Origin of background events in BPM profiles during quadrupole scan

Identify the physical origin of the background events observed in the microchannel-plate (MCP) beam profile monitor during the quadrupole scan measurement of the 100 keV accelerated positive muon beam transported by two quadrupole magnets and a horizontal bending magnet after the radio frequency quadrupole at J-PARC MUSE, which were included in the root-mean-square beam size calculation with a signal-to-noise ratio of approximately 50.

Background

To measure the transverse emittance of the 100 keV accelerated muon beam, the experiment employed the quadrupole scan method using two quadrupole magnets (QM1 and QM2), a horizontal bending magnet (BM), and an MCP-based beam profile monitor (BPM) at the end of the diagnostics line.

During the profile measurements used in the emittance fit, the authors observed background events on the BPM. While they quantified a signal-to-noise ratio of about 50 and accounted for these backgrounds in the rms size calculation, the physical source of the background remained unidentified, leaving an unresolved instrumentation or beamline-related question that could impact precision beam diagnostics and future optimization.