Abstract
The semi-inclusive difference asymmetry Ah+−h− for hadrons of opposite charge has been measured by the COMPASS experiment at CERN. The data were collected in the years 2002–2004 using a 160 GeV polarised muon beam scattered off a large polarised 6LiD target in the kinematic range 0.006<x<0.7 and 1<Q2<100 (GeV/c)2. In leading order QCD (LO) the deuteron asymmetry Ah+−h− measures the valence quark polarisation and provides an evaluation of the first moment of Δuv+Δdv which is found to be equal to 0.40±0.07(stat.)±0.06(syst.) over the measured range of x at Q2=10 (GeV/c)2. When combined with the first moment of g1d previously measured on the same data, this result favours a non-symmetric polarisation of light quarks Δu¯=−Δd¯ at a confidence level of two standard deviations, in contrast to the often assumed symmetric scenario Δu¯=Δd¯=Δs¯=Δs.
Highlights
The semi-inclusive difference asymmetry Ah+−h− for hadrons of opposite charge has been measured by the COMPASS experiment at CERN
The COMPASS experiment at CERN has recently published an evaluation of the deuteron spin-dependent structure function g1d(x) in the DIS region, based on measurements of the spin asymmetries observed in the scattering of 160 GeV longitudinally polarised muons on a longitudinally polarised 6LiD target [1]
We present here additional information on the contribution of the nucleon constituents to its spin, based on semi-inclusive spin asymmetries measured on the same data as those used in Ref. [1]
Summary
Events and does not exceed 10−3 in any x bin. The presence of possible false asymmetries due to time-dependent apparatus effects has been studied in the same way as for the inclusive asymmetries: the data sample has been divided into a large number of subsamples, each of them collected in a small time interval. The observed dispersion of the asymmetries obtained for these subsamples has been found compatible with the value expected from their statistical error. This allows to set an upper limit for this type of false asymmetries at about half of the statistical error. At higher order in QCD the difference asymmetries still determine the valence quark polarisation without any assumption on the sea and gluon densities [8]. The ratio of cross sections for negative and positive hadrons, r, depends on the event kinematics and is obtained as the product of the corresponding ratio of the number of observed hadrons N −/N + by the ratio of the geometrical acceptances a+/a−: N− N+
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