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| CMS-HIN-14-001 ; CERN-PH-EP-2015-334 | ||
| Measurement of inclusive jet production and nuclear modifications in pPb collisions at $\sqrt{ s_{\mathrm{NN} } }= $ 5.02 TeV | ||
| CMS Collaboration | ||
| 8 January 2016 | ||
| Eur. Phys. J. C 76 (2016) 372 | ||
| Abstract: Inclusive jet production in pPb collisions at a nucleon-nucleon (NN) center-of-mass energy of $\sqrt{ s_{\mathrm{NN} } }= $ 5.02 TeV is studied with the CMS detector at the LHC. A data sample corresponding to an integrated luminosity of 30.1nb$^{-1}$ is analyzed. The jet transverse momentum spectra are studied in seven pseudorapidity intervals covering the range $-2.0 < \eta_\mathrm{CM}< 1.5$ in the NN center-of-mass frame. The jet production yields at forward and backward pseudorapidity are compared and no significant asymmetry about $\eta_\mathrm{CM} = $ 0 is observed in the measured kinematic range. The measurements in the pPb system are compared to reference jet spectra obtained by extrapolation from previous measurements in pp collisions at $ \sqrt{s} = $ 7 TeV. In all pseudorapidity ranges, nuclear modifications in inclusive jet production are found to be small, as predicted by next-to-leading order perturbative QCD calculations that incorporate nuclear effects in the parton distribution functions. | ||
| Links: e-print arXiv:1601.02001 [nucl-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1-a:
a: The weighted jet spectra using prescale factors from each HLT-triggered event sample and the combined jet spectrum. A subset of the data is plotted to illustrate the procedure. b: The ratios of each individual HLT-triggered jet spectrum to the combined jet spectrum. Statistical uncertainties are shown as vertical bars, and ${p_{\mathrm {T}}}$ bin widths as horizontal bars. |
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Figure 1-b:
a: The weighted jet spectra using prescale factors from each HLT-triggered event sample and the combined jet spectrum. A subset of the data is plotted to illustrate the procedure. b: The ratios of each individual HLT-triggered jet spectrum to the combined jet spectrum. Statistical uncertainties are shown as vertical bars, and ${p_{\mathrm {T}}}$ bin widths as horizontal bars. |
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Figure 2-a:
a: Response matrix built from PYTHIA+HIJING simulation. b: The ratios of the Bayesian unfolded jet ${p_{\mathrm {T}}}$ spectrum reconstructed in the simulation and the generator-level spectrum. |
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Figure 2-b:
a: Response matrix built from PYTHIA+HIJING simulation. b: The ratios of the Bayesian unfolded jet ${p_{\mathrm {T}}}$ spectrum reconstructed in the simulation and the generator-level spectrum. |
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Figure 3:
Jet spectra at $\sqrt {s} = $ 5.02 TeV extrapolated from previous pp measurements at $\sqrt {s} = $ 7 TeV [33]. Additional scaling factors listed in the legend are applied to enhance the visibility. The horizontal bars represent the bin size, and the points are plotted in the center of the bin. The shaded boxes denote the systematic uncertainties in the extrapolation procedure. The statistical uncertainties are smaller than the symbol size. |
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Figure 4:
Inclusive jet differential cross section in pPb collisions at $ {\sqrt {s_{_\mathrm {NN}}}} = $ 5.02 TeV in six consecutive eta bins plus the range $ {| \eta _\mathrm {CM} | } < $ 1.0 . The spectra are scaled by arbitrary factors for better visibility. The horizontal bars represent the bin width, and the filled boxes indicate the systematic uncertainties. The statistical uncertainties are smaller than the symbol size. |
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Figure 5:
Mean $x$ values of partons in the Pb nucleus, $ < x_{\mathrm{pPb}} > $, corresponding to the jet ${p_{\mathrm {T}}}$ and pseudorapidity ranges covered in the measurements. The $< x_{\mathrm{pPb}} > $ values are determined using the PYTHIA event generator [43]. |
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Figure 6:
Inclusive jet asymmetry as a function of jet ${p_{\mathrm {T}}}$ for 0.5 $ < {| \eta _\mathrm {CM} | }< $ 1.0 and 1.0 $ < {| \eta _\mathrm {CM} | }< $ 1.5 . The asymmetry is calculated as the ratio between the jet yields at negative pseudorapidity (Pb beam direction) and positive pseudorapidity (proton-going side). The vertical bars represent the statistical uncertainties and the open boxes represent the systematic ones. |
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Figure 7-a:
a: Inclusive jet cross section in pPb collisions as a function of jet ${p_{\mathrm {T}}}$ normalized to the production at mid-rapidity ($ {| \eta _\mathrm {CM} | }< $ 1 ) for six $\eta _\mathrm {CM}$ intervals. The vertical bars represent the statistical uncertainties. The systematic uncertainties at mid-rapidity and in the most backward pseudorapidity are shown with open boxes. The uncertainties in the other pseudorapidity ranges have similar magnitude. b: Inclusive jet cross section in pPb collisions as a function of $\eta _\mathrm {CM}$ normalized to the cross section at $ {| \eta _\mathrm {CM} | }< $ 1 , for three jet ${p_{\mathrm {T}}}$ ranges. The open boxes represent the systematic uncertainties. The data points are shifted in pseudorapidity to enhance the visibility. The $\eta _\mathrm {CM}$bin boundaries are as specified in the left panel. The statistical uncertainties are smaller than the symbols. |
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Figure 7-b:
a: Inclusive jet cross section in pPb collisions as a function of jet ${p_{\mathrm {T}}}$ normalized to the production at mid-rapidity ($ {| \eta _\mathrm {CM} | }< $ 1 ) for six $\eta _\mathrm {CM}$ intervals. The vertical bars represent the statistical uncertainties. The systematic uncertainties at mid-rapidity and in the most backward pseudorapidity are shown with open boxes. The uncertainties in the other pseudorapidity ranges have similar magnitude. b: Inclusive jet cross section in pPb collisions as a function of $\eta _\mathrm {CM}$ normalized to the cross section at $ {| \eta _\mathrm {CM} | }< $ 1 , for three jet ${p_{\mathrm {T}}}$ ranges. The open boxes represent the systematic uncertainties. The data points are shifted in pseudorapidity to enhance the visibility. The $\eta _\mathrm {CM}$bin boundaries are as specified in the left panel. The statistical uncertainties are smaller than the symbols. |
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Figure 8:
Inclusive jet nuclear modification factor $R^{\ast }_{\mathrm{pPb}} $ as a function of jet ${p_{\mathrm {T}}}$ in $ {\sqrt {s_{_\mathrm {NN}}}} = $ 5.02 TeV pPb collisions, using a pp reference extrapolated from previous measurements [33] at $\sqrt {s} = $ 7 TeV . The vertical bars represent the statistical uncertainties, and the open boxes represent the systematic ones. The filled rectangular boxes around $R^{\ast }_{\mathrm{pPb}} = $ 1 represent the luminosity uncertainties in the pPb and pp measurements. The CMS measurements are compared to a NLO pQCD calculation [57] that is based on the EPS09 nPDFs [19]. The theoretical calculations are shown with solid lines, and the shaded bands around them represent the theoretical uncertainties. |
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Figure 9:
Inclusive jet $R^{\ast }_{\mathrm{pPb}} $ integrated over centrality and in the $ {| \eta _\mathrm {CM} | }< $ 0.5 range for anti-$ {k_{\mathrm {T}}}$ jets with distance parameter $R= $ 0.3 from this work, compared to ATLAS results [22] at $ {| y_\mathrm {CM} | }< $ 0.3 for the 0-90% most central collisions with distance parameter $R = $ 0.4 . The vertical bars show the statistical uncertainties, and the open boxes represent the systematic uncertainties. |
| Tables | |
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Table 1:
Systematic uncertainties in the measurement of the jet spectra in pPb collisions are shown in the first four lines. The sources and corresponding systematic uncertainties in the extrapolated pp reference are presented in the next four lines. The total uncertainties in the jet spectra in pPb collisions, the reference pp spectra, the jet yield asymmetry in pPb collisions, and $R^{\ast }_{\mathrm{pPb}} $ are shown in the bottom four lines. The uncertainties depend on the jet ${p_{\mathrm {T}}}$ and pseudorapidity, and the table shows representative values in two jet ${p_{\mathrm {T}}}$ and $\eta _\mathrm {CM}$ ranges. The uncertainties vary smoothly between these two ranges. Total systematic uncertainties listed for the nuclear modification factors $R^{\ast }_{\mathrm{pPb}} $ do not include the scale uncertainty of 4.3% due to the uncertainty in the integrated luminosity measurements in pPb (3.5%) and pp (2.4%) collisions. |
| Summary |
|
The inclusive jet spectra and nuclear modification factors in pPb collisions at $\sqrt{ s_{\mathrm{NN} } }= $ 5.02 TeV have been measured. The data, corresponding to an integrated luminosity of 30.1nb$^{-1}$, were collected by the CMS experiment in 2013. The jet transverse momentum spectra were measured for $ p_{\mathrm{T}} > $ 56 GeV/$c$ in six pseudorapidity intervals covering the range $ -2 <\eta_\mathrm{CM}< $ 1.5 in the NN center-of-mass system. The jet spectra were found to be softer away from mid-rapidity. The jet production at forward and backward pseudorapidity were compared, and no significant asymmetry about $\eta_\mathrm{CM} = $ 0 was observed in the measured kinematic range. The differential jet cross section results were compared with extrapolated pp reference spectra based on jet measurements in pp collisions at $\sqrt{s} = $ 7 TeV. The inclusive jet nuclear modification factors $R^{*}_\mathrm{pPb}$ were observed to have small enhancements compared to the reference pp jet spectra at low jet $p_{\mathrm{T}}$ in all $\eta_\mathrm{CM}$ ranges. In the anti-shadowing region, for $| {\eta_\mathrm{CM}} | < $ 0.5 and 56 $ < p_{\mathrm{T}} < $ 300 GeV/$c$, the value $R^{\ast}_\mathrm{pPb}=$ 1.17 $\pm$ 0.01 (stat) $\pm$ 0.12 (syst) was found. The $R^{*}_\mathrm{pPb}$ appears to be approximately independent of $p_{\mathrm{T}}$, except in the most backward pseudorapidity range. The $R^{\ast}_\mathrm{pPb}$ measurements were found to be compatible with theoretical predictions from NLO pQCD calculations that use EPS09 nPDFs. |
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