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,
μ−N → e−N , with a target single-event sensitivity of O(10−15) during Phase I,
which is expected to begin in 2028. Low-energy negative muons are produced
by collisions of a high-intensity proton beam with an upstream production tar-
get and are transported downstream to the muon stopping target. The primary
Phase I signal is a track reconstructed in the Cylindrical Drift Chamber (CDC),
produced by an electron with an energy close to the muon rest mass.
The dominant background capable of mimicking this signal originates from
cosmic-ray muons. To identify and reject these events, COMET employs a Cos-
mic Ray Veto (CRV) system based, in part, on Resistive Plate Chambers (RPCs).
For the purpose of CRV design, the detector geometry can be approximated
as a rectangular enclosure. The most challenging region for the CRV is the
front side, which is directly exposed to intense neutron and electromagnetic
backgrounds originating from the upstream target. Simulations predict back-
ground hit rates of up to 2, kHz/cm2, significantly complicating the identifi-
cation of cosmic-ray muons. To address this challenge, we propose equipping
this region with improved RPC (iRPC) chambers developed for the Phase II up-
grade of the CMS experiment at CERN. During recent beam tests at the Gamma
Irradiation Facility (GIF++) at CERN, the iRPC chambers demonstrated stable
operation at background rates exceeding 3, kHz/cm2, providing a safety mar-
gin of 1.5 with respect to the maximum rates expected in COMET. An efficiency
of 95% was measured at the nominal expected background rate.
The left and right sides of the COMET detector are exposed to significantly
lower background levels, not expected to exceed O(100, Hz/cm2). For these
regions, IHEP (Beijing, China) has proposed the use of RPC detectors devel-
oped for the ARGO-YBJ experiment, which are similar in design to the ATLAS
Phase-0 RPC chambers. Although originally designed for cosmic-ray studies,
these detectors required adaptation to satisfy the COMET operating condi-
tions. Recent tests at GIF++ demonstrated stable operation up to 150, Hz/cm2
while maintaining an efficiency well above 95%.