Photoproduction of K${}^{+}$K${}^{-}$ pairs in ultra-peripheral collisions

Abstract

K${}^{+}$K${}^{-}$ pairs may be produced in photonuclear collisions, either from the decays of photoproduced $\varphi (1020)$ mesons, or directly as non-resonant K${}^{+}$K${}^{-}$ pairs. Measurements of K${}^{+}$K${}^{-}$ photoproduction probe the couplings between the $\varphi (1020)$ 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 ultra-peripheral 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 $1.1<M_{\mathrm{KK}}<1.4$ GeV/$c^2$ above the $\varphi (1020)$ resonance, for rapidity $|y_{\mathrm{KK}}|<0.8$ and $p_{\mathrm{T},\mathrm{KK}}<0.1$ GeV/$c$, the measured coherent photoproduction cross section is $\mathrm{d}\sigma /\mathrm{d}y$ = 3.37 $\pm 0.61$ (stat.) $\pm 0.15$ (syst.) mb. The center-of-mass energy per nucleon of the photon-nucleus (Pb) system $W_{\gamma \mathrm{Pb},\mathrm{n}}$ ranges from 33 to 188 GeV, far higher than previous measurements on heavy-nucleus targets. The cross section is larger than expected for $\varphi (1020)$ photoproduction alone. The mass spectrum is fit to a cocktail consisting of $\varphi (1020)$ 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.

arXiv comment18 pages, 3 captioned figures, authors from page 12, published version, figures at http://alice-publications.web.cern.ch/node/10259

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