Charged Particle Multiplicity Research Articles

Unveiling the effects of multiple soft partonic interactions in pp collisions at s=13.6 TeV using a new event classifier

Event classifiers based either on the charged-particle multiplicity or the event shape have been extensively used in proton-proton (pp) collisions by the ALICE collaboration at the LHC. The use of these tools became very instrumental since the observation of fluid-like behavior in high-multiplicity pp collisions. In particular, the study as a function of the charged-particle multiplicity registered in the forward V0 ALICE detector allowed for the discovery of strangeness enhancement in high-multiplicity pp collisions. However, one drawback of the multiplicity-based event classifiers is that requiring a high charged-particle multiplicity biases the sample towards hard processes like multi-jet final states. These biases make it difficult to perform jet-quenching searches in high-multiplicity pp collisions. In this context, the present paper explores the use of the new event classifier, flattenicity; which uses the multiplicity calculated in the forward pseudorapidity region. To illustrate how this tool works, pp collisions at $\sqrt{s}=13.6$ TeV simulated with PYTHIA~8 are explored. The sensitivity of flattencity to multi-partonic interactions as well as to the ``hardness'' of the collision are discussed. PYTHIA 8 predictions for the transverse momentum spectra of light- and heavy-flavored hadrons as a function of flattenicity are presented.

Jet Dependence of Multiplicity of Charged Particles Produced in pp, pPb and dAu Collisions at high Energy

Jet Dependence of Multiplicity of Charged Particles Produced in pp, pPb and dAu Collisions at high Energy

Study of Isothermal Compressibility and Speed of Sound in the Hadronic Matter Formed in Heavy-Ion Collision Using Unified Formalism

The thermodynamical quantities and response functions are useful to describe the particle production in heavy-ion collisions as they reveal crucial information about the produced system. While the study of isothermal compressibility provides an inference about the viscosity of the medium, speed of sound helps in understanding the equation of state. With an aim towards understanding the system produced in the heavy-ion collision, we have made an attempt to study isothermal compressibility and speed of sound as function of charged particle multiplicity in heavy-ion collisions at sNN = 2.76 TeV, 5.02 TeV, and 5.44 TeV using unified formalism.

Two-particle angular correlations in e+e− collisions to hadronic final states in two reference coordinates at Belle

We present the analysis of two-particle angular correlations using coordinate systems defined with the conventional beam axis and the event thrust axis. We propose the latter as a good representation for the correlation structure interpretation in the e+e− collision system. The e+e− collisions to hadronic final states at center-of-mass energies of sqrt{s} = 10.52 GeV and 10.58 GeV are recorded by the Belle detector at KEKB. In this paper, results on the first dataset are supplementary to the previous Belle publication [1]. At the same time, the latter is the first two-particle correlation measurement at collision energy on the Υ(4S) resonance and is sensitive to its decay products. Measurements are reported as a function of the charged-particle multiplicity. Finally, a qualitative understanding of the correlation structure is discussed using a combination of Monte Carlo simulations and experimental data.

Experimental verification of two types of gluon jets in QCD

The Abelian decomposition of QCD tells that there are two types of gluons, the color neutral neurons and colored chromons. We propose to confirm the Abelian decomposition testing the existence of two types of gluon jets experimentally. We predict that one quarter of the gluon jet is made of the neurons which has the color factor $3/4$ and the sharpest jet radius and smallest charged particle multiplicity, while the three quarters of the gluon jet are made of the chromons with the color factor $9/4$, which have the broadest jet radius (broader than the quark jet). Moreover, we argue that the neuron jet has a distinct color flow which forms an ideal color dipole, while the quark and chromon jets have distorted dipole pattern. To test the plausibility of this proposal, we suggest to analyze the gluon distribution against the jet shape (the sphericity) and/or particle multiplicity from the existing gluon jet events and look for two distinct peaks in the distribution.

Particle Production as a Function of Underlying-event Activity and Search for Jet-like Modifications in \(pp\), \(p\)–Pb, and Pb–Pb Collisions at \(\sqrt {s_{NN}}=5.02\)\thinspace TeV with ALICE

The similarity between small- and large-collision systems is explored using the charged-particle multiplicity in the transverse region, $N^{\rm T}_{\rm ch}$, which is sensitive to the underlying event. Measurements of charged-particle production as a function of $N^{\rm T}_{\rm ch}$ in pp, p-Pb and Pb-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV in the toward, away and transverse regions are discussed. These three regions are defined relative to the track with the largest transverse momentum in the event ($p_{\rm T}^{\rm trig}$). The activity in the transverse region is subtracted from the activity in the toward and the away regions to search for jet-like modifications in small-collision systems. The jet-like signals are studied both as a function of $N^{\rm T}_{\rm ch}$ and $p_{\rm T}^{\rm trig}$. Results are compared with two general purpose Monte Carlo event generators: PYTHIA~8 and EPOS~LHC.

Quarkonium production and elliptic flow in small systems measured with ALICE

Abstract. Quarkonium production in hadronic collisions provides a unique testing ground for understanding quantum chromodynamics (QCD) since it involves both perturbative and non-perturbative regimes of this theory. A variety of experimental observables, such as the production cross-section, polarization, particle correlations, serve as insights into the phenomenology of the quarkonium production and help constrain the theoretical models. Measurements of quarkonia as a function of multiplicity probe multiple parton interactions (MPI), i.e., several parton-parton interactions occurring in a single hadron-hadron collision. Finally, measurements of the azimuthal correlation structure of emitted particles in high multiplicity pp collisions can probe the existence of collective behaviour in small systems. The ALICE detector can reconstruct inclusive quarkonia over a broad kinematical range, spanning from mid-rapidity up to forward rapidity, and down to zero transverse momentum. In addition, at midrapidity, the non-prompt charmonium contribution can be separated from the prompt contribution. In these proceedings, we present new results on the inclusive, prompt and non-prompt J/ψ production cross sections measured by ALICE in pp collisions at different collision energies. The self-normalized ψ(2S)-to-J/ψ yield ratio measurement is investigated as a function of the charged particle multiplicity in pp collisions at √S = 13. TeV. Finally, the first measurement of the J/ψ elliptic flow (v2) in high multiplicity pp collisions at √S = 13. TeV is presented. Results will be compared to theoretical models.

Analysis of the Charged-particle Multiplicity Density in Centrality from Track Lets

This paper shows the relationship between charged hadron diversity and centrality in hadron Pb-Pb collisions at a certain collision energy(2.76 TeV) per nucleon pair. It is found that the centrality from 70–80% to 0–5%, each normalized charged particle density of participating nucleon pair increases by about twice. The collision of particles in the experiment was simulated by ALICE, a detector dedicating heavy-ion physics at the Large Hadron Collider. This work compares the data sample with models based on different mechanisms for particle production in nuclear collisions. And the error analysis of data in the end of this passage ensures the correctness of the experiment.

Centrality dependence of proton and light nuclei yields as a consequence of baryon annihilation in the hadronic phase

The centrality dependence of the p/π ratio measured by the ALICE Collaboration in 5.02 TeV Pb-Pb collisions indicates a statistically significant suppression with the increase of the charged particle multiplicity once the centrality-correlated part of the systematic uncertainty is eliminated from the data. We argue that this behavior can be attributed to baryon annihilation in the hadronic phase. By implementing the BB¯↔5π reaction within a generalized partial chemical equilibrium framework, we estimate the annihilation freeze-out temperature at different centralities, which decreases with increasing charged particle multiplicity and yields Tann=132±5 MeV in 0-5% most central collisions. This value is considerably below the hadronization temperature of Thad∼160 MeV but above the thermal (kinetic) freeze-out temperature of Tkin∼100 MeV. Baryon annihilation reactions thus remain relevant in the initial stage of the hadronic phase but freeze out before (pseudo-)elastic hadronic scatterings. One experimentally testable consequence of this picture is a suppression of various light nuclei to proton ratios in central collisions of heavy ions.

Monte Carlo simulations of Upsilon meson production in p + p at = 500 GeV

Quarkonium production in proton-proton collisions serves as an essential baseline for studying the effect of quarkonium suppression in proton-nucleus and nucleus-nucleus collisions. This paper presents the main characteristics of Upsilon mesons production in proton-proton collisions at studied with two Monte Carlo event generators: Pythia and Herwig and the results compared. Main aim of these simulations is to explore the dependence of normalised Upsilon meson yield on normalised event multiplicity and the influence of the feed-down effect on Upsilon meson production. The simulations show that the normalised Upsilon yield rises stronger than linear with regards to the normalised charged particle multiplicity. A mass ordering is found for Upsilons originating from feed-down and it suggests that the observed strong increase depends on the scale of the original hard process producing the quarkonium state.

Strangeness Production from Proton–Proton Collisions at Different Energies by Using Monte Carlo Simulation

Nuclear matter, at sufficiently energy density and high temperature, undergoes a transition to a state of strongly interacting QCD matter in which quarks and gluons are not confined known as the Quark–Gluon Plasma (QGP). QGP is usually produced in high-energy collisions of heavy nuclei in the laboratory, where an enhancement of strange hadrons’ production is observed. Many of the effects which are typical of heavy ion phenomenology have been observed in high-multiplicity proton–proton (pp) collisions. The enhancement of strange particles’ production in pp collisions was reported at s=7 TeV and s=13 TeV in 2017 and 2020, respectively, and it was found that the integrated yields of strange particles, relative to pions, increase notably with the charged-particle multiplicity of events. Here, we report the multiplicity dependence of strange particles at |y|<0.5 in pp collisions at s = 7 TeV, 13 TeV, 20 TeV, and 27 TeV from a Monte Carlo simulation using PYTHIA8, EPOS-LHC, and Herwig7.

Search for Lepton Number and Flavour Violation in K+ and π0 Decays

The NA62 experiment at CERN collected a large sample of charged kaon decays into final states with multiple charged particles in Run 1 from 2016 to 2018. This sample provides sensitivities to rare decays with branching fractions as low as 10−11. Results of searches for the lepton number and/or flavour violating decays K+→π±μ∓e+ and π0→μ−e+ are reported. No evidence of these decays is found, and upper limits on the branching fractions are obtained at 90% confidence level: B(K+→π−μ+e+)<4.2×10−11, B(K+→π+μ−e+)<6.6×10−11, and B(π0→μ−e+)<3.2×10−10. These results improve by approximately one order of magnitude over previous results for these decay modes.

Collective expansion in pp collisions using the Tsallis statistics

We investigate the transverse momentum (p T) spectra of identified hadrons in minimum-bias proton–proton (pp) collisions at a centre-of-mass energy of 0.9, 2.76, 5.02, 7 and 13 TeV in the framework of Tsallis-blast wave (TBW) model. It is found that the model describes well the particle spectra up to 10 GeV/c. The radial flow (⟨β⟩) increases with the collision energy. The degrees of non-equilibrium (q) and the Tsallis temperature parameter (T) show a similar behaviour, but with a much weaker trend. With this dependence of the freeze-out parameters on the collision energy, we evaluate ⟨β⟩, T and q in pp collisions at 8 and 14 TeV and predict the particle spectra at these two energies. Moreover, in order to investigate the multiplicity dependence of the freeze-out parameters, the TBW model is extended to the spectra at different charged-particle multiplicity classes in pp collisions at 7 and 13 TeV. It is observed that at both energies the radial flow increases with the multiplicity while the degree of non-equilibrium shows an opposite behaviour, which is similar to that observed in proton–nucleus (pA) and nucleus–nucleus (AA) collisions at the Large Hadron Collider (LHC) energies. However, the Tsallis temperature parameter increases with the multiplicity, which is opposite to the trend in pA and AA collisions. At similar multiplicities, the radial flow in pp collisions is stronger than those in pA and AA collisions, indicating that the size of the colliding system has significant effects on the final state particle dynamics. Finally, we apply an additional flow correction to the Tsallis temperature parameter and find that the Doppler-corrected temperature parameter almost scales with the multiplicity in a uniform way, despite the difference in the colliding system and collision energy.

System size and flavour dependence of chemical freeze-out temperatures in ALICE data from pp, pPb and PbPb collisions at LHC energies

We present the system size and flavour dependence of the chemical freeze-out temperature (Tch) at vanishing baryo-chemical potential calculated via thermal fits to experimental yields for several multiplicity classes in pp, pPb and PbPb collisions measured by ALICE. Using the Thermal-FIST Hadron Resonance Gas model package, we compare the quality of fits across various treatments of strangeness conservation under different freeze-out conditions as a function of the charged particle multiplicity density 〈dNch/dη〉. Additionally, we examine how the anti-hadron to pion yield ratios of light and strange baryons, as well as the ϕ meson, evolve within a flavour-dependent model. Through a unique two-temperature chemical freeze-out approach, we show that flavour dependence of Tch in a Strangeness Canonical Ensemble leads to a natural explanation of strangeness enhancement from small to large systems at LHC energies without requiring any non-equilibrium particle production at small 〈dNch/dη〉.

Predictions on global properties in O+O collisions at the Large Hadron Collider using a multi-phase transport model

Oxygen (^{16}O) ions are planned to be injected at the Large Hadron Collider (LHC) in its next runs, and a day of physics run is anticipated for O+O collisions at sqrt{s_mathrm{{NN}}} = 7 TeV. As the system size of O+O collisions has the final state multiplicity overlap with those produced in pp, p+Pb and Pb+Pb collisions, the study of global properties in O+O collisions may provide a deeper insight into the heavy-ion-like behavior observed in small collision systems and its similarities/differences with a larger system like Pb+Pb collisions. In the present work, we report the predictions for global properties in O+O collisions at sqrt{s_mathrm{{NN}}} = 7 TeV using a multi-phase transport model (AMPT). We report the mid-rapidity charged-particle multiplicity, transverse mass, Bjorken energy density, pseudo-rapidity distributions, squared speed of sound, transverse momentum (p_mathrm{T}) spectra, the kinetic freeze-out parameters, and p_mathrm{T}-differential particle ratio as a function of collision centrality. Further, we have studied the transverse momentum-dependent elliptic flow of charged particles. The results are shown for Woods-Saxon and harmonic oscillator nuclear density profiles. In addition, we have compared the results with an alpha -clustered structure incorporated inside the oxygen nucleus. Average charged-particle multiplicity and the Bjorken energy density show a significant increase in most central collisions for the harmonic oscillator density profile, while other global properties show less dependence on the density profiles considered in this work. The results from the alpha -clustered structure incorporated inside the oxygen nucleus show similar initial energy density and final charged-particle multiplicity as observed for the harmonic oscillator density profile.

Minijet quenching in a concurrent jet+hydro evolution and the nonequilibrium quark-gluon plasma

Mini-jets, created by perturbative hard QCD collisions at moderate energies, can represent a significant portion of the total multiplicity of a heavy-ion collision event. Since their transverse momenta are initially larger than the typical saturation scale describing the bulk of the equilibrating QGP, their typical stopping distances are larger than the usual hydrodynamization time, so they do not in general hydrodynamize at the same pace than the bulk of the collision. Therefore, in general mini-jets cannot be described solely by a unique pre-equilibrium stage that bridges the initial, over-occupied glasma state, with the hydrodynamical evolution. In this work we make use of a new concurrent mini-jet+hydrodynamic framework in which the properties of the hydrodynamically evolving QGP are modified due to the injection of energy and momentum from the mini-jets. We study the system for different choices of the minimum transverse momentum associated to mini-jet production. In order to achieve a realistic description of charged particle multiplicity, the amount of entropy associated to the low-$x$ initial state needs to be reduced. Moreover, the fact that the injected momentum from the randomly oriented mini-jets is not correlated with the spatial gradients of the system reduces overall flow, and the value of the QGP transport coefficients needs to be reduced accordingly. They are an important new source of fluctuations, resulting in a spikier, notably modified hydrodynamical evolution. We discuss the impact of the mini-jets on a number of observables, such as $p_T$ spectra and $p_T$-differential flow $v_n$ for a wide range of centrality classes. In contrast to elliptic, triangular or quadrangular flow, we find that directed flow, $v_1$, has the strongest potential to discriminate between different mini-jet production rates.

Search for fractality and phase transition in p–p collisions at LHC energy

In this paper, a detailed study of the multiplicities of charged particles produced in [Formula: see text]–[Formula: see text] collisions, at [Formula: see text][Formula: see text]TeV, has been carried out. Analysis of UrQMD generated events along pseudo–rapidity ([Formula: see text]), azimuthal angle ([Formula: see text]) and [Formula: see text]-[Formula: see text] phase spaces is made with the help of scaled factorial moment method. The results have also been compared with the UrQMD events with finite impact parameter ([Formula: see text]) and minimum bias (MB) conditions. From the intermittency exponent ([Formula: see text]), the anomalous fractal dimension [Formula: see text] is derived and the variations of [Formula: see text] with order [Formula: see text] are investigated. The observed intermittent fluctuations are manifested by various parameters such as anomalous fractal dimension [Formula: see text], degree of multifractality ([Formula: see text]), critical exponent ([Formula: see text]), Levy index ([Formula: see text]) and multifractal specific heat. In the framework of Ginzburg–Landau theory, the second-order phase transition in the light of scaled factorial moment method has also been carried out to search for the quark-hadron phase transition. It is observed that the intermittent type of fluctuations are strong enough in [Formula: see text]-[Formula: see text] space compared to the one-dimensional [Formula: see text] space and [Formula: see text] space. The data reflects the signature of multifractality in [Formula: see text] space and [Formula: see text]-[Formula: see text] space. Whereas in case of [Formula: see text] space, it shows the evidence of monofractality. It is also interesting to note that the values of critical exponents for all impact parameters indicate the apparent existence of quark hadron phase transition except for MB events. However, no such behaviour is observed in [Formula: see text] space. From the knowledge of generalized fractal dimension [Formula: see text], multifractal specific heat is derived from scale factorial moment analysis in case of one-dimensional [Formula: see text] space, [Formula: see text] space and two-dimensional [Formula: see text]-[Formula: see text] space.

Probing quantum entanglement and collectivity effects in e+p collisions at HERA H1

Charged particle multiplicity spectra are measured at HERA H1 in DIS process at \sqrt{s}s=319 GeV. The hadron entropy determined from the multiplicity spectra is compared to the gluon entropy predicted from a quantum-entanglement model. On the other hand, observed collective behaviours in high multiplicity ppA, pppp and ultra-peripheral PbPb collisions at RHIC and LHC motivate a study in epep collisions. Two- and multi-particle correlations are studied in DIS, as well as photoproduction events with collision energy \left\langle W_{\gamma p} \right \rangle \sim⟨Wγp⟩∼270 GeV.

Two-particle Bose–Einstein correlations in { pp } collisions at mathbf {sqrt{s} = 13} TeV measured with the ATLAS detector at the LHC

This paper presents studies of Bose–Einstein correlations (BEC) in proton–proton collisions at a centre-of-mass energy of 13 TeV, using data from the ATLAS detector at the CERN Large Hadron Collider. Data were collected in a special low-luminosity configuration with a minimum-bias trigger and a high-multiplicity track trigger, accumulating integrated luminosities of 151 upmu b^{-1} and 8.4 nb^{-1}, respectively. The BEC are measured for pairs of like-sign charged particles, each with |eta | < 2.5, for two kinematic ranges: the first with particle p_{mathrm {T}} > 100 MeV and the second with particle p_{mathrm {T}} > 500 MeV. The BEC parameters, characterizing the source radius and particle correlation strength, are investigated as functions of charged-particle multiplicity (up to 300) and average transverse momentum of the pair (up to 1.5 GeV). The double-differential dependence on charged-particle multiplicity and average transverse momentum of the pair is also studied. The BEC radius is found to be independent of the charged-particle multiplicity for high charged-particle multiplicity (above 100), confirming a previous observation at lower energy. This saturation occurs independent of the transverse momentum of the pair.

Multiplicity trigger detector for the S[formula omitted]RIT experiment

A multiplicity trigger detector (MTD) was developed for the SπRIT experiment that aims to probe the high-density symmetry energy via heavy-ion collisions (HICs). The MTD is designed to measure the charged-particle multiplicity in HICs to provide a trigger signal that can be used to select high-multiplicity events induced by central collisions. The MTD consists of two side-walls segmented into 30 plastic scintillation paddles, each equipped with a multi-pixel photon counter. A custom board with EASIROC front-end ASIC is used as readout electronics. A multiplicity logic signal is generated by an on-board FPGA that processes the discriminated signals from EASIROCs. The overall latency of the whole detector system for outputting the trigger is less than 100ns, including 52ns of electronics latency for processing the signals. During the measurement of 112Sn+124Sn reactions at an incident energy of 270MeV/nucleon, the minimum multiplicity threshold of the MTD was set to 4, where the overall trigger efficiency was obtained to be about 39% for inclusive nuclear reactions. A simulation study with a numerical calculation of HICs found that the central collisions of impact parameters of less than 4fm can be triggered with a high efficiency of more than 95% by the experimental trigger condition.