Comment on the recent COMPASS data on the spin structure function g 1

Abstract

We examine the recent COMPASS data on the spin structure function g1 singlet. We show that it is rather difficult to use the data in the present form in order to draw conclusions on the initial parton densities. However, our tentative estimate is that the data better agree with positive rather than negative initial gluon densities.

First results on A_{1}^{p} and g_{1}^{p} at low x and low Q^{2} from COMPASS

“…4, the results on g p 1 are presented. No significant dependence with the virtual photon energy is observed, in contrast with the prediction of a significant dependence of the singlet structure function g S 1 with ν predicted by the model of [7].…”

“…The same caveat applies to COMPASS, which can, nonetheless, reach even lower values of x, because of its particular trigger system [4] and of its much larger data set. On the other hand, theoretical models, still to be confronted with data, allow a smooth extrapolation to both the low Q 2 and the high Q 2 regions (using resummation, vector meson dominance) [5,6,7]. With the COMPASS data taken with a proton target, A d 1 and g p 1 can be measured at low x and low Q 2 with improved precision, complementing our measurement of A d 1 and g d 1 at low x and low Q 2 [8].…”

“…The non-singlet structure function g NS 1 = g p 1 − g n 1 , which decouples from gluons, will also be extracted at a later stage. The results are presented as functions of x and ν, as suggested by [7].…”

Overview of the spin structure function g_1 at arbitrary x and Q^2

“…[12] we had shown that the arguments of Ref. [26] in favor of using the parametrization (48) in DGLAP can be used at large x only. Later, in Ref.…”

The spin structure of the proton at low x and low Q2 in two-dimensional grids from COMPASS

“…However, in fixed target experiments, x and Q 2 are highly correlated, and probing the low x region implies also entering the non-perturbative region of Q 2 < 1 (GeV/c) 2 , which is poorly known experimentally and where perturbative QCD cannot be applied. Nevertheless, this region allows studies of the transition from the non-perturbative regime of photoproduction to the perturbative region of deep inelastic scattering where several predictions were formulated [1,2,3]. Previously, the Spin Muon Collaboration (SMC) has presented its results on A p 1 and g p 1 in this region [4].…”