Scattering unitarity with effective dimension-6 operators

TL;DR

This paper investigates how higher-dimensional operators in effective field theories can lead to unitarity violations in scattering processes at the LHC, providing bounds on Wilson coefficients and identifying key channels for experimental tests.

Contribution

It analyzes the impact of dimension-6 operators on unitarity in scattering amplitudes and establishes bounds on Wilson coefficients for potential new physics detection.

Findings

Unitarity can be violated at LHC energies with small Wilson coefficients.

Certain scattering channels are most sensitive to unitarity violations.

Deviations from the Standard Model may be observed with even tiny coefficients.

Abstract

The effects of physics beyond the Standard Model may be parametrized by a set of higher-dimensional operators leading to an effective theory. The introduction of these operators makes the theory nonrenormalizable, and one may reasonably expect a violation of unitarity in $2\to 2$ scattering processes,depending on the values of the Wilson coefficients of the higher dimensional operators. Bounds on these coefficients may be obtained from demanding that there be no such unitarity violation below the scale of the effective theory. We show, at the lowest level, how the new operators affect the scattering amplitudes with longitudinal gauge bosons, scalars, and $t\bar{t}$ in the final state, and find that one may expect a violation of unitarity even at the LHC energies with small values of some of the new Wilson coefficients. For most of the others, such a violation needs large coefficients, indicating nonperturbative physics for the ultraviolet-complete theory, although a proper treatment necessitates the inclusion of even higher-dimensional operators. However, deviations from the Standard Model expectations may be observed with even smaller values for these coefficients. We find that $WW \to WW$, $WW\to ZZ$, and $ZZ\to hh$ scatterings are the best possible channels to probe unitarity violations.