Study of Inclusive J/psi Production in Two-Photon Collisions at LEP II with the DELPHI Detector

TL;DR

The paper reports the first observation of inclusive $J/\psi$ production in two-photon collisions at LEP II using the DELPHI detector. It analyzes the data to separate diffractive (VDM) and resolved photon contributions, finding the latter dominates and is sensitive to the gluon content of the photon. The measured cross-section $\sigma(J/\psi+X)=45\pm9\,\text{(stat)}\pm17\,\text{(syst)}$ pb, together with a PYTHIA-based decomposition indicating ~74% of events from resolved photons, provides a direct probe of photon structure in $\gamma\gamma$ interactions. The results show that the observed distributions in $p_T^2(J/\psi)$, $y$, and angular variables are consistent with the PYTHIA mixture, supporting the relevance of the gluon content of photons in heavy quarkonium production.

Abstract

Inclusive J/psi production in photon-photon collisions has been observed at LEP II beam energies. A clear signal from the reaction gamma gamma -> J/psi+X is seen. The number of observed N(J/psi -> mu+mu-) events is 36 +/- 7 for an integrated luminosity of 617 pb^{-1}, yielding a cross-section of sigma(J/psi+X) = 45 +/- 9 (stat) +/- 17 (syst) pb. Based on a study of the event shapes of different types of gamma gamma processes in the PYTHIA program, we conclude that (74 +/- 22)% of the observed J/psi events are due to `resolved' photons, the dominant contribution of which is most probably due to the gluon content of the photon.

Figures (8)

• Figure 1: Inclusive $J/\psi$ production in $\gamma\gamma$ processes: a) through vector-meson dominance, b) via gluon content of the photon, i.e. 'resolved' contributions.
• Figure 2: $W_{\rm vis}$ distributions for the LEP II DELPHI data, for the simulated $\gamma\gamma\rightarrow \ {\rm hadrons}$, ${e}^+{e}^-\rightarrow {Z}^0 \gamma$, ${e}^+{e}^-\rightarrow {W}^+ {W}^-$ and the sum of all above Monte-Carlo contributions
• Figure 3: $M(\mu^+\mu^-)$ distribution from the LEP II DELPHI data. The solid curve corresponds to a Gaussian fit over a second order polynomial background.
• Figure 4: $p_{_T}^2(\hbox{$J{}/{}\psi$})$ distribution from the LEP II DELPHI data, shown as points with error bars. The histogram is a combination of the normalized 'resolved' and 'diffractive' processes from PYTHIA (see text).
• Figure 5: Efficiencies for resolved and diffractive processes as functions of $p_{_T}^2$.