preliminary Simulation results
preliminary results of HBGEAN simulations
General remarks
If not stated explicitly, ARTE-01-05-r4 is used (with all the packages
like Pythia and GEANT built in).
The magnet is switched off (as planned for
this measurements) and the full detector is used - at least as dead
material, in order to provide the as much flexibillity with respect
to the detector setup as possible. As soon as the the detector setup
is settled, many calculations will be redone for updating the data.
B-field influence on particle distribution
Since the B field is being switched off, the particle distribution
will become more narrow with respect ot the x-direction - the bending
plane of the Hera-B magnet. The
radial distribution and hit distributions, separated in
x-direction and
y-direction
reveal this effect of switching off the magnet. Similar plots with the
magnet switched on can be found in the proposal. It should also be stated
that the peak structure is an artefact due to the histogram binning
and not real physics.
Technically: The magnetic field is set in two files: gufld.F and
utfeld.F. This files had to be modified (Thanks to S. Nowak for his
help).
Kinematic aspects
J/psi's are produced in proton - fixed target collisions. Without applying
cuts, the J/psi has an
average momentum
of about 100 GeV/c, an average
transverse momentum
of about 1.4 GeV/c with an
average ratio of transverse momentum and momentum of about 0.02.
Electrons, originating in the J/psi decay, have a
typical momentum of 50 GeV,
a typical transverse momentum
of 1.4 GeV and a ratio of transverse momentum and momentum of about 0.2.
Acceptance for J/psi decay products
The acceptance depends of course on the realised setups geomeetry.
A basic assumption is that the ECAL is split in two blocks of
445mm x 672 mm each. Their y-position should be symmetrical with
respect to the beam pipe and the minimum distance to the proton beam pipe
on the +x-side (in Hera-B coordinate system) is (due to the electron beam
pipe) 550 mm.
The geometrical single electron acceptance of ECAL is given by
the ratio of electrons found in ECAL and number of generated electrons.
Since this depends on the ECAL blocks x-position, the correlation between
the number of electrons found in one of the ECAL blocks and their position
is made. Please note that the x-position is given by the center.
This means a x-position of 67 cm corresponds to an ECAL blocks ranging from
45 cm to 89 cm on +x-side as well as on -x-side.
The plot shows a decreasing single electron acceptance for electrons
originating in J/psi decays as the ECAL blocks is placed more outside.
For the setup with ECAL as close to the beam pipe as possible, the
geometrical single electron acceptance is about 8%.
Since the J/psi decays into an electron and positron (the distinction is,
from the design point of view, no necessary and will be disregarded further
on), the amount of events with both electrons found in ECAL blocks is
important. Triggering events with both electrons in ECAL in the favoured
position at (x,y) = (750mm,0) gives a double electron acceptance of
slightly below 1%
Background and Trigger studies
The FLT for this setup will mainly rely on the final detectors
ECAL pretrigger system. This boards task is to identify clusters within
ECAL, determine their size, sum up the clusters energy content and estimate
the impact point by determining the center of gravity for each cluster.
These data will be fet directly to the Track parameter unit for
identifying cluster combinations with the J/psi's invariant Mass.
The trigger will presumably apply cuts on the transverse momentum, momentum
and the invariant mass. Further cuts are, of course due to the detector
acceptance.
For estimating the trigger cuts impact on J/psi events, the first step is
to look at the min. bias distributions of detected elctrons from J/psi
decays:
Applying trigger cuts on the transverse momentum (pt > 1 GeV)
and the particles energy (E > 5 GeV),
312 out of 398 events
are found. Note that there's no cut on the detector acceptance applied.
Taking the detector acceptance into account, the
acceptance on J/psi events
is about 0.67 %.
The other important point is the behaviour of the background when applying
these cuts. For these consideration is important to remind that the
used simulation takes the full detector into account. This will always
implied as long as it's not stated explicitly. This is important, since
all subdetectors will produce secondaries and enhance pair production.
Since the used setup will contain less material, the background is
crucially overestimated. Anyway: If we find to be able to live with this
disadvantegous condition, life will be even better with any realised
detector setup.
Invariant Mass analysis
Starting with an unknown particle ID, the invariant mass calculated for
all combinations of all clusters. Without any cut (neither on (transverse)
momentum nor on geometrical acceptance), the
invariant mass spectrum
The background gets reduced by applying the FLTrigger condition as
described above. This plot shows
particularly in the J/spi mass region less entries than the plot before.
Requiring both particles to be within ECAL, the
Invariant Mass histogram
using 400 J/psi events gets rather nice (but be careful: up to now I only
took the electrons/positrons into account to keep the computing time
reasonable. This would correspond to a perfect electron/pion separation
as well as a perfect photon identification).
The importance of the particle-id gets clear when looking to the
particle type distribution
This plot shows the number of clusters of ECAL as function of the
particles mass causing this cluster using 200 events. The vast majority
are photons. The same spectrum for the
mother particles
shows that photons do not mainly originate from particle decays, since
they have the mother particle mass 0.
Doing the same job for inelastic events including the applied trigger
cuts and taking the acceptance into account, one also find some entries
in the invariant mass spectrum's
region
of the J/psi. For the 500 events used for this plot, about 50 hit
combinations gave entries in this interesting region. Requiring particles
with a non-zero mass and with a mother particle fulfilling the same
criterion, the
spectrum shows a significantly lower peak at low invariant masses,
but the entries with an invariant mass near the J/psi mass are remaining.
This gives evidence to the fact that these hits doesn't originates in
pair conversion electrons but in directly produced, charged particles.
generated data files
For these investigations, HBGEAN was used for generating
event samples with
a) events with a directly produced J/psi (1.2 kEvent)
b) minimum bias events (approx. 1.2 KEvent)
for obtaining greater flexibillity in the detector setup, the full
detector in it's final design was assumed and all parts being absent
are neglected in the event digitisation.
Last update:
August-25-97 by Robert Loke