Characterizing Invisible Electroweak Particles through Single-Photon Processes at High Energy $e^+e^-$ Colliders

TL;DR

This paper investigates how high-energy $e^+e^-$ colliders can detect and characterize nearly invisible electroweak particles, such as Higgsinos, winos, and sleptons, by analyzing single-photon production processes with polarized beams.

Contribution

It introduces a method to distinguish and analyze nearly degenerate electroweak particles at $e^+e^-$ colliders using single-photon signatures and polarization, which are challenging to detect with traditional methods.

Findings

Single-photon processes can reveal the presence of nearly invisible electroweak states.

Photon energy and angular distributions allow determination of particle spin.

Polarized beams enhance the ability to characterize new particles.

Abstract

We explore the scenarios where the only accessible new states at the electroweak scale consist of a pair of color-singlet electroweak particles, whose masses are degenerate at the tree level and split only by electroweak symmetry breaking at the loop level. For the sake of illustration, we consider a supersymmetric model and study the following three representative cases with the lower-lying states as (a) two spin-1/2 Higgsino SU(2)$_L$ doublets, (b) a spin-1/2 wino SU(2)$_L$ triplet and (c) a spin-0 left-handed slepton SU(2)$_L$ doublet. Due to the mass-degeneracy, those lower-lying electroweak states are difficult to observe at the LHC and rather challenging to detect at the $e^+ e^-$ collider as well. We exploit the pair production in association with a hard photon radiation in high energy $e^+ e^-$ collisions. If kinematically accessible, such single-photon processes at $e^+e^-$ colliders with polarized beams enable us to characterize each scenario by measuring the energy and scattering angle of the associated hard photon, and to determine the spin of the nearly invisible particles unambiguously through the threshold behavior in the photon energy distribution.