arXiv subjects

M. Vepsalainen

Publications and source records attributed to M. Vepsalainen.

15 recordsLinked to original sources

arXiv2013

Dimensionally regularized Polyakov loop correlators in hot QCD

A popular observable in finite-temperature lattice QCD is the so-called singlet quark-antiquark free energy, conventionally defined in Coulomb gauge. In an effort to interpret the existing numerical data on this observable, we compute it at order O(alpha_s^2) in continuum, and analyze the result at various distance scales. At short distances (r << 1/pi T) the behaviour matches that of the gauge-independent zero-temperature potential; on the other hand at large distances (r >> 1/pi T) the singlet free energy appears to have a gauge-fixing related power-law tail. At infinite distance the result again becomes physical in the sense that it goes over to a gauge-independent disconnected contribution, the square of the expectation value of the trace of the Polyakov loop; we recompute this quantity at O(alpha_s^2), finding for pure SU(Nc) a different non-logarithmic term than in previous literature, and adding for full QCD the quark contribution. We also discuss the value of the singlet free energy in a general covariant gauge, as well as the behaviour of the cyclic Wilson loop that is obtained if the singlet free energy is made gauge-independent by inserting straight spacelike Wilson lines into the observable. Comparisons with lattice data are carried out where possible.

hep-ph↗

arXiv2011

The ultraviolet limit and sum rule for the shear correlator in hot Yang-Mills theory

We determine a next-to-leading order result for the correlator of the shear stress operator in high-temperature Yang-Mills theory. The computation is performed via an ultraviolet expansion, valid in the limit of small distances or large momenta, and the result is used for writing operator product expansions for the Euclidean momentum and coordinate space correlators as well as for the Minkowskian spectral density. In addition, our results enable us to confirm and refine a shear sum rule originally derived by Romatschke, Son and Meyer.

hep-ph↗

arXiv2011

Frequency and wave number dependence of the shear correlator in strongly coupled hot Yang-Mills theory

We use AdS/QCD duality to compute the finite temperature Green's function G(omega,k;T) of the shear operator T_12 for all omega,k in hot Yang-Mills theory. The goal is to assess how the existence of scales like the transition temperature and glueball masses affects the correlator computed in the scalefree conformal N=4 supersymmetric Yang-Mills theory. We observe sizeable effects for T close to T_c which rapidly disappear with increasing T. Quantitative agreement of these predictions with future lattice Monte Carlo data would suggest that QCD matter in this temperature range is strongly interacting.

hep-ph↗

arXiv2010

Spatial scalar correlator in strongly coupled hot N=4 Yang-Mills theory

We use AdS/CFT duality to compute in N=4 Yang-Mills theory the finite temperature spatial correlator G(r) of the scalar operator F^2, integrated over imaginary time. The computation is carried out both at zero frequency and integrating the spectral function over frequencies. The result is compared with a perturbative computation in finite T SU(N_c) Yang-Mills theory.

hep-th↗

arXiv2010

Intermediate distance correlators in hot Yang-Mills theory

Lattice measurements of spatial correlation functions of the operators FF and FF-dual in thermal SU(3) gauge theory have revealed a clear difference between the two channels at "intermediate" distances, x ~ 1/(pi T). This is at odds with the AdS/CFT limit which predicts the results to coincide. On the other hand, an OPE analysis at short distances (x << 1/(pi T)) as well as effective theory methods at long distances (x >> 1/(pi T)) suggest differences. Here we study the situation at intermediate distances by determining the time-averaged spatial correlators through a 2-loop computation. We do find unequal results, however the numerical disparity is small. Apart from theoretical issues, a future comparison of our results with time-averaged lattice measurements might also be of phenomenological interest in that understanding the convergence of the weak-coupling series at intermediate distances may bear on studies of the thermal broadening of heavy quarkonium resonances.

hep-ph↗

arXiv2010

Ultraviolet asymptotics of scalar and pseudoscalar correlators in hot Yang-Mills theory

Inspired by recent lattice measurements, we determine the short-distance (a << r << 1/pi T) as well as large-frequency (1/a >> omega >> pi T) asymptotics of scalar (trace anomaly) and pseudoscalar (topological charge density) correlators at 2-loop order in hot Yang-Mills theory. The results are expressed in the form of an Operator Product Expansion. We confirm and refine the determination of a number of Wilson coefficients; however some discrepancies with recent literature are detected as well, and employing the correct values might help, on the qualitative level, to understand some of the features observed in the lattice measurements. On the other hand, the Wilson coefficients show slow convergence and it appears uncertain whether this approach can lead to quantitative comparisons with lattice data. Nevertheless, as we outline, our general results might serve as theoretical starting points for a number of perhaps phenomenologically more successful lines of investigation.

hep-ph↗

arXiv2009

Quarkonium dissociation in the presence of a small momentum space anisotropy

We consider the dissociation of heavy quarkonium in a medium close to thermal equilibrium but with a small momentum space anisotropy. Dissociation is defined to take place when the width of the ground state equals its binding energy. We show that if the anisotropic medium is obtained isentropically from the equilibrium one, then to first order in the anisotropy parameter the dissociation temperature remains unchanged. If, in contrast, the non-equilibrium system has a smaller entropy density than the equilibrium one, then the dissociation temperature increases with respect to the isotropic case, by up to 10% for modest anisotropies.

hep-ph↗

arXiv2009

HYDJET++ heavy ion event generator and its applications for RHIC and LHC

The heavy ion event generator HYDJET++ is presented. HYDJET++ simulates relativistic heavy ion AA collisions as a superposition of the soft, hydro-type state and the hard state resulting from multi-parton fragmentation. This model is the development and continuation of HYDJET event generator. The hard parts of HYDJET and HYDJET++ are identical. The soft part of HYDJET++ contains the following important additional features as compared with HYDJET: resonance decays and more detailed treatment of thermal and chemical freeze-out hypersurfaces. HYDJET++ is capable of reproducing the bulk properties of heavy ion collisions at RHIC (hadron spectra and ratios, radial and elliptic flow, femtoscopic momentum correlations), as well as high-pT hadron spectra. Some applications of HYDJET++ at LHC are discussed.

hep-ph↗

arXiv2009

Heavy quark medium polarization at next-to-leading order

We compute the imaginary part of the heavy quark contribution to the photon polarization tensor, i.e. the quarkonium spectral function in the vector channel, at next-to-leading order in thermal QCD. Matching our result, which is valid sufficiently far away from the two-quark threshold, with a previously determined resummed expression, which is valid close to the threshold, we obtain a phenomenological estimate for the spectral function valid for all non-zero energies. In particular, the new expression allows to fix the overall normalization of the previous resummed one. Our result may be helpful for lattice reconstructions of the spectral function (near the continuum limit), which necessitate its high energy behaviour as input, and can in principle also be compared with the dilepton production rate measured in heavy ion collision experiments. In an appendix analogous results are given for the scalar channel.

hep-ph↗

arXiv2008

Heavy quarkonium in any channel in resummed hot QCD

We elaborate on the fact that quarkonium in hot QCD should not be thought of as a stationary bound state in a temperature-dependent real potential, but as a short-lived transient, with an exponentially decaying wave function. The reason is the existence of an imaginary part in the pertinent static potential, signalling the ``disappearance'', due to inelastic scatterings with hard particles in the plasma, of the off-shell gluons that bind the quarks together. By solving the corresponding Schrödinger equation, we estimate numerically the near-threshold spectral functions in scalar, pseudoscalar, vector and axial vector channels, as a function of the temperature and of the heavy quark mass. In particular, we point out a subtlety in the determination of the scalar channel spectral function and, resolving it to the best of our understanding, suggest that at least in the bottomonium case, a resonance peak can be observed also in the scalar channel, even though it is strongly suppressed with respect to the peak in the vector channel. Finally, we plot the physical dilepton production rate, stressing that despite the eventual disappearance of the resonance peak from the corresponding spectral function, the quarkonium contribution to the dilepton rate becomes more pronounced with increasing temperature, because of the yield from free heavy quarks.

hep-ph↗

arXiv2007

Applications of dimensional reduction to electroweak and QCD matter

This paper is a slightly modified version of the introductory part of a doctoral dissertation also containing the articles hep-ph/0311268, hep-ph/0510375, hep-ph/0512177 and hep-ph/0701250. The thesis discusses effective field theory methods, in particular dimensional reduction, in the context of finite temperature field theory. We first briefly review the formalism of thermal field theory and show how dimensional reduction emerges as the high-temperature limit for static quantities. Then we apply dimensional reduction to two distinct problems, the pressure of electroweak theory and the screening masses of mesonic operators in hot QCD, and point out the similarities. We summarize the results and discuss their validity, while leaving all details to original research articles.

hep-ph↗

arXiv2007

Mesonic screening masses at high temperature and finite density

We compute the first perturbative correction to the static correlation lengths of light quark bilinears in hot QCD with finite quark chemical potentials. The correction is small and positive, with mu-dependence depending on the relative sign of chemical potentials and the number of dynamical flavors. The computation is carried out using a three-dimensional effective theory for the lowest fermionic Matsubara mode. We also compute the full correlator in free theory and find a rather complicated general mu-dependence at shorter distances. Finally, rough comparisons with lattice simulations are discussed.

hep-ph↗

arXiv2006

Pressure of the Standard Model Near the Electroweak Phase Transition

We extend our previous determination of the thermodynamic pressure of the Standard Model so that the result can be applied down to temperatures corresponding to the electroweak crossover. This requires a further resummation which can be cleanly organised within the effective theory framework. The result allows for a precise determination of the expansion rate of the Universe for temperatures around the electroweak crossover.

hep-ph↗

arXiv2005

Pressure of the Standard Model at High Temperatures

We compute the pressure of the standard model at high temperatures in the symmetric phase to three loops, or to O(g^5) in all coupling constants. We find that the terms of the perturbative expansion in the SU(2) + Higgs sector decrease monotonically with increasing order, but the large values of the strong coupling constant g_s and the Yukawa coupling of the top quark g_Y make the expansion in the full theory converge more slowly. The final result is observed to be about 10% smaller than the ideal gas pressure commonly used in cosmological calculations.

hep-ph↗

arXiv2004

Mesonic correlation lengths in high-temperature QCD

We consider spatial correlation lengths ξfor various QCD light quark bilinears at temperatures above a few hundred MeV. Some of the correlation lengths (such as that related to baryon density) coincide with what has been measured earlier on from glueball-like states; others do not couple to glueballs, and have a well-known perturbative leading-order expression as well as a computable next-to-leading-order correction. We determine the latter following analogies with the NRQCD effective theory, used for the study of heavy quarkonia at zero temperature: we find (for the quenched case) ξ^{-1} = 2 πT + 0.1408 g^2 T, and compare with lattice results. One manifestation of U_A(1) symmetry non-restoration is also pointed out.

hep-ph↗