192 research outputs found
Capture rate and neutron helicity asymmetry for ordinary muon capture on hydrogen
Applying heavy-baryon chiral perturbation theory to ordinary muon capture
(OMC) on a proton, we calculate the capture rate and neutron helicity asymmetry
up to next-to-next-to-leading order. For the singlet hyperfine state, we obtain
the capture rate Gamma_0 = 695 sec^{-1} while, for the triplet hyperfine state,
we obtain the capture rate Gamma_1 = 11.9 sec^{-1} and the neutron asymmetry
alpha_1 = 0.93. If the existing formalism is used to relate these atomic
capture rates to Gamma_{liq}, the OMC rate in liquid hydrogen, then Gamma_{liq}
corresponding to our improved values of Gamma_0 and Gamma_1 is found to be
significantly larger than the experimental value, primarily due to the updated
larger value of g_A. We argue that this apparent difficulity may be correlated
to the specious anomaly recently reported for mu^- + p to n + nu_mu + gamma,
and we suggest a possibility to remove these two "problems" simply and
simultaneously by reexamining the molecular physics input that underlies the
conventional analysis of Gamma_{liq}.Comment: 14 pages, 1 figur
Aspects of radiative K^+_e3 decays
We re-investigate the radiative charged kaon decay K+- --> pi0 e+- nu_e gamma
in chiral perturbation theory, merging the chiral expansion with Low's theorem.
We thoroughly analyze the precision of the predicted branching ratio relative
to the non-radiative decay channel. Structure dependent terms and their impact
on differential decay distributions are investigated in detail, and the
possibility to see effects of the chiral anomaly in this decay channel is
emphasized.Comment: 15 pages, 6 figure
Field Transformations and the Classical Equation of Motion in Chiral Perturbation Theory
The construction of effective Lagrangians commonly involves the application
of the `classical equation of motion' to eliminate redundant structures and
thus generate the minimal number of independent terms. We investigate this
procedure in the framework of chiral perturbation theory. The use of the
'classical equation of motion' is interpreted in terms of field
transformations. Such an interpretation is crucial if one wants to bring a
given Lagrangian into a canonical form with a minimal number of terms. We
emphasize that the application of field transformations not only eliminates
structures, or, what is equivalent, expresses certain structures in terms of
already known different structures, but also leads to a modification of
coefficients of higher--order terms. This will become relevant, once one
considers effective interaction terms beyond next--to--leading order, i.e.,
beyond .Comment: TRIUMF preprint TRI-PP-94-64, 10 pages in LaTex using RevTex macr
Low-energy and low-momentum representation of the virtual Compton scattering amplitude
We perform an expansion of the virtual Compton scattering amplitude for low
energies and low momenta and show that this expansion covers the transition
from the regime to be investigated in the scheduled photon electroproduction
experiments to the real Compton scattering regime.
We discuss the relation of the generalized polarizabilities of virtual
Compton scattering to the polarizabilities of real Compton scattering.Comment: 13 pages, LaTeX2e/RevTeX, no figure
Corrections to Sirlin's Theorem in Chiral Perturbation Theory
We present the results of the first two-loop calculation of a form factor in
full Chiral Perturbation Theory. We choose a specific
linear combination of and form factors (the one
appearing in Sirlin's theorem) which does not get contributions from order
operators with unknown constants. For the charge radii, the correction to
the previous one-loop result turns out to be significant, but still there is no
agreement with the present data due to large experimental uncertainties in the
kaon charge radii.Comment: 6 pages, Latex, 2 LaTeX figure
Muon capture by a proton in heavy baryon chiral perturbation theory
The matrix element for muon capture by a proton is calculated to O(p^3)
within heavy baryon chiral perturbation theory using the new O(p^3) Lagrangian
of Ecker and Mojzis. External nucleon fields are renormalized using the
appropriate definition of the wave function renormalization factor Z_N. Our
expression for Z_N differs somewhat from that found in existing literature, but
is the one which is consistent with the Lagrangian we use and the one which
ensures, within our approach, the nonrenormalization of the vector coupling as
required by the conserved vector current. Expressions for the standard muon
capture form factors are derived and compared to experimental data and we
determine three of the coefficients of the Ecker - Mojzis Lagrangian, namely,
b_7, b_{19}, and b_{23}.Comment: 14 pages, LaTeX, using revte
Structure analysis of the virtual Compton scattering amplitude at low energies
We analyze virtual Compton scattering off the nucleon at low energies in a
covariant, model-independent formalism.
We define a set of invariant functions which, once the irregular nucleon pole
terms have been subtracted in a gauge-invariant fashion, is free of poles and
kinematical zeros.
The covariant treatment naturally allows one to implement the constraints due
to Lorentz and gauge invariance, crossing symmetry, and the discrete
symmetries.
In particular, when applied to the reaction,
charge-conjugation symmetry in combination with nucleon crossing generates four
relations among the ten originally proposed generalized polarizabilities of the
nucleon.Comment: 19 pages, LaTeX2e/RevTeX, no figures, original sections IV.-VI.
removed, to be discussed in a separate publication, none of the conclusions
change
Delta--Excitation and Exchange Corrections for NN--Bremsstrahlung
The role of the relativistic amplitudes for a number of
processes usually neglected in potential model calculations of
NN--bremsstrahlung is investigated. In particular, we consider the
--excitation pole contributions related to the one--pion and one--rho
exchange and in addition include the exchange contributions induced by the
radiative decays. The contributions are
calculated from relativistic Born amplitudes fitted to --production and
absorption data in the energy range up to 1 GeV and then used to supplement
potential model and soft photon calculations for nucleon--nucleon
bremsstrahlung. The effects on --observables, although moderate in
general, are found to be important in some kinematic domains.Comment: 15 pages in LaTex, using Revtex, 6 figures as uufile'd, compressed
Postscript file included, TRIUMF preprint TRI-PP-94-9
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
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
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