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The Importance of Flavor in SMEFT Electroweak Precision Fits

Published 31 Mar 2023 in hep-ph | (2304.00029v3)

Abstract: Effective field theory tools are essential for exploring non-Standard Model physics at the LHC in the absence of the discovery of new light particles. Predictions for observables are typically made at the lowest order in the QCD and electroweak expansions in the Standard Model effective field theory (SMEFT) and often ignore the effects of flavor. Here, we present results for electroweak precision observables (EWPOs)at the next-to-leading order QCD and electroweak expansions (NLO) of the SMEFT with an arbitrary flavor structure for the fermion operators. Numerical NLO SMEFT fits toEWPOs have a strong dependence on the assumed flavor structures and we demonstrate this using various popular assumptions for flavor symmetries.

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Summary

  • The paper demonstrates that flavor assumptions in SMEFT significantly alter electroweak precision fit constraints by affecting Wilson coefficient limits.
  • It employs next-to-leading order (NLO) analyses in both QCD and electroweak processes to compare diverse flavor scenarios.
  • The findings advocate for nuanced treatment of flavor in BSM physics, guiding future HL-LHC experiments and precision studies.

An Analysis of Flavor in SMEFT Electroweak Precision Fits

The paper "The Importance of Flavor in SMEFT Electroweak Precision Fits" by Luigi Bellafronte, Sally Dawson, and Pier Paolo Giardino addresses the significant role of flavor assumptions in the context of the Standard Model Effective Field Theory (SMEFT) when aiming to understand electroweak precision observables (EWPOs) at the High Luminosity Large Hadron Collider (HL-LHC). This investigation is pertinent due to the absence of novel light particles, which necessitates alternative strategies to explore physics beyond the Standard Model (BSM).

SMEFT Framework and Flavor Assumptions

The SMEFT framework is constructed by expanding beyond the Standard Model (SM) using higher-dimension operators that preserve the SU(3)×SU(2)×U(1)SU(3) \times SU(2) \times U(1) gauge symmetry. Such an extension allows for the capturing of potential unseen effects of BSM physics through these higher-dimension operators. The coefficients of these dimension-6 operators reflect the underlying UV complete theory, offering a means to bridge predictions at higher scales down to those measurable at collider scales such as the LHC.

Crucially, the paper puts a spotlight on the flavor structure of fermionic operators within SMEFT. Traditionally, many calculations and fits, even at leading order (LO), do not thoroughly consider flavor effects beyond a simplistic assumption. The paper acknowledges that this can lead to significant interpretational challenges, especially when SMEFT predictions are marginalized over specific flavor structures. Different assumptions about how flavor might manifest or be conserved in BSM physics lead to dramatically different constraints on the Wilson coefficients of the SMEFT operators.

Theoretical and Numerical Contributions

The authors produce a comprehensive analysis of these operators at next-to-leading order (NLO) in both QCD and electroweak processes, incorporating complex flavor structures that previous works might ignore or oversimplify. They offer insights into flavor scenarios like U(3)5U(3)^5, minimal flavor violation (MFV), U(2)5U(2)^5, and scenarios with third generation-centric or phobic tendencies. These assumptions essentially pattern how fermionic flavors will affect the integrated limits on BSM physics via SMEFT operator coefficients.

Strong numerical results and constraints emerge from this robust analysis. For example, they find substantial discrepancies in the tightness of constraints on different coefficient functions when switching among these flavor assumptions, highlighting the necessity for carefully considering flavor in precision fits. The NLO corrections generally provide small adjustments from LO predictions for 2-fermion operators, but the dependencies become far more pronounced for operators that involve 4-fermion interactions.

Implications and Future Directions

The findings in this paper suggest that caution is warranted when making broad statements or setting constraints on BSM parameters without thoroughly considering the underlying flavor assumptions. The significant impact of flavor on constraints indicates that a more nuanced approach might be required to genuinely understand the interplay between LHC results and theoretical predictions.

Future work can extend these findings by examining more global datasets that include electroweak precision tests, top quark processes, and potentially non-collider experiments, broadening the statistical base to reinforce or challenge these flavor-dependent constraints. Additionally, the theoretical framework could be expanded to include dimension-8 operators or look closer at loop-level processes in both the SMEFT context and within specific UV completions.

Conclusion

By rigorously addressing the flavor structure within SMEFT and its effects on precision fits to electroweak observables, this paper advances the methodology for testing BSM physics in light of current experimental limits. Such an approach is essential for properly constraining new physics models and for preparing for future experiments that could probe deeper into the structure of the universe.

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