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 study the internal structure of protons and neutrons and to discover more about how the property called spin arises in them, in particular how much is contributed by the motion of quarks or by the gluons that bind the quarks together. To do this the COMPASS team fires muons (particles that are like heavy electrons) and particles called pions at a polarized target.
Another important aim is to investigate the hierarchy or spectrum of particles that quarks and gluons can form. In these studies, the researchers also look for "glueballs" – exotic particles made only of gluons.
About 220 physicists from 13 countries and 25 institutions work on the COMPASS experiment. The results will help physicists to gain a better understanding of the complex world inside protons and neutrons.
AMBER, or Apparatus for Meson and Baryon Experimental Research, will be the next-generation successor of COMPASS after the experiment’s last run in 2021–2022.