共查询到20条相似文献,搜索用时 15 毫秒
1.
E.G. Drukarev M.G. Ryskin V.A. Sadovnikova 《The European Physical Journal A - Hadrons and Nuclei》1999,4(2):171-186
We calculate the contribution of pions to the $\bar qq$-expectation value κ(ρ) =<M|ˉq
q|M> in symmetric nuclear matter. We employ exact pion propagator renormalized by nucleon-hole and isobar-hole excitations. Conventional
straightforward calculation leads to the “pion condensation” at unrealistically small values of densities, causing even earlier
restoration of chiral symmetry. This requires a self-consistent approach, consisting in using the models, which include direct
dependence of in-medium mass values on κ(ρ), e.g. the Nambu–Jona-Lasinio–model. We show, that in the self-consistent approach
the ρ-dependence of the condensate is described by a smooth curve. The “pion condensate” point is removed to much higher values
of density. The chiral restoration does not take place at least while ρ < 2.8ρ0 with ρ0 being the saturation value. Validity of our approach is limited by possible accumulation of heavier baryons (delta isobars)
in the ground state of nuclear matter. For the value of effective nucleon mass at the saturation density we found m
*(ρ0) = 0.6m, consistent with nowadays results of other authors.
Received: 8 October 1998 相似文献
2.
K. Tsushima K. Saito A. W. Thomas A. Valcarce 《The European Physical Journal A - Hadrons and Nuclei》2007,31(4):626-629
We study the bound-nucleon sigma term and the quark condensate in nuclear matter. In the quark-meson coupling (QMC) model
the nuclear correction to the sigma term is small and negative, i.e., it decelerates the decrease of the quark condensate in nuclear matter. However, the quark condensate in nuclear matter is
controlled primarily by the scalar-isoscalar σ field. Compared to the leading term, it moderates the decrease more than that of the nuclear sigma term alone at densities
around and larger than the normal nuclear-matter density. 相似文献
3.
J. Hüfner Y.B. He B.Z. Kopeliovich 《The European Physical Journal A - Hadrons and Nuclei》2000,7(2):239-244
Prompt gluons are an additional source for charmonium suppression in nuclear collisions, in particular for nucleus-nucleus
collisions. These gluons are radiated as bremsstrahlung in N-N collisions and interact inelastically with the charmonium states
while the nuclei still overlap. The spectra and mean number <n
g> of the prompt gluons are calculated perturbatively and the inelastic cross section σabs
Ψg is estimated. The integrated cross sections σ(A B →J/ψX) for p-A and A-B collisions and the dependence on transverse energy for S-U and Pb-Pb can be described quantitatively with
some adjustment of one parameter <n
g>σabs
Ψg.
Received: 20 August 1999 相似文献
4.
M. J. Leitch 《The European Physical Journal A - Hadrons and Nuclei》2007,31(4):868-874
At the Relativistic Heavy-Ion Collider (RHIC) collisions of heavy ions at nucleon-nucleon energies of 200GeV appear to have
created a new form of matter thought to be a deconfined state of the partons that ordinarily are bound in nucleons. We discuss
the evidence that a thermalized partonic medium, usually called a Quark Gluon Plasma (QGP), has been produced. Then, we discuss
the effect of this high-density medium on the production of jets and their pair correlations. Next, we look at direct photons
as a clean electro-magnetic probe to constrain the initial hard scatterings. Finally, we review the developing picture for
the effect of this medium on the production of open heavy quarks and on the screening by the QGP of heavy-quark bound states. 相似文献
5.
A. Hombach W. Cassing U. Mosel 《The European Physical Journal A - Hadrons and Nuclei》1999,5(1):77-84
We study the mixing and the kinetic equilibration of projectile and target nucleons in relativistic heavy-ion collisions
in the energy regime between 150 AMeV and 2 AGeV in a coupled-channel BUU (CBUU) approach. We find that equilibrium in the
projectile-target degrees of freedom is in general not reached even for large systems at low energy where elastic nucleon-nucleon
collisions dominate. Inelastic nucleon excitations are more favorable for equilibration and their relative abundance increases
both with energy and mass. Experimentally, the projectile/target admixture can be determined by measuring the degree of isospin
equilibration in isospin asymmetric nuclear collisions. For one of the most promising systems currently under investigation,
96
44Ru +96
40Zr, we investigate the influence of the equation of state and the inelastic in-medium cross section.
Received: 23 September 1998 / Revised version: 3 December 1998 相似文献
6.
We study the static scalar susceptibility of the nuclear medium, i.e., the change of the quark condensate for a small modification of the quark mass. In the linear sigma model it is linked to
the in-medium sigma propagator and its magnitude increases due to the mixing with the softer modes of the nucleon-hole excitations.
We show that the pseudoscalar susceptibility, which is large in the vacuum, owing to the smallness of the pion mass, follows
the density evolution of the quark condensate and thus decreases. At normal nuclear matter density the two susceptibilities
become much closer, a partial chiral symmetry restoration effect as they become equal when the full restoration is achieved.
Received: 20 July 2002 / Accepted: 14 September 2002 / Published online: 21 January 2003
RID="a"
ID="a"e-mail: chanfray@ipnl.in2p3.fr
Communicated by A. Molinari 相似文献
7.
The Spin dependent gluon and sea quark distributions of the proton and the neutron are extracted in the leading order (LO)
and the next-to-leading order (NLO) QCD. The relativistic quark exchange model is used to calculate the related valence quark
spin dependent structure function. The inverse Mellin transform technique is performed to evaluate the polarized x-dependent
distributions of the gluon and the sea quark from the various moments of the valence quarks. It is shown that the calculated
spin structure functions (SSF) of the proton and the neutron are in good agreement with the available data, such as E143,
SMC, E142, E154 and Hermes experiments. A comparison is also made with the other theoretical models. Finally it is shown that
the above calculated parton distributions improve the SSF of the proton and the neutron.
Received: 4 January 1999 / Revised version: 12 April 1999 相似文献
8.
D. d'Enterria 《The European Physical Journal A - Hadrons and Nuclei》2007,31(4):816-823
We present a summary of the physics of gluon saturation and non-linear QCD evolution at small values of the parton momentum
fraction x in the proton and nucleus in the context of recent experimental results at HERA and RHIC. The rich physics potential of low-x studies at the LHC, especially in the forward region, is discussed and some benchmark measurements in pp, pA and AA collisions are introduced. 相似文献
9.
T. Hirano 《The European Physical Journal A - Hadrons and Nuclei》2006,29(1):19-22
We can establish a new picture, the perfect fluid sQGP core and the dissipative hadronic corona, of the space-time evolution
of produced matter in relativistic heavy-ion collisions at RHIC. It is also shown that the picture works well also in the
forward rapidity region through an analysis based on a new class of the hydro-kinetic model and that this is a manifestation
of the rapid increase of the entropy density in the vicinity of QCD critical temperature, namely, deconfinement. 相似文献
10.
S. Kimura A. Bonasera S. Cavallaro 《The European Physical Journal A - Hadrons and Nuclei》2006,29(1):89-93
We compute by numerical integration of the Dirac equation the number of quark-antiquark pairs produced in the classical color
fields of colliding ultrarelativistic nuclei. Results for the dependence of the number of quarks on the strength of the background
field, the quark mass and time are presented. We also perform several tests of our numerical method. While the number of qˉ pairs is parametrically suppressed in the coupling constant, we find that in this classical field model it could even be
compatible with the thermal ratio to the number of gluons. 相似文献
11.
Numerical 1D-3V solutions of the Wong-Yang-Mills equations with anisotropic particle momentum distributions are presented.
They confirm the existence of plasma instabilities for weak initial fields and of their saturation at a level where the particle
motion is affected, similar to Abelian plasmas. The isotropization of the particle momenta by strong random fields is shown
explicitly, as well as their nearly exponential distribution up to a typical hard scale, which arises from scattering off
field fluctuations. By variation of the lattice spacing we show that the effects described here are independent of the UV
field modes near the end of the Brioullin zone. 相似文献
12.
C. Beşliu A. Jipa D. Argintaru C. Argintaru R. Zaharia J. Gabriel Rican M. Iosif 《The European Physical Journal A - Hadrons and Nuclei》1998,1(1):65-75
The problem of the nuclear matter jets in nucleus-nucleus collisions at 4.5 A GeV/c is discussed. The global analysis of
experimental data, namely the sphericity tensor, is used to evidence such jets. The experimental results are compared with
those obtained in the same collisions for hydrodynamic flow and thrust. The experiments have been performed in the frame of
the SKM 200 Collaboration from JINR Dubna.
Received: 20 March 1997 / Revised version: 20 July 1997 相似文献
13.
We study the chiral phase transition at finite temperature in the linear sigma model by employing a self-consistent Hartree
approximation. This approximation is introduced by imposing self-consistency conditions on the effective meson mass equations
which are derived from the finite temperature one-loop effective potential. It is shown that in the limit of vanishing pion
mass, namely when the chiral symmetry is exact, the phase transition becomes a weak first order accompanying a gap in the
order parameter as a function of temperature. This is caused by the long range fluctuations of meson fields whose effective
masses become small in the transition region. It is shown, however, that with an explicit chiral symmetry breaking term in
the Lagrangian which generates the realistic finite pion mass the transition is smoothed out irrespective of the choice of
coupling strength.
Recieved: 19 September 1997 / Revised version: 30 October 1997 相似文献
14.
P. Costa C. A. de Sousa M. C. Ruivo Yu. L. Kalinovsky 《The European Physical Journal A - Hadrons and Nuclei》2007,31(4):842-844
We study the chiral phase transition at finite T and μ
B within the framework of the SU(3) Nambu-Jona-Lasinio (NJL) model. The QCD critical end point (CEP) and the critical line at finite temperature and baryonic
chemical potential are investigated: the study of physical quantities, such as the baryon number susceptibility near the CEP,
will provide complementary information concerning the order of the phase transition. We also analyze the information provided
by the study of the critical exponents around the CEP. 相似文献
15.
Selected results from the first five years of RHIC data taking are reviewed with emphasis on the evidence for thermalization
in central Au + Au collisions at
= 200GeV. 相似文献
16.
We investigate the evolutions with density of the QCD scalar susceptibility and of the sigma mass in a chiral relativistic theory of nuclear matter, in the mean-field approximation. In order to reach saturation we need to introduce the scalar response of the nucleons. The consequences are a quite mild density dependence of the sigma mass and the progressive decoupling of the quark density fluctuations from the nucleonic ones at large densities. 相似文献
17.
N. Jachowicz G. C. McLaughlin 《The European Physical Journal A - Hadrons and Nuclei》2006,29(1):43-47
We argue that isotropization and, consequently, thermalization of the system of gluons and quarks produced in an ultrarelativistic
heavy-ion collision does not follow from Feynman diagram analysis to any order in the coupling constant. We conclude that
the apparent thermalization of quarks and gluons, leading to success of perfect fluid hydrodynamics in describing heavy-ion
collisions at RHIC, can only be attributed to the non-perturbative QCD effects not captured by Feynman diagrams. We proceed
by modeling these non-pertrubative thermalization effects using viscous hydrodynamics. We point out that matching Color Glass
Condensate inital conditions with viscous hydrodynamics leads to a continuous evolution of all the components of the energy-momentum
tensor and, unlike the case of ideal hydrodynamics, does not give rise to a discontinuity in the longitudinal pressure. An
important consequence of such a matching is a relationship between the thermalization time and shear viscosity: we observe
that small viscosity leads to short thermalization time. 相似文献
18.
B. Kämpfer O.P. Pavlenko S. Zschocke 《The European Physical Journal A - Hadrons and Nuclei》2003,17(1):83-87
QCD sum rules predict that the change of the strange quark condensate 〈ˉss〉 in hadron matter at finite baryon density causes a shift of the peak position of the di-electron spectra from φ-meson decays.
Due to the expansion of hadron matter in heavy-ion collisions, the φ peak suffers a smearing governed by the interval of density
in the expanding fireball, which appears as an effective broadening of the di-electron spectrum in the φ region. The emerging
broadening is sensitive to the in-medium change of 〈ˉss〉. This allows to probe directly in-medium modifications of 〈ˉss〉 via di-electron spectra in heavy-ion collisions at SIS energies with HADES.
Received: 22 November 2002 / Accepted: 30 January 2003 / Published online: 29 April 2003 相似文献
19.
W.M. Alberico P. Czerski M. Nardi 《The European Physical Journal A - Hadrons and Nuclei》1999,4(2):195-203
We present a three–dimensional model for quark matter with a density dependent quark–quark (confining) potential, which allows
to describe a sort of deconfinement transition as the system evolves from a low density assembly of bound structures to a
high density free Fermi gas of quarks. We consider different confining potentials, some of which successfully utilized in
hadron spectroscopy. We find that a proper treatment of the many–body correlations induced by the medium is essential to disentangle
the different nature of the two (hadronic and deconfined) phases of the system. For this purpose the ground state energy per
particle and the pair correlation function are investigated.
Received: 10 June 1998 / Revised version: 24 September 1998 相似文献
20.
K. A. Chikin V. L. Korotkih A. P. Kryukov L. I. Sarycheva I. A. Pshenichnov J. P. Bondorf I. N. Mishustin 《The European Physical Journal A - Hadrons and Nuclei》2000,8(4):537-548
There exist several proposals to use Weizsäcker-Williams photons generated by ultrarelativistic heavy ions to produce exotic particles in γγ fusion reactions. To estimate the background conditions for such reactions we analyze various mechanisms of meson production in very peripheral collisions of ultrarelativistic heavy ions at RHIC and LHC energies. Besides the γγ fusion they include also electromagnetic γA interactions and strong nucleon-nucleon interactions in grazing AA collisions. All these processes are characterised by low multiplicities of produced particles. The γA an d AA events are simulated by corresponding Monte Carlo codes, RELDIS and FRITIOF. In each of these processes a certain fraction of pions is produced close to the mid-rapidity regionthat gives a background for the γγ events. The possibility of selecting the mesons produced in the γγ fusion events via different p t cut procedures is demonstrated. 相似文献