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We present a new measurement of the longitudinal spin asymmetry A1\ud and the spin-dependent structure function g1 of the deuteron in the range 1

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...

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