4,600 research outputs found
Intrinsic quark transverse momentum in the nucleon from lattice QCD
A better understanding of transverse momentum (k_T-) dependent quark
distributions in a hadron is needed to interpret several experimentally
observed large angular asymmetries and to clarify the fundamental role of gauge
links in non-abelian gauge theories. Based on manifestly non-local gauge
invariant quark operators we introduce process-independent k_T-distributions
and study their properties in lattice QCD. We find that the longitudinal and
transverse momentum dependence approximately factorizes, in contrast to the
behavior of generalized parton distributions. The resulting quark
k_T-probability densities for the nucleon show characteristic dipole
deformations due to correlations between intrinsic k_T and the quark or nucleon
spin. Our lattice calculations are based on N_f=2+1 mixed action propagators of
the LHP collaboration.Comment: 4 pages, 3 figure
Spin densities in the transverse plane and generalized transversity distributions
We show how generalized quark distributions in the nucleon describe the
density of polarized quarks in the impact parameter plane, both for
longitudinal and transverse polarization of the quark and the nucleon. This
density representation entails positivity bounds including chiral-odd
distributions, which tighten the known bounds in the chiral-even sector. Using
the quark equations of motion, we derive relations between the moments of
chiral-odd generalized parton distributions of twist two and twist three. We
exhibit the analogy between polarized quark distributions in impact parameter
space and transverse momentum dependent distribution functions.Comment: 23 pages, 6 eps-figure
Extraction of the x-dependence of the non-perturbative QCD b-quark fragmentation distribution component
Using recent measurements of the b-quark fragmentation distribution obtained
in events registered at the Z pole, the non-perturbative
QCD component of the distribution has been extracted independently of any
hadronic physics modelling. This distribution depends only on the way the
perturbative QCD component has been defined. When the perturbative QCD
component is taken from a parton shower Monte-Carlo, the non-perturbative QCD
component is rather similar with those obtained from the Lund or Bowler models.
When the perturbative QCD component is the result of an analytic NLL
computation, the non-perturbative QCD component has to be extended in a
non-physical region and thus cannot be described by any hadronic modelling. In
the two examples used to characterize these two situations, which are studied
at present, it happens that the extracted non-perturbative QCD distribution has
the same shape, being simply translated to higher-x values in the second
approach, illustrating the ability of the analytic perturbative QCD approach to
account for softer gluon radiation than with a parton shower generator.Comment: 13 page
The XMM-Newton Project
The abundance of high-redshift galaxy clusters depends sensitively on the
matter density \OmM and, to a lesser extent, on the cosmological constant
. Measurements of this abundance therefore constrain these fundamental
cosmological parameters, and in a manner independent and complementary to other
methods, such as observations of the cosmic microwave background and distance
measurements. Cluster abundance is best measured by the X-ray temperature
function, as opposed to luminosity, because temperature and mass are tightly
correlated, as demonstrated by numerical simulations. Taking advantage of the
sensitivity of XMM-Newton, our Guaranteed Time program aims at measuring the
temperature of the highest redshift (z>0.4) SHARC clusters, with the ultimate
goal of constraining both \OmM and .Comment: To appear in the Proceedings of the XXI Moriond Conference: Galaxy
Clusters and the High Redshift Universe Observed in X-rays, edited by D.
Neumann, F. Durret, & J. Tran Thanh Va
Heavy quark production as sensitive test for an improved description of high energy hadron collisions
QCD dynamics at small quark and gluon momentum fractions or large total
energy, which plays a major role for HERA, the Tevatron, RHIC and LHC physics,
is still poorly understood. For one of the simplest processes, namely
bottom-antibottom production, next-to-leading-order perturbation theory fails.
We show that the combination of two recently developed theoretical concepts,
the k_perp-factorization and the next-to-leading-logarithmic-approximation BFKL
vertex, gives perfect agreement with data. One can therefore hope that these
concepts provide a valuable foundation for the description of other high energy
processes.Comment: RevTeX, 4 pages, 7 figures titel and abstract changed, several
formulations modified in the text, 1 figure droppe
Transport in Nanotubes: Effect of Remote Impurity Scattering
Theory of the remote Coulomb impurity scattering in single--wall carbon
nanotubes is developed within one--electron approximation. Boltzmann equation
is solved within drift--diffusion model to obtain the tube conductivity. The
conductivity depends on the type of the nanotube bandstructure (metal or
semiconductor) and on the electron Fermi level. We found exponential dependence
of the conductivity on the Fermi energy due to the Coulomb scattering rate has
a strong dependence on the momentum transfer. We calculate intra-- and
inter--subband scattering rates and present general expressions for the
conductivity. Numerical results, as well as obtained analytical expressions,
show that the degenerately doped semiconductor tubes may have very high
mobility unless the doping level becomes too high and the inter--subband
transitions impede the electron transport.Comment: 13 pages, 4 figure
Towards a solution of the charmonium production controversy: k_t-factorization versus color octet mechanism
The cross section of \chi_{cJ} hadroproduction is calculated in the
k_t-factorization approach. We find a significant contribution of the \chi_{c1}
state due to non-applicability of the Landau-Yang theorem because of off-shell
gluons. The results are in agreement with data and, in contrast to the
collinear factorization, show a dominance of the color singlet part and a
strong suppression of the color octet contribution. Our results could therefore
lead to a solution of the longstanding controversy between the color singlet
model and the color octet mechanism.Comment: 4 pages, 5 figures, final PRL versio
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