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： In the next-to-minimal supersymmetric model (NMSSM) a light CP-odd Higgs boson is so far allowed by current experiments, which, together with a large tan beta, may greatly enhance the rare dileptonic decays B -> X(s)l(+)l(-) and B(s)-> l(+)l(-)gamma. We examine these decays paying special attention to the new operator allowed by the light CP-odd Higgs boson. We find that in the parameter space allowed by current experiments like CERN LEP II and b -> s gamma, the branching ratios of these rare decays can be greatly enhanced, and thus the existing experimental data on B -> X(s)mu(+)mu(-) can further stringently constrain the parameter space (especially the region with a superlight CP-odd Higgs boson and large tan beta). In the surviving parameter space we give the predictions for other dileptonic decay branching ratios and also show the results for the forward-backward asymmetry.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-14
摘要： Various new physics models predict a light CP-odd Higgs boson (labeled as a) and open up new decay modes for Z-boson, such as Z -> (f) over bar fa, Z -> a gamma and Z -> aaa, which could be explored at the GigaZ option of the ILC. In this work we investigate these rare decays in several new physics models, namely the type-II two Higgs doublet model (type-II 2HDM), the lepton-specific two Higgs doublet model (L2HDM), the nearly minimal supersymetric standard model (nMSSM) and the next-to-minimal supersymmetric standard model (NMSSM). We find that in the parameter space allowed by current experiments, the branching ratios can reach 10(-4) for Z -> (f) over bar fa (f = b,tau), 10(-9) for Z -> a gamma and 10(-3) for Z -> aaa, which implies that the decays Z -> (f) over bar fa and Z -> aaa may be accessible at the GigaZ option. Moreover, since different models predict different patterns of the branching ratios, the measurement of these rare decays at the GigaZ may be utilized to distinguish the models
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-14
摘要： As the most important discovery channel for a light Higgs boson at the LHC the di-photon signal gg --> h --> gamma gamma is sensitive to underlying physics. In this work we investigate such a signal in a comparative way by considering three different supersymmetric models, namely the minimal supersymmetric standard model (MSSM), the next-to-minimal supersymmetric standard model (NMSSM) and the nearly minimal supersymmetric standard model (nMSSM). Under the current collider and cosmological constraints we scan over the parameter space and obtain the following observation in the allowed parameter space: (i) In the nMSSM the signal rate is always suppressed; (ii) In the MSSM the signal rate is suppressed in most cases, but in a tiny corner of the parameter space it can be enhanced (maximally by a factor of 2); (iii) In the NMSSM the signal rate can be enhanced or suppressed depending on the parameter space, and the enhancement factor can be as large as 7. (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： Due to the limited statistics so far accumulated in the Higgs boson search at the LHC, the Higgs boson property has not yet been tightly constrained and it is still allowed for the Higgs boson to decay invisibly to dark matter with a sizable branching ratio. In this work, we perform a comparative study for the Higgs decay to neutralino dark matter by considering three different low energy SUSY models: the minimal supersymmetric standard model (MSSM), the next-to-minimal supersymmetric standard models (NMSSM) and the nearly minimal supersymmetric standard model (nMSSM). Under current experimental constraints at 2 sigma level (including the muon g - 2 and the dark matter relic density), we scan over the parameter space of each model. Then in the allowed parameter space we calculate the branching ratio of the SM-like Higgs decay to neutralino dark matter and examine its observability at the LHC by considering three production channels: the weak boson fusion VV -> h, the associated production with a Z-boson pp -> hZ + X or a pair of top quarks pp -> ht (t) over bar + X. We find that in the MSSM such a decay is far below the detectable level; while in both the NMSSM and nMSSM the decay branching ratio can be large enough to be observable at the LHC. We conclude that at the LHC the interplay of detecting such an invisible decay and the visible di-photon decay may allow for a discrimination of different SUSY models.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-14
摘要： Confronted with the LHC data of a Higgs boson around 125 GeV, different models of low energy SUSY show different behaviors: some are favored, some are marginally survived and some are strongly disfavored or excluded. In this note we update our previous scan over the parameter space of various low energy SUSY models by considering the latest experimental limits like the LHCb data for B-s -> mu(+)mu(-) and the XENON 100 (2012) data for dark matter-nucleon scattering. Then we confront the predicted properties of the SM-like Higgs boson in each model with the combined 7 TeV and 8 TeV Higgs search data of the LHC. For a SM-like Higgs boson around 125 GeV, we have the following observations: (i) The most favored model is the NMSSM, whose predictions about the Higgs boson can naturally (without any fine tuning) agree with the experimental data at 1 sigma level, better than the SM; (ii) The MSSM can fit the LHC data quite well but suffer from some extent of fine tuning; (iii) The nMSSM is excluded at 3 sigma level after considering all the available Higgs data; (iv) The CMSSM is quite disfavored since it is hard to give a 125 GeV Higgs boson mass and at the same time cannot enhance the di-photon signal rate.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-14
摘要： In light of the recent LHC Higgs search data, we investigate the pair production of a SM-like Higgs boson around 125 GeV in the MSSM and NMSSM. We first scan the parameter space of each model by considering various experimental constraints, and then calculate the Higgs pair production rate in the allowed parameter space. We find that in most cases the dominant contribution to the Higgs pair production comes from the gluon fusion process and the production rate can be greatly enhanced, maximally 10 times larger than the SM prediction (even for a TeV-scale stop the production rate can still be enhanced by a factor of 1.3). We also calculate the chi(2) value with the current Higgs data and find that in the most favored parameter region the production rate is enhanced by a factor of 1.45 in the MSSM, while in the NMSSM the production rate can be enhanced or suppressed (sigma(SUSY)/sigma(SM) varies from 0.7 to 2.4).
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