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

J. P. Lansberg, M. Anselmino, R. Arnaldi, S. J. Brodsky, V. Chambert, W. den Dunnen, J. P. Didelez, B. Genolini, E. G. Ferreiro, F. Fleuret, Y. Gao, C. Hadjidakis, I. Hrvinacova, C. Lorce, L. Massacrier, R. Mikkelsen, C. Pisano, A. Rakotozafindrabe, P. Rosier, I. Schienbein, M. Schlegel, E. Scomparin, B. Trzeciak, U. I. Uggerhoj, R. Ulrich, Z. Yang

TL;DR

The paper advocates AFTER@LHC, a high-luminosity fixed-target program using bent-crystal extraction of LHC beams, to perform spin- and TMD-related measurements in both polarised and unpolarised modes. It outlines how low- and high-P_T quarkonium production, back-to-back quarkonium+γ, and Drell-Yan channels can probe gluon and quark TMDs, including f1^g, h1^{⊥g}, and Sivers functions, especially in the target-rapidity region with large x. The initiative aims to provide unique, high-x access and complementary insights to RHIC and future EIC programs, leveraging extensive quarkonium yields and novel observables to test TMD factorisation and Sivers sign-change predictions. Overall, AFTER@LHC is presented as a versatile, multi-system platform that could significantly advance spin physics and heavy-flavour studies at the LHC energy frontier.

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.

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

TL;DR

The paper advocates AFTER@LHC, a high-luminosity fixed-target program using bent-crystal extraction of LHC beams, to perform spin- and TMD-related measurements in both polarised and unpolarised modes. It outlines how low- and high-P_T quarkonium production, back-to-back quarkonium+γ, and Drell-Yan channels can probe gluon and quark TMDs, including f1^g, h1^{⊥g}, and Sivers functions, especially in the target-rapidity region with large x. The initiative aims to provide unique, high-x access and complementary insights to RHIC and future EIC programs, leveraging extensive quarkonium yields and novel observables to test TMD factorisation and Sivers sign-change predictions. Overall, AFTER@LHC is presented as a versatile, multi-system platform that could significantly advance spin physics and heavy-flavour studies at the LHC energy frontier.

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.

Paper Structure

This paper contains 10 sections, 3 figures.

Table of Contents

  1. Introduction
  2. Beam extraction and target polarisation
  3. Short selection of highlight studies not related to spin.
  4. Spin studies with unpolarised protons: Boer-Mulders functions and related distributions
  5. Low- $P_T$ quarkonium production
  6. Back-to-back quarkonium $+ \gamma$ production
  7. Spin studies with polarised protons: Single Transverse Spin Asymmetries
  8. Looking for the gluon Sivers effect and beyond
  9. Quark Sivers effect
  10. Conclusion

Figures (3)

  • Figure 1: Instantaneous and yearly luminosities obtained for targets of various thickness with an extracted beam of (a) $5 \times 10^8$ p$^+$/s with a momentum of 7 TeV and (b) $2 \times 10^5$ Pb/s with a momentum per nucleon of 2.76 TeV.
  • Figure 2: Different contributions to the production of an isolated photon back-to-back with a $J/\psi$ from $g-g$ and $q-\bar{q}$ fusion from the CS and CO channels as a function of the invariant mass of the pair $Q_{(J/\psi+\gamma)}$ for three different rapidity regions (from left to right: $|Y|<0.5$, $-1.5< Y < -0.5$ and $-2.5< Y < -1.5$).
  • Figure 3: Typical LO Feynman graphs for (a) $\eta_Q$, (b) $\psi+\gamma$ and (c) $J/\psi$-pair production.