One of the Laboratory's newest collaborations got together at CERN in April to prepare the ground for their recently approved experiment. COMPASS, which stands for Common Muon and Proton Apparatus for Structure and Spectroscopy, will be built in the North Area reusing several components from the Spin Muon Collaboration (SMC) detector and from the Omega spectrometer. COMPASS's initial physics aims will be to continue the work of SMC into nucleon structure, and to study in detail the hadron spectrum, following the line of investigations pursued by the WA102 and WA89 experiments. For this, both muon and proton beams will be used.
The COMPASS collaboration got together at CERN in April for their first collaboration meeting since winning the approval of the Laboratory's research board.
Deep inelastic scattering experiments in which a high energy projectile scatters from a quark inside a nucleon were pioneered at SLAC, Stanford, in the 1960s. These experiments demonstrated that nucleons are made up of small pointlike objects, which subsequent studies including experiments using neutrino beams at CERN's Gargamelle bubble chamber showed to be quarks. The SMC and its predecessor, the European Muon Collaboration, have studied this phenomenon in depth, using the CERN high energy muon beam, and have shown very clearly that not all of the proton spin is contributed by its quarks. Current thinking now points to gluons as likely contributors to the nucleon's spin. This idea will be tested by the COMPASS experiment, which will identify the particles in the hadronic jet created in the collision, and provide a tag for the struck quark. In particular, by looking for charmed mesons emerging from the scattering events, COMPASS will measure the gluon contribution to the proton spin, with the aim of providing the definitive answer to a question which has been troubling physicists for over a decade.
Using a proton beam COMPASS will address long standing questions in hadron spectroscopy. It will look for so-called exotic particles made up of quarks and gluons - particles such as glueballs, composed only of gluons, quark-gluon hybrids, and quark-antiquark combinations which do not fit into existing meson multiplets. The COMPASS experiment will complement studies recently performed using the Omega spectrometer, and at LEAR, the Low Energy Antiproton Ring, where new evidence for glueballs was seen by the Crystal Barrel detector. Charmed particles also play a major role in the proton beam programme. Challenging experimental techniques will be used to study charmed hadron decays producing a pair of leptons or a lepton and another hadron. This will probe the internal structure of charmed mesons, and investigate the transitions from heavy to light quarks in heavy particle decays. Further ahead, COMPASS plans to search for baryons containing two charmed quarks, extending the currently known baryon spectrum.
With its unusual mixture of muon and proton physics, COMPASS has attracted researchers with a diverse range of backgrounds. The core of the SMC collaboration brings two decades of muon physics experience, whilst researchers from facilities as varied as LEAR and the Omega spectrometer will carry on their studies with COMPASS. But this disparity in experimental background disguises a great similarity in physics goals. Although their methods were different, COMPASS members hailing from LEAR, Omega, and the SMC have all been involved in studying the structure of hadrons, and COMPASS is a logical place for them to come together.