# A Characteristics of QGP at colliders and in the early universe

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1. Aug 4, 2017

### tom.stoer

Is there any comparison between the characteristics and the state of the QGP in collider experiments and in the early universe? Can one compare properties like temperature, pressure, specific heat, viscosity etc. using standard or non-equilibrium thermodynamics? Can one compare the "environmental conditions" like the expansion rate of the universe with the expansion rate of the QGP in collider experiments? Is it possible to compare cooling, baryonization etc.?

2. Sep 4, 2017

### vanhees71

Well at the largest beam energies as at the LHC the fireball of strongly interacting matter created in the collision is close to vanishing baryo-chemical momentum and as such it resembles the state of matter in the first few microseconds after the big bang. Unfortunately we cannot check this for the universe since all the characteristics of the QGP in the fluctuations of the cosmic microwave background are washed out. So what we can see is the stage after decoupling of photons. If one could measure the neutrino background we should be able to look at the time of the universe where the neutrinos decoupled, but that was earlier than the formation of the QGP.

3. Sep 4, 2017

### nikkkom

I thought neutrinos decouple at about 1 MeV, QGP transitions to isolated hadrons at ~200MeV. Correct me if I'm wrong.

4. Sep 4, 2017

### vanhees71

You are right. More precisely I should have said that the neutrinos decoupled much earlier than at the deconfinement-confinement transition.

5. Sep 4, 2017

### nikkkom

...And I said that this does not sound correct to me. Neutrinos decoupled later than that, not earlier. (But I'm no expert)

6. Sep 4, 2017

### Staff: Mentor

I'm confused as well. Hadrons formed after microseconds, the cosmic neutrino background was formed after a second.

7. Sep 5, 2017

### vanhees71

Yes, obviously I was wrong on that. Obviously the universe's "standard matter" has been dense enough long after hadronization so that the neutrinos only decoupled much later at a temperature of $T=1 \mathrm{MeV}$. The (pseudo-)critical temperature for the deconfinement-confinement transition is around $T=160 \; \mathrm{MeV}$. I should have checked the cosmological part of my answer better :-((.

In heavy ion collisions the mean-free path of non-strongly interacting particles (leptons, photons, $W$- and $Z$-bosons), is however much longer than the extension of the fireball, so that you can neglect final-state interactions of them with the medium. This implies that these probes, most notably dileptons (electron-positron and $\mu^+$-$\mu^-$ pairs) and photons, provide direct (space-time averaged) insight into the spectral properties of hadrons (here particularly the light vector mesons, $\rho$, $\omega$, and $\phi$) in the medium. This is important to learn more about the phase diagram of strongly interacting matter, particularly the mechanisms behind chiral-symmetry restoration (which at $\mu_{\text{B}}=0$ coincides with the deconfinement-confinement transition according to finite-temperature lattice-QCD calculations).