Abstract
The use of the SMEFT Lagrangian to quantify possible Beyond the Standard Model (BSM) effects is standard in LHC and future collider studies. One of the usual assumptions is to truncate the expansion with the dimension-$6$ operators. The numerical impact of the next terms in the series, the dimension-$8$ operators, is unknown in general. We consider a specific BSM model containing a charge-${2 3}$ heavy vector-like quark and compute the operators generated at dimension-$8$. The numerical effects of these operators are studied for the $t\bar{t}h$ process, where they contribute at tree level and we find effects at the ${\cal{O}}(0.5-2\%)$ level for allowed values of the parameters.
Highlights
One of the goals of the high luminosity LHC (HL-LHC) running is a precision physics program that enables a detailed comparison of theoretical and experimental predictions
Lacking the experimental discovery of any new particles, the tool of choice is the Standard Model effective field theory (SMEFT) which assumes that the gauge symmetries and particles of the Standard Model provide an approximate description of weak scale physics [1]
All of the new physics information resides in the coefficient functions, CðinÞ, which can be extracted from experimental data
Summary
One of the goals of the high luminosity LHC (HL-LHC) running is a precision physics program that enables a detailed comparison of theoretical and experimental predictions. The SMEFT series is usually terminated at dimension six and the amplitude is computed to Oð1=Λ2Þ, generating Oð1=Λ4Þ contributions in cross sections This leaves an uncertainty about the numerical relevance of the higher dimension operators. Within the context of this model, the coefficients of the dimensionsix and dimension-eight operators can be calculated using the covariant derivative expansion [30,31] and matched to the SMEFT This allows for a detailed numerical analysis of the various approximations frequently used when computing observables in the SMEFT. We consider tth associated production in the SMEFT limit of the TVLQ and are able to concretely determine the numerical relevance of the dimension-eight contributions to this process at tree level. The appendixes include a short summary of the relevant dimension-eight interactions and a brief discussion of one-loop matching in the TVLQ model
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