Berlin 2005 – wissenschaftliches Programm

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HK: Physik der Hadronen und Kerne

HK 30: Elektromagnetische und Hadronische Proben

HK 30.2: Vortrag

Dienstag, 8. März 2005, 17:00–17:15, TU MA001

First measurements of Collins and Sivers asymmetries at COMPASS — •Richard Webb — Physikalisches Institut, Friedrich-Alexander-Universität Erlangen-Nürnberg

The cross-section for deep inelastic lepton scattering off spin 1/2 hadrons is parametrised in leading order in terms of three quark distribution functions: the unpolarised distribution q(x), the longitudinally polarised distribution Δ q(x) and the so-called transversity distribution Δ_T q(x). This last function is chiral-odd and so can only be measured incombination with a chiral-odd fragmentation function such as the Collins function H₁^⊥(z), requiring the detection of part of the hadronic end-product (semi-inclusive measurement). An asymmetry is expected in the azimuthal production angle of the hadron in the scattering process which should depend on the Collins angle Φ_C = Φ_h − Φ_s′, where Φ_s′ is the spin angle of the fragmenting quark. The deep-inelastic cross-section also contains the product of the unpolarised fragmentation function D_q(z) and the Sivers distibution function f_1T(x). Here the dependency of the expected asymmetry is on the Sivers angle Φ_S = Φ_h − Φ_s, Φ_s being the angle of the quark spin in the initial state. COMPASS is a fixed target experiment on the SPS M2 beamline at CERN. Its ⁶LiD target can be polarised both longitudinally and transversally with respect to the polarised 160GeV/c µ⁺ beam. Approximately 20% of the beam-time is spent in the transverse configuration. Results of the analysis of the Collins and Sivers asymmetries on the basis of the first COMPASS data will be reported here. This work is supported by the BMBF.