438 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
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.
Differential Cross Section and Vector Polarization for the elastic scattering of 41 to 51 MeV Deuterons on Carbon
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
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
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
P and T Odd Asymmetries in Lepton Flavor Violating Tau Decays
We calculated the differential cross sections of the processes in which one
of the pair created tau particles at an e^+ e^- collider decays into lepton
flavor violating final states e.g. tau -> mu gamma, tau -> 3 mu, tau -> mu ee.
Using the correlations between angular distributions of both sides of tau
decays, we can obtain information on parity and CP violations of lepton flavor
non-conserving interactions. The formulae derived here are useful in
distinguishing different models, since each model of physics beyond the
standard model predicts different angular correlations. We also calculate
angular distributions of the major background process to tau -> l gamma search,
namely tau -> l nu \bar{\nu} gamma, and discuss usefulness of the angular
correlation for background suppression.Comment: 31 pages, 5 figure
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
revtex
Evidence for right-handed neutrinos at a neutrino factory
We emphasize that a muon based neutrino factory could show the existence of
light right-handed neutrinos, if a deficit in the number of detected events is
observed at a near detector. This could be as large as ~10% if the size of the
new interactions saturates the present limits from electroweak precision data,
what is not excluded by the oscillation experiments performed up to now. A
simple model realizing such a scenario can be obtained adding right-handed
neutrinos to the minimal Standard Model, together with an extra scalar doublet
and a triplet of hypercharge 1. In this case, however, the possible deficit is
reduced by a factor of ~3, and the Yukawa couplings must be adequately chosen.
This is also generically required if lepton flavour violation must be below
present bounds.Comment: Latex 18 pages, 2 eps figure
Tests of the Equivalence Principle with Neutral Kaons
We test the Principle of Equivalence for particles and antiparticles, using
CPLEAR data on tagged K0 and K0bar decays into pi^+ pi^-. For the first time,
we search for possible annual, monthly and diurnal modulations of the
observables |eta_{+-}| and phi_{+-}, that could be correlated with variations
in astrophysical potentials. Within the accuracy of CPLEAR, the measured values
of |eta_{+-}| and phi_{+-} are found not to be correlated with changes of the
gravitational potential. We analyze data assuming effective scalar, vector and
tensor interactions, and we conclude that the Principle of Equivalence between
particles and antiparticles holds to a level of 6.5, 4.3 and 1.8 x 10^{-9},
respectively, for scalar, vector and tensor potentials originating from the Sun
with a range much greater than the distance Earth-Sun. We also study
energy-dependent effects that might arise from vector or tensor interactions.
Finally, we compile upper limits on the gravitational coupling difference
between K0 and K0bar as a function of the scalar, vector and tensor interaction
range.Comment: 15 pages latex 2e, five figures, one style file (cernart.csl)
incorporate
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