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| CMS-FSQ-18-001 ; CERN-EP-2019-146 | ||
| Measurement of the average very forward energy as a function of the track multiplicity at central pseudorapidities in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 5 August 2019 | ||
| Eur. Phys. J. C 79 (2019) 893 | ||
| Abstract: The average total energy as well as its hadronic and electromagnetic components are measured with the CMS detector at pseudorapidities $-6.6 < \eta < -5.2$ in proton-proton collisions at a centre-of-mass energy $\sqrt{s} = $ 13 TeV. The results are presented as a function of the charged particle multiplicity in the region $|{\eta}| < $ 2. This measurement is sensitive to correlations induced by the underlying event structure over a very wide pseudorapidity region. The predictions of Monte Carlo event generators commonly used in collider experiments and ultra-high energy cosmic ray physics are compared to the data. | ||
| Links: e-print arXiv:1908.01750 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Top panel: Average total energy reconstructed in the CASTOR calorimeter as a function of the number of reconstructed tracks for $ {| \eta |} < $ 2. Bottom panel: Average total energy reconstructed in the CASTOR calorimeter normalised to that in the first bin ($N_{\mathrm {ch}} < $ 10) as a function of the number of reconstructed tracks for $ {| \eta |} < $ 2. In all figures, the data are shown as black circles and the corresponding systematic uncertainties with a gray band; horizontal bars are used to indicate the bin width. The predictions of various event generators are compared to the data, which are the same in both panels. The bands associated with the model predictions illustrate the model uncertainty. |
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Figure 1-a:
Average total energy reconstructed in the CASTOR calorimeter as a function of the number of reconstructed tracks for $ {| \eta |} < $ 2. The data are shown as black circles and the corresponding systematic uncertainties with a gray band; horizontal bars are used to indicate the bin width. The predictions of various event generators are compared to the data. The bands associated with the model predictions illustrate the model uncertainty. |
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Figure 1-b:
Average total energy reconstructed in the CASTOR calorimeter as a function of the number of reconstructed tracks for $ {| \eta |} < $ 2. The data are shown as black circles and the corresponding systematic uncertainties with a gray band; horizontal bars are used to indicate the bin width. The predictions of various event generators are compared to the data. The bands associated with the model predictions illustrate the model uncertainty. |
png pdf |
Figure 1-c:
Average total energy reconstructed in the CASTOR calorimeter normalised to that in the first bin ($N_{\mathrm {ch}} < $ 10) as a function of the number of reconstructed tracks for $ {| \eta |} < $ 2. The data are shown as black circles and the corresponding systematic uncertainties with a gray band; horizontal bars are used to indicate the bin width. The predictions of various event generators are compared to the data. The bands associated with the model predictions illustrate the model uncertainty. |
png pdf |
Figure 1-d:
Average total energy reconstructed in the CASTOR calorimeter normalised to that in the first bin ($N_{\mathrm {ch}} < $ 10) as a function of the number of reconstructed tracks for $ {| \eta |} < $ 2. The data are shown as black circles and the corresponding systematic uncertainties with a gray band; horizontal bars are used to indicate the bin width. The predictions of various event generators are compared to the data. The bands associated with the model predictions illustrate the model uncertainty. |
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Figure 2:
Top panel: Average electromagnetic energy reconstructed in the CASTOR calorimeter as a function of the number of reconstructed tracks for $ {| \eta |} < $ 2. Bottom panel: Average hadronic energy reconstructed in the CASTOR calorimeter as a function of the number of reconstructed tracks for $ {| \eta |} < $ 2. In all figures, the data are shown with black circles and the corresponding systematic uncertainties with a gray band; horizontal bars are used to indicate the bin width. The predictions of various event generators are compared to the data, which are the same in both panels. The bands associated with the model predictions illustrate the model uncertainty. |
png pdf |
Figure 2-a:
Average electromagnetic energy reconstructed in the CASTOR calorimeter as a function of the number of reconstructed tracks for $ {| \eta |} < $ 2. The data are shown with black circles and the corresponding systematic uncertainties with a gray band; horizontal bars are used to indicate the bin width. The predictions of various event generators are compared to the data. The bands associated with the model predictions illustrate the model uncertainty. |
png pdf |
Figure 2-b:
Average electromagnetic energy reconstructed in the CASTOR calorimeter as a function of the number of reconstructed tracks for $ {| \eta |} < $ 2. The data are shown with black circles and the corresponding systematic uncertainties with a gray band; horizontal bars are used to indicate the bin width. The predictions of various event generators are compared to the data. The bands associated with the model predictions illustrate the model uncertainty. |
png pdf |
Figure 2-c:
Average hadronic energy reconstructed in the CASTOR calorimeter as a function of the number of reconstructed tracks for $ {| \eta |} < $ 2. The data are shown with black circles and the corresponding systematic uncertainties with a gray band; horizontal bars are used to indicate the bin width. The predictions of various event generators are compared to the data. The bands associated with the model predictions illustrate the model uncertainty. |
png pdf |
Figure 2-d:
Average hadronic energy reconstructed in the CASTOR calorimeter as a function of the number of reconstructed tracks for $ {| \eta |} < $ 2. The data are shown with black circles and the corresponding systematic uncertainties with a gray band; horizontal bars are used to indicate the bin width. The predictions of various event generators are compared to the data. The bands associated with the model predictions illustrate the model uncertainty. |
png pdf |
Figure 3:
Ratio of average electromagnetic and hadronic energies reconstructed in the CASTOR calorimeter as a function of the number of reconstructed tracks for $ {| \eta |} < $ 2. The data are shown with black circles and the corresponding systematic uncertainties with a gray band; horizontal bars are used to indicate the bin width. Predictions of various event generators are compared to the data, which are the same in both panels. The bands associated with the model predictions illustrate the model uncertainty. |
png pdf |
Figure 3-a:
Ratio of average electromagnetic and hadronic energies reconstructed in the CASTOR calorimeter as a function of the number of reconstructed tracks for $ {| \eta |} < $ 2. The data are shown with black circles and the corresponding systematic uncertainties with a gray band; horizontal bars are used to indicate the bin width. Predictions of various event generators are compared to the data. The bands associated with the model predictions illustrate the model uncertainty. |
png pdf |
Figure 3-b:
Ratio of average electromagnetic and hadronic energies reconstructed in the CASTOR calorimeter as a function of the number of reconstructed tracks for $ {| \eta |} < $ 2. The data are shown with black circles and the corresponding systematic uncertainties with a gray band; horizontal bars are used to indicate the bin width. Predictions of various event generators are compared to the data. The bands associated with the model predictions illustrate the model uncertainty. |
| Tables | |
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Table 1:
Uncertainties in the average energies measured with the CASTOR calorimeter on the detector level. Ranges indicate the variation as a function of the track multiplicity. |
| Summary |
|
The average energy per event in the pseudorapidity region $-6.6 < \eta < -5.2$ was measured as a function of the observed central track multiplicity ($|{\eta}| < $ 2) in proton-proton collisions at a centre-of-mass energy of 13 TeV. The data are recorded during the first days of 13 TeV running with low beam intensities. The measurement is presented in terms of the total energy as well as its electromagnetic and hadronic components. The very forward region covered by the data contains the highest energy densities studied in proton-proton collisions at the LHC so far. This makes the present data relevant for improving the modelling of multiparticle production in event generators of ultra-high energy cosmic ray air showers. The measured average total energy as a function of the track multiplicity is described by all models with reasonable discrepancies. This demonstrates that the underlying event parameter tunes determined at central rapidity can be safely extrapolated to the very forward region within experimental uncertainties. A shape analysis indicates, however, that there are significant differences among the models and large deviations from the data. The generator SIBYLL 2.1 gives the best description of the measured multiplicity dependence of the average total energy. The data are also presented in terms of the average electromagnetic and hadronic energies per event as a function of the central track multiplicity. This is useful in the study of different particle production mechanisms, since the former is primarily due to the decay of neutral pions and the latter to the production of hadrons with longer lifetimes, mostly charged pions. All models give a good description of the electromagnetic energy dependence on the multiplicity, with the exception of SIBYLL 2.3c. Conversely, the predictions for the hadronic energy have a significantly larger spread compared to the electromagnetic case. The ratio between the electromagnetic and hadronic energies is also presented. The data exhibit a larger fraction of electromagnetic energy than the models, and disagree with the two most recent model tunes, SIBYLL 2.3c and PYTHIA8 CP5. Therefore, these models cannot explain the muon deficit in ultra-high energy air shower simulations since the data indicate that even more energy must be channelled into the electromagnetic part of the cascade and is thus lost for the generation of further hadrons [17]. |
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