Speaker
Description
The COMET experiment at J-PARC searches for the charged-lepton-flavour-
violating process of coherent muon-to-electron conversion in a muonic atom, $\mu^- N \to e^- N$, with a target single-event sensitivity of $\mathcal{O}(10^{-15})$ during Phase I, expected to start in 2028. Low-energy negative muons are produced by a high-intensity proton beam colliding with an upstream target and are transported downstream to the stopping target. The primary signal studied during Phase I is a track reconstructed in a Cylindrical Drift Chamber, produced by an electron with an energy equal to the muon mass.
The main background capable of mimicking this signal arises from muons produced in cosmic-ray showers. To identify and reject these muons, COMET employs a Cosmic Ray Veto (CRV) system based, in particular, on RPC chambers. The most challenging region of the CRV is the front area, which is especially exposed to neutron and electromagnetic backgrounds originating from the up-stream target. Simulations indicate that background hit rates can reach up to $2\rm~kHz/cm^2$, significantly complicating the identification of cosmic muons. We proposed to equip this area with improved RPC chambers designed for the Phase II upgrade of the CMS experiment at CERN. In these chambers, pickup strips embedded in large PCBs are read out from both ends by sophisticated Front-End Boards (FEBs) hosting TDCs implemented in FPGAs. The FEBs represent the largest cost component of the project. To reduce costs, we propose a modular solution: using final readout PCBs with fine segmentation from the outset, while temporarily merging adjacent strips in pairs via low-cost extension boards. This allows the FEB production to be split into two phases. However, designing extension boards capable of merging strips without degrading signal integrity is non-trivial due to the low signal amplitudes, the impulsive nature of RPC signals (as opposed to sinusoidal RF signals), ambient noise, and impedance mismatches that can generate reflections at the merging points.
In this poster, we present a proof of concept for RPC signal merging, including the design, production, and certification of the extension boards.