Function Of Charged Particle Multiplicity Research Articles

Comparative analysis of jet and underlying event properties across various models as a function of charged particle multiplicity at 7 TeV* *Supported by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2024R106), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia

In this study, a comprehensive analysis of jets and underlying events as a function of charged particle multiplicity in proton-proton (pp) collisions at a center-of-mass energy of TeV is conducted. Various Monte Carlo (MC) event generators, including Pythia8.308, EPOS1.99, EPOSLHC, EPOS4 , and EPOS4 , are employed to predict particle production. The predictions from these models are compared with experimental data from the CMS collaboration. The charged particles are categorized into those associated with underlying events and those linked to jets, and the analysis is restricted to charged particles with and GeV/c. By comparing the MC predictions with CMS data, we find that EPOS , EPOSLHC, and Pythia8 consistently reproduce the experimental results for all charged particles, underlying events, intrajets, and leading charged particles. For charged jet rates with GeV/c, EPOS4 and Pythia8 perform exceptionally well. In the case of charged jet rates with GeV/c, EPOSLHC reproduces satisfactorily good results, whereas EPOS4 exhibits good agreement with the data at higher charged particle multiplicities compared to the other models. This can be attributed to the conversion of energy into flow when "Hydro=on," leading to an increase in multiplicity. The EPOSLHC model describes the data better owing to the new collective flow effects, correlated flow treatment, and parameterization compared to EPOS1.99. However, the examination of the jet spectrum and normalized charged density reveals that EPOS4 , EPOS4 , and EPOSLHC exhibit good agreement with the experimental results, whereas Pythia8 and EPOS1.99 do not perform as well owing to the lack of correlated flow treatment.

Can quantized fragmentation replace colour reconnection models?

A brief overview of properties of the model of quantized fragmentation of a helical QCD string is followed by the discussion of colour reconnection models and of the ways to reproduce the evolution of the average transverse momentum in hadronic events, as function of charged particle multiplicity. Reference to recent experimental data is included.

Multiplicity Dependence of Quarkonium Production

Recent measurements on heavy-flavour production as a function of charged-particle multiplicity at the LHC are discussed. Focus is given to quarkonium results in small (pp or pPb) collision systems. The measurements of relative yields, i.e., the ratio of the particle yields in given multiplicity intervals to the multiplicity integrated yield are presented and compared to model calculations from Monte Carlo event generators as well as to models considering effects at play in the initial and/or final state of the collision. The absolute inclusive J/ψ yield as a function of the absolute charged-particle multiplicity is evaluated; a smooth behaviour of the absolute yield is observed across collision systems, from pp to pPb and PbPb collisions. Analogous measurements of the excited-to-ground state quarkonium ratios as a function of charged-particle multiplicity are also reviewed. Finally, the study of exotic particle production as a function of charged-particle multiplicity is introduced as a complementary tool to investigate the nature of the χc1(3872) hadron.

System-size dependence of the hadronic rescattering effect at energies available at the CERN Large Hadron Collider

The first measurements of K*(892)0 resonance production as a function of charged-particle multiplicity in Xe-Xe collisions at sNN=5.44 TeV and pp collisions ats=5.02 TeV using the ALICE detector are presented. The resonance is reconstructed at midrapidity (|y| < 0.5) using the hadronic decay channel K*0 →K±π∓. Measurements of transverse-momentum integrated yield, mean transverse-momentum, nuclear modification factor of K*0, and yield ratios of resonance to stable hadron (K*0/K) are compared across different collision systems (pp, p-Pb, Xe-Xe, and Pb-Pb) at similar collision energies to investigate how the production of K*0 resonances depends on the size of the system formed in these collisions. The hadronic rescattering effect is found to be independent of the size of colliding systems and mainly driven by the produced charged-particle multiplicity, which is a proxy of the volume of produced matter at the chemical freeze-out. In addition, the production yields of K*0 in Xe-Xe collisions are utilized to constrain the dependence of the kinetic freeze-out temperature on the system size using the hadron resonance gas–partial chemical equilibrium model. ©2024 CERN, for the ALICE Collaboration 2024 CERN

Measurement of two-particle correlations and flow coefficients in high multiplicity e+e− collisions using archived ALEPH data at 91-209 GeV

We present measurements of two-particle angular correlations of charged particles emitted in high-energy collisions using data collected by the ALEPH detector at LEP between 1992 to 2000. The correlation functions are measured over a wide range of pseudorapidity and azimuthal angle as a function of charged particle multiplicity. Previous measurement with LEP1 data at √S = 91 GeV shows no significant long-range correlations in either lab coordinate or thrust coordinate analyses, with associated yield distributions in agreement with predictions from the PYTHIA v6.1 event generator. The use of higher collision energy LEP2 data allows access to not only higher event multiplicity but also additional production channels beyond the e+e- → y* / Z → q¯q process. Notably, the highest multiplicity bin (Ntrk ≥ 50) suggests a tantalizing disagreement with MC and implies the potential to search for collective phenomena in small systems. This measurement is pushing the studies of long-range correlation to the smallest collision system limit and includes the first flow coefficient (vn) measurement in e+e− collisions, which uses a Fourier decomposition analysis to quantify the anisotropy in the azimuthal two-particle correlation as a function of charged particles’ transverse momentum. This work supplements our understanding of small-system references to long-range correlations observed in proton-proton, proton-nucleus, and nucleus-nucleus collisions.

Latest ALICE results on charm and beauty hadronisation in hadronic collisions

The study of heavy-flavour mesons and baryons in hadronic collisions provides unique access to the properties of heavy-quark hadronisation in the presence of large partonic densities, where new mechanisms of hadron formation beyond in-vacuum fragmentation can emerge. Performing these measurements in intervals of charged-particle multiplicities across different collision systems provides sensitivity to understand whether different hadronisation mechanisms are at play in small and large hadronic colliding systems. In this contribution, a selection of the latest charm and beauty production measurements in proton–proton (pp) collisions is presented, which can shed light on the modification of the heavy-quark hadronisation mechanisms with respect to leptonic collisions. New published results of the production of all prompt charm ground states in pp collisions at √S = 13 TeV, which allowed us to measure the charm fragmentation fractions and the total cc production cross section at midrapidity, will be shown. The new final measurement of non-prompt (i.e. originating from beauty-hadron decays) Λc+ baryons in the same collisions system will be discussed to provide a quantitative comparison between the hadronisation properties of beauty and charm hadrons. New measurements of Ξc0 production as a function of charged-particle multiplicity in pp collisions at √S = 13, and of Ξc0 production in p–Pb collisions at √SNN = 5.02 TeV, will be also presented, shedding further light on the hadronisation of charm-strange baryons in different colliding systems.

Correlation of heavy-flavour hadron production and charged-particle multiplicity in pp collisions measured by ALICE

Heavy-flavour (HF) quarks, i.e. charm and beauty, are produced in the early stages of ultra-relativistic collisions via hard scattering processes. The measurement of heavy flavour as a function of charged-particle multiplicity not only provides information on how the production mechanisms are influenced by the event activity, but can also be considered as a tool to test the role of multiple parton interactions. Furthermore, charged-particle multiplicity studies are essential for reference measurements, to tune theoretical models and to provide information on the global characteristics of events. In ALICE, charged-particle multiplicity and heavy-flavour production in the hadronic and electronic decay channels are measured at central rapidity while HF-decay muons and J/ψ → μ + μ − are measured at forward rapidity. In this contribution, results will be presented on relative J/ψ, average D-mesons and leptons from HF hadron decay yields as a function of charged-particle multiplicity in proton-proton (pp) collisions at , 7, 8 and 13 TeV. These results will also be compared to theoretical model calculations.

Multiplicity and rapidity dependence of textrm{K}^{*}(mathrm {{textbf {892}}})^{0} and mathrm {phi ({textbf {1020}})} production in p–Pb collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV

The transverse-momentum (p_{textrm{T}}) spectra of K^{*}(892)^{0}~and mathrm {phi (1020)}~measured with the ALICE detector up to p_{textrm{T}} = 16 GeV/c in the rapidity range -1.2< y < 0.3, in p–Pb collisions at the center-of-mass energy per nucleon–nucleon collision sqrt{s_{textrm{NN}}} = 5.02 TeV are presented as a function of charged particle multiplicity and rapidity. The measured p_{textrm{T}} distributions show a dependence on both multiplicity and rapidity at low p_{textrm{T}} whereas no significant dependence is observed at high p_{textrm{T}} . A rapidity dependence is observed in the p_{textrm{T}} -integrated yield (dN/dy), whereas the mean transverse momentum left( langle p_{textrm{T}} rangle right) shows a flat behavior as a function of rapidity. The rapidity asymmetry (Y_{textrm{asym}}) at low p_{textrm{T}} (< 5 GeV/c) is more significant for higher multiplicity classes. At high p_{textrm{T}} , no significant rapidity asymmetry is observed in any of the multiplicity classes. Both K^{*}(892)^{0}~and mathrm {phi (1020)}~show similar Y_{textrm{asym}}. The nuclear modification factor (Q_{textrm{CP}}) as a function of p_{textrm{T}} shows a Cronin-like enhancement at intermediate p_{textrm{T}} , which is more prominent at higher rapidities (Pb-going direction) and in higher multiplicity classes. At high p_{textrm{T}} (> 5 GeV/c), the Q_{textrm{CP}} values are greater than unity and no significant rapidity dependence is observed.

Study on the structure of exotic states χc1(3872) via beauty-hadron decays in pp collisions at s=8 TeV

A dynamically constrained phase-space coalescence (DCPC) model was introduced to study the exotic state $\chi_{c 1}(3872)$ yield for three possible structures: tetraquark state, nuclear-like state, and molecular state respectively, where the hadronic final states generated by the parton and hadron cascade model (PACIAE). The $\chi_{c 1}(3872)$/$\psi (2S)$ cross-section ratio from beauty-hadron decays (non-prompt) based on the $\chi_{c 1}(3872)$ or $\psi (2S)\to J/\psi{\pi^+}{\pi^-}$ bound state in the decay chains as a function of charged-particle multiplicity and transverse momentum in $pp$ collisions at $\sqrt{s}=8\,\mathrm{TeV}$ are calculated. A tetraquark state scenario from PACIAE+DCPC model shows better agreement with the LHCb and ATLAS measurements for the non-prompt $\chi_{c 1}(3872)$/$\psi(2S)$ cross-section ratio distributions, indicating that the $\chi_{c 1}(3872)$ is more likely to be a compact tetraquark state.

Model study on Υ(nS) modification in small collision systems

Quarkonium production has been studied extensively in relativistic heavy-ion collision experiments to understand the properties of the quark-gluon plasma. The experimental results on the yield modification in heavy-ion collisions relative to that in $p+p$ collisions can be described by several models considering dissociation and regeneration effects. A yield modification beyond initial-state effects has also been observed in small collision systems such as $p+\mathrm{Au}$ and $p+\mathrm{Pb}$ collisions, but it is still premature to claim any hot medium effect. A model study in various small collision systems such as $p+p$, $p+\mathrm{Pb}$, $p+\mathrm{O}$, and $\mathrm{O}+\mathrm{O}$ collisions will help quantitatively the understanding of nuclear effects on the $\mathrm{\ensuremath{\Upsilon}}(nS)$ production. A theoretical calculation considering the gluo-dissociation and inelastic parton scattering for dissociation, and their inverse reaction for regeneration, reasonably describes the modification of $\mathrm{\ensuremath{\Upsilon}}(1S)$ in $\mathrm{Pb}+\mathrm{Pb}$ collisions. Based on this calculation, a Monte Carlo simulation considering the dissociation effect is developed to more realistically incorporate the medium produced in heavy-ion collisions with event-by-event initial collision geometry and hydrodynamic evolution. We extend this framework to small systems to study the medium effects. In this work, we quantify the nuclear modification factor of $\mathrm{\ensuremath{\Upsilon}}(nS)$ as a function of charged particle multiplicity ($d{N}_{ch}/d\ensuremath{\eta}$) and transverse momentum. We also calculate the elliptic flow of $\mathrm{\ensuremath{\Upsilon}}(nS)$ in small collision systems.

Light-nuclei production in pp and pA collisions in the baryon canonical ensemble approach

The increase in yields of light nuclei with charged particle multiplicity, as reported by the ALICE collaboration at CERN in p-p and p-Pb collisions at the LHC energy is investigated in the thermal hadron resonance gas model. The model is extended to account for exact baryon number conservation. The focus is on the production of protons, deuterons, $^3$He, and $^3_\Lambda$H. A very good description of proton and deuteron yields is obtained as a function of charged particle multiplicity in the mid-rapidity region using the same fixed temperature as in central Pb-Pb collisions. The yields of light nuclei $^3$He and $^3_\Lambda$H though qualitatively explained as a function of multiplicity, are lower than the model expectation. One of the possible reasons could be that for $^{3}$He and $^{3}_{\Lambda}$H, the chemical equilibrium is not yet reached at small multiplicities.

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.

Forward rapidity J/ψ production as a function of charged-particle multiplicity in pp collisions at sqrt{s} = 5.02 and 13 TeV

The production of J/ψ is measured as a function of charged-particle multiplicity at forward rapidity in proton-proton (pp) collisions at center-of-mass energies sqrt{s} = 5.02 and 13 TeV. The J/ψ mesons are reconstructed via their decay into dimuons in the rapidity interval (2.5 < y < 4.0), whereas the charged-particle multiplicity density (dNch/dη) is measured at midrapidity (|η| < 1). The production rate as a function of multiplicity is reported as the ratio of the yield in a given multiplicity interval to the multiplicity-integrated one. This observable shows a linear increase with charged-particle multiplicity normalized to the corresponding average value for inelastic events (dNch/dη/〈dNch/dη〉), at both the colliding energies. Measurements are compared with available ALICE results at midrapidity and theoretical model calculations. First measurement of the mean transverse momentum (〈pT〉) of J/ψ in pp collisions exhibits an increasing trend as a function of dNch/dη/〈dNch/dη〉 showing a saturation towards high charged-particle multiplicities.

Measurement of Two-Particle Correlations of Hadrons in e^{+}e^{-} Collisions at Belle.

The measurement of two-particle angular correlation functions in high-multiplicity e^{+}e^{-} collisions at sqrt[s]=10.52 GeV is reported. In this study, the 89.5 fb^{-1} of hadronic e^{+}e^{-} annihilation data collected by the Belle detector at KEKB are used. Two-particle angular correlation functions are measured in the full relative azimuthal angle (Δϕ) and three units of pseudorapidity (Δη), defined by either the electron beam axis or the event-shape thrust axis, and are studied as a function of charged-particle multiplicity. The measurement in the thrust axis analysis, with mostly outgoing quark pairs determining the reference axis, is sensitive to the region of additional soft gluon emissions. No significant anisotropic collective behavior is observed with either coordinate analyses. Near-side jet correlations appear to be absent in the thrust axis analysis. The measurements are compared to predictions from various event generators and are expected to provide new constraints to the phenomenological models in the low-energy regime.

Higher-order transverse momentum fluctuations in heavy-ion collisions

In relativistic heavy-ion collisions, the event-by-event mean transverse momentum fluctuations are sensitive to overlap area and energy density fluctuations in initial state. We present a framework to calculate $p_{\mathrm{T}}$ fluctuations up to $4^{\mathrm{th}}$-order using standard and subevent methods, which is validated using the HIJING model. We observe a power-law dependence for cumulants of all orders as a function of charged particle multiplicity $N_{\mathrm{ch}}$, consistent with a simple independent source picture. The fluctuation in $pp$ collisions is observed to be larger than for $p+$Pb, Pb+Pb and Xe+Xe collisions at the same $N_{\mathrm{ch}}$ due to bias in number of contributing sources. The short-range correlations are greatly suppressed in subevent method in comparison to calculations based on the standard method. This study provides a baseline for transverse momentum fluctuations without the presence of final state effects.

Sudden increase in the degrees of freedom in dense QCD matter

In this paper, we analyzed charged particle transverse momentum spectra in high multiplicity events in proton–proton and nucleus–nucleus collisions at LHC energies from the ALICE experiment using the color string percolation model (CSPM). The color reduction factor and associated string density parameters are extracted for various multiplicity classes in [Formula: see text] collisions and centrality classes for heavy-ion collisions at various LHC energies to study the effect of collision geometry and collision energy. These parameters are used to extract the thermodynamical quantities temperature and the energy density of the hot nuclear matter. A universal scaling is observed in initial temperature when studied as a function of charged particle multiplicity scaled by transverse overlap area. From the measured initial energy density [Formula: see text] and the initial temperature T, a dimensionless quantity [Formula: see text] is constructed which is used to obtain the degrees of freedom (DOF) of the deconfined phase. A two-step behavior and a sudden increase in DOF of [Formula: see text]47 for the ideal gas, above the hadronization temperature (T [Formula: see text] 210[Formula: see text]MeV), are observed in case of heavy-ion collisions at LHC energies.

Investigating heavy-flavor vs light-flavor puzzle with event topology and multiplicity in proton + proton collisions at = 13 TeV using PYTHIA8

Heavy-flavored hadrons are unique probes to study the properties of hot and dense quantum chromodynamics medium produced in ultra-relativistic heavy-ion collisions at RHIC and the LHC. Transverse spherocity is one of the event-topology variables used to separate jetty and isotropic events from the pool of event samples. This study aims to understand the production dynamics of heavy-flavors through the transverse momentum spectra, double differential yield and mean transverse momentum of J/ψ, D 0 and as a function of charged-particle multiplicity and transverse spherocity. Further to investigate the possibility of hadronization of the charm quarks, transverse spherocity dependence ratios like /D 0 and Λ0/K − are studied. For the current analysis, the events are generated by using 4C tuned PYTHIA8 for pp at = 13 TeV, which is quite successful in explaining the heavy-flavor particle production at the LHC energies.

Two-particle azimuthal correlations in photonuclear ultraperipheral Pb+Pb collisions at 5.02 TeV with ATLAS

Two-particle long-range azimuthal correlations are measured in photonuclear collisions using 1.7 nb$^{-1}$ of 5.02 TeV Pb+Pb collision data collected by the ATLAS experiment at the LHC. Candidate events are selected using a dedicated high-multiplicity photonuclear event trigger, a combination of information from the zero-degree calorimeters and forward calorimeters, and from pseudorapidity gaps constructed using calorimeter energy clusters and charged-particle tracks. Distributions of event properties are compared between data and Monte Carlo simulations of photonuclear processes. Two-particle correlation functions are formed using charged-particle tracks in the selected events, and a template-fitting method is employed to subtract the non-flow contribution to the correlation. Significant nonzero values of the second- and third-order flow coefficients are observed and presented as a function of charged-particle multiplicity and transverse momentum. The results are compared with flow coefficients obtained in proton-proton and proton-lead collisions in similar multiplicity ranges, and with theoretical expectations. The unique initial conditions present in this measurement provide a new way to probe the origin of the collective signatures previously observed only in hadronic collisions.

Observation of Multiplicity Dependent Prompt χ_{c1}(3872) and ψ(2S) Production in pp Collisions.

The production of χ_{c1}(3872) and ψ(2S) hadrons is studied as a function of charged particle multiplicity in pp collisions at a center-of-mass energy of 8TeV, corresponding to an integrated luminosity of 2 fb^{-1}. For both states, the fraction that is produced promptly at the collision vertex is found to decrease as charged particle multiplicity increases. The ratio of χ_{c1}(3872) to ψ(2S) cross sections for promptly produced particles is also found to decrease with multiplicity, while no significant dependence on multiplicity is observed for the equivalent ratio of particles produced away from the collision vertex in b-hadron decays. This behavior is consistent with a calculation that models the χ_{c1}(3872) structure as a compact tetraquark. Comparisons with model calculations and implications for the binding energy of the χ_{c1}(3872) state are discussed.

Event Shape and Multiplicity Dependence of Freeze-Out Scenario and System Thermodynamics in Proton+Proton Collisions at s = 13 TeV Using PYTHIA8

Recent observations of QGP-like conditions in high-multiplicity pp collisions from ALICE experiment at the LHC warrant an introspection whether to use pp collisions as a baseline measurement to characterize heavy-ion collisions for the possible formation of a Quark-Gluon Plasma. A double differential study of the particle spectra and thermodynamics of the produced system as a function of charged-particle multiplicity and transverse spherocity in pp collisions would shed light on the underlying event dynamics. Transverse spherocity, one of the event shape observables, allows to separate the events in terms of jetty and isotropic events. We analyse the identified particle transverse momentum ( p T ) spectra as a function of charged-particle multiplicity and transverse spherocity using Tsallis nonextensive statistics and Boltzmann-Gibbs Blast-Wave (BGBW) model in pp collisions at s = 13 TeV using PYTHIA8 event generator. The extracted parameters such as temperature ( T ), radial flow ( β ), and nonextensive parameter ( q ) are shown as a function of charged-particle multiplicity for different spherocity classes. We observe that the isotropic events approach thermal equilibrium while the jetty ones remain far from equilibrium. We argue that, while studying the QGP-like conditions in small systems, one should separate the isotropic events from the spherocity-integrated events, as the production dynamics are different.