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Primordial Black Holes and Cosmological Phase Transitions Report ...

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PBHs <strong>and</strong> <strong>Cosmological</strong> <strong>Phase</strong> <strong>Transitions</strong> 76<br />

Figure 25: The behaviour of the sound speed c 2 s as a function of T/Tc during the<br />

QCD transition according to the Lattice model (solid line – equation 139) <strong>and</strong><br />

the numerical results obtained from quenched QCD. The dashed line represents<br />

the ideal gas case, i.e., c 2 s =1/3. When T = Tc the sound speed vanishes.<br />

The value of Tc for the QCD Crossover is not unique. Different observables<br />

lead to different numerical Tc values even in the continuum <strong>and</strong> thermodynamic<br />

limit. This is a well–known phenomenon on the water–vapour phase diagram.<br />

The peak of the renormalized chiral susceptibility predicts Tc = 151 MeV,<br />

whereas Tc based on the strange quark number susceptibility <strong>and</strong> Polyakov loops<br />

result in 175 MeV <strong>and</strong> 176 MeV, respectively (Aoki et al., 2006b). On Table 16<br />

we summarize the results obtained by Aoki et al. (2006b) <strong>and</strong> Bernard et al.<br />

(2005), considering three quark flavours.<br />

The entropy density for a Crossover can be written as (e.g. Schmid et al.,<br />

Table 16: The critcal temperature Tc for the QCD Crossover for different observables.<br />

Tc (MeV) Reference Observable<br />

151 Aoki et al. (2006b) Chiral susceptibility<br />

169 Bernard et al. (2005) Chiral susceptibility<br />

175 Aoki et al. (2006b) Strange quark number susceptibility<br />

176 Aoki et al. (2006b) Polyakov loops

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