Division of accelerators, cryogenics and magnetism

Drell-Yan Process and transverse parton distributions in the nucleon

Using a high energy pion beam and a transversally polarized nucleon target, COMPASS is the first experiment  measuring directly  spin dependent Drell-Yan events. The Drell-Yan process (annihilation of a quark and an anti-quark in a lepton pair q qbarà l+ l-) has been used in the past to study simple distributions of quarks in the nucleon. The available beams, the polarized target and the  COMPASS spectrometer allow to obtain more precise data, and particularly to access the  parton Transverse Momentum-dependent Distributions (TMD). These measurements are complementary to previous data collected at COMPASS on deep inelastic scattering. Therefore one can study the universality of TMDs, of great interest to  theoreticians. 

Experimental setup  : A 190 GeV pion beam produces antiquarks (from the pion valence region) that annihilate with the proton valence quarks. A Drell-Yan event is signed by the creation of a lepton pair, that is, in the present case a negative and a positive muon. Muons are easily identified, as they are the only particles that cross the huge tungsten  and concrete absorber. The target includes two cells where protons are transversally polarized in two opposite directions. Measurements of different azimuthal asymetries of the reaction  products, allow to determine simultaneously several  TMDs.

Results and perspectives : The first measurements give a very promising result pointing to the TMD universality. Data taking will continue in 2018. In parallel, Drell-Yan events off different nuclear targets are analyzed to study the internal structure of the pion, that is not well known. The possibility to produce beyond kaon or antiproton rich beams beyond 2023 is under investigation. It would give access to  the quark structure of the kaons.