CMS-HIN-19-007

CMS-HIN-19-007 ; CERN-EP-2021-104
Fragmentation of jets containing a prompt $\mathrm{J}/\psi$ meson in PbPb and pp collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV
CMS Collaboration
23 June 2021
Phys. Lett. B 825 (2021) 136842
Abstract: Jets containing a prompt $\mathrm{J}/\psi$ meson are studied in lead-lead collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV, using the CMS detector at the LHC. Jets are selected to be in the transverse momentum range of 30 $ < {p_{\mathrm{T}}} < $ 40 GeV. The $\mathrm{J}/\psi$ yield in these jets is evaluated as a function of the jet fragmentation variable $z$, the ratio of the $\mathrm{J}/\psi$ ${p_{\mathrm{T}}}$ to the jet ${p_{\mathrm{T}}}$. The nuclear modification factor, ${R_{\mathrm{AA}}} $, is then derived by comparing the yield in lead-lead collisions to the corresponding expectation based on proton-proton data, at the same nucleon-nucleon center-of-mass energy. The suppression of the $\mathrm{J}/\psi$ yield shows a dependence on $z$, indicating that the interaction of the $\mathrm{J}/\psi$ with the quark-gluon plasma formed in heavy ion collisions depends on the fragmentation that gives rise to the $\mathrm{J}/\psi$ meson.
Links: e-print arXiv:2106.13235 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Projections of a two-dimensional fit used to extract the prompt ${\mathrm{J}/\psi}$ yield in PbPb collisions. Left : The dimuon invariant mass distribution. Right : The pseudo-proper decay length ($l_{{\mathrm{J}/\psi}}$) distribution. The fit functions are described in the text. The lower panels display the pull distributions, defined as the difference between the fit and the data, divided by the combined statistical uncertainty in the fit and in the data.
png pdf	Figure 1-a: The dimuon invariant mass distribution, together with the projection of a two-dimensional fit used to extract the prompt ${\mathrm{J}/\psi}$ yield in PbPb collisions. The fit functions are described in the text. The lower panel displays the pull distribution, defined as the difference between the fit and the data, divided by the combined statistical uncertainty in the fit and in the data.
png pdf	Figure 1-b: The pseudo-proper decay length ($l_{{\mathrm{J}/\psi}}$) distribution, together with the projection of a two-dimensional fit used to extract the prompt ${\mathrm{J}/\psi}$ yield in PbPb collisions. The fit functions are described in the text. The lower panel displays the pull distribution, defined as the difference between the fit and the data, divided by the combined statistical uncertainty in the fit and in the data.
png pdf	Figure 2: Detector response matrices for jets containing a prompt ${\mathrm{J}/\psi}$ meson, showing the bin migration probability as a function of jet ${p_{\mathrm {T}}}$ and $z$. The response for pp collisions (left) is evaluated using PYTHIA 8. The response for PbPb collisions (right) is evaluated using PYTHIA 8 embedded into HYDJET.
png pdf	Figure 2-a: Detector response matrices for jets containing a prompt ${\mathrm{J}/\psi}$ meson, showing the bin migration probability as a function of jet ${p_{\mathrm {T}}}$ and $z$. The response for pp collisions is evaluated using PYTHIA 8.
png pdf	Figure 2-b: Detector response matrices for jets containing a prompt ${\mathrm{J}/\psi}$ meson, showing the bin migration probability as a function of jet ${p_{\mathrm {T}}}$ and $z$. The response for PbPb collisions is evaluated using PYTHIA 8 embedded into HYDJET.
png pdf	Figure 3: The main sources of systematic uncertainty, as a function of $z$, for the cross section in pp (left) and the $T_{{\mathrm {AA}}}$-scaled yield in PbPb (right) collisions. The normalization uncertainties of 1.9% for pp and 2.6% for PbPb are not shown.
png pdf	Figure 3-a: The main sources of systematic uncertainty, as a function of $z$, for the cross section in pp collisions. The normalization uncertainty of 1.9% for pp is not shown.
png pdf	Figure 3-b: The main sources of systematic uncertainty, as a function of $z$, for the $T_{{\mathrm {AA}}}$-scaled yield in PbPb collisions. The normalization uncertainty of 2.6% for PbPb is not shown.
png pdf	Figure 4: Normalized $z$ distribution in pp collisions, compared to prompt and nonprompt ${\mathrm{J}/\psi}$ in PYTHIA 8, at the generator level. Bars indicate statistical uncertainties, while systematic uncertainties are depicted as boxes.
png pdf	Figure 5: Left : The ${\mathrm{J}/\psi}$ differential cross section in pp and the $T_{{\mathrm {AA}}}$-scaled yield in PbPb collisions, as a function of $z$. The normalization uncertainties of 1.9% for pp and 2.6% for PbPb are not included in the point-by-point uncertainty boxes and not shown in the plot. Right : The nuclear modification factor ${R_ {\mathrm {AA}}}$, as a function of $z$. Bars indicate statistical uncertainties, while systematic uncertainties are depicted as boxes. The box around unity shows the normalization uncertainty.
png pdf	Figure 5-a: The ${\mathrm{J}/\psi}$ differential cross section in pp and the $T_{{\mathrm {AA}}}$-scaled yield in PbPb collisions, as a function of $z$. The normalization uncertainties of 1.9% for pp and 2.6% for PbPb are not included in the point-by-point uncertainty boxes and not shown in the plot.
png pdf	Figure 5-b: The nuclear modification factor ${R_ {\mathrm {AA}}}$, as a function of $z$. Bars indicate statistical uncertainties, while systematic uncertainties are depicted as boxes. The box around unity shows the normalization uncertainty.
png pdf	Figure 6: The nuclear modification factor ${R_ {\mathrm {AA}}}$ for two centrality selections of PbPb collisions, as a function of $z$. Due to limited statistical precision, the lowest $z$ bin is excluded. Bars indicate statistical uncertainties, while systematic uncertainties are depicted as boxes. The boxes around unity show the normalization uncertainties.

Summary

Jets containing a prompt $\mathrm{J}/\psi$ meson were studied in proton-proton (pp) and lead-lead (PbPb) collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV, for jets with transverse momentum 30 $ < {p_{\mathrm{T}}} < $ 40 GeV and pseudorapidity $|\eta| < $ 2. The distribution of the fragmentation variable $z$, the ratio of the $\mathrm{J}/\psi$ ${p_{\mathrm{T}}}$ to that of the jet, was measured in both systems. In pp collisions, prompt $\mathrm{J}/\psi$ mesons were found to have more surrounding jet activity, i.e., , to populate lower values of $z$ than predicted by PYTHIA 8 simulations, suggesting that $\mathrm{J}/\psi$ production late in the parton shower is underestimated. The pp and PbPb distributions were compared by calculating the nuclear modification factor, ${R_{\mathrm{AA}}} $, the ratio of PbPb data to the expectation based on pp data. The value of ${R_{\mathrm{AA}}} $ as a function of $z$ shows a rising trend. The suppression at low $z$ is found to be larger in the 20% most central events (i.e., "head-on'' collisions), as compared to the less central selection. The results show explicitly that the $\mathrm{J}/\psi$ produced with a large degree of surrounding jet activity are more highly suppressed than those produced in association with fewer particles. This finding emphasizes the importance of incorporating the jet quenching mechanism in models of $\mathrm{J}/\psi$ suppression.

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