J/psi Production as a Function of Charged Particle Multiplicity in pp Collisions at sqrt{s} = 7 TeV

The ALICE collaboration reports the measurement of the inclusive $\jpsi$ yield as a function of charged particle pseudorapidity density d$N_\rm{ch}$/d$\eta$ in pp collisions at $\sqrt{s}$ = 7 TeV at the LHC. $\jpsi$ particles are detected for $\pt$ > 0, in the rapidity interval |y| < 0.9 via decay into e$^+$e$^-$, and in the interval 2.5 < y < 4.0 via decay into $\mu^{+}\mu^{-}$ pairs. An approximately linear increase of the $\jpsi$ yields normalized to their event average (d$N_{\jpsi}/$d$y$)/$\langle$d$N_{\jpsi}/$d$y\rangle$ with d$N_\rm{ch}$/d$\eta$/$\langle$d$N_{ch}/$d$\eta \rangle$ is observed in both rapidity ranges, where d$N_\rm{ch}$/d$\eta$ is measured within |$\eta$| < 1 and $\pt$ > 0. In the highest multiplicity interval with $\langle$d$N_\rm{ch}$/d$\eta \rangle ({\rm {bin}})$ = 24.1, corresponding to four times the minimum bias multiplicity density, an enhancement relative to the minimum bias $\jpsi$ yield by a factor of about 5 at 2.5 < $y$ < 4 (8 at |$y$| < 0.9) is observed.

Figures

Figure 1

The distribution of the relative charged particle density (d$N_\rm{ch}$/d$\eta$)/$\langle$d$N_\rm{ch}$/d$\eta$$\rangle$ reconstructed around mid-rapidity ($|\eta| < 1.0$) after correction for SPD inefficiencies. The vertical lines indicate the boundaries of the multiplicity intervals used in this analysis
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Figure 2

Opposite sign invariant mass spectra of the selected electron[(a) + (c)] and muon [(b) + (d)] pairs (filled symbols) for the lowest [(a) + (b)] and highest [(c) + (d)] multiplicity intervals. Also shown are the estimates of the combinatorial background which are based on a fit to the $\mu^{+}\mu^-$ pair distributions (solid line), and on like-sign pairs (open circles), as well as track rotation (open squares), in the e$^{+}$e$^{-}$ case. The number of events quoted in the figures refer to the corresponding minimum bias triggered events.
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Figure 3

$\jpsi$ yield d$N_{\jpsi}/$d$y$ as a function of the charged particle multiplicity densities at mid-rapidity d$N_\rm{ch}$/d$\eta$. Both values are normalized by the corresponding value for minimum bias pp collisions ($\langle$d$N_{\jpsi}/$d$y$$\rangle$, $\langle$d$N_\rm{ch}$/d$\eta$$\rangle$). Shown are measurements at forward rapidities ($\jpsi \rightarrow \mu^{+}\mu^{-}$, $2.5 < y < 4$) and at mid-rapidity ($\jpsi \rightarrow e^{+}e^{-}$, $|y| < 0.9$) The error bars represent the statistical uncertainty on the $\jpsi$ yields, while the quadratic sum of the point-by-point systematic uncertainties on the $\jpsi$ yield as well as on d$N_\rm{ch}$/d$\eta$ is depicted as boxes.
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Figure 4

Relative $\jpsi$ yield d$N_{\jpsi}/$d$y$ as a function of relative charged particle multiplicity densities around mid-rapidity d$N_\rm{ch}$/d$\eta$ as calculated with PYTHIA 6.4 in the Perugia 2011 tune. Shown are results for directly produced $\jpsi$ in hard scatterings via the NRQCD framework at forward rapidities ($2.5 < y < 4$) and at mid-rapidity ($|y|< 0.9$)
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