Spin physics at A Fixed-Target ExpeRiment at the LHC (AFTER@LHC)
A. Rakotozafindrabe, M. Anselmino, R. Arnaldi, S. J. Brodsky, V. Chambert, J. P. Didelez, E. G. Ferreiro, F. Fleuret, B. Genolini, C. Hadjidakis, J. P. Lansberg, C. Lorce, P. Rosier, I. Schienbein, E. Scomparin, U. I. Uggerhoj
TL;DR
This paper proposes AFTER@LHC, a fixed-target spin-physics program using a bent-crystal extracted LHC proton beam to study single-spin asymmetries with a polarized target. It argues that the resulting center-of-mass energy around 115 GeV and large-$x_F$ access enable probing Sivers functions, especially the elusive gluon Sivers function, through gluon-sensitive processes like prompt photons and heavy flavors, as well as Drell–Yan and quarkonium channels. The authors detail beam extraction feasibility, target-polarization options (DNP or HD targets) within space constraints, and projected luminosities that support high-precision SSA measurements with manageable heating and damage considerations. Overall, AFTER has the potential to illuminate gluon contributions to nucleon spin and the orbital dynamics of partons at previously unexplored energies.
Abstract
We outline the opportunities for spin physics which are offered by a next generation and multi-purpose fixed-target experiment exploiting the proton LHC beam extracted by a bent crystal. In particular, we focus on the study of single transverse spin asymetries with the polarisation of the target.