Protons are made up of quarks that constantly interact by exchanging gluons, particles considered as the vectors of the strong interaction. These quarks are mobile inside the proton.

It is possible to directly probe the distribution of quark functions in a proton target using a beam of very high energy particles. These particles, muons (a heavy variety of an electron) or pions (composite particles), then interact directly with the quarks in the protons, respectively producing a measurable number of pions or muon pairs (one positive and the other negative).

When protons are prepared in two distinct quantum states at very low temperature (at only a few hundredths of a degree Kelvin), physicists expect to observe an asymmetry in their measurements with respect to the proton state. More surprisingly, they predict that their measurements with muons and pions will carry different signs. It is precisely this route that Compass researchers have chosen in order to test a theoretical prediction of quantum chromodynamics (QCD).

In 2012, a first series of measurements with muons was performed by Compass at Cern on the world's largest target, containing protons in each of the two quantum states. In 2015, this series was completed using measurements with pions of approximately equal energy, and on the same target. Several tens of thousands of muon pairs were detected. However, this impressive number does not entirely allow physicists to reach a definitive conclusion. They will have to wait until the end of 2018 to rigorously validate the QCD prediction.