Abstract
First measurements of azimuthal asymmetries in hadron-pair production in deep-inelastic scattering of muons on transversely polarised LiD6 (deuteron) and NH3 (proton) targets are presented. The data were taken in the years 2002–2004 and 2007 with the COMPASS spectrometer using a muon beam of 160 GeV/c at the CERN SPS. The asymmetries provide access to the transversity distribution functions, without involving the Collins effect as in single hadron production. The sizeable asymmetries measured on the NH3 target indicate non-vanishing u-quark transversity and two-hadron interference fragmentation functions. The small asymmetries measured on the LiD6 target can be interpreted as indication for a cancellation of u- and d-quark transversities.
Highlights
The quark content of the nucleon at twist-two level can be characterised by three independent distribution functions (DFs) for each quark flavour [1]
The analysis presented in this Letter is performed using data taken in the years 2002–2004 and 2007 with the COMPASS spectrometer [33] by scattering positive muons of 160 GeV/c from the CERN SPS off transversely polarised solid state 6LiD and NH3 targets, respectively
When comparing the results on the NH3 target to the published HERMES results on a transversely polarised proton target [28], the larger kinematic region in x and Mhh is evident. Both results cannot be directly compared for several reasons: (1) The opposite sign is due to the fact that in the extraction of the asymmetries the phase π in the angle φRS is used in the COMPASS analysis; (2) COMPASS calculates asymmetries in the photon–nucleon system, while HERMES published them in the lepton–nucleon system; both agree reasonably well when including Dnn corrections for HERMES; (3) HERMES uses identified π +π − pairs and COMPASS
Summary
The quark content of the nucleon at twist-two level can be characterised by three independent distribution functions (DFs) for each quark flavour [1]. Up to ten years ago nothing was known about the third function, the transverse spin distribution hq1(x), often referred to as transversity, which describes the difference between the number densities of quarks with their spins oriented parallel and antiparallel to the spin of a transversely polarised nucleon. Convolution integrals of quark and hadron transverse momenta have to be taken into account [3,4,8] Such asymmetries were measured by HERMES using a transversely polarised pure hydrogen target [9] and by COMPASS using transversely polarised 6LiD (deuteron) [10] and NH3 (proton) targets [11]. Due to the large acceptance of the COMPASS spectrometer and the large beam momentum of 160 GeV/c, results with high statistics were obtained covering a large kinematic range in x and Mhh, the invariant mass of the hadron pair
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