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: 2016-05-14
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: 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: 2016-05-14
摘要: 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.
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: 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 提交时间: 2016-05-09
摘要: 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.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-09
摘要: 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).
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-09
摘要: We consider the current experimental constraints on the parameter space of the MSSM and NMSSM. Then in the allowed parameter space we examine the Higgs pair production at the 14TeV LHC via b (b) over bar -> hh (h is the 125GeV SM-like Higg boson) with one-loop SUSY QCD correction and compare it with the production via gg -> hh. We obtain the following observations: (i) For the MSSM the production rate of b (b) over bar -> hh can reach 50 fb and thus can be competitive with gg -> hh, while for the NMSSM b (b) over bar -> hh has a much smaller rate than gg. hh due to the suppression of the hb (b) over bar coupling; (ii) The SUSY-QCD correction to b (b) over bar -> hh is sizable, which can reach 45% for the MSSM and 15% for the NMSSM within the 1s region of the Higgs data; (iii) In the heavy SUSY limit (all soft mass parameters become heavy), the SUSY effects decouple rather slowly from the Higgs pair production (especially the gg -> hh process), which, for M S U S Y = 5TeV and m A < 1TeV, can enhance the production rate by a factor of 1.5 and 1.3 for the MSSM and NMSSM, respectively. So, the Higgs pair production may be helpful for unraveling the effects of heavy SUSY.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-09
摘要: Motivated by the future precision test of the Higgs boson at an e(+)e(-) Higgs factory, we calculate the production e(+)e(-) -> ZH gamma in the Standard Model with complete next-to-leading order electroweak corrections. We find that for p s root s = 240 (350) GeV the cross section of this production is sizably reduced by the electroweak corrections, which is 1.03 (5.32) fb at leading order and 0.72 (4.79) fb at next-to-leading order. The transverse momentum distribution of the photon in the final states is also presented.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-09
摘要: In SUSY, a light dark matter is usually accompanied by light scalars to achieve the correct relic density, which opens new decay channels of the SM-like Higgs boson. Under current experimental constraints including the latest LHC Higgs data and the dark matter relic density, we examine the status of a light neutralino dark matter in the framework of NMSSM and confront it with the direct detection results of CoGeNT, CDMS-II and LUX. We have the following observations: (i) A dark matter as light as 8 GeV is still allowed and its scattering cross section off the nucleon can be large enough to explain the CoGeNT/CDMS-II favored region; (ii) The LUX data can exclude a sizable part of the allowed parameter space, but still leaves a light dark matter viable; (iii) The SM-like Higgs boson can decay into the light dark matter pair with an invisible branching ratio reaching 30% under the current LHC Higgs data, which may be tested at the 14 TeV LHC experiment.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-09
摘要: In light of the Higgs discovery and the nonobservation of sparticles at the LHC, we revisit the supersymmetric theory (SUSY) induced top quark flavor-changing decay into the Higgs boson. We perform a scan over the relevant SUSY parameter space by considering the constraints from the Higgs mass measurement, the LHC search for SUSY, the vacuum stability, the precision electroweak observables as well as B -> X-s gamma. We make the following observations: (1) In the Minimal Supersymmetric Standard Model (MSSM), the branching ratio of t -> ch can only reach 3.0 x 10(-6), which is about one order smaller than previous results obtained before the advent of the LHC. Among the considered constraints, the Higgs mass and the LHC search for sparticles are found to play an important role in limiting the prediction. (2) In the singlet extension of the MSSM, since the squark sector is less constrained by the Higgs mass, the branching ratio of t -> ch can reach the order of 10(-5) in the allowed parameter space. (3) The chiral-conserving mixings delta(LL) and delta(RR) may have remanent effects on t -> ch in the heavy SUSY limit. In the MSSM with squarks above 3 TeV and gluino above 4 TeV and meanwhile the CP-odd Higgs boson mass around 1 TeV, the branching ratio of t -> ch can still reach the order of 10(-8) under the constraints.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-09
摘要: In some supersymmetric models like split supersymmetry or models with nonuniversal gaugino mass, 'bino [lightest sparticle (LSP)] and wino [next-to-lightest sparticle (NLSP)] may have rather small mass splitting in order to provide the correct dark matter relic density through bino/wino coannihilation. Such a scenario with the compressed bino/wino is difficult to explore at the LHC. In this work we propose to probe this scenario from pp -> j (chi) over tilde (0)(2)(chi) over tilde (+/-)(1) followed by (chi) over tilde (0)(2) -> gamma(chi) over tilde (0)(1) and (chi) over tilde (+/-)(1) -> W*(chi) over tilde (0)(1) -> l(+/-)nu(chi) over tilde (0)(1) (this method is also applicable to the compressed bino/Higgsino scenario). Through a detailed Monte Carlo simulation for both the signal and the backgrounds, we find that for a mass splitting Delta M similar to 10-15 GeV between bino (LSP) and wino (NLSP), the 14 TeV LHC with a luminosity of 1000 fb(-1) can probe the wino up to 150 GeV (the sensitivity can reach 3.4 sigma for Delta M = 10 GeV and 2 sigma for Delta M = 15 GeV). We also investigate the dark matter detection sensitivities for this scenario and find that the planned XENON-1T (2017) cannot fully cover the parameter space with a wino below 150 GeV allowed by relic density and the LUX limits.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-09
摘要: Measuring the Higgs self-coupling is one of the crucial physics goals at the LHC Run-2 and other future colliders. In this work, we attempt to figure out the size of SUSY effects on the trilinear self-coupling of the 125 GeV Higgs boson in the MSSM and NMSSM after the LHC Run-1. Taking account of current experimental constraints, such as the Higgs data, flavor constraints, electroweak precision observables and dark matter detections, we obtain the observations: (1) In the MSSM, the ratio lambda(MSSM)(3h)/lambda(SM)(3h) has been tightly constrained by the LHC data, which can be only slightly smaller than 1 and minimally reach 97%; (2) In the NMSSM with lambda 0.7, a large enhancement or reduction -1.1 < lambda(MSSM)(3h1)/lambda(SM)(3h1) < 2 can occur, which is accompanied by a sizable change of h(1)tau(+)tau(-) coupling. The future colliders, such as the HL-LHC and ILC, will have the capacity to test these large deviations in the NMSSM. (C) 2015 The Authors. Published by Elsevier B.V.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-05-09
摘要: We attempt to explain the recent ATLAS 3 sigma excess of dilepton events with an invariant mass near Z peak through the gluino-mediated sbottom production in a simplified scenario inspired by the minimal supersymmetric Standard Model. The additional Z bosons can be produced through the cascade decay chain (g) over tilde -> b (b) over tilde1(dagger) -> b (b) over bar(chi) over tilde (0)(2,3) -> b (b) over bar(chi) over tilde (0)(1)Z, in which (b) over tilde (1) is the right-handed sbottom, (chi) over tilde (0)(1) is the bino-like lightest supersymmetric particle (LSP) and (chi) over tilde (0)(2,3) are two nearly degenerate Higgsino-like next-to LSPs. Taking into account the constraint from the LHC search for the gluino-mediated sbottom production in the final states with the missing transverse energy and at least three b-jets, we find that the ATLAS on-Z excess can only be marginally explained at the 2 sigma level. We also note that within the scenario where the gluino predominantly decays to the right-handed stop instead of the sbottom, the excess can hardly be explained, since the Z-boson is not produced in the stop dominant decay channel, (t) over tilde (1) -> b (chi) over tilde (+)(1).
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.