DIRAC Collaboration status report to SPSC, October 2021

DIRAC experiment investigated in the reaction p (24 GeV/c) + Ni the particle pairs K+K−, π+π- and pp¯ with relative momentum Q in the pair center mass system (c.m.s.) less than 100 MeV/c. Because of background studies, DIRAC explored three subsamples of K+K− pairs, obtained by subtracting – using time-of-flight (TOF) technique – background from initial Q distributions with K+K− populations more than 70%, 50% and 30%. In the two runs with the data sets DATA2 and DATA3, DIRAC identified about 11000 K+K− pairs (30% subsample). Half of these pairs lie in the effective mass interval 2mK to 2mK + 0.8 MeV. The corresponding pair distributions in Q and in its longitudinal projection QL were analyzed first in a Coulomb model, which takes into account only Coulomb final state interaction (FSI) and assuming point-like pair production. In order to study contributions from strong interaction, a second more sophisticated model was applied, considering besides Coulomb FSI also strong FSI via the resonances f0(980) and a0(980) and a variable distance r∗ between the produced K mesons. This analysis was based on the three theoretical models with different parameter sets for the pairs production description.