Speaker
Description
The MEGII experiment at the Paul Scherrer Institute (PSI) has set the best upper limit in the world on the branching ratio of the Charged Lepton Flavor Violating (CLFV) decay $\mu^+ \rightarrow e^+ \gamma$, equal to $\mathcal{B}(\mu^{+} \rightarrow e^+ \gamma)<3.1\times 10^{-13} \text{ (90% confidence level)}$. If this decay is observed experimentally it would indicate evidence of New Physics (NP) beyond the Standard Model (SM), while limits on its $\mathcal{B}$ will constrain NP models' parameters.
To improve the results of MEGII, which aims to reach a sensitivity of $6 \times 10^{-14}$ by collecting data until 2026, efforts are ongoing at PSI and elsewhere to increase the muon beam intensity up to $\mathcal{O}(10^{10})\mu$/s. In such high-rate environments, the detector components must be upgraded accordingly. A convenient method for reconstructing the photon could be to convert it into an $e^+e^-$ pair and track the emitted leptons to measure the $\gamma$ energy and direction.
Currently, a radial Time Projection Chamber (TPC) with a drift length of $\sim$10cm, a Resistive Micro-Wells ($\mu$RWells) amplification stage and orthogonal strips readout is under study, satisfying the requirements of a light and compact tracker for the $e^+e^-$ pairs. The $\mu$RWells are micro pattern gaseous detectors, characterized by a resistive layer between the amplification and the readout regions, which allows to reach high rates without the formation of permanent discharges.
This kind of detector is also being considered by the n_TOF collaboration, which is working on an experiment that aims to probe the existence of a new boson, named X17, that could explain the anomalies observed for the first time at the ATOMKI laboratory (Hungary). Four TPCs with $\mu$RWells and strips readout will be used to track the $e^+e^-$ ejected from the performed nuclear reactions.
A prototype detector with a drift length of 3cm was characterized in a test beam at ATOMKI in May 2024, showing a position resolution of $\sim 800\mu$m and an angular resolution of $\sim 2$°. Some construction criticalities of the considered device have been addressed, and potential ideas have been suggested to improve the design and the data analysis. Further studies are ongoing and tests of improved prototypes are in program.
