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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 √sNN= 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 (paveT) 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 paveT 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 Summary References CMS Publications
Figures

png pdf 		Figure 1:
(left) Correlation between xPb/xp and dijet pseudorapidity ηdijet. The dashed line corresponds to the expected correlation without ISR or FSR effects. (right) Mean Bjorken x of the parton from the lead ion <xPb> obtained from PYTHIA 6 events as a function of ηdijet in different dijet pTave intervals.

png pdf 		Figure 1-a:
Correlation between xPb/xp and dijet pseudorapidity ηdijet. The dashed line corresponds to the expected correlation without ISR or FSR effects.

png pdf 		Figure 1-b:
Mean Bjorken x of the parton from the lead ion <xPb> obtained from PYTHIA 6 events as a function of ηdijet in different dijet pTave intervals.

png pdf 		Figure 2:
Ratio of pPb to pp ηdijet 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.

png pdf 		Figure 3:
Ratio of theory to data, for the ratio of the pPb to pp ηdijet spectra for 115 <pTave< 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.

png pdf 		Figure A1:
The measured pp dijet pseudorapidity spectra, shifted to match the ηdijet 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.

png pdf 		Figure A2:
The measured pPb dijet pseudorapidity spectra in bins of pTave, 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.

png pdf 		Figure A3:
The measured pPb dijet pseudorapidity spectra in bins of pTave, 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 (ηdijet) in different average transverse momentum (paveT) intervals in pPb and pp collisions at a nucleon-nucleon center-of-mass energy √sNN= 5.02 TeV are reported. Ratios of the pPb and pp ηdijet spectra using the pp reference data are also reported. Significant modifications of the ηdijet distributions are observed in pPb data compared to the pp reference in all paveT 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 pT and high mass particle production at collider energies.
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