23 research outputs found
Model for Polarized and Unpolarized Parton Density Functions in the Nucleon
We present a physical model for polarized and unpolarized structure functions
and parton density functions (PDFs) of the proton and the neutron. It
reproduces the data on F_2^p(x,Q^2) for 0.00001<x<1 and 2.5<Q^2<5000 GeV^2,
F_2^p(x)-F_2^n(x), F_2^n(x)/F_2^p(x), xg(x), dbar(x)-ubar(x), d(x)/u(x), the
Gottfried sum, the fractional momentum of charged partons and the polarized
structure functions g_1^{p,n}(x), at various Q^2. We present for the first
time, proton and neutron PDFs which do not assume charge symmetry. Contrary to
the common practice, we explain polarized and unpolarized data with a single
model.Comment: version to appear in Phys. Lett. B; a note added at the end of the
paper; no other change; latex, 10 pages, 4 ps figure
Eta meson rescattering effects in the p + 6Li --> eta + 7Be reaction near threshold
The p + 6Li --> eta + 7Be reaction has been investigated with an emphasis on
the eta meson and 7Be interaction in the final state. Considering the 6Li and
7Be nuclei to be alpha-d and alpha-3He clusters respectively, the reaction is
modelled to proceed via the p + d [alpha] --> 3He [\alpha] + eta reaction with
the alpha remaining a spectator. The eta meson interacts with 7Be via multiple
scatterings on the 3He and alpha clusters inside 7Be. The individual eta-3He
and eta-alpha scatterings are evaluated using few body equations for the eta-3N
and eta-4N systems with a coupled channel eta-N interaction as an input.
Calculations including four low-lying states of 7Be lead to a double hump
structure in the total cross section corresponding to the and angular momentum states. The humps
arise due to the off-shell rescattering of the eta meson on the 7Be nucleus in
the final state.Comment: New results and references adde
Determining the size of the proton
A measurement of the Lamb shift of 49,881.88(76) GHz in muonic hydrogen in
conjunction with theoretical estimates of the proton structure effects was
recently used to deduce an accurate but rather small radius of the proton. Such
an important shift in the understanding of fundamental values needs
reconfirmation. Using a different approach with electromagnetic form factors of
the proton, we obtain a new expression for the transition energy, , in muonic hydrogen and deduce
a proton radius, fm.Comment: 20 pages LaTe
Statistical model for pionic partons
We present a model for the structure of the pion. Based on ideas of a
recently developed statistical model of the nucleon, we assume the pion to be a
gas of partons. The finite-size corrections (FSC) are incorporated through two
parameters. Using the same two FSC parameters for the proton and pion we
reproduce quantitatively the data on Drell-Yan
production and valence quark distribution of the pion.Comment: revised manuscript, 11 pages, LaTex, including 4 figures, to appear
in Phys. Lett.
Breit type equation for mesonic atoms
The finite size effects and relativistic corrections in pionic and kaonic
hydrogen are evaluated by generalizing the Breit equation for a spin-0 -
spin-1/2 amplitude with the inclusion of the hadron electromagnetic form
factors. The agreement of the relativistic corrections to the energies of the
mesonic atoms with other methods used to evaluate them is not exact, but
reasonably good. The precision values of the energy shifts due to the strong
interaction, extracted from data, are however subject to the hadronic form
factor uncertainties.Comment: 11 pages Late
Collision times in pi-pi and pi-K scattering and spectroscopy of meson resonances
Using the concept of collision time (time delay) introduced by Eisenbud and
Wigner and its connection to on-shell intermediate unstable states, we study
mesonic resonances in pi-pi and pi-K scattering. The time-delay method proves
its usefulness by revealing the spectrum of the well-known rho- and K*-mesons
and by supporting some speculations on rho-mesons in the 1200 MeV region. We
use this method further to shed some light on more speculative meson
resonances, among others the enigmatic scalars. We confirm the existence of
chiralons below 1 GeV in the unflavoured and strange meson sector.Comment: 22 pages LaTex, 8 figure
A Naturally Narrow Positive Parity Theta^+
We present a consistent color-flavor-spin-orbital wave function for a
positive parity Theta^+ that naturally explains the observed narrowness of the
state. The wave function is totally symmetric in its flavor-spin part and
totally antisymmetric in its color-orbital part. If flavor-spin interactions
dominate, this wave function renders the positive parity Theta^+ lighter than
its negative parity counterpart. We consider decays of the Theta^+ and compute
the overlap of this state with the kinematically allowed final states. Our
results are numerically small. We note that dynamical correlations between
quarks are not necessary to obtain narrow pentaquark widths.Comment: 10 pages, 1 figure, Revtex4, two-column format, version to be
published in Phys. Rev. D, includes numerical estimates of decay width
Z^* Resonances: Phenomenology and Models
We explore the phenomenology of, and models for, the Z^* resonances, the
lowest of which is now well established, and called the Theta. We provide an
overview of three models which have been proposed to explain its existence
and/or its small width, and point out other relevant predictions, and potential
problems, for each. The relation to what is known about KN scattering,
including possible resonance signals in other channels, is also discussed.Comment: 29 pages, uses RevTeX4; expanded version (published form
On Born approximation in black hole scattering
A massless field propagating on spherically symmetric black hole metrics such
as the Schwarzschild, Reissner-Nordstr\"{o}m and Reissner-Nordstr\"{o}m-de
Sitter backgrounds is considered. In particular, explicit formulae in terms of
transcendental functions for the scattering of massless scalar particles off
black holes are derived within a Born approximation. It is shown that the
conditions on the existence of the Born integral forbid a straightforward
extraction of the quasi normal modes using the Born approximation for the
scattering amplitude. Such a method has been used in literature. We suggest a
novel, well defined method, to extract the large imaginary part of quasinormal
modes via the Coulomb-like phase shift. Furthermore, we compare the numerically
evaluated exact scattering amplitude with the Born one to find that the
approximation is not very useful for the scattering of massless scalar,
electromagnetic as well as gravitational waves from black holes
Dwell-time distributions in quantum mechanics
Some fundamental and formal aspects of the quantum dwell time are reviewed,
examples for free motion and scattering off a potential barrier are provided,
as well as extensions of the concept. We also examine the connection between
the dwell time of a quantum particle in a region of space and flux-flux
correlations at the boundaries, as well as operational approaches and
approximations to measure the flux-flux correlation function and thus the
second moment of the dwell time, which is shown to be characteristically
quantum, and larger than the corresponding classical moment even for freely
moving particles.Comment: To appear in "Time in Quantum Mechanics, Vol. 2", Springer 2009, ed.
by J. G. Muga, A. Ruschhaupt and A. del Camp