Missing energy signature from invisible decays of dark photons at the CERN SPS

TL;DR

This paper investigates the potential of the NA64 experiment at CERN SPS to detect invisible decays of dark photons via missing energy signatures, providing detailed simulations and sensitivity estimates for unexplored parameter space.

Contribution

It presents a detailed simulation and analysis of the missing energy signature for dark photon decays at NA64, including sensitivity estimates and parameter extraction methods.

Findings

NA64 can probe mixing strengths from 10^{-6} to 10^{-2}

Sensitivity to dark photon masses up to 1 GeV

Potential to observe and characterize dark photons related to the Be-8 anomaly

Abstract

The dark photon ($A'$) production through the mixing with the bremsstrahlung photon from the electron scattering off nuclei can be accompanied by the dominant invisible $A'$ decay into dark-sector particles. In this work we discuss the missing energy signature of this process in the experiment NA64 aiming at the search for $A'\to invisible$ decays with a high-energy electron beam at the CERN SPS. We show the distinctive distributions of variables that can be used to distinguish the $A'\to invisible$ signal from background. The results of the detailed simulation of the detector response for the events with and without $A'$ emission are presented. The efficiency of the signal event selection is estimated. It is used to evaluate the sensitivity of the experiment and show that it allows to probe the still unexplored area of the mixing strength $10^{-6}\lesssim \epsilon \lesssim 10^{-2}$ and masses up to $M_{A'} \lesssim 1$ GeV. The results obtained are compared with the results from other calculations. In the case of the signal observation, a possibility of extraction of the parameters $M_{A'}$ and $\epsilon$ by using the missing energy spectrum shape is discussed. We consider as an example the $A'$ with the mass 16.7 MeV and mixing $\epsilon \lesssim 10^{-3}$, which can explain an excess of events recently observed in nuclear transitions of an excited state of $^8$Be. We show that if such $A'$ exists its invisible decay can be observed in NA64 within a month of running, while data accumulated during a few months would allow also to determine the $\epsilon$ and $M_{A'}$ parameters.