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| CMS-PAS-HIN-18-006 | ||
| Jet shapes for isolated photon-tagged jets in PbPb and pp collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}= $ 5.02 TeV | ||
| CMS Collaboration | ||
| May 2018 | ||
| Abstract: The modification of jet shapes in PbPb collision, relative to those in pp collision, is studied for jets associated with an isolated photon, using data collected with the CMS detector at the CERN LHC at a nucleon-nucleon center-of-mass energy of 5.02 TeV. Jet shapes are constructed in annuli around the axes of jets of transverse momentum $p_\mathrm{T}^\mathrm{jet} > $ 30 GeV/$c$ that are associated with an isolated photon of $p_\mathrm{T}^\gamma > $ 60 GeV/$c$, using charged particles of $p_\mathrm{T}^\mathrm{trk} > $ 1 GeV/$c$. The jet shape distributions are consistent between peripheral PbPb collisions and pp collisions, and they become more different for more central PbPb collisions. The modifications are indicative of a larger fraction of the jet momenta being carried at larger distances from the jet axes, following the interaction between the medium and the traversing partons. | ||
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These preliminary results are superseded in this paper, PRL 122 (2019) 152001. The superseded preliminary plots can be found here. |
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| Figures | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1:
Top: The differential jet shape, $ {\rho (r)} $, for jets associated with an isolated photon for 50-100%, 30-50%, 10-30%, 0-10% PbPb (full crosses), pp (open crosses) collisions, and PYTHIA MC (histogram). Middle: The ratios of the PbPb over pp distributions. Bottom: The ratios of the MC over pp distributions. The vertical lines through the points represent statistical uncertainties, while the colored boxes indicate systematic uncertainties in data. |
| Summary |
| In summary, the differential jet shapes for jets associated with isolated photons are measured in pp and PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV for the first time. They are constructed using charged particles with $p_\mathrm{T}^\mathrm{trk} > $ 1 GeV/$c$, for jets with $p_\mathrm{T}^\mathrm{jet} > $ 30 GeV/$c$, which are associated with an isolated photon with $p_\mathrm{T}^\gamma > $ 60 GeV/$c$. While the distribution from the most peripheral (50--100%) PbPb collisions are consistent with pp, a modification of the PbPb jet shape is observed in central collisions, where the 0--10% (10--30%) PbPb $\rho (r)$ is enhanced for $r \geq $ 0.15 ($r \geq $ 0.2 ). No significant suppression is seen at intermediate $r$. The modifications demonstrate conclusively that for hard scatterings which produce dominantly quarks with very similar momentum distribution in pp and PbPb, as identified by the photon tag, the jet momentum is distributed fractionally at greater radial distance in PbPb. This significant redistribution of energy observed in central PbPb collisions, compared with pp and peripheral PbPb collisions, is interpreted as a direct observation of jet broadening in the QGP. |
| Additional Figures | |
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Additional Figure 1:
Top: The centrality dependence of differential jet shape, $ {\rho (r)} $, for jets associated with an isolated photon for PbPb (full markers) and pp (open markers) collisions. Bottom: The ratios of the PbPb over pp distributions. The vertical lines through the points represent statistical uncertainties, while the colored boxes indicate systematic uncertainties. |
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Additional Figure 2:
Top: The differential jet shape, $ {\rho (r)} $, for jets associated with an isolated photon for pp (full markers) collisions compared to {pythia} MC (histogram). Bottom: The ratios of the MC over pp data distributions. The vertical lines through the points represent statistical uncertainties, while the colored boxes indicate systematic uncertainties in data. |
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Additional Figure 3:
Top: The differential jet shape, $ {\rho (r)} $, from two event centralities for jets associated with an isolated photon for PbPb (full markers) and pp (open markers) collisions Bottom: The ratios of the PbPb over pp distributions. The vertical lines through the points represent statistical uncertainties, while the colored boxes indicate systematic uncertainties. |
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Additional Figure 4:
The ratio of PbPb over pp differential jet shape, $ {\rho (r)} $, for 0-10% centrality interval. |
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Additional Figure 5:
Comparison of the ratios of PbPb over pp differential jet shape, $ {\rho (r)} $, for 0-10% centrality interval with $\text {SCET}_\text {G}$ [1,2,3] and LBT [4] theoretical models. The $\text {SCET}_\text {G}$ band represents the variation of physics scale. |
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Additional Figure 6:
The ratio of PbPb over pp differential jet shape, $ {\rho (r)} $, for 30-100% (left) and 0-30% (right) centrality intervals. |
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