The HMPID has been
designed to measure the Cherenkov angle with an accuracy of a
few mrad, required to discriminate p, K and p in the relevant
momentum range. Moreover,
the chosen pad size and the analog readout allow charged tracks with good
spatial resolution and high multi-hit recognition efficiency to be detected.
The Cherenkov angle resolution per track
is a function not only of the chromatic, geometric and localization errors but
also of the number of detected photons, being inversely proportional to the
square root of that number. Therefore much effort has been made to successfully
develop a standardized procedure of manufacturing CsI photocathodes of large
area with a reproducible high QE.
Presently, an average of 19 reconstructed photon hits per Cherenkov ring
are obtained with b=1 particles, yielding an overall resolution (intrinsic plus pattern
recognition) of 3 mrad per track at a pad occupancy of 15%, corresponding to 50
A 1.2 m2 prototype (2/3 of one HMPID module) was installed in the STAR experiment at BNL and took data for two years from the start-up of RHIC Au–Au collisions. Its successful operation yielded not only invaluable expertise for the realization of the HMPID array in ALICE but also important physics results.