Spin physics and TMD studies at A Fixed-Target ExpeRiment at the LHC (AFTER@LHC)

TL;DR

The paper discusses the potential of the AFTER@LHC fixed-target experiment to advance spin physics and TMD studies using high-energy LHC beams, offering unprecedented luminosity and access to large-x regions for polarized nucleon investigations.

Contribution

It introduces the scientific opportunities and experimental capabilities of the proposed AFTER@LHC fixed-target setup for spin and TMD research at multi-TeV energies.

Findings

High luminosity surpassing RHIC by over 3 orders of magnitude.

Ability to measure TMDs like Boer-Mulders and gluon distributions.

Potential to study transverse single-spin asymmetries in polarized targets.

Abstract

We report on the opportunities for spin physics and Transverse-Momentum Dependent distribution (TMD) studies at a future multi-purpose fixed-target experiment using the proton or lead ion LHC beams extracted by a bent crystal. The LHC multi-TeV beams allow for the most energetic fixed-target experiments ever performed, opening new domains of particle and nuclear physics and complementing that of collider physics, in particular that of RHIC and the EIC projects. The luminosity achievable with AFTER@LHC using typical targets would surpass that of RHIC by more that 3 orders of magnitude in a similar energy region. In unpolarised proton-proton collisions, AFTER@LHC allows for measurements of TMDs such as the Boer-Mulders quark distributions, the distribution of unpolarised and linearly polarised gluons in unpolarised protons. Using the polarisation of hydrogen and nuclear targets, one can measure transverse single-spin asymmetries of quark and gluon sensitive probes, such as, respectively, Drell-Yan pair and quarkonium production. The fixed-target mode has the advantage to allow for measurements in the target-rapidity region, namely at large x^uparrow in the polarised nucleon. Overall, this allows for an ambitious spin program which we outline here.