Investigation of K+K- pairs in the effective mass region near 2mK

The DIRAC experiment at CERN investigated in the reaction p(24 GeV/c) + Ni the particle pairs K+K-, pi(+pi)-, and p p over line with relative momentum Q in the pair system less than 100 MeV/c. Because of background influence studies, DIRAC explored three subsamples of K+K- pairs, obtained by subtracting -using the time-of-flight (TOF) technique-the background from initial Q distributions with K+K- sample fractions more than 70%, 50%, and 30%. The corresponding pair distributions in Q and in its longitudinal projection Q(L) were analyzed first in a Coulomb model, which takes into account only the Coulomb final -state interaction (FSI) and assuming pointlike pair production. This Coulomb model analysis leads to a K+K- yield increase of about four at Q(L) = 0.5 MeV/c compared to 100 MeV/c. In order to study contributions from strong interaction, a second more sophisticated model was applied, considering also strong FSI via the resonances f(0)(980) and a(0)(980) and a variable distance r* between the produced K mesons besides Coulomb FSI. This analysis was based on three different parameter sets for the pair production. For the 70% subsample and with the best parameters, 3680 +/- 370 K+K- pairs were found to be compared to 3900 +/- 410 K+K- extracted by means of the Coulomb model. Knowing the efficiency of the TOF cut for background suppression, the total number of detected K+K- pairs was evaluated to be around 40000 +/- 10%, which agrees with the result from the 30% subsample. The K+K- pair number in the 50% subsample differs from the two other values by about three standard deviations, confirming-as discussed in the paper-that experimental data in this subsample is less reliable. In summary, the upgraded DIRAC experiment observed increased K+K- production at small relative momentum Q. The pair distribution in Q is well described by Coulomb FSI, whereas a potential influence from strong interaction in this Q region is insignificant within experimental errors.