COMPASS experiment

COMPASS experiment. The Common Muon and Proton Apparatus for Structure and Spectroscopy (COMPASS) is a fixed-target particle physics experiment located at the Super Proton Synchrotron (SPS) accelerator at CERN in Geneva. It is a multi-purpose facility designed to investigate the internal structure and dynamics of hadrons, primarily using high-energy beams of muons and hadrons. The collaboration's research has significantly advanced the understanding of quantum chromodynamics (QCD), particularly in the realms of nucleon spin structure and hadron spectroscopy.

Overview

The experiment was formally proposed in 1996 and began taking data in 2002, building upon the legacy of earlier European Muon Collaboration (EMC) and Spin Muon Collaboration (SMC) experiments at CERN. Its apparatus is installed in the North Area of the SPS, utilizing one of the world's most intense muon and hadron beams. The COMPASS setup is characterized by its large acceptance spectrometer, which allows for the detection of a wide variety of final-state particles produced in deep-inelastic scattering and hadronic reactions. This versatility has enabled a broad and enduring physics program that continues to evolve.

Physics goals

The primary scientific objectives are divided into two main themes. The first is the study of the spin structure of the nucleon, aiming to precisely determine the contributions of quark spins, gluon spins, and orbital angular momenta to the overall spin of the proton, a puzzle initiated by the famous EMC experiment. The second major theme is hadron spectroscopy, focusing on the search for and investigation of exotic hadronic states like glueballs and hybrids, which are predicted by lattice QCD calculations. Additional goals include precise measurements of hadron properties and the study of transverse-momentum-dependent parton distribution functions.

Experimental setup

The apparatus is a two-stage, large-acceptance spectrometer approximately 60 meters in length. A high-intensity beam, either a polarized muon beam from pion decay or a hadron beam (pions, kaons, or protons), impinges on various polarized or unpolarized fixed targets, such as a polarized deuteron target or a liquid hydrogen target. The spectrometer employs a wide array of particle detection technologies, including tracking detectors like silicon microstrips and drift chambers, particle identification via RICH detectors, electromagnetic and hadronic calorimetry, and a sophisticated muon filter system. Key magnets, like the large SM1 magnet and the SM2 magnet, provide momentum analysis.

Key results and discoveries

The collaboration has produced landmark results in spin physics, notably providing the world's most precise determination of the gluon polarization contribution to the proton's spin. In hadron spectroscopy, COMPASS made the first compelling observation of a spin-exotic resonance, the a1(1420), challenging conventional quark model interpretations. The experiment has also conducted high-precision measurements of the charged pion polarizability, delivered extensive data on transverse spin asymmetries, and performed detailed studies of light meson spectra, providing crucial input for theoretical models like those from the Glasgow group and the JPAC collaboration.

Collaboration and institutions

The COMPASS collaboration is a large international team comprising several hundred physicists, engineers, and technicians from nearly 30 institutions across more than a dozen countries. Major participating institutes include the CEA in France, the Technische Universität München, the Joint Institute for Nuclear Research in Dubna, the University of Trieste, and the Charles University in Prague. The collaboration is managed through a spokesperson system and its work is supported by various national funding agencies, including the Deutsche Forschungsgemeinschaft and the IN2P3.

Category:Particle physics experiments Category:CERN experiments