CMS-FSQ-16-002

CMS-FSQ-16-002 ; CERN-EP-2016-313
Measurement of the inclusive energy spectrum in the very forward direction in proton-proton collisions at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
30 January 2017
J. High Energy Phys. 08 (2017) 046
Abstract: The differential cross section for inclusive particle production as a function of energy in proton-proton collisions at a center-of-mass energy of 13 TeV is measured in the very forward region of the CMS detector. The measurement is based on data collected with the CMS apparatus at the LHC, and corresponds to an integrated luminosity of 0.35 $\mu$b$^{-1}$. The energy is measured in the CASTOR calorimeter, which covers the pseudorapidity region $-6.6<\eta<-5.2$. The results are given as a function of the total energy deposited in CASTOR, as well as of its electromagnetic and hadronic components. The spectra are sensitive to the modeling of multiparton interactions in pp collisions, and provide new constraints for hadronic interaction models used in collider and in high energy cosmic ray physics.
Links: e-print arXiv:1701.08695 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: Spectra of the energy reconstructed in CASTOR, normalized to the number of events that pass the offline event selection, compared to the detector-level predictions of various event generators. The total energy spectrum is shown in the left panel, the electromagnetic in the middle, and the hadronic in the right. Statistical (systematic) uncertainties are shown with error bars (yellow band, discussed in Section 4).
png pdf	Figure 1-a: Spectra of the energy reconstructed in CASTOR, normalized to the number of events that pass the offline event selection, compared to the detector-level predictions of various event generators. The total energy spectrum is shown. Statistical (systematic) uncertainties are shown with error bars (yellow band, discussed in Section 4).
png pdf	Figure 1-b: Spectra of the energy reconstructed in CASTOR, normalized to the number of events that pass the offline event selection, compared to the detector-level predictions of various event generators. The electromagnetic energy spectrum is shown. Statistical (systematic) uncertainties are shown with error bars (yellow band, discussed in Section 4).
png pdf	Figure 1-c: Spectra of the energy reconstructed in CASTOR, normalized to the number of events that pass the offline event selection, compared to the detector-level predictions of various event generators. The hadronic energy spectrum is shown. Statistical (systematic) uncertainties are shown with error bars (yellow band, discussed in Section 4).
png pdf	Figure 2: Distributions of reconstructed energy as a function of the particle-level energy for PYTHIA 8 CUETP8M1 for the total (left), electromagnetic (middle), and hadronic (right) energy in CASTOR. The color indicates the number of events. The selection $\xi > 10^{-6}$ is explained in the text.
png pdf	Figure 3: Differential cross section as a function of the total energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$. The left panel shows the data compared to MC event generators mostly developed for cosmic ray induced air showers, and the right panel to different PYTHIA 8 tunes.
png pdf	Figure 3-a: Differential cross section as a function of the total energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$. The panel shows the data compared to MC event generators mostly developed for cosmic ray induced air showers.
png pdf	Figure 3-b: Differential cross section as a function of the total energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$. The panel shows the data compared to different PYTHIA 8 tunes.
png pdf	Figure 4: Differential cross section as a function of the electromagnetic energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$. The left panel shows the data compared to MC event generators mostly developed for cosmic ray induced air showers, and the right panel to different PYTHIA 8 tunes.
png pdf	Figure 4-a: Differential cross section as a function of the electromagnetic energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$. The panel shows the data compared to MC event generators mostly developed for cosmic ray induced air showers.
png pdf	Figure 4-b: Differential cross section as a function of the electromagnetic energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$. The panel shows the data compared to different PYTHIA 8 tunes.
png pdf	Figure 5: Differential cross section as a function of the hadronic energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$. The left panel shows the data compared to MC event generators mostly developed for cosmic ray induced air showers, and the right panel to different PYTHIA 8 tunes.
png pdf	Figure 5-a: Differential cross section as a function of the hadronic energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$. The panel shows the data compared to MC event generators mostly developed for cosmic ray induced air showers.
png pdf	Figure 5-b: Differential cross section as a function of the hadronic energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$. The panel shows the data compared to different PYTHIA 8 tunes.

Tables
png pdf	Table 1: Uncertainties on the differential cross sections at a few selected values of the total, electromagnetic, and hadronic energies.

Summary

The electromagnetic, hadronic, and total energy spectra of particles produced at very forward pseudorapidities ($-6.6<\eta < -5.2$) have been measured with the CASTOR calorimeter of the CMS experiment in proton-proton collisions at a center-of-mass energy of 13 TeV. The experimental distributions, fully corrected for detector effects, are compared to the predictions of various Monte Carlo event generators commonly used in high energy cosmic ray physics (EPOS, QGSJetII, and Sibyll), and those of different tunes of PYTHIA 8. None of the generators considered describe all features seen in the data. The present measurements are particularly sensitive to the modeling of multiparton interactions (MPI) that dominate particle production in the underlying event at forward rapidities in pp collisions. PYTHIA 8 CUETP8M1 without MPI is ruled out by the data, which exhibit much harder spectra than predicted by the model. The shape of the spectra are significantly influenced by the MPI-related settings in PYTHIA 8. The present results can therefore contribute to improvements in future Monte Carlo parameter tunes. Event generators developed for modeling high energy cosmic ray air showers, tuned to LHC measurements at 0.9, 7, and 8 TeV, agree better with the present data than those tuned to Tevatron results alone. This is especially true for QGSJetII and Sibyll. However, all these models underestimate the muon production rate in extensive air showers because of their inaccurate description of the hadronic shower component [31]. The present results provide new constraints for improving the modeling of hadron production in event generators commonly used in high energy particle and cosmic ray physics.

Additional Figures
png pdf	Additional Figure 1: The distribution of the ratio of electromagnetic to total reconstructed energy in CASTOR. The error bars denote statistical uncertainties, while the yellow band indicates the systematic uncertainty. The data without correction for any detector effect agree within experimental uncertainties with event generator predictions after a full GEANT4 detector simulation.
png pdf	Additional Figure 2: Distributions of reconstructed energy as a function of the particle-level energy for EPOS LHC for the total (left), electromagnetic (middle), and hadronic (right) energy in CASTOR. The color indicates the number of events. The selection $\xi > 10^{-6}$ is explained in the paper.
png pdf	Additional Figure 3: Distributions of reconstructed energy as a function of the particle-level energy for PYTHIA 8 4C+MBR for the total (left), electromagnetic (middle), and hadronic (right) energy in CASTOR. The color indicates the number of events. The selection $\xi > 10^{-6}$ is explained in the paper.
png pdf	Additional Figure 4: Statistical covariance matrix of the total energy spectrum. For illustration, absolute values are shown.
png pdf	Additional Figure 5: Statistical covariance matrix of the electromagnetic energy spectrum. For illustration, absolute values are shown.
png pdf	Additional Figure 6: Statistical covariance matrix of the hadronic energy spectrum. For illustration, absolute values are shown.
png pdf	Additional Figure 6-a: Statistical covariance matrix of the hadronic energy spectrum. For illustration, absolute values are shown.
png pdf	Additional Figure 6-b: Statistical covariance matrix of the hadronic energy spectrum. For illustration, absolute values are shown.
png pdf	Additional Figure 7: Differential cross section as a function of the total energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$, zoomed into the low energy region. Panel (a) shows the data compared to MC event generators mostly developed for cosmic ray induced air showers, and panel (b) to different PYTHIA 8 tunes.
png pdf	Additional Figure 7-a: Differential cross section as a function of the total energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$, zoomed into the low energy region. The panel shows the data compared to MC event generators mostly developed for cosmic ray induced air showers.
png pdf	Additional Figure 7-b: Differential cross section as a function of the total energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$, zoomed into the low energy region. The panel shows the data compared to different PYTHIA 8 tunes.
png pdf	Additional Figure 8: Differential cross section as a function of the electromagnetic energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$, zoomed into the low energy region. Panel (a) shows the data compared to MC event generators mostly developed for cosmic ray induced air showers, and panel (b) to different PYTHIA 8 tunes.
png pdf	Additional Figure 8-a: Differential cross section as a function of the electromagnetic energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$, zoomed into the low energy region. The panel shows the data compared to MC event generators mostly developed for cosmic ray induced air showers.
png pdf	Additional Figure 8-b: Differential cross section as a function of the electromagnetic energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$, zoomed into the low energy region. The panel shows the data compared to different PYTHIA 8 tunes.
png pdf	Additional Figure 9: Differential cross section as a function of the hadronic energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$, zoomed into the low energy region. Panel (a) shows the data compared to MC event generators mostly developed for cosmic ray induced air showers, and panel (b) to different PYTHIA 8 tunes.
png pdf	Additional Figure 9-a: Differential cross section as a function of the hadronic energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$, zoomed into the low energy region. The panel shows the data compared to MC event generators mostly developed for cosmic ray induced air showers.
png pdf	Additional Figure 9-b: Differential cross section as a function of the hadronic energy in the region $-6.6<\eta < -5.2$ for events with $\xi >10^{-6}$, zoomed into the low energy region. The panel shows the data compared to different PYTHIA 8 tunes.

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