COMPASS, a “COmmon Muon and Proton Apparatus for Structure and Spectroscopy”, is a multipurpose high-energy physics experiment at the Super Proton Synchrotron (SPS). Following the approval of the project in 1997 the spectrometer was installed in 1999 - 2000 and was commissioned during a technical run in 2001 with the first data taken back in 2002. Today, nearly 240 researchers from 11 countries and 28 institutions are working in the study of hadron structure and the hadron spectrum with high-rate muon and hadron beams.

A glimpse inside the COMPASS solenoid magnet 

In tune with its name, COMPASS points to several different directions of research in order to study the strong interactions. A major aim of COMPASS is to reveal more about how intrinsic angular momentum, called spin, arises in protons and neutrons and in particular how much is contributed by the gluons that bind the quarks together via the strong force. Another important strand of research is to investigate the hierarchy or spectrum of bound states formed by quarks and gluons. For this purpose, the experiment uses a beam of pions while in the spin programme, a muon beam is used. Moreover, COMPASS searcheds for "glueballs" - particles made of gluons and other unusual configuratons of quarks and gluons. In 2010 an extension of the COMPASS programme has been approved by the CERN Research Board. It tackles measurements to study the structure of hadrons in Deep Virtual Compton Scattering (DVCS), Hard Exclusive Meson Production (HEMP), SIDIS, Polarized Drell-Yan and Primakoff reactions. 

Last month, in a paper published in the journal Physical Review Letters, the COMPASS experiment reported a key measurement on the strong interaction. The strong interaction not only binds quarks into protons and neutrons, it also binds protons and neutrons into nuclei. Inside those nuclei, pions – made up of a quark and an antiquark – take part in mediating the interaction. Strong interaction theory makes a precise prediction on the polarisability of pions – the degree to which their differently-charged constituents can be separated in an electromagnetic field. This has baffled scientists since the 1980s, when the first measurements appeared to be at odds with the theory. Today's COMPASS result is in agreement with theory. "The theory of the strong interaction is one of the cornerstones of our understanding of nature at the level of the fundamental particles," said Fabienne Kunne and Andrea Bressan, spokespersons of the COMPASS experiment, "so this result, in good agreement with the theory, is a very important one." After a long preparatory phase, including a pilot run in the year 2004 and a detailed analysis of these test data, the data published now have been collected in autumn 2009.

To measure the polarisability of the pion, COMPASS shot a beam of pions at onto a target of nickel. As the incoming pions approached the individual electricaly charged nickel nuclei at distances of only twice a few times the radius of the particles themselves, they experienced the very strong electric field of the nickel nucleus, which caused the pions to deform and change trajectory, emitting a photon in the process. It is by measuring this photon and the deflection of the pion for a large sample of 63,000 pions that the polarisability could be measured. The result shows the pion to be significantly less polarisable than suggested by previous measurements, and rather just as much as expected from strong interaction theory. “Despite the high energies available at CERN, the experiment is a big challenge, as the pion polarisability is tiny and its effect hard to isolate,” said Jan Friedrich, researcher at the Technische Universität München and leading scientist in the project. A key point in reaching the desired precision, which is on the one-percent level for the measured cross-section shape, was to perform control measurements with the muon beam, for which there is no free parameter in the theory. (see Figure.1). 

This result is admirably complementary to the studies of fundamental interactions performed at the LHC and a testimony to the diversity and strength of CERN’s research programme,” said CERN Director General Rolf Heuer.