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| CMS-HIN-16-003 ; CERN-EP-2018-091 | ||
| Constraining gluon distributions in nuclei using dijets in proton-proton and proton-lead collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| 13 May 2018 | ||
| Phys. Rev. Lett. 121 (2018) 062002 | ||
| Abstract: The pseudorapidity distributions of dijets as functions of their average transverse momentum ($p_{\mathrm{T}}^{\text{ave}}$) are measured in proton-lead (pPb) and proton-proton (pp) collisions. The data samples were collected by the CMS experiment at the CERN LHC, at a nucleon-nucleon center-of-mass energy of 5.02 TeV. A significant modification of the pPb spectra with respect to the pp spectra is observed in all $p_{\mathrm{T}}^{\text{ave}}$ intervals investigated. The ratios of the pPb and pp distributions are compared to next-to-leading order perturbative quantum chromodynamics calculations with unbound nucleon and nuclear parton distribution functions (PDFs). These results give the first evidence that the gluon PDF at large Bjorken $x$ in lead ions is strongly suppressed with respect to the PDF in unbound nucleons. | ||
| Links: e-print arXiv:1805.04736 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
(left) Correlation between $x_{{\mathrm {Pb}}}/x_{{\mathrm {p}}}$ and dijet pseudorapidity ${\eta _{\text {dijet}}}$. The dashed line corresponds to the expected correlation without ISR or FSR effects. (right) Mean Bjorken $x$ of the parton from the lead ion $ < x_{{\mathrm {Pb}}} > $ obtained from PYTHIA 6 events as a function of ${\eta _{\text {dijet}}}$ in different dijet ${{p_{\mathrm {T}}} ^{\text {ave}}}$ intervals. |
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Figure 1-a:
Correlation between $x_{{\mathrm {Pb}}}/x_{{\mathrm {p}}}$ and dijet pseudorapidity ${\eta _{\text {dijet}}}$. The dashed line corresponds to the expected correlation without ISR or FSR effects. |
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Figure 1-b:
Mean Bjorken $x$ of the parton from the lead ion $ < x_{{\mathrm {Pb}}} > $ obtained from PYTHIA 6 events as a function of ${\eta _{\text {dijet}}}$ in different dijet ${{p_{\mathrm {T}}} ^{\text {ave}}}$ intervals. |
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Figure 2:
Ratio of pPb to pp ${\eta _{\text {dijet}}}$ spectra compared to NLO pQCD calculations with DSSZ [18] and EPS09 [14] nPDFs, using CT14 [58] as the baseline nucleon PDF. Red boxes indicate systematic uncertainties in data and the height of the NLO pQCD calculation boxes represent the nPDF uncertainties. |
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Figure 3:
Ratio of theory to data, for the ratio of the pPb to pp ${\eta _{\text {dijet}}}$ spectra for 115 $ < {{p_{\mathrm {T}}} ^{\text {ave}}} < $ 150 GeV. Theory points are from the NLO pQCD calculations of DSSZ [18], EPS09 [14], nCTEQ15 [15], and EPPS16 [16] nPDFs, using CT14 [58] as the baseline PDF. Red boxes indicate the total (statistical and systematic) uncertainties in data, and the error bars on the points represent the nPDF uncertainties. |
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Figure A1:
The measured pp dijet pseudorapidity spectra, shifted to match the $ {\eta _{\text {dijet}}} $ range of the pPb collisions, in bins of {{p_{\mathrm {T}}} ^{\text {ave}}}, overlaid with NLO pQCD calculations with the CT14 [58] and MMHT14 [59] nucleon PDFs. The red boxes indicate systematic uncertainties in data and the height of the NLO pQCD calculation boxes represent the PDF uncertainties. |
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Figure A2:
The measured pPb dijet pseudorapidity spectra in bins of ${{p_{\mathrm {T}}} ^{\text {ave}}}$, overlaid with the NLO pQCD calculations of DSSZ [18], EPS09 [14], and nCTEQ15 [15] nPDFs, using the CT14 [58] as the baseline PDF. The red boxes indicate systematic uncertainties in data and the height of the NLO pQCD calculation boxes represent the nPDF uncertainties. |
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Figure A3:
The measured pPb dijet pseudorapidity spectra in bins of ${{p_{\mathrm {T}}} ^{\text {ave}}}$, overlaid with the NLO pQCD calculations of DSSZ [18], EPS09 [14], and nCTEQ15 [15] nPDFs, using the MMHT14 [59] as the baseline PDF. The red boxes indicate systematic uncertainties in data and the height of the NLO pQCD calculation boxes represent the nPDF uncertainties. |
| Summary |
| In summary, measurements of the dijet pseudorapidity ($ {\eta _{\text {dijet}}} $) in different average transverse momentum ($ p_{\mathrm{T}}^{\text{ave}} $) intervals in pPb and pp collisions at a nucleon-nucleon center-of-mass energy ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV are reported. Ratios of the pPb and pp $ {\eta _{\text {dijet}}} $ spectra using the pp reference data are also reported. Significant modifications of the $ {\eta _{\text {dijet}}} $ distributions are observed in pPb data compared to the pp reference in all $ p_{\mathrm{T}}^{\text{ave}} $ intervals, which show shadowing, antishadowing, and EMC effects in nuclear parton distribution functions. The ratios of the pPb and pp distributions are compared to next-to-leading order calculations with nucleon and nPDFs. Based on these comparisons, the results present the first evidence that the gluon PDF at large Bjorken $x$ in lead ions is strongly suppressed with respect to that in unbound nucleons. The data are incompatible with predictions using nucleon PDFs or using nPDFs without large-$x$ gluon suppression. Based on a statistical analysis, the EPS09 nPDF provides the best overall agreement with the data. This data can be used to place strong constraints on the next-generation of nPDFs, which are crucial for the understanding of high $ p_{\mathrm{T}} $ and high mass particle production at collider energies. |
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