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CMS-HIN-16-014 ; CERN-EP-2017-337
Observation of medium induced modifications of jet fragmentation in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV using isolated-photon-tagged jets
Phys. Rev. Lett. 121 (2018) 242301
Abstract: Measurements of fragmentation functions for jets associated with an isolated photon are presented for the first time in pp and PbPb collisions. The analysis uses data collected with the CMS detector at the CERN LHC at a nucleon-nucleon center-of-mass energy of 5.02 TeV. Fragmentation functions are obtained for jets with $ p_{\mathrm{T}}^\text{jet} > $ 30 GeV/$c$ in events containing an isolated photon with $ p_{\mathrm{T},\gamma} > $ 60 GeV/$c$, using charged tracks with transverse momentum $ p_{\mathrm{T}}^\text{trk} > $ 1 GeV/$c$ in a cone around the jet axis. The association with an isolated photon constrains the initial $ p_{\mathrm{T}} $ and azimuthal angle of the parton whose shower produced the jet. For central PbPb collisions, modifications of the jet fragmentation functions are observed when compared to those measured in pp collisions, while no significant differences are found in the 50% most peripheral collisions. Jets in central PbPb events show an excess (depletion) of low (high) $ p_{\mathrm{T}} $ particles, with a transition around 3 GeVc. This measurement shows for the first time the in-medium shower modifications of partons (quark dominated) with well defined initial kinematics. It constitutes a new well-controlled reference for testing theoretical models of the parton passage through the QGP.
Figures Summary Additional Figures References CMS Publications
Figures

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Figure 1:
Top: The centrality dependence of the $ \xi^\text{jet} $ distribution for jets associated with an isolated photon for PbPb (full crosses) and pp (open crosses) collisions. The pp results are smeared for each PbPb centrality bin, and data for each centrality bin are shifted vertically as indicated. Bottom: The ratios of the PbPb over smeared pp distributions. The vertical bars through the points represent statistical uncertainties, while the colored boxes indicate systematic uncertainties.

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Figure 2:
Top: The centrality dependence of the $ \xi^\gamma_{\mathrm{T}} $ distribution for jets associated with an isolated photon for PbPb (full crosses) and pp (open crosses) collisions. The pp results are smeared for each PbPb centrality bin, and data for each centrality bin are shifted vertically as indicated. Bottom: The ratios of the PbPb over smeared pp distributions. The vertical bars through the points represent statistical uncertainties, while the colored boxes indicate systematic uncertainties.
Summary
In summary, the fragmentation functions of jets associated with isolated photons are measured for the first time in pp and PbPb data collected at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV by CMS. Fragmentation patterns as functions of $ \xi^\text{jet} = \ln{\left[|\vec{p}^{ \, \text{jet}}|^{2}/(\vec{p}^{ \, \text{trk}}\cdot \vec{p}^{ \, \text{jet}})\right]} $ and $ \xi^\gamma_{\mathrm{T}} = \ln{\left[-|\vec{p}_{\mathrm{T}}^\gamma|^{2}/(\vec{p}_{\mathrm{T}}^{ \, \text{trk}} \cdot \vec{p}_{\mathrm{T}}^\gamma)\right]} $ are constructed using charged particles with $ p_{\mathrm{T}}^\text{trk} > $ 1 GeV/$c$, for jets with $ p_{\mathrm{T}}^\text{jet} > $ 30 GeV/$c$ tagged by an isolated photon with $ p_{\mathrm{T},\gamma} > $ 60 GeV/$c$. When compared to the pp results, the $ \xi^\text{jet} $ and $ \xi^\gamma_{\mathrm{T}} $ distributions in central PbPb collisions show an excess of low-$ p_{\mathrm{T}} $ particles and a depletion of high-$ p_{\mathrm{T}} $ particles inside the jet. This observation is more apparent in the $ \xi^\gamma_{\mathrm{T}} $ distributions, where the photon-based selection allows for tagging the properties of the initial parton before quenching occurs. This measurement shows for the first time the in-medium parton shower modifications for events with well-defined initial parton kinematics, and constitutes a new well-controlled reference for testing theoretical models of the parton's passage through the QGP.
Additional Figures

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Additional Figure 1:
Top: The centrality dependence of the $ \xi^\mathrm{jet} $ distribution for jets associated with an isolated photon for PbPb (full markers) and pp (open markers) collisions. The pp results are smeared for each PbPb centrality bin. Bottom: The ratios of the PbPb over smeared pp distributions. The vertical bars through the points represent statistical uncertainties, while the colored boxes indicate systematic uncertainties.

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Additional Figure 2:
Top: The centrality dependence of the $ \xi^\gamma_{\mathrm{T}} $ distribution for jets associated with an isolated photon for PbPb (full markers) and pp (open markers) collisions. The pp results are smeared for each PbPb centrality bin. Bottom: The ratios of the PbPb over smeared pp distributions. The vertical bars through the points represent statistical uncertainties, while the colored boxes indicate systematic uncertainties.

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Additional Figure 3:
Top: The $ \xi^\mathrm{jet} $ distribution from two event centralities for jets associated with an isolated photon for PbPb (full markers) and pp (open markers) collisions. The pp results are smeared for each PbPb centrality bin. Bottom: The ratios of the PbPb over smeared pp distributions. The vertical bars through the points represent statistical uncertainties, while the colored boxes indicate systematic uncertainties.

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Additional Figure 4:
Top: The $ \xi^\gamma_{\mathrm{T}} $ distribution from two event centralities for jets associated with an isolated photon for PbPb (full markers) and pp (open markers) collisions. The pp results are smeared for each PbPb centrality bin. Bottom: The ratios of the PbPb over smeared pp distributions. The vertical bars through the points represent statistical uncertainties, while the colored boxes indicate systematic uncertainties.

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Additional Figure 5:
Comparison of the ratios of PbPb over smeared pp $ \xi^\mathrm{jet} $ distributions, for 10-30% (left) and 0-10% (right) centrality intervals, with several theoretical models: $ \text{SCET}_\text{G} $ [55], CoLBT-hydro [56], and Hybrid [57,34,58,59]. The $ \text{SCET}_\text{G} $ band represents the variation of coupling strength between the jet and the QCD medium. The Hybrid band represents the variation of the dimensionless constant $ \kappa $ in the model.

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Additional Figure 6:
Comparison of the ratios of PbPb over smeared pp $ \xi^\gamma_{\mathrm{T}} $ distributions, for 10-30% (left) and 0-10% (right) centrality intervals, with several theoretical models: CoLBT-hydro [56] and Hybrid [57,34,58,59]. The Hybrid band represents the variation of the dimensionless constant $ \kappa $ in the model.

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Additional Figure 7:
Comparison of the smeared pp $ \xi^\mathrm{jet} $ distributions, for 10-30% (left) and 0-10% (right) centrality intervals, with pp calculations from CoLBT-hydro [56] and Hybrid [57,34,58,59] models.

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Additional Figure 8:
Comparison of the smeared pp $ \xi^\gamma_{\mathrm{T}} $ distributions, for 10-30% (left) and 0-10% (right) centrality intervals, with pp calculations from CoLBT-hydro [56] and Hybrid [57,34,58,59] models.

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Additional Figure 9:
Comparison of the PbPb $ \xi^\mathrm{jet} $ distributions, for 10-30% (left) and 0-10% (right) centrality intervals, with CoLBT-hydro [56] and Hybrid [57,34,58,59] models. The Hybrid band represents the variation of the dimensionless constant $ \kappa $ in the model.

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Additional Figure 10:
Comparison of the PbPb $ \xi^\gamma_{\mathrm{T}} $ distributions, for 10-30% (left) and 0-10% (right) centrality intervals, with CoLBT-hydro [56] and Hybrid [57,34,58,59] models. The Hybrid band represents the variation of the dimensionless constant $ \kappa $ in the model.

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Additional Figure 11:
The ratios of PbPb over smeared pp $ \xi^\mathrm{jet} $ distributions, for 30-100% (left) and 0-30% (right) centrality intervals.

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Additional Figure 12:
The ratios of PbPb over smeared pp $ \xi^\gamma_{\mathrm{T}} $ distributions, for 30-100% (left) and 0-30% (right) centrality intervals.

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Additional Figure 13:
The ratios of PbPb over smeared pp $ \xi^\mathrm{jet} $ distributions, for 10-30% (left) and 0-10% (right) centrality intervals.

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Additional Figure 14:
The ratios of PbPb over smeared pp $ \xi^\gamma_{\mathrm{T}} $ distributions, for 10-30% (left) and 0-10% (right) centrality intervals.

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Additional Figure 15:
Subtraction of the background tracks (red crosses) from the $ \xi^\mathrm{jet} $ distribution for jets associated with an isolated photon (black squares) for 0-10% centrality PbPb collisions. The distribution after background track subtraction (blue circles) is further subtracted for background jets (corresponding to orange squares in Fig. 16).

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Additional Figure 16:
Subtraction of the background jets (violet crosses) from the $ \xi^\mathrm{jet} $ distribution for background track subtracted jets associated with an isolated photon (orange squares) for 0-10% centrality PbPb collisions. The subtraction gives the background track and background jet subtracted distribution (green circles).

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Additional Figure 17:
Distribution of the sum of photon isolation variables in Pythia+Hydjet simulation for signal (red histogram) and background (green histogram) events.

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Additional Figure 18:
$ p_{\mathrm{T}}^\mathrm{jet} $ calculated as a function of $ \xi^\mathrm{jet} $ for three different $ p_{\mathrm{T}}^\mathrm{trk} $ selections where $ \Delta R $ between the track and the jet is 0.

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Additional Figure 19:
$ p_{\mathrm{T}}^\mathrm{trk} $ calculated as a function of $ \xi^\mathrm{jet} $ for three different $ p_{\mathrm{T}}^\mathrm{jet} $ selections where $ \Delta R $ between the track and the jet is 0.

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Additional Figure 20:
$ p_{\mathrm{T}}^\mathrm{jet} $ calculated as a function of $ \xi^\mathrm{jet} $ for four different $ \eta^\mathrm{jet} $ and for each of them two different $ |\eta^\mathrm{trk}| - |\eta^\mathrm{jet}| $ selections where $ p_{\mathrm{T}}^\mathrm{trk} = $ 1 GeV/$c$ and $ \Delta \phi $ between the track and the jet is 0.

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Additional Figure 21:
$ p_{\mathrm{T}}^\mathrm{trk} $ calculated as a function of $ \xi^\mathrm{jet} $ for four different $ \eta^\mathrm{jet} $ and for each of them two different $ |\eta^\mathrm{trk}| - |\eta^\mathrm{jet}| $ selections where $ p_{\mathrm{T}}^\mathrm{jet} = $ 30 GeV/$c$ and $ \Delta \phi $ between the track and the jet is 0.

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Additional Figure 22:
$ p_{\mathrm{T},\gamma} $ calculated as a function of $ \xi^\gamma_{\mathrm{T}} $ for three different $ p_{\mathrm{T}}^\mathrm{trk} $ selections where $ \Delta \phi $ between the track and the photon is $ \pi $.

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Additional Figure 23:
$ p_{\mathrm{T}}^\mathrm{trk} $ calculated as a function of $ \xi^\gamma_{\mathrm{T}} $ for three different $ p_{\mathrm{T},\gamma} $ selections where $ \Delta \phi $ between the track and the photon is $ \pi $.

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Additional Figure 24:
$ p_{\mathrm{T},\gamma} $ calculated as a function of $ \xi^\gamma_{\mathrm{T}} $ for two different selections of the $ \Delta \phi $ between the track and the photon where the $ p_{\mathrm{T}}^\mathrm{trk} $ is 1 GeV/$c$.

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Additional Figure 25:
$ p_{\mathrm{T}}^\mathrm{trk} $ calculated as a function of $ \xi^\gamma_{\mathrm{T}} $ for two different selections of the $ \Delta \phi $ between the track and the photon where the $ p_{\mathrm{T},\gamma} $ is 60 GeV/$c$.
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