Results are reported from the HERMES experiment at HERA on a measurement of the neutron spin structure function ~(x, Q2) in deep inelastic scattering using 27.5 GeV longitudinally polarized positrons incident on a polarized 3He internal gas target. The data cover the kinematic range 0.023 < x < 0.6 and 1 (GeV/c) 2 < Q2 < 15 (GeV/c) 2. The integral fo~i0623 ~(x) dx evaluated at a fixed Qz of 2.5 (GeV/c) 2 is-0.0344-0.013(stat.)+0.005(syst.). Assuming Regge behavior at low x, the first moment F'~ = fl ~(x)dx is-0.037 ± 0.013(stat.)±0.005(syst.)±0.006(extrapol.

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Spin asymmetries of semi-inclusive cross sections for the production of positively and negatively charged hadrons have been measured in deep-inelastic scattering of polarized positrons on polarized hydrogen and He-3 targets, in the kinematic range 0.023 < x < 0.6 and 1 GeV2 < Q(2) < 10 GeV2. Polarized quark distributions are extracted as a function of x for up (u + (u) over bar) and down (d + (d) over bar) flavors. The up quark polarization is positive and the down quark polarization is negative in the measured range. The polarization of the sea is compatible with zero. The first moments of the polarized quark distributions are presented. The isospin non-singlet combination dq, is consistent with the prediction based on the Bjorken sum rule. The moments of the polarized quark distributions are compared to predictions based on SU(3)(f) flavor symmetry and to a prediction from lattice QCD. (C) 1999 Published by Elsevier Science B.V. All rights reserved

The s e l e c t i v i t y and t h e s u r p r i s i n g s i m p l i c i t y of response function f o r spin-isospin t r a n s i t i o n s with t h e (p,n) r e a c t i o n a t intermediate energies has been AL=O t r a n s f e r s f o r 5 4 9 5 6~e t a r g e t s . I n t h e case of used t o study the spin-isospin c o r r e l a t i o n s i n n u c l e i t h e N i i s o t o p e s , a comparison may be made with t h e M1 (see Ref. 1 and o t h e r references within). The zero s p e c t r a obtained from e l e c t r o -e x c i t a t i o n . 3 9 degree s p e c t r a have been used t o o b t a i n t h e AL = 0 The experimental r e s u l t s and a n a l y s i s f o r the response function of n u c l e i while t h e measured2 energy 5 8~i ( p , n ) 5 8~u r e a c t i o n have been published3; t h e dependence of the e f f e c t i v e n u c l e o n n u c l e u s i n t e r a c t i o n a n a l y s i s f o r t h e o t h e r i s o t o p e s is i n progress.r e s u l t i n g from the spin-isospin terms i n the The double d i f f e r e n t i a l c r o s s s e c t i o n of L = 0 nucleonnucleon f o r c e has been employed t o i d e n t i f y t h e s t r e n g t h observed a t ( 3~ = 0' f o r t h e 5 8~i ( p , n ) 5 8~u spin-isospin o r Gamow-Teller strength. Other r e a c t i o n is presented i n Fig. l b and i s compared i n multipoles observed a t higher e x c i t a t i o n energies Fig. l c with the B(M1)C s t r e n g t h reported4 f o r t h e i n d i c a t e a maximum d i f f e r e n t i a l c r o s s s e c t i o n a t 5 8~i ( e , e ' ) reaction. The l o c a t i o n s of known 1+ e x c i t e ds l i g h t l y higher angles and have been i n t e r p r e t e d a s s t a t e s i n A = 58 n u c l e i a r e shown i n Fig. la. (AL = 1, AS = 1) and (AL = 2, AS = 1) e x c i t a t i 0 n s . l *permanent address: C e n t r a l Research I n s t i t u t e f o r W e have obtained 160 MeV (p,n) d a t a on 5 4 * 5 6~e and Physics, Budapest, Hungary.5 8 * 6 0~i and 120 MeV d a t a f o r t h e 5 8~i ( p , n ) 5 8~u r e a c t i o n 1) C. Gaarde e t a l . , Nucl. Phys. A369, 258 (1981).i n order t o study t h e response function of 5 8 * 6 0~i 2) T.N. Taddeucci e t a l . , Phys. Rev. C 25, 1094 (1981).t a r g e t s t o spin-isospin t r a n s i t i o n s characterized with 3) J. Rapaport e t al., Phys. Lett. 119B, 61 (1982). AL = 0, AL = 1 and AL = 2 t r a n s f e r s and t o study t h e 4) R.A. Lindgren e t a l . , Phys. Rev. C 3, 1789 (1976).

We report a measurement of the asymmetry in spin-dependent quasielastic scattering of longitudinally polarized electrons from a polarized 3~e target. The neutron magnetic form factor G' ;, has been extracted from the measured asymmetry based on recent PWIA calculations using spin-dependent spectral functions. Our determination of G& at ~~= 0 . 1 9 (G~VIC)' agrees with the dipole parametrization. This experiment represents the first measurement of the neutron magnetic form factor using spin-dependent electron scattering. PACS nuniber(s): 25.30. Fj, 13.40.Gp, 14.20.Dh, 24.70.fs Electromagnetic form factors are of fundamental importance for an understanding of the underlying structure of nucleons. Knowledge of the distribution of charge and magnetization within the nucleons provides a sensitive test of models based on QCD, as well as a basis for calculations of processes involving the electromagnetic interaction with complex nuclei. Due to the lack of a free neutron target, the neutron electromagnetic form factors are known with less precision than the proton electric and magnetic form factors. They have been deduced in the past from elastic or quasielastic electron-deuteron scattering. This procedure involves considerable model dependence. The development of polarized targets and beams has allowed more complete studies of electromagnetic structure than has been possible with unpolarized reactions. In quasielastic scattering, the spin degrees of freedom introduce new response functions into the inclusive cross section, thus providing additional information on nuclear structure [I].3~e is an interesting nucleus for polarization studies because its ground state wave function is predominantly a spa- tially symmetric S state in which the spin of the nucleus is carried mainly by the unpaired neutron. Therefore, inelastic scattering of polarized electrons from polarized '~e in the vicinity of the quasielastic peak should be useful for studying the neutron electromagnetic form factors. This idea was first investigated by Blankleider and Woloshyn in closure approximation [2]. Friar et al. [3] have studied the model dependence in the spin structure of the 3~e wave function and its effect on the quasielastic asymmetry. Recently the plane wave impulse approximation (PWIA) calculations performed independently by two groups [4,5] using a spin-dependent spectral function show that the spin-dependent asymmetry is very sensitive to the neutron electric or magnetic form factors at certain kinematics near the top of the quasielastic peak. Two previous experiments [6,7] measured the spindependent asymmetry in quasielastic scattering of polarized electrons from oolarized h e . and demonstrated that this new experimental technique is feasible for studying the neutron electromagnetic structure. As a result, new experimental programs utilizing polarized electrons and polarized %e targets to study the neutron electromagnetic structure and the nucleon spin structure are under way at several electron accelerator laboratories (SLAC, MIT-B...