The COMPASS experiment (Common Muon and Proton Aparatus for Structure and Spectroscopy) is a particle physics experiment on the hadron and muon beam from the SPS accelerator at CERN, close to Geneva. Several physics topics are adressed:
- Nucleon structure in quarks and gluons, in which the DPhN is particlarly involved. This topics includes several subjects: contribution of the quarks and the gluons on the proton spin, transverse distribution of parton positions and momenta, 3D structure of the nucleon, specific processes like the Drell-Yan one to study the nucleon structure
- Hadron spectroscopy: charmed hadrons, search for glueballs, exotic states of hadrons
The COMPASS experiment uses 160 to 200 GeV muons and hadrons beam built from the SPS proton beam impinging a tungstene target, with final beam intensities up to 2.10⁸ p/s. The beam is focalised on several kinds of targets: NH3 or ⁶LiD polarised target using a solenoid magnet, liquid hydrogen target, solid target. The COMPASS collaboration groups 26 institutes from 14 countries including Germany, Italia, France, USA and Russia.
Several topics are studied or were recently studied:
- 2015-2018: Transverse momentum parton distribution in the nucleon using the Drell-Yan process (q q annihilation into 2 leptons), using for the first time a transversely polarized target
- 2012-2016: Generalized parton distribution in the nucleon studies using the deep virtual Compton scattering process (DVCS)
- 2002-2011: Studies on the spin structure of the nucleon, in particular the contribution of the gluon spin to the nucleon spin. Results are in accordance with those of other experiments on a positive contribution of the gluon spon in the studied kinematical area.
3D tomography of the nucleon (2012, 2016-2017)
New theoretical concepts like the generalized parton distributions (GPD), or transverse momentum disctributions (TMD) lead to a improved description of the nucleon structure, and more generaly of the confinment of quarks in hadrons. GPDs are describing the correlations between transverse position and longitudinal momenta of partons and give a 3D mapping of the nucleon called nucleon tomography. A new program named COMPASS-II started in 2012 to study these both descriptions.
GPDs are measured with exclusive process like the DVCS process, or the deep virtual meson production (DVMP) process. High energy positive or negative polarized muon beams are an important advantage of the COMPASS experiment. Based on the DVCS process and its interference with the Bethe-Heitler (BH) process, sums and differences of polarized cross sections with positive and negative muons provide measurements of the real and imaginary parts of the DVCS amplitude.
Study of transverse parton distributions in the nucleon using polarized Drell-Yan process (2015, 2018)
The Drel-Yan process take place in the interaction of an hadron beam, here pions, on a nucleon target, here protons. In the present study the target is transversaly polarized in order to study spin effects. In the Drell-Yan process a quark of the target and an antiquark of the impinging pion interact to produce a virtual photon and then a pair of µ⁺ and µ⁻. Compared to previous Drell-Yan experiments the COMPASS experiment benefits of a polarized target and a high precision spectrometer which give access to the TMDs of the proton. These measurements are complementary to the TMD ones done at COMPASS with the deep inelastic scattering processes. The universality of the TMD distributions can therefore be verified with the COMPASS experiment, a result which is largely expected by theoretical physicists.
Experimental setup
The Drell-Yan process is recognized by the emitted pair of leptons, here a positive and a negative muon. Muons are easilly identified as they are the only particle able to cross an huge absorber made of tungsten and concrete. The target is formed of two cells where the protons are transversally polarized in two opposed directions. Measuring several azimutal assymetries of the lepton pair between the two cells give access to the determination of several TMDs.
Results and prospects
First results from the 2015 data are very promissing, giving hints on the universality of the TMDs. A second year of data taking takes place in 2018 to improve the measurement. Drell-Yan events from interactions on several types of nuclear targets are also analyzed in order to study the internal structure of the pion which is largely unknown. The possibility to produce hadron beams enriched in kaons or antiprotons (after 2025) is also under study. It will lead to the determination of the internal structure of the kaons.
The spin structure of the nucleon (2002-2011)
To study the spin structure of the nucleon a naurally polarized muon beam with an energy of 160 GeV is scattered on 2 or 3 polarized targets with opposite polarization and formed of deutons (⁶LiD) or protons (NH3). By measured assymetries of deep inelastic scattering cross sections between two different longitudinal polarization states of the muon beam, one can access to the polarized parton distributions of the nucleons. Previous measurements showed that quark spins have a small contribution to the nucleon spin, one the COMPASS goals was to measure the contribution of the gluon spins.
This measurement was done by studying specific events called photon-gluon fusions (PGF), where the virtual photon and the gluon interact through the creation of a quark-antiquark pair. Two PGF processes were specifically studied: production of D⁰ charmed mesons, identified from the remnant of their decay; and production of meson with high transverse momenta. Since 2002 many papers were published based on these studies, and they show, in he limits of the precision of the measurement, a positive contribution of the gluons in the measured kinematical area. the contribution of the quark spins was also measured, with a outcome according to what 25% of the proton spin comes from the quark spins, well in agreement of the last lattice QCD results.
Strange quarks in the nucleon
Thanks to the large luminosity (4.10³²cm⁻².s⁻¹) compared to previous experiments and the availability of a RICH detector, the COMPASS experiment also determined with a better precistion the polarized parton distributions of the sea quarks at low x including the strange quarks. Associated with multiplicity measurements of pions and kaons, these measurements also lead to the extraction of the strange quark fragmentation functions.
The IRFU technical contributions
The IRFU has important contributions to the major detectors of the COMPASS spectrometer as well as to the supraconductive magnet of the polarized target.
- Micromegas detectors: development of 12 new Micromegas detectors which have a important 40×40 cm² large active surface associated with a fast read-out electronics. These detectors can handle important particle flux up to 200 kHz/cm² with a spatial resolution better than 70 µm and a time resolution of the order of 10 ns. Compared to the former detectors, their centers which were inactive before are now active and read with pixel electrodes on a 5 cm-diameter area. These new hybrid detectors also features a preamplifying GEM foil which helps to decrease the impact of discharges at high hadron flux. The detectors were built in 2015 in partnership with the ELVIA company.
- Drift chambers: development of large drift chambers (1.2×1.2 m² and 2×2 m²) with 8 planes per chambers and a 8 m gap, and the associated electronics. These chambers feature a 200 µm spatial resolution. The IRFU transfered its know-how to the Illinois University (UIUC) to allow it to realize a second 2×2 m² large drift chamber for the Drell-Yan studies.
- Supraconductive magner for the polarized target: The IRFU contributed to the refurbishing of the old magnet used in the SMC experiment for COMPASS data taking between 2002 and 2004. Afterward the IRFU worked on he cold tests, the characterization and the slow control of the solenoid magnet at large angle built at Oxford Danphysik and commissioned in 2006.
- CAMERA time of flight detector: development of a recoil proton detector for the DVCS studies. This 5 ton weight, 4 m long and 2 m diameter detector is based on two concentric cylinders of plastic scintillators. It is installed around a 2.5m long hydrogen target since 2012, and has been used for the DVCS data taking in 2012, 2016 and 2017.
Scientific responsabilities of the DPhN group in the COMPASS collaboration
Spokesperson of the collaboration: 2003 to 2015
Run coordination: 2002, 2003, 2006 and 2007
Physics analysis coordination: 1999 – 2001, 2010 – 2012, 2014 – 2017 Physics sub-groups coordination (DIS, GPD): 2002 – 2017
Technical coordination: 2003 – 2006, 2014
Publication committee: 2001 – 2017
Major publications
Quark Helicity Distributions from Longitudinal Spin Asymmetries in Muon-Proton and Muon-Deuteron Scattering, Phys. Lett. B 693 (2010) 227-235
Leading order determination of the gluon polarisation from DIS events with high-pThadron pairs, Phys. Lett. B 718 (2013) 922
Leading and Next-to-Leading Order Gluon Polarisation in the Nucleon and Longitudinal Double Spin Asymmetries from Open Charm Muoproduction, Phys. Rev. D 87 (2013) 052018
The COMPASS-II Proposal, CERN-SPSC-2010-014