Photoproduction of K⁺K⁻ Pairs in Ultraperipheral Collisions

Abstract

K⁺K⁻ pairs may be produced in photonuclear collisions, either from the decays of photoproduced ϕ(1020) mesons or directly as nonresonant K⁺K⁻ pairs. Measurements of K⁺K⁻ photoproduction probe the couplings between the ϕ(1020) 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 K⁺K⁻ pairs on lead ions in ultraperipheral collisions using the ALICE detector, including the first investigation of direct K⁺K⁻ production. There is significant K⁺K⁻ production at low transverse momentum, consistent with coherent photoproduction on lead targets. In the mass range 1.1 < Mĸĸ < 1.4 GeV=c2 above the ϕ(1020) resonance, for rapidity |Yĸĸ| < 0.8 and Pᴛ,ĸĸ < 0.1 GeV=c, the measured coherent photoproduction cross section is dσ = dy ¼ 3.37 ± 0.61(stat) ± 0.15(syst) 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 ϕ(1020) photoproduction alone. The mass spectrum is fit to a cocktail consisting of ϕ(1020) decays, direct K⁺K⁻ photoproduction, and interference between the two. The confidence regions for the amplitude and relative phase angle for direct K⁺K⁻ photoproduction are presented.