CMS-HIN-15-012

CMS-HIN-15-012 ; CERN-EP-2016-274
Measurements of the charm jet cross section and nuclear modification factor in pPb collisions at $\sqrt{ s_{\mathrm{NN}} } =$ 5.02 TeV
CMS Collaboration
29 December 2016
Phys. Lett. B 772 (2017) 306
Abstract: The CMS Collaboration presents the first measurement of the differential cross section of jets from charm quarks produced in proton-lead (pPb) collisions at a nucleon-nucleon center-of-mass energy of $\sqrt{ s_{\mathrm{NN}} } =$ 5.02 TeV, as well as results from charm quark jets in proton-proton (pp) collisions at $\sqrt{s} =$ 2.76 and 5.02 TeV. By comparing the yields of the pPb and pp collision systems at the same energy, a nuclear modification factor for charm jets in pPb collisions at $\sqrt{ s_{\mathrm{NN}} } =$ 5.02 TeV of $R_{\mathrm{ p }\mathrm{A}} =$ 0.92 $\pm$ 0.07 (stat) $\pm$ 0.11 (syst) is obtained. This is consistent with an absence of final-state energy loss for charm quarks in pPb collisions. In addition, the fraction of jets coming from charm quarks is found to be consistent with that predicted by PYTHIA 6 for pp collisions at $\sqrt{s} = $ 2.76 and 5.02 TeV, and is independent of the jet transverse momentum from 55 to 400 GeV/$c$.
Links: e-print arXiv:1612.08972 [nucl-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Efficiency of tagging b jets (left) and light parton jets (right) for the high-purity (3+track), and high-efficiency (2+track) versions of the simple secondary vertex (SSV) tagger as a function of c jet tagging efficiency. The charm-to-bottom discrimination power is virtually unchanged between the high-efficiency and high-purity versions of the tagger, while the light parton jet mistag rate is reduced by a factor of three at the analysis working point, shown as the closed red cross on the plots.
png pdf	Figure 2: Corrected secondary vertex mass distributions from PYTHIA+HIJING for c jets (green), light parton jets (blue), and b jets (red) in the jet ${p_{\mathrm {T}}}$ range 55-80 GeV/$c$ (left) and 120-170 GeV/$c$ (right). Relative normalizations of these three distributions are fit to a distribution from pPb collision data (black). Statistical uncertainties are shown in black for data and for individual simulated flavor components and are shown in blue for the sum of the simulated distribution. The bottom panels of both plots show the ratio of data to simulation.
png pdf	Figure 2-a: Corrected secondary vertex mass distributions from PYTHIA+HIJING for c jets (green), light parton jets (blue), and b jets (red) in the jet ${p_{\mathrm {T}}}$ range 55-80 GeV/$c$. Relative normalizations of these three distributions are fit to a distribution from pPb collision data (black). Statistical uncertainties are shown in black for data and for individual simulated flavor components and are shown in blue for the sum of the simulated distribution. The bottom panel shows the ratio of data to simulation.
png pdf	Figure 2-b: Corrected secondary vertex mass distributions from PYTHIA+HIJING for c jets (green), light parton jets (blue), and b jets (red) in the jet ${p_{\mathrm {T}}}$ range 120-170 GeV/$c$. Relative normalizations of these three distributions are fit to a distribution from pPb collision data (black). Statistical uncertainties are shown in black for data and for individual simulated flavor components and are shown in blue for the sum of the simulated distribution. The bottom panel shows the ratio of data to simulation.
png pdf	Figure 3: Corrected secondary vertex mass from a PYTHIA-6, tune Z2 simulation for c jets (green), light parton jets (blue) and b jets (red) in the jet ${p_{\mathrm {T}}}$ range 55-80 GeV/$c$ (left) and 120-170 GeV/$c$ (right). Relative normalizations of these three distributions are fit to a distribution from pp collision data (black). Statistical uncertainties are shown in black for data and for individual simulated flavor components and are shown in blue for the sum of the simulated distribution. The bottom panels of both plots show the ratio of data to simulation.
png pdf	Figure 3-a: Corrected secondary vertex mass from a PYTHIA-6, tune Z2 simulation for c jets (green), light parton jets (blue) and b jets (red) in the jet ${p_{\mathrm {T}}}$ range 55-80 GeV/$c$. Relative normalizations of these three distributions are fit to a distribution from pp collision data (black). Statistical uncertainties are shown in black for data and for individual simulated flavor components and are shown in blue for the sum of the simulated distribution. The bottom panel shows the ratio of data to simulation.
png pdf	Figure 3-b: Corrected secondary vertex mass from a PYTHIA-6, tune Z2 simulation for c jets (green), light parton jets (blue) and b jets (red) in the jet ${p_{\mathrm {T}}}$ range 120-170 GeV/$c$. Relative normalizations of these three distributions are fit to a distribution from pp collision data (black). Statistical uncertainties are shown in black for data and for individual simulated flavor components and are shown in blue for the sum of the simulated distribution. The bottom panel shows the ratio of data to simulation.
png pdf	Figure 4: Tagging purity (left) and efficiency (right) for the working point selection of $\mathrm {SSVHP} > $ 1.68 in pPb collisions at 5.02 TeV for simulation (open red squares) and data (closed black points).
png pdf	Figure 4-a: Tagging purity for the working point selection of $\mathrm {SSVHP} > $ 1.68 in pPb collisions at 5.02 TeV for simulation (open red squares) and data (closed black points).
png pdf	Figure 4-b: The efficiency (right) for the working point selection of $\mathrm {SSVHP} > $ 1.68 in pPb collisions at 5.02 TeV for simulation (open red squares) and data (closed black points).
png pdf	Figure 5: The tagging purity (left) and efficiency (right) for the working point selection of $\mathrm {SSVHP} > $ 1.68 in pp collisions at 5.02 TeV (square markers) and at 2.76 TeV (circular markers). Purity curves from simulation (open red markers) and data (closed markers) are shown, obtained by fitting templates to the data. The right plot shows efficiency curves from simulation (open red markers) and the cross-check based on JP tagging.
png pdf	Figure 5-a: The tagging purity for the working point selection of $\mathrm {SSVHP} > $ 1.68 in pp collisions at 5.02 TeV (square markers) and at 2.76 TeV (circular markers). Purity curves from simulation (open red markers) and data (closed markers) are shown, obtained by fitting templates to the data.
png pdf	Figure 5-b: The efficiency for the working point selection of $\mathrm {SSVHP} > $ 1.68 in pp collisions at 5.02 TeV (square markers) and at 2.76 TeV (circular markers). The plot shows efficiency curves from simulation (open red markers) and the cross-check based on JP tagging.
png pdf	Figure 6: The c jet spectra (top) and fraction (bottom) as a function of c jet ${p_{\mathrm {T}}}$ for 5.02 TeV (left) and 2.76 TeV pp data (right), compared to predictions from PYTHIA-6. Systematic uncertainties are shown as filled boxes.
png pdf	Figure 6-a: The c jet spectra (top) and fraction (bottom) as a function of c jet ${p_{\mathrm {T}}}$ for 5.02 TeV pp data, compared to predictions from PYTHIA-6. Systematic uncertainties are shown as filled boxes.
png pdf	Figure 6-b: The c jet spectra (top) and fraction (bottom) as a function of c jet ${p_{\mathrm {T}}}$ for 2.76 TeV pp data, compared to predictions from PYTHIA-6. Systematic uncertainties are shown as filled boxes.
png pdf	Figure 7: The c jet spectrum (top) and ${R_{\mathrm{ p } \mathrm {A}}}$ (bottom) as a function of c jet ${p_{\mathrm {T}}}$ for 5.02 TeV pPb and pp data. Statistical uncertainties are solid black lines, while systematic uncertainties are shown as filled colored boxes. Integrated luminosity uncertainties for pp and pPb data are shown as filled boxes around unity.

Summary

The transverse momentum differential cross-section for c jets has been obtained for pPb collisions at $\sqrt{ s_{\mathrm{NN}} }= $ 5.02 TeV, as well as for pp collisions at $\sqrt{s} = $ 2.76 and 5.02 TeV. The c jet fraction of $\approx$6% is consistent with PYTHIA simulations for pp collisions at both center-of-mass energies. By comparing the cross sections for pPb and pp collisions, a $p_{\mathrm{T}}$-independent ${R_{\mathrm{ p }\mathrm{A}}} $ value of 0.92 $\pm$ 0.07 (stat) $\pm$ 0.11 (syst) is observed for c jets at 5.02 TeV, indicating that no significant jet energy modification is present in pPb collisions for c jets with $p_{\mathrm{T}}> $ 55 GeV/$c$. These measurements indicate that proton-lead initial state effects on c jets between 55-400 GeV/$c$ are small and that charm jet quenching in lead-lead collisions should not be influenced by such effects.

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