531 research outputs found
Spin Observables in Transition-Distribution-Amplitude Studies
Exclusive hadronic reactions with a massive lepton pair (l^+l^-) in the final
state will be measured with PANDA at GSI-FAIR and with Compass at CERN, both in
p+p-bar -> l^+l^-+pi and pi+N -> N'+l^+l^-. Similarly, electroproduction of a
meson in the backward region will be studied at JLAB. We discuss here how the
spin structure of the amplitude for such processes will enable us to
disentangle various mechanisms. For instance, target-transverse-spin
asymmetries are specific of a partonic description, where the amplitude is
factorised in terms of baryon to meson or meson to baryon Transition
Distribution Amplitudes (TDAs) as opposed to what is expected from
baryon-exchange contributions.Comment: Contributed to the 19th International Spin Physics Symposium (SPIN
2010), September 27 - October 2, 2010, Juelich, Germany. 8 pages, 7 figures,
uses jpconf.cls, jpconf11.clo, iopams.sty (included
Pion Structure Function and Violation of the Momentum Sum Rule
We present a method to evaluate the pion structure functions from a box
diagram calculation. Pion and constituent quark fields are coupled through the
simplest pseudoscalar coupling. The gamma^* pi -> q \bar q cross-section is
evaluated and related to the structure functions. We then show that the
introduction of non-perturbative effects, related to the pion size and
preserving gauge invariance, provides us with a straighforward relation with
the quark distribution. It is predicted that higher-twist terms become
negligible for Q^2 larger than about 2 GeV^2 and that quarks in the pion have a
momentum fraction smaller than in the proton. We enlarge the discussion
concerning this violation of the momentum sum rule, emphasizing that the sum
rule is recovered in the chiral limit and also when the finite size condition
is not imposed.Comment: Presented by J.P. Lansberg at the II International Workshop on Hadron
Physics, 25-29 September, 2002, Coimbra, Portugal, 14 pages, 9 figures,
LaTeX, uses aip-6s.clo, aipproc.cls and aipxfm.sty (included
A toy model for generalised parton distributions
We give the results of a simple model for the diagonal and off-diagonal
valence quark distributions of a pion. We show that structure can be
implemented in a gauge-invariant manner. This explicit model questions the
validityof the momentum sum rule, and gives an explicit counter-example to the
Wandzura-Wilczek ansatz for twist-3 GPD's.Comment: 11 pages, presented at HADRONIC PHYSICS (HLPR 2004): Joint
Meeting-Heidelberg-Liege-Paris-Rostock, Spa, Belgium, 16-18 December 2004, 11
pages, 11 figures, LaTe
The Valence Quark Distribution of the Pion
The pion structure function is investigated in a simple model, where the pion
and its constituent quark fields are coupled through the simplest pseudoscalar
coupling. The imaginary part of the forward gamma* pi -> gamma* pi scattering
amplitude is evaluated and related to the structure functions. It is shown that
the introduction of non-perturbative effects, linked to the size of the pion,
allows a connection with the quark distribution. It is predicted that
higher-twist terms become negligible for Q^2 larger than ~2 GeV^2, that quarks
in the pion have a momentum fraction smaller than in the proton case, and that
the momentum sum rule is violated for the pion.Comment: Presented by J. P. Lansberg at the XXII Physics in Collision
Conference (PIC02), Stanford, California, June 20-22, 2002, 3 pages, 4
figures, LaTeX, uses pic02.sty (included
Confinement and cut-off: a model for the pion quark distribution function
The pion structure function is investigated in a simple pseudo-scalar
coupling model of pion and constituent quark fields. The imaginary part of the
forward Compton scattering amplitude is evaluated. We show that the
introduction of non-perturbative effects, linked through a cut-off to the size
of the pion, allows the reproduction of important features of the pion quark
distribution function.Comment: 3 pages, 1 figur
A look at the influence of the transverse momentum on shadowing
Stringent physical constraints relate the produced at a given
transverse momentum to the Bjorken- of the initial gluons. We present
a new approach which takes them into account in order to explicitely
investigate the -dependence of the shadowing effect on the
production. Using the rapidity and spectra extracted from
p + p data from PHENIX, we build a Glauber
Monte-Carlo code which includes shadowing in two alternative ways: multiple
scattering corrections or evolution of parton densities. We present our
results in d + Au collisions at the same energy, notably providing the first
prediction of the nuclear modification factor as a function of ,
and compare them to the available data by adding some nuclear absorption
effect.Comment: Proceedings prepared for the HLPW08 conference (Three days of strong
interactions and astrophysics, March 2008, Belgium) and will be published in
the "AIP conference proceedings series
The effect of changing the resultant linear acceleration relative to the subject on nystagmus generated by angular acceleration
Effect of changing resultant linear acceleration relative to human subject on nystagmus generated by angular acceleratio
On the mechanisms of heavy-quarkonium hadroproduction
We discuss the various mechanisms potentially at work in hadroproduction of
heavy quarkonia in the light of computations of higher-order QCD corrections
both in the Colour-Singlet (CS) and Colour-Octet (CO) channels and the
inclusion of the contribution arising from the s-channel cut in the CS channel.
We also discuss new observables meant to better discriminate between these
different mechanisms.Comment: Invited review talk at 3rd International Conference On Hard And
Electromagnetic Probes Of High-Energy Nuclear Collisions (HP2008), 8-14 June
2008, Illa da Toxa, Galicia, Spain. 11 pages, 21 figures, LaTeX, uses
svjour.cls and svepj.clo (included
Gravitational and higher-order form factors of the pion in chiral quark models
The gravitational form factor of the pion is evaluated in two chiral quark
models and confronted to the recent full-QCD lattice data. We find good
agreement for the case of the Spectral Quark Model, which builds in the
vector-meson dominance for the charge form factor. We derive a simple relation
between the gravitational and electromagnetic form factors, holding in the
considered quark models in the chiral limit. The relation implies that the
gravitational mean squared radius is half the electromagnetic one. We also
analyze higher-order quark generalized form factors of the pion, related to
higher moments in the symmetric Bjorken X-variable of the generalized parton
distribution functions, and discuss their perturbative QCD evolution, needed to
relate the quark-model predictions to the lattice data. The values of the
higher-order quark form factors at t=0, computed on the lattice, also agree
with our quark model results within the statistical and method uncertainties.Comment: 12 pages, 4 figures, discussion and references adde
- …