Charged Particle Multiplicity Research Articles

Studying the mechanisms for strange particle production with ALICE at LHC

The main goal of the ALICE experiment is to study the physics of strongly interacting matter, focusing on the properties of the quark-gluon plasma (QGP). The relative production of strange hadrons with respect to non-strange hadrons in heavy-ion collisions was historically considered as one of the signatures of QGP formation. However, the latest results in proton-proton (pp) and proton-lead (p-Pb) collisions have revealed an increasing trend in the yield ratio of strange hadrons to pions with the charged-particle multiplicity in the event, showing a smooth evolution across different collision systems and energies. We present the new studies which are performed with the aim of better understanding the production mechanisms for strange particles and hence the strangeness enhancement phenomenon in small collision systems. In one of the recent studies, the very forward energy transported by beam remnants (spectators) and detected by the Zero Degree Calorimeters (ZDC) is used to classify events. The contribution of the effective energy and the particle multiplicity on strangeness production is studied using a multi-differential approach in order to disentangle initial and final state effects. In the second study, the origin of strangeness enhancement with multiplicity in pp has been further investigated by separating the contribution of soft and hard processes, such as jets, to strange hadron production. Techniques involving full jet reconstruction or two-particle correlations have been exploited. The results indicate that the increased relative strangeness production emerges from the growth of the underlying event, being disconnected from initial state properties, and suggest that soft (out-of-jets) processes are the dominant contribution to strange hadron production.

Neutral to charged kaon yield fluctuations in Pb – Pb collisions at [formula omitted] TeV

We present the first measurement of event-by-event fluctuations in the kaon sector in Pb – Pb collisions at sNN=2.76 TeV with the ALICE detector at the LHC. The robust fluctuation correlator νdyn is used to evaluate the magnitude of fluctuations of the relative yields of neutral and charged kaons, as well as the relative yields of charged kaons, as a function of collision centrality and selected kinematic ranges. While the correlator νdyn[K+,K−] exhibits a scaling approximately in inverse proportion of the charged particle multiplicity, νdyn[KS0,K±] features a significant deviation from such scaling. Within uncertainties, the value of νdyn[KS0,K±] is independent of the selected transverse momentum interval, while it exhibits a pseudorapidity dependence. The results are compared with HIJING, AMPT and EPOS–LHC predictions, and are further discussed in the context of the possible production of disoriented chiral condensates in central Pb – Pb collisions.

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.

Pseudorapidity window size dependence of multiplicity fluctuations in high-energy collisions with system size and beam energies

An investigation of the critical behavior of strongly interacting quantum chromodynamics (QCD) matter has been performed by analyzing fluctuation observables on event-by-event (ebe) basis measured in high-energy collision experiments. The fluctuation analysis is performed using nuclear interactions at different target sizes and at different colliding beam energies as a function of varying width of pseudorapidity interval. For the sake of comparison, ebe multiplicity fluctuations in hadronic and heavy ion collisions (p–H, p–A and A–B) are studied within the framework of the Lund Monte Carlo-based FRITIOF model. Charged particle multiplicity and the variance of the multiplicity distribution are estimated for the interactions involving different target sizes and beam momenta, i.e., p–H, p–CNO, p–AgBr at 200 A GeV/c and [Formula: see text]O–AgBr collisions at 14.6 A, 60 A and 200 A GeV/c. Further, multiplicity fluctuations are quantified in terms of intensive quantity, the scaled variances [Formula: see text] and the strongly intensive quantity [Formula: see text] derived from the charged particle multiplicity and the width of the multiplicity distribution. Strongly intensive quantity [Formula: see text] is a quantity of great significance to extract information about short- and long-range multiplicity correlations. Furthermore, the collision centrality and centrality bin width-dependent behavior of the multiplicity fluctuation have been examined in the framework of Lund Monte Carlo-based FRITIOF model. Results based on the fluctuation analysis carried out in this study are interpreted in terms of dynamics of collision process and the possibility of related QCD phase transition.

Multiplicity dependence of charged-particle jet production in pp collisions at mathbf {sqrt{s}}=mathbf {13~TeV}

The multiplicity dependence of jet production in pp collisions at the centre-of-mass energy of sqrt{s} = 13 {mathrm {TeV}} is studied for the first time. Jets are reconstructed from charged particles using the anti-k_mathrm {T} algorithm with resolution parameters R varying from 0.2 to 0.7. The jets are measured in the pseudorapidity range |eta _{mathrm{jet}}|< 0.9-R and in the transverse momentum range 5<p_mathrm {T,jet}^{mathrm{ch}}<140 {mathrm {GeV}}/c. The multiplicity intervals are categorised by the ALICE forward detector V0. The p_{mathrm T} differential cross section of charged-particle jets are compared to leading order (LO) and next-to-leading order (NLO) perturbative quantum chromodynamics (pQCD) calculations. It is found that the data are better described by the NLO calculation, although the NLO prediction overestimates the jet cross section below 20 {mathrm {GeV}}/c. The cross section ratios for different R are also measured and compared to model calculations. These measurements provide insights into the angular dependence of jet fragmentation. The jet yield increases with increasing self-normalised charged-particle multiplicity. This increase shows only a weak dependence on jet transverse momentum and resolution parameter at the highest multiplicity. While such behaviour is qualitatively described by the present version of PYTHIA, quantitative description may require implementing new mechanisms for multi-particle production in hadronic collisions.

Modeling of charged-particle multiplicity and transverse-momentum distributions in pp collisions using a DNN

A machine learning technique is used to fit multiplicity distributions in high energy proton-proton collisions and applied to make predictions for collisions at higher energies. The method is tested with Monte Carlo event generators. Charged-particle multiplicity and transverse-momentum distributions within different pseudorapidity intervals in proton-proton collisions were simulated using the PYTHIA event generator for center of mass energies sqrt{s}= 0.9, 2.36, 2.76, 5, 7, 8, 13 TeV for model training and validation and at 10, 20, 27, 50, 100 and 150 TeV for model predictions. Comparisons are made in order to ensure the model reproduces the relation between input variables and output distributions for the charged particle multiplicity and transverse-momentum. The multiplicity and transverse-momentum distributions are described and predicted very well, not only in the case of the trained but also in the case of untrained energy values. The study proposes a way to predict multiplicity distributions at a new energy by extrapolating the information inherent in the lower energy data. Using real data instead of Monte Carlo, as measured at the LHC, the technique has the potential to project the multiplicity distributions for different intervals at very high collision energies, e.g. 27 TeV or 100 TeV for the upgraded HE-LHC and FCC-hh respectively, using only data collected at the LHC, i.e. at center of mass energies from 0.9 up to 13 TeV.

Quarkonium production in p+p collisions at RHIC and LHC

Quarkonium production mechanism is still an unresolved question. This paper provides an overview of recent quarkonium production studies performed at RHIC and LHC in p + p collisions at a wide range of collision energies. After a brief explanation of representative theoretical models, experimental results of the quarkonium spectra and polarizations are presented with comparison to the model calculations. The spectra are overall rather well described by production model calculations, with a few exceptions, while no significant polarization is observed beyond large uncertainties for J/ψ and ϒ, and thus no constraint on the models is obtained at present. Recently, new observables have been measured such as charged particle multiplicity dependence of quarkonium production, which provides a way to understand the interplay between hard and soft processes, and associated production of quarkonium (J/ψ in jets and double production), which provides more information about production mechanism. A trend of non-linear increase of quarkonium production with increasing the particle multiplicity is observed both at RHIC and LHC, but it is reproduced by most models and further studies and more data are needed in order to differentiate them. Meanwhile, the first measurements of J/ψ in jets show that it is produced with larger activity within a jet than simulated by PYTHIA. A comment on the study of J/ψ-hadron femtoscopic correlations to measure their mutual interaction is also given. In summary, the production mechanism remains a puzzle, but more precise studies will be possible with upcoming RHIC and LHC runs.

Study of the KNO scaling in pp collisions at sqrt{s} from 0.9 to 13 TeV using results of the ATLAS at the LHC

The comparisons of the charged-particle multiplicity and the average transverse momentum distributions on the scaled multiplicity, KNO scale, using the results of the ATLAS collaboration at the LHC are presented. These distributions were measured in proton–proton collisions at centre-of-mass energies from 0.9 to 13 TeV for the absolute pseudorapidity region less than 2.5 and two samples of events with each charged-particle transverse momentum greater than 100 and 500 MeV, respectively. The shape evolution of the multiplicity distributions with a collision energy is studied in terms of KNO scaling variables. The charged-particle multiplicity distributions on the KNO scale have the similar shape and decrease with increasing collision energy. The KNO distributions tend to be independent of energy for the highest energies. The average transverse momentum distributions on the KNO scale have a similar shape and increase with increasing collision energy.

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.

Observation of a multiplicity dependence in the pT-differential charm baryon-to-meson ratios in proton–proton collisions at [formula omitted] TeV

The production of prompt D0, Ds+, and Λc+ hadrons, and their ratios, Ds+/D0 and Λc+/D0, are measured in proton–proton collisions at s=13 TeV at midrapidity (|y|<0.5) with the ALICE detector at the LHC. The measurements are performed as a function of the charm-hadron transverse momentum (pT) in intervals of charged-particle multiplicity, measured with two multiplicity estimators covering different pseudorapidity regions. While the strange to non-strange Ds+/D0 ratio indicates no significant multiplicity dependence, the baryon-to-meson pT-differential Λc+/D0 ratio shows a multiplicity-dependent enhancement, with a significance of 5.3σ for 1<pT<12 GeV/c, comparing the highest multiplicity interval with respect to the lowest one. The measurements are compared with a theoretical model that explains the multiplicity dependence by a canonical treatment of quantum charges in the statistical hadronisation approach, and with predictions from event generators that implement colour reconnection mechanisms beyond the leading colour approximation to model the hadronisation process. The Λc+/D0 ratios as a function of pT present a similar shape and magnitude as the Λ/KS0 ratios in comparable multiplicity intervals, suggesting a potential common mechanism for light- and charm-hadron formation, with analogous multiplicity dependence. The pT-integrated ratios, extrapolated down to pT=0, do not show a significant dependence on multiplicity within the uncertainties.

Possible formation of a perfect fluid in pp, p–Pb, Xe–Xe and Pb–Pb collisions at the Large Hadron Collider energies: a color string percolation approach

Isothermal compressibility ($\kappa_{\rm T}$) is an important thermodynamic observable which gives information about the deviation of a system from perfect fluid behavior. In this work, for the first time we have estimated the isothermal compressibility of QCD matter formed in high energy hadronic and nuclear collisions using color string percolation model (CSPM), where we investigate the change in $\kappa_{\rm T}$ as a function of final state charged particle multiplicity and initial percolation temperature across various collision species. The estimated initial percolation temperature for different collision systems at different collision energies helps us to have a better understanding of the system at the initial phase of evolution. The comparison of the CSPM results for isothermal compressibility with that for the well known fluids, indicates that the matter formed in heavy-ion collisions might be the {\it closest perfect fluid} found in nature. This estimation complements the well-known observation of minimum shear viscosity to entropy density ratio for a possible QGP medium created in heavy-ion collision experiments. A threshold of pseudorapidity density of charged particles, $\langle dN_{\rm ch}/d\eta \rangle \geq 20 $ in the final state event multiplicity is observed, after which one may look for a possible QGP formation at the LHC energies.

Investigating charm production and fragmentation via azimuthal correlations of prompt D mesons with charged particles in pp collisions at mathbf {sqrt{ s} = 13} TeV

Angular correlations of heavy-flavour and charged particles in high-energy proton–proton collisions are sensitive to the production mechanisms of heavy quarks and to their fragmentation as well as hadronisation processes. The measurement of the azimuthal-correlation function of prompt D mesons with charged particles in proton–proton collisions at a centre-of-mass energy of sqrt{s} = 13 TeV with the ALICE detector is reported, considering mathrm D^{0} , mathrm D^{+} , and mathrm D^{*+} mesons in the transverse-momentum interval 3< p_{mathrm{T}} < 36 GeV/c at midrapidity (|y| < 0.5), and charged particles with p_{mathrm{T}} > 0.3 GeV/c and pseudorapidity |eta | < 0.8. This measurement has an improved precision and provides an extended transverse-momentum coverage compared to previous ALICE measurements at lower energies. The study is also performed as a function of the charged-particle multiplicity, showing no modifications of the correlation function with multiplicity within uncertainties. The properties and the transverse-momentum evolution of the near- and away-side correlation peaks are studied and compared with predictions from various Monte Carlo event generators. Among those considered, PYTHIA8 and POWHEG+PYTHIA8 provide the best description of the measured observables. The obtained results can provide guidance on tuning the generators.

Production of light (anti)nuclei in pp collisions at sqrt{s} = 5.02 TeV

The study of the production of nuclei and antinuclei in pp collisions has proven to be a powerful tool to investigate the formation mechanism of loosely bound states in high-energy hadronic collisions. In this paper, the production of protons, deuterons and ^{3}mathrm {He} and their charge conjugates at midrapidity is studied as a function of the charged-particle multiplicity in inelastic pp collisions at sqrt{s}=5.02 TeV using the ALICE detector. Within the uncertainties, the yields of nuclei in pp collisions at sqrt{s}=5.02 TeV are compatible with those in pp collisions at different energies and to those in p–Pb collisions when compared at similar multiplicities. The measurements are compared with the expectations of coalescence and Statistical Hadronisation Models. The results suggest a common formation mechanism behind the production of light nuclei in hadronic interactions and confirm that they do not depend on the collision energy but on the number of produced particles.

Jet Transport Coefficient at the Large Hadron Collider Energies in a Color String Percolation Approach

Within the color string percolation model (CSPM), jet transport coefficient, q^, is calculated for various multiplicity classes in proton-proton and centrality classes in nucleus-nucleus collisions at the Large Hadron Collider energies for a better understanding of the matter formed in ultra-relativistic collisions. q^ is studied as a function of final state charged particle multiplicity (pseudorapidity density at midrapidity), initial state percolation temperature and energy density. The CSPM results are then compared with different theoretical calculations from the JET Collaboration those incorporate particle energy loss in the medium.

Multiplicity Dependencies of Midrapidity Transverse Momentum Distributions of Identified Charged Particles in Proton-Proton Collisions at (s)1/2 = 7 TeV at the LHC

Dependencies of midrapidity pt distributions of the charged pions and kaons, protons and antiprotons on charged-particle multiplicity density (&lt;dNch/dη&gt;) in inelastic proton-proton collisions at (s)1/2 = 7 TeV at the LHC, measured by ALICE Collaboration, are investigated. The simultaneous minimum χ2 fits with the Tsallis function with thermodynamical consistence and the Hagedorn function with included transverse flow have well-described the pt spectra of the particle species in the ten studied groups of charged-particle multiplicity density. The effective temperatures, T, of the Tsallis function with thermodynamical consistence have shown a steady rise with increasing the charged-particle multiplicity in proton-proton collisions at (s)1/2 = 7 TeV, in agreement with the similar result obtained recently in proton-proton collisions at (s)1/2 = 13 TeV at the LHC. The respective T versus &lt;dNch/dη&gt; dependence in proton-proton collisions at (s)1/2 = 7 TeV is reproduced quite well by the simple power function with the same value (≈ 1/3) of the exponent parameter as that extracted in proton-proton collisions at (s)1/2 = 13 TeV. The identical power dependence T~ε1/3 between the initial energy density and effective temperature of the system has been observed in proton-proton collisions at (s)1/2 = 7 and 13 TeV. We have observed that the transverse radial flow emerges at &lt;dNch/dη&gt; ≈ 6 and then increases, becoming substantial at larger multiplicity events in proton-proton collisions at (s)1/2 = 7 TeV. We have estimated, analyzing T0 and ⟨βt⟩ versus &lt;dNch/dη&gt; dependencies, that the possible onset of deconfinement phase transition in proton-proton collisions at (s)1/2 = 7 TeV occurs at &lt;dNch/dη&gt; ≈ 6.1 ± 0.3, which is close to the corresponding recent estimate (&lt;dNch/dη&gt; ≈ 7.1 ± 0.2) in proton-proton collisions at (s)1/2 = 13 TeV. The corresponding critical energy densities for probable onset of deconfinement phase transition in proton-proton collisions at (s)1/2 = 7 and 13 TeV at the LHC have been estimated to be 0.67 ± 0.03 and 0.76 ± 0.02 GeV/fm3, respectively.

Dynamics of particle production in Pb-Pb collisions at sNN=2.76 TeV using the PYTHIA8 Angantyr model

We study the dynamics of identified, strange, and multistrange particle production in Pb-Pb collisions at $\sqrt{{s}_{NN}}$ = 2.76 TeV using the recently developed Angantyr model, incorporated within pythia8. We show the interplay between multiparton interactions (MPIs) and color reconnection (CR) on the experimentally measured quantities. We also investigate the role of string shoving within the rope hadronization framework and its impact in particle production. The charged-particle multiplicity (${N}_{\mathrm{ch}}$) and mean transverse momentum ($\ensuremath{\langle}{p}_{\mathrm{T}}\ensuremath{\rangle}$) distributions are well explained by pythia8 Angantyr with proper tuning, as presented in this paper. Predictions of ${p}_{\mathrm{T}}$ spectra, $\ensuremath{\langle}{p}_{\mathrm{T}}\ensuremath{\rangle}$, and ${p}_{\mathrm{T}}$ integrated yields of the identified, strange, and multistrange particles are studied. To provide insight into the collective nature of the produced particles, we look into the ratio of particle yields to pions and kaons. pythia8 Angantyr with MPI$+$CR and hadronization via string shoving is seen to mimic signs of collectivity due to different implementations in string interactions.

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.

Properties of charged particle multiplicities in anti-neutrino interactions of the CHORUS experiment

In this study, the charged particle multiplicity distribution produced in anti-neutrino interactions in nuclear emulsion is investigated in terms of the log-normal parametric model and KNO-G scaling for the first time. The results are based on the data collected by the CHORUS experiment in the nu _{mu } beam from CERN SPS. The analysis of 369 bar{nu }_{mu }CC events showed that anti-neutrino charged-particle multiplicity data shape is well described by a log-normal distribution. Furthermore, KNO-G scaling has been successfully applied to the data and results confirm the validity of KNO-G scaling for the anti-neutrino interactions. Finally, results are presented in detail in the form of figures and tables.

Exact strangeness conservation in heavy ion collisions

We investigate the increase in strangeness production with charged particle multiplicity (dNch/dy) seen by the ALICE collaboration at CERN in p-p, p-Pb and Pb-Pb collisions using the hadron resonance gas model. The strangeness canonical ensemble is used taking into account the interactions among hadrons using S-matrix corrections based on known phase shift analyses. We show the essential role of constraints due to the exact conservation of strangeness which is instrumental to describing observed features of strange particle yields and their scaling with dNch/dy. Furthermore, the results on comparing the hadron resonance gas model with and without S-matrix corrections, are presented. We observe that the interactions introduced by the phase shift analysis via the S-matrix formalism are essential for a better description of the yields data independent of collision system. This work is based on our longlasting collaboration and most recent publications with Jean Cleymans.

Investigating the origin of strangeness enhancement in small systems through multi-differential analyses

The main goal of the ALICE experiment is to study the physics of strongly interacting matter, including the properties of the quark-gluon plasma (QGP). The relative production of strange hadrons with respect to non-strange hadrons in heavy-ion collisions was historically considered as one of the signatures of QGP formation. However, measurements at the LHC in pp and p-Pb collisions have shown similar features to those observed in Pb-Pb, measuring an increase in the production of strange hadrons relative to pions with the charged-particle multiplicity in the event. In order to better understand the role of the event multiplicity to the observed enhancement in pp, two new complementary analyses have been performed. The first exploits the angular correlation between strange and high-pT hadrons in the event to classify in-jet and out-of-jet strangeness production. The second uses the concept of the effective energy available in the event for particle production which is estimated by an anti-correlation with the energy deposited in ALICE’s Zero Degree Calorimeters. The results suggest that strangeness enhancement emerges from the growth of the underlying event and is not connected to initial state properties.