Longitudinal double-spin asymmetry $A_1^{\rm p}$ and spin-dependent structure function $g_1^{\rm p}$ of the proton at small values of $x$ and $Q^2$

COMPASS collaboration (226 authors); Bordalo, P.; Franco, C.; Nunes, A.S.; Quaresma, M.; Quintans, C.; Ramos, S.; Silva, L.; Stolarski, M.

http://hdl.handle.net/10400.26/27466

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COMPASS collaboration (226 authors)

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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 .