# Probing Nuclear PDF and Gluon Saturation At The LHC with Forward Direct Photons

## TL;DR

This paper proposes a new forward calorimeter upgrade for the ALICE experiment at the LHC to measure direct photon production, aiming to probe nuclear PDFs and gluon saturation at very small x values.

## Contribution

Introduction of a high-granularity forward calorimeter (FoCal) upgrade for ALICE to enable measurements of direct photons in pA collisions, exploring gluon saturation effects.

## Key findings

- Performance studies demonstrate the detector's capability.
- R&D results show feasibility of high-granularity calorimetry.
- Proposed measurements will access gluon distributions at x~10^{-5}.

## Abstract

In relativistic nuclear collisions some of the important aspects to be addressed are the effects of the nuclear PDF and the gluon saturation. In the LHC the best way to address these questions is by means of pA collisions and in particular through the measurement of direct photon production in the forward direction. In order to achieve this measurement a new forward calorimeter (FoCal) is proposed as an upgrade to the ALICE experiment. The proposed detector will cover the range 3.5 < $\eta$ < 5, probing the gluon distributions at x$\sim$10$^{-5}$ and Q$\sim p_{\rm T}$ > 4 GeV. We will discuss performance studies and demonstrate that extremely high-granularity calorimetry is required for such measurement. We will also present a few results from R\&D for this project.