COMPASS spins in new directions

The COMPASS experiment is preparing for a new phase in its physics programme: COMPASS-II. Due to start in 2014, COMPASS-II will bring a powerful new look at the building blocks of protons: quarks and gluons.


The COMPASS installation.

It’s an exciting and busy time for COMPASS. As one of the few experiments in the world capable of studying the internal structure of protons with high precision, COMPASS uses secondary beams from the SPS accelerator to study a variety of quark and gluon properties. This includes their distribution within nucleons, their contribution to nucleon spin and the way they form hadrons when pulled out from the nucleon - all properties that may also improve the understanding of 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,” explains Fabienne Kunne, COMPASS co-spokesperson. “The first will begin in 2014, colliding 190 GeV negative pion beams into a polarised target. This will allow us to make the first polarised 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. Using measurements of the azimuthal distribution of particles in this process, the COMPASS collaboration will look for differences with previous COMPASS studies in order to test current Standard Model theory. To accomplish these measurements, COMPASS will be moving the polarised target further upstream of the beamline and inserting 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 Andrea Bressan, COMPASS co-spokesperson. “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.”

For this study, the collaboration will use 160 GeV muon beams and a new liquid-hydrogen target. This target is currently being constructed by CERN’s Cryogenics Group (TE) and will be installed 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,” concludes Fabienne.

by Katarina Anthony