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 $Higgs$ ($LRTH$) model is a concrete realization of the twin $Higgs$ mechanism, which predicts the existence of the top partner $T$. In this paper, we consider production of $T$ associated with the top quark $t$ at the high energy linear $e^{+}e^{-}$ collider ($ILC$) and the $LHC$ experiments, and its single production in future linac-ring type $ep$ collider experiment. To compare our results with those of the littlest $Higgs$ model with $T$-parity, we also estimate production of the $T$-even top partner $T_{+}$ 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 ttˉγt\bar{t}\gamma 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 $e^{+}e^{-} \to t\bar{t}\gamma$ at the International Linear Collider (ILC), and find that the $K$-factor can be as large as 1.238 (1.105, 1.060) for a center-of-mass energy $\sqrt{s}=500 (800, 1500)$ 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 ($A^{t}_{FB}$) in $t\bar{t}\gamma$ production at NLO, which is found to be 45.82 (55.25, 55.89)% for $\sqrt{s}=500 (800, 1500)$ 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

O(αs){\cal O}(\alpha_{s}) QCD and O(αew){\cal O}(\alpha_{ew}) electroweak corrections to ttˉh0t\bar{t}h^0 production in γγ\gamma \gamma collision

We calculate the ${\cal O}(\alpha_{s})$ QCD and ${\cal O}(\alpha_{{\rm ew}})$ electroweak one-loop corrections in the Standard Model framework, to the production of an intermediate Higgs boson associated with $t\bar{t}$ pair via $\gamma \gamma$ fusion at an electron-positron linear collider (LC). We find the ${\cal O}(\alpha_{s})$ QCD corrections can be larger than the ${\cal O}(\alpha_{{\rm ew}})$ electroweak ones, with the variations of the Higgs boson mass $m_{h}$ and $e^+e^-$ colliding energy $\sqrt{s}$. 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 $\sqrt{s}=800$ GeV and $m_h=200$ GeV, while those from electroweak can be -13.1%, -15.8% and -12.0%, at $\sqrt{s} = 800$ GeV, 1 TeV and 2 TeV respectively.Comment: 38 pages, 16 figure