分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-16
摘要： The O(alpha m(t)(2)/m(w)(2)) Yukawa corrections to top quark pair production in photon-photon collisions ate calculated in the standard model (SM), the general two-Higgs-doublet model (2HDM), as well as the minimal supersymmetric model (MSSM). We found that the correction to the cross section can only reach a few percent in the SM, but can be quite significant (> 10%) in the 2HDM and MSSM for favorable parameter values, which may be observable at high energy e(+)e(-) colliders.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-16
摘要： We list all possible dimension-six CP-conserving SUc(3)xSU(L)(2)xU(Y)(1)-invariant operators involving the third-family quarks which could be generated by new physics at a higher scale. Expressions for these operators after electroweak gauge symmetry breaking and the induced effective couplings Wt(b) over bar, Xb(b) over bar, and Xt(t) over bar (X = Z, gamma,g, H) are presented. Analytic expressions for the tree level contributions of all these operators to the observables R-b and A(FB)(b) at CERN LEP I, sigma(e(+)e(-) --> ) and A(FB)(b) at LEP II, sigma(e(+)e(-) --> ) and A(FB)(t) at the Next Linear Collider (NLC). as well as sigma( --> + X) at the Fermilab Tevatron upgrade, are provided. The effects of these operators on different electroweak observables are discussed and numerical examples presented. Numerical analyses show that in the coupling region allowed by R-b and A(FB)(b) at LEP I, some of the new physics operators can still have significant contributions at LEP II, the Tevatron, and the NLC.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-15
摘要： We examine the possibility of observing exotic top quark decays via R-parity violating SUSY interactions in e(+)e(-) collisions at roots = 500 GeV. We present cross-sections for tt production followed by the subsequent decay of either the t or (t) over bar via the R-parity violating interaction while the other undergoes the SM decay. We discuss kinematic cuts that allow the exotic SUSY decays to be detected over standard model backgrounds. Discovery limits for R-parity violating couplings in the top sector are presented assuming an integrated luminosity of 100 fb(-1). (C) 2001 Published by Elsevier Science B.V.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-15
摘要： In the minimal supersymmetric standard model the R-parity violating interactions can induce anomalous top pair productions at the LHC through the t-channel process d(R)(R)((d) over bar) -> t(L)(L)((t) over bar) by exchanging a slepton or by the u-channel process d(R)(R)((d) over bar) -> t(R)(R)(<(t)over bar) exchanging a squark. Such top pair productions with a certain chirality cause top-quark polarization in the top pair events. We found that at the LHC, due to the large statistics, the statistical significance of the polarization observable, and thus the probing ability for the corresponding R-parity violating couplings, is much higher than at the Tevatron upgrade.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-16
摘要： We calculate the one-loop QCD corrections to t-->(t) over tilde(1)(0)(j) using the dimensional reduction scheme, including QCD and supersymmetric QCD corrections. The analytic expressions for the corrections to the decay width are given, which can easily be extended to t-->(+)(j)(b) over tilde(i). The numerical results show that the correction amounts to more than a 10% reduction in the partial width relative to the tree level result. We also compare the corrections in the no-mixing top squark case with those in the mixing top squark case.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-15
摘要： In split supersymmetry, gauginos and higgsinos are the only supersymmetric particles possibly accessible at foreseeable colliders like the CERN Large Hadron Collider (LHC) and the International Linear Collider (ILC). In order to account for the cosmic dark matter measured by WMAP, these gauginos and higgsinos are stringently constrained and could be explored at the colliders through their direct productions and/or virtual effects in some processes. The clean environment and high luminosity of the ILC render the virtual effects at percent level meaningful in unraveling the new physics effects. In this work we assume split supersymmetry and calculate the virtual effects of the WMAP-allowed gauginos and higgsinos in the Higgs productions e(+) e(-) -> Zh and e(+) e(-) ->nu(e)(nu) over bar (e)h through WW fusion at the ILC. We find that the production cross section of e+ e-. Zh can be altered by a few percent in some part of the WMAP-allowed parameter space, while the correction to the WW fusion process e(+) e(-) ->nu(e) (nu) over bar (e)h is below 1%. Such virtual effects are correlated with the cross sections of chargino pair productions and can offer complementary information in probing split supersymmetry at the colliders.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-15
摘要： We propose to probe the top-color-assisted technicolor (TC2) model from the top-charm associated production at the CERN LHC, which is highly suppressed in the standard model. Because of the flavor-changing couplings of the top quark with the scalars (top-pions and top-Higgs-bosons) in the TC2 model, the top-charm associated production can occur via both the s-channel and t-channel parton processes by exchanging a scalar field at the LHC. We examine these processes through Monte Carlo simulation and find that they can reach the observable level at the LHC in quite a large part of the parameter space of the TC2 model.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-16
摘要： We list all possible dimension-six CP-violating SUc(3) x SUL(2) x U-Y(1)-invariant operators involving the third-family quarks, which can be generated by new physics at a higher energy scale. The expressions of these operators after electroweak symmetry breaking and the induced effective couplings Wt (b) over bar,Xb (b) over bar, and Xt (t) over bar (X = Z,gamma,g,H) are also presented. We evaluate sample contributions of these operators to CP-odd asymmetries of transverse polarization of the top quark in single top quark production at the upgraded Fermilab Tevatron, the same polarization effect in top-quark-top-antiquark pair production at the NLC, and the CP-odd observables of momentum correlations among the top quark decay products at the NLC. The energy and luminosity sensitivity in probing this CP-violating new physics is also studied.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-15
摘要： The top quark forward-backward asymmetry A(FB)(t) measured at the Tevatron is above the standard model prediction by more than 2 sigma deviation, which might be a harbinger for new physics. In this work we examine the contribution to A(FB)(t) in two different new physics models: one is the minimal supersymmetric model without R parity which contributes to A(FB)(t) via sparticle-mediated t channel process d (d) over bar -> t (t) over bar; the other is the third-generation enhanced left-right model which contributes to A(FB)(t) via Z'-mediated t channel or s channel processes. We find that in the parameter space allowed by the t (t) over bar production rate and the t (t) over bar invariant mass distribution at the Tevatron, the left-right model can enhance A(FB)(t) to within the 2 sigma region of the Tevatron data for the major part of the parameter space, and in optimal case A(FB)(t) can reach 12% which is slightly below the 1 sigma lower bound. For the minimal supersymmetric model without R parity, only in a narrow part of the parameter space can the lambda '' couplings enhance A(FB)(t) to within the 2 sigma region while the lambda' couplings just produce negative contributions to worsen the fit.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-14
摘要： Extensions of the standard model often predict new chiral interactions for top quarks, which will contribute to top quark spin correlation and polarization in t (t) over bar production at the LHC. In this work, under the constraints from the current Tevatron measurements, a comparative study of the spin correlation and polarization is performed in three new physics models: the minimal supersymmetric model without R-parity, the third-generation enhanced left-right model, and the axigluon model. We find that the polarization asymmetry may be enhanced to the accessible level in all these models, while the correction to the spin correlation may be detectable in the axigluon model and the minimal supersymmetric model without R-parity with lambda '' couplings.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-15
摘要： The top-charm associated productions via e(+)e(-), e(-)gamma and gamma gamma collisions at linear colliders, which are extremely suppressed in the standard model ( SM), could be significantly enhanced in some extensions of the SM. In this article we calculate the full contribution of the top-color-assisted technicolor ( TC2) to these productions and then compare the results with the existing predictions of the SM, the general two-Higgs-doublet model and the minimal supersymmetric model. We find that the TC2 model predicts much larger production rates than other models and the largest-rate channel is gamma gamma -> t (c) over bar, which exceeds 10 fb for a large part of the parameter space. From the analysis of the observability of such productions at the future linear colliders, we find that the predictions of the TC2 model can reach the observable level for a large part of the parameter space while the predictions of other models are hardly accessible.
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