CMS-FSQ-15-005

CMS-FSQ-15-005 ; CERN-EP-2018-004
Measurement of the inelastic proton-proton cross section at $\sqrt{s} =$ 13 TeV
CMS Collaboration
7 February 2018
JHEP 07 (2018) 161
Abstract: A measurement of the inelastic proton-proton cross section with the CMS detector at a center-of-mass energy of $\sqrt{s} =$ 13 TeV is presented. The analysis is based on events with energy deposits in the forward calorimeters, which cover pseudorapidities of $-6.6 < \eta < -3.0$ and $+3.0 < \eta < +5.2$ . An inelastic cross section of 68.6 $\pm$ 0.5 (syst) $\pm$ 1.6 (lumi) mb is obtained for events with $M_\mathrm{X} >$ 4.1 GeV and/or $M_\mathrm{Y} >$ 13 GeV, where $M_\mathrm{X}$ and $M_\mathrm{Y}$ are the masses of the diffractive dissociation systems at negative and positive pseudorapidities, respectively. The results are compared with those from other experiments as well as to predictions from high-energy hadron-hadron interaction models.
Links: e-print arXiv:1802.02613 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures

png pdf

Figure 1:
Proton-proton inelastic cross section at

$\sqrt {s} =$ 13 TeV in two phase space regions, where

$\xi = M^2/s$ , compared to different models and to the ATLAS result. The inner error bar on the CMS and ATLAS data points indicates the systematic uncertainty only, while the full error bar reflects the total (including the integrated luminosity) uncertainty. The horizontal placement of the points within each phase space bin is arbitrary.

Tables
png pdf	Table 1: Overview of the noise-subtracted fraction of events ( $N_{\text {cor}}/(N_{\mathrm {ZB}}\, + \, F_{\mathrm {ZB}})$ ), average pileup ( $\lambda$ ), and fiducial cross section (with the statistical uncertainty) for all runs used in this analysis. The uncertainties in the noise-subtracted fraction of events and pileup (not shown) have a negligible contribution to the uncertainty in the fiducial cross section.
png pdf	Table 2: Overview of systematic uncertainties from various sources that contribute to the cross section uncertainty in the two phase space regions considered in this analysis.
png pdf	Table 3: Relative increase in the cross section from the $\xi > 10^{-6}$ to the $\xi _{\mathrm {X}} > 10^{-7}$ or $\xi _{\mathrm {Y}} > 10^{-6}$ fiducial region. The uncertainty in the cross section ratio in data is obtained using systematic uncertainties only, varying each uncertainty source simultaneously for both phase space regions and obtaining the total uncertainty from the variation of the ratio (thus assuming full correlation between both measurements).

Summary

A measurement of the inelastic proton-proton cross section at

$\sqrt{s} =$ 13 TeV with the CMS detector at the LHC has been presented. An inelastic cross section of 67.5

$\pm$ 0.8 (syst)

$\pm$ 1.6 (lumi) mb is obtained for

$\xi = M^2/s > 10^{-6}$ (corresponding to

$M >$ 13 GeV), with

$M$ the larger of

$M_{\mathrm{X}}$ and

$M_{\mathrm{Y}}$ , where

$M_{\mathrm{X}}$ and

$M_{\mathrm{Y}}$ are the masses of the diffractive dissociation systems with negative and positive pseudorapidities, respectively, consistent with a previous measurement in the same phase space [21]. In addition, an inelastic cross section of 68.6

$\pm$ 0.5 (syst)

$\pm$ 1.6 (lumi) mb is obtained in the enlarged phase space

$\xi_{\mathrm{X}} > 10^{-7}$ and/or

$\xi_{\mathrm{Y}} > 10^{-6}$ (corresponding to

$M_{\mathrm{X}} >$ 4.1 GeV and/or

$M_{\mathrm{Y}} >$ 13 GeV). The measured cross sections are smaller than those predicted by the majority of models for hadron-hadron scattering. In contrast, the same models generally describe reasonably well the measurements of the total inelastic cross section at

$\sqrt{s} =$ 13 TeV [21,22]. Given that the difference between the two sets of measurements is dominated by the contribution from low-mass diffractive processes, the data-model discrepancies observed here suggest a theoretical underestimation of the cross section for such events.

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