Explaining the t-tbar asymmetry with a light axigluon

We propose an axigluon with mass between 400 and 450 GeV and flavor universal couplings to quarks to explain the Tevatron t-tbar forward-backward asymmetry. The model predicts a small negative asymmetry for t-tbar pairs with invariant mass below 450 GeV and a large positive asymmetry above 450 GeV. The asymmetry arises from interference between s-channel gluon and axigluon diagrams and requires a relatively weakly coupled axigluon ($g_{a} = g_{qcd}/3$). Axigluon-gluon interference does not contribute to the t-tbar cross section. New contributions to the cross section arise only at fourth order in the axigluon coupling and are very small for a sufficiently broad axigluon. Dijet measurements do not significantly constrain the axigluon couplings. We propose several possible UV completions of the phenomenological axigluon which explain the required small couplings and large width. Such UV completions necessarily contain new colored fermions or scalars below the axigluon mass and predict multi-jet events with large cross sections at the Tevatron and LHC.Comment: references adde

New self-dual solutions of SU(2) Yang-Mills theory in Euclidean Schwarzschild space

We present a systematic study of spherically symmetric self-dual solutions of SU(2) Yang-Mills theory on Euclidean Schwarzschild space. All the previously known solutions are recovered and a new one-parameter family of instantons is obtained. The newly found solutions have continuous actions and interpolate between the classic Charap and Duff instantons. We examine the physical properties of this family and show that it consists of dyons of unit (magnetic and electric) charge.Comment: 12 pages, 5 figures. To appear in Phys Rev

New physics at the weak scale: axigluon models, scale invariance and naturalness, and interacting dark matter

The Standard Model of particle physics describes all known elementary particles and their interactions. Despite its great experimental success, we know that the Standard Model is not a complete description of Nature and therefore new phenomena should be observed at higher energies. In the coming years the Large Hadron Collider will test the Standard Model by colliding protons with center of mass energies of up to 14 TeV providing some of the most stringent tests on the Standard Model. Experimental searches for Dark Matter provide a complementary program to test physics at the weak scale. In the near future new experimental data coming from direct detection experiments, and from satellites and telescopes will drastically improve our sensitivity to weak scale dark matter. This could lead to the first direct observation of dark matter, and thus of physics beyond the Standard Model. In this thesis I propose different extensions of the Standard Model and discuss their experimental consequences. I first discuss models for Axigluons, which are spin one particles in the adjoint representation of the SU(3) color gauge group. These models were motivated by the measurement of higher than predicted forward-backward asymmetry in top quark pair production at the Tevatron. I study different scenarios for Axigluon models that can explain the Tevatron result and explore their signatures at the Large Hadron Collider. Second I discuss the implications of ultraviolet scale invariance for the Standard Model, which has been advocated as a solution to the hierarchy problem. I show that in order to solve the hierarchy problem with scale invariance, new physics is required not far from the weak scale. In the last part of this thesis I propose a new model for dark matter, in which dark matter is charged under a hidden non-Abelian gauge group. This leads to modifications in the sensitivity of the usual experimental searches for dark matter in addition to distinct signatures in the Cosmic Microwave Background and in Large Scale Structure data

Solving 2D QCD with an adjoint fermion analytically

We present an analytic approach to solving 1+1 dimensional QCD with an adjoint Majorana fermion. In the UV this theory is described by a trivial CFT containing free fermions. The quasi-primary operators of this CFT lead to a discrete basis of states which is useful for diagonalizing the Hamiltonian of the full strongly interacting theory. Working at large-$N$, we find that the decoupling of high scaling-dimension quasi-primary operators from the low-energy spectrum occurs exponentially fast in their scaling-dimension. This suggests a scheme, whereby, truncating the basis to operators of dimension below $\Delta_{max}$, one can calculate the low-energy spectrum, parametrically to an accuracy of $e^{-\Delta_{max}}$ (although the precise accuracy depends on the state). Choosing $\Delta_{max} =9.5$ we find very good agreement with the known spectrum obtained earlier by numerical DLCQ methods. Specifically, below the first three-particle threshold, we are able to identify all six single-particle bound-states, as well as several two-particle thresholds.Comment: 26 pages, 5 figures; v2: some typos correcte

Non-Abelian dark matter and dark radiation

We propose a new class of dark matter models with unusual phenomenology. What is ordinary about our models is that dark matter particles are WIMPs, they are weakly coupled to the Standard Model and have weak scale masses. What is unusual is that they come in multiplets of a new "dark" non-Abelian gauge group with milli-weak coupling. The massless dark gluons of this dark gauge group contribute to the energy density of the universe as a form of weakly self-interacting dark radiation. In this paper we explore the consequences of having i.) dark matter in multiplets ii.) self-interacting dark radiation and iii.) dark matter which is weakly coupled to dark radiation. We find that i.) dark matter cross sections are modified by multiplicity factors which have significant consequences for collider searches and indirect detection, ii.) dark gluons have thermal abundances which affect the CMB as dark radiation. Unlike additional massless neutrino species the dark gluons are interacting and have vanishing viscosity and iii.) the coupling of dark radiation to dark matter represents a new mechanism for damping the large scale structure power spectrum. A combination of additional radiation and slightly damped structure is interesting because it can remove tensions between global $\Lambda$CDM fits from the CMB and direct measurements of the Hubble expansion rate ($H_0$) and large scale structure ($\sigma_8$).Comment: 25 pages, 8 figures; v2: minor improvements, references added; v3: added references and an acknowledgement note to J. Lesgourgues; accepted for publication in PR

Instantons in curved spaces

Orientador: Ricardo Antonio MosnaDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: Neste trabalho estudamos os instantons da teoria de Yang-Mills nos espaços de Schwarzs-child e de Reissner-Nordstrom com grupo de gauge SU(2).Instantons são soluções clássicas da teoria de Yang-Mills definida em um espaço com métrica riemanniana (positiva-definida) e com ação finita. Primeiramente revisamos a formulação geométrica da teoria de Yang-Mills em uma variedade 4-dimensional,identificando os campos de gauge com conexões em um fibrado principal. Em seguida apresentamos os principais resultados clássicos relacionados aos instantons no espaço plano. Na segunda parte da dissertação realizamos um estudo sistemático das soluções da teoria de Yang-Mills nos espaços de Schwarzschild e de Reissner-Nordstrom euclidianos. Esta abordagem nos permitiu descobrir novas famílias de instantons neste contexto.Ainda,os resultados obtidos mostram que o número de famílias de instantons no espaço de Reissner- Nordstrom depende diretamente da carga elétrica que caracteriza esta geometriaAbstract: In this work we study instanton solutions of the Yang-Mills theory in Schwarzschild and Reissner-Nordstrom spaces with gauge group SU(2).Instantons are solutions to the classical field equations of Yang-Mills theory defined in a space with Riemannian (positive de finite)metric with finite action. We begin with a review of the geometric setting of Yang-Mills theory on a four dimensional manifold,which relates the gauge fields to connections on a fiber bundle.We proceed by presenting the main results related to instantons in flat space. In the second part of this thesis we perform a systematic study of the solutions of Yang-Mills theory in Euclidian Schwarzschild and Reissner-Nordstrom spaces.This approach led us to discover a new family of instantons de fined in those backgrounds. Moreover, our results show that the number of instanton families in the Reissner-Nordstrom space depends directly on the eletric charge which caracterizes this geometryMestradoFísica das Particulas Elementares e CamposMestre em Físic