The COMPASS experiment at CERN

✍ Scribed by F. Bradamante

• Publisher: Elsevier Science
• Year: 1997
• Tongue: English
• Weight: 759 KB
• Volume: 622
• Category: Article
• ISSN: 0375-9474
• DOI: 10.1016/s0375-9474(97)00332-1

✦ Synopsis

An important goal of the COMPASS experiment is the study of semi-inclusive polarised muon scattering from a polarised target. The identification of the hadrons will allow, for the first time, a direct measurement of the gluon polarisation AG/G from the cross-section asymmetry of open charm production, and, more generally, a flavour analysis of the spin structure function 91(x). With a transversely polarised target the spin dependent hadron azimuthal asymmetries will be measured. These asymmetries depend on the hitherto unmeasured twist-2 distribution functions ATq of transverse spin and the Collins asymmetries in fragmentation.

The present measurements of gl and g~ will also be improved. This programme could be completed in a four year measurement with a proportion of four to one for the running time with longitudinally and transversely polarised targets.

1 Physics Objectives

The original EMC discovery [1,2] that the singlet axial vector current matrix element AE is smaller than the value predicted by the Ellis-Jaffe sum rule is by now firmly established. The new experiments carried out at CERN [3][4][5] and SLAC [6-8] have confirmed the EMC result for the proton and obtained a similar result for the deuteron.

A clear-cut explanation of this result is still missing. It has been suggested [9][10][11] that, due to the axial anomaly the flavour singlet part of the structure functions gl of polarised nucleons might be substantially reduced. In an alternative model [12] negatively polarised strange quarks are responsible. Inclusive measurements of g~(x),g~(x) and 1 Presently at CERN, PPE Division, Geneva