CMS-HIN-16-002

CMS-HIN-16-002 ; CERN-EP-2017-285
Study of jet quenching with isolated-photon+jet correlations in PbPb and pp collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV
CMS Collaboration
27 November 2017
Phys. Lett. B 785 (2018) 14
Abstract: Measurements of azimuthal angle and transverse momentum (${p_{\mathrm{T}}}$) correlations of isolated photons and associated jets are reported for pp and PbPb collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV. The data were recorded with the CMS detector at the CERN LHC. For events containing a leading isolated photon with ${p_{\mathrm{T}}^{\gamma}} > $ 40 GeV/$c$ and an associated jet with ${p_{\mathrm{T}}^{\text{jet}}} > $ 30 GeV/$c$ , the photon+jet azimuthal correlation and ${p_{\mathrm{T}}}$ imbalance in PbPb collisions are studied as functions of collision centrality and ${p_{\mathrm{T}}^{\gamma}}$. The results are compared to pp reference data collected at the same collision energy and to predictions from several theoretical models for parton energy loss. No evidence of broadening of the photon+jet azimuthal correlations is observed, while the ratio ${p_{\mathrm{T}}^{\text{jet}}}/{p_{\mathrm{T}}^{\gamma}}$ decreases significantly for PbPb data relative to the pp reference. All models considered agree within uncertainties with the data. The number of associated jets per photon with ${p_{\mathrm{T}}^{\gamma}} > $ 80 GeV/$c$ is observed to be shifted towards lower ${p_{\mathrm{T}}^{\text{jet}}}$ values in central PbPb collisions compared to pp collisions.
Links: e-print arXiv:1711.09738 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The centrality dependence of the shower shape variable $\sigma _{\eta \eta}$ for photons with $ {{p_{\mathrm {T}}} ^\gamma} \in $ (40,50) GeV/$c$. The black points show the PbPb experimental results, the red histograms are the signal templates from PYTHIA+HYDJET simulations, and the green histograms are the background templates obtained from a nonisolated sideband region in data.
png pdf	Figure 2: The azimuthal correlation of photons and jets in five ${{p_{\mathrm {T}}} ^\gamma}$ intervals for 0-30% centrality (top, full circles) and 30-100% centrality (bottom, full squares) PbPb collisions. The smeared pp data (open symbols) are included for comparison. The vertical lines (bands) through the points represent statistical (systematic) uncertainties.
png pdf	Figure 3: Distribution of $ {x_{\mathrm {j}\gamma}} = {{p_{\mathrm {T}}} ^\text {jet}} / {{p_{\mathrm {T}}} ^\gamma} $ in five ${{p_{\mathrm {T}}} ^\gamma}$ intervals for 0-30% centrality (top, full circles) and 30-100% centrality (bottom, full squares) PbPb collisions. The smeared pp data (open symbols) are included for comparison. The vertical lines (bands) through the points represent statistical (systematic) uncertainties.
png pdf	Figure 4: The $ < x_{\mathrm {j}\gamma} > $ values (top) and ${R_{\mathrm {j}\gamma}}$, the number of associated jets per photon (bottom), in 0-30% centrality (left, full circles) and 30-100% centrality (right, full squares) PbPb collisions. The smeared pp data (open symbols) are added for comparison. The vertical lines (bands) through the points represent statistical (systematic) uncertainties.
png pdf	Figure 5: The ${\mathrm {I_\mathrm {AA}^\text {jet}}} $ vs. $ {{p_{\mathrm {T}}} ^\text {jet}}$ for 0-30% centrality (top) and 30-100% centrality (bottom) PbPb collisions. The vertical lines (bands) through the points represent statistical (systematic) uncertainties.
png pdf	Figure 6: The centrality dependence of ${x_{\mathrm {j}\gamma}}$ of photon+jet pairs normalized by the number of photons for PbPb (full markers) and smeared pp (open markers) data. The vertical lines (bands) through the points represent statistical (systematic) uncertainties.
png pdf	Figure 7: The $ < x_{\mathrm {j}\gamma} > $ (top) and ${R_{\mathrm {j}\gamma}}$ (bottom) as a function of $ < N_{\text {part}} > $ for $ {{p_{\mathrm {T}}} ^\gamma} > $ 60 GeV/$c$ (left) and $ {{p_{\mathrm {T}}} ^\gamma} > $ 80 GeV/$c$ (right). The PbPb results (full markers) are compared to pp results (open markers) smeared by the relative jet energy resolution corresponding to each centrality interval. The vertical lines (bands) through the points represent statistical (systematic) uncertainties.
png pdf	Figure 8: The azimuthal correlation of photons and jets in five ${{p_{\mathrm {T}}} ^\gamma}$ intervals for 0-30% centrality (top, full circles) and 30-100% centrality (bottom, full squares) PbPb collisions. The data points shown are identical to those in Fig. 2. Theoretical calculations from jewel [57,33], LBT [30], and hybrid model [31,32] are included for comparison.
png pdf	Figure 9: The $ {x_{\mathrm {j}\gamma}}$ distributions in five ${{p_{\mathrm {T}}} ^\gamma}$ intervals for 0-30% centrality (top, full circles) and 30-100% centrality (bottom, full squares) PbPb collisions. The data points shown are identical to those in Fig. 3. Theoretical calculations from jewel [57,33], LBT [30], and hybrid model [31,32] are included for comparison.

Tables
png pdf	Table 1: Jet resolution parameters for pp and PbPb collisions. A global uncertainty of 7% is assigned to the smearing parameters, evaluated as described in text.
png pdf	Table 2: Summary of the relative systematic uncertainties (in%) for $ {{p_{\mathrm {T}}} ^\gamma} > $ 40 GeV/$c$.
png pdf	Table 3: Summary of the absolute systematic uncertainties on $({1}/{N_{{\mathrm {j}}\gamma}}) ({{\mathrm {d}}{}N}/{{\mathrm {d}}{}\phi _{\mathrm {j}\gamma}})$ for $ {{p_{\mathrm {T}}} ^\gamma} > $ 40 GeV/$c$, averaged over the ${\Delta \phi _{\mathrm {j}\gamma}}$ distributions.

Summary

Correlations of isolated photons with transverse momentum ${p_{\mathrm{T}}^{\gamma}} > $ 40 GeV/$c$ and pseudorapidity $ | {{\eta^{\gamma}}} | < $ 1.44 and associated jets with ${p_{\mathrm{T}}^{\text{jet}}} > $ 30 GeV/$c$ and ${\eta^{\text{jet}}} < $ 1.6, have been studied for the first time in pp and PbPb collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV, using a large data sample collected by the CMS experiment. No significant azimuthal angular broadening between photons and the associated jets is observed in PbPb data as compared to pp data, for all event centralities and multiple photon ${p_{\mathrm{T}}}$ intervals. The ${x_{\mathrm{j}\gamma}} = {p_{\mathrm{T}}^{\text{jet}}}/{p_{\mathrm{T}}^{\gamma}}$ and the average number of associated jets per photon, ${R_{\mathrm{j}\gamma}} $, are studied in different leading photon ${p_{\mathrm{T}}}$ and PbPb collision centrality intervals. For all ${p_{\mathrm{T}}^{\gamma}} > $ 60 GeV/$c$ intervals, the ${< x_{\mathrm{j}\gamma} >}$ and ${R_{\mathrm{j}\gamma}} $ values in the 0-30% most central PbPb collisions are found to be lower than those in the corresponding pp reference data, indicating that a larger fraction of jets lose energy and thus fall below 30 GeV/$c$ in PbPb collisions. The differences between the pp and PbPb results increase as collisions become more central. By comparing the yields of associated jets in PbPb and pp events that include photons with ${p_{\mathrm{T}}^{\gamma}} > $ 80 GeV/$c$, a shift of the jet spectra towards lower ${p_{\mathrm{T}}^{\text{jet}}}$ is observed. These new results are qualitatively similar to those reported at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 2.76 TeV and to calculations from various theoretical models. The better statistical precision of the new higher energy data provides an opportunity to test theoretical models against data over a wide kinematic range in ${p_{\mathrm{T}}^{\gamma}}$ and ${x_{\mathrm{j}\gamma}} $, and for different event centralities, using a selection of partons with well-defined flavor and initial kinematics.

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