CMS-PAS-HIN-16-002

CMS-PAS-HIN-16-002
Study of isolated-photon+jet correlations in PbPb and pp collisions at $\sqrt{s_{NN}} =$ 5.02 TeV
CMS Collaboration
September 2016

Abstract: Measurements of correlations in isolated-photon and jet pairs in pp and PbPb collisions at $\sqrt{s_{NN}} =$ 5.02 TeV are reported. Jets are reconstructed with an anti- $k_\mathrm{T}$ clustering algorithm with a distance parameter of 0.3. For events containing a leading isolated photon with transverse momentum $p_\mathrm{T}^\gamma >$ 40 GeV/ $c$ and an associated jet with $p_\mathrm{T}^\mathrm{Jet} >$ 30 GeV/ $c$ , the photon+jet transverse momentum balance in PbPb collisions is studied as a function of collision centrality and $p_\mathrm{T}^\gamma$ . The results are compared to pp reference data collected at the same collision energy. A significant decrease in the ratio $p_\mathrm{T}^\mathrm{Jet}/p_\mathrm{T}^\gamma$ relative to that in the pp reference is observed.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 785 (2018) 14. The superseded preliminary plots can be found here.

Figures	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: The shower shape variable ( $\sigma _{\eta \eta }$ ) of photons from mid-central (30-50%) PbPb collisions. The black points are PbPb data, the red histogram is the signal template from PYTHIA+HYDJET, and the green histogram is the background template from a non-isolated data sideband. The purity is defined as the fraction of isolated photons in the signal region $\sigma _{\eta \eta }<$ 0.01 used in the isolated-photon + jet analysis.
png pdf	Figure 2: Azimuthal correlation of photons and jets in each ${p_{\mathrm {T}}^\gamma }$ bin (from left to right) for central PbPb (top) and peripheral PbPb (bottom) after mixed event background subtraction. The correlation is shown on a logarithmic scale and its range is restricted to ${\Delta \phi _{\mathrm {J}\gamma }} > \frac {3\pi }{5}$ . The PbPb data is compared to smeared pp data. The lines through the points represent the statistical uncertainty while the shaded boxes represent the systematic uncertainty.
png pdf	Figure 3: Distribution of ${x_{\mathrm {J}\gamma }} = {p_{\mathrm {T}}^\mathrm {Jet}} / {p_{\mathrm {T}}^\gamma }$ in each ${p_{\mathrm {T}}^\gamma }$ bin (from left to right) for central PbPb (top) and peripheral PbPb (bottom). The PbPb data are compared to smeared pp data. The lines through the points represent the statistical uncertainty while the shaded boxes represent the systematic uncertainty.
png pdf	Figure 4: Average jet over photon transverse momentum ratio ( $< x_{\mathrm {J}\gamma } >$ ) of the recoiled jets in (left) smeared pp and central PbPb, and (right) smeared pp and peripheral PbPb. The pp results are smeared by the relative jet energy resolution in order to account for the underlying event fluctuations when compared to PbPb data. The lines through the points represent the statistical uncertainty while the shaded boxes represent the systematic uncertainty.
png pdf	Figure 5: Average number of associated jets per photon ( ${R_{\mathrm {J}\gamma }}$ ) as a function of leading photon ${p_{\mathrm {T}}}$ in (left) smeared pp and central PbPb, and (right) smeared pp and peripheral PbPb. The jet energy in the pp data is smeared by the relative jet energy resolution in order to account for the underlying event fluctuations when compared to PbPb data. The lines through the points represent the statistical uncertainty while the shaded boxes represent the systematic uncertainty.
png pdf	Figure 6: Ratio of jet yield in PbPb collisions to smeared pp. In the low ${p_{\mathrm {T}}^\gamma }$ events, the yields in central PbPb events are smaller than in pp for all ${p_{\mathrm {T}}^\mathrm {Jet}}$ bins. As ${p_{\mathrm {T}}^\gamma }$ increases, yields at low ${p_{\mathrm {T}}^\mathrm {Jet}}$ are greater in PbPb than smeared pp. The lines through the points represent the statistical uncertainty while the shaded boxes represent the systematic uncertainty.
png pdf	Figure 7: Distribution of ${x_{\mathrm {J}\gamma }}$ of photon+jet pairs of pp and PbPb collisions normalized by the number of photon+jet pairs. The momenta of jets in pp are smeared by the relative jet energy resolution to be used as the reference of each centrality bin. The lines through the points represent the statistical uncertainty while the shaded boxes represent the systematic uncertainty.
png pdf	Figure 8: Comparison of ${\Delta \phi _{\mathrm {J}\gamma }}$ width in pp and PbPb collisions shown for different ${p_{\mathrm {T}}^\mathrm {Jet}}$ bins as a function of the average number of participants weighted by the number of collisions. The momenta of jets in pp are smeared by the relative jet energy resolution to be used as the reference of each centrality bin. The lines through the points represent the statistical uncertainty while the shaded boxes represent the systematic uncertainty.
png pdf	Figure 9: Comparison of ${< x_{\mathrm {J}\gamma } > }$ in pp and PbPb collisions as a function of the average number of participants weighted by the number of collisions. The momenta of jets in pp are smeared by the relative jet energy resolution to be used as the reference of each centrality bin. The lines through the points represent the statistical uncertainty while the shaded boxes represent the systematic uncertainty.
png pdf	Figure 10-a: Comparison of ${R_{\mathrm {J}\gamma }}$ in pp and PbPb collisions as a function of the average number of participants weighted by the number of collisions. The momenta of jets in pp are smeared by the relative jet energy resolution to be used as the reference of each centrality bin. The lines through the points represent the statistical uncertainty while the shaded boxes represent the systematic uncertainty.
png pdf	Figure 10-b: Comparison of ${R_{\mathrm {J}\gamma }}$ in pp and PbPb collisions as a function of the average number of participants weighted by the number of collisions. The momenta of jets in pp are smeared by the relative jet energy resolution to be used as the reference of each centrality bin. The lines through the points represent the statistical uncertainty while the shaded boxes represent the systematic uncertainty.

Summary

Studies of isolated-photon+jet correlations in pp and PbPb at

$\sqrt{s_{NN}} =$ 5.02 TeV are reported. The photon+jet transverse momentum ratio,

$x_{\mathrm{J}\gamma} = p_{\mathrm{T}}^{\text{Jet}}/p_{\mathrm{T}}^{\gamma}$ , and the fraction of photons with an associated jet,

$R_{\mathrm{J}\gamma}$ , are studied in bins of leading photon

$p_{\mathrm{T}}$ and PbPb collision centrality. For all

$p_{\mathrm{T}}^{\gamma}$ bins,

$< x_{\mathrm{J}\gamma} >$ and

$R_{\mathrm{J}\gamma}$ in the 0-30% central PbPb collisions are found to be lower than corresponding pp reference values, indicating that a larger fraction of jets lose energy and fall below 30 GeV/

$c$ in the PbPb system. By comparing the yields of jets in PbPb and pp collisions triggered by photons above 80 GeV/

$c$ , a shift of the jet spectra toward the lower

$p_{\mathrm{T}}^{\text{Jet}}$ direction is observed.

Additional Figures
png pdf	Additional Figure 1: The shower shape variable ( $\sigma _{\eta \eta }$ ) of photons from central (0-30%) PbPb collisions. The black points are PbPb data, the red histogram is the signal template from Pythia+Hydjet, and the green histogram is the background template from a non-isolated data sideband. The purity is defined as the fraction of isolated photons in the signal region $\sigma _{\eta \eta } <$ 0.01 used in the isolated-photon+jet analysis.
png pdf	Additional Figure 2: Azimuthal correlation of photons and jets in each $p_{\mathrm {T},\gamma }$ bin (from left to right) for pp smeared to match central resolution (top) and pp smeared to match peripheral resolution (bottom). The correlation is shown on a logarithmic scale with the range restricted to $\Delta \phi _{J\gamma } > \frac {3\pi }{5}$ . The pp reference is compared to several theoretical predictions: JEWEL: [1], [2], LBT 2017: [3], Hybrid Model: [4] [5]. Update Jan 2017: LBT calculations were changed following a correction of the authors of a bug that affected the normalization of the hydrodynamic profile.
png pdf	Additional Figure 3: Distribution of $x_{J\gamma }$ in each $p_{\mathrm {T},\gamma }$ bin (from left to right) for pp smeared to match central resolution (top) and pp smeared to match peripheral resolution (bottom). The pp reference is compared to several theoretical predictions: JEWEL: [1], [2], LBT 2017: [3], Hybrid Model: [4] [5]. Update Jan 2017: LBT calculations were changed following a correction of the authors of a bug that affected the normalization of the hydrodynamic profile.
png pdf	Additional Figure 4: Average jet over photon transverse momentum ratio ( $< x_{J\gamma } >$ ) of the recoiled jets in (left) pp smeared to match central resolution, and (right) and pp smeared to match peripheral resolution. The pp reference is compared to several theoretical predictions: JEWEL: [1], [2], LBT 2017: [3], Hybrid Model: [4] [5]. Update Jan 2017: LBT calculations were changed following a correction of the authors of a bug that affected the normalization of the hydrodynamic profile.
png pdf	Additional Figure 5: Average number of associated jets per photon ( $R_{J\gamma }$ ) as a function of leading photon ${p_{\mathrm {T}}}$ in (left) pp smeared to match central resolution, and (right) and pp smeared to match peripheral resolution. The pp reference is compared to several theoretical predictions: JEWEL: [1], [2], LBT 2017: [3], Hybrid Model: [4] [5]. Update Jan 2017: LBT calculations were changed following a correction of the authors of a bug that affected the normalization of the hydrodynamic profile.
png pdf	Additional Figure 6: Distribution of $x_{J\gamma }$ of photon+jet pairs of pp collisions normalized by the number of photon+jet pairs. The pp reference is compared to several theoretical predictions: JEWEL: [1], [2], Hybrid Model: [4] [5].
png pdf	Additional Figure 7: Comparison of $\Delta \phi _{J\gamma }$ width in pp collisions shown for different $p_{\mathrm {T}}^\mathrm {Jet}$ bins. The pp reference is compared to several theoretical predictions: JEWEL: [1], [2], LBT 2017: [3], Hybrid Model: [4] [5], pQCD jet E-loss: [6]. Update Jan 2017: LBT calculations were changed following a correction of the authors of a bug that affected the normalization of the hydrodynamic profile.
png pdf	Additional Figure 8: Comparison of $< x_{J\gamma } >$ in pp collisions. The pp reference is compared to several theoretical predictions: JEWEL: [1], [2], Hybrid Model: [4] [5].
png pdf	Additional Figure 9: Comparison of $R_{J\gamma }$ in pp collisions. The pp reference is compared to several theoretical predictions: JEWEL: [1], [2].
png pdf	Additional Figure 9-a: Comparison of $R_{J\gamma }$ in pp collisions. The pp reference is compared to several theoretical predictions: JEWEL: [1], [2].
png pdf	Additional Figure 9-b: Comparison of $R_{J\gamma }$ in pp collisions. The pp reference is compared to several theoretical predictions: JEWEL: [1], [2].
png pdf	Additional Figure 10: Azimuthal correlation of photons and jets in each $p_{\mathrm {T},\gamma }$ bin (from left to right) for central PbPb (top) and peripheral PbPb (bottom). The correlation is shown on a logarithmic scale with the range restricted to $\Delta \phi _{J\gamma } > \frac {3\pi }{5}$ . The PbPb data is compared to several theoretical predictions: JEWEL: [1], [2], LBT 2017: [3], Hybrid Model: [4] [5]. Update Jan 2017: LBT calculations were changed following a correction of the authors of a bug that affected the normalization of the hydrodynamic profile.
png pdf	Additional Figure 11: Distribution of $x_{J\gamma }$ in each $p_{\mathrm {T},\gamma }$ bin (from left to right) for central PbPb (top) and peripheral PbPb (bottom). The PbPb data is compared to several theoretical predictions: JEWEL: [1], [2], LBT 2017: [3], Hybrid Model: [4] [5]. Update Jan 2017: LBT calculations were changed following a correction of the authors of a bug that affected the normalization of the hydrodynamic profile.
png pdf	Additional Figure 12: Average jet over photon transverse momentum ratio ( $< x_{J\gamma } >$ ) of the recoiled jets in (left) central PbPb, and (right) and peripheral PbPb. The PbPb data is compared to several theoretical predictions: JEWEL: [1], [2], LBT 2017: [3], Hybrid Model: [4] [5]. Update Jan 2017: LBT calculations were changed following a correction of the authors of a bug that affected the normalization of the hydrodynamic profile.
png pdf	Additional Figure 13: Average number of associated jets per photon ( $R_{J\gamma }$ ) as a function of leading photon ${p_{\mathrm {T}}}$ in (left) central PbPb, and (right) and peripheral PbPb. The PbPb data is compared to several theoretical predictions: JEWEL: [1], [2], LBT 2017: [3], Hybrid Model: [4] [5]. Update Jan 2017: LBT calculations were changed following a correction of the authors of a bug that affected the normalization of the hydrodynamic profile.
png pdf	Additional Figure 14: Ratio of jet yield in PbPb collisions to smeared pp. The PbPb data is compared to several theoretical predictions. The solid purple line is the prediction of the pQCD jet E-loss model with collisional energy loss, while the dashed purple line is the prediction of the same model without collisional energy loss. JEWEL: [1], [2], LBT 2017: [3], Hybrid Model: [4] [5], pQCD jet E-loss: [6]. Update Jan 2017: LBT calculations were changed following a correction of the authors of a bug that affected the normalization of the hydrodynamic profile.
png pdf	Additional Figure 15: Distribution of $x_{J\gamma }$ of photon+jet pairs of PbPb collisions normalized by the number of photon+jet pairs. The PbPb data is compared to several theoretical predictions: JEWEL: [1], [2], Hybrid Model: [4] [5].
png pdf	Additional Figure 16: Comparison of $\Delta \phi _{J\gamma }$ width in PbPb collisions shown for different $p_{\mathrm {T}}^\mathrm {Jet}$ bins. The PbPb data is compared to several theoretical predictions: JEWEL: [1], [2], Hybrid Model: [4] [5].
png pdf	Additional Figure 17: Comparison of $< x_{J\gamma } >$ in PbPb collisions. The PbPb data is compared to several theoretical predictions: JEWEL: [1], [2], LBT 2017: [3], Hybrid Model: [4] [5]. Update Jan 2017: LBT calculations were changed following a correction of the authors of a bug that affected the normalization of the hydrodynamic profile.
png pdf	Additional Figure 18: Comparison of $R_{J\gamma }$ in PbPb collisions. The PbPb data is compared to several theoretical predictions: JEWEL: [1], [2].
png pdf	Additional Figure 18-a: Comparison of $R_{J\gamma }$ in PbPb collisions. The PbPb data is compared to several theoretical predictions: JEWEL: [1], [2].
png pdf	Additional Figure 18-b: Comparison of $R_{J\gamma }$ in PbPb collisions. The PbPb data is compared to several theoretical predictions: JEWEL: [1], [2].

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