COMPASS investigates how quarks and gluons interact to give the particles we observe

The Common Muon and Proton Apparatus for Structure and Spectroscopy (COMPASS) experiment is a multipurpose experiment at CERN’s Super Proton Synchrotron (SPS).

The experiment looks at the complex ways in which the elementary quarks and gluons work together to give the particles we observe, from the humble proton to the huge variety of more exotic particles.

A major aim is to discover more about how the property called spin arises in protons and neutrons, in particular how much is contributed by the gluons that bind the quarks together via the strong force. To do this the COMPASS team fire muons (particles that are like heavy electrons) at a polarized target.

Another important aim is to investigate the hierarchy or spectrum of particles that quarks and gluons can form. To do this the experiment uses a beam of particles called pions. In these studies, the researchers will also look for "glueballs" - particles made only of gluons.

About 240 physicists from 11 countries and 28 institutions work on the COMPASS experiment. The results will help physicists to gain a better understanding of the complex world inside protons and neutrons.