50,733 research outputs found
Bounding the Tau Neutrino Magnetic Moment from Single Photon Searches at LEP
We show that single photon searches at LEP constrain the tau neutrino
magnetic moment to be less than . This bound is
competitive with low energy () single photon searches.Comment: 5 pgs. LaTeX, one reference fixed in revised version,
JHU-TIPAC-940004, UM-TH-94-1
Recent achievements in ab initio modelling of liquid water
The application of newly developed first-principle modeling techniques to
liquid water deepens our understanding of the microscopic origins of its
unusual macroscopic properties and behaviour. Here, we review two novel ab
initio computational methods: second-generation Car-Parrinello molecular
dynamics and decomposition analysis based on absolutely localized molecular
orbitals. We show that these two methods in combination not only enable ab
initio molecular dynamics simulations on previously inaccessible time and
length scales, but also provide unprecedented insights into the nature of
hydrogen bonding between water molecules. We discuss recent applications of
these methods to water clusters and bulk water.Comment: 23 pages, 17 figure
Electronic signature of the instantaneous asymmetry in the first coordination shell of liquid water
Interpretation of the X-ray spectra of water as evidence for its asymmetric
structure has challenged the conventional symmetric nearly-tetrahedral model
and initiated an intense debate about the order and symmetry of the hydrogen
bond network in water. Here, we present new insights into the nature of local
interactions in water obtained using a novel energy decomposition method. Our
simulations reveal that while a water molecule forms, on average, two strong
donor and two strong acceptor bonds, there is a significant asymmetry in the
energy of these contacts. We demonstrate that this asymmetry is a result of
small instantaneous distortions of hydrogen bonds, which appear as fluctuations
on a timescale of hundreds of femtoseconds around the average symmetric
structure. Furthermore, we show that the distinct features of the X-ray
absorption spectra originate from molecules with high instantaneous asymmetry.
Our findings have important implications as they help reconcile the symmetric
and asymmetric views on the structure of water.Comment: Accepted by Nature Commu
SU(3) Family Gauge Symmetry and the Axion
We analyze the structure of a recently proposed effective field theory (EFT)
for the generation of quark and lepton mass ratios and mixing angles, based on
the spontaneous breaking of an SU(3) family gauge symmetry at a high scale F.
We classify the Yukawa operators necessary to seed the masses, making use of
the continuous global symmetries that they preserve. One global U(1), in
addition to baryon number and electroweak hypercharge, remains unbroken after
the inclusion of all operators required by standard-model-fermion
phenomenology. An associated vacuum symmetry insures the vanishing of the
first-family quark and charged-lepton masses in the absence of the family gauge
interaction. If this U(1) symmetry is taken to be exact in the EFT, broken
explicitly by only the QCD-induced anomaly, and if the breaking scale F is
taken to lie in the range 10 to 9 - 10 to 12 GeV, then the associated
Nambu-Goldstone boson is a potential QCD axion.Comment: References added and clarifications in Vacuum Structure sectio
Neutrinos and SU(3) Family Gauge Symmetry
We include the standard-model (SM) leptons in a recently proposed framework
for the generation of quark mass ratios and Cabibbo-Kobayashi-Maskawa (CKM)
mixing angles from an SU(3) family gauge interaction. The set of SM-singlet
scalar fields describing the spontaneous breaking is the same as employed for
the quark sector. The imposition at tree-level of the experimentally correct
Pontecorvo-Maki-Nakagawa-Sakata (PMNS) mixing matrix, in the form of a tri-bi
maximal structure, fixes several of the otherwise free parameters and renders
the model predictive. The normal hierarchy among the neutrino masses emerges
from this scheme.Comment: 9 pages, 3 tables; a comment added to clarify the effects of
additional Yukawa operators; final version in PR
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