Subject Grouping: Physics

Pentaquarks, bound states of five quarks predicted in the first formulation of the quark model in 1964, have had a troubled history. Following disputed claims of the discovery of light-flavour species over 20 years ago, pentaquarks with hidden charm are now well-established members of the hadronic spectrum. The breakthrough was achieved by the LHCb experiment in 2015 with the observation of P_c⁺ states in the J/ψ p system.

The P_c⁺ quark content (uudcc) implies that decays to two open-charm hadrons, such as Λ_c⁺ D⁰ or Λ_c⁺ D*⁰, are possible. The rates of such decays are important for understanding more about the nature of the P_c⁺ states, as different models predict rates that differ by orders of magnitude. Distinguishing between the proposed mechanisms by which pentaquarks, and excited hadrons in general, are produced and bound allows a better understanding of the dynamics of the strong interaction in the non-perturbative regime.

A new analysis by LHCb of the open-charm hadrons in Λ_b decays was presented at the International Conference on Meson-Nucleon Physics and the Structure of the Nucleon, held in Mainz in October. It concerns the first observation and measurement of the branching fractions of Λ_b⁰ → Λ_c⁺ D(*)⁰ K^– and Λ_b⁰ → Λ_c⁺ D_s^*– decays using proton–proton collision data collected during LHC Run 2.

All branching fractions are measured relative to the known Λ_b⁰ → Λ_c⁺ D_s^– decay mode, which is reconstructed with the same set of six final-state hadrons: p K^– π⁺ K⁺ π^– K^–. Many systematic uncertainties in the measured ratios therefore cancel out, making the precision on the relative branching fraction of Λ_b⁰ → Λ_c⁺ D⁰ K^– statistically limited. For Λ_b⁰ → Λ_c⁺ D^0* K^– and Λ_b⁰ → Λ_c⁺ D*^– the resulting branching fractions are systematically limited. This is because either a photon or neutral pion is not reconstructed, so their shape in the invariant mass spectrum of the reconstructed particles is more difficult to describe and more affected by the backgrounds (see figure 1, where the components with a missing photon for which a branching fraction is calculated are shown in orange and those with a missing neutral pion in green).

The partially reconstructed Λ_b⁰ → Λ_c⁺ D_s^*– decay cannot be used directly to search for pentaquarks, but it is an important input to model calculations. In addition, as a two-body decay, it is a powerful test of factorisation assumptions in heavy-quark effective theory.

In the Λ_b⁰ → Λ_c⁺ D(*)⁰  K^– decay, the production process of the P_c⁺ pentaquarks is the same as in the discovery channel, Λ_b⁰ → J/ψ p K^–. A comparison between the measured branching fractions and observed signal yields can thus be used to estimate the expected sensitivity for observing P_c⁺ signals in the open-charm channels. In particular, the rate of a Λ_b⁰ decay to Λ_c⁺ D⁰ K^– is about six times greater than to J/ψ p K^–; however, more than 60 times as much data would be needed to match the currently available Λ_b⁰ → J/ψ p K^– signal yield.

A factor of about 24 in this calculation comes from the branching fractions ratio of J/ψ and open-charm hadrons, given their reconstructed decay modes. The rest is from reconstruction and selection inefficiencies, which favour the four-prong μ⁺μ^– p K^– over the fully hadronic six-body final state. With the upgraded Run 3 detector and now triggerless detector readout, a large part of the inefficiency for fully hadronic final states is recoverable, making pentaquark searches in double open-charm final states more favourable compared to the situation in Run 2.

When lead ions collide head-on at the LHC they deposit most of their kinetic energy in the collision zone, forming new matter at extremely high temperatures and energy densities. The hot and dense zone quickly expands and cools down, leading to the production of approximately equal numbers of particles and antiparticles at mid-rapidity. However, in reality the balance between matter and antimatter can be slightly distorted.

The collision starts with matter only, i.e. protons and neutrons from the incoming beam. During the collision process, incoming lead nuclei interact while penetrating each other, and most of their quantum numbers are carried away by particles travelling close to the beam direction. Due to strong interactions among the quarks and gluons, quantum numbers of the colliding ions are transported to mid-rapidity rather than to the ions themselves. This leads to an imbalance of baryons originating from the initial state, which has more baryons than antibaryons.

This matter–antimatter imbalance can be quantified by determining two global system properties: the chemical potentials associated with the electric charge and baryon number (denoted μ_Q and μ_B, respectively). In a thermodynamic description, the chemical potentials determine the net electric-charge and baryon-number densities of the system. Thus, μ_B measures the imbalance between matter and antimatter, with a vanishing value indicating a perfect balance.

In a new, high-precision measurement, the ALICE collaboration reports the most precise characterisation so far of the imbalance between matter and antimatter in collisions between lead nuclei at a centre-of-mass energy per nucleon pair of 5.02 TeV. The study was carried out by measuring the antiparticle-to-particle yield ratios of light-flavour hadrons, which make up the bulk of particles produced in heavy-ion collisions. The measurement using the ALICE central barrel detectors included identified charged pions, protons and multi- strange Ω^– baryons, in addition to light nuclei, ³He, triton and the hypertriton (a bound state of a proton, a neutron and a Λ-baryon). The larger baryon content of these light nuclei makes them more sensitive to baryon-asymmetry effects.

The medium created in lead–lead collisions at the LHC is nearly electrically neutral and baryon-number-free at mid-rapidity

The analysis reveals that in head-on lead–ion collisions, for every 1000 produced protons, approximately 986 ± 6 antiprotons are produced. The chemical potentials extracted from the experimental data are μ_Q = -0.18 ± 0.90 MeV and μ_B = 0.71 ± 0.45 MeV. These values are compatible with zero, showing that the medium created in lead–lead collisions at the LHC is nearly electrically neutral and baryon-number-free at mid-rapidity. This observation holds for the full centrality range, from collisions where the incoming ions peripherally interact with each other up to the most violent head-on processes, indicating that quantum-number transport at the LHC is independent of the size of the system formed.

The values of μ_B are shown in figure 1 as a function of the centre-of-mass energy of the colliding nuclei, along with lower-energy measurements at other facilities. The recent ALICE result is indicated by the red solid circle, along with a phenom­enological parametrisation of μ_B. The decreasing trend of μ_B observed as a function of increasing collision energy indicates that different net-baryon-number density conditions can be explored by varying the beam energy, reaching almost vanishing net-baryon content at the LHC. The inset gives the μ_B values extracted at two LHC energies. It shows that the new ALICE result is almost one order of magnitude more precise than the previous estimate (violet), thanks to a more refined study of systematic uncertainties.

The present study with improved precision characterises the vanishing baryon-asymmetry at the LHC, posing stringent limits to models describing baryon-number transport effects. Using the data samples collected in LHC Run 3, these studies will be extended to the strangeness sectors, enabling a full characterisation of quantum-number transport at the LHC.