CMS-PAS-FSQ-18-001

CMS-PAS-FSQ-18-001
Measurement of the average very forward energy as a function of the track multiplicity at central rapidities in proton-proton collisions at $\sqrt{s}=$ 13 TeV
CMS Collaboration
March 2019

Abstract: The average total energy as well as the hadronic and electromagnetic components of it are measured with the CMS detector at pseudorapidities $-6.6<\eta<-5.2$ in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=$ 13 TeV. The results are presented as a function of the multiplicity of charged particle tracks in the region $\|\eta\|<$ 2. This measurement is sensitive to correlations induced by the underlying event structure over very wide pseudorapidity regions. We test Monte Carlo event generator predictions commonly used in collider and ultra-high energy cosmic ray physics with respect to these new data. It is very interesting that some of the most recent event generator tunes have the largest tension with respect to the data.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, EPJC 79 (2019) 893. The superseded preliminary plots can be found here.

Figures	Summary	References	CMS Publications

Figures
png pdf	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.
png pdf	Figure 1-a: 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.
png pdf	Figure 1-b: 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.
png pdf	Figure 1-c: 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.
png pdf	Figure 1-d: 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.
png pdf	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: 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-b: 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-c: 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-d: 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 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, which are the same in both panels. 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, which are the same in both panels. The bands associated with the model predictions illustrate the model uncertainty.

Summary

The average energy per event in the pseudorapidity region

$-6.6<\eta<-5.2$ has been measured as a function of the observed central track multiplicity (

$|\eta|<$ 2) in proton-proton collision at a centre-of-mass energy of 13 TeV. Data recorded during the first days of the LHC Run 2, with low beam intensities, are used. 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. This makes the data in particular relevant for improving the modeling of multiparticle production in event generators used for the simulation of ultra-high energy cosmic ray air showers.
The observables introduced provide a new approach to characterise particle production, and to study the properties of the underlying event. The measured average total energy as function of the track multiplicity is described with only minor tension by all models. This is a very good indication that underlying event parameter tunes performed at mid-rapidity can be extrapolated to the very forward direction within experimental uncertainties. However, it is also found that in a shape analysis of the same data we see very significant model differences and partly large deviations from the data. Thus, there is remaining opportunity to further improve the particle production models in this very forward phase space. Among all models, SIBYLL 2.1 shows the best reproduction of the measured multiplicity dependence of the average total energy.
The data is also presented separately for the average electromagnetic and hadronic energy per event as a function of central track multiplicity. This is useful to study different underlying particle production mechanisms, since the former is mostly due to decaying neutral pions, and the latter related to the production of non-resonant hadrons; most commonly charged pions. We find a general good description of all models of the electromagnetic energy -- with the exception of SIBYLL 2.3c. Notably, the predicted energy in hadrons reveals a significantly larger spread compared to the electromagnetic energy between the different models.
The data are also presented in terms of the ratio between the electromagnetic and hadronic energies. The data exhibit a larger fraction of electromagnetic energy compared to the models, and disagree with the two most recent model tunes, SIBYLL 2.3c and PYTHIA 8 CP5. This deficiency implies an increased difficulty to solve the muon deficit in ultra-high energy air shower simulations since more energy will be channelled into the electromagnetic part of the cascade and will subsequently be lost for the generation of further hadrons [15].

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