22 research outputs found
Radiative Tau Decays with One Pseudoscalar Meson
We have calculated the decay . We present
the photon energy spectrum, the meson-photon invariant mass spectrum and the
integrated rate as a function of a photon energy cut or an invariant mass cut.
Both the internal bremsstrahlung and the structure dependent radiation have
been taken into account. To this aim we have parametrized the form factors
and , which determine the structure dependent radiation. Observables
especially suited for the measurement of the structure dependent form factors
are found and implications on the width of the discussed.Comment: p.20, TTP93-1, LaTe
About estimations of an one-photon background in neutrino oscillation experiments at low neutrino energies
The possible sources of one-photon radiation as background for quasi-elastic
reaction nu_mu+n to mu^-+p are considered. The ones are relevant in experiments
on determination of oscillation parameters at low neutrino energies (E_nu^lab~1
GeV). The estimation for the cross section of reaction nu_mu+n to mu^-+p+gamma
is given at E_nu^lab=0.7 GeV as 0.65% from the corresponding cross section of
quasi-elastic reaction.Comment: 17 pages, 10 figure
Pion photo- and electroproduction and the partially-conserved axial current
The relevance of the axial current for pion production processes off the
nucleon with real or virtual photons is revisited. Employing the hypothesis of
a partially conserved axial current (PCAC), it is shown that, when all of the
relevant contributions are taken into account, PCAC does not provide any
additional constraint for threshold production processes that goes beyond the
Goldberger-Treiman relation. In particular, it is shown that pion
electroproduction processes at threshold cannot be used to extract any
information regarding the weak axial form factor. The relationships found in
previous investigations are seen to be an accident of the approximations
usually made in this context.Comment: 4 pages, 3 figures; typos corrected; references updated; some
rewording; conclusions unchange
Neutral Pion Photoproduction on Nuclei in Baryon Chiral Perturbation Theory
Threshold neutral pion photoproduction on light nuclei is studied in the
framework of baryon chiral perturbation theory. We obtain a general formula for
the electric dipole amplitude in the special case of neutral pion
photoproduction on a nucleus. To third order in small momenta, the amplitude is
a sum of 2- and 3-body interactions with no undetermined parameters. With
reasonable input from the single nucleon sector, our result for neutral pion
photoproduction on the deuteron is in agreement with experiment.Comment: 24 pages, 4 uuencoded postscript figures, uses LaTex and epsf.tex.
Added footnote and references. Minor changes in text and forma
Pion electroproduction, PCAC, chiral Ward identities, and the axial form factor revisited
We re-investigate Adler's PCAC relation in the presence of an external
electromagnetic field within the framework of QCD coupled to external fields.
We discuss pion electroproduction within a tree-level approximation to chiral
perturbation theory and explicitly verify a chiral Ward identity referred to as
the Adler-Gilman relation. We critically examine soft-momentum techniques and
point out how inadmissable approximations may lead to results incompatible with
chiral symmetry. As a result we confirm that threshold pion electroproduction
is indeed a tool to obtain information on the axial form factor of the nucleon.Comment: 33 pages, RevTex, 9 figure
Quasifree eta photoproduction from nuclei and medium modifications of resonances
We investigate the sensitivity of the differential cross section, recoil
nucleon polarization and the photon asymmetry to changes in the elementary
amplitude, medium modifications of the resonance masses, as
well as nuclear target effects. All calculations are performed within a
relativistic plane wave impulse approximation formalism resulting in analytical
expressions for all observables. The spin observables are shown to be unique
tools to study subtle effects that are not accessible by only looking at the
unpolarized differential cross section.Comment: 27 pages, 8 figures, Revtex, To be published in Phys. Rev.
The standard model at low energies
The hadronic sector of the standard model at low energies is described by a
non--decoupling effective field theory, chiral perturbation theory. An
introduction is given to the construction of effective chiral Lagrangians, both
in the purely mesonic sector and with inclusion of baryons. The connection
between the relativistic formulation and the heavy baryon approach to chiral
perturbation theory with baryons is reviewed.Comment: Lectures given at the 6th Indian-Summer School on Intermediate Energy
Physics, Prague, Aug. 1993, Latex, 26 pages (with a4.sty), UWThPh-1993-3
Observation of a unitary cusp in the threshold {\gamma}p -> \pi^{0}p reaction
A rigorous multipole analysis of the recent {\gamma}p -> \pi^{0}p cross
section measurement is presented. The data were taken using the photon
spectrometer TAPS at the tagged photon beam of the Mainz microtron. The s and p
wave multipoles were extracted using minimal model assumptions. The predicted
unitary cusp for the s wave multipole E_{0+} due to the two step {\gamma}p ->
\pi^{+}n -> \pi^{0}p reaction was observed. The results are consistent with one
loop chiral perturbation theory calculations for which three low energy
constants have been determined by a fit to the data. The uncertainties in the
analysis and the need for polarization observables are discussed.Comment: 22 pages, 10 figures, 2 tables. Uses LaTex 2e (will not work with
previous versions). This paper is also available via an anonymous ftp site
ftp://marie.mit.edu/pub/bernstein/ Submitted to Phys. Rev.
Effective Lagrangian Approach to the Theory of Eta Photoproduction in the Region
We investigate eta photoproduction in the resonance region
within the effective Lagrangian approach (ELA), wherein leading contributions
to the amplitude at the tree level are taken into account. These include the
nucleon Born terms and the leading -channel vector meson exchanges as the
non-resonant pieces. In addition, we consider five resonance contributions in
the - and - channel; besides the dominant , these are:
and . The amplitudes for the
and the photoproduction near threshold have significant
differences, even as they share common contributions, such as those of the
nucleon Born terms. Among these differences, the contribution to the
photoproduction of the -channel excitation of the is the most
significant. We find the off-shell properties of the spin-3/2 resonances to be
important in determining the background contributions. Fitting our effective
amplitude to the available data base allows us to extract the quantity
, characteristic of the
photoexcitation of the resonance and its decay into the
-nucleon channel, of interest to precise tests of hadron models. At the
photon point, we determine it to be from
the old data base, and from a
combination of old data base and new Bates data. We obtain the helicity
amplitude for to be from the old data base, and from the combination of the old data base and new Bates
data, compared with the results of the analysis of pion photoproduction
yielding , in the same units.Comment: 43 pages, RevTeX, 9 figures available upon request, to appear in
Phys. Rev.