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 >> 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 explore the possibility that the observed baryon asymmetry of the Universe is the result of an earlier phase transition in which an extended gauge sector breaks down into the SU(3)(C)xSU(2)(L)xU(1)(Y) of the standard model. Our proto-typical example is the topflavor model, in which there is a separate SU(2)(1) for the third generation from the SU(2)(2) felt by the first two generations. We show that the breakdown of SU(2)(1)xSU(2)(2)-> SU(2)(L) results in lepton number being asymmetrically distributed throughout the three families, and provided the SM electroweak phase transition is not strongly first order, results in a nonzero baryon number, which for parameter choices that can be explored at the LHC may explain the observed baryon asymmetry.
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: 2016-07-11
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: 2016-07-11
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: 2016-07-08
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: 2016-07-08
摘要: We explicitly show that split-universal extra dimension (split-UED), a recently suggested extension of universal extra dimension (UED) model, can nicely explain recent anomalies in cosmic-ray positrons and electrons observed by PAMELA and ATIC/PPB-BETS. Kaluza-Klein (KK)dark matters mainly annihilate into leptons because the hadronic branching fraction is highly suppressed by large KK quark masses and the antiproton flux agrees very well with the observation where no excess is found. The flux of cosmic gamma-rays from pion decay is also highly suppressed and hardly detected in low energy region (E-gamma less than or similar to 20GeV). Collider signatures of colored KK particles at the LHC, especially q(1)q(1) production, are studied in detail. Due to the large split in masses of KK quarks and other particles, hard p(T) jets and missing E-T are generated, which make it possible to suppress the standard model background and discover the signals.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-07-08
摘要: We discuss general on-shell couplings of a scalar with two Z bosons using an operator analysis. In addition to the operator that originated from the Higgs mechanism, two dimension-five operators, one CP even and one CP odd, are generated only at the loop level. Simple formulas are derived for the differential decay distributions when the Z pair subsequently decays into four leptons by computing the helicity amplitudes, from which it is shown the CP-odd operator merely induces a phase shift in the azimuthal angular distribution between the two decay planes of the Z bosons. We also investigate new physics scenarios giving rise to loop-induced decays of a scalar into the ZZ pair, and argue that the total decay width of such a scalar would be an order-of-magnitude smaller than that of a Higgs boson, should such decays be observed in the early running of the LHC. Therefore, the total decay width alone is a strong indicator of the Higgs nature, or the lack thereof, of a scalar resonance in ZZ final states. In addition, we study the possibility of using the azimuthal angular distribution to disentangle effects among all three operators.
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: 2016-07-08
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.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-07-05
摘要: We discuss the neutrino constraints from solar and terrestrial dark matter (DM) annihilations in the inelastic dark matter (iDM) scenario after the recent CDMS II results. To reconcile the DAMA/LIBRA data with constraints from all other direct experiments, the iDM needs to be light (m(chi) , and tau(+)tau(-). For soft channels such as b (b) over bar and c (c) over bar, the limits are loose, but could be tested or further constrained by future IceCube plus DeepCore. For neutrino constraints from the DM annihilation in the Earth, due to the weaker gravitational effect of the Earth and inelastic capture condition, the constraint exists only for small mass splitting delta < 40 keV and m(chi) similar to (10, 50) GeV even in the tau(+)tau(-) channel.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-07-05
摘要: We propose a gauged Ud(1)(H) horizontal symmetry around a TeV scale that is a subgroup of a SU(3)(H) horizontal gauge symmetry broken at O(10(14)) GeV. The breaking generates right-handed Majorana neutrino masses through a SU(3)(H) sextet scalar. A particular Majorana right-handed neutrino mass matrix explicitly determines the remnant U(1)(H) at low energy which only couples to b - s and mu - tau in the gauge eigenstate. The dangerous K - (K) over bar, D - (D) over bar mixing and B-s -> mu(+) mu(-) are kept to be safe because the relevant couplings are suppressed through high powers of small mixing angles in the fermion rotation matrix. Our analysis which applies to the general case shows that the Tevatron di-muon anomaly can be explained through the B-s and B-d mixing while keeping all the other experimental constraints within 90% C.L. For the B meson decay, the B-s -> mu(+/-) tau(-/+) is the leading leptonic decay channel which is several orders of magnitude below current experimental bounds.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2016-07-05
摘要: 5D UED is not automatically minimally flavor violating. This is due to flavor asymmetric counter-terms required on the branes. Additionally, there are likely to be higher dimensional operators which directly contribute to flavor observables. We document a mostly unsuccessful attempt at utilizing localization in a flat extra dimension to resolve these flavor constraints while maintaining KK-parity as a good quantum number. It is unsuccessful insofar as we seem to be forced to add brane operators in such a way as to precisely mimic the effects of a double throat warped extra dimension. In the course of our efforts, we encounter and present solutions to a problem common to many extra dimensional models in which fields are "doubly localized:" ultra-light modes. Under scrutiny, this issue seems tied to an intrinsic tension between maintaining Kaluza-Klein parity and resolving mass hierarchies via localization.
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: Recent studies of precision electroweak observables have led to the conclusion that a fourth generation is highly constrained. However, we point out that a long-lived fourth generation can reopen a large portion of the parameter space. In addition, it preserves baryon and lepton asymmetries against sphaleron erasure even if B - L = 0. It opens up the possibility of exact B - L symmetry and hence Dirac neutrinos. The fourth generation can be observed at the LHC with unique signatures of long-lived particles in the near future. (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: 2016-07-05
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 address the possibility that CP violation in B-s-(B) over bar (s) mixing may help explain the origin of the cosmic baryon asymmetry. We propose a new baryogenesis mechanism-"electroweak beautygenesis"-explicitly showing that these two CP-violating phenomena can be sourced by a common CP phase. As an illustration, we work in the two-Higgs-doublet model. Because the relevant CP phase is flavor off diagonal, this mechanism is less severely constrained by null results of electric dipole moment searches than other scenarios. We show how measurements of flavor observables by the D0, CDF, and LHCb collaborations test this scenario.
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: 2016-06-30
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.