CMS-FSQ-17-001

CMS-FSQ-17-001 ; CERN-EP-2018-325
Measurement of inclusive very forward jet cross sections in proton-lead collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV
CMS Collaboration
4 December 2018
JHEP 05 (2019) 043
Abstract: Measurements of differential cross sections for inclusive very forward jet production in proton-lead collisions as a function of jet energy are presented. The data were collected with the CMS experiment at the LHC in the laboratory pseudorapidity range $-6.6 < \eta < -5.2$. Asymmetric beam energies of 4 TeV for protons and 1.58 TeV per nucleon for Pb nuclei were used, corresponding to a center-of-mass energy per nucleon pair of ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. Collisions with either the proton (p+Pb) or the ion (Pb+p) traveling towards the negative $\eta$ hemisphere are studied. The jet cross sections are unfolded to stable-particle level cross sections with ${p_{\mathrm{T}}} \geq $ 3 GeV, and compared to predictions from various Monte Carlo event generators. In addition, the cross section ratio of p+Pb and Pb+p data is presented. The results are discussed in terms of the saturation of gluon densities at low fractional parton momenta. None of the models under consideration describes all the data over the full jet-energy range and for all beam configurations. Discrepancies between the differential cross sections in data and model predictions of more than two orders of magnitude are observed.
Links: e-print arXiv:1812.01691 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: A schematic drawing of one half of the CASTOR calorimeter and its mechanical support structure. The diameter of CASTOR is roughly 0.6 m and it is approximately 1.6 m in length. The transversal and longitudinal segmentation in eight sectors and fourteen modules, respectively, can be clearly distinguished. The 112 small cylinders represent the photomultiplier tubes of CASTOR. These are mounted on light guides, which transport the Cherenkov radiation out of the detector. It may be observed that CASTOR has only transverse and no $\eta $ segmentation.
png pdf	Figure 2: Detector-level differential cross sections for inclusive forward jet production as a function of calibrated jet energy in p+Pb (left) and Pb+p (right) collisions. Model predictions are shown for EPOS-LHC, HIJING, and QGSJETII-04.
png pdf	Figure 2-a: Detector-level differential cross sections for inclusive forward jet production as a function of calibrated jet energy in p+Pb collisions. Model predictions are shown for EPOS-LHC, HIJING, and QGSJETII-04.
png pdf	Figure 2-b: Detector-level differential cross sections for inclusive forward jet production as a function of calibrated jet energy in Pb+p collisions. Model predictions are shown for EPOS-LHC, HIJING, and QGSJETII-04.
png pdf	Figure 3: Detector-level ratio of differential cross sections for inclusive forward jet production in p+Pb to Pb+p data vs. calibrated jet energy. Model predictions are shown for EPOS-LHC, HIJING, and QGSJETII-04.
png pdf	Figure 4: Stable-particle-level differential jet cross section as a function of jet energy measured in p+Pb collisions at 5.02 TeV, compared to the EPOS-LHC, HIJING, and QGSJETII-04 (left), and KATIE and AAMQS (right) predictions. The band associated with the nonlinear KATIE curve accounts for the 50-100% variation of the strength of the parton saturation effects in this model.
png pdf	Figure 4-a: Stable-particle-level differential jet cross section as a function of jet energy measured in p+Pb collisions at 5.02 TeV, compared to the EPOS-LHC, HIJING, and QGSJETII-04 predictions. The band associated with the nonlinear KATIE curve accounts for the 50-100% variation of the strength of the parton saturation effects in this model.
png pdf	Figure 4-b: Stable-particle-level differential jet cross section as a function of jet energy measured in p+Pb collisions at 5.02 TeV, compared to the KATIE and AAMQS predictions. The band associated with the nonlinear KATIE curve accounts for the 50-100% variation of the strength of the parton saturation effects in this model.
png pdf	Figure 5: Stable-particle-level differential jet cross section as a function of jet energy in proton-lead collisions at 5.02 TeV. The Pb+p measurement is depicted (left), and the ratio of the p+Pb to Pb+p cross sections is displayed (right). The data are compared to model predictions from EPOS-LHC, HIJING, and QGSJETII-04.
png pdf	Figure 5-a: Stable-particle-level differential jet cross section as a function of jet energy in proton-lead collisions at 5.02 TeV. The Pb+p measurement is depicted. The data are compared to model predictions from EPOS-LHC, HIJING, and QGSJETII-04.
png pdf	Figure 5-b: Stable-particle-level differential jet cross section as a function of jet energy in proton-lead collisions at 5.02 TeV. The ratio of the p+Pb to Pb+p cross sections is displayed. The data are compared to model predictions from EPOS-LHC, HIJING, and QGSJETII-04.

Tables
png pdf	Table 1: The contribution in percentage (%) of various sources of systematic uncertainty in the highest and lowest common energy bins for the p+Pb, Pb+p, and p+Pb/Pb+p spectra.

Summary

Measurements of the differential inclusive forward jet cross sections in proton-lead collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV have been presented. The measurements are performed in the laboratory pseudorapidity range $-6.6 < \eta < -5.2$, and as a function of jet energy over the range $E\approx $ 150-2500 GeV. Collisions with either the incoming proton (p+Pb) or the incoming ion (Pb+p) directed towards the negative $\eta$ hemisphere are studied. The jet cross sections are unfolded to stable-particle level cross sections with ${p_{\mathrm{T}}} \geq $ 3 GeV and compared to predictions from various Monte Carlo event generators. The cross section ratio for p+Pb to Pb+p data as a function of jet energy has also been measured, and exhibits a much smaller systematic uncertainty than the individual spectra.

The so-far unexplored kinematic phase space covered by this measurement is sensitive to the parton densities and their evolution at low fractional momenta. Models incorporating various implementations of gluon saturation have been confronted with data. No model is, however, currently able to describe all aspects of the data.

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