Abstract
We present a precise measurement of the proton longitudinal double-spin asymmetry A1p and the proton spin-dependent structure function g1p at photon virtualities 0.006(GeV/c)2<Q2<1(GeV/c)2 in the Bjorken x range of 4×10−5<x<4×10−2. The results are based on data collected by the COMPASS Collaboration at CERN using muon beam energies of 160GeV and 200GeV. The statistical precision is more than tenfold better than that of the previous measurement in this region. In the whole range of x, the measured values of A1p and g1p are found to be positive. It is for the first time that spin effects are found at such low values of x.
Highlights
The spin-dependent structure function of the proton, g1p, has been extensively studied in the last few decades
In order to provide a suitable description of the non-perturbative region and of the transition region between ‘soft’ and ‘hard’ physics, it is tried in phenomenological calculations to extrapolate ideas based on the parton model to the low- Q 2 region and add mechanisms like vector meson dominance, (G)VMD, supplemented by the Regge model
In this Letter, we present new results obtained on the longitudinal double-spin asymmetry Ap1 and the spin-dependent structure function g1p for the proton, in the kinematic region 0.0062 (GeV/c)2 < Q 2 < 1 (GeV/c)2 and 4 × 10−5 < x < 4 × 10−2
Summary
The spin-dependent structure function of the proton, g1p, has been extensively studied in the last few decades. Measurements at low x and low Q 2 are scarce as they put very high demands on event triggering and reconstruction In spindependent leptoproduction they were performed only by the Spin Muon Collaboration (SMC) using proton and deuteron targets [7] and by the COMPASS Collaboration using a deuteron target [8]. The latter, very precise results do not reveal any spin effects in g1d over the whole measured interval of x. In this Letter, we present new results obtained on the longitudinal double-spin asymmetry Ap1 and the spin-dependent structure function g1p for the proton, in the kinematic region 0.0062 (GeV/c)2 < Q 2 < 1 (GeV/c) and 4 × 10−5 < x < 4 × 10−2.
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