All Results

Top-quark FCNC productions at CERN LHC in topcolor-assisted technicolor model

Cao, Junjie; Liu, Guoli; Yang, Jin Min; Zhang, HuanjunSubjects: Physics >> The Physics of Elementary Particles and Fields

We evaluate the top-quark FCNC productions induced by the topcolor-assisted technicolor (TC2) model at the LHC. These productions proceed, respectively, through the parton-level processes gg -> t (c) over bar, cg -> t, cg -> tg, cg -> tZ, and cg -> t gamma. We show the dependence of the production rates on the relevant TC2 parameters and compare the results with the predictions in the minimal supersymmetric model. We find that for each channel the TC2 model allows for a much larger production rate than the supersymmetric model. All these rare productions in the TC2 model can be enhanced above the 3 sigma sensitivity of the LHC. Since in the minimal supersymmetric model only cg -> t is slightly larger than the corresponding LHC sensitivity, the observation of these processes will favor the TC2 model over the supersymmetric model. In case of unobservation, the LHC can set meaningful constraints on the TC2 parameters. |

Probing R-parity violating interactions from top-quark polarization at LHC

Li, Peiying; Lu, Gongru; Yang, Jin Min; Zhang, HuanjunSubjects: Physics >> The Physics of Elementary Particles and Fields

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. |

Subjects: Physics >> The Physics of Elementary Particles and Fields

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. |

SUSY dark matter in light of CDMS II results: a comparative study for different models

Cao, Junjie; Hikasa, Ken-ichi; Wang, Wenyu; Yang, Jin Min; Yu, Li-XinSubjects: Physics >> The Physics of Elementary Particles and Fields

We perform a comparative study of the neutralino dark matter scattering on nucleon in three popular supersymmetric models: the minimal (MSSM), the next-to-minimal (NMSSM) and the nearly minimal (nMSSM). First, we give the predictions of the elastic cross section by scanning over the parameter space allowed by various direct and indirect constraints, which are from the measurement of the cosmic dark matter relic density, the collider search for Higgs boson and sparticles, the precision electroweak measurements and the muon anomalous magnetic moment. Then we demonstrate the property of the allowed parameter space with/without the new limits from CDMS II. We obtain the following observations: (i) For each model the new CDMS limits can exclude a large part of the parameter space allowed by current collider constraints; (ii) The property of the allowed parameter space is similar for MSSM and NMSSM, but quite different for nMSSM; (iii) For each model the future SuperCDMS can cover most of the allowed parameter space given that all soft breaking parameters are below 1 TeV. |

Rare Z-decay into light CP-odd Higgs bosons: a comparative study in different new physics models

Cao, Junjie; Heng, Zhaoxia; Yang, Jin MinSubjects: Physics >> The Physics of Elementary Particles and Fields

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 |

Subjects: Physics >> The Physics of Elementary Particles and Fields

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. |

Light dark matter in NMSSM and implication on Higgs phenomenology

Cao, Junjie; Hikasa, Ken-ichi; Wang, Wenyu; Yang, Jin MinSubjects: Physics >> The Physics of Elementary Particles and Fields

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. |

Di-photon Higgs signal at the LHC: A comparative study in different supersymmetric models

Cao, Junjie; Heng, Zhaoxia; Liu, Tao; Yang, Jin MinSubjects: Physics >> The Physics of Elementary Particles and Fields

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. |

Split-SUSY dark matter in light of direct detection limits

Cao, Junjie; Wang, Wenyu; Yang, Jin MinSubjects: Physics >> The Physics of Elementary Particles and Fields

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. |

Higgs decay to dark matter in low energy SUSY: is it detectable at the LHC?

Cao, Junjie; Heng, Zhaoxia; Yang, Jin Min; Zhu, JingyaSubjects: Physics >> The Physics of Elementary Particles and Fields

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. |