COMPASS – COmmon Muon and Proton Apparatus for Structure and Spectroscopy

Particles built from quarks and gluons

COMPASS is a multi-purpose experiment taking place at CERN’s Super Proton Synchrotron accelerator.

It is looking into the complex ways in which the elementary quarks and gluons work together to give particles we observe, from the humble proton to the huge variety of more complex 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 experiment fires ‘heavy electrons’ (particles called muons) 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 particles called glueballs, which are 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.