Inclusive J/psi production in pp collisions at sqrt(s) = 2.76 TeV

TL;DR

ALICE measures inclusive J/psi production in pp collisions at sqrt(s)=2.76 TeV in two rapidity regions using J/psi→e+e− and J/psi→μ+μ− decays. The analysis yields integrated and differential cross sections with comprehensive acceptance, efficiency, and polarization systematics, and compares results to 7 TeV data and to NLO NRQCD predictions, finding reasonable agreement. The study provides a key reference point for Pb–Pb analyses at the same energy and demonstrates ALICE's capability to access low-pT J/psi production, enriching the energy evolution picture of charmonium production at the LHC.

Abstract

The ALICE Collaboration has measured inclusive J/psi production in pp collisions at a center of mass energy sqrt(s)=2.76 TeV at the LHC. The results presented in this Letter refer to the rapidity ranges |y|<0.9 and 2.5<y<4 and have been obtained by measuring the electron and muon pair decay channels, respectively. The integrated luminosities for the two channels are L^e_int=1.1 nb^-1 and L^mu_int=19.9 nb^-1, and the corresponding signal statistics are N_J/psi^e+e-=59 +/- 14 and N_J/psi^mu+mu-=1364 +/- 53. We present dsigma_J/psi/dy for the two rapidity regions under study and, for the forward-y range, d^2sigma_J/psi/dydp_t in the transverse momentum domain 0<p_t<8 GeV/c. The results are compared with previously published results at sqrt(s)=7 TeV and with theoretical calculations.

Figures (7)

• Figure 1: Top panel: invariant mass distributions for opposite-sign (OS) and like-sign (LS) electron pairs ($|y|<0.9$, all $p_{\rm t}$). The background estimate from the TrkRot method (see text for details) is also shown. Bottom panel: the difference of the OS and LS distributions with the normalized MC signal shape superimposed.
• Figure 2: Invariant mass distribution for opposite-sign muon pairs ($2.5<y<4$, all $p_{\rm t}$), in the mass region $2<m_{\mu\mu}<5$ GeV/$c^2$, with the result of the fit (see text for details). The fitted J/$\psi$ and $\psi(2S)$ contributions, as well as the background, are also shown.
• Figure 3:
• Figure 4: Double differential J/$\psi$ production cross section at $\sqrt{s}=2.76$ TeV compared to previous ALICE results at $\sqrt{s}=7$ TeV Aam11. The vertical error bars represent the statistical errors while the boxes correspond to the systematic uncertainties. The systematic uncertainties on luminosity are not included. The results are compared with a NLO NRQCD calculation But11a performed in the region $p_{\rm t}>3$ GeV/$c$.
• Figure 5: The $\sqrt{s}$-dependence of $\langle p_{\rm t}\rangle$ for inclusive J/$\psi$ production, for various fixed-target and collider experiments. For the ALICE points the error bars represent the quadratic sum of statistical and systematic uncertainties. The points for $\sqrt{s}=7$ TeV have been slightly shifted to improve visibility.