24 research outputs found
Semileptonic and Exclusive Rare B Decays
The exclusive rare decay B \ra K^\ast \gamma takes place in a region of
maximum recoil, , posing a problem for nonrelativistic quark models
which are usually thought to be most reliable at zero recoil. The
Bauer--Stech--Wirbel (BSW) model, formulated in the infinite--momentum--frame
(IMF) formalism, is designed to work at . We show in this model that the
ratio relating the decay B \ra K^\ast \gamma and the --spectrum of the
semileptonic decay B\ra \rho e {\bar \nu}, becomes independent of the wave
function in the SU(3) flavor symmetry limit. We show that this feature is also
true in relativistic quark models formulated in the IMF or light--cone
formalism, if the quark is infinitely heavy. In fact, these relativistic
models, which have a different spin structure from the BSW case, reduce to the
BSW model in the heavy --quark limit. A direct measurement of the
--spectrum of the semileptonic decay can therefore provide accurate
information for the exclusive rare decay.Comment: 13 pages, Latex, no figure, UTPT--94--0
The PAMELA excess from neutralino annihilation in the NMSSM
We examine whether the cosmic ray positron excess observed by PAMELA can be
explained by neutralino annihilation in the Next-to-Minimal Supersymmetric
Standard Model (NMSSM). The main dark matter annihilation products are the
lightest CP-even scalar h1 plus the lightest CP-odd scalar a1, with the a1
decaying into two muons. The energetic positrons needed to explain PAMELA are
thus obtained in the NMSSM simply from kinematics. The required large
annihilation cross section is obtained from an s-channel resonance with the
heavier CP-odd scalar a2. Various experiments constrain the PAMELA-favored
NMSSM parameter space, including collider searches for a light a1. These
constraints point to a unique corner of the NMSSM parameter space, having a
lightest neutralino mass around 160 GeV and a very light pseudoscalar mass less
than a GeV. A simple parameterized formula for the charge-dependent solar
modulation effects reconciles the discrepancy between the PAMELA data and the
estimated background at lower energies. We also discuss the electron and gamma
ray spectra from the Fermi LAT observations, and point out the discrepancy
between the NMSSM predictions and Fermi LAT preliminary results and possible
resolution. An NMSSM explanation of PAMELA makes three striking and uniquely
correlated predictions: the rise in the PAMELA positron spectrum will turn over
at around 70 GeV, the dark matter particle mass is less than the top quark
mass, and a light sub-GeV pseudoscalar will be discovered at colliders.Comment: 25 pages, 9 figures; final version for PR
Three-Fermion Bound States on the Light Front
We investigate the stability of the relativistic three-fermion system with a
zero-range force in the light front form. In particular, introducing an
invariant cut-off, we study the dependence of the bound state on the coupling
strength also for cases where the two-fermion system is unbound. The
relativistic Thomas collapse is discussed by solving the fully coupled integral
equation system. Furthermore, we explicitly investigate the ground state mass
of the three-fermion system and compare to previous simplified calculations.Comment: 23 pages, 6 figure
Status of the N* Program at Jefferson Lab
Recent results from JLab on the electromagnetic excitation of nucleon
resonances are presented, and confronted with theoretical predictions.
Preliminary data in the search for undiscovered states are discussed as well.Comment: 7 pages, 11 figures, talk presented at Electron-Nucleus Scattering
VII, Elba, June 24-28,2002, added reference in section 2 and section 4.1,
corrected misleading typographical error in section 4.
Perturbative QCD Analysis of the Nucleon's Pauli Form Factor F_2(Q^2)
We perform a perturbative QCD analysis of the nucleon's Pauli form factor
in the asymptotically large limit. We find that the leading
contribution to has a power behavior, consistent with the
well-known result in the literature. Its coefficient depends on the leading-
and subleading-twist light-cone wave functions of the nucleon, the latter
describing the quarks with one unit of orbital angular momentum. We also derive
at the logarithmic accurary the asymptotic scaling which describes recent Jefferson Lab data well.Comment: 4 papes, 3 figures include
Nucleon and pion electromagnetic form factors in a light-front constituent quark model
Nucleon and pion electromagnetic form factors are evaluated in the spacelike
region within a light-front constituent quark model, where eigenfunctions of a
mass operator, reproducing a large set of hadron energy levels, are adopted and
quark form factors are considered in the one-body current. The hadron form
factors are sharply affected by the high momentum tail generated in the wave
function by the one-gluon-exchange interaction. Useful information on the
electromagnetic structure of light constituent quarks can be obtained from the
comparison with nucleon and pion experimental data.Comment: 9 pages, latex, 4 figures available as separate .uu fil
On the radiative decays of light vector and axial-vector mesons
We study the light vector and axial-vector mesons. According to the
hadrogenesis conjecture the nature of the two types of states is distinct. The
axial-vector mesons are generated dynamically by coupled-channel interactions
based on the chiral Lagrangian written down in terms of the Goldstone bosons
and the light vector mesons. We propose a novel counting scheme that arises if
the chiral Lagrangian is supplemented by constraints from large-N_c QCD in the
context of the hadrogenesis conjecture. The counting scheme is successfully
tested by a systematic study of the properties of vector mesons. The spectrum
of light axial-vector mesons is derived relying on the leading order
interaction of the Goldstone bosons with the vector mesons supplemented by a
phenomenology for correction terms. The f_1(1282), b_1(1230), h_1(1386),
a_1(1230) and K_1(1272) mesons are recovered as molecular states. Based on
those results the one-loop contributions to the electromagnetic decay
amplitudes of axial-vector molecules into pseudo-scalar or vector mesons are
evaluated systematically. In order to arrive at gauge invariant results in a
transparent manner we choose to represent the vector particles by
anti-symmetric tensor fields. It is emphasized that there are no tree-level
contributions to a radiative decay amplitude of a given state if that state is
generated by coupled-channel dynamics. The inclusion of the latter would be
double counting. At present we restrict ourselves to loops where a vector and a
pseudo-scalar meson couple to the axial-vector molecule. We argue that final
and predictive results require further computations involving intermediate
states with two vector mesons. The relevance of the latter is predicted by our
counting rules.Comment: added appendix concerning double-counting issue
Light Cone Sum Rules for gamma* N -> Delta Transition Form Factors
A theoretical framework is suggested for the calculation of gamma* N -> Delta
transition form factors using the light-cone sum rule approach. Leading-order
sum rules are derived and compared with the existing experimental data. We find
that the transition form factors in a several GeV region are dominated by the
``soft'' contributions that can be thought of as overlap integrals of the
valence components of the hadron wave functions. The ``minus'' components of
the quark fields contribute significantly to the result, which can be
reinterpreted as large contributions of the quark orbital angular momentumComment: 38 pages, 10 figures; some typos fixed and references added, to
appear in Phys. Rev.
Baryon Current Matrix Elements in a Light-Front Framework
Current matrix elements and observables for electro- and photo-excitation of
baryons from the nucleon are studied in a light-front framework. Relativistic
effects are estimated by comparison to a nonrelativistic model, where we use
simple basis states to represent the baryon wavefunctions. Sizeable
relativistic effects are found for certain transitions, for example, to radial
excitations such as that conventionally used to describe to the Roper
resonance. A systematic study shows that the violation of rotational covariance
of the baryon transition matrix elements stemming from the use of one-body
currents is generally small.Comment: 32 pages, LaTeX, 10 postscript figures, uses epsf.sty; figures
uuencoded with uufiles (or available by request in .ps or hardcopy form
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