COMPASS (particle physics experiment)

COMPASS (particle physics experiment)
Name	COMPASS
Location	CERN, Geneva
Institution	CERN
Start	2002
Status	Active

COMPASS (particle physics experiment) is a fixed-target experiment at CERN located on the Super Proton Synchrotron beamline, designed to investigate the structure and spectroscopy of hadrons. The collaboration combines expertise from institutions such as CERN, DESY, INFN, IHEP, and Joint Institute for Nuclear Research to study phenomena including spin structure, parton distributions, and exotic mesons using high-energy muon and hadron beams.

Introduction

The experiment began data taking in 2002 on the M2 beamline of the Super Proton Synchrotron with a program influenced by proposals associated with European Organization for Nuclear Research, Nikolaus Meier-era planning, and follow-up studies linked to HERMES, COMPASS-II proposals, and legacy results from EMC (particle physics experiment). The collaboration unites groups from CERN, Max Planck Society, CNRS, University of Oxford, and Massachusetts Institute of Technology to address questions raised by earlier measurements at SLAC, DESY HERA, and Fermilab. Key motivations derive from anomalies observed in experiments such as EMC effect studies and spectroscopy signals reported by E852, Crystal Ball, and VES.

Experimental Apparatus

The apparatus centers on a two-stage open spectrometer incorporating large-aperture dipole magnets, tracking detectors, calorimetry, and particle identification systems developed by teams from CERN, Paul Scherrer Institute, University of Trieste, and Institut de Physique Nucléaire d'Orsay. Tracking uses technologies including GEM, Micromegas, and drift chambers contributed by INFN, CEA Saclay, and University of Birmingham. Particle identification employs a ring-imaging Cherenkov detector (RICH) built with components from DESY, ETH Zurich, and NIKHEF, while electromagnetic calorimeters have been provided by University of Glasgow, University of Geneva, and JINR. The target region accommodates polarized targets realized via expertise from University of Ferrara, Institute for High Energy Physics (Protvino), and Ruhr University Bochum, enabling measurements with muon beams sourced from PS (Proton Synchrotron) and hadron beams derived from the Super Proton Synchrotron.

Physics Program and Key Measurements

COMPASS pursues a broad physics program concentrating on nucleon spin structure, transverse momentum dependent distributions, generalized parton distributions, and hadron spectroscopy. Spin studies follow up on the EMC (particle physics experiment) spin crisis by measuring polarized structure functions similar to programs at SLAC E155, HERMES, and Jefferson Lab, while transverse spin investigations relate to results from RHIC Spin and BELLE. Drell–Yan measurements with pion beams address sign-change predictions associated with Collins effect and theoretical frameworks from QCD factorization, reflecting analyses comparable to those at Fermilab SeaQuest. Hadron spectroscopy searches target exotic states such as hybrids and tetraquarks following hints from GlueX, BESIII, BaBar, and LHCb.

Data Analysis and Results

Results have included precision determinations of spin-dependent structure functions, flavor-separated parton distribution information complementary to CTEQ, MSTW, and NNPDF global fits, and measurements of transverse spin asymmetries relevant to theoretical work by Xiangdong Ji, John Collins, and Andre H. Mueller. Spectroscopy analyses reported candidates for exotic mesons that were compared with signals from E852, COMPASS results 2004, and amplitude analyses techniques developed alongside groups from University of Regensburg and Ruhr University Bochum. Drell–Yan data provided tests of non-universality predicted in perturbative Quantum Chromodynamics and feed into global phenomenology alongside measurements from NA10 and E866/NuSea.

Collaboration and Organization

The collaboration comprises hundreds of physicists, engineers, and students from institutions such as CERN, DESY, INFN, CNRS, JINR, IHEP, University of Manchester, University of Michigan, and University of Tokyo. Governance follows models used at CERN experiments with an elected spokesperson, technical coordinator, and physics coordinators, and close interactions with facility groups responsible for the Super Proton Synchrotron and CERN accelerator complex. Workshops and analysis schools have been organized with partners including IPPP Durham, CERN Summer Student Programme, and SPS Physics Working Group.

Impact and Future Prospects

COMPASS results have influenced global parton distribution fits such as NNPDF and MSTW, informed experimental designs at Jefferson Lab 12 GeV Upgrade, and guided spectroscopy programs at GlueX and PANDA. Planned upgrades and future runs aim to extend Drell–Yan measurements, deepen generalized parton distribution studies, and refine hadron spectroscopy with technological developments in detectors from CERN EP-ESE collaborations and institutes like INFN Sezione di Trieste and CEA. The experiment remains a key component of the European hadron physics landscape, interfacing with theory efforts at DESY Theory Group, IPPP Durham, and Institut de Physique Théorique.

Category:Particle physics experiments Category:Experiments at CERN