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: The topcolor-assisted technicolor (TC2) theory predicts tree-level flavor-changing neutral-current (FCNC) top quark Yukawa couplings with top-pions. Such FCNC interactions will induce like-sign top quark pair productions at CERN Large Hadron Collider (LHC). While these rare productions are far below the observable level in the Standard Model and other popular new physics models such as the Minimal Supersymmetric Model, we find that in a sound part of parameter space the TC2 model can enhance the production cross sections to several tens of fb and thus may be observable at the LHC due to rather low backgrounds. Searching for these productions at the LHC will serve as an excellent probe for the TC2 model.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-15
摘要: We examined Zb (b) over bar and Zl (l) over bar couplings in the minimal supersymmetric model with explicit trilinear R-parity violating interactions. We found that the top quark couplings lambda (i3k)' and lambda (3j3)" can give sizable contributions through top quark loops. When deriving the bounds from R-b and R-l data, we also take into account the loop contributions of R-parity conserving interactions. The bounds from R-l are found to be stronger than those from R-b and serve as the hitherto strongest bounds for some couplings.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-15
摘要: We examined Zb (b) over bar and Zl (l) over bar couplings in the minimal supersymmetric model with explicit trilinear R-parity violating interactions. We found that the top quark couplings lambda (i3k)' and lambda (3j3)" can give sizable contributions through top quark loops. When deriving the bounds from R-b and R-l data, we also take into account the loop contributions of R-parity conserving interactions. The bounds from R-l are found to be stronger than those from R-b and serve as the hitherto strongest bounds for some 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 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: 2016-05-15
摘要: We examine various direct and indirect constraints on the lepton-specific two-Higgs-doublet model and scrutinize the property of the Higgs bosons in the allowed parameter space. We find that in the allowed parameter space the CP-odd Higgs boson A is rather light (m(A) AA -> 4 tau with a large decay width, which will make the Higgs discovery more difficult at the LHC, whereas this scenario predicts a branching ratio Br(Z -> tau(+) tau(-) A) ranging from 10(-5) to 10(-4), which may be accessible at the GigaZ option of the International Linear Collider.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-15
摘要: Since the top quark FCNC processes are extremely supressed in the Standard Model (SM) but could be greatly enhanced in some new physics models, they could serve as a smoking gun for new physics hunting at the LHC. In this brief review we summarize the new physics predictions for various top quark FCNC processes at the LHC by focusing on two typical models: the minimal supersymmetric model (MSSM) and the topcolor-assisted technicol or (TC2) model. The conclusion is: (1) Both new physics models can greatly enhance the SM predictions by several orders; (2) The TC2 model allows for largest enhancement, and for each channel the maximal prediction is much larger than in the MSSM; (3) Compared with the 3 sigma sensitivity at the LHC, only a couple of channels are accessible for the MSSM while most channles are accesible for the TC2 model.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-15
摘要: This paper briefly reviews the lepton flavor violating Z-decays at GigaZ as a probe of supersymmetry by focusing on Z -> l(i)(l) over bar (j) in two representative supersymmetric models: the minimal supersymmetric model without R-parity and the supersymmetric seesaw model. The work concludes that under the current experimental constraints from LEP and l(i) -> l(j)gamma, these rare decays can still be enhanced to reach the sensitivity of the GigaZ. Therefore, supersymmetry can be probed via these decays at GigaZ.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-15
摘要: We briefly review the supersymmetric explanation for the cosmic dark matter. Although the neutralino in the minimal supersymmetric model (MSSM), the next-to-minimal supersymmetric model (NMSSM) and the nearly minimal supersymmetric model (nMSSM) can naturally explain the dark matter relic density, the PAMELA result can hardly be explained in these popular models. In the general singlet extension of the MSSM,both the PAMELA result and the relic density can be explained by the singlino-like neutralino. Such singlino-like neutralinos annihilate into the singlet-like Higgs bosons, which are light enough to decay dominantly to muons or electrons, and the annihilation cross section can be greatly enhanced by the Sommerfeld effect via exchanging alight CP-even singlet-like Higgs boson. In this scenario, in order to meet the stringent LEP constraints, the SM-like Higgs boson tends to decay into the singlet Higgs pairs instead of b (b) over bar and consequently it will give a multi-muon signal h(SM) -> aa -> 4 mu or h(SM) -> hh -> 4a -> 8 mu at the LHC.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-14
摘要: For the experimental search of neutralino dark matter, it is important to know its allowed mass and scattering cross section with the nucleon. In order to figure out how light a neutralino dark matter can be predicted in low energy supersymmetry, we scan over the parameter space of the NMSSM (next-to-minimal supersymmetric model), assuming all the relevant soft mass parameters to be below TeV scale. We find that in the parameter space allowed by current experiments the neutralino dark matter can be as light as a few GeV and its scattering rate off the nucleon can reach the sensitivity of XENON100 and CoGeNT. As a result, a sizable parameter space is excluded by the current XENON100 and CoGeNT data (the plausible CoGeNT dark matter signal can also be explained). The future 6000 kg-days exposure of XENON100 will further explore (but cannot completely cover) the remained parameter space. Moreover, we find that in such a light dark matter scenario a light CP-even or CP-odd Higgs boson must be present to satisfy the measured dark matter relic density. Consequently, the SM-like Higgs boson h(SM) may decay predominantly into a pair of light Higgs bosons or a pair of neutralinos so that the conventional decays like h(SM) -> gamma gamma is much suppressed. (C) 2011 Elsevier B.V. All rights reserved.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-14
摘要: We examine the present and future XENON limits on the neutralino dark matter in split supersymmetry (split-SUSY). Through a scan over the parameter space under the current constraints from collider experiments and the WMAP measurement of the dark matter relic density, we find that in the allowed parameter space a large part has been excluded by the present XENON100 limits and a further largish part can be covered by the future exposure (6000 kg day). In case of unobservation of dark matter with such an exposure in the future, the lightest neutralino will remain bino-like and its annihilation is mainly through exchanging the SM-like Higgs boson in order to get the required relic density. (C) 2011 Elsevier B.V. All rights reserved.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-14
摘要: We consider the natural supersymmetry scenario in the framework of the R-parity conserving minimal supersymmetric standard model (called natural MSSM) and examine the observability of stop pair production at the LHC. We first scan the parameters of this scenario under various experimental constraints, including the SM-like Higgs boson mass, the indirect limits from precision electroweak data and B-decays. Then in the allowed parameter space we study the stop pair production at the LHC followed by the stop decay into a top quark plus a lightest neutralino or into a bottom quark plus a chargino. From detailed Monte Carlo simulations of the signals and backgrounds, we find the two decay modes are complementary to each other in probing the stop pair production, and the LHC with root s = 14 TeV and 100 fb(-1) luminosity is capable of discovering the stop predicted in natural MSSM up to 450 GeV. If no excess events were observed at the LHC, the 95% C.L. exclusion limits of the stop masses can reach around 537 GeV.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-14
摘要: Inspired by the recent LHC Higgs data and null search results of supersymmetry (SUSY), we scan the parameter space of the Minimal Supersymmetric Standard Model (MSSM) with relatively heavy sparticles (1-3 TeV). Then in the parameter space allowed by current collider experiments and dark matter detections, we calculate the complete one) SUSY QCD corrections to pp -> b (b) over barh at the LHC with root s = 14 TeV and obtain the following observations: (1) For the large values of tan beta and low values of m(A), the SUSY QCD effects can be quite large, which, however, have been excluded by the latest results of LHC search for H/A -> tau(+)tau(-); (ii) For modest values of tan beta and m(A) which so far survived all experimental constraints, the SUSY QCD corrections can maximally reach about -9%.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-09
摘要: In this paper, we interpret the 750 GeV diphoton excess in the Zee-Babu extension of the two-Higgs-doublet model by introducing a top partner (T)/bottom partner (B). In the alignment limit, the 750 GeV resonance is identified as the heavy CP-even Higgs boson (H), which can be sizably produced via the QCD process pp -> T (T) over bar or pp -> B (B) over bar followed by the decay T -> Ht or B -> Hb. The diphoton decay rate of His greatly enhanced by the charged singlet scalars predicted in the Zee-Babu extension and the total width of H can be as large as 7 GeV. Under the current LHC constraints, we scan the parameter space and find that such an extension can account for the observed diphoton excess. (C) 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-09
摘要: In the simplest Higgs-portal scalar dark matter model, the dark matter mass has been restricted to be either near the resonant mass (m(h)/2) or in a large-mass region by the direct detection at LHC Run 1 and LUX. While the large-mass region below roughly 3 TeV can be probed by the future Xenon1T experiment, most of the resonant mass region is beyond the scope of Xenon1T. In this paper, we study the direct detection of such scalar dark matter in the narrow resonant mass region at the 14 TeV LHC and the future 100 TeV hadron collider. We show the luminosities required for the 2 sigma exclusion and 5 sigma discovery. (C) 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license.