Hot strong matter

Abstract

The recent results on relativistic heavy-ion collisions are discussed. The most convincing quarkgluon plasma signatures at the LHC and the top RHIC energies are presented. Moreover, the possible methods of evaluating the energy threshold for deconfinement (onset of deconfinement) are described, and the corresponding results from the RHIC Beam Energy Scan and the SPS programs are shown. Additionally, the first signatures of creating dense and collectively behaving systems in collisions of small nuclei (or even in elementary interactions) are discussed. Finally, the current status of experimental search for the critical point of strongly interacting matter is summarized.

Some intriguing aspects of multiparticle production processes

“…Therefore, in both cases we encounter an inhomogeneous medium with r-dependent velocity of sound, c(r), and refractive index, n(r). These findings agree with the fact that in nuclear collisions a higher speed of sound is actually observed, as demonstrated by the NA61/SHINE collaboration at SPS energies 153,154 (note, however, that what is measured is a parameter in the equation of state of hadronic matter described by a hydrodynamical model, c 2 s ). Actually, this is not unexpected because, considering the connection of the isothermal compressibility of nuclear matter, κ T = −(1/V ) (∂V /∂P ) T , and remembering that fluctuations of the multiplicity of the produced secondaries is represented by the relative variance, ̟, of multiplicity fluctuations, one finds that 155,156 T…”

“…Note that higher velocity of sound c corresponds to lower fluctuations of multiplicity ̟. From the experimental data 153,154,157 one can deduce that c P b+P b c p+p ≃ 1.04 and ̟ p+p ̟ P b+P b ≃ 1.29 ± 0.04.…”

Sound waves in hadronic matter

“…These findings seem to agree with the fact that in nuclear collisions one really observes a higher speed of sound as demonstrated by the NA61/SHINE collaboration at SPS energies [24,25] (note, however, that what is measured is a parameter in the equation of state of hadronic matter described by a hydrodynamical model, c 2 s ). This is not so unexpected because, considering the connection of the isothermal compressibility of nuclear matter, κ T = −(1/V) (∂V/∂P) T , and fluctuations of the multiplicity of produced secondaries represented by the relative variance, ̟, of multiplicity fluctuations, one finds that [26,27]…”

“…From the experimental data shown in Fig. 4 [24,25,28] one has that c Pb+Pb c p+p ≃ 1.04 and ̟ p+p ̟ Pb+Pb ≃ 1.29 ± 0.04 (29) From them, using Eq. (28), one can deduce the expected ratio of the factors γT for p + p and Pb + Pb collisions at a beam energy of 158 GeV:…”

“…This results in the correlation function becoming negative which, in turn, leads to a diminishing of fluctuations of multiplicity (because the variance of the total multiplicity from a number of particular collisions is smaller that the sum of the variances of independent nucleon-nucleon collisions). [24,25] compared with the ratio ω p+p /ω Pb+Pb taken from [28].…”