Nuclear Physics Division

The physicists from the Compass collaboration at Cern, which comprises a team from Irfu, have just published the results of a new measurement of the quark structure of the proton [1]. This uncommon but eagerly awaited measurement tends to confirm one of the basic assumptions of the theory of the strong interaction, the Quantum Chromodynamics (QCD). According to QCD, the complex reaction between two particles in a collision of sufficiently high energy can be separated (factorized) into two contributions: the interaction itself and the quark distribution functions inside the interacting particles. To examine the concept of factorization, the experimenters measured the same quantity called asymmetry, but using two different physical processes: first with a muon beam and then with a pion beam. The result is uncommon because paradoxically, in order to confirm the QCD predictions, the two processes must produce opposite-sign results.