Pseudorapidity distributions of charged particles as a function of mid- and forward rapidity multiplicities in pp collisions at $$\sqrt{s}$$ = 5.02, 7 and 13 TeV

The multiplicity dependence of the pseudorapidity density of charged particles in proton–proton (pp) collisions at centre-of-mass energies $$\sqrt{s}~=~5.02$$ s = 5.02 , 7 and 13 TeV measured by ALICE is reported. The analysis relies on track segments measured in the midrapidity range ($$|\eta | < 1.5$$ | η | < 1.5 ). Results are presented for inelastic events having at least one charged particle produced in the pseudorapidity interval $$|\eta |<1$$ | η | < 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, $$\eta /s$$ η / 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, $$G_2$$ 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 $$G_2$$ 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 ($${\mathrm{d}}N_{\mathrm{ch}}$$ d N ch /d$$\eta )^{1/3}$$ η ) 1 / 3 . Estimates of $$\eta /s$$ η / s based on ALICE data exhibit little to no dependence on collision centrality at LHC energy, while estimates obtained from STAR data hint that $$\eta /s$$ η / s might be a function of collision centrality at top RHIC energy.

J/ψ 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 $$ \sqrt{s_{\mathrm{NN}}} $$ s 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.

Lifetime estimations from RHIC Au+Au data

We discuss a recently found family of exact and analytic, finite and accelerating, [Formula: see text]-dimensional solutions of perfect fluid relativistic hydrodynamics to describe the pseudorapidity densities and longitudinal HBT-radii and to estimate the lifetime parameter and the initial energy density of the expanding fireball in Au[Formula: see text]+[Formula: see text]Au collisions at RHIC with [Formula: see text] GeV and 200 GeV colliding energies. From these exact solutions of relativistic hydrodynamics, we derive a simple and powerful formula to describe the pseudorapidity density distributions in high-energy proton–proton and heavy-ion collisions, and derive the scaling of the longitudinal HBT radius parameter as a function of the pseudorapidity density. We improve upon several oversimplifications in Bjorken’s famous initial energy density estimate, and apply our results to estimate the initial energy densities of high-energy reactions with data-driven pseudorapidity distributions. When compared to similar estimates at the LHC energies, our results indicate a surprising and nonmonotonic dependence of the initial energy density on the energy of heavy-ion collisions.

Data structure in the HIJING 2.0 Monte Carlo program

The Heavy-Ion Jet Interaction Generator (HIJING) Monte Carlo model was developed to simulate hadron production in proton–proton, proton–nucleus and nucleus–nucleus collisions. It has been updated recently with the latest parton distributions functions (PDFs) and new set of the parameters in the two-component mini-jet model that controls total [Formula: see text] cross-section and the central pseudorapidity density. We will discuss these new elements in the HIJING 2.0 model, derive the two-component model from the eikonal formalism of hadron–hadron collisions and review the hadron spectra and multiplicity distributions as compared to recent experimental data at the LHC energies. We will review in particular the data structure of the Monte Carlo program and discuss future improvements.

Viscous Hydrodynamic Description of the Pseudorapidity Density and Energy Density Estimation for Pb+Pb and Xe+Xe Collisions at the LHC

Based on the analytical solution of accelerating relativistic viscous fluid hydrodynamics and Buda–Lund model, the pseudorapidity distributions of the most central Pb+Pb and Xe+Xe collisions are presented. Inspired by the CNC model, a modified energy density estimation formula is presented to investigate the dependence of the initial energy density estimation on the viscous effect. This new energy density estimation formula shows that the bulk energy is deposited to the neighboring fluid cells in the presence of the shear viscosity and bulk viscosity. In contrast to the well-known CNC energy density estimation formula, a 4.9% enhancement of the estimated energy density at the LHC kinematics is shown.