SciPost Thesis Link
| Title: | Higgs Effective Field Theories - Systematics and Applications | |
| Author: | Claudius Krause | |
| As Contributor: | Claudius Krause | |
| Type: | Ph.D. | |
| Field: | Physics | |
| Specialties: |
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| Approaches: | Theoretical, Phenomenological | |
| URL: | https://doi.org/10.5282/edoc.19873 | |
| Degree granting institution: | LMU Munich | |
| Supervisor(s): | Gerhard Buchalla | |
| Defense date: | 2016-09-15 |
Abstract:
Researchers of the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) announced on July 4th, 2012, the observation of a new particle. The properties of the particle agree, within the relatively large experimental uncertainties, with the properties of the long-sought Higgs boson. Particle physicists around the globe are now wondering, "Is it the Standard Model Higgs that we observe; or is it another particle with similar properties?" We employ effective field theories (EFTs) for a general, model-independent description of the particle. We use a few, minimal assumptions - Standard Model (SM) particle content and a separation of scales to the new physics - which are supported by current experimental results. By construction, effective field theories describe a physical system only at a certain energy scale, in our case at the electroweak-scale $v$. Effects of new physics from a higher energy-scale, $\Lambda$, are described by modified interactions of the light particles. In this thesis, "Higgs Effective Field Theories - Systematics and Applications", we discuss effective field theories for the Higgs particle, which is not necessarily the Higgs of the Standard Model. In particular, we focus on a systematic and consistent expansion of the EFT. The systematics depends on the dynamics of the new physics. We distinguish two different consistent expansions. EFTs that describe decoupling new-physics effects and EFTs that describe non-decoupling new-physics effects. We briefly discuss the first case, the SM-EFT. The focus of this thesis, however, is on the non-decoupling EFTs. We argue that the loop expansion is the consistent expansion in the second case. We introduce the concept of chiral dimensions, equivalent to the loop expansion. Using the chiral dimensions, we expand the electroweak chiral Lagrangian up to next-to-leading order, $\mathcal{O}(f^2/\Lambda^2)=\mathcal{O}(1/16\pi^2)$. Further, we discuss how different assumptions on the custodial symmetry in the Higgs sector influences the list of operators in the basis. Finally, we compare the decoupling and the non-decoupling EFT. We also consider scenarios in which the new-physics sector is non-decoupling at a scale $f$, far above the electroweak-scale $v$. We discuss the relevance of the resulting double expansion in $\xi=v^2/f^2$ and$ f^2/\Lambda^2$ for the data analysis at the LHC.In the second part of this thesis, we discuss the applications of the EFTs, especially of the electroweak chiral Lagrangian. First, we connect the EFT with explicit models of new physics. This illustrates how the power counting works in a specific example. We show how different regions of the parameter space of the same model generate a decoupling and a non-decoupling EFT. Second, we use the expansion at leading order to describe the current LHC Higgs data. We show how the current parametrization of the Higgs data, which is used by the experimentalists at CERN (the $\kappa$-framework), can be justified quantum field theoretically by the EFT. The result of a fit does therefore not only indicate whether we observe the SM-Higgs, but also, in case there are deviations, what kind of new physics is preferred. In this thesis, we fit the data of Run-1 (2010-2013). The effective Lagrangian describing this data can be reduced to six free parameters. The result of this fit is consistent with the SM. It has, however, statistical uncertainties of about ten percent.