所有结果

分类： 物理学 >> 基本粒子与场物理学

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

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) < 30 GeV with 95% possibility), which is composed dominantly by the leptonic Higgs and decays dominantly into tau(+) tau(-); while the standard-model-like Higgs boson h (responsible largely for electroweak symmetry breaking) decays dominantly in the mode h -> 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

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. |

Top quark forward-backward asymmetry and charge asymmetry in the left-right twin Higgs model

Wang, Lei; Wu, Lei; Yang, Jin Min分类： 物理学 >> 基本粒子与场物理学

In order to explain the Tevatron anomaly of the top quark forward-backward asymmetry A(FB)(t) in the left-right twin Higgs model, we choose to give up the lightest neutral particle of (h) over cap field as a stable dark matter candidate. Then a new Yukawa interaction for (h) over cap is allowed, which can be free from the constraint of same-sign top pair production and contribute sizably to A(FB)(t). Considering the constraints from the production rates of the top pair (t (t) over bar), the top decay rates, and t (t) over bar invariant mass distribution, we find that this model with such new Yukawa interaction can explain A(FB)(t) measured at the Tevatron while satisfying the charge asymmetry A(C)(t) measured at the LHC. Moreover, this model predicts a strong correlation between A(C)(t) at the LHC and A(FB)(t) at the Tevatron, i.e., A(C)(t) increases as A(FB)(t) increases. |

Probing topcolor-assisted technicolor from top charge asymmetry and triple-top production at the LHC

Han, Chengcheng; Liu, Ning; Wu, Lei; Yang, Jin Min分类： 物理学 >> 基本粒子与场物理学

In a topcolor-assisted technicolor model (TC2) with large FCNC top quark couplings, we study its correlated contributions to the top quark forward-backward asymmetry (A(FB)) at the Tevatron, the top charge asymmetry (A(C)) and the triple-top production at the LHC. Under current constraints on the top quark from the LHC and Tevatron (such as the total and differential production rates), we scan the parameter space of such a TC2 model. We find that in the allowed parameter space the TC2 model can explain the Tevatron measured A(FB) at 2 sigma level, but meanwhile significantly enhance A(C) at the LHC. Such enhanced A(C), albeit currently allowed by the LHC measurement at 2 sigma level, will serve as a test of TC2 with the improvement of measurement precision at the LHC. Then with all the constraints (including the requirement to explain A(FB) at 2 sigma level and satisfying the current LHC measurement of A(C) at 2 sigma level), we find that the TC2 model can induce sizable triple-top production at the 14 TeV LHC (the production rate can maximally reach 16 pb). Due to the low SM backgrounds, the triple-top production can also be a good probe for TC2 model, complementary to A(C). (C) 2012 Elsevier B.V. All rights reserved. |

Probing natural SUSY from stop pair production at the LHC

Cao, Junjie; Han, Chengcheng; Wu, Lei; Yang, Jin Min; Zhang, Yang分类： 物理学 >> 基本粒子与场物理学

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

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%. |

Current experimental bounds on stop mass in natural SUSY

Han, Chengcheng; Hikasa, Ken-ichi; Wu, Lei; Yang, Jin Min; Zhang, Yang分类： 物理学 >> 基本粒子与场物理学

Motivated by the recent progress of direct search for the productions of stop pair and sbottom pair at the LHC, we examine the constraints of the search results on the stop ( (t) over tilde (1)) mass in natural SUSY. We first scan the parameter space of natural SUSY in the framework of MSSM, considering the constraints from the Higgs mass, B-physics and electroweak precision measurements. Then in the allowed parameter space we perform a Monte Carlo simulation for stop pair production followed by (t) over tilde (1). t (chi) over tilde (0)(1) or (t) over tilde (1). b (chi) over tilde (+)(1) and sbottom pair production followed by (b) over tilde (1) -> b (chi) over tilde (0)(1) or (b) over tilde (1) -> t (chi) over tilde (-)(1). Using the combined results of ATLAS with 20.1 fb(-1) from the search of l + jets + (sic)(T), hadronic t (t) over bar + (sic)(T) and 2b + (sic)(T), we find that a stop lighter than 600 GeV can be excluded at 95% CL in this scenario. |

分类： 物理学 >> 基本粒子与场物理学

The excess of top-quark forward-backward asymmetry (A(FB)(t)) reported by the Tevatron and the enhancement of the Higgs decay to diphoton observed at the LHC may point to a same origin of new physics. In this note we examined such anomalies in the two-Higgs-doublet model with a color-triplet scalar. We found that under current experimental constraints this model can simultaneously explain both anomalies at 1 sigma level. Also, we examined the Higgs decay h -> Z gamma and displayed its correlation with h -> gamma gamma. We found that unlike other models, this model predicts a special correlation between h -> Z gamma and h -> gamma gamma, i.e., the Z gamma rate is highly suppressed while the gamma gamma rate is enhanced. This behavior may help to distinguish this model in the future high luminosity run of the LHC. |

Probing light higgsinos in natural SUSY from monojet signals at the LHC

Han, Chengcheng; Kobakhidze, Archil; Liu, Ning; Saavedra, Aldo; Wu, Lei; Yang, Jin Min分类： 物理学 >> 基本粒子与场物理学

We investigate a strategy to search for light, nearly degenerate higgsinos within the natural MSSM at the LHC. We demonstrate that the higgsino mass range mu in 100 - 160 GeV, which is preferred by the naturalness, can be probed at 3 sigma significance through the monojet search at 14TeV HL-LHC with 3000 fb(-1) luminosity. The proposed method can also probe certain region in the parameter space for the lightest neutralino with a high higgsino purity, that cannot be reached by planned direct detection experiments at XENON-1T(2017). |