Search for the QCD critical point in nuclear collisions at158A GeV at the CERN Super Proton Synchrotron (SPS)

Abstract

Pion production in nuclear collisions at the Super Proton Synchrotron (SPS) is investigated with the aim to search, in a restricted domain of the phase diagram, for power laws in the behavior of correlations that are compatible with critical QCD. We analyzed interactions of nuclei of different sizes (p + p, C + C, Si + Si, Pb + Pb) at 158A GeV adopting, as appropriate observables, scaled factorial moments in a search for intermittent fluctuations in transverse dimensions. The analysis is performed for π + π − pairs with an invariant mass very close to the two-pion threshold. In this sector one may capture critical fluctuations of the sigma component in a hadronic medium, even if the σ meson has no well-defined vacuum state. It turns out that for the Pb + Pb system the proposed analysis technique cannot be applied without entering the invariant mass region with strong Coulomb correlations. As a result the treatment becomes inconclusive in this case. Our results for the other systems indicate the presence of power-law fluctuations in the freeze-out state of Si + Si approaching in size the prediction of critical QCD.

Viscosity of a net-baryon fluid near the QCD critical point

“…In this framework one may proceed to a semi-quantitative treatment of shear and bulk viscosity, near the QCD critical point, on the basis of eqs. (1,2,4). For the functions (1) we adopt a simple model inspired by a perturbative treatment of conventional fluids, in the vicinity of liquid-gas critical point, sharing the same universality class (3d Ising) with QCD [7]:…”

“…where ξ + 1 fm a typical scale of the correlation length and a set of critical values (T c , µ c , n c ) can be taken from reference [10] where a study of baryon-number susceptibility near the critical point is performed and also from NA49 measurements in a search for critical fluctuations [4,17]: [18,19], the constants f (i) increase by a factor 1.7 but the overall prefactors f (s) M ± , f (b) N ± in eq. (9) remain unchanged, fixed by the constraint of Kovtun-Son-Starinets (KSS) bound, described in the discussion below.…”

“…As a result, one may claim that although in the study of static properties of the QCD critical point (critical fluctuations, divergence of baryon-number susceptibility) both components of the order parameter (σ, n b ) are relevant [3], in any attempt to investigate the dynamic properties of the QCD critical point (divergence of transport coefficients) only the baryon-number density n b is a relevant order parameter [1]. Experimentally, one may state that in the search for the location of the critical point in the phase diagram, the investigation of critical fluctuations of π + π − pairs may simulate σ-field critical fluctuations [4]. However, dynamic properties of the QCD critical point cannot be revealed within a study of transport coefficients (viscosity) of a meson gas produced in high-energy nuclear collisions [5].…”

Mapping the phases of quantum chromodynamics with beam energy scan

“…Their main focus is the search for possible structures in the QCD phase diagram with various fluctuation and correlation observables for various energies and colliding systems [210]. In particular, NA49 and NA61/SHINE obtained results on the scaled variance [211][212][213][214], strongly intensive quantities [211,[215][216][217][218][219][220], intermittency [221][222][223], and many others [224][225][226][227][228]. Also, the HADES experiment at GSI Darmstadt, which measures at very low collision energies, has some capability to address fluctuation observables.…”

Photon emissivity in the vicinity of a critical point – A case study within the quark meson model

“…The onset can be related to a critical end point (CEP) with presently rather uncertain coordinates (T CEP , µ CEP ). Such an option of a CEP in the QCD phase diagram has triggered a lot of dedicated activities, both experimentally [10][11][12][13] and theoretically, applying lattice techniques e.g. reweighting [5], Taylor expansion in µ B [8], analytic continuation from imaginary µ B [6] or density of state methods [7] as well as Dyson-Schwinger [14], chiral model [15][16][17][18][19][20] or quasiparticle aproaches [21] giving widespread results [9].…”