Qing-Hong Cao

Forward-Backward Asymmetry of Top Quark Pair Production

We adopt a Markov chain Monte Carlo method to examine various new physics models which can generate the forward-backward asymmetry in top quark pair production observed at the Tevatron by the CDF Collaboration. We study the following new physics models: (1) exotic gluon G{sup }, (2) extra Z{sup } boson with flavor-conserving interaction, (3) extra Z{sup } with flavor-violating u-t-Z{sup } interaction, (4) extra W{sup } with flavor-violating d-t-W{sup } interaction, and (5) extra scalars S and S{sup {+-}}with flavor-violating u-t-S and d-t-S{sup {+-}}interactions. After combining the forward-backward asymmetry with the measurement of the top pair production cross section and the tt invariant mass distribution at the Tevatron, we find that an axial vector exotic gluon G{sup } of mass about 1 TeV or 2 TeV or a W{sup } of mass about 2TeV offer an improvement over the standard model. The other models considered do not fit the data significantly better than the standard model. We also emphasize a few points that have been long ignored in the literature for new physics searches: (1) heavy resonance width effects, (2) renormalization scale dependence, and (3) next-to-leading order corrections to the tt invariant mass spectrum. We argue that these three effectsmorexa0» are crucial to test or exclude new physics effects in the top quark pair asymmetry.«xa0less

Dark matter: The leptonic connection

Abstract Recent observations of high-energy positrons and electrons by the PAMELA and ATIC experiments may be an indication of the annihilation of dark matter into leptons and not quarks. This leptonic connection was foreseen already some years ago in two different models of radiative neutrino mass. We discuss here the generic interactions ( ν η 0 − l η + ) χ and l c ζ − χ c which allow this to happen, where χ and/or χ c are fermionic dark-matter candidates. We point out in particular the importance of χ χ → l + l − γ to both positron and gamma-ray signals within this framework.

Observable T{sub 7} Lepton Flavor Symmetry at the Large Hadron Collider

More often than not, models of flavor symmetry rely on the use of nonrenormalizable operators (in the guise of flavons) to accomplish the phenomenologically successful tribimaximal mixing of neutrinos. We show instead how a simple renormalizable two-parameter neutrino mass model of tribimaximal mixing can be constructed with the non-Abelian discrete symmetry T{sub 7} and the gauging of B-L. This is also achieved without the addition of auxiliary symmetries and particles present in almost all other proposals. Most importantly, it is verifiable at the Large Hadron Collider.

Nonzero θ 13 for neutrino mixing in a supersymmetric B − L gauge model with T 7 lepton flavor symmetry

We discuss how �13 6 0 is accommodated in a recently proposed renormalizable model of neutrino mixing using the non-Abelian discrete symmetry T7 in the context of a supersymmetric extension of the Standard Model with gauged U(1)B−L. We predict a correlation between �13 and �23, as well as the effective neutrino mass mee in neutrinoless double beta decay.

Next-to-leading order cross sections for new heavy fermion production at hadron colliders.

We evaluate the cross sections for new heavy fermion production at three Large Hadron Collider energies accurate to next-to-leading order in perturbative quantum chromodynamics. We treat the cases of pair production of heavy quarks via strong interactions, single heavy quark production via electroweak interactions, and the production of heavy leptons. Theoretical uncertainties associated with the choice of the renormalization scale and the parton distribution functions are specified. We derive a simple and useful parametrization of our results which should facilitate phenomenological studies of new physics models that predict new heavy quarks and/or leptons.

W plus two jets from a quasi-inert Higgs doublet

We show that, the result recently reported by the CDF collaboration showing an excess in the invariant mass distribution of jet pairs produced in association with a W-boson can be explained by a simple extension of the Standard Model (SM) with an additional quasi-inert Higgs doublet. The two additional neutral Higgs states H0 and A0 have a mass of about 150xa0GeV and decay into a pair of jets. W±H0/A0 pairs are produced from the decay of the heavier charged Higgs boson H±. Depending on the precise masses of the neutral and charged Higgs bosons, the model is shown to be in agreement with constraints from electroweak precision tests and from flavor physics for a broad range of the Standard Model-like Higgs mass from 100xa0GeV to several hundreds of GeV. Other possible signals of this model at the Tevatron and the LHC are discussed.

Measuring Top Quark Polarization in Top Pair plus Missing Energy Events

The polarization of a top quark can be sensitive to new physics beyond the standard model. Since the charged lepton from top-quark decay is maximally correlated with the top-quark spin, it is common to measure the polarization from the distribution in the angle between the charged lepton and the top-quark directions. We propose a novel method based on the charged lepton energy fraction and illustrate the method with a detailed simulation of top-quark pairs produced in supersymmetric top squark pair production. We show that the lepton energy ratio distribution that we define is very sensitive to the top-quark polarization but insensitive to the precise measurement of the top-quark energy.

A Dark Matter Model with Non-Abelian Gauge Symmetry

We propose a dark-matter model in which the dark sector is gauged under a new SU(2) group. The dark sector consists of SU(2) dark gauge fields, two triplet dark Higgs fields, and two dark fermion doublets (dark-matter candidates in this model). The dark sector interacts with the standard model sector through kinetic and mass mixing operators. The model explains both PAMELA and Fermi LAT data very well and also satisfies constraints from both the dark-matter relic density and standard model precision observables. The phenomenology of the model at the LHC is also explored.

Probing lepton flavor triality with Higgs boson decay

If neutrino tribimaximal mixing is explained by a non-Abelian discrete symmetry such as A{sub 4}, T{sub 7}, {Delta}(27), etc., the charged-lepton Higgs sector has a Z{sub 3} residual symmetry (lepton flavor triality), which may be observed directly in the decay chain H{sup 0}{yields}{psi}{sub 2}{sup 0}{psi}{sub 2}{sup 0}, then {psi}{sub 2}{sup 0}({psi}{sub 2}{sup 0}){yields}l{sub i}{sup +}l{sub j}{sup -}(i{ne}j), where H{sup 0} is a standard-model-like Higgs boson and {psi}{sub 2}{sup 0} is a scalar particle needed for realizing the original discrete symmetry. If kinematically allowed, this unusual and easily detectable decay is observable at the LHC with 1 fb{sup -1} for E{sub cm}=7 TeV.

Model Independent Constraints Among the Wtb, Zb anti-b, and Zt anti-t Couplings

Using an effective Lagrangian approach, we perform a model-independent analysis of the interactions among electroweak gauge bosons and the third generation quarks, i.e. the Wtb, Zt{bar t}, and Zb{bar b} couplings. After one imposes the known experimental constraint on the Zb{sub L}b{sub L} coupling, we show that the electroweak SU(2){sub L} x U(1){sub Y} symmetry of the standard model specifies the pattern of deviations of the Zt{sub L}t{sub L} and Wt{sub L}b{sub L} couplings, independent of underlying new physics scenarios. We study implications of the predicted pattern with data on the single top quark and Zt{bar t} associated production processes at the Large Hadron Collider. Such an analysis could in principle allow for a determination of the Wtb coupling without prior knowledge of |V{sub tb}|, which is otherwise difficult to achieve.