arXiv subjects

E. V. Shuryak

Publications and source records attributed to E. V. Shuryak.

At least 73 records · Page 4Linked to original sources

arXiv1997

Physics of Heavy Ions Collisions: The Summary of Moriond-97

CERN dilepton experiments have provided the most exciting data. Strong enhancement at low masses observed by CERES and HELIOS3 indicate strong modification in the vector channel in matter compared to vacuum properties. NA50 data on $J/ψ$ suppression in PbPb collisions show surprising deviation from the previous trend. The question is whether it is the expected early-time signal of QGP, or due to late-time hadronic interactions. Theoretical and experimental suggestions have been made to resolve this issue. BNL and SPS experiments have also provided rather complete data with heavy beams (Au and Pb, respectively). Very strong collective flow effects have been observed at both energies, which allow for the first time to restrict the EOS of the hadronic matter. Several observables (flow, Coulomb effects and HBT) suggest rather long evolution of systems created in heavy ion collisions and very low freeze-out densities relative to previous studies. Theory of jet stopping in QGP is becoming quantitative.

hep-ph↗

arXiv1997

The QCD Phase Transition: From the Microscopic Mechanism to Signals Talk at Renconres de Moriond - 97

This talk consists of two very different parts: the first one deals with non-perturbative QCD and physics of the chiral restoration, the second with rather low-key (and still unfinished) work aiming at obtaining EOS and other properties of hot/dense hadronic matter from data on heavy ion collisions. The microscopic mechanism for chiral restoration phase transition is a transition from randomly placed tunneling events (instantons) at low T to a set of strongly correlated tunneling-anti-tunneling events (known as instanton - anti-instanton molecules) at high T. Many features of the transition can be explained in this simple picture, especially the critical line and its dependence on quark masses. This scenario predicts qualitative change of the basic quark-quark interactions around the phase transition line, with some states (such as pion-sigma ones) probably surviving even at $T>T_c$. In the second half of the talk we discuss experimental data on collective flow in heavy ion collision, its hydro-based description and relation to equation of state (EOS). A distinct feature of the QCD phase transition region is high degree of ``softness'', (small ratio pressure/energy density). We present some preliminary results indicated that it is indeed needed to explain the radial flow at SPS energies.

hep-ph↗

arXiv1996

Dilepton/Photon Production in Heavy Ion Collisions and the QCD Phase Transition

We calculate electromagnetic production from highly excited hadronic matter created in heavy ion collisions. The rates include the usual lowest order processes in quark-gluon plasma plus the usual reactions in the hadronic phase, related with rho,a_1 mesons. The space-time integration is done using a hydrodynamical model. We have found that dilepton mass spectrum agrees with results of other previous works, but *disagrees* with the CERES dilepton data. In order to explain these data, some "unconventional" production mechanism need to be incorporated: we discuss especially the notion of modified rho,a_1 masses, which indeed may explain the data. Other suggestions (e.g. the longer-lived fireball) to increase the production of low mass dileptons seem to be insufficient for the task. The results for direct photon production are below the current WA80 experimental bounds, for all variants considered.

hep-ph↗

arXiv1995

Hadronic Correlation Functions in the Interacting Instanton Liquid

We study hadronic correlation functions in the interacting instanton liquid model, both at zero and nonzero temperature $T$. At zero $T$ we investigate the dependence of the correlators on the instanton ensemble, in particular the effect of the fermionic determinant. We demonstrate that quark-induced correlations between instantons are important, especially in the repulsive $η'$ and $δ$-meson channels. We also calculate a large number of mesonic and baryonic correlation functions as a function of temperature. We find three different types of behavior as $T\to T_c$. The vector channels $ρ,a_1,Δ$ show a gradual melting of the resonance contribution and approach free quark behavior near the chiral phase transition. The light pseudoscalars and scalars $π,σ$, as well as the nucleon show stable resonance contributions, probably even surviving above $T_c$. Correlation functions in the heavy scalar channels $η',δ$ are enhanced as $T\to T_c$.

hep-ph↗

arXiv1995

Bose Condensate made of Atoms with Attractive Interaction is Metastable

Recent experiments with trapped cooled atoms have produced evidences for Bose-Einstein condensation. Among the atoms used are $^7Li$ ones, with attractive low energy interaction. Stability of their condensate is studied, with a conclusion that for a number of atoms below the critical value there exist a metastable minimum, separated from collapse by a potential barrier. The lifetime due to tunneling is estimated: it is unlikely to happen in the present experiment. Although in experimental conditions most of the atoms are not in the condensate, we argue that they only slightly affect the stability conditions for the condensate evaluated at T=0.

cond-mat↗

arXiv1995

The instanton liquid in QCD at zero and finite temperature

In this paper we study the statistical mechanics of the instanton liquid in QCD. After introducing the partition function as well as the gauge field and quark induced interactions between instantons we describe a method to calculate the free energy of the instanton system. We use this method to determine the equilibrium density and the equation of state from numerical simulations of the instanton ensemble in QCD for various numbers of flavors. We find that there is a critical number of flavors above which chiral symmetry is restored in the groundstate. In the physical case of two light and one intermediate mass flavor the system undergoes a chiral phase transition at $T\simeq 140$ MeV. We show that the mechanism for this transition is a rearrangement of the instanton liquid, going from a disordered, random, phase at low temperatures to a strongly correlated, molecular, phase at high temperature. We also study the behavior of mesonic susceptibilities near the phase transition.

hep-ph↗

arXiv1995

Glueballs and Instantons

We study correlation functions and Bethe Salpeter amplitudes for the scalar, the pseudoscalar and the tensor glueballs using an instanton-based model of the QCD vacuum. We consider both the pure gauge case and the situation for real QCD with two light quark flavors. We show that instantons lead to a strong modification of the correlation functions as compared to their perturbative behavior. In particular, we find a strong attractive force in the $J^{CP}=0^{++}$ channel and repulsion in the $0^{+-}$ channel. Due to the strong classical field of the instantons, these effects are much larger than the spin splittings observed in mesons made of quarks. The resulting masses, coupling constants and wave functions appear to be in agreement with lattice gauge simulations.

hep-ph↗

arXiv1995

Can hadrons survive the chiral phase transition ?

We study mesonic and baryonic correlation functions in the temporal directions in the vicinity of the critical temperature $T\simeq T_c$ using the instanton liquid model. In this model chiral symmetry restoration is driven by the formation of instanton-antiinstanton molecules. Although we find the signals for all hadronic poles to be drastically reduced, some hadronic states seem to survive the phase transition, as loosely bound $U(N_f)\times U(N_f)$ chiral multiplets in the plasma phase. These states are the $0^+ - 0^-$ mesons $(π,σ,η',δ)$ and the $N(\frac{1}{2}^+) - N(\frac{1}{2}^-)$ baryons.

hep-ph↗

arXiv1995

Instanton size distribution: repulsion or the infrared fixed point?

We discuss available information about the instanton size distribution $d(ρ)$, which comes from lattice simulations and the interacting instanton liquid model. Not only they are remarkably consistent, but one can also reproduce $d(ρ)$ with an alternative idea, based on the infrared fixed point. We show, that lattice non-perturbative beta-function (for $bare$ charge renormalization) also suggests such a hint. We also discuss whether it is possible to reconcile the instanton physics with reasonable large-number-of-colors limit, concluding that our fits for SU(2) and SU(3) theories are actually very close to the critical value $8π^2/g^2= N_c*5.4$ when it is the case.

hep-ph↗

arXiv1994

Hydrodynamics near the QCD Phase Transition: Looking for the Longest-Lived Fireball

We propose a new strategy for the experimental search of the QCD phase transition in heavy ion collisions: One may tune collision energy around the point where the lifetime of the fireball is expected to be longest. We demonstrate that the hydrodynamic evolution of excited nuclear matter does change dramatically as the initial energy density goes through the "softest point" (where the pressure to energy density ratio reaches its minimum). For our choice of equation of state, this corresponds to epsilon_i approx. = 1.5 GeV/fm^3 and collision energy E_lab/A approx. = 30 GeV (for Au+Au). Various observables seem to show distinct changes near the softest point.

hep-ph↗

arXiv1994

Screening of the topological charge in a correlated instanton vacuum

Screening of the topological charge due to he fermion-induced interactions is an important phenomenon, closely related with the resolution of the strong CP and U(1) problems. We study the mechanism of such screening in a 'correlated instanton vacuum', as opposed to the 'random' one. Both scalar and pseudoscalar gluonic correlators are analyzed by means of an observable that minimizes finite size effects. Screening of the topological charge is established. This allows us to calculate the $η'$ mass without having to invert the Dirac operator. We suggest that this method might be used in lattice QCD calculations as well. Our results for the screening of the topological charge are in agreement with the chiral Ward identities, and the scalar gluonic correlator satisfies a low energy theorem first derived by Novikov et al. \cite{Novikov-etal}. We also propose to evaluate the topological susceptibility in the Witten-Veneziano formula not in an infinite box in an world $without$ fermions but in an infinitesimal box in a world $with$ fermions.

hep-lat↗

arXiv1994

Where the excess photons and dileptons in SPS nuclear collisions come from?

Recently the first single photon spectra from CERN energy heavy-ion collisions were reported by WA80, while NA34/3 and NA38 have obtained the spectra for dileptons with the mass up to 4-5 GeV. The production rates for photons and dileptons significantly increase when reactions involving the $A_1$ meson are included. However, with the conventional expansion scenario, the absolute yields are still significantly smaller than the observed ones. It may indicate that expansion in the ``mixed state" takes much more time.

hep-ph↗

arXiv1994

Quark-Induced Correlations between Instantons Drive the Chiral Phase Transition

A simple model for the instanton ensemble at finite temperature T is proposed, including ``random" and strongly correlated ``molecular" component. T-dependence of fermionic zero modes naturally leads to chiral symmetry restoration, without instanton suppression. Moreover, at $T=(1-2) T_c$ the non-perturbative effects due to ``molecules" are so strong, that they even dominate the global thermodynamics. % two figures are enclosed as .ps files

hep-ph↗

arXiv1994

Hadronic Wave Functions in the Instanton Model

We study hadronic wave functions using an instanton model for the QCD vacuum. The wave functions are defined in terms of gauge invariant Bethe Salpeter amplitudes which we have determined numerically using a Monte Carlo simulation of the instanton ensemble. We find that the pion and the proton, as well as the rho meson and the delta have very similar wave functions but observe a sizeable splitting between mesons or baryons with different spin. We compare our results with data obtained in lattice gauge simulations.

hep-ph↗

arXiv1993

Weinberg-Type Sum Rules at Zero and Finite Temperature

We consider sum rules of the Weinberg type at zero and nonzero temperatures. On the basis of the operator product expansion at zero temperature we obtain a new sum rule which involves the average of a four-quark operator on one side and experimentally measured spectral densities on the other. We further generalize the sum rules to finite temperature. These involve transverse and longitudinal spectral densities at each value of the momentum. Various scenarios for the relation between chiral symmetry restoration and these finite temperature sum rules are discussed.

hep-ph↗

arXiv1993

Baryonic Correlation Functions in the Random Instanton Vacuum

We study baryon and diquark correlation functions in the framework of an instanton model for the QCD vacuum. The model naturally accomodates a light scalar diquark. As a consequence, the correlation functions for the nucleon and delta are found to be qualitatively different. Using a complete set of all available correlation functions we determine masses and coupling constants for these particles. We compare our results with predictions from QCD sum rules and recent lattice measurement of point-to-point correlators.

hep-ph↗

arXiv1993

Quark Propagation in the Random Instanton Vacuum

This is the first of a series of papers devoted to a systematic study of QCD correlation functions in a framework of 'instanton vacuum' models. The topic of this paper is to work out approximate formulae for quark propagators in a multi-instanton environment. As an application, and also as a necessary step toward understanding the correlation functions, we study the propagators of scalar and spinor quarks, using the simplest possible model, the so called 'random instanton vacuum' (RIV). Results related to heavy-light mesons, are found to be very consistent with phenomenology.

hep-ph↗

arXiv1993

Mesonic Correlation Functions in the Random Instanton Vacuum

A general model-independent discussion of mesonic correlation functions is given. We derive new inequalities, including one stronger than Weingarten's inequality. Mesonic correlation functions are calculated in the random instanton vacuum and are compared with phenomenological expectations and lattice results. Both diagonal and non-diagonal correlators of all strange and light flavored currents, as well as the most important unflavored ones are considered. Our results are used to extract the masses and the coupling constants of the corresponding mesons. Not only the qualitative behaviour is reproduced in all channels, but in several channels the model works with amazing accuracy

hep-ph↗