Charged-particle Pseudorapidity Density Research Articles

Particle production and entropy measurement in ALICE

One of the main goals in the study of hadronic interactions at LHC energies is the attempt to characterize the mechanisms involved in particle production in different regimes. The charged-particle multiplicity is one of the most interesting observables in these kind of studies. Measurements of chargedparticle pseudorapidity densities in pp collisions at √s = 13.6 TeV are presented, for the first time. Furthermore, we present a method for better understanding the collision dynamics. On the one hand, the pseudorapidity dependence of charged-particle production provides information on the partonic structure of the colliding hadrons and is sensitive to non-linear QCD evolution in the initial state. On the other hand to understand the thermal-like behavior and particle yields in pp collisions, a possible approach is to address the principles of quantum states and their entanglement in the produced system. The entanglement in the initial state has a measurable effect on the evolution of the system and is the driving mechanism behind the thermal-like behavior and particle yields observed. We describe a method to understand the level of entanglement in the initial and final states of the collision using a calculation of entropy, the final-state entropy being calculated from multiplicity distributions.

Measurement of ψ (2S) production as a function of charged-particle pseudorapidity density in pp collisions at sqrt{s} = 13 TeV and p–Pb collisions at sqrt{s_{textrm{NN}}} = 8.16 TeV with ALICE at the LHC

Production of inclusive charmonia in pp collisions at center-of-mass energy of sqrt{s} = 13 TeV and p–Pb collisions at center-of-mass energy per nucleon pair of sqrt{s_{textrm{NN}}} = 8.16 TeV is studied as a function of charged-particle pseudorapidity density with ALICE. Ground and excited charmonium states (J/ψ, ψ(2S)) are measured from their dimuon decays in the interval of rapidity in the center-of-mass frame 2.5 < ycms< 4.0 for pp collisions, and 2.03 < ycms< 3.53 and −4.46 < ycms< −2.96 for p–Pb collisions. The charged-particle pseudorapidity density is measured around midrapidity (|η| < 1.0). In pp collisions, the measured charged-particle multiplicity extends to about six times the average value, while in p-Pb collisions at forward (backward) rapidity a multiplicity corresponding to about three (four) times the average is reached. The ψ(2S) yield increases with the charged-particle pseudorapidity density. The ratio of ψ(2S) over J/ψ yield does not show a significant multiplicity dependence in either colliding system, suggesting a similar behavior of J/ψ and ψ(2S) yields with respect to charged-particle pseudorapidity density. Results for the ψ(2S) yield and its ratio with respect to J/ψ agree with available model calculations.

System-size dependence of the charged-particle pseudorapidity density at [formula omitted] for pp, p[sbnd]Pb, and Pb[sbnd]Pb collisions

We present the first systematic comparison of the charged-particle pseudorapidity densities for three widely different collision systems, pp, pPb, and PbPb, at the top energy of the Large Hadron Collider (sNN=5.02TeV) measured over a wide pseudorapidity range (−3.5<η<5), the widest possible among the four experiments at that facility. The systematic uncertainties are minimised since the measurements are recorded by the same experimental apparatus (ALICE). The distributions for pPb and PbPb collisions are determined as a function of the centrality of the collisions, while results from pp collisions are reported for inelastic events with at least one charged particle at midrapidity. The charged-particle pseudorapidity densities are, under simple and robust assumptions, transformed to charged-particle rapidity densities. This allows for the calculation and the presentation of the evolution of the width of the rapidity distributions and of a lower bound on the Bjorken energy density, as a function of the number of participants in all three collision systems. We find a decreasing width of the particle production, and roughly a smooth ten fold increase in the energy density, as the system size grows, which is consistent with a gradually higher dense phase of matter.

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.

Pseudorapidity distributions of charged particles as a function of mid- and forward rapidity multiplicities in pp collisions at sqrt{s}\xa0=\xa05.02, 7 and 13 TeV

The multiplicity dependence of the pseudorapidity density of charged particles in proton–proton (pp) collisions at centre-of-mass energies sqrt{s}~=~5.02, 7 and 13 TeV measured by ALICE is reported. The analysis relies on track segments measured in the midrapidity range (|eta | < 1.5). Results are presented for inelastic events having at least one charged particle produced in the pseudorapidity interval |eta |<1. The multiplicity dependence of the pseudorapidity density of charged particles is measured with mid- and forward rapidity multiplicity estimators, the latter being less affected by autocorrelations. A detailed comparison with predictions from the PYTHIA 8 and EPOS LHC event generators is also presented. The results can be used to constrain models for particle production as a function of multiplicity in pp collisions.

Extraction of the specific shear viscosity of quark-gluon plasma from two-particle transverse momentum correlations

The specific shear viscosity, eta /s, of the quark-gluon plasma formed in ultrarelativistic heavy-ion collisions at RHIC and LHC is estimated based on the progressive longitudinal broadening of transverse momentum two-particle correlators, G_2, reported as a function of collision centrality by the STAR and ALICE experiments. Estimates are computed as a function of collision centrality using the Gavin ansatz which relates the G_2 longitudinal broadening to the specific shear viscosity. Freeze out times required for the use of the ansatz are computed using a linear fit of freeze out times reported as a function of the cubic root of the charged particle pseudorapidity density ({mathrm{d}}N_{mathrm{ch}}/deta )^{1/3}. Estimates of eta /s based on ALICE data exhibit little to no dependence on collision centrality at LHC energy, while estimates obtained from STAR data hint that eta /s might be a function of collision centrality at top RHIC energy.

Multiplicity dependence of inclusive J/ψ production at midrapidity in pp collisions at [formula omitted] TeV

Measurements of the inclusive J/ψ yield as a function of charged-particle pseudorapidity density dNch/dη in pp collisions at s=13 TeV with ALICE at the LHC are reported. The J/ψ meson yield is measured at midrapidity (|y|<0.9) in the dielectron channel, for events selected based on the charged-particle multiplicity at midrapidity (|η|<1) and at forward rapidity (−3.7<η<−1.7 and 2.8<η<5.1); both observables are normalized to their corresponding averages in minimum bias events. The increase of the normalized J/ψ yield with normalized dNch/dη is significantly stronger than linear and dependent on the transverse momentum. The data are compared to theoretical predictions, which describe the observed trends well, albeit not always quantitatively.

J/\u03c8 production as a function of charged-particle multiplicity in p-Pb collisions at sqrt{s_{mathrm{NN}}} = 8.16 TeV

Inclusive J/ψ yields and average transverse momenta in p-Pb collisions at a center-of-mass energy per nucleon pair sqrt{s_{mathrm{NN}}} = 8.16 TeV are measured as a function of the charged-particle pseudorapidity density with ALICE. The J/ψ mesons are reconstructed at forward (2.03 < ycms< 3.53) and backward (−4.46 < ycms< −2.96) center-of-mass rapidity in their dimuon decay channel while the charged-particle pseudorapidity density is measured around midrapidity. The J/ψ yields at forward and backward rapidity normalized to their respective average values increase with the normalized charged-particle pseudorapidity density, the former showing a weaker increase than the latter. The normalized average transverse momenta at forward and backward rapidity manifest a steady increase from low to high charged-particle pseudorapidity density with a saturation beyond the average value.

Influence of electromagnetic fields in proton-nucleus collisions at relativistic energy

We study proton-gold collisions at RHIC energy $\sqrt{s_{NN}}=200$ GeV within the Parton-Hadron-String Dynamics (PHSD) off-shell transport approach, investigating the influence of the intense electromagnetic fields generated in these small systems. We show the space-time evolution of the magnetic and electric components, emphasizing the huge values of the latter one, in particular the electric field $E_x$ along the impact parameter direction whose magnitude is comparable to the magnetic field $B_y$ perpendicular to the reaction plane. We find a fair agreement of the charged particle pseudorapidity density of the high-multiplicity events with respect to the experimental result of the PHENIX Collaboration. Focusing on the most central collision, we show rapidity distributions and spectra as well as the flow coefficients $v_1$ and $v_2$ and we discuss the impact of the electromagnetic fields on identified particle observables. We compute the directed flow $v_1$ of $\pi^+$, $\pi^-$, $K^+$, $K^-$ for collisions at fixed impact parameter and predict that an electromagnetically-induced splitting in the $v_1$ of positively and negatively charged particles is generated in the Au-going side of p+Au reaction mainly driven by the huge $E_x$ component. We find that this effect is stronger for the strange mesons and increases for increasing impact parameter. Furthermore, we highlight the amount of directed flow generated in the deconfined phase, finding that it constitutes the main contribution in the central rapidity region, especially for kaons. Thus, we support the idea that the directed flow is a promising probe for the electromagnetic fields generated in relativistic nuclear collisions and show that in proton-induced reactions the electric component along the impact parameter axis is the primary origin of a charge-odd $v_1$ of pions and kaons.

Charged-particle pseudorapidity density at mid-rapidity in p\u2013Pb collisions at pmb {sqrt{s_{scriptscriptstyle {mathrm{NN}}}}} = 8.16 TeV

The pseudorapidity density of charged particles, mathrm {d}N_{mathrm{ch}}/mathrm {d}eta , in p–Pb collisions has been measured at a centre-of-mass energy per nucleon–nucleon pair of sqrt{s_{scriptscriptstyle {mathrm{NN}}}} = 8.16 TeV at mid-pseudorapidity for non-single-diffractive events. The results cover 3.6 units of pseudorapidity, |eta |<1.8. The mathrm {d}N_{mathrm{ch}}/mathrm {d}eta value is 19.1pm 0.7 at |eta |<0.5. This quantity divided by langle N_{mathrm{part}} rangle / 2 is 4.73pm 0.20, where langle N_{mathrm{part}} rangle is the average number of participating nucleons, is 9.5% higher than the corresponding value for p–Pb collisions at sqrt{s_{scriptscriptstyle {mathrm{NN}}}} = 5.02 TeV. Measurements are compared with models based on different mechanisms for particle production. All models agree within uncertainties with data in the Pb-going side, while HIJING overestimates, showing a symmetric behaviour, and EPOS underestimates the p-going side of the mathrm {d}N_{mathrm{ch}}/mathrm {d}eta distribution. Saturation-based models reproduce the distributions well for eta >-1.3. The mathrm {d}N_{mathrm{ch}}/mathrm {d}eta is also measured for different centrality estimators, based both on the charged-particle multiplicity and on the energy deposited in the Zero-Degree Calorimeters. A study of the implications of the large multiplicity fluctuations due to the small number of participants for systems like p–Pb in the centrality calculation for multiplicity-based estimators is discussed, demonstrating the advantages of determining the centrality with energy deposited near beam rapidity.

ALICE results on system-size dependence of charged-particle multiplicity density in p–Pb, Pb–Pb and Xe–Xe collisions

Particle production at LHC energies involves the interplay of hard (perturbative) and soft (non-perturbative) QCD processes. Global observables, such as the charged-particle multiplicity, are related to the initial geometry and the energy density produced in the collision. They are important to characterise relativistic heavy-ion collisions and to constrain model calculations. The LHC produced Xenon–Xenon collisions for the first time in October 2017. New results on the primary charged-particle pseudorapidity density, and its pseudorapidity and centrality dependence are presented for this lighter and deformed nucleus, and compared to measurements obtained for lead ions. New results will also be presented for p-Pb collisions at the highest energy of 8.16 TeV, as part of an overview of all the measurements at LHC Run 1 and 2 energies. These studies allow us to investigate the evolution of particle production with energy and system size and to compare models based on various particle production mechanisms and different initial conditions.

Scaling Properties of the Mean Multiplicity and Pseudorapidity Density in e−+e+, e±+p, p( p ¯ )+p, p+A and A+A(B) Collisions

The charged-particle pseudorapidity density ( d N ch / d η ) for p( p ¯ )+p, p+A and A+A(B) collisions and the mean multiplicity 〈 N ch 〉 for e − +e + , e ± + p , and p( p ¯ )+p collisions are studied for a wide range of beam energies ( s ). Characteristic scaling patterns are observed for both d N ch / d η and 〈 N ch 〉 , consistent with a thermal particle production mechanism for the bulk of the soft particles created in all of these systems. The scaling patterns found also validate an essential role for quark participants in these collisions. The measured values for d N ch / d η and 〈 N ch 〉 are observed to factorize into contributions that depend on log ( s ) and the number of nucleon or quark participant pairs N pp . The quantification of these contributions gives expressions that serve to systematize d N ch / d η and 〈 N ch 〉 measurements spanning nearly 4 orders of magnitude in s and to predict their values as a function of s and N pp .

Nucleon shadowing effects in Cu + Cu and Au + Au collisions at RHIC within the HIJING code

The centrality dependence of pseudorapidity density of charged particles () in Cu + Cu (Au + Au) collisions at Relativistic Heavy Ion Collider energy of , 62.4 and 200 (19.6, 62.4 and 200) GeV, is investigated within an improved HIJING code. The standard HIJING model is enhanced by a prescription for collective nucleon–nucleon (NN) interactions and more modern parton distribution functions. The collective NN-interactions are used to induce both cascade and nucleon shadowing effects. We find collective cascade broadens the pseudorapidity distributions in the tails (at ) above 25%–30% collision centrality to be consistent with the data at . The overall contribution of nucleon shadowing is shown to depress the whole shape of in the primary interaction region (at ) for semiperipheral (20%–25%) and peripheral () Cu + Cu (Au + Au) interactions at , in accordance with the PHOBOS data.

Constraining the magnitude of the Chiral Magnetic Effect with Event Shape Engineering in Pb–Pb collisions at [formula omitted

In ultrarelativistic heavy-ion collisions, the event-by-event variation of the elliptic flow v2 reflects fluctuations in the shape of the initial state of the system. This allows to select events with the same centrality but different initial geometry. This selection technique, Event Shape Engineering, has been used in the analysis of charge-dependent two- and three-particle correlations in Pb–Pb collisions at sNN=2.76 TeV. The two-particle correlator 〈cos⁡(φα−φβ)〉, calculated for different combinations of charges α and β, is almost independent of v2 (for a given centrality), while the three-particle correlator 〈cos⁡(φα+φβ−2Ψ2)〉 scales almost linearly both with the event v2 and charged-particle pseudorapidity density. The charge dependence of the three-particle correlator is often interpreted as evidence for the Chiral Magnetic Effect (CME), a parity violating effect of the strong interaction. However, its measured dependence on v2 points to a large non-CME contribution to the correlator. Comparing the results with Monte Carlo calculations including a magnetic field due to the spectators, the upper limit of the CME signal contribution to the three-particle correlator in the 10–50% centrality interval is found to be 26–33% at 95% confidence level.

J/ψ production as a function of charged-particle pseudorapidity density in p–Pb collisions at [formula omitted

We report measurements of the inclusive J/ψ yield and average transverse momentum as a function of charged-particle pseudorapidity density dNch/dη in p–Pb collisions at sNN=5.02TeV with ALICE at the LHC. The observables are normalised to their corresponding averages in non-single diffractive events. An increase of the normalised J/ψ yield with normalised dNch/dη, measured at mid-rapidity, is observed at mid-rapidity and backward rapidity. At forward rapidity, a saturation of the relative yield is observed for high charged-particle multiplicities. The normalised average transverse momentum at forward and backward rapidities increases with multiplicity at low multiplicities and saturates beyond moderate multiplicities. In addition, the forward-to-backward nuclear modification factor ratio is also reported, showing an increasing suppression of J/ψ production at forward rapidity with respect to backward rapidity for increasing charged-particle multiplicity.

Constraints on rapidity-dependent initial conditions from charged-particle pseudorapidity densities and two-particle correlations

We study the initial three-dimensional spatial configuration of the quark-gluon plasma produced in relativistic heavy-ion collisions using centrality and rapidity-dependent measurements of charged particle pseudorapidity densities and two-particle correlations. A cumulant-generating function is used to parametrize the rapidity dependence of local entropy deposition and extend arbitrary boost-invariant initial conditions to nonzero beam rapidities. The model is compared to p+Pb and Pb+Pb single-particle distributions and systematically optimized using Bayesian parameter estimation to extract high-probability initial condition parameters. The optimized initial conditions are then compared to a number of experimental observables including two-particle rapidity correlations, the rapidity dependence of anisotropic flow, and event-plane decorrelations.

System-size dependence of the charged-particle pseudorapidity density at [formula omitted] with ALICE

We present the charged-particle pseudorapidity density in pp, p–Pb, and Pb–Pb collisions at sNN=5.02 TeV over a broad pseudorapidity range. The distributions are determined using the same experimental apparatus and methodologies, thereby minimizing systematic uncertainties, and providing clear and model-independent observations on the system-size dependence of the particle production at relativistic energies. An increase of particle production in Pb–Pb collisions near mid-rapidity, relative to pp collisions, is observed. The relative linearity in p–Pb collisions of the ratio to the smaller system indicate a coherent particle production throughout the longitudinal extend of the collision zone.

Constraints on rapidity-dependent initial conditions from charged particle pseudorapidity densities and correlations at the LHC

The initial three-dimensional entropy distribution of the quark-gluon plasma produced in relativistic heavy-ion collision is systematically studied using parametric initial conditions. We apply a cumulant generating function approach to parametrize the rapidity dependence of local entropy deposition and extend the boost-invariant initial condition model TRENTo to include longitudinal structure. Two different parametrizations are tested and their parameters optimized by comparing to centrality dependent charged particle pseudorapidity densities of p+Pb (5.02A TeV) and Pb+Pb (2.76A TeV) using Bayesian inference. Successful parametrizations are then selected based on their ability to describe two-particle psuedorapidity correlations. Finally, we predict the pseudorapidity dependent flows and event-plane decorrelations using the optimized initial conditions and a 3+1D hybrid hydro+micro model.

Strangeness production and nuclear modification at LHC energies

We report results on strangeness production in Pb–Pb collisions at sNN=2.76 TeV, p-Pb collisions at sNN=5.02 TeV and pp collisions at s=7 TeV from the ALICE collaboration. In heavy-ion collisions, the nuclear modification factor RAA of charged pions, kaons and protons shows a strong, centrality dependent suppression with respect to pp collisions. RAA for Ξ is similar to the one for protons. In 0–10% central collisions, RAA for Ω is significantly higher than RAA for Ξ and exceeds unity. The hyperon-to-pion yields measured in pp collisions as a function of the pseudorapidity density of primary charged particles at midrapidity exhibit a pronounced multiplicity dependence similar to p–Pb collisions. The production rate of strange particles increases faster with multiplicity than that of non-strange hadrons. No enhancement is observed for particles with no strange quark content, demonstrating that the observed effect is strangeness rather than mass related.

Centrality dependence of the pseudorapidity density distribution for charged particles in Pb–Pb collisions at [formula omitted

We present the charged-particle pseudorapidity density in Pb–Pb collisions at sNN=5.02 TeV in centrality classes measured by ALICE. The measurement covers a wide pseudorapidity range from −3.5 to 5, which is sufficient for reliable estimates of the total number of charged particles produced in the collisions. For the most central (0–5%) collisions we find 21400±1300, while for the most peripheral (80–90%) we find 230±38. This corresponds to an increase of (27±4)% over the results at sNN=2.76 TeV previously reported by ALICE. The energy dependence of the total number of charged particles produced in heavy-ion collisions is found to obey a modified power-law like behaviour. The charged-particle pseudorapidity density of the most central collisions is compared to model calculations — none of which fully describes the measured distribution. We also present an estimate of the rapidity density of charged particles. The width of that distribution is found to exhibit a remarkable proportionality to the beam rapidity, independent of the collision energy from the top SPS to LHC energies.