Subject Grouping: Physics

At the LHC, light nuclei and antinuclei are produced both in proton–proton and in heavy-ion collisions. Unstable nuclei, called hypernuclei, are also produced. First observed in cosmic rays in 1953, hypernuclei are formed by a mix of protons, neutrons and hyperons containing one or more strange quarks and undergo weak decays. Almost 70 years since their discovery, hypernuclei are still a source of fascination for nuclear physicists since their production is very rare and the measurement of their properties is extremely challenging.

The only hypernucleus observed so far at the LHC is the hypertriton (³_ΛH), composed of a Lambda baryon (Λ), a proton and a neutron. While, traditionally, hypernuclei are studied in low-energy nuclear experiments, the hundreds of hypertritons and antihypertritons produced in each lead–lead run at the LHC provide one of the largest data samples for their study. The hypertritons fly for a few centimetres in the experimental apparatus before decaying into a ³He nucleus and a charged pion, which are then identified by the detectors.

The ALICE collaboration recently completed a new analysis of the largest Run 2 data sample, achieving the most precise measurements to date of the hypertriton lifetime and its Λ-separation energy (the energy required to separate the Λ from the rest of the hypertriton). The lifetime, measured from the distribution of reconstructed two-body decay lengths, was found to be 253 ± 11 (stat.) ± 6 (syst.) ps, while the separation energy, obtained from the hypertriton invariant-mass distribution, was measured to be 72 ± 63 (stat.) ± 36 (syst.) keV.

These two quantities are fundamental to understand the structure of this hypernucleus and therefore the nature of the strong interaction. While the strong force binding neutrons and protons inside atomic nuclei is well understood, the characteristics of the strong force binding nucleons and hyperons are not precisely known.

The study of this interaction is not only interesting per se, but it is also an input for modelling of the dense core of neutron stars. Indeed, the creation of hyperons is energetically favoured compared to ordinary nucleonic matter in the inner core of neutron stars. Therefore, detailed knowledge of the interactions between nucleons and hyperons is required to understand these compact astrophysical objects.

The new ALICE measurements indicate that the interaction between the hyperon inside the hypertriton and the other two nucleons is extremely feeble (see figure 1). This is also confirmed by the lifetime of the hypertriton, which is compatible with the free Λ-baryon lifetime. Finally, since at the LHC matter and antimatter are produced in the same amount, the ALICE collaboration could compare the lifetimes of the antihypertriton and the hypertriton. Within the experimental uncertainty, the lifetimes were found to be compatible, as expected from CPT invariance.

During LHC Run 3, ALICE will extend its studies to heavier hypernuclei, putting tighter constraints on the interaction models among hyperons and nucleons. 

Understanding hadronic final states is key to a successful physics programme at the LHC. The quarks and gluons flying out from proton–proton collisions instantly hadronise into sprays of particles called jets. Each jet has a unique composition that makes their flavour identification and energy calibration challenging. While the performance of jet-classification schemes has been increased by the fast-paced evolution of machine-learning algorithms, another, more subtle, revolution is ongoing in terms of precision jet-energy corrections.

CMS physicists have taken advantage of the data collected during LHC Run 2 to observe jets in many different final states and systematically understand their differences in detail. The main differences originate from the varying fractions of gluons making up the jets and the different amounts of final-state radiation (FSR) in the events, causing an imbalance between the leading jet and its companions. The gluon uncertainty was constrained by splitting the Z+jet sample by flavour, using a combination of quark–gluon likelihood and b/c-quark tagging, while FSR was constrained by combining the missing-E_T projection fraction (MPF) and direct balance (DB) methods. The MPF and DB methods have been well established at the LHC since Run 1: while in the DB method the jet response is evaluated by comparing the reconstructed jet momentum directly to the momentum of the reference object, the MPF method considers the response of the whole hadronic activity in the event, recoiling versus the reference object. Figure 1 shows the agreement achieved with the Run 2 data after carefully accounting for these biases for samples with different jet-flavour compositions.

Precise jet-energy corrections are critical for some of the recent high-profile measurements by CMS, such as an intriguing double dijet excess at high mass, a recent exceptionally accurate top-quark mass measurement, and the most precise extraction of the strong coupling constant at hadron colliders using inclusive jets.

The expected increase of pileup in Run 3 and at the High-Luminosity LHC will pose additional challenges in the derivation of precise jet-energy corrections, but CMS physicists are well prepared: CMS will adopt the next-generation particle-flow algorithm (PUPPI, for PileUp Per Particle Id) as the default reconstruction algorithm to tackle pileup effects within jets at the single-particle level.

Jets can be used to address some of the most intriguing puzzles of the Standard Model (SM), in particular: is the SM vacuum metastable, or do some new particles and fields stabilise it? The top-quark mass and strong-coupling-constant measurements address the former question via their interplay with the Higgs-boson mass, while dijet-resonance searches tackle the latter. 

Underlying these studies are the jet-energy corrections and the awareness that each jet flavour is unique. 

Photon-induced reactions are regularly studied in ultra-peripheral nucleus–nucleus collisions (UPCs) at the LHC. In these collisions, the accelerated ions, which carry a strong electromagnetic field, pass by each other with an impact parameter (the distance between their centres) larger than the sum of their nuclear radii. Hadronic interactions between nuclei are therefore strongly suppressed. At LHC energies, the photo­production of charmonium (a bound state of charm and anti-charm quarks) in UPCs is sensitive to the gluon distributions in nuclei over a wide low Bjorken-x range. In particular, in coherent interactions, the photon emitted by one of the nuclei couples to the other nucleus as a whole, leaving it intact, while a J/ψ meson is emitted with a characteristic low transverse momentum (p_T) of about 60 MeV, which is roughly of the order of the inverse of the nuclear radius.

Surprisingly, in 2016 ALICE measured an unexpectedly large yield of J/ψ mesons at very low p_T in peripheral, not ultra-peripheral, PbPb collisions at a centre-of-mass energy of 2.76 TeV. The excess with respect to expectations from hadronic J/ψ-meson production was interpreted as the first indication of coherent photoproduction of J/ψ mesons in PbPb collisions with nuclear overlap. This effect comes with many theoretical challenges. For instance, how can the coherence condition survive in the photon–nucleus interaction if the latter is broken up during the hadronic collision? Do only the non-interacting spectator nucleons participate in the coherent process? Can the photoproduced J/ψ meson be affected by interactions with the formed and fast-expanding quark–gluon plasma (QGP) created in nucleus–nucleus collisions? Recent theoretical developments on the subject are based on calculations for UPCs in which the J/ψ meson photoproduction-cross section is computed as the product of an effective photon flux and an effective photonuclear cross section for the process γPb → J/ψPb, with both terms usually modified to account for the nuclear overlap.

The ALICE experiment has recently measured the coherently photoproduced J/ψ mesons in PbPb collisions at a centre-of-mass energy of 5.02 TeV, using the full Run 2 data sample. The measurement is performed at forward rapidity (2.5 < y < 4) in the dimuon decay channel. For the first time, a significant (> 5σ) coherently photoproduced J/ψ-meson signal is observed even in semi-central PbPb collisions. In figure 1, the coherently photoproduced J/ψ cross section is shown as a function of the mean number of nucleons participating in the hadronic interaction (<N_part>). In this representation, the most central head-on PbPb collisions correspond to large <N_part> values close to 400. The photoproduced J/ψ cross section does not exhibit a strong dependence on collision centrality (i.e. on the amount of nuclear overlap) within the current experimental precision. A UPC-like model (the red line in figure 1) reproduces the semi-central to central PbPb data if a modified photon flux and photonuclear cross section to account for the nuclear overlap are included.

To clarify the theory behind this experimental observation of coherent J/ψ photoproduction, the upcoming Run 3 data will be crucial in several aspects. ALICE expects to collect a much larger data sample, thereby measuring a statistically significant signal in most central collisions. At midrapidity, the larger data sample and the excellent momentum resolution of the detector will allow for p_T-differential cross-section measurements, which will shed light on the role of spectator nucleons for the coherence condition. By extending the coherently photo-produced J/ψ cross-section measurement towards most central PbPb collisions, ALICE will study the possible interaction of these charmonia with the QGP. Photoproduced J/ψ mesons could therefore turn out to be a completely new probe of the charmonium dissociation in the QGP.

The top quark – the heaviest known elementary particle – differs from the other quarks by its much larger mass and a lifetime that is shorter than the time needed to form hadronic bound states. Within the Standard Model (SM), the top quark decays almost exclusively into a W boson and a b quark, and the dominant production mechanism in proton–proton (pp) collisions is top-quark pair (tt) production.

Measurements of tt production at various pp centre-of-mass energies at the LHC probe different values of Bjorken-x, the fraction of the proton’s longitudinal momentum carried by the parton participating in the initial interaction. In particular, the fraction of tt events produced through quark–antiquark annihilation increases from 11% at 13 TeV to 25% at 5.02 TeV. A measurement of the tt production cross-section thus places additional constraints on the proton’s parton distribution functions (PDFs), which describe the probabilities of finding quarks and gluons at particular x values.

In November 2017, the ATLAS experiment recorded a week of pp-collision data at a centre-of-mass energy of 5.02 TeV. Although the main motivation of this 5.02 TeV dataset is to provide a proton reference sample for the ATLAS heavy-ion physics programme, it also provides a unique opportunity to study top-quark production at a previously unexplored energy in ATLAS. The majority of the data was recorded with a mean number of two inelastic pp collisions per bunch crossing compared to roughly 35 collisions during the 13 TeV runs. Due to much lower pileup conditions, the ATLAS calorimeter cluster noise thresholds were adjusted accordingly, and a dedicated jet-energy scale calibration was performed.

Now, the ATLAS collaboration has released its measurement of the tt production cross-section at 5.02 TeV in two final states. Events in the dilepton channel were selected by requiring opposite-charge pairs of leptons, resulting in a small, high-purity sample. Events in the single-lepton final states were separated into subsamples with different signal-to-background ratios, and a multivariate technique was used to further separate signal from background events. The two measurements were combined, taking the correlated systematic uncertainties into account.

The measured cross section in the dilepton channel (65.7 ± 4.9 pb) corresponds to a relative uncertainty of 7.5%, of which 6.8% is statistical. The single-lepton measurement (68.2 ± 3.1 pb), on the other hand, has a 4.5% uncertainty that is primarily systematic. This measurement is slightly more precise than the single-lepton measurement at 13 TeV, despite the much smaller (almost a factor of 500!) integrated luminosity. The combination of the two measurements gives 67.5 ± 2.6 pb, corresponding to an uncertainty of just 3.9%.

The new ATLAS result is consistent with the SM prediction and with a measurement by the CMS collaboration, though with a total uncertainty reduced by almost a factor of two. It thus improves our understanding of the top-quark production at different centre-of-mass energies and allows an important test of the compatibility with predictions from different PDF sets (see figure 1). The result also provides a new measurement of high-x proton structure and shows a 5% reduction in the gluon PDF uncertainty in the region around x = 0.1, which is relevant for Higgs-boson production. Moreover, the measurement paves the way for the study of top-quark production in collisions involving heavy ions.