The kaon electromagnetic form factor in the light-front formalism

Numerical calculations are performed and compared to the experimental data for the electromagnetic form factor of the kaon, extracted from both components of the electromagnetic current, J(+) and J(-), with a pseudo-scalar coupling of the quarks to the kaon. In the case of J(+), there is no pair term contribution in the Drell-Yan frame (q(+) = 0). However, for J-, the pair term contribution is different from zero and is necessary in order to preserve the rotational symmetry of the current. The free parameters are the quark masses and the regulator mass.

Search for the supersymmetric partner of the top quark in pp̅ collisions at √s=1.96 TeV

We present a search for the lightest supersymmetric partner of the top quark in proton-antiproton collisions at a center-of-mass energy √s=1.96  TeV. This search was conducted within the framework of the R parity conserving minimal supersymmetric extension of the standard model, assuming the stop decays dominantly to a lepton, a sneutrino, and a bottom quark. We searched for events with two oppositely-charged leptons, at least one jet, and missing transverse energy in a data sample corresponding to an integrated luminosity of 1  fb-1 collected by the Collider Detector at Fermilab experiment. No significant evidence of a stop quark signal was found. Exclusion limits at 95% confidence level in the stop quark versus sneutrino mass plane are set. Stop quark masses up to 180  GeV/c2 are excluded for sneutrino masses around 45  GeV/c2, and sneutrino masses up to 116  GeV/c2 are excluded for stop quark masses around 150  GeV/c2.

Electroweak precision constraints on the Lee-Wick standard model

We perform an analysis of the electroweak precision observables in the Lee-Wick Standard Model. The most stringent restrictions come from the S and T parameters that receive important tree level and one loop contributions. In general the model predicts a large positive S and a negative T. To reproduce the electroweak data, if all the Lee-Wick masses are of the same order, the Lee-Wick scale is of order 5 TeV. We show that it is possible to find some regions in the parameter space with a fermionic state as light as 2.4-3.5 TeV, at the price of rising all the other masses to be larger than 5-8 TeV. To obtain a light Higgs with such heavy resonances a fine-tuning of order a few per cent, at least, is needed. We also propose a simple extension of the model including a fourth generation of Standard Model fermions with their Lee-Wick partners. We show that in this case it is possible to pass the electroweak constraints with Lee-Wick fermionic masses of order 0.4-1.5 TeV and Lee-Wick gauge masses of order 3 TeV.

Calculable lepton masses, seesaw relations, and four neutrino mixings in a 3-3-1 model with an extra U(1) symmetry

We propose a scheme in which the masses of the heavier leptons obey seesaw type relations. The light lepton masses, except the electron and the electron neutrino ones, are generated by one loop level radiative corrections. We work in a version of the 3-3-1 electroweak model that predicts singlets (charged and neutral) of heavy leptons beyond the known ones. An extra U(1)(Omega) symmetry is introduced in order to avoid the light leptons getting masses at the tree level. The electron mass induces an explicit symmetry breaking at U(1)(Omega). We discuss also the mixing matrix among four neutrinos. The new energy scale required is not higher than a few TeV.

Hierarchy and anarchy in quark mass matrices, or can hierarchy tolerate anarchy?

The consequences of adding random perturbations (anarchy) to a baseline hierarchical model of quark masses and mixings are explored. Even small perturbations of the order of 5% of the smallest non-zero element can already give deviations significantly affecting parameters of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, so any process generating the anarchy should in general be limited to this order of magnitude. The regularities of quark masses and mixings thus appear to be far from a generic feature of randomness in the mass matrices, and more likely indicate an underlying order. (C) 2001 Published by Elsevier B.V. B.V.

Nucleon flavor asymmetry in a statistical quark model

A statistical model of linear-confined quarks is applied to obtain the flavor asymmetry of the nucleon sea. The model parametrization is fixed by the experimental available data, where a temperature parameter is used to fit the Gottfried sum rule violation. Results are presented for the ratios of light quark and antiquark distributions, d/u and (d) over bar/(u) over bar.

Description of Heavy Quark Mass by Lippmann-Schwinger Equation

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Measurement of the top quark mass in final states with two leptons

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Study of the (D)over-barN Interaction in a QCD Coulomb Gauge Quark Model

We study the (D) over barN interaction at low energies with a quark model inspired in the QCD Hamiltonian in Coulomb gauge. The model Hamiltonian incorporates a confining Coulomb potential extracted from a self-consistent quasiparticle method for the gluon degrees of freedom, and transverse-gluon hyperfine interaction consistent with a finite gluon propagator in the infrared. Initially a constituent-quark mass function is obtained by solving a gap equation and baryon and meson bound-states are obtained in Fock space using a variational calculation. Next, having obtained the constituent-quark masses and the hadron waves functions, an effective meson-nucleon interaction is derived from a quark-interchange mechanism. This leads to a short range meson-baryon interaction and to describe long-distance physics vector- and scalar-meson exchanges described by effective Lagrangians are incorporated. The derived effective (D) over barN potential is used in a Lippmann-Schwinger equation to obtain phase shifts. The results are compared with a recent similar calculation using the nonrelativistic quark model.

Search for heavy quarks decaying into a top quark and a W or Z boson using lepton + jets events in pp collisions at √s = 7 TeV

Results are presented from a search for the pair-production of heavy quarks, QQ̄, that decay exclusively into a top quark and a W or Z boson. The search is performed using a sample of proton-proton collisions at √s = 7 TeV corresponding to an integrated luminosity of 5.0 fb-1, collected by the Compact Muon Solenoid experiment. The signal region is defined using a sample of events containing one electron or muon, missing transverse momentum, and at least four jets with large transverse momenta, where one jet is likely to originate from the decay of a bottom quark. No significant excess of events is observed with respect to the standard model expectations. Assuming a strong pair-production mechanism, quark masses below 675 (625) GeV decaying into tW (tZ) are excluded at the 95 % confidence level.[Figure not available: see fulltext.] © 2013 CERN for the benefit of the CMS collaboration.

D̄N interaction in a color-confining chiral quark model

We investigate the low-energy elastic D̄N interaction using a quark model that confines color and realizes dynamical chiral symmetry breaking. The model is defined by a microscopic Hamiltonian inspired in the QCD Hamiltonian in Coulomb gauge. Constituent quark masses are obtained by solving a gap equation, and baryon and meson bound-state wave functions are obtained using a variational method. We derive a low-energy meson-nucleon potential from a quark-interchange mechanism whose ingredients are the quark-quark and quark-antiquark interactions and baryon and meson wave functions, all derived from the same microscopic Hamiltonian. The model is supplemented with (σ, ρ, ω, a0) single-meson exchanges to describe the long-range part of the interaction. Cross sections and phase shifts are obtained by iterating the quark-interchange plus meson-exchange potentials in a Lippmann-Schwinger equation. Once coupling constants of long-range scalar σ and a0 meson exchanges are adjusted to describe experimental phase shifts of the K+N and K0N reactions, predictions for cross sections and s-wave phase shifts for the D̄0N and D-N reactions are obtained without introducing new parameters. © 2013 American Physical Society.

Linking natural supersymmetry to flavour physics

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Sensitivity of the Top quark mass measurement with the CMS experiment at LHC using t-tbar multijet simulated events

This thesis comes after a strong contribution on the realization of the CMS computing system, which can be seen as a relevant part of the experiment itself. A physics analysis completes the road from Monte Carlo production and analysis tools realization to the final physics study which is the actual goal of the experiment. The topic of physics work of this thesis is the study of tt events fully hadronic decay in the CMS experiment. A multi-jet trigger has been provided to fix a reasonable starting point, reducing the multi-jet sample to the nominal trigger rate. An offline selection has been provided to reduce the S/B ratio. The b-tag is applied to provide a further S/B improvement. The selection is applied to the background sample and to the samples generated at different top quark masses. The top quark mass candidate is reconstructed for all those samples using a kinematic fitter. The resulting distributions are used to build p.d.f.’s, interpolating them with a continuous arbitrary curve. These curves are used to perform the top mass measurement through a likelihood comparison

Baryons in the chiral regime

Quantum Chromodynamics (QCD) is the theory of strong interactions, one of the four fundamental forces in our Universe. It describes the interaction of gluons and quarks which build up hadrons like protons and neutrons. Most of the visible matter in our universe is made of protons and neutrons. Hence, we are interested in their fundamental properties like their masses, their distribution of charge and their shape. \\rnThe only known theoretical, non-perturbative and {\it ab initio} method to investigate hadron properties at low energies is lattice Quantum Chromodynamics (lattice QCD). However, up-to-date simulations (especially for baryonic quantities) do not achieve the accuracy of experiments. In fact, current simulations do not even reproduce the experimental values for the form factors. The question arises wether these deviations can be explained by systematic effects in lattice QCD simulations.rnrnThis thesis is about the computation of nucleon form factors and other hadronic quantities from lattice QCD. So called Wilson fermions are used and the u- and d-quarks are treated fully dynamically. The simulations were performed using gauge ensembles with a range of lattice spacings, volumes and pion masses.\\rnFirst of all, the lattice spacing was set to be able to make contact between the lattice results and their experimental complement and to be able to perform a continuum extrapolation. The light quark mass has been computed and found to be $m_{ud}^{\overline{\text{MS}}}(2\text{ GeV}) = 3.03(17)(38)\text{ MeV}$. This value is in good agreement with values from experiments and other lattice determinations.\\rnElectro-magnetic and axial form factors of the nucleon have been calculated. From these form factors the nucleon radii and the coupling constants were computed. The different ensembles enabled us to investigate systematically the dependence of these quantities on the volume, the lattice spacing and the pion mass.\newpage Finally we perform a continuum extrapolation and chiral extrapolations to the physical point.\\rnIn addition, we investigated so called excited state contributions to these observables. A technique was used, the summation method, which reduces these effects significantly and a much better agreement with experimental data was achieved. On the lattice, the Dirac radius and the axial charge are usually found to be much smaller than the experimental values. However, due to the carefully investigation of all the afore-mentioned systematic effects we get $\langle r_1^2\rangle_{u-d}=0.627(54)\text{ fm}^2$ and $g_A=1.218(92)$, which is in agreement with the experimental values within the errors.rnrnThe first three chapters introduce the theoretical background of form factors of the nucleon and lattice QCD in general. In chapter four the lattice spacing is determined. The computation of nucleon form factors is described in chapter five where systematic effects are investigated. All results are presented in chapter six. The thesis ends with a summary of the results and identifies options to complement and extend the calculations presented. rn

Measurement of jet shapes in top-quark pair events at sqrts = 7 TeV using the ATLAS detector

A measurement of jet shapes in top-quark pair events using 1.8 fb−1 of s√=7 TeV pp collision data recorded by the ATLAS detector at the LHC is presented. Samples of top-quark pair events are selected in both the single-lepton and dilepton final states. The differential and integrated shapes of the jets initiated by bottom-quarks from the top-quark decays are compared with those of the jets originated by light-quarks from the hadronic W-boson decays W→qq¯′ in the single-lepton channel. The light-quark jets are found to have a narrower distribution of the momentum flow inside the jet area than b-quark jets.