15 research outputs found
Vector Meson Production in Ultraperipheral Heavy Ion Collisions
The ultraperipheral heavy ion collisions (UPC's) are an important alternative
to study the QCD dynamics until the next generation of
colliders become reality. Due to the coherent action of all the protons in the
nucleus, the electromagnetic field is very strong and the resulting flux of
equivalent photons is large, which allows to study two-photon as well as
photonuclear interactions at high energies. In this paper we present a brief
review of the vector meson production in UPC's at high energies using the QCD
color dipole approach to describe their photonuclear production and the
perturbative QCD Pomeron (BFKL dynamics) to describe the double meson
production in photon-photon process. Predictions for rates and integrated cross
sections are presented for energies of RHIC and LHC.Comment: 16 pages, 1 figure. Version to be published in Journal of Physics G:
Nuclear and Particle Physic
The Polarised Valence Quark Distribution from semi-inclusive DIS
The semi-inclusive difference asymmetry A^{h^{+}-h^{-}} for hadrons of
opposite charge has been measured by the COMPASS experiment at CERN. The data
were collected in the years 2002-2004 using a 160 GeV polarised muon beam
scattered off a large polarised ^6LiD target and cover the range 0.006 < x <
0.7 and 1 < Q^2 < 100 (GeV/c)^2. In leading order QCD (LO) the asymmetry
A_d^{h^{+}-h^{-}} measures the valence quark polarisation and provides an
evaluation of the first moment of Delta u_v + Delta d_v which is found to be
equal to 0.40 +- 0.07 (stat.) +- 0.05 (syst.) over the measured range of x at
Q^2 = 10 (GeV/c)^2. When combined with the first moment of g_1^d previously
measured on the same data, this result favours a non-symmetric polarisation of
light quarks Delta u-bar = - Delta d-bar at a confidence level of two standard
deviations, in contrast to the often assumed symmetric scenario Delta u-bar =
Delta d-bar = Delta s-bar = Delta s.Comment: 7 pages, 3 figures, COMPASS, revised: details added, author list
update
The Deuteron Spin-dependent Structure Function g1d and its First Moment
We present a measurement of the deuteron spin-dependent structure function
g1d based on the data collected by the COMPASS experiment at CERN during the
years 2002-2004. The data provide an accurate evaluation for Gamma_1^d, the
first moment of g1d(x), and for the matrix element of the singlet axial
current, a0. The results of QCD fits in the next to leading order (NLO) on all
g1 deep inelastic scattering data are also presented. They provide two
solutions with the gluon spin distribution function Delta G positive or
negative, which describe the data equally well. In both cases, at Q^2 = 3
(GeV/c)^2 the first moment of Delta G is found to be of the order of 0.2 - 0.3
in absolute value.Comment: fits redone using MRST2004 instead of MRSV1998 for G(x), correlation
matrix adde
Double spin asymmetry in exclusive rho^0 muoproduction at COMPASS
The longitudinal double spin asymmetry A_1^rho for exclusive leptoproduction
of rho^0 mesons, mu + N -> mu + N + rho, is studied using the COMPASS 2002 and
2003 data. The measured reaction is incoherent exclusive rho^0 production on
polarised deuterons. The Q^2 and x dependence of A_1^rho is presented in a wide
kinematical range: 3x10^-3 < Q^2 < 7 (GeV/c)^2 and 5x10^-5 < x < 0.05. The
presented results are the first measurements of A_1^rho at small Q2 (Q2 < 0.1
(GeV/c)^2) and small x (x < 3x10^-3). The asymmetry is in general compatible
with zero in the whole kinematical range.Comment: 6 Figures, 15 pages, version 2 with updated author list, technical
latex problem fixe
Collins and Sivers asymmetries for pions and kaons in muon-deuteron DIS
The measurements of the Collins and Sivers asymmetries of identified hadrons
produced in deep-inelastic scattering of 160 GeV/c muons on a transversely
polarised 6LiD target at COMPASS are presented. The results for charged pions
and charged and neutral kaons correspond to all data available, which were
collected from 2002 to 2004. For all final state particles both the Collins and
Sivers asymmetries turn out to be small, compatible with zero within the
statistical errors, in line with the previously published results for not
identified charged hadrons, and with the expected cancellation between the u-
and d-quark contributions.Comment: 16 pages, 9 figures, added author Efremov, calculated pure kaon
asymmetries instead of those for experimental kaon/pion mixture (mainly error
affected
Diffractive Vector Meson Production at HERA: from soft to hard QCD
Experimental results from HERA on diffractive vector meson production and
their theoretical interpretation within microscopic QCD are reviewed with an
emphasis on the BFKL color dipole and kt-factorization approaches.Comment: 178 pages including 78 figure
Double spin asymmetry in exclusive rho(0) muoproduction at COMPASS
Alekseev M, Alexakhin VY, Alexandrov Y, et al. Double spin asymmetry in exclusive rho(0) muoproduction at COMPASS. Eur.Phys.J. C. 2007;52(2):255-265.The longitudinal double spin asymmetry A(1)(rho) for exclusive leptoproduction of rho(0) mesons, mu+N ->mu+N+rho, is studied using the COMPASS 2002 and 2003 data. The measured reaction is incoherent exclusive rho(0) production on polarised deuterons. The Q(2) and x dependence of A(1)(rho) is presented in a wide kinematical range, 3x10(-3) rho at small Q(2) (Q(2) < 0.1 (GeV/c)(2) ) and small x (x < 3x10(-3) ). The asymmetry is in general compatible with zero in the whole kinematical range
Measurement of photoproduction at large momentum transfer at HERA
The proton-dissociative diffractive photoproduction of J/psi mesons has been studied in ep collisions with the ZEUS detector at HERA using an integrated luminosity of 112 pb(-1). The cross section is presented as a function of the photon-proton centre-of-mass energy and of the squared four-momentum transfer at the proton vertex. The results are compared to perturbative QCD calculations