Two-pion femtoscopic correlations in Be+Be collisions at $$\sqrt{s_{\text {NN}}} = 16.84$$ GeV measured by the NA61/SHINE at CERN

Abstract This paper reports measurements of two-pion femtoscopic correlations in Be+Be collisions at a beam momentum of 150 $$A\,\hbox {GeV}\!/\!c$$ A GeV / c (energy available in the center-of-mass system for nucleon pair $$\sqrt{s_{\text {NN}}} = 16.84$$ s NN = 16.84 GeV) by the NA61/SHINE experiment at the CERN SPS accelerator. The obtained momentum space correlation functions can be well described by a Lévy distributed source model. The transverse mass dependence of the Lévy source parameters is presented, and their possible theoretical interpretations are discussed. The results show that the Lévy exponent $$\alpha $$ α is approximately constant as a function of $$m_{\text {T}}$$ m T , and far from both the Gaussian case of $$\alpha = 2$$ α = 2 or the conjectured value at the critical endpoint, $$\alpha = 0.5$$ α = 0.5 . The radius scale parameter R shows a slight decrease in $$m_{\text {T}}$$ m T , which can be explained as a signature of transverse flow. Finally, an approximately constant trend of the intercept parameter $$\lambda $$ λ as a function of $$m_{\text {T}}$$ m T was observed, similar to previous NA44 S + Pb results (obtained with a Gaussian approximation, but unlike RHIC results).