The deuteron spin-dependent structure function g1d and its first moment

Abstract

We present a measurement of the deuteron spin-dependent structure function g d 1 based on the data collected by the COMPASS experiment at CERN during the years [2002][2003][2004]. The data provide an accurate evaluation for Γ d 1 , the first moment of g d 1 (x), and for the matrix element of the singlet axial current, a 0 . The results of QCD fits in the next to leading order (NLO) on all g 1 deep inelastic scattering data are also presented. They provide two solutions with the gluon spin distribution function ∆G positive or negative, which describe the data equally well. In both cases, at Q 2 = 3(GeV/c) 2 the first moment of ∆G(x) is found to be of the order of 0.2 -0.3 in absolute value.Keywords: Deep inelastic scattering; Spin; Structure function; QCD analysis; A1; g1 (To be Submitted to Physics Letters B)The COMPASS Collaboration [4]. Due to the relatively low incident energy, the DIS events collected in those experiments cover only a limited range of x for Q 2 > 1(GeV/c) 2 , x > 0.015 and x > 0.03, respectively. Further measurements covering the low x region were also performed at CERN (see [5] and references therein). Besides its general interest for the understanding of the spin structure of the nucleon, g d 1 is specially important because its first moment is directly related to the matrix element of the singlet axial vector current a 0 . A precise measurement of g d 1 can thus provide an evaluation of the fraction of nucleon spin carried by quarks, on the condition that the covered range extends far enough to low x to provide a reliable value of the first moment.Here we present new results from the COMPASS experiment at CERN on the deuteron spin asymmetry A As previous fits were found to be in disagreement with our data at low x, we have performed a new QCD fit at NLO. The resulting polarised parton distribution functions (PDF) are also presented in this paper and discussed in relation with the new data, however without a full investigation of the theoretical uncertainties due, for instance, to the values of the factorisation and renormalisation scales.The COMPASS data acquisition system is triggered by coincidence signals in hodoscopes, defining the direction of the scattered muon behind the spectrometer magnets, and by signals in the hadron calorimeters [7]. Triggers due to halo muons are eliminated by veto counters installed upstream of the target. Inclusive triggers, based on muon detection only, cover the full range of x and are dominant in the medium (x, Q 2 ) region. Semi-inclusive triggers, based on the muon energy loss and the presence of a hadron signal in the calorimeters, contribute mainly at low x and low Q 2 . Purely calorimetric triggers, based on the energy deposit in the hadron calorimeter without any condition on the scattered muon, account for most events at large Q 2 . The relative contributions of these three trigger types are shown in Fig. 1 as a function of x. The minimum hadron energy deposit required for the purely calorimetric trigger has been reduced to 10 GeV for the...

Spin-polarized high-energy scattering of charged leptons on nucleons

“…This leadingorder result is in fair agreement with the results of NLO fits of g 1 from [72] shown as curves in figure 11. The value of the unpolarized gluon distribution from the CTEQ6 set of PDFs [81] is G(x)| x=0.1 ≈ 10, so that for the gluon helicity distribution…”

Deep inelastic scattering with the SPS muon beam

“…34 Recent COMPASS results indicate a somewhat larger quark spin contribution of a 0 = 0.33 ± 0.03 ± 0.05 and a similar strange quark contribution ∆s + ∆s = −0.08 ± 0.01 ± 0.02. 43 Still the original EMC conclusion that the quark spins do not account for most of the proton spin holds.…”

Structure of the nucleon in the unquenched quark model

“…Recent experiments show that approximately one third of the proton spin is carried by the quarks and antiquarks [24,25], and that the gluon contribution is rather small (either positive or negative) and compatible with zero [26]. This rules out the possibility that most of the missing spin be carried by the gluon and indicates that the origen of the missing spin of the proton has to be attributed to other mechanisms.…”

“…An analysis of the experimental DIS data for the proton [24,25] in combination with Eq. (6) shows that the strange quarks (and antiquarks) carry about 65 % of the Λ spin, while the up and down quarks (and antiquarks) account for a negative polarization of -32 %.…”