120 research outputs found
Twist-2 Light-Cone Pion Wave Function
We present an analysis of the existing constraints for the twist-2 light-cone
pion wave function. We find that existing information on the pion wave function
does not exclude the possibility that the pion wave function attains its
asymptotic form. New bounds on the parameters of the pion wave function are
presented.Comment: 7 pages, LaTeX, 1 PS-figure, one reference added, minor changes in
the tex
Pion Interactions in Chiral Field Theories
We study in various chiral models the pion charge radius, form
factor ratio, amplitude, charge pion
polarizabilities, amplitude at low
energies and the s-wave I = 0 scattering length. We find that a
quark-level linear sigma-model approach (also being consistent with tree-level
vector meson dominance) is quite compatible with all of the above data.Comment: 12 pages, 9 eps figure
Trouble in Asymptopia---the Hulthen Model on the Light Front
We use light-front dynamics to calculate the electromagnetic form-factor for
the Hulthen model of the deuteron. For small momentum transfer Q^2 < 5 GeV^2
the relativistic effects are quite small. For Q^2 = 11 GeV^2 there is about a
13% discrepancy between the relativistic and non-relativistic approaches. For
asymptotically large momentum transfer, however, the light-front form factor,
log Q^2 /Q^4, markedly differs from the non-relativistic version, 1/Q^4. This
behavior is also present for any wave function, such as those obtained from
realistic potential models, which can be represented as a sum of Yukawa
functions. Furthermore, the asymptotic behavior is in disagreement with the
Drell-Yan-West relation. We investigate precisely how to determine the
asymptotic behavior and confront the problem underlying troublesome form
factors on the light front.Comment: 20 pages, 8 figures Accepted by Phys. Rev
Pion Generalized Dipole Polarizabilities by Virtual Compton Scattering
We present a calculation of the cross section and the event generator of the
reaction . This reaction is sensitive to the pion
generalized dipole polarizabilities, namely, the longitudinal electric
, the transverse electric , and the magnetic
which, in the real-photon limit, reduce to the ordinary electric
and magnetic polarizabilities and , respectively.
The calculation of the cross section is done in the framework of chiral
perturbation theory at . A pion VCS event generator has been
written which is ready for implementation in GEANT simulation codes or for
independent use.Comment: 33 pages, Revtex, 15 figure
Electroproduction of Charmonia off Nuclei
In a recent publication we have calculated elastic charmonium production in
ep collisions employing realistic charmonia wave functions and dipole cross
sections and have found good agreement with the data in a wide range of s and
Q^2. Using the ingredients from those calculations we calculate exclusive
electroproduction of charmonia off nuclei. Here new effects become important,
(i) color filtering of the c-cbar pair on its trajectory through nuclear
matter, (ii) dependence on the finite lifetime of the c-cbar fluctuation
(coherence length) and (iii) gluon shadowing in a nucleus compared to the one
in a nucleon. Total coherent and incoherent cross sections for C, Cu and Pb as
functions of s and Q^2 are presented together with some differential cross
sections. The results can be tested with future electron-nucleus colliders or
in peripheral collisions of ultrarelativistic heavy ions.Comment: 21 pages of Latex including 14 figures; few misprints are fixe
Charm and Bottom Semileptonic Decays
We review the present status of theoretical attempts to calculate the
semileptonic charm and bottom decays and then present a calculation of these
decays in the light--front frame at the kinematic point . This allows us
to evaluate the form factors at the same value of , even though the
allowed kinematic ranges for charm and bottom decays are very different. Also,
at this kinematic point the decay is given in terms of only one form factor
. For the ratio of the decay rates given by the E653 collaboration we
show that the determination of the ratio of the Cabibbo--Kobayashi--Maskawa
(CKM) matrix elements is consistent with that obtained from the unitarity
constraint. At present, though, the unitarity method still has greater
accuracy. Since comparisons of the semileptonic decays into and either
electrons or muons will be available soon from the E791 Fermilab experiment, we
also look at the massive muon case. We show that for a range of the
symmetry breaking is small even though the contributions of the
various helicity amplitudes becomes more complicated. For decays, the decay
at involves an extra form factor
coming from the photon contribution and so is not amenable to the same kind of
analysis, leaving only the decay as a
possibility. As the mass of the decaying particle increases we note that the
symmetry becomes badly broken at .Comment: Latex, 19 pages, two figures are attached, a minor change in the
manuscript related to thi
Relativistic Correction to the First Moment of the Spin-Dependent Structure Function of the Deuteron in the Light-Cone Formalism
The deuteron is considered as a superposition of two-nucleon Fock states with
the invariant mass depending on the relative momentum in a proton-neutron pair.
The condition of the transversality of the polarization vectors should be
imposed at the Fock component level and these vectors depend on the invariant
mass of the Fock component. Such "running" longitudinal polarization vector was
not used in early estimates of relativistic effects. The technique for the
calculation of the average helicity of the proton in the deuteron has been
considered in the light-cone variables. A receipt has been proposed for the
consistent calculation of relativistic nuclear corrections to the average
helicity of the proton in the deuteron and to the first moment of the
spin-dependent structure function of the deuteron.Comment: 34 pages, 6 figures, an extended version of the paper published in
Journal of Experimental and Theoretical Physics, 201
Generalized dipole polarizabilities and the spatial structure of hadrons
We present a phenomenological discussion of spin-independent, generalized
dipole polarizabilities of hadrons entering the virtual Compton scattering
process gamma* h -> gamma h. We introduce a new method of obtaining a tensor
basis with appropriate Lorentz-invariant amplitudes which are free from
kinematical singularities and constraints. We then motivate a gauge-invariant
separation into a generalized Born term containing ground-state properties
only, and a residual contribution describing the model-dependent internal
structure. The generalized dipole polarizabilities are defined in terms of
Lorentz-invariant residual amplitudes. Particular emphasis is laid on a
physical interpretation of these quantities as characterizing the spatial
distributions of the induced electric polarization and magnetization of
hadrons. It is argued that three dipole polarizabilities, namely the
longitudinal electric alpha_L(q^2), the transverse electric alpha_T(q^2), and
the magnetic beta(q^2) ones are required in order to fully reconstruct local
polarizations induced by soft external fields in a hadron. One of these
polarizabilities, alpha_T(q^2), describes an effect of higher order in the soft
final-photon momentum q'. We argue that the associated spatial distributions
obtained via the Fourier transforms in the Breit frame are meaningful even for
such a light particle as the pion. The spatial distributions are determined at
large distances r ~ 1/m_pi for pions, kaons, and octet baryons by use of ChPT.Comment: 41 pages, 5 figures, RevTex fil
Chiral Anomaly and Eta-Eta' Mixing
We determine the mixing angle via a procedure relatively
independent of theoretical assumptions by simultaneously fitting
reactions involving the anomaly--.
We extract reasonably precise renormalized values of the octet and singlet
pseudoscalar decay constants as well as the mixing angle .Comment: 12 page standard Latex file, three figures, added comment
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