Critical QCD in nuclear collisions

Abstract

A detailed study of correlated scalars, produced in collisions of nuclei and associated with the σ-field fluctuations, (δσ) 2 =< σ 2 >, at the QCD critical point (critical fluctuations), is performed on the basis of a critical event generator (Critical Monte-Carlo) developed in our previous work.The aim of this analysis is to reveal suitable observables of critical QCD in the multiparticle environment of simulated events and select appropriate signatures of the critical point, associated with new and strong effects in nuclear collisions.The existence of a critical point in the phase diagram of QCD, for nonzero baryonic density, is of fundamental significance for our understanding of strong interactions and so its experimental verification has become an issue of high priority [1]. For this purpose an extensive programme of event-by-event searches for critical fluctuations in the pion sector is in progress in experiments with heavy ions from SPS to RHIC energies [2]. In [3] we have emphasized, however, that in order to reveal critical density fluctuations in multiparticle environment, one has to look for unconventional properties in the momentum distribution of reconstructed dipions (π + π − -pairs) [9] with invariant mass just above the two-pion threshold.In fact, the QCD critical point, if it exists, communicates with a zero mass scalar field (σfield) which at lower temperatures (T < T c ) may reach the two-pion threshold and decay in very short time scales owing to the fact that its coupling to the two-pion system is strong. Obviously, the fundamental, underlying pattern of σ-field fluctuations, built-up near the critical point by the universal critical exponents of QCD [3], is phenomenologically within reach if and only if the study of correlated sigmas, reconstructed near the two-pion threshold, becomes feasible. In the present work we perform a detailed feasibility study of the observables related to the detection of the QCD critical point in nuclear collisions. In

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“…Significant σ-field fluctuations are expected at the CP (density fluctuations of zero mass σ-particles produced in abundance at the CP) [20]. σ particles at T < T c may reach the two-pion threshold (2m π ) and then decay into two pions, therefore density fluctuations of di-pions with m π + π − close to the two pion mass incorporate σ-field fluctuations at the CP.…”

“…Then second factorial moments F 2 (M ) in transverse momentum space were computed for real data and for artificially produced mixed events where only statistical fluctuations are present. The combinatorial background subtracted (by use of mixed events) moments ∆F 2 in transverse momentum space are expected to follow a power-law behavior ∆F 2 ∼ (M 2 ) φ 2 , with φ 2 = 2/3 for systems freezing-out at CP [20]. Figure 9 shows that ∆F 2 for Si+Si at the top SPS energy measures fluctuations approaching in size the prediction of critical QCD (the remaining departure, φ 2,max ≈ 0.33±0.04 instead of 2/3, may be due to freezing out at a distance from the CP).…”

Improved intermittency analysis of proton density fluctuations in NA49 ion collisions at 158 AGeV

“…In order to detect the chiral critical point of QCD in nuclear collision experiments, it is necessary to employ suitable observables [1][2][3][4][5][6][7][8][9][10]. Such observables are connected to the fluctuations of the chiral condensate, q(x)q(x) , which is the order parameter of the chiral phase transition (q(x) is the quark field).…”

“…However, the chiral condensate, though likely to be formed in heavy ion collisions, is not directly observable, as it is unstable and decays mainly into pions at time scales characteristic of the strong nuclear interaction. Its critical properties are transferred to (π + , π − ) pairs with invariant mass slightly above their production threshold, which are experimentally observable [10]. At finite baryochemical potential, there is evidence that the sigma-field will induce critical density fluctuations also in the baryonic sector [11][12][13][14][15][16].…”

Search for critical fluctuations of the proton density in central A+A collisions at maximum SPS energy

“…In a heavy ion collision the chiral condensate is likely to be formed, however it will be unstable, decaying mainly into pions at time scales characteristic for the strong interaction. The critical properties of the condensate are transferred to (π + π − ) pairs with invariant mass just above their production threshold which are experimentally observable [9]. If the baryochemical potential is different from zero then we expect that the sigma-field will induce critical density fluctuations also in the baryonic sector [10][11][12][13][14][15].…”