Search for hyper-triton in Ni+Ni collisions at 1.91A GeV

Production of hypernuclei in heavy-ion collisions is a unique way to study the interaction between strange baryons and the surrounding nuclear matter [1]. At ultrarelativistic energies, the production of (anti-)hyper-tritons has been already measured by the STAR experiment at RHIC [2]. In this contribution, we report on the first measurement of the production of hyper-triton in Ni+Ni collisions at 1.91A GeV, carried out with the FOPI spectrometer at SIS18 in GSI/Darmstadt. The FOPI spectrometer was described in more detail in [3]. Hyper-triton is the lightest hypernuclei. It decays weekly, but its half-life is not known precisely, and can be in the order of 100 300 ps [2]. In the FOPI experiment, hyper-tritons can be identified by reconstructing the invariant mass of their charged decay-products. In this contribution, we show the identification in the two-body decay channel: Λt→π−+3He. π− were registered in the Central Drift Chamber (CDC) and were identified by correlating their momenta with specific energy loss. Proper identification of He required in addition information about the velocity of the particle, measured by the Time-of-Flight Barrel, which surrounds the CDC, is made of Multi-strip Multi-gap Resistive Plate Chamber (MMRPC) and has an excellent intrinsic TOF resolution of about 60 ps. In order to extend the geometrical acceptance of the CDC and the RPC system towards mid-rapidity, the target used for this experiment was shifted along the beam axis by 40 cm, upstream from its normal position in the center of the CDC. The huge combinatorial background was suppressed by imposing stringent selection criteria on the quality and topology of reconstructed trajectories of π− and He pairs. This involved, among others, detailed analysis of the decay kinematics and its influence on the correlations between the selected decay-products of hyper-tritons, e.g., distances of closest approach of the reconstructed trajectories to the primary interaction point, position of the decay-vertex, emission angles and momenta of the involved particles. The solid, red histogram in figure 1(a) depicts the reconstructed invariant mass of (π−,3He) pairs, selected in 56 million semi-central collisions. The dashed, blue line in figure 1(a) corresponds to the combinatorial background, which was reconstructed by the mixed-event method and normalized to the signal spectrum in the region depicted by the horizontal line (3.03-3.6 GeV/c). The red histogram in figure 1(b) shows the distribution obtained after subtracting the normalized background from the signal spectrum. The error bars correspond to the statistical fluctuations only. As depicted by the black line in figure 1(b), the distribution