Jefferson Lab to Probe Dark Photons
A simulation study outlines a proposed experiment using a 500 MeV positron beam on a hydrogen target to search for light dark photons in the 7-19 MeV mass
A new simulation study details a proposed experiment to search for light dark photons using a positron beam at Jefferson Lab. The research, described in a paper submitted to arXiv, evaluates the sensitivity of a missing-mass search employing a 500 MeV positron beam incident on a thin liquid-hydrogen target.
This investigation is motivated by ongoing research and development toward a low-energy positron facility at Jefferson Lab. The proposed experiment would use a high-resolution forward electromagnetic calorimeter to reconstruct final-state photons.
The Missing-Mass Technique
The core of the proposed search is the missing-mass technique applied to the annihilation-in-flight process where a positron and an electron annihilate to produce a photon and a hypothetical dark photon, denoted as A'. This method is crucial because it provides sensitivity to the A' particle independently of its decay mode. The experiment would not need to detect how the dark photon decays, which is often unknown.
Researchers conducted a Geant4-based simulation to model the experiment. They evaluated expected backgrounds from standard quantum electrodynamics (QED) processes. These include annihilation and bremsstrahlung, which could mimic or obscure a dark photon signal.
Projected Sensitivity and Mass Range
The simulation projects the experiment's sensitivity to the kinetic-mixing parameter, ε², across a range of dark photon masses. Kinetic mixing is a key parameter that describes how strongly the hypothetical dark photon interacts with ordinary photons. A lower ε² value indicates a weaker interaction, making the particle harder to detect.
With the 500 MeV beam, the experiment is designed to probe the dark photon, or A', across a specific mass range. The study presents the projected sensitivity as a function of the A' mass.
| Beam Energy | A' Mass Range | Projected ε² Sensitivity at 19 MeV |
|---|---|---|
| 500 MeV | 7-19 MeV | 4×10⁻⁸ |
The projected sensitivity on ε² reaches 4×10⁻⁸ for a dark photon with a mass of 19 MeV. This defines the experiment's potential to discover or constrain the properties of these hypothetical particles.
Motivation and Future Steps
The work is explicitly motivated by the development of a low-energy positron facility at Jefferson Lab. Such a facility would enable this and other precision experiments. The simulation study provides a foundation for the experimental design and sensitivity estimates needed to advance the proposal.
By focusing on the annihilation-in-flight process and the missing-mass technique, the proposed setup aims to circumvent uncertainties related to dark photon decays. The study evaluates backgrounds to ensure the signal would be distinguishable from known Standard Model processes.
The final projected sensitivity curve, showing ε² across the 7-19 MeV mass range, charts the experiment's potential reach in the search for light dark matter force carriers.
