404 research outputs found
Pole Mass, Width, and Propagators of Unstable Fermions
The concepts of pole mass and width are extended to unstable fermions in the
general framework of parity-nonconserving gauge theories, such as the Standard
Model. In contrast with the conventional on-shell definitions, these concepts
are gauge independent and avoid severe unphysical singularities, properties of
great importance since most fundamental fermions in nature are unstable
particles. General expressions for the unrenormalized and renormalized dressed
propagators of unstable fermions and their field-renormalization constants are
presented.Comment: 9 page
Renormalization in general theories with inter-generation mixing
We derive general and explicit expressions for the unrenormalized and
renormalized dressed propagators of fermions in parity-nonconserving theories
with inter-generation mixing. The mass eigenvalues, the corresponding mass
counterterms, and the effect of inter-generation mixing on their determination
are discussed. Invoking the Aoki-Hioki-Kawabe-Konuma-Muta renormalization
conditions and employing a number of very useful relations from Matrix Algebra,
we show explicitly that the renormalized dressed propagators satisfy important
physical properties.Comment: 14 pages; to appear in Phys. Rev.
The Ademollo-Gatto theorem for lattice semileptonic decays
We present the results of the calculation of the Kl3 semileptonic form factor
at zero momentum transfer, f(0), obtained at one-loop in partially quenched
Chiral Perturbation Theory (with either Nf=2, or Nf=3, and with generic valence
and sea quark masses). We show that for Nf=2, when the masses of the valence
and sea light quarks are equal, the correction is of the order (MK^2-Mpi^2)^3.
The formulae presented here can be useful for the mass extrapolation of the
results obtained in lattice simulations to the physical point.Comment: 7 page
Electroweak Radiative Corrections to Muon Capture
Electroweak radiative corrections to muon capture on nuclei are computed and
found to be sizable. They enhance the capture rates for hydrogen and helium by
2.8% and 3.0% respectively. As a result, the value of the induced pseudoscalar
coupling, g_P^exp, extracted from a recent hydrogen 1S singlet capture
experiment is increased by about 21% to g_P^exp = 7.3 +/- 1.2 and brought into
good agreement with the prediction of chiral perturbation theory,
g_P^theory=8.2 +/- 0.2. Implications for helium capture rate predictions are
also discussed.Comment: 6 page
Resonance Propagation and Threshold Singularities
We consider the problem of propagation of an unstable particle in the
framework of Quantum Field Theory. Using unitarity, we show that a real
renormalization constant free of threshold singularities naturally arises.Comment: 5 pages, no figures, revte
Radiative Corrections to W and Quark Propagators in the Resonance Region
We discuss radiative corrections to W and quark propagators in the resonance
region, |s-M^2| \lsim M*Gamma. We show that conventional mass renormalization,
when applied to photonic or gluonic corrections, leads in next to leading order
(NLO) to contributions proportional to [M*Gamma/(s-M^2)]^n, (n=1,2...), i.e. to
a non-convergent series in the resonance region, a difficulty that affects all
unstable particles coupled to massless quanta. A solution of this problem,
based on the concepts of pole mass and width, is presented. It elucidates the
issue of renormalization of amplitudes involving unstable particles and
automatically circumvents the problem of apparent on-shell singularities. The
roles of the Fried-Yennie gauge and the Pinch Technique prescription are
discussed. Because of special properties of the photonic and gluonic
contributions, and in contrast with the Z case, the gauge dependence of the
conventional on-shell definition of mass is unbounded in NLO. The evaluations
of the width in the conventional and pole formulations are compared and shown
to agree in NLO but not beyond.Comment: 19 pages, 7 figures, LaTeX (uses epsfig). Slight rewording of the
abstract and one of the sentences of the text. Minor misprints corrected. To
appear in Phys. Rev.
Eliminating the Hadronic Uncertainty
The Standard Model Lagrangian requires the values of the fermion masses, the
Higgs mass and three other experimentally well-measured quantities as input in
order to become predictive. These are typically taken to be ,
and . Using the first of these, however, introduces a hadronic
contribution that leads to a significant error. If a quantity could be found
that was measured at high energy with sufficient precision then it could be
used to replace as input. The level of precision required for this to
happen is given for a number of precisely-measured observables. The boson
mass must be measured with an error of \,MeV, to \,MeV
and polarization asymmetry, , to that would seem to be the
most promising candidate. The r\^ole of renormalized parameters in perturbative
calculations is reviewed and the value for the electromagnetic coupling
constant in the renormalization scheme that is consistent
with all experimental data is obtained to be .Comment: 8 pages LaTeX2
QED Corrections to Neutrino Electron Scattering
We evaluate the O(alpha) QED corrections to the recoil electron energy
spectrum in the process nu_l + e --> nu_l + e (+gamma), where (+gamma)
indicates the possible emission of a photon and l=e, mu or tau. The soft and
hard bremsstrahlung differential cross sections are computed for an arbitrary
value of the photon energy threshold. We also study the O(alpha) QED
corrections to the differential cross section with respect to the total
combined energy of the recoil electron and a possible accompanying photon.
Their difference from the corrections to the electron spectrum is investigated.
We discuss the relevance and applicability of both radiative corrections,
emphasizing their role in the analysis of precise solar neutrino electron
scattering experiments.Comment: 14 pages + 10 figures. Minimal changes, published versio
Supersymmetric Electroweak Corrections to Single Top Quark Production at the Fermilab Tevatron
We have calculated the supersymmetric
electroweak corrections to single top quark production via at the Fermilab Tevatron in the minimal supersymmetric model. The
supersymmetric electroweak corrections to the cross section are a few percent
for , and can exceed 10% for . The combined effects
of SUSY electroweak corrections and the Yukawa corrections can exceed 10% for
favorable parameter values, which might be observable at a high-luminosity
Tevatron.Comment: 13 pages, 4 figures available at reques
Finite Width Effects and Gauge Invariance in Radiative Production and Decay
The naive implementation of finite width effects in processes involving
unstable particles can violate gauge invariance. For the example of radiative
production and decay, , at tree level, it is
demonstrated how gauge invariance is restored by including the imaginary part
of triangle graphs in addition to resumming the imaginary contributions to the
vacuum polarization. Monte Carlo results are presented for the Fermilab
Tevatron.Comment: 10 pages, Revtex, 3 figures submitted separately as uuencoded tarred
postscript files, the complete paper is available at
ftp://phenom.physics.wisc.edu/pub/preprints/1995/madph-95-878.ps.Z or
http://phenom.physics.wisc.edu/pub/preprints/1995/madph-95-878.ps.
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