Event reconstruction algorithm for BGO endcap calorimeter of CMD-2 detector

Paper: 391
Session: A (poster)
Presenter: Kazanin, Vasilii, Budker Institute of Nuclear Physics, Novosibirsk
Keywords: algorithms, analysis, simulation, trigger algorithms

Event reconstruction algorithm for BGO endcap
calorimeter of CMD-2 detector.

Kazanin Vasilii

Budker Institute of Nuclear Physics, Russia,
Novosibirsk 630090, Lavrentieva Avenue 11

CMD-2 collaboration


An event reconstruction algorithm for BGO endcap
calorimeter of CMD-2 detector is described. Detector operates
on VEPP-2M electron-positron collider with c.m. energy range
from 0.36 to 1.4 GeV.
The calorimeter consists of two endcaps, built of rectangu-
lar BGO crystals. Crystal size is 25*25*150 mm**3 that corres-
ponds to 13.5 radiation length for normal incidence. Total
number of crystals in the calorimeter is 680. The presence of
1T magnetic field caused the use of vacuum phototriods for
light readout. They can operate in high magnetic field, but
their current gain is about 10 only, instead of 10**5 --
10**7, typical to PMT.
The low gain and relatively low lightoutput for BGO crys-
tals cause substantial electronic noise level of about 1 MeV
per channel in average. The contribution of electronic noise
to energy resolution is especially important for CMD-2 calori-
meter due to low energy of photons to be detected. Maximum
photon energy is 700 MeV, while in the most interesting reac-
tions, such as radiative decays of light vector mesons, pho-
tons have typical energies of 100 MeV. Due to these reasons a
special algorithm with detailed noise consideration is needed.
Algorithm has been realized as a set of subroutines inclu-
ded into general CMD-2 event reconstruction program. The main
feature of this algorithm is that selection criteria are tuned
individually for each crystal as a function of measured elect-
ronic noise. Noise measurement is performed during data taking
approximately once per week. Two selection criteria for trig-
gered crystals are used in the algorithm: absolute energy de-
position and ratio of signal to electronic noise sigma.
Soft selection marks crystal hits. Reconstruction of event in
the calorimeter begins with the formation of cluster nuclei.
Cluster nucleus is formed of standing side by side crystals,
which have passed hard selection. Afterwards cluster is built
from cluster nucleus by adding all neighbouring crystal hits.
To prevent information loss all crystal hits which don't be-
long to any cluster are saved separately. Energy, position
and other parameters of clusters are calculated after each
Reconstruction program uses HEPDB data base to extract ca-
libration data, noise data and parameters of calorimeter data
storage. Reconstruction code is used on SGI Super Server Chal-
lenge-L. The algorithm has been tested with simulated events
and has been applied to elastic electron-positron scattering
and cosmic events. Preliminary energy resolution for endcap
calorimeter is 6% for 510 MeV electrons produced by elastic
scattering. Algorithm provides good resolution and stability.