Beyond the Standard Model: Dark Mesons and Custodial Symmetry
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Date
2020-09-24
Authors
Tong, Tom
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Publisher
University of Oregon
Abstract
We describe our investigations on possible new physics beyond the Standard Model that reveal their connections with custodial symmetry.
First, we consider several strongly-coupled dark sectors with fermions that transform under the electroweak group. We construct the non-linear sigma model describing the dark pions and match the ultraviolet theory onto a low energy effective theory that provides the leading interactions of the lightest dark pions with the Standard Model. We uncover two distinct classes of effective theories: “Gaugephilic” and “Gaugephobic”.
Second, we demonstrate such a dark sector could be accessible to current searches by studying the production and decay of dark mesons at the LHC. Dark pions can be pair-produced and decay in one of two distinct ways: “gaugephilic” or “gaugephobic”. We recast a vast set of existing LHC searches to determine the current constraints on the dark meson. We find the relative insensitivity of LHC searches, especially at 13 TeV, can be blamed mainly on their penchant for high mass objects or large missing energy. Future dedicated searches would undoubtedly improve sensitivity.
Finally, we consider custodially-symmetric UV physics, mapping their effects onto higher-dimensional operators in a custodial basis. This basis explicitly identifies the global SU(2)R symmetries of the Higgs and flavor sector with custodial preserving and violating operators. Custodially symmetric UV physics that contributes purely to oblique corrections at leading matching order leads to the electroweak \rho-parameter = 1 at tree-level. Nevertheless, such UV physics can also generate non-oblique corrections, and thus \rho = 1 is insufficient to claim custodial violation. We therefore identify a set of observables that are able to capture the leading tree-level effects of integrating out a custodially-symmetric UV sector. We illustrate our results with four examples: a heavy singlet scalar; a heavy Z' transforming under U(1)B-L; heavy W's and Z's transforming under SU(2)L X SU(2)R X U(1)B-L; and a heavy W'L coupling purely to left-handed fields. These examples demonstrate that our observables could be used to “fingerprint” custodial symmetry of UV physics.
This dissertation consists of previously published and unpublished co-authored material.
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Keywords
Custodial Symmetry, Dark Matter, Dark Meson, Effective Theory, SMEFT, Strongly-Coupled