3ΛH and 4ΛH production in Ni+Ni collisions at 1.91A GeV

3ΛH and 4ΛH are identified from their two-body π--decay channel in Ni+Ni collisions at 1.91A GeV for the first time. The experiment was performed by the FOPI spectrometer at the SIS18 of GSI. The analyzed data sample consists of 56 x 106 events covering the most central 60% of the total reaction cross section. The reconstructed invariant mass of 3ΛH is 2.9926±0.0017 GeV/c2 with a width of 5.6±1.0 MeV/c2, and the one of 4ΛH is 3.9226±0.0010 GeV/c2 with a width of 4.0±1.6 MeV/c2. The significance of the 3ΛH and 4ΛH signal is 5.6 and 4.2, respectively. The 3ΛH and 4ΛH signal is observed in a certain phase space region in the pt/m-ylab plane only. By applying the differential decay time efficiency obtained from MC simulation, the mean lifetime of 3ΛH and 4ΛH is found to be 263±64(sta)±44(sys) ps and 196±75(sta)±43(sys) ps, respectively. By taking reconstruction efficiency and the decay branching ratio into account, the yield of 3ΛH and 4ΛH for the analysed event sample is 7.5 x 10-4 and 1.3 x 10-4 per event, respectively. The 3ΛH and 4ΛH production in Ni+Ni at 1.91A GeV via coalescence process is naively discussed and it is shown that the observed results are not compatible with the expectations. The yield ratio of 3ΛH and 4ΛH/4He is compared with the predictions of the thermal model for hadron chemistry.