Photoproduction of K+K− pairs in ultraperipheral collisions

Abstract

KþK− pairs may be produced in photonuclear collisions, either from the decays of photoproduced ϕð1020Þ mesons or directly as nonresonant KþK− pairs. Measurements of KþK− photoproduction probe the couplings between the ϕð1020Þ and charged kaons with photons and nuclear targets. The kaon-proton scattering occurs at energies far above those available elsewhere. We present the first measurement of coherent photoproduction of KþK− pairs on lead ions in ultraperipheral collisions using the ALICE detector, including the first investigation of direct KþK− production. There is significant KþK− production at low transverse momentum, consistent with coherent photoproduction on lead targets. In the mass range 1.1 < MKK < 1.4 GeV=c2 above the ϕð1020Þ resonance, for rapidity jyKKj < 0.8 and pT;KK < 0.1 GeV=c, the measured coherent photoproduction cross section is dσ=dy ¼ 3.37 0.61ðstatÞ 0.15ðsystÞ mb. The center-of-mass energy per nucleon of the photon-nucleus (Pb) system WγPb;n ranges from 33 to 188 GeV, far higher than previous measurements on heavy-nucleus targets. The cross section is larger than expected for ϕð1020Þ photoproduction alone. The mass spectrum is fit to a cocktail consisting of ϕð1020Þ decays, direct KþK− photoproduction, and interference between the two. The confidence regions for the amplitude and relative phase angle for direct KþK− photoproduction are presented.