2,801 research outputs found
Masses and semileptonic decays of doubly heavy baryons in a nonrelativistic quark model
We evaluate masses and semileptonic decay widths for the ground state of
doubly heavy and baryons in the framework of a nonrelativistic
quark model. We solve the three-body problem by means of a variational ansatz
made possible by heavy-quark spin symmetry constraints. Our masses are
comparable to the ones obtained in relativistic calculations and we get one of
the best agreements with lattice data. Our simple wave functions are used to
evaluate semileptonic decays of doubly heavy and baryons. Our results for the decay widths are in reasonable
agreement with calculations done in a relativistic calculation in the
quark-diquark approximation. We also check that our wave functions comply with
what it is expected in the infinite heavy quark mass limit.Comment: 3 latex pages, two tables. To appear in the INPC07 proceeding
Has a Higgs-flavon with a GeV mass been detected at the LHC13?
Higgs-flavon fields appear as a part of the Froggatt-Nielsen (FN) mechanism,
which attempts to explain the hierarchy of Yukawa couplings. We explore the
possibility that the 750 GeV diphoton resonance recently reported at the LHC13,
could be identified with a low-scale Higgs-flavon field and find the
region of the parameter space consistent with CMS and ATLAS data. It is found
that the extra vector-like fermions of the ultraviolet completion of the FN
mechanism are necessary in order to reproduce the observed signal. We consider
a standard model (SM) extension that contains two Higgs doublets (a standard
one and an inert one) and one complex FN singlet. The inert doublet includes a
stable neutral boson, which provides a viable dark matter candidate, while the
mixing of the standard doublet and the FN singlet induces flavor violation in
the Higgs sector at the tree-level. Constraints on the parameters of the model
are derived from the LHC Higgs data, which include the search for the lepton
flavor violating decay of the SM Higgs boson . It is also
found that in some region of the parameter space the model may give rise to a
large branching ratio for the decay, of the order of 0.1, which
could be searched for at the LHC.Comment: 18 pages, 7 Figures, includes updated files to match published
versio
Seasonal Growth Curves of Perennial Ryegrass in Mexico
An experiment was established at Colegio de Postgraduados Research Station, Texcoco, México to evaluate the seasonal pattern of growth curve of perennial ryegrass (Lolium perenne) swards. Growth rate increases rapidly from a low level immediately after defoliation, and eventually reaches an equilibrium level as the amount of green leaf in the sward stabilize. The time required to reach this state varied according to the season of year, with the four seasons ranked in the order: spring (4 weeks) \u3c summer (5 weeks) \u3c autumn and winter (6 weeks). Once swards reached equilibrium, growth rates started to decline as a consequence of an increase in dead material and pseudostem. These increases were highest in spring and lowest in winter. The results of this study suggest that sward management to maximise herbage production and utilisation in ryegrass swards must be done once the highest green leaf mass is reached. In this trial, it was reached at 5, 6, 6 and 4 weeks in summer, autumn, winter and spring, respectively
Rare top quark decay t --> u_1 \bar{u}_2 u_2 in the standard model
The one-loop induced top quark decay () is
calculated in the context of the standard model. The dominant contribution to
this top quark decay arises from the Feynman diagrams induced by the off-shell
vertex, whereas the box diagrams are negligibly small. In contrast
with the on-shell vertex, which only gives rise to a pure dipolar
effect, the off-shell coupling also involves a monopolar term, which
gives a larger contribution than the dipolar one. It is found that the
branching ratio for the three-body decay is about of the
same order of magnitude of the two-body decay , the latter being
slightly larger, which stems from the fact that the three-body decay is
dominated by a monopolar term.Comment: 8 pages, 3 figures, branching ratio is evaluated using the running
quark masses at the top quark mass scale, results and conclusion changes
slightly, new results agree with those presented in hep-ph/0601151, missing
references are adde
Study of the leptonic decays of pseudoscalar and vector mesons and of the semileptonic and decays
We present results for different observables in weak decays of pseudoscalar
and vector mesons with a heavy or quark. The calculations are done in a
nonrelativistic constituent quark model improved at some instances by heavy
quark effective theory constraints. We determine pseudoscalar and vector meson
decay constants that within a few per cent satisfy , a
result expected in heavy quark symmetry when the heavy quark masses tend to
infinity. We also analyze the semileptonic and decays for
which we evaluate the different form factors. Here we impose heavy quark
effective theory constraints among form factors that are not satisfied by a
direct quark model calculation. The value of the form factors at zero recoil
allows us to determine, by comparison with experimental data, the value of the
Cabbibo-Kobayashi-Maskawa matrix element. From the
semileptonic decay we get in perfect agreement with
our previous determination based on the study of the semileptonic decay and also in excellent agreement with a recent experimental
determination by the DELPHI Collaboration. We further make use of the partial
conservation of axial current hypothesis to determine the strong coupling
constants and . The
ratio agrees with the heavy quark symmetry
prediction of 1.Comment: 19 Latex pages,6 figures, references added, corrected typos, content
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Shared binding sites for the Bacillus thuringiensis proteins Cry3Bb, Cry3Ca and Cry7Aa in the African sweetpotato pest Cylas puncticollis (Brentidae).
CP-odd static electromagnetic properties of the W gauge boson and the t quark via the anomalous tbW coupling
In the framework of the electroweak chiral Lagrangian, the one-loop induced
effects of the anomalous coupling, which includes both left- and
right-handed complex components, on the static electromagnetic properties of
the boson and the quark are studied. The attention is focused mainly on
the CP-violating electromagnetic properties. It is found that the
anomalous coupling can induce both CP-violating moments of the boson,
namely, its electric dipole () and magnetic quadrupole
() moments. As far as the quark is concerned, a potentially
large electric dipole moment can arise due to the anomalous
coupling. The most recent bounds on the left- and right-handed parameters from
meson physics lead to the following estimates e-cm and e-cm, which
are 7 and 14 orders of magnitude larger than the standard model (SM)
predictions, whereas may be as large as e-cm, which is about 8
orders of magnitude larger than its SM counterpart.Comment: This paper has been merged with hep-ph/0612171 for publication in
Physical Review
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