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| CMS-HIN-18-016 ; CERN-EP-2020-032 | ||
| The production of isolated photons in PbPb and pp collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| 28 March 2020 | ||
| JHEP 07 (2020) 116 | ||
| Abstract: The transverse energy ($E_{\mathrm{T}}^{\gamma}$) spectra of photons isolated from other particles are measured using proton-proton (pp) and lead-lead (PbPb) collisions at the LHC at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV with integrated luminosities of 27.4 pb$^{-1}$ and 404 $\mu$b$^{-1}$ for pp and PbPb data, respectively. The results are presented for photons with 25 $ < E_{\mathrm{T}}^{\gamma} < $ 200 GeV in the pseudorapidity range $|{\eta}| < $ 1.44, and for different centrality intervals for PbPb collisions. Photon production in PbPb collisions is consistent with that in pp collisions scaled by the number of binary nucleon-nucleon collisions, demonstrating that photons do not interact with the quark-gluon plasma. Therefore, isolated photons can provide information about the initial energy of the associated parton in photon+jet measurements. The results are compared with predictions from the next-to-leading-order JETPHOX generator for different parton distribution functions (PDFs) and nuclear PDFs (nPDFs). The comparisons can help to constrain the nPDFs global fits. | ||
| Links: e-print arXiv:2003.12797 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Template fit of the shower shape variable $\sigma _{\eta \eta}$ for 40 $ < {{E_{\mathrm {T}}} ^\gamma} < $ 50 GeV in the 10-30% centrality class. The black points show the PbPb experimental data. The red histogram is the signal template obtained from PYTHIA+HYDJET simulations, and the green histogram is the background template estimated from the data for the nonisolated sideband region. |
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Figure 2:
Efficiency of the isolated photon detection as a function of ${{E_{\mathrm {T}}} ^\gamma}$ for PbPb collisions in the 0-10% centrality range (left) and for pp data (right). The different colors represent various selection criteria. |
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Figure 3:
Isolated photon spectra (upper) measured as a function of ${{E_{\mathrm {T}}} ^\gamma}$ for 0-10%, 10-30%, 30-50%, 50-100%, and 0-100% PbPb collisions (scaled by ${T_\mathrm {AA}}$) at 5.02 TeV. The spectra are scaled by the factors shown in the legend for clarity. The symbols are placed at the center of the bin. The vertical bars associated with symbols indicate the statistical uncertainties and the horizontal bars reflect the bin width. Some of the statistical uncertainties are smaller than the symbols. The total systematic uncertainties are shown as boxes in each ${{E_{\mathrm {T}}} ^\gamma}$ bin. The spectra in the 0-100% centrality bin are compared to the NLO JETPHOX calculations with EPPS16+CT14 nPDFs (left) and nCTEQ15 nPDFs (right). The ratio of the data in the 0-100% centrality class to JETPHOX is shown in the lower panels. The gray boxes indicate the total systematic uncertainties of the data. The blue and red hatched boxes correspond to the JETPHOX PDF and scale uncertainties, respectively. |
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Figure 3-a:
Isolated photon spectra (upper) measured as a function of ${{E_{\mathrm {T}}} ^\gamma}$ for 0-10%, 10-30%, 30-50%, 50-100%, and 0-100% PbPb collisions (scaled by ${T_\mathrm {AA}}$) at 5.02 TeV. The spectra are scaled by the factors shown in the legend for clarity. The symbols are placed at the center of the bin. The vertical bars associated with symbols indicate the statistical uncertainties and the horizontal bars reflect the bin width. Some of the statistical uncertainties are smaller than the symbols. The total systematic uncertainties are shown as boxes in each ${{E_{\mathrm {T}}} ^\gamma}$ bin. The spectra in the 0-100% centrality bin are compared to the NLO JETPHOX calculations with EPPS16+CT14 nPDFs. The ratio of the data in the 0-100% centrality class to JETPHOX is shown in the lower panels. The gray boxes indicate the total systematic uncertainties of the data. The blue and red hatched boxes correspond to the JETPHOX PDF and scale uncertainties, respectively. |
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Figure 3-b:
Isolated photon spectra (upper) measured as a function of ${{E_{\mathrm {T}}} ^\gamma}$ for 0-10%, 10-30%, 30-50%, 50-100%, and 0-100% PbPb collisions (scaled by ${T_\mathrm {AA}}$) at 5.02 TeV. The spectra are scaled by the factors shown in the legend for clarity. The symbols are placed at the center of the bin. The vertical bars associated with symbols indicate the statistical uncertainties and the horizontal bars reflect the bin width. Some of the statistical uncertainties are smaller than the symbols. The total systematic uncertainties are shown as boxes in each ${{E_{\mathrm {T}}} ^\gamma}$ bin. The spectra in the 0-100% centrality bin are compared to the NLO JETPHOX calculations with nCTEQ15 nPDFs. The ratio of the data in the 0-100% centrality class to JETPHOX is shown in the lower panels. The gray boxes indicate the total systematic uncertainties of the data. The blue and red hatched boxes correspond to the JETPHOX PDF and scale uncertainties, respectively. |
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Figure 4:
Isolated photon cross section (upper) measured as a function of ${{E_{\mathrm {T}}} ^\gamma}$ in pp collisions at 5.02 TeV. The symbols are placed at the center of the bin. The vertical bars associated with symbols indicate the statistical uncertainties and the horizontal bars reflect the bin width. Some of the statistical uncertainties are smaller than the symbols. The total systematic uncertainties are shown as boxes in each ${{E_{\mathrm {T}}} ^\gamma}$ bin. The data are compared to the NLO JETPHOX calculations with CT14 PDFs. The ratio of the data to JETPHOX is shown in the lower panel. The yellow boxes indicate the total systematic uncertainties of the data. The blue and red hatched boxes correspond to JETPHOX PDF and scale uncertainties, respectively. |
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Figure 5:
Nuclear modification factors ${R_{\mathrm {AA}}}$ as a function of the photon ${{E_{\mathrm {T}}} ^\gamma}$ measured in the 0-10%, 10-30%, 30-50%, and 50-100% centrality ranges in PbPb. The symbols are placed at the center of the bin. The vertical bars associated with symbols indicate the statistical uncertainties and the horizontal bars reflect the bin width. The total systematic uncertainties without the ${T_\mathrm {AA}}$ uncertainty are shown as the colored boxes. The ${T_\mathrm {AA}}$ uncertainty, common to all points for a given centrality range, is indicated by the gray box centered at unity on the left side of each panel. The 2.3% integrated luminosity uncertainty for pp data is shown as the brown box at unity at the leftmost position. |
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Figure 6:
Nuclear modification factors ${R_{\mathrm {AA}}}$ as a function of the photon ${{E_{\mathrm {T}}} ^\gamma}$ measured in the 0-100% centrality range in PbPb. The symbols are placed at the center of the bin. The vertical bars indicate the statistical uncertainties and the horizontal bars reflect the bin width. The total systematic uncertainties without the ${T_\mathrm {AA}}$ uncertainty are shown by the colored boxes. The 3.4% ${T_\mathrm {AA}}$ uncertainty, common to all points, is indicated by the gray box centered at unity on the left side of the panel. The luminosity uncertainty of the pp data is shown as the brown box at unity at the leftmost position. The three different NLO JETPHOX calculations of EPPS16+CT14 nPDFs, nCTEQ15 nPDFs, and CT14 PDFs for PbPb collisions are divided by the NLO JETPHOX calculations with CT14 PDFs for pp collisions, and compared to the data. The hatched boxes correspond to JETPHOX (n)PDF uncertainties. |
| Tables | |
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Table 1:
Average numbers of the nuclear overlap function (${T_\mathrm {AA}}$) and their uncertainties for various centrality ranges used in this analysis. |
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Table 2:
Summary of the contributions from various sources to the estimated systematic uncertainties in the cross section of isolated photons in pp and PbPb collisions. When ranges are shown, they indicate the ${{E_{\mathrm {T}}} ^\gamma}$-dependent variations of the uncertainties. |
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Table 3:
Summary of the contributions from various sources to the estimated systematic uncertainties in the nuclear modification factors calculated from pp and PbPb data. When ranges are shown, they indicate the ${{E_{\mathrm {T}}} ^\gamma}$-dependent variations of the uncertainties. |
| Summary |
|
The differential cross sections of photons isolated from nearby particles are reported at pseudorapidity $|{\eta^{\gamma}}| < $ 1.44 for transverse energy from 25 to 200 GeV in proton-proton (pp) and lead-lead (PbPb) collisions at a center-of-mass energy per nucleon pair ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV with the CMS detector. No significant modification of isolated photon cross sections in PbPb collisions with respect to scaled pp collisions is observed in the explored kinematic ranges at all collision centralities. Thus, isolated photons are not affected by the strongly interacting medium produced in heavy ion collisions, and they can be a valuable tool to access the initial ${p_{\mathrm{T}}}$ of the associated parton in photon+jet events. The data are compared with the next-to-leading order perturbative quantum chromodynamics calculations using the generator JETPHOX with CT14 parton distribution functions (PDFs) for pp data and EPPS16 and nCTEQ15 nuclear PDFs for PbPb data. The predictions are found to be consistent with the cross sections for both pp and PbPb collisions. The current measurements significantly improve the precision compared to the previous CMS results at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 2.76 TeV and can be valuable inputs for global fits of nuclear PDFs. |
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