In an isentropic expansion of a baryon-rich quark-gluon plasma (QGP) formed in heavy-ion collisions and undergoing a first-order phase transition into a hadron-resonance gas, it is found that an analysis of the production cross sections of dimuons in the region where the invariant mass M is greater than 2.25 GeV is enough for the detection of a QGP, whereas an analysis in the region with M Z 7.5 GeV is required for photon pairs. While the production cross section of dimuons is found to be almost independent of the equation of state used for the QGP, that for photon pairs depends on the equation of state of the QGP. We conclude that for the detection of the QGP muon pairs are better signals than photon pairs, whereas for a study of the equation of state of the QGP photon pairs seem to be more suitable than muon pairs.
We analyze the baryon-number density contrast R between the quark-gluon-plasma (QGP) phase and the hadronic phase during a first-order quark-hadron phase transition in the early Universe. Without the inclusion of baryonic resonances in the hadronic phase, we can reduce R by including interactions between quarks and gluons in the QGP phase. We also show that there is no need to use a large value of the bag constant to lower the value of R. Our calculations may be useful in nonstandard big-bang nucleosynthesis.Recently there has been considerable interest in the possibility of the early Universe undergoing a first-order quark-hadron phase transition which produces large inhomogeneities in the baryon-number density. '-" These might significantly modify the predictions of the standard scenario of the primordial nucleosynthesis.'-l2 If the baryon-number density contrast R between the quarkgluon-plasma (QGP) and hadronic phases during the cosmic quark-hadron phase transition is below 10, the nonstandard big-bang theory could also reproduce all the observed light-element abundances in addition to solving the outstanding cosmological problem of the "missing mass,=1,7,12 Several theoretical investigations have been made to study R."'In these calculations it has been found that R is very large if the neutron and the proton (and their antiparticles) are the only baryonic states in the hadronic phase. In order to reduce R , they included all the observed hadronic resonances in the hadronic phase; R is found to be -7 when the critical temperature T, k 160 MeV. They treated the Q G P as an ideal gas of quarks and gluons and the hadrons as nonrelativistic point particles.The purpose of this paper is to study the effects upon R of the inclusion of interactions between quarks and gluons in the Q G P phase via the running coupling constant a,, together with the bag term.I3-l5 We use relativistic quantum statistics for particles in both the phases and Hagedorn's pressure ensemble correction for the finite size of the hadrons in the hadronic phase. ' "18 The hadronic phase contains only nucleons and antinucleons and the Q G P phase is made up of u , d, ii, d quarks and gluons.The thermodynamic potential for the Q G P phase, CIQGP, is given byl4-I7 (with fi= 1 = c = k )where PQGP is the pressure, V is the volume, T is the temperature, pq is the quark chemical potential, and B is the bag constant. From the study of hadron spectroscopy'9-22 the value of the Q C D bag constant B 'I4 is found to lie between 150 MeV and 235 MeV; see also Ref. 23.As in Refs. 4-6, 10, and 18, we have chosen the quark chemical potential to be flavor independent: p, =pd =p, =pq. Nf is the number of quark flavors considered (2 if u and d quarks are included; 3 if the s quark is also in thermal equilibrium and r e l a t i v i s t i~'~~'~) .The net baryon density in the Q G P phase, pb,QGp, is given byThe running coupling constant a, which accounts for the interactions between the quarks and gluons is given byI4where A is the scale-fixing parameter in QCD. In...
We consider a first-order phase transition from a quark-gluon plasma (QGP) to a hadronresonance gas (HRG) in the early Universe. We use relativistic quantum statistics for particles in both the phases and include Hagedorn's pressure ensemble correction for the finite size of the hadrons in the HRG phase. In our model we can find the pressure, temperature, and baryon-chemical potential equilibria between the QGP phase and the HRG phase even at very large values of the bag constant B. The ratio of the baryon-number densities in the QGP and HRG phases at the critical temperature shows that the primordial nucleosynthesis will not be affected significantly if the transition temperature 2 125 MeV. The inclusion of sufficiently light strange quarks in the QGP phase leads to an increase in the baryon density contrast.
A study of the energy distribution of muon pairs and photon pairs produced in ultrarelativistic heavyion collisions shows that muon pairs may be better signatures than photon pairs for the detection of a baryon-rich quark-gluon plasma (QGP). The value of the transverse mass of the muon pairs at which the p-meson peak disappears from their invariant-mass spectrum is sensitive to the equation of state of the QGP. Hence dimuons are useful not only for the detection of a QGP but also for a study of its equation of state.PACS numberk): 25.75.+r, 12.38.Mh, 24.85.+p
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