Measurements of two-pion HBT correlations in Be+Be collisions at 150$A$ GeV/$c$ beam momentum at the NA61/SHINE experiment at CERN

This paper reports measurements of two-pion Bose-Einstein (HBT) correlations in Be+Be collisions at a beam momentum of 150$A$ GeV/$c$ by the NA61/SHINE experiment at the CERN SPS accelerator. The obtained momentum space correlation functions can be well described by a Levy distributed source model. The transverse mass dependence of the Levy source parameters is presented, and their possible theoretical interpretations are discussed. The results show that the Lévy exponent $\alpha$ is approximately constant as a function of $m_T$, and far from both the Gaussian case of $\alpha = 2$ or the conjectured value at the critical endpoint, $\alpha = 0.5$. The radius scale parameter $R$ shows a slight decrease in $m_T$ , which can be explained as a signature of transverse flow. Finally, an approximately constant trend of the intercept parameter $\lambda$ as a function of $m_T$ was observed, different from measurement results at RHIC.