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Register a new userFluctuations as probe of the QCD phase transition and freeze-out in heavy ion collisions at LHC and RHIC
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We discuss the relevance of higher order moments of net baryon number fluctuations for the analysis of freeze-out and critical conditions in heavy ion collisions at LHC and RHIC. Using properties of O(4) scaling functions, we discuss the generic structure of these higher moments at vanishing baryon chemical potential and apply chiral model calculations to explore their properties at non-zero baryon chemical potential. We show that the ratios of the sixth to second and eighth to second order moments of the net baryon number fluctuations change rapidly in the transition region of the QCD phase diagram. Already at vanishing baryon chemical potential they deviate considerably from the predictions of the hadron resonance gas model which reproduce the second to fourth order moments of the net proton number fluctuations at RHIC. We point out that the sixth order moments of baryon number and electric charge fluctuations remain negative at the chiral transition temperature. Thus, they offer the possibility to probe the proximity of the thermal freeze-out to the crossover line.
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The experimental data from the RHIC and LHC experiments of invariant pT spectra in A+A and p + p collisions are analysed with Tsallis distributions in different approaches. The information about the freeze-out surface in terms of freeze-out volume, temperature, chemical potential and radial flow velocity for different particle species are obtained. Further, these parameters are studied as a function of the mass of the secondary particles. A mass-dependent differential freeze-out is observed which does not seem to distinguish between particles and their antiparticles. Further a mass-hierarchy in the radial flow is observed, meaning heavier particles suffer lower radial flow. Tsallis distribution function at finite chemical potential is used to study the mass dependence of chemical potential. The peripheral heavy-ion and proton-proton collisions at the same energies seem to be equivalent in terms of the extracted thermodynamic parameters.
We present a determination of chemical freeze-out conditions in heavy ion collisions based on ratios of cumulants of net electric charge fluctuations. These ratios can reliably be calculated in lattice QCD for a wide range of chemical potential values by using a next-to-leading order Taylor series expansion around the limit of vanishing baryon, electric charge and strangeness chemical potentials. From a computation of up to fourth order cumulants and charge correlations we first determine the strangeness and electric charge chemical potentials that characterize freeze-out conditions in a heavy ion collision and confirm that in the temperature range 150 MeV < T < 170 MeV the hadron resonance gas model provides good approximations for these parameters that agree with QCD calculations on the (5-15)% level. We then show that a comparison of lattice QCD results for ratios of up to third order cumulants of electric charge fluctuations with experimental results allows to extract the freeze-out baryon chemical potential and the freeze-out temperature.
High energy heavy-ion collisions in laboratory produce a form of matter that can test Quantum Chromodynamics (QCD), the theory of strong interactions, at high temperatures. One of the exciting possibilities is the existence of thermodynamically distinct states of QCD, particularly a phase of de-confined quarks and gluons. An important step in establishing this new state of QCD is to demonstrate that the system has attained thermal equilibrium. We present a test of thermal equilibrium by checking that the mean hadron yields produced in the small impact parameter collisions as well as grand canonical fluctuations of conserved quantities give consistent temperature and baryon chemical potential for the last scattering surface. This consistency for moments up to third order of the net-baryon number, charge, and strangeness is a key step in the proof that the QCD matter produced in heavy-ion collision attains thermal equilibrium. It is a clear indication for the first time, using fluctuation observables, that a femto-scale system attains thermalization. The study also indicates that the relaxation time scales for the system are comparable to or smaller than the life time of the fireball.
We calculate the mean and variance of net-baryon number and net-electric charge distributions from Quantum Chromodynamics (QCD) using a next-to-leading order Taylor expansion in terms of temperature and chemical potentials. We compare these expansions with experimental data from STAR and PHENIX, determine the freeze-out temperature in the limit of vanishing baryon chemical potential, and, for the first time, constrain the curvature of the freeze-out line through a direct comparison between experimental data on net-charge fluctuations and a QCD calculation. We obtain a bound on the curvature coefficient, kappa_2^f < 0.011, that is compatible with lattice QCD results on the curvature of the QCD transition line.
The LHC data on event-by-event harmonic flow coefficients measured in PbPb collisions at center-of-mass energy 2.76 TeV per nucleon pair are analyzed and interpreted within the HYDJET++ model. To compare the model results with the experimental data the unfolding procedure is employed. The essentially dynamical origin of the flow fluctuations in hydro-inspired freeze-out approach has been established. It is shown that the simple modification of the model via introducing the distribution over spatial anisotropy parameters permits HYDJET++ to reproduce both elliptic and triangular flow fluctuations and related to it eccentricity fluctuations of the initial state at the LHC energy.
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