Highlights from NA61/SHINE: Proton Intermittency Analysis

Abstract

The NA61/SHINE experiment at CERN SPS searches for the critical point of strongly interacting matter via scanning the phase diagram by changing beam momenta (13A-150A GeV/c) and system size (p + p, p + Pb, Be + Be, Ar + Sc, Xe + La). An observation of local proton-density fluctuations that scale as a power law of the appropriate universality class as a function of phase space bin size would signal the approach of the system to the vicinity of the possible critical point. An investigation of this phenomenon was performed in terms of the second-scaled factorial moments (SSFMs) of proton density in transverse momentum space with subtraction of a noncritical background. New NA61/SHINE preliminary analysis of Ar + Sc data at 150A GeV/c revealed a nontrivial intermittent behavior of proton moments. A similar effect was observed by NA49 in "Si" + Si data at 158A GeV/c. At the same time, no intermittency signal was detected in "C" + C and Pb + Pb events by NA49, as well as in Be + Be collisions by NA61/SHINE. EPOS1.99 also fails to describe the power-law scaling of SSFMs in Ar + Sc. Qualitatively, the effect is more pronounced with the increase of collision-peripherality and proton-purity thresholds, but a quantitative estimate is to be properly done via power-law exponent fit using the bootstrap method and compared to intermittency critical index φ 2 , derived from 3D-Ising effective action.

Machine learning phase transitions of the three-dimensional Ising universality class

“…In relativistic heavy-ion collisions, we expect the QCD critical point to have similar critical behaviors as those in the 3D Ising model due to the same universality that is dictated by the symmetry of the systems. In heavy-ion experiments, the measured high-order cumulants [15,70,71] and intermittency [72][73][74] both at SPS and RHIC energies suggest that there exists large density fluctuations near the QCD critical point. Here we have proved that the state-of-the-art ML method can learn the important features which discriminate different types of phase transitions.…”

Intermittency analysis of proton numbers in heavy-ion collisions at energies available at the BNL Relativistic Heavy Ion Collider

“…It is argued that in heavy-ion collisions large baryon density fluctuation may provide a unique signal to the phase transition in the QCD phase diagram. It is expected to observe the critical density fluctuations as a power-law pattern on available phase space resolution if system freezes out right in the vicinity of the critical point [15,20].…”

“…In high energy experiments, the power-law or intermittency behavior can be measured by calculations of SFMs of baryon number density [15,20,38]. For this purpose, an available region of momentum space is partitioned into M D equal-size bins, F q (M ) is defined as:…”

“…In this section, we have demonstrated that the cell-bycell method could be served as a precise and effective way of efficiency correction on SFMs. It can be easily applied to current studies at STAR [21], NA49 [19], NA61 [20] and other heavy-ion experiments in the intermittency analysis. It should also be noted that one needs to consider how to treat the momentum resolution in different experiments.…”

“…It can be revealed in transverse momentum spectrum as a power-law behavior of scaled factorial moment [12,15]. In current high energy heavy-ion experiments at CERN SPS [19,20], the NA49 and NA61 collaborations have explored the critical fluctuations by employing an intermittency analysis with various sizes of colliding nuclei. The power-law behavior has been observed in Si + Si collisions at 158A GeV [19].…”