# Measurements of very-forward energy with the CASTOR calorimeter of CMS

## TL;DR

This paper reports on measurements of very-forward energy using the CASTOR calorimeter at CMS, providing insights into particle production at extreme pseudorapidities relevant for understanding ultra-high energy cosmic ray air showers.

## Contribution

It presents the first detailed energy measurements in the very-forward region at the LHC using CASTOR, with implications for cosmic ray physics.

## Key findings

- Energy deposition peaks at high pseudorapidity observed
- Results impact models of cosmic ray air shower development
- Data constrains particle production in ultra-forward phase-space

## Abstract

The very-forward energy production in hadron collisions is of paramount importance for the understanding of ultra-high energy cosmic ray air showers. The CASTOR calorimeter of CMS is located at $-6.6 < \eta < -5.2$ in the phase-space where the peak of energy is deposited at LHC. The composition and characteristics of the particles in this phase-space have a determining impact on the formation of air shower cascades. An overview of various energy measurements performed with CASTOR is reported and possible implications for cosmic ray physics are outlined.