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X. Zhang

Publications and source records attributed to X. Zhang.

378 records · Page 21Linked to original sources
arXiv1996

Cosmic Strings in Theories With Dynamical Symmetry Breaking

We study the classical cosmic string solution in a theory with dynamical $U(1)$ symmetry breaking. We calculate the energy per unit length of the string and compare it to that obtained in a model with a fundamental Higgs. We find that the predictions of the standard Abelian Higgs model are quite stable towards the addition of higher dimension operators expected in the effective Lagrangian for the order parameter in a model with dynamical symmetry breaking, at least for small coupling constant $λ$.

hep-ph↗
arXiv1996

Non-Universal Correction to $Z \to b {\overline b}$ and Single Top Quark Production at Tevatron

New physics associated with the heavy top quark can affect top quark production and the partial decay width of $Z \rightarrow b \bar b$. In this paper, we examine the correlated effects of possible new physics on ${R_b}$ measured at LEP I and the single top quark production rate at Tevatron by using an effective lagrangian technique. We point out that certain operators in the effective lagrangian, constrained by the measured value of $R_b$, can lead to significant and potentially observable effects in single top production.

hep-ph↗
arXiv1996

Implications Of A Non-Standard Light Higgs Boson

Analyses of the vacuum stability of the electroweak theory indicate that new physics occur at a scale of order of 1 TeV if a light Higgs is discovered at LEP II. In this paper, we parameterize the effects of new physics in the effective Lagrangian approach and examine its implication on the Higgs boson production at LEP II. We consider the effect of higher dimension operator on the Higgs potential and calculate the lower bound on the Higgs boson mass from the requirement of vacuum stability. We show that if a Higgs boson is seen at LEP II then under favourable conditions the deviation of the production cross section from the standard model value could be significant and therefore the presence of the new physics is detectable at LEP II.

hep-ph↗
arXiv1996

Searching for $t \to c g$ at the Fermilab Tevatron

We examine the experimental observability of the decay mode $t \rightarrow c g$ at the Fermilab Tevatron via the flavor-changing neutral current vertex $t \bar c g$. We find that with the existing data, one should be able to probe the $t \bar c g$ coupling to a value smaller than indirect limits previously obtained from $b\rightarrow s γ$ and the measured branching fraction for $t\rightarrow bW$, reaching $BF(t \rightarrow cg) \sim$ 15\% - 28\%. A data sample of 1~fb$^{-1}$ (10~fb$^{-1}$) at $\sqrt{s}=2$~TeV may probe the $t \rightarrow c g$ branching fraction to a level of 2\% (0.5\%).

hep-ph↗
arXiv1996

Top-Quark Decay Via the Anomalous Coupling $\bar t c γ$ at Hadron Colliders

We determine the constraints on the anomalous top-quark coupling associated with the flavor-changing neutral current vertex $\bar t c γ$ from the limits on the $b$-quark rare decay $b\rightarrow s γ$ and non-standard top-quark decays. Based on these constraints, we discuss the experimental observability of the rare decay mode $t \rightarrow c γ$, both at the Fermilab Tevatron with a luminosity-upgrade and at the LHC.

hep-ph↗
arXiv1995

Top-Quark Decay Via Flavor-Changing Neutral Currents at Hadron Colliders

We study low energy experimental constraints on an anomalous top-quark coupling associated with the flavor-changing neutral current vertex $Z \bar t c$. In view of these constraints, we discuss the experimental observability of the induced rare decay mode $t \rightarrow Zc$, both at the Fermilab Tevatron (with the Main Injector or a luminosity-upgrade) and at the LHC.

hep-ph↗
arXiv1995

Non-Universal Correction To $Z \to b {\bar{b}}$ And Flavor Changing Neutral Current Couplings

A non-universal interaction associated with top quark induces flavor changing neutral currents (FCNC) among light fermions. The size of the FCNC effect depends crucially on the dynamics of the fermion mass generation. In this paper, we study the effect of a non-universal interaction on $Z b b$, $Z b s$ {\it etc}, by using an effective lagrangian technique and assuming the quark mass matrices in the form of a generalized Fritzsch ansatz. We point out that if fitting $R_b$ to the LEP data within $1 σ$, the induced FCNC couplings are very close to the experimental limits.

hep-ph↗
arXiv1994

Unitarity And Anomalous Top-Quark Yukawa Couplings

Unitarity constraints on anomalous top-Higgs couplings are examined. We also compare the unitarity constraints with the constraints from electroweak baryogenesis and electric dipole moments derived earlier.

hep-ph↗
arXiv1994

Phenomenology of a non-standard top quark Yukawa coupling

There are theoretical speculations that the top quark may have different properties from that predicted by the standard model. We use an effective Lagrangian technique to model such a non-standard top quark scenario and study its effects on the electroweak baryogenesis, low and high energy physics.

hep-ph↗
arXiv1994

The Chiral Lagrangian Approach to the Non-universal Effects in the Zbb Vertex

We calculate the top quark loop corrections to the Zbb vertex in the chiral lagrangian of the dynamical electroweak symmetry breaking theory. We use the effective lagrangian based on coset space SU(2) X U(1) / U(1) with the gauge interactions switched off, so these corrections are directly related to the Physics associated with the heavy top quark and the mass generation in the dynamical models.

hep-ph↗
arXiv1994

An Automatic Invisible Axion In The SUSY Preon Model

It is shown that the recently proposed preon model which provides a unified origin of the diverse mass scale and an explanation of family replication as well as of inter-family mass-hierarchy, possesses a Peccei-Quinn symmetry whose spontaneous breaking leads to an automatic invisible axion. Existence of the PQ-symmetry is simply a consequence of supersymmetry and requirement of minimality in the field-content and interactions, which propose that the lagrangian should possess only those terms which are dictated by the gauge principle and no others. In addition to the axion, the model also generates two superlight Goldstone bosons and their superpartners which are cosmologically safe.

hep-ph↗
arXiv1993

Effects of $e^- e^+ ν_e$ Decays of Tau Neutrinos Near A Supernova

We revisit the constraints implied by SN 1987 A observations on the decay rate of a multi-MeV $ν_τ$ decaying into the visible channel $ν_τ\rightarrow e^+ e^- ν_e$, if its lifetime is more than 10 {\it sec.}. We discuss its implication for the minimal left-right symmetric model with see-saw mechanism for neutrino masses. We also speculate on the possible formation of a ``giant Capacitor" in intergalactic space due to the decay of "neutronization" $ν_τ$'s and spin allignment possibility in the supernova.

hep-ph↗
arXiv1993

Effective Lagrangian Approach to Electroweak Baryogenesis: Higgs mass limit and Electric dipole moments of fermion

A natural solution to the hierachy problem of the standard model is to assume new physics to appear at the TeV scale. We parametrize the effects of this new physics in terms of an effective lagrangian and examine its impacts on electroweak baryogenesis. We point out that with such an effective lagrangian successful electroweak baryogenesis implies: i) Higgs boson lies within the reach of LEP II; ii)electric dipole moments of electron and neutron are detectable in the near future.

hep-ph↗
arXiv1993

Lepton Flavor Violation and the Tau Neutrino Mass

We point out that, in the left-right symmetric model of weak interaction, if $ν_τ$ mass is in the keV to MeV range, there is a strong correlation between rare decays such as $τ\rightarrow 3 μ, τ\rightarrow 3 e$ and the $ν_τ$ mass. In particular, we point out that a large range of $ν_τ$ masses are forbidden by the cosmological constraints on $m_{ν_τ}$ in combination with the present upper limits on these processes.

hep-ph↗
arXiv1993

Cosmological Constraint on the Scale of the Supersymmetric Singlet Majoron

In a SUSY theory with a spontaneously broken global symmetry, G, if the scale of SUSY breaking, $M_s$ is smaller than the scale $M_G$ of the global symmetry, the Nambu-Goldstone boson, $χ$ is accompanied by two massive superpartners ( a fermionic, $Ψ_χ$ and a scalar $σ_χ$) with mass of order $M_s$. Cosmological considerations imply stringent constraints on the couplings of $Ψ_χ$ and $σ_χ$. Application of these considerations to the supersymmetric singlet Majoron model leads to an upper limit on the scale $V_{BL}$ of global $U(1)_{B-L}$ symmetry to be less than $10^4 GeV$, for resonable values of parameters in the theory.

hep-ph↗
arXiv1993

Operator Analysis for the Higgs Potential and Cosmological Bound on the Higgs-Boson Mass

Using effective lagrangian, we examine the impacts of new physics on the electroweak baryogenesis. By analysing the high dimensional operators relevent to the Higgs potential we point out that the Higgs mass bound required by electroweak baryogenesis can be relaxed to the region allowed by experiments, provided that new physics appears at the TeV scale.

hep-ph↗