Eur. Phys. J. C (2023) 83: 919
https://doi.org/10.1140/epjc/s10052-023-11997-8

Regular Article - Experimental Physics

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

¹ National Nuclear Research Center, Baku, Azerbaijan
² Faculty of Physics, University of Sofia, Sofia, Bulgaria
³ Ruđer Bošković Institute, Zagreb, Croatia
⁴ LPNHE, University of Paris VI and VII, Paris, France
⁵ Karlsruhe Institute of Technology, Karlsruhe, Germany
⁶ University of Frankfurt, Frankfurt, Germany
⁷ Wigner Research Centre for Physics, Budapest, Hungary
⁸ Eötvös Loránd University, Budapest, Hungary
⁹ Institute for Particle and Nuclear Studies, Tsukuba, Japan
¹⁰ Okayama University, Okayama, Japan
¹¹ University of Bergen, Bergen, Norway
¹² University of Oslo, Oslo, Norway
¹³ Jan Kochanowski University, Kielce, Poland
¹⁴ Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland
¹⁵ National Centre for Nuclear Research, Warsaw, Poland
¹⁶ Jagiellonian University, Cracow, Poland
¹⁷ AGH-University of Science and Technology, Cracow, Poland
¹⁸ University of Silesia, Katowice, Poland
¹⁹ University of Warsaw, Warsaw, Poland
²⁰ University of Wrocław, Wrocław, Poland
²¹ Warsaw University of Technology, Warsaw, Poland
²² Institute for Nuclear Research, Moscow, Russia
²³ Joint Institute for Nuclear Research, Dubna, Russia
²⁴ National Research Nuclear University (Moscow Engineering Physics Institute), Moscow, Russia
²⁵ St. Petersburg State University, St. Petersburg, Russia
²⁶ University of Belgrade, Belgrade, Serbia
²⁷ Fermilab, Batavia, USA
²⁸ University of Notre Dame, Notre Dame, USA
²⁹ University of Colorado, Boulder, USA
³⁰ University of Hawaii at Manoa, Honolulu, USA
³¹ University of Pittsburgh, Pittsburgh, USA

^dj bporfy@cern.ch

Received: 14 February 2023
Accepted: 15 August 2023
Published online: 11 October 2023

Abstract

This paper reports measurements of two-pion femtoscopic correlations in Be+Be collisions at a beam momentum of 150$A\text{GeV}/c$ (energy available in the center-of-mass system for nucleon pair $\sqrt{s_{\text{NN}}}=16.84$ 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 $\alpha$ is approximately constant as a function of $m_{\text{T}}$ , 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 $m_{\text{T}}$ , which can be explained as a signature of transverse flow. Finally, an approximately constant trend of the intercept parameter $\lambda$ as a function of $m_{\text{T}}$ was observed, similar to previous NA44 S + Pb results (obtained with a Gaussian approximation, but unlike RHIC results).