15 research outputs found
The Beam Conditions Monitor of the LHCb Experiment
The LHCb experiment at the European Organization for Nuclear Research (CERN)
is dedicated to precision measurements of CP violation and rare decays of B
hadrons. Its most sensitive components are protected by means of a Beam
Conditions Monitor (BCM), based on polycrystalline CVD diamond sensors. Its
configuration, operation and decision logics to issue or remove the beam permit
signal for the Large Hadron Collider (LHC) are described in this paper.Comment: Index Terms: Accelerator measurement systems, CVD, Diamond, Radiation
detector
Measurement of the Collins and Sivers asymmetries on transversely polarised protons
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.Comment: 9 Pages, 5 figure
The Spin-dependent Structure Function of the Proton g_1^p and a Test of the Bjorken Sum Rule
The inclusive double-spin asymmetry, A_1^p, has been measured at COMPASS in
deepinelastic polarised muon scattering off a large polarised NH3 target. The
data, collected in the year 2007, cover the range Q2 > 1 (GeV/c)^2, 0.004 < x <
0.7 and improve the statistical precision of g_1^p(x) by a factor of two in the
region x < 0.02. The new proton asymmetries are combined with those previously
published for the deuteron to extract the non-singlet spin-dependent structure
function g_1^NS(x,Q2). The isovector quark density, Delta_q_3(x,Q2), is
evaluated from a NLO QCD fit of g_1^NS. The first moment of Delta_q3 is in good
agreement with the value predicted by the Bjorken sum rule and corresponds to a
ratio of the axial and vector coupling constants g_A/g_V =
1.28+-0.07(stat)+-0.10(syst).Comment: 12 pages, 5 figure
Quark helicity distributions from longitudinal spin asymmetries in muon-proton and muon-deuteron scattering
Double-spin asymmetries for production of charged pions and kaons in
semi-inclusive deep-inelastic muon scattering have been measured by the COMPASS
experiment at CERN. The data, obtained by scattering a 160 GeV muon beam off a
longitudinally polarised NH_3 target, cover a range of the Bjorken variable x
between 0.004 and 0.7. A leading order evaluation of the helicity distributions
for the three lightest quarks and antiquark flavours derived from these
asymmetries and from our previous deuteron data is presented. The resulting
values of the sea quark distributions are small and do not show any sizable
dependence on x in the range of the measurements. No significant difference is
observed between the strange and antistrange helicity distributions, both
compatible with zero. The integrated value of the flavour asymmetry of the
helicity distribution of the light-quark sea, \Delta u-bar - \Delta d-bar, is
found to be slightly positive, about 1.5 standard deviations away from zero.Comment: 13 pages, 5 figure
Measurement of the Longitudinal Spin Transfer to Lambda and Anti-Lambda Hyperons in Polarised Muon DIS
The longitudinal polarisation transfer from muons to lambda and anti-lambda
hyperons, D_LL, has been studied in deep inelastic scattering off an
unpolarised isoscalar target at the COMPASS experiment at CERN. The spin
transfers to lambda and anti-lambda produced in the current fragmentation
region exhibit different behaviours as a function of x and xF . The measured x
and xF dependences of D^lambda_LL are compatible with zero, while
D^anti-lambda_LL tends to increase with xF, reaching values of 0.4 - 0.5. The
resulting average values are D^lambda_LL = -0.012 +- 0.047 +- 0.024 and
D^anti-lambda_LL = 0.249 +- 0.056 +- 0.049. These results are discussed in the
frame of recent model calculations.Comment: 13 pages, 7 figure
Rosseland and Planck Mean Opacities for Protoplanetary Discs
In this paper, we present mean gas and dust opacities relevant to the
physical conditions typical of protoplanetary discs. As the principal absorber
for temperatures below ~1,500 K, we consider spherical and aggregate dust
particles of various sizes, chemical structure, and porosity, consisting of
ice, organics, troilite, silicates, and iron. For higher temperatures, ions,
atoms, molecules, and electrons are included as the main opacity sources.
Rosseland and Planck mean opacities are calculated for temperatures between 5 K
and 10,000 K and gas densities ranging from 10^{-18} g/ccm to 10^{-7} g/ccm.
The dependence on the adopted model of dust grains is investigated. We compare
our results with recent opacity tables and show how different opacity models
affect the calculated hydrodynamical structure of accretion discs.Comment: 12 pages, 4 figures, to be published in A&A, 200
Azimuthal asymmetries of charged hadrons produced by high-energy muons scattered off longitudinally polarised deuterons
Azimuthal asymmetries in semi-inclusive production of positive (h^+) and
negative hadrons (h^-) have been measured by scattering 160 GeV muons off
longitudinally polarised deuterons at CERN. The asymmetries were decomposed in
several terms according to their expected modulation in the azimuthal angle phi
of the outgoing hadron. Each term receives contributions from one or several
spin and transverse-momentum-dependent parton distribution and fragmentation
functions. The amplitudes of all phi-modulation terms of the hadron asymmetries
integrated over the kinematic variables are found to be consistent with zero
within statistical errors, while the constant terms are nonzero and equal for
h^+ and h^- within the statistical errors. The dependencies of the
phi-modulated terms versus the Bjorken momentum fraction x, the hadron
fractional momentum z, and the hadron transverse momentum p_h^T were studied.
The x dependence of the constant terms for both positive and negative hadrons
is in agreement with the longitudinal double-spin hadron asymmetries, measured
in semi-inclusive deep-inelastic scattering. The x dependence of the sin
phi-modulation term is less pronounced than that in the corresponding HERMES
data. All other dependencies of the phi-modulation amplitudes are consistent
with zero within the statistical errors.Comment: 12 pages, 11 Figures; revision 1 signs in Eq 5 corrected, polishe