On the new COMPASS measurement of the deuteron spin - dependent structure function

Very recently, a new measurement of the deuteron spin-dependent structure function g d 1 (x) was reported by the COMPASS group. A main change from the old SMC measurement is a considerable improvement of the statistical accuracy in the low x region 0.004 < x < 0.03. We point out that the new COMPASS data for g d 1 (x) as well as their QCD fits for ∆Σ and ∆s + ∆¯ s are all remarkably close to our theoretical predictions given several years ago based on the chiral quark soliton model. If the intrinsic quark spin carries little of the total nucleon spin, what carries the rest of the nucleon spin? Quark orbital angular momentum (OAM) L Q ? Gluon OAM L g ? Or gluon polarization ∆g? That is a still unsolved fundamental puzzle of QCD [1]. Toward the solution of the problem, remarkable progress has been made for the past few years. First, the new COMPASS measurement of the quasi-real photoproduction of high-p T hadron pairs indicates that ∆g cannot be very large at least below Q 2 ≤ 3 GeV 2 [2]. (The small gluon polarization is also indicated by PHENIX measurement of neutral pion double longitudinal spin asymmetry in the proton-proton collisions [3] and also by the STAR measurement of the double longitudinal spin asymmetry in inclusive jet production in polarized proton-proton collisions [4],[5].) There also appeared an interesting paper by Brodsky and Gardner [6], in which, based on the conjecture on the relation between the Sivers mechanism [7] and the quark and gluon OAM [8], it was argued that small single-spin asymmetry observed by the COMPASS collaboration on the deuteron target is an indication of small gluon OAM. These observations together with the progress of the physics of generalized parton distribution functions [9] arose a growing interest on the role of quark OAM in the nucleon.