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

The Collins and Sivers asymmetries for charged hadrons produced in deeply inelastic scattering on transversely polarised protons have been extracted from the data collected in 2007 with the CERN SPS muon beam tuned at 160 GeV/c. At large values of the Bjorken x variable non-zero Collins asymmetries are observed both for positive and negative hadrons while the Sivers asymmetry for positive hadrons is slightly positive over almost all the measured x range. These results nicely support the present theoretical interpretation of these asymmetries, in terms of leading-twist quark distribution and fragmentation functions.

The COMPASS experiment at the CERN SPS has studied the diffractive dissociation of negative pions into the π- π- π+ final state using a 190  GeV/c pion beam hitting a lead target. A partial wave analysis has been performed on a sample of 420,000 events taken at values of the squared 4-momentum transfer t' between 0.1 and 1  GeV2/c2. The well-known resonances a1(1260), a2(1320), and π2(1670) are clearly observed. In addition, the data show a significant natural-parity exchange production of a resonance with spin-exotic quantum numbers J(PC)=1-+ at 1.66  GeV/c2 decaying to ρπ. The resonant nature of this wave is evident from the mass-dependent phase differences to the J(PC)=2-+ and 1++ waves. From a mass-dependent fit a resonance mass of (1660±10(-64)(+0))  MeV/c2 and a width of (269±21(-64)(+42))  MeV/c2 are deduced, with an intensity of (1.7±0.2)% of the total intensity.

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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...