Abstract
We present a LO evaluation of helicity densities of valence, Δuv+Δdv, non-strange sea, Δu¯+Δd¯, and strange quarks, Δs (assumed to be equal to Δs¯). They have been obtained from the inclusive asymmetry A1,d and the semi-inclusive asymmetries A1,dπ+, A1,dπ−, A1,dK+, A1,dK− measured in polarised deep inelastic muon–deuteron scattering. The full deuteron statistics of COMPASS (years 2002–2004 and 2006) has been used. The data cover the range Q2>1 (GeV/c)2 and 0.004<x<0.3. Both non-strange densities are found to be in a good agreement with previous measurements. The distribution of Δs(x) is compatible with zero in the whole measured range, in contrast to the shape of the strange quark helicity distribution obtained in most LO and NLO QCD fits. The sensitivity of the values of Δs(x) upon the choice of fragmentation functions used in the derivation is discussed.
Highlights
A m ong the various sea quarks contributing to the nucleon spin, the strange quark is the only one accessible in inclusive lepton-nucleon scattering experim ents.T he rst m om ent ofthe strange quark helicity distribution, s+ s, has been found to be negative already twenty years ago in the EM C experim ent [1] under the assum ption of SU (3)F sym m etry in hyperon decays.T his result has been con rm ed w ith im proved precision by recentm easurem entsperform ed by H ER M ES [2]and by C O M PA SS w hich has obtained s+ s = 0:09 0:01(stat:) 0:02(syst:) (1)at Q 2 = 10(G eV =c)2 at leading order (LO ) in Q C D [3]
K 1;d m easured in polarised deep inelastic m uon-deuteron scattering.T he fulldeuteron statistics ofC O M PA SS has been used.T he data cover the range Q 2 > 1(G eV /c)2 and 0:004 < x < 0:3.B oth nonstrange densities are found to be in a good agreem ent w ith previous m easurem ents.T he distribution of s(x)iscom patible w ith zero in the w hole m easured range,in contrastto the shape ofthe strange quark helicity distribution obtained in m ost LO and N LO Q C D ts.T he sensitivity ofthe values of s(x) upon the choice offragm entation functions used in the derivation is discussed
1 Introduction A m ong the various sea quarks contributing to the nucleon spin, the strange quark is the only one accessible in inclusive lepton-nucleon scattering experim ents.T he rst m om ent ofthe strange quark helicity distribution, s+ s, has been found to be negative already twenty years ago in the EM C experim ent [1] under the assum ption of SU (3)F sym m etry in hyperon decays.T his result has been con rm ed w ith im proved precision by recentm easurem entsperform ed by H ER M ES [2]and by C O M PA SS w hich has obtained s+ s = 0:09 0:01(stat:) 0:02(syst:)
Summary
W e present a LO evaluation of helicity densities of valence, u v+ d v, non-strange sea, u+ d, and strange quarks, s (assum ed to be equalto s).T hey have been obtained from the inclusive asym m etry. K 1;d m easured in polarised deep inelastic m uon-deuteron scattering.T he fulldeuteron statistics ofC O M PA SS (years 2002{2004 and 2006) has been used.T he data cover the range Q 2 > 1(G eV /c) and 0:004 < x < 0:3.B oth nonstrange densities are found to be in a good agreem ent w ith previous m easurem ents.T he distribution of s(x)iscom patible w ith zero in the w hole m easured range,in contrastto the shape ofthe strange quark helicity distribution obtained in m ost LO and N LO Q C D ts.T he sensitivity ofthe values of s(x) upon the choice offragm entation functions used in the derivation is discussed.
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