The Standard Model as an Effective Field Theory
Dr. Xiaochuan Lu
Department of Physics
University of Oregon
The Standard Model (SM) is a great success, yet it needs to be viewed as a low-energy Effective Field Theory (EFT) in light of various unanswered questions. In this talk, I will discuss how to take this point of view to study BSM physics. First, SM should be viewed as the decoupling limit of SMEFT, which is defined to include non-renormalizable interactions that preserve SM symmetries to form a robust parameterization of BSM effects. Once truncated, SMEFT has only a finite number of counterterms/operators, making it well defined. This can be systematically explored with the method of Hilbert series. With the framework established, I will discuss how to use SMEFT. When used as an interpretation of the experimental limits, global fitting results show that the current bounds are typically of a few TeV, which naturally raises a validity question regarding using SMEFT for BSM searches at the LHC. I will explain how PDF effects could be important for this concern and sometimes salvage the validty of SMEFT. When used as a guidance for UV model building, a matching and running procedure is needed to connect SMEFT with a given UV theory. Functional methods can signifcantly facilitate these calculations. It is also interesting to consider imposing further symmetries/constraints on SMEFT to reduce the number of degrees of freedom, which is apt for certain classes of BSM physics. Finally, I will revisit the framework and emphasize that SMEFT is not always enough --- certain non-decoupling BSM physics is still viable but can only be captured by HEFT.
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