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Register a new userFluctuations at finite temperature and density
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Authors
Szabolcs Borsanyi
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Fluctuations of conserved charges in a grand canonical ensemble can be calculated as derivatives of the free energy with respect to the respective chemical potential. They are directly related to experimentally available observables that describe the hadronization in heavy ion collisions. The same derivatives can be used to extrapolate zero density results to finite chemical potential. We review the recent lattice calculations in the staggered formalism and discuss its implications to phenomenology and resummed perturbation theory.
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Fluctuations of conserved charges are sensitive to the QCD phase transition and a possible critical endpoint in the phase diagram at finite density. In this work, we compute the baryon number fluctuations up to tenth order at finite temperature and density. This is done in a QCD-assisted effective theory that accurately captures the quantum- and in-medium effects of QCD at low energies. A direct computation at finite density allows us to assess the applicability of expansions around vanishing density. By using different freeze-out scenarios in heavy-ion collisions, we translate these results into baryon number fluctuations as a function of collision energy. We show that a non-monotonic energy dependence of baryon number fluctuations can arise in the non-critical crossover region of the phase diagram. Our results compare well with recent experimental measurements of the kurtosis and the sixth-order cumulant of the net-proton distribution from the STAR collaboration. They indicate that the experimentally observed non-monotonic energy dependence of fourth-order net-proton fluctuations is highly non-trivial. It could be an experimental signature of an increasingly sharp chiral crossover and may indicate a QCD critical point. The physics implications and necessary upgrades of our analysis are discussed in detail.
We study the phase structure of lattice QCD with heavy quarks at finite temperature and density by a histogram method. We determine the location of the critical point at which the first-order deconfining transition in the heavy-quark limit turns into a crossover at intermediate quark masses through a change of the shape of the histogram under variation of coupling parameters. We estimate the effect of the complex phase factor which causes the sign problem at finite density, and show that, in heavy-quark QCD, the effect is small around the critical point. We determine the critical surface in 2+1 flavor QCD in the heavy-quark region at all values of the chemical potential mu including mu=infty.
Neither the chiral limit nor finite baryon density can be simulated directly in lattice QCD, which severely limits our understanding of the QCD phase diagram. In this review I collect results for the phase structure in an extended parameter space of QCD, with varying numbers of flavours, quark masses, colours, lattice spacings, imaginary and isospin chemical potentials. Such studies help in understanding the underlying symmetries and degrees of freedom, and are beginning to provide a consistent picture constraining the possibilities for the physical phase diagram.
QCD matter at finite temperature and density is a subject that has witnessed very impressive theoretical developments in the recent years. In this review I will discuss some new insights on the microscopic degrees of freedom of the QCD medium near the chiral crossover transition from lattice QCD. Latest high precision lattice data on the fluctuations and correlations between conserved charges like the baryon number, strangeness can help us to understand and distinguish between different models of interacting hadrons. Furthermore, the latest constraints on the location of the critical end-point and the curvature of the critical line will be discussed. In the later part of this review I will discuss about the insights on the thermal nature of the medium created in heavy ion collision experiments that have come from the theoretical analysis of the particle yields, and to what extent the lattice data on correlations and fluctuations of conserved charges can give us any information about the fireball at freezeout.
Meson properties at finite temperature and density are studied in lattice QCD simulations with two-flavor Wilson fermions. For this purpose, we investigate screening masses of mesons in pseudo-scalar (PS) and vector (V) channels. The simulations are performed on $16^3times 4$ lattice along the lines of constant physics at $m_{rm PS}/m_{rm V}|_{T=0}=0.65$ and 0.80, where $m_{rm PS}/m_{rm V}|_{T=0}$ is a ratio of meson masses in PS and V channels at $T=0$. A temperature range is $T/T_{rm pc}=(0.8 - 4.0)$, where $T_{rm pc}$ is the pseudo-critical temperature. We find that the temperature dependence of the screening masses normalized by temperature, $M_0/T$, shows notable structure around $T_{rm pc}$, and approach $2pi$ at high temperature in both channels, which is consistent with twice the thermal mass of a free quark in high temperature limit. The screening masses at low density are also investigated by using the Taylor expansion method with respect to the quark chemical potential. We find that the expansion coefficients in the leading order become positive in the temperature range, and thermal and density effect on the meson screening-masses becomes apparent in the quark-gluon plasma phase. The meson screening-masses are also compared with the gluon (Debye) screening masses at finite temperature and density.
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