Precision measurement of the neutron spin asymmetries and spin-dependent structure functions in the valence quark region

Abstract

We report on measurements of the neutron spin asymmetries A n 1,2 and polarized structure functions g n 1,2 at three kinematics in the deep inelastic region, with x = 0.33, 0.47 and 0.60 and Q 2 = 2.7, 3.5 and 4.8 (GeV/c) 2 , respectively. These measurements were performed using a 5.7 GeV longitudinally-polarized electron beam and a polarized 3 He target. The results for A n 1 and g n 1 at x = 0.33 are consistent with previous world data and, at the two higher x points, have improved the precision of the world data by about an order of magnitude. The new A n 1 data show a zero crossing around x = 0.47 and the value at x = 0.60 is significantly positive. These results agree with a next-to-leading order QCD analysis of previous world data. The trend of data at high x agrees with constituent quark model predictions but disagrees with that from leading-order perturbative QCD (pQCD) assuming hadron helicity conservation. Results for A n 2 and g n 2 have a precision comparable to the best world data in this kinematic region. Combined with previous world data, the moment d n 2 was evaluated and the new result has improved the precision of this quantity by about a factor of two. When combined with the world proton data, polarized quark distribution functions were extracted from the new g n 1 /F n 1 values based on the quark parton model. While results for ∆u/u agree well with predictions from various models, results for ∆d/d disagree with the leading-order pQCD prediction when hadron helicity conservation is imposed.

Measurements of the Double-Spin Asymmetry A₁ on Helium-3: Toward a Precise Measurement of the Neutron A₁

“…4. In Figures 7.5 and 7.6, we plot our A 1 and A 2 measurements along with existing 3 He data from the E142 experiment at SLAC [51] and from the E99-117 experiment in Hall A of Jefferson Lab [41]. We note that, in these kinematics, our measurements are in the deep-inelastic scattering region only up to about x ∼ 0.55; at higher x, we are in the resonance region.…”

“…Weigel and Gamberg Soffer and Bourelly g2_WW [118], the results of Jefferson Lab experiments E94-010 [119] and E99-117 [41], and the results of SLAC experiment E155X [120]. The quantityd n 2 is equal to d n 2 with the elastic contribution (x = 1) subtracted from the integral.…”

“…Circles represent data from HERMES [39]; triangles pointing up show SLAC data [40]; triangles pointing up correspond to data from Hall A of Jefferson Lab [41]; and squares show CLAS data [42].…”

“…The resulting contribution is negative and quite large at low x, but approaches 0 as x → 1, giving a possible explanation to the spin puzzle and suggesting that A p 1 is negative [41]. Unfortunately, it is not straightforward to apply this model to the neutron, as the theory is extremely sensitive to the precise mechanics of SU(6) symmetry breaking in the valencequark distribution functions.…”

“…Following the convention of previous experiments (e.g. E99-117 [41]), the numerator of the longitudinal asymmetry is the number of counts in the parallel spin configuration, subtracted from the number of counts in the antiparallel spin configuration, as in Equation 1. 34.…”