The search for experimental signatures of the critical point (CP) of strongly interacting matter is one of the main objectives of the NA61/SHINE experiment at CERN SPS. In the course of the experiment, a beam momentum (13A – 150A GeV/$c$) and system size (p+p, p+Pb, Be+Be, Ar+Sc, Xe+La, Pb+Pb) scan is performed. Local proton density fluctuations in transverse momentum space represent an order parameter of the chiral phase transition and are expected to scale according to a universal power-law in the vicinity of the CP. They can be probed through an intermittency analysis of the proton second scaled factorial moments (SSFMs) in transverse momentum space. Previous such analyses  revealed power-law behavior in NA49 Si+Si collisions at 158A GeV/$c$, the fitted power-law exponent being consistent with the theoretically expected critical value, within errors. Probes of NA61/SHINE systems at the maximum SPS energy revealed no intermittency effect in Be+Be, whereas Ar+Sc analysis is inconclusive due to large uncertainties. The analysis has recently been extended to Pb+Pb collision data at lower energies.
We present a summary of the current status of NA61/SHINE intermittency analysis, and review novel techniques developed and employed to subtract non-critical background and estimate statistical and systematic uncertainties. In particular, the issue of obtaining a reliable estimate of the intermittency index (power-law exponent) $\phi_2$ from correlated data is addressed through the use of toy models. Additionally, we use Monte Carlo simulations to assess the statistical significance of the observed intermittency effect.
 T. Anticic et al, Eur. Phys. J. C 75: 587 (2015).
|Collaboration / Activity||The NA61/SHINE Collaboration|