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Abstract
Subthreshold production of ΛΛ pairs in nucleus–nucleus collisions is largely unexplored and is of particular interest for studying strangeness-production mechanisms in dense baryonic matter.
In 2023, the fixed-target BM@N experiment at the NICA–Nuclotron accelerator complex collected data from Xe + CsI collisions at a beam kinetic energy of 3.8A GeV. This work presents the procedure used to reconstruct Λ hyperons and extract the ΛΛ signal from these data.
Λ hyperons are reconstructed via the weak decay Λ → pπ⁻ using a displaced-vertex topology, with the secondary decay vertex separated from the primary interaction vertex. Events containing two Λ candidates are analysed using the two-dimensional distribution of the invariant masses of the two pπ⁻ combinations. The number of events containing ΛΛ pairs is determined by estimating the contribution from combinatorial background in the two-dimensional invariant-mass plane. The stability of the extracted ΛΛ signal is assessed by varying the Λ-candidate selection criteria and the methods used to model the combinatorial background.
