443 research outputs found
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
Decay of polarized muon at rest as a source of polarized neutrino beam
In this paper, we indicate the theoretical possibility of using the decay of
polarized muons at rest as a source of the transversely polarized electron
antineutrino beam. Such a beam can be used to probe new effects beyond standard
model. We mean here new tests concerning CP violation, Lorentz structure and
chirality structure of the charged current weak interactions. The main goal is
to show how the energy and angular distribution of the electron antineutrinos
in the muon rest frame depends on the transverse components of the antineutrino
beam polarization. Our analysis is model-independent and consistent with the
current upper limits on the non-standard couplings. The results are presented
in a limit of infinitesimally small mass for all particles produced in the
decay.Comment: elsart style, 11 pages, 2 eps figures, submitted do publicatio
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
A Method to Determine the Tau Neutrino Helicity Using Polarized Taus
A method is presented to extract the tau neutrino helicity, or equivalently,
the chirality parameter , independent of any tau
polarization which may be present. The method is thus well-suited to
measurements using taus produced from the and is complementary to
analyses using tau correlations since it provides the sign of the chirality
parameter which is otherwise unavailable without recourse to lower energy
experiments where taus are unpolarized. Results of Monte Carlo studies and
comments regarding the use of the technique in experiments are also included.Comment: 13 pages, uuencoded postscript fil
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
Search for the Lepton Number Violating Process nu_mu-bar e^- -> mu^- nu_e-bar
The NuTeV experiment at Fermilab has used a sign-selected neutrino beam to
perform a search for the lepton number violating process nu_mu-bar e^- -> mu^-
nu_e-bar, and to measure the cross-section of the Standard Model inverse muon
decay process nu_mu e^- -> mu^- nu_e. NuTeV measures the inverse muon decay
asymptotic cross-section sigma/E to be 13.8 +/- 1.2 +/- 1.4 x 10^-42 cm^2/GeV.
The experiment also observes no evidence for lepton number violation and places
one of the most restrictive limits on the LNV/IMD cross-section ratio at < 1.7%
at 90% C.L. for V-A couplings and < 0.6% for scalar couplings.Comment: 4 pages, 4 figures, 2 tables. Submitted to Physics Review Letter
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