Snowmass 2021/22 Letter of Interest: A Forward Calorimeter at the LHC

TL;DR

The proposed FoCal upgrade for the ALICE experiment aims to enable high-precision measurements of small-x parton structure in nucleons and nuclei at the LHC, expanding the physics capabilities of the collider.

Contribution

This paper introduces a new forward calorimeter (FoCal) for ALICE, designed to explore small-x physics with unprecedented precision at the LHC.

Findings

FoCal will enable measurements down to x~10^{-6}

It will provide high-precision inclusive photon and jet data

FoCal will complement other LHC small-x probes

Abstract

A forward electromagnetic and hadronic calorimeter (FoCal) was proposed as an upgrade to the ALICE experiment, to be installed during LS3 for data-taking in 2027--2029 at the LHC. The FoCal extends the scope of ALICE, which was designed for the comprehensive study of hot and dense partonic matter, by adding new capabilities to explore the small-$x$ parton structure of nucleons and nuclei. The primary objective of the FoCal is high-precision inclusive measurement of direct photons and jets, as well as coincident gamma-jet and jet-jet measurements, in pp and p--Pb collisions. These measurements by FoCal constitute an essential part of a comprehensive small-$x$ program at the LHC down to $x\sim10^{-6}$ and over a large range of $Q^2$ with a broad array of complementary probes, comprising -- in addition to the photon measurements by FoCal and LHCb -- Drell-Yan and open charm measurements planned by LHCb, as well as photon-induced reactions performed by all LHC experiments.