Application of FHCal for Heavy-Ion Collision Centrality Determination in MPD/NICA Experiment

Two approaches related to the centrality determination in heavy-ion Multi-Purpose Detector (MPD) experiments, using charge-particles multiplicity in Time Projection Chamber (TPC) and the energy deposition in Forward Hadron Calorimeter (FHCal) are discussed. The main features of the FHCal are the fine transverse segmentation and the beam holes in the center of the calorimeters. Leaking the heavy non-interacting fragments (spectators) leads to ambiguity in the dependence of energy deposition in the FHCal on the collision centrality. However, the calorimeter transverse segmentation allows one to measure the energy distributions in each of the FHCal modules and to construct combined observables to resolve the problems associated with the beam hole. The comparison of these approaches in the collision centrality measurements is discussed.

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.

About Transverse Momentum Distributions of Negative Pions in p12C and π–12C Collisions at High Energies

Collision centrality dependences of the transverse momentum distributions of negative pions produced in p12C and p−12C collisions at 9.9 and 40 GeV/c, respectively, are investigated. The shapes and widths of the normalized transverse momentum distributions, as well as the average values of the transverse momentum of the negative pions, do not depend within the uncertainties on the collision centrality in p12C and п−12C collisions at 9.9 and 40 GeV/c in experiment, which is confirmed by the modified FRITIOF model calculations and minimum X2 fits of the experimental spectra with the two- and three-temperature Hagedorn model functions. Modified FRITIOF model calculations underestimate the average values of transverse momenta of the negative pions in experiment and do not reproduce the tail of the experimental transverse momentum distributions of the negative pions in both collision types. It is obtained that the transverse momentum distributions of the negative pions exhibit two temperature (two-slope) shapes in peripheral and central p12C collisions at 9.9 GeV/c in agreement with the early works on nucleus-nucleus collisions at incident energies of the order of a few GeV/c. However, the transverse momentum distributions of the negative pions in peripheral and central p−12C collisions at 40 GeV/c can be described well assuming the three temperature (three-slope) shapes of these spectra.

Kaon Isospin Fluctuations in ALICE at the LHC

The first measurements of isospin fluctuations in the kaon sector is presented in Pb-Pb collisions at s N N = 2.76 TeV. A robust statistical observable was used to extract the isospin fluctuations of neutral and charged kaons as a function of collision centrality. The results show a significant variation in the behaviour of ν d y n in data when compared to Monte-Carlo models such as HIJING and AMPT. The deviation from 1/n scaling in data indicates possible isospin fluctuation in the kaon sector in heavy ion collisions.

Centrality Dependences of Characteristics of Negative Pions in π− +12C Interactions at 40 GeV/c

The collision centrality dependences of the mean multiplicities and kinematical characteristics of negative pions produced in п−+12C collisions at 40 GeV/с are investigated. It is obtained that the average values of the partial inelasticity coefficients of negative pions (without “leading” pions) do not depend on the п−+12C collision centrality. It is shown that the normalized transverse momentum distributions, as well as the average values of the transverse momentum of negative pions, do not depend within the uncertainties on the п−+12C collision centrality. The average value of the emission angle of the negative pions was found to increase with the п−+12C collision centrality. The comparison of the normalized emission angle distributions of the negative pions in the peripheral and central п−+12C collisions at 40 GeV/с supported the assumption that such an increase in the average emission angle is likely due to an increase in the number of binary collisions and multiple rescatterings with the п−+12C collision centrality. The obtained experimental findings are reproduced qualitatively well by the Modified FRITIOF model calculations.

Strongly Intensive Observables in the Model with String Fusion

We calculate the strongly intensive observables for multiplicities in two rapidity windows in the model with independent identical strings taking into account the charge sign of particles. We express the observables through the string pair correlation functions describing the correlations between the same and opposite sign particles produced in a string decay. We extract these charge-wise string two-particle correlation functions from the ALICE data on the forward-backward correlations and the balance function. Using them we predict the behavior of the charge-wise strongly intensive observables in the model with independent identical strings. We also show that the observable between multiplicities in two acceptance windows separated in rapidity, which is a strongly intensive in the case with independent identical strings, loses this property, when we take into account string fusion effects and a formation of strings of a few different types takes place in a collision. We predict the changes in the behaviour of this observable with energy and collision centrality, arising due to the string fusion phenomena.

Collision centrality dependencies of charged pion production in 12C+ 181 Ta collisions at 4.2 A GeV/c

The collision centrality dependencies of the average kinematical characteristics of the negative and positive pions, produced in [Formula: see text] collisions at [Formula: see text], were investigated. The ratio [Formula: see text] proved to be [Formula: see text], [Formula: see text] and [Formula: see text] in the peripheral, semicentral, and central [Formula: see text] collision events, respectively, decreasing noticeably with increasing collision centrality. The suppression (decrease) of the ratio [Formula: see text] was observed in the semicentral and central [Formula: see text] collisions as compared to the ratio [Formula: see text], estimated using the simple model for [Formula: see text] collision system. The ratio [Formula: see text] estimated using the simple model agreed well with the corresponding ratio [Formula: see text], estimated for [Formula: see text] collisions at [Formula: see text] based on the Wounded Nucleon Model (WNM). Comparison of the emission angle as well as momentum distributions of the charged pions in the peripheral and central [Formula: see text] collisions revealed the significant decrease of the fraction of the relatively fast charged pions (with smaller emission angles) and increase of the fraction of the relatively slow charged pions (with larger emission angles) with an increase in collision centrality. The results of the present analysis can be useful for analysis of the centrality dependence of the charged pion production in heavy ion collisions at high energies.

The Construction and Parameters of Forward Hadron Calorimeter (FHCAL) at MPD/NICA

Forward hadron calorimeter (FHCAL) at MPD/NICA experimental setup is intended for the measurements of the geometry of heavy ions collisions, namely, the collision centrality and the orientation of the reaction plane. FHCAL consists of two identical arms placed at the left/right sides from the beam collision point. This is a modular lead- scintillator compensating calorimeter designed to measure the energy distribution of the projectile nuclei fragments (spectators) and forward going particles close to the beam rapidity.

Centrality dependences of soft and hard components of pt distributions of negative pions in 4He+12C collisions at 4.2A GeV/c

The dependences of the shapes of transverse momentum distributions of the negative pions, produced in minimum bias 4 He +12 C collisions at a momentum of 4.2 GeV/c per nucleon, on collision centrality and fitting range of pt were analyzed systematically. To study the change in slopes (temperatures) of the pt spectra of π- with changing collision centrality and fitting pt range, the pt spectra, extracted from the experimental data and quark–gluon string model (QGSM) calculations, were fitted by the one- and two-temperature Hagedorn and Boltzmann functions. Fitting of the experimental pt distributions of π- in both the whole pt and pt = 0.1–1.2 GeV/c intervals required the two-temperature functions for adequate description of spectra, in agreement with the previous findings for different sets of colliding nuclei and various energies. On the whole, the absolute values of the extracted temperatures were lower in case of fitting range pt = 0.1–0.7 GeV/c as compared to the fitting interval pt = 0.1–1.2 GeV/c. The one-temperature functions were sufficient for fitting satisfactorily the experimental pt distributions of the negative pions in range pt = 0.1–0.7 GeV/c. In contrast to the experimental temperatures, the extracted QGSM temperatures were not sensitive to collision centrality and fitting range of pt. The collision centrality dependences of the temperatures of soft (pt = 0.1–0.5 GeV/c) and hard (pt = 0.5–1.2 GeV/c) components of the experimental pt distributions of the negative pions in 4 He +12 C collisions at 4.2A GeV/c were studied separately. The extracted temperatures of both soft and hard components of pt distributions of π- depended on geometry (size) and degree of overlap of colliding nuclei in peripheral, semicentral and central 4 He +12 C collisions. The temperature of soft pt component of the negative pions was consistently larger in semicentral and central 4 He +12 C collisions than that in peripheral interactions. The temperature of hard pt component of π- in 4 He +12 C collisions decreased consistently with an increase in collision centrality. The physical interpretations of the observed centrality dependences of temperature (T) of soft and hard pt components of the negative pions in 4 He +12 C collisions were given.