4,926 research outputs found
Semileptonic decays of polarised top quarks: V + A admixture and QCD corrections
The semileptonic decays of polarised top quarks are analysed for a general
chirality-conserving tbW vertex. We calculate double differential distributions
for the charged lepton and the neutrino to order \alpha_s in the QCD coupling.
We present these QCD corrections in terms of compact parameterisations that
should be useful for the future investigation of the structure of the top decay
vertex on the basis of large data samples.Comment: 7 pages, 2 figures, Late
Emergence of structural and dynamical properties of ecological mutualistic networks
Mutualistic networks are formed when the interactions between two classes of
species are mutually beneficial. They are important examples of cooperation
shaped by evolution. Mutualism between animals and plants plays a key role in
the organization of ecological communities. Such networks in ecology have
generically evolved a nested architecture independent of species composition
and latitude - specialists interact with proper subsets of the nodes with whom
generalists interact. Despite sustained efforts to explain observed network
structure on the basis of community-level stability or persistence, such
correlative studies have reached minimal consensus. Here we demonstrate that
nested interaction networks could emerge as a consequence of an optimization
principle aimed at maximizing the species abundance in mutualistic communities.
Using analytical and numerical approaches, we show that because of the
mutualistic interactions, an increase in abundance of a given species results
in a corresponding increase in the total number of individuals in the
community, as also the nestedness of the interaction matrix. Indeed, the
species abundances and the nestedness of the interaction matrix are correlated
by an amount that depends on the strength of the mutualistic interactions.
Nestedness and the observed spontaneous emergence of generalist and specialist
species occur for several dynamical implementations of the variational
principle under stationary conditions. Optimized networks, while remaining
stable, tend to be less resilient than their counterparts with randomly
assigned interactions. In particular, we analytically show that the abundance
of the rarest species is directly linked to the resilience of the community.
Our work provides a unifying framework for studying the emergent structural and
dynamical properties of ecological mutualistic networks.Comment: 10 pages, 4 figure
A particle-hole model approach for hypernuclei
A particle-hole model is developed to describe the excitation spectrum of
single lambda hypernuclei and the possible presence of collective effects is
explored by making a comparison with the mean-field calculations. Results for
the spectra of 12C, 16O, 40Ca, 90Zr and 208Pb single lambda hypernuclei are
shown. The comparison with the available experimental data is satisfactory. We
find that collective phenomena are much less important in hypernuclei than in
ordinary nuclei.Comment: 24 pages, 5 eps figures, accepted for publication in Nucl. Phys.
Phase transitions in Ising magnetic films and superlattices
Within the framework of mean field theory, we examine the phase transitions
in Ising magnetic films and superlattices. By transfer matrix method, we derive
two general nonlinear equations for phase transition temperatures of Ising
magnetic films and superlattices, respectively. The equations can be applied to
the films and superlattices with arbitrary exchange interaction constants and
arbitrary layer number. Numerical results for phase transition temperatures as
a function of exchange interaction constants are presented.Comment: 10 pages, 3 figures, accepted by Solid State Communication
Deviations from Tribimaximal Neutrino Mixing
Current neutrino data are consistent with the so-called tribimaximal mixing
scenario, which predicts \sin^2 \theta_{12}=1/3, zero U_{e3} and maximal
\theta_{23}. This implies a special form of the neutrino mass matrix.
Introducing small breaking terms in this mass matrix generates deviations from
the tribimaximal scheme and leads to testable correlations between the
parameters. They depend on where the perturbation is located in the mass
matrix. A special case of such perturbations are radiative corrections.
Alternative deviations from tribimaximal mixing may stem from contributions of
the charged lepton sector. If there is quark-lepton-unification and it is the
CKM matrix which corrects the tribimaximal mixing scheme, then almost maximal
CP violation and sizable deviation from zero U_{e3} are implied.Comment: 17 pages, 4 figures; to appear in PL
Single production of the top partners at high energy colliders
The left-right twin () model is a concrete realization of the
twin mechanism, which predicts the existence of the top partner . In
this paper, we consider production of associated with the top quark at
the high energy linear collider () and the experiments,
and its single production in future linac-ring type collider experiment.
To compare our results with those of the littlest model with
-parity, we also estimate production of the -even top partner via
the corresponding processes in these high energy collider experiments. A simply
phenomenological analysis is also given.Comment: 21 pages, 10 figures; to be published in Nucl. Phys.
QCD corrections to the production of at the ILC
A precise calculation of the top quark pair production associated with a hard
photon is essential for testing the electroweak property of the top quark in
the Standard Model (SM). We investigate the one-loop QCD corrections to the
process at the International Linear Collider
(ILC), and find that the -factor can be as large as 1.238 (1.105, 1.060) for
a center-of-mass energy GeV. The transverse momentum
distributions of the top quark and photon are respectively shown at leading
order (LO) and next-to-leading order(NLO). Due to the asymmetric rapidity
distribution of the top (anti-top) quark, we also study the top quark
forward-backward asymmetry () in production at
NLO, which is found to be 45.82 (55.25, 55.89)% for
GeV.Comment: 12 pages, 5 figures, version acceptd by Phys. Lett.
Neutrino Masses, Dark Matter and B-L Symmetry at the LHC
We establish a hybrid seesaw mechanism to explain small neutrino masses and
predict cold dark matter candidate in the context of the B-L gauge symmetry
extension of the Standard Model. In this model a new scalar doublet and two new
fermion singlets are introduced at loop-level beyond the minimal Type I seesaw.
The lightest particle inside the loop can be dark matter candidate. We study in
detail the constraints from neutrino oscillation data, lepton flavor violating
processes and cosmological observation. We also explore the predictions of the
decays of the new charged scalars in each spectrum of neutrino masses and show
the most optimistic scenarios to distinguish the spectra. We consider the pair
production of the stable fermion associated with two observable SM charged
leptons at the LHC, which occurs in a two-step cascade decay of the new gauge
boson Z' and the new charged scalars stand as intermediate particles. The
masses of missing dark matter and its parent particle can be well-determined in
such production topology.Comment: 20 pages, 9 figures and 2 tables, revised version accepted for
publication in Nucl. Phys.
Leptogenesis with SU(5)-inspired mass matrices
In the baryogenesis via leptogenesis framework the baryonic asymmetry depends
on lepton mass matrices. In a previous paper we used SO(10)-inspired mass
matrices and we found few possibilities to obtain a sufficient level of
asymmetry. In the present paper we use SU(5)-inspired mass matrices, which also
allow to check the dependence of the baryonic asymmetry on Dirac neutrino
masses. In particular, we find that the large mixing matter solution to the
solar neutrino problem, which within SO(10) gives too small asymmetry, can now
be favoured.Comment: RevTex, 14 pages with 4 figure
QCD and electroweak corrections to production in collision
We calculate the QCD and
electroweak one-loop corrections in the Standard Model framework, to the
production of an intermediate Higgs boson associated with pair via
fusion at an electron-positron linear collider (LC). We find
the QCD corrections can be larger than the electroweak ones, with the variations of the Higgs boson
mass and colliding energy . Both corrections may
significantly decrease or increase the Born cross section. The numerical
results show that the relative corrections from QCD to the process \eep may
reach 34.8%, when GeV and GeV, while those from
electroweak can be -13.1%, -15.8% and -12.0%, at GeV, 1 TeV
and 2 TeV respectively.Comment: 38 pages, 16 figure
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