Pb-Pb Collisions Research Articles

Elliptic Flow of Electrons from Beauty-Hadron Decays in Pb-Pb Collisions at sNN=5.02 TeV

The elliptic flow of electrons from beauty hadron decays at midrapidity ($|y|$ $<$ 0.8) is measured in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV with the ALICE detector at the LHC. The azimuthal distribution of the particles produced in the collisions can be parameterized with a Fourier expansion, in which the second harmonic coefficient represents the elliptic flow, $v_{\rm 2}$. The $v_{\rm 2}$ coefficient of electrons from beauty-hadron decays is measured for the first time in the transverse momentum ($p_{\rm{T}}$) range 1.3-6 GeV/$c$ in the centrality class 30-50%. The measurement of electrons from beauty-hadron decays exploits their larger mean proper decay length $c\tau \approx$ 500 $\mu$m compared to that of charm hadrons and most of the other background sources. The $v_{\rm 2}$ of electrons from beauty hadron decays at midrapidity is found to be positive with a significance of 3.75$\sigma$. The results provide insights on the degree of thermalization of beauty quarks in the medium. A model assuming full thermalization of beauty quarks is strongly disfavoured by the measurement at high $p_{\rm{T}}$, but is in agreement with the results at low $p_{\rm{T}}$. Transport models including substantial interactions of beauty quarks with an expanding strongly-interacting medium describe the measurement.

Measurement of prompt D0 and [formula omitted] meson azimuthal anisotropy and search for strong electric fields in PbPb collisions at [formula omitted

The strong Coulomb field created in ultrarelativistic heavy ion collisions is expected to produce a rapidity-dependent difference (Δv2) in the second Fourier coefficient of the azimuthal distribution (elliptic flow, v2) between D0 (u‾c) and D‾0 (uc‾) mesons. Motivated by the search for evidence of this field, the CMS detector at the LHC is used to perform the first measurement of Δv2. The rapidity-averaged value is found to be 〈Δv2〉=0.001±0.001(stat)±0.003(syst) in PbPb collisions at sNN=5.02TeV. In addition, the influence of the collision geometry is explored by measuring the D0 and D‾0mesons v2 and triangular flow coefficient (v3) as functions of rapidity, transverse momentum (pT), and event centrality (a measure of the overlap of the two Pb nuclei). A clear centrality dependence of prompt D0 meson v2 values is observed, while the v3 is largely independent of centrality. These trends are consistent with expectations of flow driven by the initial-state geometry.

Estimation of initial-state structures in high-energy heavy-ion collisions using principal component analysis

In high-energy heavy-ion collisions, structures in the initial collision zone are a matter of intense investigation, both from theory and experimental points of views. A large number of models have been developed to represent the initial state of the collision including the Glauber model and color glass condensate (CGC), among others. Another important aspect of the study is to investigate proper observables that will be sensitive to the initial collision zone. In this work, we discuss a formalism to implement the spatial clusters at the partonic level in the string melting version of the ampt model for PbPb collisions at $\sqrt{{s}_{NN}}=200\phantom{\rule{4pt}{0ex}}\mathrm{GeV}$. These clusters are then propagated through the ampt hadronization scheme. Principal component analysis (PCA) has been used on the $\ensuremath{\eta}$, $\ensuremath{\phi}$, and ${p}_{T}$ distributions of the produced charged particles and the eigenvalues have been compared before and after the implementation of the clustering. It is found that for all these three different distributions, all the prominent PCA modes show sensitivity to the clustering. A centrality dependent study has also been performed on those eigenvalues.

The effect of hadronic scatterings on the measurement of vector meson spin alignments in heavy-ion collisions * *The work of D. Y. Shen and J. H. Chen was Supported in part by the Strategic Priority Research Program of Chinese Academy of Sciences (XDB34030000), the Guangdong Major Project of Basic and Applied Basic Research (2020B0301030008) and the National Natural Science Foundation

Spin alignments of vector mesons and hyperons in relativistic heavy-ion collisions have been proposed as signals of global polarization. The STAR experiment first observed the polarization. Recently, the ALICE collaboration measured the transverse momentum ( ) and the collision centrality dependence of , and spin alignments during Pb-Pb collisions at = 2.76 TeV. A large signal is observed in the low region of mid-central collisions for , while the signal is much smaller for , and these have not been understood yet. Since vector mesons have different lifetimes and their decay products have different scattering cross sections, they suffer from different hadronic effects. In this paper, we study the effect of hadronic interactions on the spin alignment of , , and mesons in relativistic heavy-ion collisions with a multi-phase transport model. We find that hadronic scatterings lead to a deviation of the observed spin alignment matrix element away from the true value for and mesons (with a bigger effect on ) while the effect is negligible for the meson. The effect depends on the kinematic acceptance: the observed value is lower than the true value when the pseudorapidity ( ) coverage is small, while there is little effect when the coverage is large. Hence, this study provides valuable information to understand the vector meson spin alignment signals observed during the experiments.

Exclusive dilepton production at forward rapidities in PbPb collisions

The dilepton production at forward rapidities in diffractive and exclusive processes present in ultraperipheral PbPb collisions at the LHC is investigated. Predictions for the e^+ e^-, mu ^+ mu ^- and tau ^+ tau ^- cross sections are presented taking into account of realistic cuts that can be implemented by the LHCb Collaboration in a future experimental analysis. Our results indicate that the background associated with the diffractive production can be strongly suppressed and the exclusive processes can be cleanly separated. For the tau ^+ tau ^- production, the semi and purely leptonic decay channels are considered. Our results indicate that a future experimental analysis of the dilepton production at the LHCb is feasible and can be useful to search for BSM physics.

Role of nuclear gluon distribution on particle production in heavy ion collisions

The transverse momentum spectra of hadrons are calculated from the unintegrated gluon distribution (UGD) within the ${k}_{T}$-factorization framework at small $x$. Starting from $pp$ collisions, the modification caused by the nuclear medium is incorporated in the UGD at high energies, which is related to the nuclear shadowing phenomenon. Moreover, we consider that particle production from minijet decay is not enough to explain the ${p}_{T}$ spectra in $AA$ collisions due to collective phenomena that take place after the hard collision. The Boltzmann-Gibbs blast wave distribution is utilized in order to evaluate the distribution of particle production in equilibrium. Data from the ALICE Collaboration for PbPb collisions at $\sqrt{s}=2.76\text{ }\text{ }\mathrm{TeV}$ are analyzed, and the nuclear modification factor for pion production is computed.

Study of K*(892)0 and ϕ (1020) meson production in proton–proton and Pb–Pb collisions at = 2.76 TeV

In this paper, we describe a study of charged particle yield as a function of p T for K *(892)0 and ϕ (1020) mesons in proton–proton (pp) and Pb–Pb collisions at = 2.76 TeV in the central rapidity region of ∣y∣ < 0.5, in a p T range of 0 < p T < 15 GeV c −1 in pp collisions and in a p T range of 0 < p T < 20 GeV c −1 in Pb-Pb collisions. We also investigated a very important ratio, the nuclear modification factor, to study the effects of the medium in the most central region, i.e. 0%–5% centrality. For data simulation, we used the EPOS-LHC and EPOS-1.99 models. To check the validity of these models’ simulations, we compared the data obtained from these Monte Carlo simulation programs with ALICE experimental data for = 2.76 TeV. It was concluded that the models’ predictions for the ϕ-meson in pp and for the most central Pb-Pb collisions disagreed with the ALICE data, and that the difference increased with p T . This may be connected with the essential role of collective parton behaviors which could not have been taken into account by the models. For K *0 mesons, both programs gave almost the same predictions, and with p T in the interval p T > 3 GeV c −1, the predictions were very close to the experimental data. Both models gave higher predictions for the soft p T interval and lower predictions for the hard interactions. The values of the R AA distributions were lower than unity and both models were very close to the ALICE data. It is very interesting that the models were not able to describe the p T distributions, but they gave good predictions for their ratios. This may possibly be due to parton collective behaviors. We observed some additional suppression of K ∗0 at low values of p T with respect to ϕ-mesons, which may be related to the role of the masses of the particles in soft interactions. The rising trend for R AA in the region from p T = 10 GeV c −1 to 20 GeV c −1 observed by the ALICE experiment was absent for the ϕ-mesons.

Multiplicity dependence of (multi)strange baryons in the canonical ensemble with phase shift corrections

The increase in strangeness production with charged particle multiplicity, as seen by the ALICE collaboration at CERN in $p\text{\ensuremath{-}}p$, $p$-Pb, and Pb-Pb collisions, is investigated in the hadron resonance gas model taking into account interactions among hadrons using $S$-matrix corrections based on known phase shift analyses. Strangeness conservation is taken into account in the framework of the canonical strangeness ensemble. A very good description is obtained for the variation of the strangeness content in the final state as a function of the number of charged hadrons in the midrapidity region using the same fixed temperature value as obtained in the most central Pb-Pb collisions and with a fixed strangeness suppression factor ${\ensuremath{\gamma}}_{s}=1$. It is shown that the number of charged hadrons is linearly proportional to the volume of the system. For small multiplicities the canonical ensemble with local strangeness conservation restricted to midrapidity leads to a stronger suppression of (multi)strange baryons than seen in the data. This is compensated by introducing a global conservation of strangeness in the whole phase-space which is parametrized by the canonical correlation volume larger than the fireball volume at the midrapidity. The results on comparing the hadron resonance gas model with and without S-matrix corrections, are presented in detail. It is shown that the interactions introduced by the phase shift analysis via the $S$-matrix formalism are essential for a better description of the yields data.

Deciphering the Nature of X(3872) in Heavy Ion Collisions.

Exploring the nature of exotic multiquark candidates such as the X(3872) plays a pivotal role in understanding quantum chromodynamics (QCD). Despite significant efforts, consensus on their internal structures is still lacking. As a prime example, it remains a pressing open question to decipher the X(3872) state between two popular exotic configurations: a loose hadronic molecule or a compact tetraquark. We demonstrate a novel approach to help address this problem by studying the X(3872) production in heavy ion collisions, where a hot fireball with ample light as well as charm (anti-)quarks is available for producing the exotics. Adopting a multiphase transport model (AMPT) for describing such collisions and implementing appropriate production mechanism of either molecule or tetraquark picture, we compute and compare a series of observables for X(3872) in Pb-Pb collisions at the Large Hadron Collider. We find the fireball volume plays a crucial role, leading to a 2-order-of-magnitude difference in the X(3872) yield and a markedly different centrality dependence between hadronic molecules and compact tetraquarks, thus offering a unique opportunity for distinguishing the two scenarios. We also make the first prediction of X(3872) elliptic flow coefficient to be tested by future experimental measurements.

Simulation of hypertriton productions in Pb-Pb collisions at ALICE energies using PACIAE model

In this work, the parton and hadron cascade model (PACIAE) and the dynamically constrained phase space coalescence model (DCPC) are used to investigate the production of hypertriton and antihypertriton in Pb-Pb collisions at , 2.76, 5.02 and 7 TeV. The model parameters are fixed by comparing the PACIAE result with ALICE data on Λ and p productions in Pb-Pb collisions at TeV. Then, the predetermined parameters are used in PACIAE and DCPC models to simulate the hypertriton production in Pb-Pb collisions at TeV. The hypertriton and antihypertriton yields in Pb-Pb collisions at , 5.02 and 7 TeV are predicted in this work.

Hadronic Resonance Gas Model and Multiplicity Dependence in \(p\)–\(p\), \(p\)–Pb, Pb–Pb Collisions: Strangeness Enhancement

Recently the ALICE collaboration has observed an interesting systematic behaviour of ratios of identified particles to pions yields at the LHC, showing that they depend solely on the charged-particle multiplicity in pp, pPb and PbPb collisions. In particular, the yields of (multi)strange particles relative to pions, increases with multiplicity and the enhancement is mode pronounced with increasing strangeness content. We will argue, that such a pattern of arises naturally in the thermal model taking into account exact strangeness conservation. Furthermore, extending the thermal model by including hadron interactions within the S-matrix approach, the ALICE data can be well quantified by the thermal particle yields at the chiral-crossover temperature, as previously found in central Pb-Pb collisions.

Deep neural network techniques in the calibration of space-charge distortion fluctuations for the ALICE TPC

The Time Projection Chamber (TPC) of the ALICE experiment at the CERN LHC was upgraded for Run 3 and Run 4. Readout chambers based on Gas Electron Multiplier (GEM) technology and a new readout scheme allow continuous data taking at the highest interaction rates expected in Pb-Pb collisions. Due to the absence of a gating grid system, a significant amount of ions created in the multiplication region is expected to enter the TPC drift volume and distort the uniform electric field that guides the electrons to the readout pads. Analytical calculations were considered to correct for space-charge distortion fluctuations but they proved to be too slow for the calibration and reconstruction workflow in Run 3. In this paper, we discuss a novel strategy developed by the ALICE Collaboration to perform distortion-fluctuation corrections with machine learning and convolutional neural network techniques. The results of preliminary studies are shown and the prospects for further development and optimization are also discussed.

Initial state fluctuations and the sub-leading flow modes from the experimental data and HYDJET++ model

A few microseconds after the birth of the Universe, the Universe was filled with the matter consisting of quarks and gluons, called quark gluon plasma (QGP). That primordial QGP lasts for about a few $$\upmu s$$ until the Universe cooled down and expanded enough that colored quarks had to confine within the colorless new formed hadrons. In high-energy nuclear collisions, where a high baryon density, or a high temperature could be achieved, small pieces of the QGP can be recreated and studied experimentally. Such created QGP undergoes an explosion, called the Little Bang. In spite of its small size (about 1000 $$fm^{3}$$ ) and short duration (a few fm/c, where c is the speed of light), the QGP is well described by relativistic hydrodynamics, including even the small perturbations on top of the explosion. In high-energy nucleus–nucleus (AA) collisions which have been performed at the Relativistic Heavy Ion Collider and at the Large Hadron Collider, the QGP was created with extremely high temperature and the baryon density close to zero. One of observables used to study QGP is azimuthal anisotropy. It was found that the initial state fluctuations have a significant influence on azimuthal anisotropies. We present results on azimuthal anisotropies measured in ultra-central PbPb collisions at $$\sqrt{s}_{NN}$$ = 2.76 TeV by the CMS and ALICE collaborations, as well as the leading and sub-leading flow modes for the elliptic and triangular anisotropies. The measured flow modes are also compared with the predictions from the HYDJET++ model.

Analyses of multi-pion Bose-Einstein correlations for granular sources with coherent pion-emission droplets * *Supported by the National Natural Science Foundation of China (11675034, 11275037)

The ALICE Collaboration measured three- and four-pion Bose-Einstein correlations (BECs) for Pb-Pb collisions at the Large Hadron Collider (LHC). It is speculated that the observed significant suppression of multi-pion BECs is owing to a considerable degree of coherent pion emission in these collisions. Here, we study multi-pion BEC functions for granular sources with coherent pion-emission droplets. We find that the intercepts of the multi-pion correlation functions at the relative momenta near zero are sensitive to the number of droplets in the granular source. They decrease with the droplet number. The three-pion correlation functions for evolving granular sources with momentum-dependent partially coherent pion-emission droplets basically agree with the experimental data for Pb-Pb collisions at TeV at the LHC. However, the model results for the four-pion correlation function are inconsistent with the experimental data. Investigations into normalized multi-pion correlation functions of granular sources suggest an interesting enhancement of the normalized four-pion correlation function in the moderate relative-momentum region.

Bound-free pair production from nuclear collisions and the steady-state quench limit of the main dipole magnets of the CERN Large Hadron Collider

During its Run 2 (2015-2018), the Large Hadron Collider (LHC) operated at almost twice higher energy, and provided Pb-Pb collisions with an order of magnitude higher luminosity, than in the previous Run 1. In consequence, the power of the secondary beams emitted from the interaction points by the bound-free pair production (BFPP) process increased by a factor ~20, while the propensity of the bending magnets to quench increased with the higher magnetic field. This beam power is about 35 times greater than that contained in the luminosity debris from hadronic interactions and is focused on specific locations that fall naturally inside superconducting magnets. The risk of quenching these magnets has long been recognized as severe and there are operational limitations due to the dynamic heat load that must be evacuated by the cryogenic system. High-luminosity operation was nevertheless possible thanks to orbit bumps that were introduced in the dispersion suppressors around the ATLAS and CMS experiments to prevent quenches by displacing and spreading out these beam losses. Further, in 2015, the BFPP beams were manipulated to induce a controlled quench, thus providing the first direct measurement of the steady-state quench level of an LHC dipole magnet. The same experiment demonstrated the need for new collimators that are being installed around the ALICE experiment to intercept the secondary beams in the future. This paper discusses the experience with BFPP at luminosities very close to the future High Luminosity LHC (HL-LHC) target, gives results on the risk reduction by orbit bumps and presents a detailed analysis of the controlled quench experiment.

Studies of Quark and Gluon Contributions to Jets using Jet Charge Measurements in pp and PbPb Collisions

Jet charge, defined as the momentum-weighted sum of the electric charges of particles inside a jet is sensitive to the electric charge of the initiating parton and can be used to study the color charge dependence of the parton energy loss in the QGP. In this paper, the first measurements of jet charge in heavy-ion collisions are presented using lead-lead (PbPb) collision data and compared to results from proton-proton (pp) data at the same collision energy. The measurements are unfolded for detector and background effects and are studied differentially in pT and additionally as a function of collision centrality in PbPb collisions. We also present a template fitting technique for estimating the fractions of quark- and gluon-initiated jets in pp and PbPb collisions based on Monte Carlo templates. This analysis uses pp and PbPb collision data collected by the CMS experiment at sNN=5.02TeV.

Exploring Longitudinal Observables with 3+1D IP-Glasma

We present a formulation of the initial state of heavy ion collisions that generalizes the 2+1D boost invariant IP-Glasma [B. Schenke, P. Tribedy, R. Venugopalan, Fluctuating Glasma initial conditions and flow in heavy ion collisions, Phys. Rev. Lett. 108 (2012) 252301.] to 3+1D through JIMWLK rapidity evolution of the pre-collision Wilson lines. The rapidity dependence introduced by the JIMWLK evolution leads us to modify the initial condition for the gauge fields, and to solve Gauss' law iteratively in order to allow for temporal evolution on a 3-dimensional lattice.While the transverse physics of QGP has been studied nearly exhaustively, the effect of longitudinal fluctuations introduced by the JIMWLK evolution has yet to be studied in detail phenomenologically. Hence, we couple our 3+1D IP-Glasma model to MUSIC+UrQMD, for completely 3+1D simulations of heavy ion collisions. Specifically, we consider Pb-Pb collisions at s=2.76TeV and study the rapidity dependence of the charged hadron νn(η) via the η-dependent flow factorization ratios rn(ηa,ηb) as measured by CMS [V. Khachatryan, et al., Evidence for transverse momentum and pseudorapidity dependent event plane fluctuations in PbPb and pPb collisions, Phys. Rev. C 92 (3) (2015) 034911.], as well as the charged hadron multiplicity dNch/dη.

Nuclear modification factor of isolated prompt photons in pp and PbPb collisions at [formula omitted] with the CMS detector

High-transverse-momentum prompt photons, as colorless objects, do not interact strongly with the medium and provide a direct way to test perturbative QCD and the nuclear parton distribution functions (nPDFs). The transverse energy spectra and the nuclear modification factors of isolated photons are measured in pp and PbPb collisions at sNN=5.02TeV using the CMS detector. The data are compared to jetphox next-to-leading-order calculations and found to be consistent with the prediction of the nuclear modification factor. The measurements significantly improved the accuracy compared to the previous CMS results at sNN=2.76TeV. No significant modification of isolated photon cross-sections in PbPb collisions with respect to pp collisions is observed in the pseudorapidity range |η| < 1.44 for EγT between 25 and 200 GeV at various collision centrality intervals.

Anisotropic flow fluctuations of charged and identified hadrons in Pb-Pb collisions with the ALICE detector

Anisotropic flow fluctuations can be used to constrain the initial conditions and probe the properties and evolution of the system created in heavy-ion collisions. In this proceeding, the pT-differential flow coefficient vn with 2-, 4-, 6- and 8-particle cumulants for charged hadrons are investigated, which allows to study the higher order moments, including skewness and kurtosis, of v2 probability density function (p.d.f.). In addition, the v2 fluctuations of π±, K±, p+p¯, ϕ, KS0, Λ+Λ¯ and Ξ−+Ξ¯+ are also measured at mid rapidity in Pb-Pb collisions at sNN=5.02TeV cumulants. The implications of the results for understanding the properties of the medium are discussed.

Evidence for X(3872) in PbPb collisions and studies of its prompt production at [formula omitted

The first evidence for X(3872) in heavy ion collisions is reported. Prompt X(3872) production is studied with the CMS detector in PbPb collisions at a center-of-mass energy of sNN=5.02TeV per nucleon pair, using the decay chain X(3872)→ J/ψπ+π− → μ+μ−π+π−. The measurement is performed in a kinematic range in which the X(3872) candidates have a transverse momentum 15 <pT < 50GeV/c and rapidity |y| < 1.6. The statistical significance of the X(3872) signal is in excess of 3 standard deviations. The production and survival of the prompt X(3872) in the quark-gluon plasma is expected to depend upon its internal structure. The ratio of the X(3872) and ψ(2S) yields is found to be R = 1.10 ± 0.51 (stat.) ± 0.53 (syst.) and is compared to the ratio measured in pp collisions at s=7and8TeV.