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

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

Figure 1

Top panel: invariant mass distributions for opposite-sign (OS) andlike-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 OSand LS distributions with the normalized MC signal shape superimposed.
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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 ofthe fit (see text for details). The fitted J/$\psi$ and $\psi(2S)$ contributions, as well as thebackground, are also shown.
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Figure 3

Invariant mass spectra for OS muon pairs ($2.5< y< 4$), in bins of $p_{\rm t}$. The results of the fits are also shown.
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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~ The vertical error bars represent the statistical errorswhile the boxes correspond to the systematic uncertainties. The systematic uncertainties on luminosity are not included The results are compared with a NLO NRQCD calculation~ performed in the region $p_{\rm t}>3$ GeV/$c$.
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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.
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Figure 6

Differential J/$\psi$ production cross section at $\sqrt{s}=2.76$~TeV compared to previous ALICE results at $\sqrt{s}=7$~TeV~ The vertical error bars represent the statistical errorswhile the boxes correspond to the systematic uncertainties. The systematic uncertainties on luminosity are not included.
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Figure 7

$\sqrt{s}$-dependence of the inclusive J/$\psi$ production cross section ${\rm d}\sigma/{\rm d}y$, at central rapidity for various collider experiments.
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