Search for the critical point of strongly interacting matter in the NA61/SHINE experiment at CERN SPS

The NA61/SHINE experiment at the CERN SPS is a multipurpose fixed-target spectrometer for charged and neutral hadron measurements. Its research program includes studies of strong interactions as well as reference measurements for neutrino and cosmic-ray physics. One major goal of its strong interaction program is to determine the existence and pinpoint the location of the critical point (CP) of strongly interacting matter. The critical point (CP) is an important hypothesized feature of the phase diagram of Quantum Chromodynamics (QCD) – the plot in temperature and baryon density that maps the states of strongly interacting matter (nuclear matter, hadron gas, quark-gluon plasma) and the phase transitions between them. Experimental detection of the CP and mapping of its exact location and characteristics is a highly sought-after goal of high-energy ion collision experiments in large particle accelerators, such as the SPS; while also being the driving force behind considerable theoretical and computational effort (effective models, lattice QCD).

This presentation will summarize the current status of a diverse collection of NA61/SHINE critical point searches in nucleus-nucleus collisions, in the collision energy range $\sqrt{s_{NN}} = 5 - 17$ GeV. The review includes studies of multiplicity and net-electric charge fluctuations, femtoscopy analysis of $\pi^+$ / $\pi^-$ pairs, as well as intermittency analyses of protons and negatively charged hadrons probed via the scaling of factorial moments of particle multiplicities at different transverse momentum scales. The presentation will also touch on the recent significant methodological progress in properly handling bin-by-bin correlations – a long-standing problem in experimental intermittency analysis, particularly for datasets with a small number of events and small particle multiplicity. We report on novel methods that solve the aforementioned problem and provide a more accurate handling of systematics and uncertainties.