2,635 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
Constraints on R-parity violating couplings from LEP/SLD hadronic observables
We analyze the one loop corrections to hadronic Z decays in an R-parity
violating extension to the Minimal Supersymmetric Standard Model (MSSM).
Performing a global fit to all the hadronic observables at the Z-peak, we
obtain stringent constraints on the R-violating couplings constants lambda' and
lambda''. As a result of the strong constraints from the b asymmetry parameters
A_b and A_FB(b), we find that the couplings lambda'{i31}, lambda'{i32}, and
lambda''{321} are ruled out at the 1 sigma level, and that lambda'{i33} and
lambda''{33i} are ruled out at the 2 sigma level. We also obtain Bayesian
confidence limits for the R-violating couplings.Comment: 30 pages, 19 postscript figures, REVTeX, new section 8 on Bayesian
confidence limits adde
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
QED Corrections to the Scattering of Solar Neutrinos and Electrons
We discuss recent calculations of the O(alpha) QED corrections to the recoil
electron energy spectrum in neutrino electron scattering, and to the spectrum
of the combined energy of the recoil electron and a possible accompanying
photon emitted in the scattering process. We then examine the role of these
corrections in the interpretation of precise measurements from solar neutrino
electron scattering experiments.Comment: (16 Pages, 4 Figures) Presented at the Symposium in Honor of
Professor Alberto Sirlin's 70th Birthday: ``50 Years of Precision Electroweak
Physics'', New York University, October 27-28, 200
Constraints on R-parity violating couplings from lepton universality
We analyze the one loop corrections to leptonic W and Z decays in an R-parity
violating extension to the Minimal Supersymmetric Standard Model (MSSM). We
find that lepton universality violation in the Z line-shape variables alone
would strengthen the bounds on the magnitudes of the lambda' couplings, but a
global fit on all data leaves the bounds virtually unchanged at |lambda'_{33k}|
< 0.42 and |lambda'_{23k}| < 0.50 at the 2 sigma level. Bounds from W decays
are less stringent: |lambda'_{33k}| < 2.4 at 2 sigma, as a consequence of the
weaker Fermilab experimental bounds on lepton universality violation in W
decays. We also point out the potential of constraining R-parity violating
couplings from the measurement of the Upsilon invisible width.Comment: 26pages, 8 postscript figures, REVTeX. Updated references. Typos
correcte
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
Electroweak and QCD corrections to Higgs production via vector-boson fusion at the LHC
The radiative corrections of the strong and electroweak interactions are
calculated at next-to-leading order for Higgs-boson production in the
weak-boson-fusion channel at hadron colliders. Specifically, the calculation
includes all weak-boson fusion and quark--antiquark annihilation diagrams to
Higgs-boson production in association with two hard jets, including all
corresponding interferences. The results on the QCD corrections confirm that
previously made approximations of neglecting s-channel diagrams and
interferences are well suited for predictions of Higgs production with
dedicated vector-boson fusion cuts at the LHC. The electroweak corrections,
which also include real corrections from incoming photons and leading
heavy-Higgs-boson effects at two-loop order, are of the same size as the QCD
corrections, viz. typically at the level of 5-10% for a Higgs-boson mass up to
\sim 700 GeV. In general, both types of corrections do not simply rescale
differential distributions, but induce distortions at the level of 10%. The
discussed corrections have been implemented in a flexible Monte Carlo event
generator.Comment: 33 pages, LaTeX, 24 postscript figure
Four-fermion production with RACOONWW
RACOONWW is an event generator for e+e- --> WW --> 4fermions(+gamma) that
includes full tree-level predictions for e+e- --> 4f and e+e- --> 4f+gamma as
well as O(alpha) corrections to e+e- --> 4f in the so-called double-pole
approximation. We briefly sketch the concept of the calculation on which this
generator is based and present some numerical results.Comment: 9 pages, latex, 6 postscript files, to appear in the proceedings of
the UK Phenomenology Workshop on Collider Physics, Durham, UK, 19-24
September, 199
Decays of Scalar and Pseudoscalar Higgs Bosons into Fermions: Two-loop QCD Corrections to the Higgs-Quark-Antiquark Amplitude
As a first step in the aim of arriving at a differential description of
neutral Higgs boson decays into heavy quarks, , to second
order in the QCD coupling , we have computed the
amplitude at the two-loop level in QCD for a general neutral Higgs boson which
has both scalar and pseudoscalar couplings to quarks. This amplitude is given
in terms of a scalar and a pseudoscalar vertex form factor, for which we
present closed analytic expressions in terms of one-dimensional harmonic
polylogarithms of maximum weight 4. The results hold for arbitrary
four-momentum squared, , of the Higgs boson and of the heavy quark mass,
. Moreover we derive the approximate expressions of these form factors near
threshold and in the asymptotic regime .Comment: 56 pages, 2 figure
Quadratic Word Equations with Length Constraints, Counter Systems, and Presburger Arithmetic with Divisibility
Word equations are a crucial element in the theoretical foundation of
constraint solving over strings, which have received a lot of attention in
recent years. A word equation relates two words over string variables and
constants. Its solution amounts to a function mapping variables to constant
strings that equate the left and right hand sides of the equation. While the
problem of solving word equations is decidable, the decidability of the problem
of solving a word equation with a length constraint (i.e., a constraint
relating the lengths of words in the word equation) has remained a
long-standing open problem. In this paper, we focus on the subclass of
quadratic word equations, i.e., in which each variable occurs at most twice. We
first show that the length abstractions of solutions to quadratic word
equations are in general not Presburger-definable. We then describe a class of
counter systems with Presburger transition relations which capture the length
abstraction of a quadratic word equation with regular constraints. We provide
an encoding of the effect of a simple loop of the counter systems in the theory
of existential Presburger Arithmetic with divisibility (PAD). Since PAD is
decidable, we get a decision procedure for quadratic words equations with
length constraints for which the associated counter system is \emph{flat}
(i.e., all nodes belong to at most one cycle). We show a decidability result
(in fact, also an NP algorithm with a PAD oracle) for a recently proposed
NP-complete fragment of word equations called regular-oriented word equations,
together with length constraints. Decidability holds when the constraints are
additionally extended with regular constraints with a 1-weak control structure.Comment: 18 page
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