THE SPIN STRUCTURE OF THE NUCLEON

The status of polarised deep inelastic scattering experiments is summarised. Special emphasis is given to new data which became available during the last year. Those have lead to a precise knowledge of the spin dependent structure function g1(x) over a rel

THE SPIN STRUCTURE OF THE NUCLEONMPI fur Kernphysik Heidelberg, Postfach 103390 D-69029 Heidelberg, Germany E-mail: Antje.Bruell@desy.deThe status of polarised deep inelastic scattering experiments is summarised. Special emphasis is given to new data which became available during the last year. Those have lead to a precise knowledge of the spin dependent structure function g1 (x) over a relatively wide kinematic range. However, recent theoretical developements have shown that the interpretation of these data is not yet fully understood. Especially a possible polarisation of the gluons is essentially unconstraint by the present measurements and leads to a large uncertainty in the low x extrapolation of g1 . First measurements of semi-inclusive hadron and pion asymmetries at HERMES are presented and compared to the only other existing hadron data from SMC. Finally the prospects of future measurements of the gluon polarisation are presented.A. BRULL1 IntroductionDeep inelastic lepton nucleon scattering provides a unique possibility to study the internal structure of the nucleon. Since the discovery of the point-like substructure of the nucleon at SLAC and the introduction of the quark parton model a large number of experiments at SLAC, CERN, FERMILAB and DESY have measured the unpolarised structure functions of the nucleon and thus the momentum distributions of the quarks and gluons inside the nucleon with high precision over a large kinematic range. The comparison of these measurements with perturbative QCD represents one of the most stringent tests of the theory of strong interaction and has lead to a precise determination of its coupling constant. To investigate also the spin structure of the nucleon deep inelastic scattering experiments using polarised beams and targets began at SLAC already shortly after the discovery of scaling. In 1984/85 the EMC experiment extended the kinematic range and announced in 1987 the surprising result that only a small fraction of the nucleon's spin is carried by the quark spins. This result triggered a new round of experiments at CERN (SMC), at SLAC (E142, E143, E154 and E155) and at DESY (HERMES) as well as an enormous theoretical e ort to understand the implications of this measurement. It lead to a new insight into the role of the axial anomaly and demonstrated the possible importance of contributions from gluons and/or angular momenta. Still, the basic spin sum rule of the nucleon 1 = 1 + L + g+ L (1) Q g 2 2 where is the intrinsic spin carried by the quarks, LQ is the angular momentum of the quarks, g is the intrinsic spin carried by the gluons and nally Lg is the angular momentum carried by the gluons, is far from being understood experimentally. 1

The status of polarised deep inelastic scattering experiments is summarised. Special emphasis is given to new data which became available during the last year. Those have lead to a precise knowledge of the spin dependent structure function g1(x) over a rel

2 Spin dependent deep inelastic scatteringIn the deep inelastic scattering of a charged lepton on a nucleon the di erential cross section for the one-photon exchange can be expressed as a product of a leptonic tensor, L , and a hadronic tensor, W ,2 d3 = Q4 y L W ; dx dy d' 2(2)where is the ne structure constant and ?Q2 the four-momentum transfer squared. squared, E the energy of the incident lepton and m the lepton mass. The two scaling variables x and y are de ned as x = Q2=2M and y = =E , where is the virtual photon energy in the laboratory frame, M the proton mass and E the energy of the incident lepton. ' represents the azimuthal angle between the scattering plane and the plane containing the lepton and target spins (see g. 1). The tensors L and W involve the leptonic and hadronic electromagnetic currents, respectively. While the leptonic current is well known from QED, the hadronic current contains the unknown non-perturbative structure of the nucleon. Requiring Lorentz invariance, P and T invariance and conservation of the lepton current the di erential cross section is given by2 d3 = Q4 y L (S)W (S) ? L (A) W (A) ; dx dy d' 2(3)where the rst term corresponds to the spin-averaged and the second term to the spindependent part of the cross section involving both, the lepton's and the nucleon's polarisation vectors. Considering only longitudinally polarised leptons (H` = 1 for right and left-handed incident leptons) the Born cross section nally has the form:3 3 d3 d3 = dxd y d' ? H` cos dx dykd' ? H` sin cos ' dd dy?d' ; dx dy d' d x(4)where refers to the spin-averaged cross section and k and ? denote the cross section for longitudinal and transverse orientation of the target spin, respectively. The angle between the lepton momentum and the target spin is 0 and the azimuthal angle between the scattering plane and the plane containing the lepton and target spins is 0 ' 2 (see g. 1). Only ? explicitly depends on '. 2

The status of polarised deep inelastic scattering experiments is summarised. Special emphasis is given to new data which became available during the last year. Those have lead to a precise knowledge of the spin dependent structure function g1(x) over a rel

Spin Plane S s = Hl k leptonβN k' Scattering PlaneFigure 1: Kinematics of polarised deep-inelastic lepton-nucleon scattering.1 The hadronic tensor for a spin- 2 target can be expressed by four dimensionless structure functions F1 , F2 , g1 and g2 . This allows to rewrite the three terms of the cross section by: 2 2 2 1 d3 = 4 2 y F1 + 2xy 1 ? y ? y 4 F2 ; dx dy d' Q 2 ) ( ! d3 k 4 2 1 ? y ? y 2 2 g ? y 2g ; 1 2 2 dx dy d' = Q2 2 4 8 s 9 3 ? = 2< 2 2 y d = 4 2 : 1?y? y g1 + g 2 …… 此处隐藏：28566字，全部文档内容请下载后查看。喜欢就下载吧 ……