CMS-HIN-18-006

CMS-HIN-18-006 ; CERN-EP-2018-249
Jet shapes of isolated photon-tagged jets in PbPb and pp collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV
CMS Collaboration
23 September 2018
Phys. Rev. Lett. 122 (2019) 152001
Abstract: The modification of jet shapes in PbPb collisions, relative to those in pp collisions, is studied for jets associated with an isolated photon. The data were collected with the CMS detector at the LHC at a nucleon-nucleon center-of-mass energy of 5.02 TeV. Jet shapes are constructed from charged particles with track transverse momenta (${p_{\mathrm{T}}}$) above 1 GeV/$c$ in annuli around the axes of jets with ${p_{\mathrm{T}}} > $ 30 GeV/$c$ associated with an isolated photon with ${p_{\mathrm{T}}} > $ 60 GeV/$c$. The jet shape distributions are consistent between peripheral PbPb and pp collisions, but are modified for more central PbPb collisions. In these events, a larger fraction of the jet momentum is observed at larger distances from the jet axis, reflecting the interaction between the partonic medium created in heavy ion collisions and the traversing partons.
Links: e-print arXiv:1809.08602 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures	Summary	Additional Figures	References	CMS Publications

Figures

png pdf

Figure 1:
Upper: The differential jet shape, ${\rho (r)}$, for jets associated with an isolated photon for (from left to right) 50-100%, 30-50%, 10-30%, 0-10% PbPb (full circles), and pp (open circles) collisions and from PYTHIA simulation (histogram). Lower: The ratios of the PbPb and pp distributions. For the pp results, the ratios is to the PYTHIA distribution. The vertical lines through the points represent statistical uncertainties, while the shaded colored boxes indicate the total systematic uncertainties in data.

Summary

In summary, the differential jet shapes for jets associated with isolated photons are measured in pp and PbPb collisions for the first time. They are constructed using charged particles with transverse momentum $p_{\mathrm{T}}^{\text{trk}} > $ 1 GeV/$c$, for jets with $p_{\mathrm{T}}^{\text{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 is consistent with that in pp data, a modification of the jet shape in PbPb collisions is observed in more central events. The 0--10% (10--30%) PbPb $\rho(r)$ is enhanced for the distance between the track and the jet axis $ r \gtrsim$ 0.15 (0.20). No significant suppression is seen at intermediate $ r $. The modifications demonstrate that for hard scatterings that predominantly produce quarks with very similar momentum distributions in pp and PbPb collisions, as identified by the photon tag, the jet momentum is distributed at greater radial distance in PbPb collisions. This significant redistribution of energy observed in central PbPb collisions, compared with pp and peripheral PbPb collisions, can be interpreted as a direct observation of jet broadening in the quark-gluon plasma (QGP). This first measurement of radial momentum density profile for jets tagged by an isolated photon, which constrains the information about the jet energy before any loss occurred while traversing the QGP, constitutes a new unambiguous reference for testing theoretical models of parton-medium interactions.

Additional Figures
png pdf	Additional Figure 1: Upper: The differential jet shape, ${\rho (r)}$, for jets associated with an isolated photon for (from left to right) 50-100%, 30-50%, 10-30%, 0-10% PbPb (full circles), and pp (open circles) collisions. Lower: The ratios of the PbPb and pp distributions. The vertical lines through the points represent statistical uncertainties, while the shaded colored boxes indicate the total systematic uncertainties in data.
png pdf	Additional Figure 2: Upper: The differential jet shape, ${\rho (r)}$, for jets associated with an isolated photon for pp (full circles) collisions compared to PYTHIA simulation (histogram). Lower: The ratios of the MC over pp data distributions. The vertical lines through the points represent statistical uncertainties, while the shaded colored boxes indicate the total systematic uncertainties in data.
png pdf	Additional Figure 3: Upper: The differential jet shape, ${\rho (r)}$, for jets associated with an isolated photon for (from left to right) 30-100%, 0-30% PbPb (full circles), and pp (open circles) collisions. Lower: The ratios of the PbPb and pp distributions. The vertical lines through the points represent statistical uncertainties, while the shaded colored boxes indicate the total systematic uncertainties in data.
png pdf	Additional Figure 4: The ratio of PbPb over pp differential jet shape, ${\rho (r)}$, for 0-10% centrality interval.
png pdf	Additional Figure 5: Comparison of the ratios of PbPb over pp differential jet shape, ${\rho (r)}$, for 0-10% centrality interval with $\text {SCET}_\text {G}$ [41,58,31] and LBT [59] theoretical models. The $\text {SCET}_\text {G}$ band represents the variation of physics scale.
png pdf	Additional Figure 6: The ratio of PbPb over pp differential jet shape, ${\rho (r)}$, for 30-100% (left) and 0-30% (right) centrality intervals.
png pdf	Additional Figure 7: The ratio of PbPb over pp differential jet shape, ${\rho (r)}$, for 10-30% (left) and 0-10% (right) centrality intervals.
png pdf	Additional Figure 8: Relative contributions of fragmentation (red), photon+quark jet (grey), and photon+gluon jet (blue) processes to the production of isolated photons in PYTHIA events. Isolation requires that the total transverse energy of generated particles inside a cone of radius $\Delta R = $ 0.4 around the photon is less than 5 GeV/$c$.

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