation and shadowing in unprecedented detail, using a broad array of reactions, collision species and energy levels. By providing a multidimensional map of gluonic behaviour, the EIC will address fundamental questions such as the origin of mass and nuclear spin.
Before then, a tenfold increase in PbPb statistics in LHC Runs 3 and 4 will allow a transformative leap in low Bjorken-x physics. Though not originally designed for low Bjorken-x physics, the LHC’s unparalleled energy reach and diverse range of colliding systems offers unique opportunities to explore gluon dynamics at the highest energies.
Enhanced capabilities
Surpassing the gains from increased luminosity alone, ALICE’s new triggerless detector readout mode will offer a vast improvement over previous runs, which were constrained by dedicated triggers and bandwidth limitations. Subdetector upgrades will also play an important role. The muon forward tracker has already enhanced ALICE’s capabilities, and the high-granularity forward calorimeter set to be installed in time for Run 4 is specifically designed to improve sensitivity to small Bjorken-x physics (see “Saturation specific” figure).
Ultraperipheral-collision physics at the LHC is far more than a technical exploration of QCD. Gluons govern the structure of all visible matter. Saturation, hotspots and shadowing shed light on the origin of 99% of the mass of the visible universe.
The 14th Higgs Hunting workshop took place from 23 to 25 September 2024 at Orsay’s IJCLab and Paris’s Laboratoire Astroparticule et Cosmologie. More than 100 participants joined lively discussions to decipher the latest developments in theory and results from the ATLAS and CMS experiments.
The portrait of the Higgs boson painted by experimental data is becoming more and more precise. Many new Run 2 and first Run 3 results have developed the picture this year. Highlights included the latest di-Higgs combinations with cross-section upper limits reaching down to 2.5 times the Standard Model (SM) expectations. A few excesses seen in various analyses were also discussed. The CMS collaboration reported a brand new excess of top–antitop events near the top–antitop production threshold, with a local significance of more than 5σ above the background described by perturbative quantum chromodynamics (QCD) only, that could be due to a pseudoscalar top–antitop bound state. A new W-boson mass measurement by the CMS collaboration – a subject deeply connected to electroweak symmetry breaking – was also presented, reporting a value consistent with the SM prediction with a very accurate precision of 9.9 MeV (CERN Courier November/December 2024 p7).
Parton shower event generators were in the spotlight. Historical talks by Torbjörn Sjöstrand (Lund University) and Bryan Webber (University of Cambridge) described the evolution of the PYTHIA and HERWIG generators, the crucial role they played in the discovery of the Higgs boson, and the role they now play in the LHC’s physics programme. Differences in the modelling of the parton–shower systematics by the ATLAS and CMS collaborations led to lively discussions!
The vision talk was given by Lance Dixon (SLAC) about the reconstruction of scattering amplitudes directly from analytic properties, as a complementary approach to Lagrangians and Feynman diagrams. Oliver Bruning (CERN) conveyed the message that the HL-LHC accelerator project is well on track, and Patricia McBride (Fermilab) reached a similar conclusion regarding ATLAS and CMS’s Phase-2 upgrades, enjoining new and young people to join the effort, to ensure they are ready and commissioned for the start of Run 4.
The next Higgs Hunting workshop will be held in Orsay and Paris from 15 to 17 July 2025, following EPS-HEP in Marseille from 7 to 11 July.
Thirty years ago, physicists from Harvard University set out to build a portable antiproton trap. They tested it on electrons, transporting them 5000 km from Nebraska to Massachusetts, but it was never used to transport antimatter. Now, a spin-off project of the Baryon Antibaryon Symmetry Experiment (BASE) at CERN has tested their own antiproton trap, this time using protons. The ultimate goal is to deliver antiprotons to labs beyond CERN’s reach.
“For studying the fundamental properties of protons and antiprotons, you need to take extremely precise measurements – as precise as you can possibly make it,” explains principal investigator Christian Smorra. “This level of precision is extremely difficult to achieve in the antimatter factory, and can only be reached when the accelerator is shut down. This is why we need to relocate the measurements – so we can get rid of these problems and measure anytime.”
The team has made considerable strides to miniaturise their apparatus. BASE-STEP is far and away the most compact design for an antiproton trap yet built, measuring just 2 metres in length, 1.58 metres in height and 0.87 metres across. Weighing in at 1 tonne, transportation is nevertheless a complex operation. On 24 October, 70 protons were introduced into the trap and lifted onto a truck using two overhead cranes. The protons made a round trip through CERN’s main site before returning home to the antimatter factory. All 70 protons were safely transported and the experiment with these particles continued seemlessly, successfully demonstrating the trap’s performance.
Antimatter needs to be handled carefully, to avoid it annihilating with the walls of the trap. This is hard to achieve in the controlled environment of a laboratory, let alone on a moving truck. Just like in the BASE laboratory, BASE–STEP uses a Penning trap with two electrode stacks inside a single solenoid. The magnetic field confines charged particles radially, and the electric fields trap them axially. The first electrode stack collects antiprotons from CERN’s antimatter factory and serves as an “airlock” by protecting antiprotons from annihilation with the molecules of external gases. The second is used for long-term storage. While in transit, non-destructive image-current detection monitors the particles and makes sure they have not hit the walls of the trap.
“We originally wanted a system that you can put in the back of your car,” says Smorra. “Next, we want to try using permanent magnets instead of a superconducting solenoid. This would make the trap even smaller and save CHF 300,000. With this technology, there will be so much more potential for future experiments at CERN and beyond.”
With or without a superconducting magnet, continuous cooling is essential to prevent heat from degrading the trap’s ultra-high vacuum. Penning traps conventionally require two separate cooling systems – one for the trap and one for the superconducting magnet. BASE-STEP combines the cooling systems into one, as the Harvard team proposed in 1993. Ultimately, the transport system will have a cryocooler that is attached to a mobile power generator with a liquid-helium buffer tank present as a backup. Should the power generator be interrupted, the back-up cooling system provides a grace period of four hours to fix it and save the precious cargo of antiprotons. But such a scenario carries no safety risk given the miniscule amount of antimatter being transported. “The worst that can happen is the antiprotons annihilate, and you have to go back to the antimatter factory to refill the trap,” explains Smorra.
With the proton trial-run a success, the team are confident they will be able to use this apparatus to successfully deliver antiprotons to precision laboratories in Europe. Next summer, BASE-STEP will load up the trap with 1000 antiprotons and hit the road. Their first stop is scheduled to be Heinrich Heine University inGermany.
“We can use the same apparatus for the antiproton transport,” says Smorra. “All we need to do is switch the polarity of the electrodes.”
To provide the best experiences, we use technologies like cookies to store and/or access device information. Consenting to these technologies will allow us to process data such as browsing behavior or unique IDs on this site. Not consenting or withdrawing consent, may adversely affect certain features and functions.
Functional
Always active
The technical storage or access is strictly necessary for the legitimate purpose of enabling the use of a specific service explicitly requested by the subscriber or user, or for the sole purpose of carrying out the transmission of a communication over an electronic communications network.
Preferences
The technical storage or access is necessary for the legitimate purpose of storing preferences that are not requested by the subscriber or user.
Statistics
The technical storage or access that is used exclusively for statistical purposes.The technical storage or access that is used exclusively for anonymous statistical purposes. Without a subpoena, voluntary compliance on the part of your Internet Service Provider, or additional records from a third party, information stored or retrieved for this purpose alone cannot usually be used to identify you.
Marketing
The technical storage or access is required to create user profiles to send advertising, or to track the user on a website or across several websites for similar marketing purposes.
