Speaker
Description
The X17 is a hypothetical light boson (${\sim}17\,\text{MeV}/c^2$) proposed to explain anomalous $e^+e^-$ pair excesses observed in nuclear de-excitation decays at ATOMKI. No independent experiment has yet definitively confirmed or refuted the anomaly. We present the first search for X17 using a neutron beam, conducted at CERN's n_TOF facility (EAR2). Neutron capture on a pressurized $^3\text{He}$ target produces excited $^4\text{He}^*$ at ${\sim}20\,\text{MeV}$, which may de-excite via X17 production. The subsequent decay to MeV-scale $e^+e^-$ should be kinematically peaked at an opening angle of around $120^\circ$, making precise and efficient MIP tracking the central detector requirement.
The experimental setup relies on four $40 \times 40\,\text{cm}$ resistive Micromegas detectors developed at CEA Saclay, arranged to detect coincident $e^+e^-$ pairs. These detectors feature a novel pad-strip readout geometry with 512 channels per axis. A top layer of pads are connected by vias to $X$ and $Y$ strip layers below to facilitate equal charge sharing. With a $3\,\text{cm}$ drift gap, the detectors are designed for $\mu$TPC mode operation to provide full 3D track reconstruction --- to our knowledge, the first application of Micromegas in $\mu$TPC mode for a dedicated physics experiment. Performance characterized with cosmic muons has demonstrated high detection efficiency, uniform response, and an angular resolution better than $2^\circ$ for track inclinations above $5^\circ$, validating the $\mu$TPC approach for precise vertex and angular measurements.
Operating these detectors in the n_TOF EAR2 environment presents significant challenges, primarily the intense ``gamma-flash'' produced when each proton bunch strikes the lead spallation target, which can generate sufficient charge to saturate the readout electronics. To mitigate these effects, we explore a range of gas mixtures to minimize photon and neutron sensitivity while preserving high efficiency for Minimum Ionizing Particles (MIPs). Additionally, we describe the implementation of an adjustable drift window designed to tune the active volume, reducing the charge produced from the prompt flash.
| Name of the speaker | Dylan Neff |
|---|---|
| Eligible for the Georges Charpak Young Scientist Award. | yes |