分类： 物理学 >> 基本粒子与场物理学 提交时间： 2017-08-24
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 × SU(2)L × U(1)Y of the Standard Model. Our prototypical 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 × SU(2)2 → SU(2)L results in lepton number being asymmetrically distributed through-out the three families, and provided the SM electroweak phase transition is not strongly first order, results in a non-zero baryon number, which for parameter choices that can be explored at the LHC, may explain the observed baryon asymmetry.
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 explore the possibility that the right-handed top quark is composite. We examine the consequences that compositeness would have on tt production at the Tevatron, and derive a weak constraint on the scale of compositeness of order a few hundred GeV from the tt inclusive cross section. More detailed studies of differential properties of tt production could potentially improve this limit. We find that a composite top can result in an enhancement of the tttt production rate at the LHC (of as much as 103 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： 2017-08-24
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 蟽 compared to the SM 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炉 production within measured uncertainties. Such particles have good discovery prospects by study of the kinematic structure of t t炉+jets at the LHC.
分类： 物理学 >> 核物理学 提交时间： 2016-08-30
摘要： We explore a dark matter model extending the standard model particle content by one fermionic SU(2)L triplet and two fermionic SU(2)L quadruplets, leading to a minimal realistic UV-complete model of electroweakly interacting dark matter which interacts with the Higgs doublet at tree level via two kinds of Yukawa couplings. After electroweak symmetry-breaking, the physical spectrum of the dark sector consists of three Majorana fermions, three singly charged fermions, and one doubly charged fermion, with the lightest neutral fermion χ01 serving as a dark matter candidate. A typical spectrum exhibits a large degree of degeneracy in mass between the neutral and charged fermions, and we examine the one-loop corrections to the mass differences to ensure that the lightest particle is neutral. We identify regions of parameter space for which the dark matter abundance is saturated for a standard cosmology, including coannihilation channels, and find that this is typically achieved for mχ01∼2.4 TeV. Constraints from precision electroweak measurements, searches for dark matter scattering with nuclei, and dark matter annihilation are important, but leave open a viable range for a thermal relic.
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