分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-07-11
摘要: If a new massive vector boson with nonzero axial couplings to fermions will be observed at LHC, then an upper limit on the scale of new physics could be derived from unitarity of S matrix. The new physics will involve either new massive fermions, or scalars, or even a strongly coupled sector. We derive a model independent bound on the scale of new physics. If M-G/g(A) < 3 TeV and the fermion is a top quark, the upper limit is 78 TeV.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-06-28
摘要: We observe a generic connection between LHC Higgs data and electroweak baryogenesis: the particle that contributes to the CP-odd hgg or h gamma gamma vertex would provide the CP-violating source during a first-order phase transition. It is illustrated in the two Higgs doublet model that a common complex phase controls the lightest Higgs properties at the LHC, electric dipole moments, and the CP-violating source for electroweak baryogenesis. We perform a general parametrization of Higgs effective couplings and a global fit to the LHC Higgs data. Current LHC measurements prefer a nonzero phase for tan beta less than or similar to 1 and electric dipole moment constraints still allow an order-one phase for tan beta similar to 1, which gives sufficient room to generate the correct cosmic baryon asymmetry. We also give some prospects in the direct measurements of CP violation in the Higgs sector at the LHC.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-07-08
摘要: Motivated by the recent observation of the high energy electron and positron excesses in cosmic ray by PAMELA and ATIC/PPB-BETS, we suggest an anomaly-free scenario for the universal extra dimension that localizes the standard model quarks and splits the spectrum of Kaluza-Klein (KK) quarks from KK leptons. When the SM quarks are "well localized" at the boundaries, the most stringent bound of the model (1/R > 510 GeV) comes from the resonance search for the Tevatron dijet channels. Even at the early stage of LHC, one can discover the second KK gluon for masses up to 4 TeV.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-07-25
摘要: In the presence of background fields that spontaneously violate Lorentz invariance, a matter-antimatter asymmetry can be generated even in thermal equilibrium. In this paper we systematically investigate models of this type, showing that either high-energy or electroweak versions of baryogenesis are possible, depending on the dynamics of the Lorentz-violating fields. In addition to the previously studied models of spontaneous baryogenesis and quintessential baryogenesis, we identify two scenarios of interest: baryogenesis from a weak-scale pseudo-Nambu-Goldstone boson with intermediate-scale baryon-number violation, and sphaleron-induced baryogenesis driven by a constant-magnitude vector with a late-time phase transition.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-07-05
摘要: We consider the spin determination of new colored particles in the missing energy plus jets channel at the early stage of the discovery. We use a three site moose model to describe the low energy Lagrangian of all same spin partner (LHT or UED like) models and check the gauge invariance of the amplitude. For the benchmark production and decay channel pp -> (UU(R))-U-(R) -> uuB(H)B(H), in contrast to those in supersymmetric models, there are spin correlations which affect the polar and azimuthal angle distributions of the quarks from the heavy partner U(R) decay. We show such effects would be visible in the E-Tmiss/M-eff distribution and the reconstructed azimuthal angle correlation using MAOS reconstruction.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-07-05
摘要: The improved standard model prediction for total top quark forward-backward asymmetry of 8.9% at the Tevatron has significantly reduced the long-term discrepancy between the theory value and the experimental observations. The seemingly "last" over-3 sigma anomaly is the CDF measurement of A(FB)(M-t (t) over bar > 450 GeV) and it is not seen by the D0 Collaboration. In this paper, we take the CDF measurement to obtain the best-fit parameter space for various previously proposed models, including axigluon, t-channel neutral current, charged current, and diquark models, then study the predictions of the corresponding parameter space on the direct search or indirect constrain. The particular axigluon model is excluded by the LHC dijet search. The t-channel Z' model suffer from the Tevatron same-sign dilepton search bound. The t-channel W' model and the diquark model both predict significant increase in the production rate of inclusive t (t) over bar search at the LHC which is not seen. We conclude none of the models are favored by the direct search bound.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-06-30
摘要: We construct a general class of pseudo-Goldstone composite Higgs models, within the minimal SO(5)/SO(4) coset structure, that are not necessarily of moose-type. We characterize the main properties these models should have in order to give rise to a Higgs mass around 125 GeV. We assume the existence of relatively light and weakly coupled spin 1 and 1/2 resonances. In absence of a symmetry principle, we introduce the Minimal Higgs Potential (MHP) hypothesis: the Higgs potential is assumed to be one-loop dominated by the SM fields and the above resonances, with a contribution that is made calculable by imposing suitable generalizations of the first and second Weinberg sum rules. We show that a 125 GeV Higgs requires light, often sub-TeV, fermion resonances. Their presence can also be important for the models to successfully pass the electroweak precision tests. Interestingly enough, the latter can also be passed by models with a heavy Higgs around 320 GeV. The composite Higgs models of the moose-type considered in the literature can be seen as particular limits of our class of models.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-06-28
摘要: We consider the Higgs portal through which light scalars contribute both to the Higgs production and decay and Higgs effective potential at finite temperature via quantum loops. The positive Higgs portal coupling required by a strongly first order electroweak phase transition is disfavored by the current Higgs data if we consider one such scalar. We observe that by introducing a second scalar with negative Higgs portal coupling, one can not only improve the Higgs fits, but also enhance the strength of first order EWPT. We apply this mechanism to the light stop scenario for electroweak baryogenesis in the MSSM and find a light sbottom could play the role as the second scalar, which allows the stop to be relatively heavier. Non-decoupled effects on the Higgs or sbottom self-interactions from physics beyond MSSM are found to be indispensable for this scenario to work. A clear prediction from the picture is the existence of a light sbottom (below 200 GeV) and a light stop (can be as heavy as 140 GeV), which can be directly tested in the near future.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-06-27
摘要: We analyze the observational potential of single t' production in both the t' -> bW and t' -> th decay channels at 8 TeV LHC using an integrated luminosity of 25 fb(-1). Our analysis is based on a simplified model with minimal coset SO(5)/SO(4) in which the t' is a singlet of the unbroken SO(4). The single t' production, as a consequence of electroweak symmetry breaking, is less kinematically suppressed, associated with a light forward jet and has boosted decay products at the 8 TeV LHC. Therefore it provides the most promising channel in searching for a heavy t'. We have exploited the above kinematical features and used the jet substructure method to reconstruct the boosted Higgs in th decay channel. It is shown that a strong constraint on the t'bW coupling (g(t'bW)/g(tbW,SM) < 0.2 similar to 0.3) at the 95% C. L. can be obtained for m(t') subset of (700, 1000) GeV.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-06-24
摘要: We present a class of cancellation conditions for suppressing the total contributions of Barr-Zee diagrams to the electron electric dipole moment (eEDM). Such a cancellation is of particular significance after the new eEDM upper limit was released by the ACME Collaboration, which strongly constrains the allowed magnitude of CP violation in Higgs couplings and hence the feasibility of electroweak baryogenesis (EWBG). Explicitly, if both the CP-odd Higgs-photon-photon (Z boson) and the CP-odd Higgs-electron-positron couplings are turned on, a cancellation may occur either between the contributions of a CP-mixing Higgs boson, with the other Higgs bosons being decoupled, or between the contributions of CP-even and CP-odd Higgs bosons. With a cancellation, large CP violation in the Higgs sector is still allowed, yielding successful EWBG. The reopened parameter regions would be probed by future neutron, mercury EDM measurements, and direct measurements of Higgs CP properties at the Large Hadron Collider Run II and future colliders.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-06-24
摘要: The W W production is the primary channel to directly probe the triple gauge couplings. We first analyze the e (+) e (-) -> W (+) W (-) process at the future lepton collider, China's proposed Circular Electron-Positron Collider (CEPC). We use the five kinematical angles in this process to constrain the anomalous triple gauge couplings and relevant dimension six operators at the CEPC up to the order of magnitude of 10(-4). The most sensible information is obtained from the distributions of the production scattering angle and the decay azimuthal angles. We also estimate constraints at the 14 TeV LHC, with both 300 fb(-1) and 3000 fb(-1) integrated luminosity from the leading lepton p (T) and azimuthal angle difference Delta I center dot (ll) distributions in the di-lepton channel. The constrain is somewhat weaker, up to the order of magnitude of 10(-3). The limits on the triple gauge couplings are complementary to those on the electroweak precision observables and Higgs couplings. Our results show that the gap between sensitivities of the electroweak and triple gauge boson precision can be significantly decreased to less than one order of magnitude at the 14 TeV LHC, and that both the two sensitivities can be further improved at the CEPC.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-06-24
摘要: We present a new probe of strongly coupled electroweak symmetry breaking at the 14 TeV LHC by measuring a phase shift in the event distribution of the decay azimuthal angles in massive gauge boson scattering. One generically expects a large phase shift in the longitudinal gauge boson scattering amplitude due to the presence of broad resonances. This phase shift is observable as an interference effect between the strongly interacting longitudinal modes and the transverse modes of the gauge bosons. We find that even very broad resonances of masses up to 900 GeV can be probed at 3 sigma significance with a 3000 fb(-1) run of the LHC by using this technique. We also present the estimated reach for a future 50 TeV proton-proton collider.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-07-11
摘要: We study the decay of a Z' boson into two Z bosons by extending the Landau-Yang theorem to a parent particle decaying into two Z bosons. For a spin-1 parent the theorem predicts that (1) there are only two possible couplings and (2) the normalized differential cross section depends on kinematics only through a phase shift in the azimuthal angle between the two decay planes of the Z boson. When the parent is a Z' the two possible couplings are anomaly induced and CP violating, respectively. At the CERN Large Hadron Collider their effects could be disentangled when both Z bosons decay leptonically.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-07-08
摘要: A flavor-nonuniversal chiral color model is introduced. It is used for comparison to the recent data on (p) over barp -> (t) over bart. We concluded that the data are consistent with interpretation as an axigluon exchange within 1 sigma and a unique rise and fall behavior is predicated with regard to the asymmetry A(FB)(t) as a function of t (t) over bar invariant mass, which can distinguish our model from others before one discovers the axigluon resonance. Further aspects of the model are discussed. (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: 2016-06-30
摘要: Both CDF and Dempty set report a forward-backward asymmetry in t (t) over bar production that is above the standard model prediction. We review new physics models that can give a large forward backward asymmetry in t (t) over bar production at the Tevatron and the constraints these models face from searches for dijet resonances and contact interactions, from flavor physics and the t (t) over bar cross section. Expected signals at the LHC are also reviewed.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-06-24
摘要: We explain the recent diphoton excesses around 750 GeV by both ATLAS and CMS as a singlet scalar Phi which couples to SM gluon and neutral gauge bosons only through higher-dimensional operators. A natural explanation is that Phi is a pseudo-Nambu-Goldstone boson (pNGB) which receives parity violation through anomaly if there exists a hidden strong dynamics. The singlet and other light pNGBs will decay into two SM gauge bosons and even serves as the metastable colored states which can be probed in the future. By accurately measuring their relative decay and the total production rate in the future, we will learn the underlying strong dynamics parameter. The lightest baryon in this confining theory could serve as a viable dark matter candidate.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-06-18
摘要: With the assistance of a complex singlet, and an effective operator involving CP violations, the dark matter relic abundance and baryon asymmetry of the universe have been addressed simultaneously. We studied the electroweak baryogenesis mechanism systematically. The electroweak phase transition analysis indicates that the strong first order phase transition takes place by one-step or two-step type due to the dynamics of the energy gap between the electroweak vacuum and the vacuum of the complex singlet. The relation between the magnitude of baryon asymmetry of the universe and the phase transition type and strength has been explored in the framework of electroweak baryogenesis.
分类: 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间: 2016-07-19
摘要: We study the properties of g(1), the first excited state of the gluon in representative variants of the Randall-Sundrum (RS) model with the standard model (SM) fields in the bulk. We find that measurements of the coupling to light quarks (from the inclusive cross section for pp -> g(1)-> t (t) over bar), the coupling to bottom quarks (from the rate of pp -> g(1)b), as well as the overall width can provide powerful discriminants between the models. In models with large brane kinetic terms, the g(1) resonance can even potentially be discovered decaying into dijets against the large QCD background. We also derive bounds based on existing Tevatron searches for resonant t (t) over bar production and find that they require M(g)(1)greater than or similar to 950 GeV. In addition, we explore the pattern of interference between the g(1) signal and the nonresonant SM background, defining an asymmetry parameter for the invariant mass distribution. The interference probes the relative signs of the couplings of the g(1) to light quark pairs and to t (t) over bar, and thus provides an indication that the top is localized on the other side of the extra dimension from the light quarks, as is typical in the RS framework.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-07-11
摘要: We explore the possibility that the right-handed top quark is composite. We examine the consequences that compositeness would have on t (t) over bar production at the Tevatron, and derive a weak constraint on the scale of compositeness of order a few hundred GeV from the t (t) over bar inclusive cross section. More detailed studies of differential properties of t (t) over bar production could potentially improve this limit. We find that a composite top can result in an enhancement of the t (t) over bart (t) over bar production rate at the LHC (of as much as 10(3) compared to the Standatd Model four top rate). We explore observables which allow us to extract the four top rate from the backgrounds, and show that the LHC can either discover or constrain top compositeness for wide ranges of parameter space.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-07-08
摘要: We consider the recent measurement of the top quark forward-backward asymmetry at the Fermilab Tevatron, which shows a discrepancy of slightly more than 2 sigma compared to the standard model prediction. We find that t-channel exchange of a color sextet or triplet scalar particle can explain the measurement, while leaving the cross section for t (t) over bar production within measured uncertainties. Such particles have good discovery prospects by study of the kinematic structure of t (t) over bar + jets at the LHC.