分类： 物理学 >> 基本粒子与场物理学 提交时间： 2017-08-24
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 MG/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： 2017-08-24
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. We identify two scenarios of interest: baryogenesis from a weak-scale pseudo-Nambu-Goldstone boson with intermediatescale 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： 2017-08-24
摘要： Kibble mechanism drastically underestimates the production of topological defects, as confirmed recently in atomic and condensed matter systems. If non-thermally produced, they can be cosmological dark matter of mass 1–10 PeV. If thermalized, skyrmion of mass 1–10 TeV is also a viable dark matter candidate, whose decay may explain e ± spectra in cosmic rays recently measured by PAMELA, FERMI, and HESS. Models that produce magnetic monopoles below the inflation scale, such as Pati–Salam unification, are excluded
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2017-08-03
摘要： All the possible CP-conserving non-linear operators up to the p4 -order in theLagrangian expansion are analysed here for the left-right symmetric model in the non-linear electroweak chiral context coupled to a light dynamical Higgs. The low energy effects will be triggered by an emerging new physics field content in the nature, more specifically, from spin-1 resonances sourced by the straightforward extension of the SM local gauge symmetry to the larger local group SU(2)L ⊗SU(2)R ⊗U(1)B−L. Low energy phenomenology will be altered by integrating out the resonances from the physical spectrum, being manifested through induced corrections onto the left handed operators. Such modifications are weighted by powers of the scales ratio implied by the symmetries of the model and will determine the size of the effective operator basis to be used. The recently observed diboson excess around the invariant mass 1.8–2 TeV entails a scale suppression that suggests to encode the low energy effects via a much smaller set of effective operators.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2017-08-02
摘要： We explore lepton-flavored electroweak baryogenesis, driven by CP-violation in leptonic Yukawa sector, using the τ −µ system in the two Higgs doublet model as an example. This setup generically yields, together with the flavor-changing decay h → τµ, a tree-level Jarlskog-invariant that can drive dynamical generation of baryon asymmetry during a first-order electroweak phase transition and results in CP-violating effect in the decay h → τ τ . We find that the observed baryon asymmetry can be generated in parameter space compatible with current experimental results for the decays h → τµ, h → τ τ The excess events reported by the ATLAS Collaboration in the W Z–final state, and by the CMS Collaboration in the e +e−jj, W h and jj–final states, may be induced by the decays of a heavy boson W0 in the 1.8–2 TeV mass range, here modelled via the larger local group SU(2)L × SU(2)R × U(1)B−L in a non–linear dynamical Higgs scenario. The W0 –production cross section at the 13 TeV LHC is around 700–1200 fb. This framework also predicts a heavy Z' boson with a mass of 2.5–4 TeV, and some decay channels testable in the LHC Run II. We determine the cross section times branching fractions for the dijet, dilepton and top–pair Z' –decay channels at the 13 TeV LHC around 2.3, 7.1, 70.2 fb respectively, for MZ' = 2.5 TeV, while one/two orders of magnitude smaller for the dijet/dilepton and top–pair modes at MZ' = 4 TeV. Non-zero contributions from the effective operators, and the underlying Higgs sector of the model, will induce sizeable enhancement in the W+W− and Zh–final states that could be probed in the future LHC Run II.and τ → µγ, as well as the present bound on the electric dipole moment of the electron. The baryon asymmetry generated is intrinsically correlated with the CP-violating decay h → τ τ and the flavor-changing decay h → τµ, which thus may serve as “smoking guns” to test lepton-flavored electroweak baryogenesis.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2017-08-24
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： 2017-08-24
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 SM quarks and splits the spectrum of 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： 2017-08-24
摘要： Kaluza-Klein dark matter is an attractive weakly interacting massive particle in universal extra dimension model. In the recent extension ”split-UED”, annihilation of Kaluza-Klein dark matter with a mass range 600−1000 GeV provides excellent fits to the recently observed excesses in cosmic electron and positron fluxes of Pamela, ATIC and Fermi-LAT experiments. The cosmic gamma-ray flux in the same process can be significant around 300 GeV, thus can be observed or constrained by the forthcoming Fermi-LAT diffuse gamma-ray data. The collider signal at the LHC is the resonance in the dijets channels and the large missing energy in the missing energy plus jets.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-07-26
摘要： Kibble mechanism drastically underestimates the production of point-like topological defects, as confirmed recently in atomic and condensed matter systems If non-thermally produced, they can be cosmological dark matter of mass 1-10 PeV or heavier If thermalized, skyrimon of mass 1-10 TeV is also a viable dark matter candidate, whose decay may explain e(+/-) spectra in cosmic rays recently measured by PAMELA, FERMI, and HESS Collaborations Models that produce magnetic monopoles below the inflation scale, such as Pati-Salam unification, are ruled out unless new annihilation mechanism for monopoles is introduced (C) 2010 Elsevier B V All rights reserved
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We study the possibility of probing new physics accounting for $(g-2)_\mu$ anomaly and gravitational waves with pulsar timing array measurements. The model we consider is either a light gauge boson or neutral scalar interacting with muons. We show that the parameter spaces of dark $U(1)$ model with kinetic mixing explaining $(g-2)_\mu$ anomaly can realize a first-order phase transition, and the yield-produced gravitational wave may address the common red noise observed in the NANOGrav 12.5-yr dataset.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2017-08-03
摘要： We analyze various models for the top quark forward-backward asymmetry ( A t F B) at the Tevatron, using the latest CDF measurements on different At F Bs and the total cross section. The axigluon model in Ref. [5] has difficulties in explaining the large rapidity dependent asymmetry and mass dependent asymmetry simultaneously and the parameter space relevant to A t F B is ruled out by the latest dijet search at ATLAS. In contrast to Ref. [8], we demonstrate that the large parameter space in this model with a U(1) d flavor symemtry is not ruled out by flavor physics. The t-channel flavor-violating Z ′ , W ′ and diquark models all have parameter regions that satisfy different AF B measurements within 1 σ. However, the heavy Z ′ model which can be marginally consistent with the total cross section is severely constrained by the Tevatron direct search of samesign top quark pair. The diquark model suffers from too large total cross section and is difficult to fit the t t¯ invariant mass distribution. The electroweak precision constraints on the W ′ model based on Z ′ - Z mixings is estimated and the result is rather weak ( m Z ′ > 450 GeV). Therefore, the heavy W ′ model seems to give the best fit for all the measurements. The W ′ model predicts the t t¯ + j signal from tW ′ production and is 10%-50% of SM t t¯ at the 7 TeV LHC. Such t + j resonance can serve as the direct test of the W ′ model.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2017-08-03
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 3σ 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： 2017-08-03
摘要： We present a class of cancellation mechanisms to suppress the total contributions of Barr-Zee diagrams to the electron electric dipole moment (eEDM). This class of mechanisms are of particular significance after the new eEDM upper limit, which strongly constrains the allowed magnitude of CP-violation in Higgs couplings and hence the feasibility of electroweak baryogenesis (EWBG), were released by the ACME collaboration in 2013. We point out: if both the CP-odd Higgs-photonphoton (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 a CP-even and a CP-odd Higgs bosons. With the assistance of the cancellation mechanisms, a large CP-phase in Higgs couplings with viable electroweak baryogenesis (EWBG) is still allowed. The reopened parameter regions would be probed by the future neutron, mercury EDM measurements, and direct measurements of Higgs CP-properties at the LHC and future colliders.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2017-08-03
摘要： The WW 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 pT and azimuthal angle difference ∆ φll 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： 2017-08-03
摘要： New physics field content in the nature, more specifically, from spin–1 resonances sourced by the extension of the SM local gauge symmetry to the larger local group SU(2)L⊗SU(2)R⊗U(1)B−L, may induce CP–violation signalling NP effects from higher energy regimes. In this work we completely list and study all the CP–violating operators up to the p4–order in the Lagrangian expansion, for a non–linear left–right electroweak chiral context and coupled to a light dynamical Higgs. Heavy right handed fields can be integrated out from the physical spectrum, inducing thus a physical impact in the effective gauge couplings, fermionic electric dipole moment, and CP-violation in the decay h → ZZ∗ → 4 l that are briefly analysed. The final relevant set of effective operators have also been identified at low energies.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2017-08-24
摘要： We study the properties of g 1 , the first excited state of the gluon in representative variants of the Randall Sundrum model with the Standard Model fields in the bulk. We find that measurements of the coupling to light quarks (from the inclusive cross-section for pp → g 1 → tt), 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 tt production and find that they require Mg 1 & 950 GeV. In addition we explore the pattern of interference between the g 1 signal and the non-resonant 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 tt, 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： 2017-08-24
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: 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： 2017-08-03
摘要： We consider a simple solution to explain the recent diboson excess observed by ATALS and CMS Collaborations in models with custodial symmetry SU(2)L ×SU(2)R → SU(2)c. The SU(2)L triplet vector boson ρ with mass range of 1.8 ∼ 2 TeV would be produced through the Drell-Yan process with sizable diboson decay branching to account for the excess. The other SU(2)L × SU(2)R bidoublet axial vector boson a would cancel all deviations of electroweak obervables induced by ρ even if the SM fermions mix with some heavy vector like (composite) fermions which couple to ρ (“non-universally partially composite”), therefore allows arbitrary couplings between each SM fermion and ρ. We present our model in the “General Composite Higgs” framework with SO(5) × U(1)X → SO(4) × U(1)X breaking at scale f and demand the first Weinberg sum rule and positive gauge boson form factors as the theoretical constraints. We find that our model can fit the diboson excess very well if the left-handed SM light quarks, charged leptons and tops have zero, zero/moderately small and moderate/large composite components for reasonable values of gρ and f. The correlation between tree level S parameter and the h → Zγ suggest a large a contribution to h → Zγ and it is indeed a O(1) effect in our parameter space which provides a strong hint for our scenario if this diboson excess is confirmed by the 13 ∼ 14 TeV LHC Run II.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2017-08-02
摘要： We study in this work a scenario that the universe undergoes a two step phase transition with the first step happened to the dark matter sector and the second step being the transition between the dark matter and the electroweak vacuums, where the barrier between the two vacuums, that is necessary for a strongly first order electroweak phase transition (EWPT) as required by the electroweak baryogenesis mechanism, arises at the tree-level. We illustrate this idea by working with the standard model (SM) augmented by a scalar singlet dark matter and an extra scalar singlet which mixes with the SM Higgs boson. We study the conditions for such pattern of phase transition to occur and especially for the strongly first order EWPT to take place, as well as its compatibility with the basic requirements of a successful dark matter, such as observed relic density and constraints of direct detections. We further explore the discovery possibility of this pattern EWPT by searching for the gravitational waves generated during this process in spaced based interferometer, by showing a representative benchmark point of the parameter space that the generated gravitational waves fall within the sensitivity of eLISA, DECIGO and BBO.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The measurement of gravitational waves produced by binary black-hole mergers at the Advanced LIGO has encouraged extensive studies on the stochastic gravitational wave background. Recent studies have focused on gravitational wave sources made of the same species, such as mergers from binary primordial black holes or those from binary astrophysical black holes. In this paper, we study a new possibility -- the stochastic gravitational wave background produced by mergers of one primordial black hole and one astrophysical black hole. Such systems are necessarily present if primordial black holes exist. We study the isotropic gravitational wave background produced through the history of the Universe. We find it is very challenging to detect such a signal. We also demonstrate that it is improper to treat the gravitational waves produced by such binaries in the Milky Way as a directional stochastic background, due to a very low binary formation rate.
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