Spectator induced electromagnetic effects on charged meson production in nucleus-nucleus collisions from NA61/SHINE at CERN SPS

The SPS Heavy Ion and Neutrino Experiment (NA61/SHINE) studies the properties of hadron emission in collisions of beams of hadrons and nuclei with fixed hadronic and nuclear targets. In recent years, the NA61/SHINE physics program has been supplemented with studies of electromagnetic (EM) effects, induced by the spectator charge on spectra and ratios of emitted charged mesons. This paper reports the first results on spectator induced EM effects on charged pion ($\pi^+/\pi^-$) ratios, for $\pi$ mesons emitted in Ar+Sc collisions at 40 A GeV/$c$ beam momentum ($\sqrt{s_{NN}}$ = 8.76 GeV). The presented data include the first-ever measurement of these effects in a small peripheral nucleus-nucleus system at the CERN SPS, and the first analysis of the full range of centrality in Ar+Sc collisions in NA61/SHINE.