Non-Decoupling New Particles

Abstract

We initiate the study of a new class of beyond the Standard Model states that we call "Loryons." They have the defining characteristic of being nondecoupling, in the sense that their physical mass is dominated by a contribution from the vacuum expectation value of the Higgs boson. The stakes are high: the discovery of a Loryon would tell us that electroweak symmetry must be non-linearly realized in the effective field theory of the Standard Model. Loryons have their masses bounded from above by perturbative unitarity considerations and thus define a finite parameter space for exploration. After providing a complete catalog of Loryon representations under mild assumptions, we turn to examining the constraints on the parameter space from Higgs couplings measurements, precision electroweak tests, and direct collider searches. We show that most fermionic candidates are already ruled out (with some notable exceptions), while much of the scalar Loryon parameter space is still wide open for discovery.

On amplitudes and field redefinitions

“…This makes the invariance of amplitudes under the restricted set of field redefinitions completely manifest. This approach has seen recent applications to the scalar sector of the Standard Model [64][65][66][67][68][69][70][71][72][73][74], and has also led to new insights into the properties of amplitudes for both scalars and particles of higher spins [75][76][77][78][79][80][81][82][83][84][85][86].…”

Model-Agnostic Exploration of the Mass Reach of Precision Higgs Boson Coupling Measurements

“…In models with a Z 2 symmetry that forbids this term, substantial reach for effects of the singlet field can still be obtained through loop effects; see [15].…”

Unitarity Bounds on Effective Field Theories at the LHC

“…In this paper, we focus on the s-wave component for simplicity. It is somewhat tedious to keep track of all the S-matrix components corresponding to the various spin configurations when fermions are involved in the scattering (see example calculations in [23,25,47,110,138] and [75,80] for recent reviews). Minimizing this technical complication is the reason for studying the scalar toy models introduced in Sec.…”

“…• Refs. [138,[143][144][145] argued that one must include all orders in the field expansion in order to correctly identify if a BSM EFT can be matched onto SMEFT (as opposed to being forced to match onto the more general formulation with non-linearly realized electroweak symmetry breaking). There is additionally a close relation between unitarity violation and inclusive amplitudes involving an arbitrary number of fields in the final state [146,147].…”