Energy Dependence of Direct-Quarkonium Production in pp Collisions from Fixed-Target to LHC Energies: Complete One-Loop Analysis

We compute the energy dependence of the P_T-integrated cross section of directly produced quarkonia in pp collisions at next-to-leading order (NLO), namely up to alpha_s^3, within nonrelativistic QCD (NRQCD). Our analysis is based on the idea that the P_T-integrated and the P_T-differential cross sections can be treated as two different observables. The colour-octet NRQCD parameters needed to predict the P_T-integrated yield can thus be extracted from the fits of the P_T-differential cross sections at mid and large P_T. For the first time, the total cross section is evaluated in NRQCD at full NLO accuracy using the recent NLO fits of the P_T-differential yields at RHIC, the Tevatron and the LHC. Both the normalisation and the energy dependence of the J/psi, psi' and Upsilon(1S), we obtained, are in disagreement with the data irrespective of the fit method. The same is true if one uses CEM-like colour-octet NRQCD parameters. If, on the contrary, one disregards the colour-octet contribution, the existing data in the TeV range are well described by the alpha_s^3 contribution in the colour-singlet model --which, at alpha_s^4, however shows an unphysical energy dependence. A similar observation is made for eta(c,b). This calls for a full NNLO or for a resummation of the initial-state radiation in this channel. In any case, past claims that colour-octet transitions are dominantly responsible for low-P_T quarkonium production are not supported by our results. This may impact the interpretation of quarkonium suppression in high-energy proton-nucleus and nucleus-nucleus collisions.Comment: 15 pages, 22 Figures, LaTeX uses svepjc3.clo, svglov3.clo, svjour3.cls (included

Study of the rare decay J/ψ→2γ+hadronsJ/\psi \to 2\gamma+hadrons at the BESIII

Two-photon radiative decay process $J/\psi \to 2\gamma+hadrons$ is studied and the branch ratio of the leading contribution $J/\psi \to 2\gamma + g g g$ is calculated, which is $(1.39 \pm 0.02)\times10^{-6}.$ With the specific condition at the BESIII, this rare decay process and the main background process $e^{+} e^{-} \to \gamma \gamma q \bar{q}$ are investigated. The results show that the ratio of signal to background can reach 27% with the optimized selection criteria at the BESIII. In addition, a few distribution of the signal and background are presented, and the results show that the signal is large enough to be measured in experiment.Comment: 7 pages, 13 figure