The PMTs of the CMS Hadron Forward calorimeter were found to generate a
large size signal when their windows were traversed by energetic charged
particles. This signal, which is due to. Cerenkov light production at
the PMT window, could interfere with the calorimeter signal and mislead
the measurements. In order to find a viable solution to this problem,
the response of four different types of PMTs to muons traversing their
windows at different orientations is measured at the H2 beam-line at
CERN. Certain kinds of PMTs with thinner windows show significantly
lower response to direct muon incidence. For the four anode PMT, a
simple and powerful algorithm to identify such events and recover the
PMT signal using the signals of the quadrants without window hits is
also presented. For the measurement of PMT responses to. Cerenkov light,
the Hadron Forward calorimeter signal was mimicked by two different
setups in electron beams and the PMT performances were compared with
each other. Superior performance of particular PMTs was observed