8 research outputs found
Novo određivanje pionskih faktora oblika
We have used the PIBETA large-acceptance detector for a precise measurement of the π + → e +νγ radiative decay at rest, with a broad phase space coverage. Using the CVC value for the pion vector form factor, FV = 0.0259(5), we have obtained a new value of the pion axial-vector form factor γ = FA/FV = 0.429(14). However, significant deviations from the SM predictions are evident in our data. The discrepancy can be accounted for by introducing a pion tensor form factor of FT = −0.0016(3). The question of the true nature of the observed deviations remains open pending further theoretical and experimental study.Primijenili smo detektor PIBETA, koji pokriva velik prostorni kut, za točno mjerenje radioaktivnog raspada π + → e +νγ mirnih π +, uz veliko zahvaćanje faznog prostora. Uzevši CVC vrijednost pionskog vektorskog faktora oblika, FV = 0.0259(5), dobili smo novu vrijednost pionskog aksijalno-vektorskog faktora oblika γ = FA/FV = 0.429(14). Naši podaci ukazuju na velika odstupanja od predviđanja standardnog modela. Ta se odstupanja mogu objasniti uvođenjem tenzorskog faktora oblika FT = −0.0016(3). Daljnja teorijska i eksperimentalna istraživanja su nužna da bi se utvrdila stvarna priroda opaženih odstupanja
A Review of Rare Pion and Muon Decays
After a decade of no measurements of pion and muon rare decays, PIBETA, a new
experimental program is producing its first results. We report on a new
experimental study of the pion beta decay, Pi(+) -> Pi(0) e(+) Nu, the Pi(e2
gamma) radiative decay, Pi(+) -> e(+) Nu Gamma, and muon radiative decay, Mu ->
e Nu Gamma. The new results represent four- to six-fold improvements in
precision over the previous measurements. Excellent agreement with Standard
Model predictions is observed in all channels except for one kinematic region
of the Pi(e2 gamma) radiative decay involving energetic photons and
lower-energy positrons.Comment: 10 pages, 6 figures, 2 tables, invited talk presented at MESON 2004,
8th Int'l. Workshop on Meson Production, Properties and Interaction, Krakow,
Poland 4-8 June 200
Nab: Measurement Principles, Apparatus and Uncertainties
The Nab collaboration will perform a precise measurement of 'a', the
electron-neutrino correlation parameter, and 'b', the Fierz interference term
in neutron beta decay, in the Fundamental Neutron Physics Beamline at the SNS,
using a novel electric/magnetic field spectrometer and detector design. The
experiment is aiming at the 10^{-3} accuracy level in (Delta a)/a, and will
provide an independent measurement of lambda = G_A/G_V, the ratio of
axial-vector to vector coupling constants of the nucleon. Nab also plans to
perform the first ever measurement of 'b' in neutron decay, which will provide
an independent limit on the tensor weak coupling.Comment: 12 pages, 6 figures, 1 table, talk presented at the International
Workshop on Particle Physics with Slow Neutrons, Grenoble, 29-31 May 2008; to
appear in Nucl. Instrum. Meth. in Physics Research
The neutron and its role in cosmology and particle physics
Experiments with cold and ultracold neutrons have reached a level of
precision such that problems far beyond the scale of the present Standard Model
of particle physics become accessible to experimental investigation. Due to the
close links between particle physics and cosmology, these studies also permit a
deep look into the very first instances of our universe. First addressed in
this article, both in theory and experiment, is the problem of baryogenesis ...
The question how baryogenesis could have happened is open to experimental
tests, and it turns out that this problem can be curbed by the very stringent
limits on an electric dipole moment of the neutron, a quantity that also has
deep implications for particle physics. Then we discuss the recent spectacular
observation of neutron quantization in the earth's gravitational field and of
resonance transitions between such gravitational energy states. These
measurements, together with new evaluations of neutron scattering data, set new
constraints on deviations from Newton's gravitational law at the picometer
scale. Such deviations are predicted in modern theories with extra-dimensions
that propose unification of the Planck scale with the scale of the Standard
Model ... Another main topic is the weak-interaction parameters in various
fields of physics and astrophysics that must all be derived from measured
neutron decay data. Up to now, about 10 different neutron decay observables
have been measured, much more than needed in the electroweak Standard Model.
This allows various precise tests for new physics beyond the Standard Model,
competing with or surpassing similar tests at high-energy. The review ends with
a discussion of neutron and nuclear data required in the synthesis of the
elements during the "first three minutes" and later on in stellar
nucleosynthesis.Comment: 91 pages, 30 figures, accepted by Reviews of Modern Physic