Evidence for Colour-Octet Mechanism from CERN LEP2 gamma gamma -> J/psi + X Data

TL;DR

Practical predictions for the transverse-momentum distribution of J/psi mesons promptly produced in gammagamma collisions within the factorization formalism of nonrelativistic quantum chromodynamics are presented.

Abstract

We present theoretical predictions for the transverse-momentum distribution of J/psi mesons promptly produced in gamma gamma collisions within the factorization formalism of nonrelativistic quantum chromodynamics, including the contributions from both direct and resolved photons, and we perform a conservative error analysis. The fraction of J/psi mesons from decays of bottom-flavoured hadrons is estimated to be negligibly small. New data taken by the DELPHI Collaboration at LEP2 nicely confirm these predictions, while they disfavour those obtained within the traditional colour-singlet model.

Figures (3)

• Figure 1: The cross section $d\sigma/dp_T^2$ of $e^+e^-\to e^+e^-J/\psi+X$ measured by DELPHI delphi as a function of $p_T^2$ is compared with the theoretical predictions of NRQCD and the CSM. The solid and dashed lines represent the central predictions obtained with the ME sets referring to the MRST98LO mrst (default) and CTEQ5L cteq PDFs, respectively, while the shaded bands indicate the theoretical uncertainties on the default predictions. The arrows indicate the NRQCD prediction for $p_T=0$ and its ${}^3\!P_J^{(1)}$, ${}^1\!S_0^{(8)}$, ${}^3\!S_1^{(8)}$, and ${}^3\!P_J^{(8)}$ components.
• Figure 2: Data-over-theory representation of Fig. \ref{['f1']}.
• Figure 3: The normalized cross section $(1/\sigma)d\sigma/dp_T^2$ of $e^+e^-\to e^+e^-J/\psi+X$ measured by DELPHI delphi as a function of $p_T^2$ is compared with the default theoretical predictions of NRQCD and the CSM. The shaded bands indicate the theoretical uncertainties.