Multiplicity dependence of $Υ$ production at forward rapidity in pp collisions at $\sqrt{s}$ = 13 TeV

The measurement of $\Upsilon$(1S), $\Upsilon$(2S), and $\Upsilon$(3S) yields as a function of the charged-particle multiplicity density ${\rm d}N_{\rm ch}/{\rm d}\eta$, using the ALICE experiment at the LHC, is reported in pp collisions at $\sqrt{s}$ = 13 TeV. The $\Upsilon$ meson yields are measured at forward rapidity ($2.5 <~ y <~ 4$) in the dimuon decay channel, whereas the charged-particle multiplicity is defined at central rapidity ($|\eta| <~ 1$). Both quantities are normalized to their average value in minimum bias events. The increase of the self-normalized $\Upsilon$(1S), $\Upsilon$(2S), and $\Upsilon$(3S) yields is found to be compatible with a linear scaling with the self-normalized ${\rm d}N_{\rm ch}/{\rm d}\eta$, within the uncertainties. The measured $\Upsilon$ excited-to-ground state self-normalized yield ratios are compatible with unity within uncertainties. Similarly, the measured double ratio of the self-normalized $\Upsilon$(1S) to the self-normalized J/$\psi$ yields, both measured at forward rapidity, is compatible with unity for self-normalized charged-particle multiplicity beyond one. The measurements are compared with theoretical predictions incorporating initial or final state effects.

 

Submitted to: PLB
e-Print: arXiv:2209.04241 | PDF | inSPIRE
CERN-EP-2022-174
Figure group

Figure 1

Dimuon invariant mass distribution for low-multiplicity pp collisions, corresponding to the $N_{\mathrm {trk}}^{\mathrm {corr}}$ interval bin $[1, 8]$ (left) and for high-multiplicity pp collisions, corresponding to the $N_{\mathrm {trk}}^{\mathrm {corr}}$ interval bin $[42, 50]$ (right)..

Figure 2

Self-normalized yield of $\Upsilon$(nS) states as a function of normalized charged-particle multiplicity for $p_{\rm{T}} > 0$. The error bars represent the statistical uncertainty on the $\Upsilon$ yields, while the quadratic sum of the point-by-point systematic uncertainties on the $\Upsilon$ yield as well as on $\textrm{d}N_{\mathrm{ch}} / \textrm{d}\eta$ / $\langle \textrm{d}N_{\mathrm{ch}}/\textrm{d}\eta \rangle$ is depicted as boxes. The dashed line shown in the top panel represents a linear function with the slope equal to unity..

Figure 3

Self-normalized yield of $\Upsilon$(nS) states as a function of normalized charged-particle multiplicity for $p_{\rm{T}} > 0$, compared to 3-pomeron CGC approach , PYTHIA 8.2  and CPP . The dashed line represents a linear function with the slope equal to unity. .

Figure 4

Top: Excited-to-ground state self-normalized yield ratio ($\Upsilon$(2S) over $\Upsilon$(1S)) as a function of self-normalized charged-particle multiplicity, compared to model predictions from 3-pomeron CGC approach , PYTHIA 8.2 , comovers , and CPP  predictions; Bottom: Excited-to-ground state self-normalized yield ratio ($\Upsilon$(3S) over $\Upsilon$(1S)) as a function of self-normalized multiplicity, compared to PYTHIA 8.2 and comovers predictions..

Figure 5

Top: Self-normalized yield of $\Upsilon$ as a function of self-normalized charged-particle multiplicity, compared to inclusive J/$\psi$ measured in the forward rapidity region at 5.02 TeV , 7 TeV , and 13 TeV , and to inclusive J/$\psi$ measured in the central rapidity region at 13 TeV . The error bars represent the statistical uncertainty on the quarkonium yields, while the quadratic sum of the point-by-point systematic uncertainties on the quarkonium yields as well as on $\textrm{d}N_{\mathrm{ch}} / \textrm{d}\eta$ / $\langle \textrm{d}N_{\mathrm{ch}}/\textrm{d}\eta \rangle$ is depicted as boxes. Bottom: Self-normalized yield ratio of $\Upsilon$(1S) over J/$\psi$ as a function of self-normalized charged-particle multiplicity, compared to model computations from 3-pomeron CGC approach , PYTHIA 8.2 , comovers , and CPP ..