Search

We present predictions at next-to-leading order (NLO) accuracy in QCD for top-quark pair production induced by an anomalous chromomagnetic dipole moment of the top quark. Our results are obtained for total as well as fully differential cross sections, including matching to parton shower simulations. This process is expected to provide the most stringent direct limits on top-quark chromomagnetic dipole moment. We find that NLO corrections increase the contribution from the dipole moment by about 50% at the LHC, and significantly reduce the renormalization and factorization scale dependence. Using the NLO prediction, we update the current limit from the Tevatron and the LHC measurements. Apart from total cross section, we also study other observables relevant for LHC phenomenology. [ABSTRACT FROM AUTHOR]

We present analytical results for top-quark decay processes, in an effective field theory beyond the standard model, at next-to-leading order in QCD. We parametrize new physics effects using dimension-six operators, and consider all operators that give rise to nonstandard interactions of the top quark. We investigate both the flavor-conserving and flavor-changing decay modes, including their two-body and three-body semileptonic final states. The QCD mixing among relevant operators is also taken into account. These results provide all information needed for a complete model-independent study of top-quark decay at next-to-leading order accuracy, paving the way to global analyses for new physics effects in an effective field theory approach. [ABSTRACT FROM AUTHOR]

Neutral flavor-changing transitions are hugely suppressed in the Standard Model and therefore they are very sensitive to new physics. We consider the decay rate of t → uih where ui = u,c using an effective field theory approach. We perform the calculation at next-to-leading order (NLO) in QCD including the relevant dimension-six operators. We find that at NLO the contribution from the flavor-changing chromomagnetic operator is as important as the standard QCD correction to the flavor-changing Yukawa coupling. In addition to improving the accuracy of the theoretical predictions, the NLO calculation provides information on the operator mixing under the renormalization group. [ABSTRACT FROM AUTHOR]

We study nonstandard top quark couplings in the effective field theory approach. All nine dimension-six operators that generate anomalous couplings between the electroweak gauge bosons and the third-generation quarks are included. We calculate their contributions at tree level and one loop to all major precision electroweak observables. The calculations are compared with data to obtain constraints on eight of these operators. [ABSTRACT FROM AUTHOR]

The amplitude for Higgs decay to two photons is calculated in renormalizable and unitary gauges using dimensional regularization at intermediate steps. The result is finite, gauge independent, and in agreement with previously published results. The large Higgs mass limit is examined using the Goldstone-boson equivalence theorem as a check on the use of dimensional regularization and to explain the absence of decoupling. [ABSTRACT FROM AUTHOR]

We discuss new physics in top-quark interactions, using an effective-field-theory approach. We consider top-quark decay, single top production, and top-quark pair production. We identify 15 dimension-six operators that contribute to these processes, and we compute the deviation from the standard model induced by these operators. The results provide a systematic way of searching for (or obtaining bounds on) physics beyond the standard model. [ABSTRACT FROM AUTHOR]

Weak vector boson scattering (VBS) is a sensitive probe of new physics effects in the electroweak symmetry breaking. Currently, experimental results at the LHC are interpreted in the effective field theory approach, where possible deviations from the Standard Model in the quartic-gauge-boson couplings are often described by 18 dimension-eight operators. By assuming that an UV completion exists, we derive a new set of theoretical constraints on the coefficients of these operators; i.e., certain combinations of coefficients must be positive. These constraints imply that the current effective approach to VBS has a large redundancy: only about 2% of the full parameter space leads to an UV completion. By excluding the remaining unphysical region of the parameter space, these constraints provide guidance for future VBS studies and measurements. [ABSTRACT FROM AUTHOR]

We propose a new method for measuring the top-quark width based on the on-/off-shell ratio of b-charge asymmetry in pp → Wbj production at the LHC. The charge asymmetry removes virtually all backgrounds and related uncertainties, while remaining systematic and theoretical uncertainties can be taken under control by the ratio of cross sections. Limited only by statistical error, in an optimistic scenario, we find that our approach leads to good precision at high integrated luminosity, at a few hundred MeV assuming 300-3000 fb-1 at the LHC. The approach directly probes the total width, in such a way that model-dependence can be minimized. It is complementary to existing cross section measurements which always leave a degeneracy between the total rate and the branching ratio, and provides valuable information about the properties of the top quark. The proposal opens up new opportunities for precision top measurements using a b-charge identification algorithm. [ABSTRACT FROM AUTHOR]

We adopt a fully gauge-invariant effective-field-theory approach for parametrizing top-quark flavorchanging- neutral-current interactions. It allows for a global interpretation of experimental constraints (or measurements) and the systematic treatment of higher-order quantum corrections. We discuss some recent results obtained at next-to-leading-order accuracy in QCD and perform, at that order, a first global analysis of a subset of the available experimental limits in terms of effective operator coefficients. We encourage experimental collaborations to adopt this approach and extend the analysis by using all information they have prime access to. [ABSTRACT FROM AUTHOR]

We derive bounds from oblique parameters on the dimension-6 operators of an effective field theory of electroweak gauge bosons and the Higgs doublet. The loop-induced contributions to the ∆S, ∆T, and ∆U oblique parameters are sensitive to these contributions, and we pay particular attention to the role of renormalization when computing loop corrections in the effective theory. Limits on the coefficients of the effective theory from loop contributions to oblique parameters yield complementary information to direct Higgs production measurements. [ABSTRACT FROM AUTHOR]

We derive bounds on nine dimension-six operators involving electroweak gauge bosons and the Higgs boson from precision electroweak data. Four of these operators contribute at tree level, and five contribute only at one loop. Using the full power of effective field theory, we show that the bounds on the five loop-level operators are much weaker than previously claimed, and thus much weaker than bounds from tree-level processes at high-energy colliders. [ABSTRACT FROM AUTHOR]

The phenomenology of an unstable particle, including searches and exclusion limits at the LHC, depends significantly on its lineshape. When the width of the resonance is large with respect to its mass, off-shell effects become relevant and the very same definition of width becomes nontrivial. Taking a heavy Higgs boson as an example, we propose a new formulation to describe the lineshape via an effective field theory approach. Our method leads to amplitudes that are gauge invariant, respect unitarity, and can appropriately describe the lineshape of broad resonances. The application of the method to the following relevant processes for the LHC phenomenology have been considered: gluon fusion, vector boson scattering, and tt production via weak boson fusion. [ABSTRACT FROM AUTHOR]

Computations at next-to-leading order in the Standard Model offer new technical challenges in the presence of higher dimensional operators. We introduce a framework that, starting from the top-quark effective field theory at dimension six, allows one to make predictions for cross sections as well as distributions in a fully automatic way. As an application, we present the first complete results at next-toleading order in QCD for flavor-changing neutral interactions including parton shower effects, for tZ, th, tγ associated production at the LHC. [ABSTRACT FROM AUTHOR]

A correction to the article "Effective field theory approach to the Higgs lineshape" in the 2013 issue is presented.

We present the automation of one-loop computations in the standard-model effective field theory at dimension 6. Our general implementation, dubbed smeft@nlo, covers all types of operators: bosonic and two- and four-fermion ones. Included ultraviolet and rational counterterms presently allow for fully differential predictions, possibly matched to the parton shower, up to the one-loop level in the strong coupling or in four-quark operator coefficients. Exact flavor symmetries are imposed among light quark generations, and an initial focus is set on top-quark interactions in the fermionic sector. We illustrate the potential of this implementation with novel loop-induced and next-to-leading-order computations relevant for top-quark, electroweak, and Higgs-boson phenomenology at the LHC and future colliders. [ABSTRACT FROM AUTHOR]