456 research outputs found
Global analysis of muon decay measurements
We have performed a global analysis of muon decay measurements to establish
model-independent limits on the space-time structure of the muon decay matrix
element. We find limits on the scalar, vector and tensor coupling of right- and
left-handed muons to right- and left-handed electrons. The limits on those
terms that involve the decay of right-handed muons to left-handed electrons are
more restrictive than in previous global analyses, while the limits on the
other non-standard model interactions are comparable. The value of the Michel
parameter eta found in the global analysis is -0.0036 \pm 0.0069, slightly more
precise than the value found in a more restrictive analysis of a recent
measurement. This has implications for the Fermi coupling constant G_F.Comment: 5 pages, 3 table
A new experiment to search for the invisible decay of the orthopositronium
We propose an experiment to search for invisible decays of orthopositronium
(o-Ps) with a 90% confidence sensitivity in the branching ratio as low as
. Evidence for this decay mode would unambigously signal new physics:
either the existence of extra--dimensions or fractionally charged particles or
new light gauge bosons. The experimental approach and the detector components
of the proposed experiment are described.Comment: Based on a talk given at Workshop on Positronium Physics, Zurich,
Switzerland, 30-31 May 200
Measurement of the Michel Parameter xi" in Polarized Muon Decay and Implications on Exotic Couplings of the Leptonic Weak Interaction
The Michel parameter xi" has been determined from a measurement of the
longitudinal polarization of positrons emitted in the decay of polarized and
depolarized muons. The result, xi" = 0.981 +- 0.045stat +- 0.003syst, is
consistent with the Standard Model prediction of unity, and provides an order
of magnitude improvement in the relative precision of this parameter. This
value sets new constraints on exotic couplings beyond the dominant V-A
description of the leptonic weak interaction.Comment: 15 pages, 16 figures, 3 tables; submitted to Phys. Rev.
CLEO Results on Tau Michel Parameters
We present measurements of the tau Michel Parameters made by the CLEO
experiment. Three different analyses are performed: a spin-independent lepton
spectrum analysis and a second spin-dependent analysis using \vs\
events, and a third spin-dependent analysis using \vs\
events. the results are used to derive limits on the general
four-fermion couplings, the mass of the charged Higgs in the MSSM, and a
right-handed in left-right models. Many of these measurements are more
precise than the PDG world averages.Comment: Invited talk at the TAU'98 Workshop, 14-17 September 1998, Santander,
Spain. 7 pages, 7 figures, LaTeX -> postscrip
Virtual and Soft Pair Corrections to Polarized Muon Decay Spectrum
Radiative corrections to the muon decay spectrum due to soft and virtual
electron--positron pairs are calculated.Comment: 10pp, 2 PS figs, details of calculations are adde
Matter effects in the D0-D0bar system
We discuss the impact of matter effects in the D0-D0bar system. We show that
such effects could, in principle, be measured, but that they cannot be used to
probe the mass difference x_D or the lifetime difference y_D. This occurs
because the mixing effects and the matter effects decouple at short times. We
also comment briefly on the B systems.Comment: 6 pages, RevTe
An Improved Limit on Invisible Decays of Positronium
The results of a new search for positronium decays into invisible final
states are reported. Convincing detection of this decay mode would be a strong
evid ence for new physics beyond the Standard Model (SM): for example the
existence of extra--dimensions, of milli-charged particles, of new light gauge
bosons or of mirror particles. Mirror matter could be a relevant dark matter
candidate.
In this paper the setup and the results of a new experiment are presented. In
a collected sample of about orthopositronium decay
s, no evidence for invisible decays in an energy window [0,80] keV was found
and an upper limit on the branching ratio of orthopositronium \invdecay could
be set: \binvdecay<4.2\times 10^{-7} (90% C.L.)
Our results provide a limit on the photon mirror-photon mixing strength
(90% C.L.) and rule out particles lighter
than the electron mass with a fraction of the
electron charge. Furthermore, upper limits on the branching ratios for the
decay of parapositronium (90%
C.L.) and the direct annihilation (90% C.L.) could be set.Comment: 17 pages, 7 figures, added references, fixed limit on millicharged
particles and changed two plots accordingl
Improved limits on nuebar emission from mu+ decay
We investigated mu+ decays at rest produced at the ISIS beam stop target.
Lepton flavor (LF) conservation has been tested by searching for \nueb via the
detection reaction p(\nueb,e+)n. No \nueb signal from LF violating mu+ decays
was identified. We extract upper limits of the branching ratio for the LF
violating decay mu+ -> e+ \nueb \nu compared to the Standard Model (SM) mu+ ->
e+ nue numub decay: BR < 0.9(1.7)x10^{-3} (90%CL) depending on the spectral
distribution of \nueb characterized by the Michel parameter rho=0.75 (0.0).
These results improve earlier limits by one order of magnitude and restrict
extensions of the SM in which \nueb emission from mu+ decay is allowed with
considerable strength. The decay \mupdeb as source for the \nueb signal
observed in the LSND experiment can be excluded.Comment: 10 pages, including 1 figure, 1 tabl
Left-handed neutrino disappearance probe of neutrino mass and character
We explore the sensitivity to a non vanishing neutrino mass offered by
dynamical observables, i.e., branching ratios and polarizations. The
longitudinal polarization in the C.M. frame decreases by a 4% for and MeV. Taking advantage of the
fact that the polarization is a Lorentz variant quantity, we study the
polarization effects in a boosted frame. By means of a neutrino beam, produced
by a high velocity boosted parent able to flip the neutrino helicity, we find
that an enhanced left-handed neutrino deficit, induced by a Wigner rotation,
appears.Comment: 8 pages and 2 figures. Last version accepted in PRL, new references
and better analysis of experimental possibilitie
Search for Time-Reversal Violation in the beta-Decay of Polarized 8Li Nuclei
The transverse polarization of electrons emitted in the beta-decay of
polarized 8Li nuclei has been measured. For the time reversal violating triple
correlation parameter we find R = (0.9 +- 2.2)x10^{-3}. This result is in
agreement with the standard model and yields improved constraints on exotic
tensor contributions to the weak interaction. It also provides a new limit on
the mass of a possible scalar leptoquark, m_{LQ} > 560 GeV/c^2 (90% C.L.).Comment: Submitted to Physical Review Letters, 4 pages, 3 figures, uses
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