arXiv subjects

E. V. Shuryak

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

At least 37 records · Page 2Linked to original sources

arXiv2002

Matter-induced modification of resonances at RHIC freezeout

We discuss the physical effects causing a modification of resonance masses, widths and even shapes in a dilute hadronic gas at late stages of heavy ion collisions. We quantify the conditions at which resonances are produced at RHIC, and found that it happens at $T\approx 120 MeV$. Although in the pp case the ``kinematic'' effects like thermal weighting of the states is sufficient, in AA we see a clear effect of dynamical interaction with matter, both due to a variety of s-channel resonances and due to t-channel scalar exchanges. The particular quantity we focus mostly on is the $ρ$ meson mass, for which these dynamical effects lead to about -50 MeV shift, on top of about -20 MeV of a thermal effect: both agree well with preliminary data from STAR experiment at RHIC. We also predict a complete change of shape of $f_0(600)$ resonance, even by thermal effects alone.

hep-ph↗

arXiv2002

Instanton Contribution to the Proton and Neutron Electric Form Factors

We study the instanton contribution to the proton and neutron electric form factors. Using the single instanton approximation, we perform the calculations in a mixed time-momentum representation in order to obtain the form factors directly in momentum space. We find good agreement with the experimentally measured electric form factor of the proton. For the neutron, our result falls short of the experimental data. We argue that this discrepancy is due to the fact that we neglect the contribution of the sea quarks. We compare to lattice calculations and a relativistic version of the quark-diquark model.

hep-ph↗

arXiv2002

Instanton Contribution to the Pion Electro-Magnetic Formfactor at Q^2 > 1 GeV^2

We study the effects of instantons on the charged pion electro-magnetic formfactor at intermediate momenta. In the Single Instanton Approximation (SIA), we predict the pion formfactor in the kinematic region Q^2=2-15 GeV^2. By developing the calculation in a mixed time-momentum representation, it is possible to maximally reduce the model dependence and to calculate the formfactor directly. We find the intriguing result that the SIA calculation coincides with the vector dominance monopole form, up to surprisingly high momentum transfer Q^2~10 GeV^2. This suggests that vector dominance for the pion holds beyond low energy nuclear physics.

hep-ph↗

arXiv2002

The Azimuthal Asymmetry at large p_t seem to be too large for a ``Jet Quenching''

We discuss simple generic model of ``jet quenching'' in which matter absorption is defined by one parameter. We show that as absorption grows, the azimuthal asymmetry v_2 grows as well, reaching the finite limit with a simple geometric interpretation. It turns out, that this limit is still below the experimental values for 6 > p_t > 2 GeV, according to preliminary data from STAR experiment at RHIC. We thus conclude that ``jet quenching'' models alone cannot account for the observed phenomenon, and speculate about alternative scenarios.

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arXiv2002

Jet Quenching in High Energy Heavy Ion Collisions by QCD Synchrotron-like Radiation

We consider synchrotron-like radiation in QCD by generalizing Schwinger's treatment of quantum synchrotron radiation in QED to the case of a constant chromomagnetic field. We suggest a novel mechanism for {\em jet quenching} in heavy ion collisions, whereby high-$p_t$ partons get depleted through strong (classical) color fields. The latters are encountered in the color glass condensate or in the form of expanding shells of exploding sphalerons. Unlike bremsstrahlung radiation through multiple soft rescattering, synchrotron radiation converts a jet into a wide shower of soft gluons. We estimate the energy loss through this mechanism and suggest that it contributes significantly to the unexpectedly strong jet quenching observed at RHIC.

hep-ph↗

arXiv2002

Forced Tunneling and Turning State Explosion in Pure Yang-Mills Theory

We consider forced tunneling in QCD, described semiclassically by instanton-antiinstanton field configurations. By separating topologically different minima we obtain details of the effective potential and study the turning states, which are similar to the sphaleron solution in electroweak theory. These states are alternatively derived as minima of the energy under the constraints of fixed size and Chern-Simons number. We study, both analytically and numerically, the subsequent evolution of such states by solving the classical Yang-Mills equations in real time, and find that the gauge field strength is quickly localized into an expanding shell of radiating gluons. The relevance to high-energy collisions of hadrons and nuclei is briefly discussed.

hep-ph↗

arXiv2002

Anti-Baryon Puzzle in Ultra-Relativistic Heavy-Ion Collisions

The evolution of (non-strange) antibaryon abundances in the hadronic phase of central heavy-ion collisions is studied within a thermal equilibrium framework, based on the well-established picture of subsequent chemical and thermal freezeout. Due to large annihilation cross sections, antiprotons are, a priori, not expected to comply with this scheme. However, we show that a significant regeneration of their abundance occurs upon inclusion of the inverse reaction of multipion fusion, $n_ππ\to p\bar p$ (with $n_π$=5-6), necessary to ensure detailed balance. Especially at SPS energies, the build-up of large pion-chemical potentials between chemical and thermal freezeout reinforces this mechanism, rendering the $\bar p/p$ ratio in reasonable agreement with the observed one (reflecting chemical freezeout). Explicit solutions of the pertinent rate equation, which account for chemical off-equilibrium effects, corroborate this explanation.

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arXiv2002

Limits on Cosmological Variation of Strong Interaction and Quark Masses from Big Bang Nucleosynthesis, Cosmic, Laboratory and Oklo Data

Recent data on cosmological variation of the electromagnetic fine structure constant from distant quasar (QSO) absorption spectra have inspired a more general discussion of possible variation of other constants. We discuss variation of strong scale and quark masses. We derive the limits on their relative change from (i) primordial Big-Bang Nucleosynthesis (BBN); (ii) Oklo natural nuclear reactor, (iii) quasar absorption spectra, and (iv) laboratory measurements of hyperfine intervals.

hep-ph↗

arXiv2001

A Hydrodynamic Description of Heavy Ion Collisions at the SPS and RHIC

A hydrodynamic + cascade model of relativistic heavy ion collisions is presented and compared to available hadronic data from the SPS to RHIC. The model consistently reproduces the radial and elliptic flow data for different particles, collision energies, and impact parameters. Three ingredients are essential to the success: (a) a reasonable EOS exhibiting the hard and soft features of the QCD phase transition, (b) thermal hadronization at the phase boundary, and (c) subsequent hadronic rescattering. Some features of the RHIC data are readily explained: (i) the observed elliptic flow and its dependence on $p_{T}$ and mass, (ii) the anomalous $\bar{p}/π^{-}$ ratio for $p_{T} \approx 2.0$ GeV, (iii) the difference in the slope parameters measured by the STAR and PHENIX collaborations, and (iv) the respectively strong and weak impact parameter dependence of the $\bar{p}$ and $ϕ$ slope parameters. For an EOS without the hard and soft features of the QCD phase transition, the broad consistency with the data is lost.

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arXiv2001

Instanton-induced Semi-hard Parton Interactions and Phenomenology of High Energy Hadron Collisions

We phenomenologically study whether partonic collisions responsible for the growth of hadron-hadron cross sections at high energy can be ascribed to instanton-induced processes. Although non-perturbative in nature, these interactions occur at the semi-hard scale $Q\sim 1-2$ GeV, and should therefore be described using information from deep inelastic leptonic scattering on the partonic constituents in nucleons, pions, and photons. After considering shadowing corrections in nucleon-nucleon scattering, we fix a free instanton tail suppression parameter and determine the effective quark-quark cross section. The resulting contributions to $NN$, $πN$, $γN$, and $γγ$ cross sections all $increase$ with energy differently, but in reasonable agreement with experimental data. We then proceed to an estimate of the number of such processes present in high energy Au-Au collisions at RHIC, finding that the amount of entropy produced by instanton/sphaleron events matches the observed amount.

hep-ph↗

arXiv2001

Domain Wall Bubbles in High Energy Heavy Ion Collisions

It has been recently shown that meta-stable domain walls exist in high-density QCD ($μ\neq 0$) as well as in QCD with large number of colors ($N_c\to\infty$), with the lifetime being exponentially long in both cases. Such metastable domain walls may exist in our world as well, especially in hot hadronic matter with temperature close to critical. In this paper we discuss what happens if a bubble made of such wall is created in heavy ion collisions, in the mixed phase between QGP and hadronic matter. We show it will further be expanded to larger volume $\sim 20 fm^3$ by the pion pressure, before it disappears, either by puncture or contraction. Both scenarios leave distinctive experimental signatures of such events, negatively affecting the interference correlations between the outgoing pions.

hep-ph↗

arXiv2001

The QCD Phase Diagram, Equation of State, and Heavy Ion Collisions

After some historic remarks and a brief summary of recent theoretical news about the QCD phases, we turn to the issue of $freeze-out$ in heavy ion collisions. We argue that the chemical freeze-out line should actually consists of two crossing lines of different nature. We also consider some inelatic reactions which occure $after$ chemical freeze-out, emphasizing the role of overpopulation of pions. The $hydrodynamics$ (with or without hadronic afterburner) explaines SPS/RHIC data on radial and elliptic flow in unexpected details,for different particles, collision energies, and impact parameters. Apart of Equation of State (EoS), it has basically no free parameters. The EoS which describe these data best agrees quite well with the lattice predictions, with the QGP latent heat $Δε\approx 800 Mev/fm^3$. Other phenomena at RHIC, such as ``jet quenching'' and huge ellipticity at large $p_t$, also point toward very rapid entropy production. Its mechanism remains an outstanding open problem: at the end we discuss recent application of the instanton/sphaleron mechanism. The gg collisions with $\sqrt{s}=2-3 GeV$ may result not in mini-jets but rather in production of sphaleron-like gluomagnetic clusters, which are classically unstable and promptly decay into several gluons and quarks, in sperical mini-Bangs.

hep-ph↗

arXiv2001

Do Instantons and Strings Cluster when the Number of Colors is Large?

We consider the $N_c\to\infty$ limit of QCD using a toy model in which instantons exchange color-singlet scalar fields which do not self-interact. Our main observation is that collective attraction leads the formation of large clusters containing $O(N_c)$ nonperturbative objects. We further show that this clustering of instantons is limited due to a non-trivial repulsion inherent in the ADHM multi-instanton solution. As a result the vacuum is very different from that at low $N_c$, notably being more inhomogeneous, in ways which will affect chiral symmetry breaking of light quarks. We also briefly discuss a similar phenomenon for color strings in baryons made of medium-mass (charm-like) quarks.

hep-ph↗

arXiv2001

Systematic Study of the Single Instanton Approximation in QCD

The Single-instanton approximation (SIA) is often used to evaluate analytically instanton contribution to euclidean correlation functions in QCD, at small distances. We discuss how this approximation can be consistently derived from the theory of the instanton ensemble and give precise definitions to a number of different "effective masses", generalizing the parameter $m*$, which was introduced long ago to account for the collective contribution of the whole ensemble. We test numerically the range of applicability of the SIA for different quantities. Furthermore, we determine all the effective masses (for random and interacting instanton liquid models) as well as from phenomenology, and discuss to what extent those are universal.

hep-ph↗

arXiv2001

When can long-range charge fluctuations serve as a QGP signal?

We critically discuss recent suggestion to use long-range modes of charge (electric or baryon) fluctuations as a signal for the presence of Quark-Gluon Plasma at the early stages of a heavy ion collision. We evaluate the rate of diffusion in rapidity for different secondaries, and argue that for conditions of the SPS experiments, it is strong enough to relax the magnitude of those fluctuations almost to its equilibrium values, given by hadronic ``resonance gas''. We further argue that experimental data from SPS agree with this conclusion. We evaluate the detector acceptance needed to measure such ``primordial'' long-range fluctuations at RHIC conditions. We conclude with an application of the charge fluctuation analysis to the search for the QCD critical point.

hep-ph↗

arXiv2001

Hydro+Cascade, Flow, the Equation of State, Predictions and Data

A Hydro+Cascade model has been used to describe radial and elliptic flow at the SPS and successfully predicted the radial and elliptic flow measured by the both STAR and PHENIX collaborations . Furthermore, a combined description of the radial and elliptic flow for different particle species, restricts the Equation of State(EoS) and points towards an EoS with a phase transition to the Quark Gluon Plasma(QGP) .

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arXiv2001

Flow at the SPS and RHIC as a Quark Gluon Plasma Signature

Radial and elliptic flow in non-central heavy ion collisions can constrain the effective Equation of State(EoS) of the excited nuclear matter. To this end, a model combining relativistic hydrodynamics and a hadronic transport code(RQMD [17]) is developed. For an EoS with a first order phase transition, the model reproduces both the radial and elliptic flow data at the SPS. With the EoS fixed from SPS data, we quantify predictions at RHIC where the Quark Gluon Plasma(QGP) pressure is expected to drive additional radial and elliptic flow. Currently, the strong elliptic flow observed in the first RHIC measurements does not conclusively signal this nascent QGP pressure. Additional measurements are suggested to pin down the EoS.

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arXiv2001

Regeneration of Anti-Protons in Ultrarelativistic Heavy-Ion Collisions

The production and annihilation of antiprotons in the hadronic phase of heavy-ion collisions is evaluated within a thermal equilibrium approach. It is shown that the inverse reaction of $p\bar p$ annihilation (i.e., multi-pion annihilation $N_ππ\to p\bar p$ with $N_π\simeq$~5-7) in connection with oversaturation of pion phase space (i.e., finite pion chemical potentials) plays an important role in understanding the observed antiproton yields at SpS energies within the standard picture of subsequent chemical and thermal freezeout. Implications for RHIC energies are also addressed.

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