Statistical Hadronization Model Research Articles

Nonuniversality of heavy quark hadronization in elementary high-energy collisions

It has been traditionally hypothesized that the heavy quark (charm c and bottom b) fragmentation is universal across different collision systems, based on the notion that hadronization as a soft process should occur at the characteristic nonperturbative quantum chromodynamics (QCD) scale, ΛQCD. However, this universality hypothesis has recently been challenged by the observation that the c- and b-baryon production relative to their meson counterparts in minimum bias proton-proton (pp) collisions at the CERN Large Hadron Collider (LHC) energies is significantly enhanced as compared to the electron-positron (e+e−) collisions. The conception of nonuniversality is unambiguously reinforced by the latest measurement of the charged-particle multiplicity dependence of the b-baryon–to–meson yield ratio, Λb/B, by the LHCb experiment in s=13TeVpp collisions at the LHC, evolving continuously from the saturation value in minimum bias pp collisions toward the small value in e+e− collisions as the system size gradually reduces. We address the multiplicity dependence of b-baryon production in the canonical statistical hadronization model with input b-hadron spectrum augmented with many hitherto unobserved states from quark model predictions. We demonstrate that the decreasing trend of the Λb/B toward low multiplicities can be quantitatively understood from the canonical suppression on the yield of Λb, as caused by the requirement of strict conservation of baryon number in sufficiently small systems. We have therefore proposed a plausible scenario for understanding the origin of the nonuniversality of heavy quark fragmentation in elementary collisions. Published by the American Physical Society 2024

Unveiling the dynamics of little-bang nucleosynthesis

High-energy nuclear collisions provide a unique site for the synthesis of both nuclei and antinuclei at temperatures of kT ≈ 100 − 150 MeV. In these little bangs of transient collisions, a quark-gluon plasma (QGP) of nearly vanishing viscosity is created, which is believed to have existed in the early universe within the first few microseconds after the Big Bang. Analyses of identified particles produced in these little bangs based on the statistical hadronization model for the QGP have suggested that light (anti)nuclei are produced from the QGP as other hadrons and their abundances are little affected by later hadronic dynamics. Here, we find a strong reduction of the triton yield by about a factor of 1.8 in high-energy heavy-ion collisions based on a kinetic approach that includes the effects of hadronic re-scatterings, particularly that due to pion-catalyzed multi-body reactions. This finding is supported by the latest experimental measurements and thus unveils the important role of hadronic dynamics in the little-bang nucleosynthesis.

Measurements of (anti)(hyper)nuclei with ALICE

The investigation of the production mechanism of light (anti)(hyper)nuclei in ultrarelativistic hadronic collisions is one of the main topics in modern nuclear physics. The ALICE Collaboration has significantly contributed to this specific field of research with systematic measurements of the production of (anti)(hyper)nuclei in different collision systems and centre-ofmass energies provided by the Large Hadron Collider. Measurements of the properties of hypernuclei, such as their lifetimes and binding energies, provide information on the hadronic interaction between hyperons and nucleons, which is complementary to that obtained from correlation measurements. In this contribution, recent results on the production of (anti)(hyper)nuclei measured with ALICE during the LHC Run 2 in different collision systems will be presented. These results will be discussed within the context of the statistical hadronization model and baryon coalescence. For the first time, the observations of the (anti)hyperhydrogen-4 and (anti)hyperhelium-4 in Pb–Pb collisions at 5.02 TeV will be shown. Moreover, new results on (anti)(hyper)nuclei measurements obtained using the LHC Run 3 data will be presented.

An augmented QCD phase portrait: Mapping quark-hadron deconfinement for hot, dense, rotating matter under magnetic field

The quark-hadron transition that happens in ultra-relativistic heavy-ion collisions is expected to be influenced by the effects of rotation and magnetic field, both present due to the geometry of a generic non-head-on impact. We augment the conventional T–μB planar phase diagram for QCD matter by extending it to a multi-dimensional domain spanned by temperature T, baryon chemical potential μB, external magnetic field B and angular velocity ω. Using two independent approaches, one from a rapid rise in entropy density and another dealing with a dip in the squared speed of sound, we identify deconfinement in the framework of a modified statistical hadronization model. We find that the deconfinement temperature TC(μB,ω,eB) decreases nearly monotonically with increasing μB,ω and eB with the most prominent drop (by nearly 40 to 50 MeV) in TC occurring when all the three quasi-control (via collision energy and centrality) parameters are simultaneously tuned to finite values that are typically achievable in present and upcoming heavy-ion colliders.

F0(980) production in inelastic pp collisions at [formula omitted] = 5.02 TeV

The measurement of the production of f0(980) in inelastic pp collisions at s=5.02 TeV is presented. This is the first reported measurement of inclusive f0(980) yield at LHC energies. The production is measured at midrapidity, |y|<0.5, in a wide transverse momentum range, 0<pT<16 GeV/c, by reconstructing the resonance in the f0(980)→π+π− hadronic decay channel using the ALICE detector. The pT-differential yields are compared to those of pions, protons and ϕ mesons as well as to predictions from the HERWIG 7.2 QCD-inspired Monte Carlo event generator and calculations from a coalescence model that uses the AMPT model as an input. The ratio of the pT-integrated yield of f0(980) relative to pions is compared to measurements in e+e− and pp collisions at lower energies and predictions from statistical hadronisation models and HERWIG 7.2. A mild collision energy dependence of the f0(980) to pion production is observed in pp collisions from SPS to LHC energies. All considered models underpredict the pT-integrated 2f0(980)/(π+ +π−) ratio. The prediction from the canonical statistical hadronisation model assuming a zero total strangeness content of f0(980) is consistent with the data within 1.9σ and is the closest to the data. The results provide an essential reference for future measurements of the particle yield and nuclear modification in p–Pb and Pb–Pb collisions, which have been proposed to be instrumental to probe the elusive nature and quark composition of the f0(980) scalar meson.

Measurement of the production of (anti)nuclei in p–Pb collisions at [formula omitted

Measurements of (anti)proton, (anti)deuteron, and (anti)3He production in the rapidity range −1<y<0 as a function of the transverse momentum and event multiplicity in p–Pb collisions at a center-of-mass energy per nucleon–nucleon pair sNN=8.16TeV are presented. The coalescence parameters B2 and B3, measured as a function of the transverse momentum per nucleon and of the mean charged-particle multiplicity density, confirm a smooth evolution from low to high multiplicity across different collision systems and energies. The ratios between (anti)deuteron and (anti)3He yields and those of (anti)protons are also reported as a function of the mean charged-particle multiplicity density. A comparison with the predictions of the statistical hadronization and coalescence models for different collision systems and center-of-mass energies favors the coalescence description for the deuteron-to-proton yield ratio with respect to the canonical statistical model.

Results on light (anti)hypernuclei production with ALICE at the LHC

The high collision energies reached at the LHC lead to significant production yields of light (anti)hypernuclei in proton–proton (pp), proton–lead (p–Pb) and, in particular, Pb–Pb collisions. The lightest known hypernucleus is the hypertriton, which is a bound state of a proton, a neutron, and a Λ hyperon. It decays weakly with a decay length of a few centimeters. The excellent tracking and particle identification capabilities of the ALICE detector, exploiting the energy loss measurement of the Time Projection Chamber (TPC) and using the Inner Tracking System (ITS) to distinguish between primary and secondary (decay) vertices, allow for the determination of the hypertriton yield across different collision systems, its lifetime, and its binding energy. The latest hypertriton lifetime measurement in Pb–Pb collisions performed in the 2-body decay channel will be presented. This measurement contributes to the solution of the hypertriton lifetime puzzle. In addition, the hypertriton production in different collision systems and at different energies will be compared to model predictions. Due to its low binding energy, and hence to its large size, the hypertriton is the ideal candidate to distinguish between statistical hadronization and coalescence models. With the precision of the presented yield measurements some variants of the aforementioned models can be excluded.

Measurement of the {Lambda}_c^{+} to D0 production ratio in peripheral PbPb collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV

We report on a measurement of the {Lambda}_c^{+} to D0 production ratio in peripheral PbPb collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV with the LHCb detector in the forward rapidity region 2 < y < 4.5. The {Lambda}_c^{+} (D0) hadrons are reconstructed via the decay channel {Lambda}_c^{+} → pK−π+ (D0→ K−π+) for 2 < pT< 8 GeV/c and in the centrality range of about 65–90%. The results show no significant dependence on pT, y or the mean number of participating nucleons. They are also consistent with similar measurements obtained by the LHCb collaboration in pPb and Pbp collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV. The data agree well with predictions from PYTHIA in pp collisions at sqrt{s} = 5 TeV but are in tension with predictions of the Statistical Hadronization model.

Light (anti)nuclei production in Pb-Pb collisions at sNN=5.02 TeV

The measurement of the production of deuterons, tritons and He3 and their antiparticles in Pb-Pb collisions at sNN=5.02TeV is presented in this article. The measurements are carried out at midrapidity (|y|< 0.5) as a function of collision centrality using the ALICE detector. The pT-integrated yields, the coalescence parameters and the ratios to protons and antiprotons are reported and compared with nucleosynthesis models. The comparison of these results in different collision systems at different center-of-mass collision energies reveals a suppression of nucleus production in small systems. In the Statistical Hadronisation Model framework, this can be explained by a small correlation volume where the baryon number is conserved, as already shown in previous fluctuation analyses. However, a different size of the correlation volume is required to describe the proton yields in the same data sets. The coalescence model can describe this suppression by the fact that the wave functions of the nuclei are large and the fireball size starts to become comparable and even much smaller than the actual nucleus at low multiplicities.Received 10 February 2023Accepted 16 May 2023DOI:https://doi.org/10.1103/PhysRevC.107.064904Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.©2023 CERN, for the ALICE CollaborationPhysics Subject Headings (PhySH)Research AreasParticle & resonance productionParticle correlations & fluctuationsRelativistic heavy-ion collisionsPropertiesA ≤ 5Accelerators & BeamsNuclear Physics

Sigma (1385)^{pm } resonance production in Pb–Pb collisions at sqrt{s_{textrm{NN}}}~=~5.02 TeV

Hadronic resonances are used to probe the hadron gas produced in the late stage of heavy-ion collisions since they decay on the same timescale, of the order of 1–10 fm/c, as the decoupling time of the system. In the hadron gas, (pseudo)elastic scatterings among the products of resonances that decayed before the kinetic freeze-out and regeneration processes counteract each other, the net effect depending on the resonance lifetime, the duration of the hadronic phase, and the hadronic cross sections at play. In this context, the Sigma (1385)^{pm } particle is of particular interest as models predict that regeneration dominates over rescattering despite its relatively short lifetime of about 5.5 fm/c. The first measurement of the Sigma (1385)^{pm } resonance production at midrapidity in Pb–Pb collisions at sqrt{s_{textrm{NN}}}= 5.02 TeV with the ALICE detector is presented in this Letter. The resonances are reconstructed via their hadronic decay channel, Lambda pi , as a function of the transverse momentum (p_textrm{T}) and the collision centrality. The results are discussed in comparison with the measured yield of pions and with expectations from the statistical hadronization model as well as commonly employed event generators, including PYTHIA8/Angantyr and EPOS3 coupled to the UrQMD hadronic cascade afterburner. None of the models can describe the data. For Sigma (1385)^{pm }, a similar behaviour as textrm{K}^{*} (892)^{0} is observed in data unlike the predictions of EPOS3 with afterburner.

X(3872) and Tcc : Structures and productions in heavy ion collisions

We argue why the recently observed ${T}_{cc}$ could either be a compact multiquark configuration or a loosely bound molecular configuration composed of charmed mesons, whereas the $X(3872)$ is most likely a molecular configuration. The argument is based on different short range interactions for these tetraquark states coming from the color-color and color-spin interaction in a quark model, and the presence of a common strong $D$-wave mixing at larger distance similar to the deuteron case, which for the molecular configurations leads to large sizes. Such an analogy at large distance allows us to calculate the transverse momentum dependence of the loosely bound molecular configuration of tetraquarks produced in heavy ion collisions using the coalescence model that successfully reproduces the deuteron data using the proton spectra. The ratio of the integrated $X(3872)$ yield obtained from our method to the $\ensuremath{\psi}(2S)$ yield obtained from the statistical hadronization model method is calculated to be $0.806\ifmmode\pm\else\textpm\fi{}0.234$, which is a factor of 2.47 larger than that obtained by using statistical model predictions for both particles and in line with the data from the CMS experiment. As the previously calculated transverse momentum distribution of the ${T}_{cc}$ assuming the structure to be a compact multiquark configuration is markedly different, experimental measurements of the transverse distribution of the tetraquark states will discriminate between their two possible structures.

Interpretation of particle yields in pp interactions at √S = 8.8, 12.3 and 17.3 GeV within statistical hadronization model

The statistical hadronization model ThermalFist was applied to numerous hadron yields measured in p+p collisions at √S = 8.8, 12.3 and 17.3 GeV, including recently published yields of ϕ-mesons, measured by the NA61/SHINE Collaboration. We consistently used the energy-dependent widths of Breit-Wigner mass distributions of hadronic resonances, as this approach was generally found to provide better agreement with experimental data. The well-established experimental ϕ meson yields are consequently accounted for (although neglecting this particle was found to improve the fit quality). The canonical treatment of particles with open strangeness with the grand canonical approach for non-strange particles gave a moderately reasonable agreement with the measured yields, only when the volume of strange particles was allowed to vary freely. In all the studied cases this volume is found to be greater than the canonical one.

(Anti)nucleosynthesis in heavy-ion collisions and (anti)nuclei as "baryonmeter" of the collision

The production mechanism of light (anti)nuclei in heavy-ion collisions has been extensively studied experimentally and theoretically. Two competing (anti)nucleosynthesis models are typically used to describe light (anti)nuclei yields and their ratios to other hadrons in heavy-ion collisions: the statistical hadronization model (SHM) and the nucleon coalescence model. The possibility to distinguish these phenomenological models calls for new experimental observables. Given their large baryon number, light (anti)nuclei have a high sensitivity to the baryon chemical potential (μB) of the system created in the collision. In this talk, the first measurement of event-by-event antideuteron number fluctuations in heavy-ion collisions is presented and compared with expectations of the SHM and coalescence model. In addition, the antinuclei-to-nuclei ratios are used to obtain a measurement of μB in heavy-ion collisions with unprecedented precision.

Measurement of the production of (anti)(hyper)nuclei

In recent years, ALICE has extensively studied the production of light (anti)(hyper)nuclei in different collision systems and center-of-mass energies. Nevertheless, the production mechanisms of light (hyper)nuclei is still unclear and under intense debate in the scientific community. Two classes of models are typically used to describe nuclear production: the statistical hadronisation model and the coalescence ones. In heavy-ion collisions, both models describe well the production yields of light nuclei and their ratios to the yields of hadrons, making it difficult to distinguish between the two. On the contrary, small collision systems, such as pp and p–Pb collisions, are ideal to study the (anti)(hyper)nuclei production mechanisms, thanks to the large separation between the different predictions. In this paper, recent results on light nuclei production measured with ALICE at the LHC in small collision systems are discussed in the context of the nucleosynthesis models, allowing us to exclude some configurations of the SHM and coalescence models in order to set tighter constraints to the available theoretical predictions.

Synthesis of light nuclei in hadronic collisions

Light-nuclei production yields in heavy-ion collisions are well described in the framework of Statistical Hadronization Models (SHM) but a thorough understanding of the underlying dynamics is still missing. In a complementary approach, synthesis of light nuclei can be modeled in terms of final-state coalescence of nucleons. While yielding an equally good description in central heavy-ion collisions, coalescence predictions are substantially different to those from SHM in small collision systems, in particular for the loosely bound hypertriton. This should allow a firm distinction of the two production scenarios in small collision systems. Comprehensive data on light-nuclei and hypertriton production in pp and p–Pb collisions from the ALICE Collaboration are presented in this contribution. Complementary to the measurement of production yields, the dynamics of nuclear cluster formation can be inferred from the measurement of final-state correlations of nucleons and light nuclei. Preliminary p-d correlation results from high-multiplicity pp collisions at √ 11 s = 13 TeV are compared to calculations based on experimental scattering parameters and discussed in the context of nuclear cluster formation.

Results on light (anti)nuclei production in Pb–Pb collisions with ALICE at the LHC

The high collision energies reached at the LHC lead to significant production yields of light (anti)nuclei in proton-proton, p–Pb and Pb–Pb collisions. Light (anti)nuclei are identified using their specific energy loss (dE/dx), measured in the Time Projection Chamber, and their velocity using the Time-Of-Flight detector. The excellent tracking and particle identification capabilities of the ALICE experiment, as well as its low material budget, make this detector unique for measurements of these rarely produced particles. Results on (anti)deuteron, (anti)triton, (anti)3He and (anti) He production in Pb–Pb collisions at p sNN = 5.02 TeV, including their transverse momentum (pT) spectra, production yields and coalescence parameters BA, are presented. These results will be compared to the expectations of coalescence and statistical hadronization models to obtain information on the production mechanism of light (anti)nuclei in heavy-ion collisions. Furthermore, the first measurements of the d and 3He absorption cross section are shown.

Impact of the Coulomb field on charged-pion spectra in few-GeV heavy-ion collisions

In nuclear collisions the incident protons generate a Coulomb field which acts on produced charged particles. The impact of these interactions on charged-pion transverse-mass and rapidity spectra, as well as on pion–pion momentum correlations is investigated in Au + Au collisions at sqrt{s_text {NN}} = 2.4 GeV. We show that the low-m_t region (m_t < 0.2 GeV/c^2) can be well described with a Coulomb-modified Boltzmann distribution that also takes changes of the Coulomb field during the expansion of the fireball into account. The observed centrality dependence of the fitted mean Coulomb potential energy deviates strongly from a A_{text {part}}^{2/3} scaling, indicating that, next to the fireball, the non-interacting charged spectators have to be taken into account. For the most central collisions, the Coulomb modifications of the HBT source radii are found to be consistent with the potential extracted from the single-pion transverse-mass distributions. This finding suggests that the region of homogeneity obtained from two-pion correlations coincides with the region in which the pions freeze-out. Using the inferred mean-square radius of the charge distribution at freeze-out, we have deduced a baryon density, in fair agreement with values obtained from statistical hadronization model fits to the particle yields.

The origin of the K ∗ suppression at the LHC

When relativistic heavy ions collide, a plasma of quarks and gluons is created. It expands, cools down and eventually reaches the hadronization temperature, when a hadron gas phase is formed. In this phase, K ∗ and K can interact with light mesons and change their abundances with respect to the predictions of statistical hadronization models. In a previous article, we studied the effect of the cooling of the gas, the freeze-out temperature and the interaction cross sections on the K ∗/K yield ratio. In the present work, we investigate the effect of the different reaction mechanisms on this ratio. Our analysis showns that, out of all the possible interactions that the K ∗ and K mesons can undergo in the hadron gas, only the K ∗ decay and its inverse mechanism are indeed necessary to give a good description the observed yield in several collision systems studied by the ALICE collaboration.

Hypertriton Production in p-Pb Collisions at sqrt[s_{NN}]=5.02 TeV.

The study of nuclei and antinuclei production has proven to be a powerful tool to investigate the formation mechanism of loosely bound states in high-energy hadronic collisions. The first measurement of the production of _{Λ}^{3}H in p-Pb collisions at sqrt[s_{NN}]=5.02 TeV is presented in this Letter. Its production yield measured in the rapidity interval -1<y<0 for the 40% highest-multiplicity p-Pb collisions is dN/dy=[6.3±1.8(stat)±1.2(syst)]×10^{-7}. The measurement is compared with the expectations of statistical hadronization and coalescence models, which describe the nucleosynthesis in hadronic collisions. These two models predict very different yields of the hypertriton in charged particle multiplicity environments relevant to small collision systems such as p-Pb, and therefore the measurement of dN/dy is crucial to distinguish between them. The precision of this measurement leads to the exclusion with a significance larger than 6.9σ of some configurations of the statistical hadronization model, thus constraining the theory behind the production of loosely bound states at hadron colliders.

Measurement of prompt [formula omitted]-meson production and azimuthal anisotropy in Pb–Pb collisions at [formula omitted

The production yield and angular anisotropy of prompt Ds+ mesons were measured as a function of transverse momentum (pT) in Pb–Pb collisions at a centre-of-mass energy per nucleon pair sNN=5.02TeV collected with the ALICE detector at the LHC. Ds+ mesons and their charge conjugates were reconstructed at midrapidity (|y|<0.5) from their hadronic decay channel Ds+→ϕπ+, with ϕ→K−K+, in the pT intervals 2<pT<50GeV/c and 2<pT<36GeV/c for the 0–10% and 30–50% centrality intervals. For pT>10GeV/c, the measured Ds+-meson nuclear modification factor RAA is consistent with the one of non-strange D mesons within uncertainties, while at lower pT a hint for a Ds+-meson RAA larger than that of non-strange D mesons is seen. The enhanced production of Ds+ relative to non-strange D mesons is also studied by comparing the pT-dependent Ds+/D0 production yield ratios in Pb–Pb and in pp collisions. The ratio measured in Pb–Pb collisions is found to be on average higher than that in pp collisions in the interval 2<pT<8GeV/c with a significance of 2.3σ and 2.4σ for the 0–10% and 30–50% centrality intervals. The azimuthal anisotropy coefficient v2 of prompt Ds+ mesons was measured in Pb–Pb collisions in the 30–50% centrality interval and is found to be compatible with that of non-strange D mesons. The main features of the measured RAA, Ds+/D0 ratio, and v2 as a function of pT are described by theoretical calculations of charm-quark transport in a hydrodynamically expanding quark–gluon plasma including hadronisation via charm-quark recombination with light quarks from the medium. The pT-integrated production yield of Ds+ mesons is compatible with the prediction of the statistical hadronisation model.