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

TL;DR

The paper measures the average forward energy in the CASTOR calorimeter as a function of central track multiplicity in proton-proton collisions at $\sqrt{s}=13$ TeV, using a forward-folding technique to compare generator predictions with detector-level data. It correlates very forward activity ($-6.6<\eta<-5.2$) with central multiplicity ($|\eta|<2$) to probe the underlying event and diffractive processes over a broad rapidity range. The study compares a wide set of MC models, finding that all overestimate the fraction of energy going into hadrons, with varying degrees of success in reproducing the multiplicity dependence and the electromagnetic-to-hadronic energy ratio. The results provide important constraints for tuning hadron production models used in collider and cosmic-ray simulations, particularly for forward physics and air-shower development. Overall, some models describe the trend reasonably, but persistent shape differences point to areas for model refinement, with implications for muon production in extensive air showers.

Abstract

The average total energy as well as its hadronic and electromagnetic components are measured with the CMS detector at pseudorapidities $-$6.6 $<$ $η$ $<-$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 $|η|$ $<$ 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. All generators considered overestimate the fraction of energy going into hadrons.

Figures (3)

• Figure 1: Top panel: Average total energy reconstructed in the CASTOR calorimeter as a function of the number of reconstructed tracks for $\lvert \eta \rvert<2$. 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 $\lvert \eta \rvert<2$. 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.
• Figure 2: Top panel: Average electromagnetic energy reconstructed in the CASTOR calorimeter as a function of the number of reconstructed tracks for $\lvert \eta \rvert<2$. Bottom panel: Average hadronic energy reconstructed in the CASTOR calorimeter as a function of the number of reconstructed tracks for $\lvert \eta \rvert<2$. 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.
• Figure 3: Ratio of average electromagnetic and hadronic energies reconstructed in the CASTOR calorimeter as a function of the number of reconstructed tracks for $\lvert \eta \rvert<2$. 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.