Investigation of the spin structure of the nucleon at the COMPASS experiment
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Abstract
The spin structure of the nucleon is studied at the COMPASS experiment. Their subject is of special interest since the surprising findings of the European muon collaboration (EMC) that the contribution from the quark spins to the nucleon spin is rather small. This finding started the search for the various contributions to the nucleon spin. Their contributions are given by the spins of the quarks and gluons and their orbital angular momenta, where the contribution from the quark spins is about 30%. At the COMPASS experiment, a polarised muon beam is scattered off a polarised fixed target made out of either lithium deuteride or ammonia. In such reactions, the longitudinal double spin asymmetry and the spin-dependent structure function are measured.
The longitudinal double spin asymmetry is obtained in two different kinematic regions. At low photon virtualities, Q^2 < 1 (GeV/c)^2, precise results are obtained from the data taken in 2007 and 2011 using ammonia as a polarised proton target. The results show a small asymmetry of about 1%, which differs from zero, even at very small Bjorken-x. This is the first observation of spin effects at such low Bjorken-x. At high photon virtualities, Q^2 > 1 (GeV/c)^2, the results for the asymmetry obtained from the 2006 and 2011 data taking are presented. The results from the 2006 data taking improve the statistical precision of the spin-dependent structure function of the deuteron. For the 2011 data taking, the nominal beam momentum was increased from 160 GeV/c to 200 GeV/c. This extends the kinematic range towards lower Bjorken-x and higher photon virtualities. These results complete the previous results for the spin-dependent structure function of the proton using the data taken in 2007.
The results on the spin-dependent structure function are used together with the world data on the spin-dependent structure function of the proton, deuteron and neutron measured in deep inelastic scattering in a QCD fit to obtain the parton helicity distributions. They can be interpreted similar as the parton distribution function, which describe the momentum fraction carried by quarks within a certain Bjorken-x range dx. The parton helicity distributions describe the contribution from the quark spins to the total nucleon spin within dx. From the QCD fit, the contribution from the quark spins to the total spin of the nucleon is obtained as well as the the contributions from the various quark flavours.
The measured results on the spin-dependent structure functions are also used to evaluate their first moments, int_0^1 g_1(x,Q^2) dx. They are used to test QCD sum rules like the Bjorken and the Ellis-Jaffe sum rule. A violation of the Ellis-Jaffe sum rule is already known since the surprising results from EMC and was confirmed by various experiments. The Bjorken sum rule is of special interest since it connects the non-singlet structure function, g_1^NS = g_1^p - g_1^n, with the ratio g_A/g_V. Further on the evolution of the non-singlet structure function is independent of the poorly known gluon helicity distribution. Here, the Bjorken sum rule is confirmed at the level of 9%. The first moment of the spin-dependent structure function of the deuteron allows for the determination of the singlet axial charge a_0 = 0.32 +/- 0.02_{stat} +/- 0.04_{syst} pm 0.05_{evol}, which is identified with the contribution from the quark spins to the total nucleon spin in the MSbar scheme.