The triplet photoproduction on a free electron as a possible way to search for a dark photon

TL;DR

This paper investigates the triplet photoproduction process on a free electron as a potential method to detect dark photons through their virtual production and decay into electron-positron pairs, focusing on resonance effects and background suppression.

Contribution

It introduces a novel approach to search for dark photons using triplet production, emphasizing the resonance contribution of the dark photon in Compton-like diagrams and analyzing the kinematic regions for optimal detection.

Findings

Identifies kinematic regions where dark photon signals can be distinguished from QED background.

Estimates the sensitivity to the kinetic mixing parameter $$ as a function of dark photon mass.

Provides calculations of invariant mass distributions relevant for experimental searches.

Abstract

The process of the triplet production on a free electron, $\gamma e^-\to e^+e^-e^-$, has been investigated as a reaction where a dark photon, $A'$, is produced as a virtual state with subsequent decay into a $e^+e^-$-pair. This effect arises due to the so-called kinetic mixing and is characterized by the small parameter $\epsilon$ describing the coupling strength relative to the electric charge e. The search of $A'$ in this process has advantage because the background to the $A'$ signal is pure QED. This QED background is described by eight Feynman diagrams taking into account the identity of final electrons. As concern $A'$, we leave its contribution in Compton-like diagrams only since, in this case, the virtual dark photon is time-like and its propagator has the Breit-Wigner form. So, near the resonance $A'$ can manifest itself. The contribution of $A'$ in Borsellino diagrams is negligible since, in this case, the virtual dark photon is space-like, the $A'$ propagator does not peak and effect is proportional at least to $\epsilon^2$. We calculate the distributions over the invariant masses of the both produced $e^+e^-$ pairs and search for the kinematical region where the Compton-like diagrams contribution is not suppressed as compared with the Borsellino one. We estimate what value of the parameter $\epsilon$, as a function of the dark photon mass, can be obtained at given number of the measured events.