COMPASS to point in new directions

It's an exciting and busy time for COMPASS. The experiment is one of the few in the world capable of studying the internal structure of protons with high precision. It uses secondary beams from the SPS accelerator to study various properties of quarks and gluons. These include their distribution within nucleons, their contribution to nucleon spin and the way they form hadrons when pulled out from the nucleon – information that could feed into understanding proton collisions in the LHC.

In 2014, a new chapter will begin for the COMPASS collaboration. "We have two new phases planned for COMPASS-II," says COMPASS co-spokesperson Fabienne Kunne. "The first will begin in 2014, colliding 190 GeV negative-pion beams into a polarized target. This will allow us to make the first polarized measurements of the Drell-Yan process."

The Drell-Yan process occurs when a quark and an antiquark annihilate at high energies, producing two leptons in the final state. The COMPASS collaboration will look for differences in the azimuthal distribution of particles in this process compared to previous COMPASS studies, in order to test Standard Model theory. To make these measurements, COMPASS will move the polarized target further upstream of the beamline and insert an absorption medium behind it to track the muon pairs that dominate the collisions.

"The second phase of COMPASS-II will begin in 2015 and will focus on gathering a kind of 3D picture of the nucleon," says COMPASS co-spokesperson Andrea Bressan. "By analysing the general distribution of particles inside the nucleon and taking account of their spin and transverse momentum, we hope to gain a new understanding of the internal dynamics of the nucleon. We also hope to gather the first understanding of the quark-orbital angular momentum, which has yet to be measured." The collaboration will use 160 GeV muon beams and a new liquid-hydrogen target for this study. CERN's Cryogenics Group (TE) is building the target and will install it in September 2012. "We will be conducting a pilot run of this new set-up, gathering preliminary data to ensure the 2015 run goes as smoothly as possible," says Kunne.