3,413 research outputs found
Classical sum rules and spin correlations in photoabsorption and photoproduction processes
In this paper we study the possibility of generalizing the classical
photoabsorption () sum rules, to processes
and crossed helicity amplitudes. In the first case, using detailed balance, the
sum rule is written as where is a kinematical constant which depends only
on the mass of the particles and the center of mass energy. For other crossed
helicity amplitudes, we show that there is a range of values of and for
which the differential cross section for the process or in which the helicities of the photon and particle have
specific values, is equal to the differential cross section for the process in
which one of these two helicities is reversed (parallel-antiparallel spin
correlation).Comment: 9 pages, 2 figure
AdS/QCD and Light Front Holography: A New Approximation to QCD
The combination of Anti-de Sitter space (AdS) methods with light-front
holography leads to a semi-classical first approximation to the spectrum and
wavefunctions of meson and baryon light-quark bound states. Starting from the
bound-state Hamiltonian equation of motion in QCD, we derive relativistic
light-front wave equations in terms of an invariant impact variable zeta which
measures the separation of the quark and gluonic constituents within the hadron
at equal light-front time. These equations of motion in physical space-time are
equivalent to the equations of motion which describe the propagation of spin-J
modes in anti--de Sitter (AdS) space. Its eigenvalues give the hadronic
spectrum, and its eigenmodes represent the probability distributions of the
hadronic constituents at a given scale. Applications to the light meson and
baryon spectra are presented. The predicted meson spectrum has a string-theory
Regge form ; i.e., the square of the
eigenmass is linear in both L and n, where n counts the number of nodes of the
wavefunction in the radial variable zeta. The space-like pion and nucleon form
factors are also well reproduced. One thus obtains a remarkable connection
between the description of hadronic modes in AdS space and the Hamiltonian
formulation of QCD in physical space-time quantized on the light-front at fixed
light-front time. The model can be systematically improved by using its
complete orthonormal solutions to diagonalize the full QCD light-front
Hamiltonian or by applying the Lippmann-Schwinger method in order to
systematically include the QCD interaction terms.Comment: Invited talk, presented by SJB at the Fifth International Conference
On Quarks and Nuclear Physics (QNP09), 21-26 Sep 2009, Beijing, China. Figure
update
Light-Cone Quantization and Hadron Structure
In this talk, I review the use of the light-cone Fock expansion as a
tractable and consistent description of relativistic many-body systems and
bound states in quantum field theory and as a frame-independent representation
of the physics of the QCD parton model. Nonperturbative methods for computing
the spectrum and LC wavefunctions are briefly discussed. The light-cone Fock
state representation of hadrons also describes quantum fluctuations containing
intrinsic gluons, strangeness, and charm, and, in the case of nuclei, "hidden
color". Fock state components of hadrons with small transverse size, such as
those which dominate hard exclusive reactions, have small color dipole moments
and thus diminished hadronic interactions; i.e., "color transparency". The use
of light-cone Fock methods to compute loop amplitudes is illustrated by the
example of the electron anomalous moment in QED. In other applications, such as
the computation of the axial, magnetic, and quadrupole moments of light nuclei,
the QCD relativistic Fock state description provides new insights which go well
beyond the usual assumptions of traditional hadronic and nuclear physics.Comment: LaTex 36 pages, 3 figures. To obtain a copy, send e-mail to
[email protected]
Renormalisation Flow and Universality for Ultracold Fermionic Atoms
A functional renormalisation group study for the BEC-BCS crossover for
ultracold gases of fermionic atoms is presented. We discuss the fixed point
which is at the origin of universality for broad Feshbach resonances. All
macroscopic quantities depend only on one relevant parameter, the concentration
a k_F, besides their dependence on the temperature in units of the Fermi
energy. In particular, we compute the universal ratio between molecular and
atomic scattering length in vacuum. We also present an estimate to which level
of accuracy universality holds for gases of Li and K atoms.Comment: 19 pages, 3 figures, to be published in PR
Probing Intrinsic Charm with Semileptonic B Decays
We discuss semileptonic B decays of the form B -> J/Psi e nu X as possible
probes of intrinsic charm. We calculate the leading order perturbative
contribution to the process B- -> J/Psi e- nu_e X and find it to be
unobservably small, with a branching ratio ~ 10^-10. We propose a modified
spectator model to estimate the intrinsic charm contribution and find that it
can be significantly larger, with a branching ratio for B -> (c cbar) e- nu_e X
as large as 5 X 10^-7. We show that the process could be observed at these
levels by CDF assuming a Run II integrated luminosity of 15 fb^-1, making this
a useful reaction to probe the idea of intrinsic charm.Comment: 25 pages, LaTeX, 7 figures, uses epsf.sty. Version substantially
revise
Thermal Field Theory and Generalized Light Front Coordinates
The dependence of thermal field theory on the surface of quantization and on
the velocity of the heat bath is investigated by working in general coordinates
that are arbitrary linear combinations of the Minkowski coordinates. In the
general coordinates the metric tensor is non-diagonal. The
Kubo, Martin, Schwinger condition requires periodicity in thermal correlation
functions when the temporal variable changes by an amount
. Light front quantization fails since
, however various related quantizations are possible.Comment: 10 page
Large corrections to asymptotic and in the light-cone perturbative QCD
The large- behavior of - and -
transition form factors, and
are analyzed in the framework of light-cone perturbative QCD with the heavy
quark ( and ) mass effect, the parton's transverse momentum dependence
and the higher helicity components in the light-cone wave function are
respected. It is pointed out that the quark mass effect brings significant
modifications to the asymptotic predictions of the transition form factors in a
rather broad energy region, and this modification is much severer for
than that for due to the
-quark being heavier than the -quark. The parton's transverse momentum
and the higher helicity components are another two factors which decrease the
perturbative predictions. For the transition form factor
, they bring sizable corrections in the present
experimentally accessible energy region (). For the
transition form factor , the corrections coming from
these two factors are negligible since the -quark mass is much larger than
the parton's average transverse momentum. The coming collider (LEP2)
will provide the opportunity to examine these theoretical predictions.Comment: 8 pages, RevTex, 5 PostScript figure
Muon Pair Production by Electron-Photon Scatterings
The cross section for muon pair productions by electrons scattering over
photons, , is calculated analytically in the leading order. It is
pointed out that for the center-of-mass energy range, ,
the cross section for is less than b. The differential
energy spectrum for either of the resulting muons is given for the purpose of
high-energy neutrino astronomy. An implication of our result for a recent
suggestion concerning the high-energy cosmic neutrino generation through this
muon pair is discussed.Comment: a comment added, to appear in Phys. Rev. D, Rapid Communicatio
Timelike form factors at high energy
The difference between the timelike and spacelike meson form factors is
analysed in the framework of perturbative QCD with Sudakov effects included. It
is found that integrable singularities appear but that the asymptotic behavior
is the same in the timelike and spacelike regions. The approach to asymptotia
is quite slow and a rather constant enhancement of the timelike value is
expected at measurable large . This is in agreement with the trend
shown by experimental data.Comment: 17 pages, report DAPNIA/SPhN 94 0
Optimal Renormalization Scale and Scheme for Exclusive Processes
We use the BLM method to fix the renormalization scale of the QCD coupling in
exclusive hadronic amplitudes such as the pion form factor and the
photon-to-pion transition form factor at large momentum transfer.
Renormalization-scheme-independent commensurate scale relations are established
which connect the hard scattering subprocess amplitudes that control exclusive
processes to other QCD observables such as the heavy quark potential and the
electron-positron annihilation cross section. The commensurate scale relation
connecting the heavy quark potential, as determined from lattice gauge theory,
to the photon-to-pion transition form factor is in excellent agreement with
data assuming that the pion distribution amplitude is
close to its asymptotic form . We also reproduce the
scaling and normalization of the data at large
momentum transfer. Because the renormalization scale is small, we argue that
the effective coupling is nearly constant, thus accounting for the nominal
scaling behavior of the data. However, the normalization of the space-like pion
form factor obtained from electroproduction experiments is
somewhat higher than that predicted by the corresponding commensurate scale
relation. This discrepancy may be due to systematic errors introduced by the
extrapolation of the electroproduction data to the
pion pole.Comment: 22 pages, Latex, 7 Latex figures. Several references added,
discussion of scale fixing revised for clarity. Final version to appear in
Phys. Rev.
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