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: 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: 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: 2016-05-15
摘要: We examine the constraints on F-term hybrid inflation by considering the flat directions in the Minimal Supersymmetric Standard Model (MSSM). We find that some coupling terms between the flat direction fields and the field which dominates the energy density during inflation are quite dangerous and can cause the no-exit of hybrid inflation even if their coupling strength is suppressed by Planck scale. Such couplings must be forbidden by imposing some symmetry for a successful F-term hybrid inflation. At the same time, we find that in the D-term inflation these couplings can be avoided naturally. Further, given the tachyonic preheating, we discuss the feasibility of Affleck-Dine baryogenesis after the F-term and D-term inflations. (c) 2006 Elsevier B.V. All rights reserved.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-15
摘要: The left-right twin Higgs model predicts a light stable scalar (S) over cap, which is a candidate for WIMP dark matter. We study its scattering on nucleon and find that the cross section is below the CDMS II upper bound but can reach the SuperCDMS sensitivity. Then we study the Higgs phenomenology by paying special attention to the decay h -> (S) over cap(S) over cap which is strongly correlated with the dark matter scattering on nucleon. We find that such an invisible decay can be sizable, which can severely suppress the conventional decay modes like h -> VV(V = W, Z) and h -> b (b) over bar. On the other hand, compared to the SM prediction, the rates of Higgs boson productions at the LHC via gluon-gluon fusion, weak boson fusion or in association with top quark pairs are all reduced significantly, e. g., the gluon-gluon fusion channel can be suppressed by about 30%.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-15
摘要: We study the Higgs boson decays and production via gluon fusion at the LHC as a probe of two typical littlest Higgs models which introduce a top quark partner with different (even and odd) T parity to cancel the Higgs mass quadratic divergence contributed by the top quark. For each model, we consider two different choices for the down-type quark Yukawa couplings. We first examine the branching ratios of the Higgs boson decays and then study the production via gluon fusion followed by the decay into two photons or two weak gauge bosons. We find that the predictions can be quite different for different models or different choices of down-type quark Yukawa couplings, and all these predictions can sizably deviate from the standard model predictions. So the Higgs boson processes at the LHC can be a sensitive probe for these littlest Higgs models.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-15
摘要: We study the direct mediation of metastable supersymmetry breaking by a Phi(2)-deformation to the ISS model and extend it by splitting both Tr Phi and Tr Phi(2) terms in the superpotential and gauging the flavor symmetry. We find that with such an extension enough-long-lived metastable vacua can be obtained and the proper gaugino masses can be generated. Also, this allows for constructing a kind of models which can avoid the Landau pole problem. Especially, in our metastable vacua there exist a large region for the parameter m(3) which can satisfy the phenomenology requirements and allow for a low SUSY-breaking scale (h mu(2) similar to 100 TeV). Copyright (C) EPLA, 2009
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-15
摘要: Higgs boson productions at the LHC will serve as a sensitive probe of various little Higgs models. In this work we comparatively study two littlest Higgs models with different T-parity constructions through examining their effects in three production processes of the Higgs boson at the LHC, namely, the productions of a single Higgs, a Higgs-pair, as well as a Higgs boson associated with a pair of top and antitop quarks. The two models are characterized by predicting a top partner with, respectively, even and odd T-parity, which cancels the Higgs mass quadratic divergence caused by the top quark. We find that both models can alter the standard model cross sections sizably and their corrections also differ significantly. Therefore, the Higgs boson productions at the LHC might shed some light on these two models or even distinguish them.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-14
摘要: Little Higgs theory naturally predicts a light Higgs boson whose most important discovery channel at the LHC is the diphoton signal pp -> h -> gamma gamma. In this work, we perform a comparative study for this signal in some typical little Higgs models, namely, the littlest Higgs model, two littlest Higgs models with T-parity (named LHT-I and LHT-II), and the simplest little Higgs models. We find that compared with the standard model prediction, the diphoton signal rate is always suppressed and the suppression extent can be quite different for different models. The suppression is mild (<= 10%) in the littlest Higgs model but can be quite severe (similar or equal to 90%) in other three models. This means that discovering the light Higgs boson predicted by the little Higgs theory through the diphoton channel at the LHC will be more difficult than discovering the standard model Higgs boson.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-15
摘要: We study the process gamma gamma -> h -> b (b) over bar at ILC as a probe of different little Higgs models, including the simplest little Higgs model (SLH), the littlest Higgs model (LH), and two types of littlest Higgs models with T-parity (LHT-I, LHT-II). Compared with the Standard Model (SM) prediction, the production rate is found to be sizably altered in these little Higgs models and, more interestingly, different models give different predictions. We find that the production rate can be possibly enhanced only in the LHT-II for some part of the parameter space, while in all other cases the rate is suppressed. The suppression can be 10% in the LH and as much as 60% in both the SLH and the LHT-I/LHT-II. The severe suppression in the SLH happens for a large tan beta and a small m(h), in which the new decay mode h -> eta eta (eta is a light pseudo-scalar) is dominant; while for the LHT-I/LHT-II the large suppression occurs when integral and m(h) are both small so that the new decay mode h -> A(H)A(H) is dominant. Therefore, the precision measurement of such a production process at the ILC will allow for a test of these models and even distinguish between different scenarios.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-15
摘要: If the sparticles are relatively heavy (a few TeV) while the Higgs sector is not so heavy (m(A) is not so large), the Higgs boson Yukawa couplings can harbor sizable quantum effects of sparticles and these large residual effects may play a special role in probing supersymmetry at foreseeable colliders. In this work, focusing on the supersymmetric QCD effects in the hb (b) over bar coupling (h is the lightest CP-even Higgs boson), we give a comparative study for the two popular supersymmetric models: the MSSM and NMSSM. While for both models the supersymmetric QCD can leave over large residual quantum effects in hb (b) over bar coupling, the NMSSM can allow for a much broader region of such effects. Since these residual effects can be over 20% for the hb (b) over bar coupling (and thus over 40% for the ratio Br(h -> b (b) over bar)/Br(h -> tau(+)tau(-))), future measurements may unravel the effects of heavy sparticles or even distinguish the two models. (C) 2009 Elsevier B.V. All rights reserved.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-15
摘要: To understand the origin of the left-right symmetry, we study a partial unification model based on SU(4)(W) x U(1)(B-L) which can be broken down to the mini mal left-right model either through the Higgs mechanism in four dimensions or through the five-dimensional orbifolding gauge symmetry breaking, especially we propose to use the rank reducing outer automorphism orbifolding breaking mechanism. We scrutinized all these breaking mechanisms and found that for the orbifold breaking in five dimensions, the rank-reducing outer automorphism is better than the inner automorphism and can make the low energy theory free of the U(1)Z anomaly. It is possible for the outer automorphism orbifolding breaking mechanism to be non-anomalous without Chern-Simons terms and new localized fermions. For the four-dimensional model with the Higgs mechanism, we study in detail both its structure and its typical phenomenology. It turns out that this four-dimensional scenario may predict some new phenomenology since the new mirror fermions (which are introduced in order to fill the SM fermions into SU(4)W without anomaly) are preserved at low energy scale and mix with the SM fermions. We also examine the gauge coupling unification in each case, and discuss the possibility for unifying this partial unification group with the Pati-Salam group SU(4)(PS) to realize a grand unification. (C) 2009 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: 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: 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: 2016-05-14
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: 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.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-09
摘要: We recalculate the two-loop beta functions for three gauge couplings taking into account all low energy threshold corrections in split supersymmetry(split-SUSY) which assumes a very high scalar mass scale M-S. We find that in split-SUSY with a gaugino mass unification assumption and with a large M-S the gauge coupling unification requires a lower bound on the gaugino mass. Combined with the constraints from the dark matter relic density and direct detection limits, we find that split-SUSY is very restricted and for dark matter mass below 1 TeV the allowed parameter space can be fully covered by XENON-1T(2017).