Chapter 8
Single Photon Efficiencies
The selection efficiency for the single photon events is considered the
rate at which e+e-® n[`(n)]g events are selected to the rate at which they occur.
For all practical purposes, this efficiency can be broken into the following
three distinct areas:
(1) The Trigger Efficiency - The efficiency for the OPAL detector
to trigger on e+e-® n[`(n)]g events.
Since this efficiency is closely related to the expected event type, only those
events which have at least activity in the electromagnetic calorimeter will
be considered.
The calorimeter triggering efficiencies for barrel events will be determined
from the copious single electrons of the radiative Bhabha sample.
(2) The Selection Efficiency - The efficiency for an e+e-® n[`(n)]g event
to produce a shower in the electromagnetic calorimeter by converting in the
coil and initiating activity in the presampler calorimeter.
Since e+e-® n[`(n)]g events are required to convert in the coil, those events which
convert in the central detector and have a track are not accepted.
Useful in determining these efficiencies are the single electrons from
e+e-® e+e-g events, the single photons from e+e-® l+l-g events, and the double photons from
e+e-® gg events.
(3) Veto Efficiencies - The efficiencies for the events to
fulfill the detector veto requirements.
The single electrons from e+e-® e+e-g events as well as the random beam
crossings will be used to determine these efficiencies.
This chapter presents the results and some of the details of the each type of
efficiency studied.
8.1 Trigger Efficiencies
The efficiency of the OPAL detector to trigger on e+e-® n[`(n)]g events is determined
from single electron events which are triggered by a superset of the same
triggers for the single photon events as shown in Table 4.5.
A sample of 14,946 single electron events was selected from the 1991 data as
discussed in Section 7.2.
With this sample of single electron events, the trigger efficiencies are
determined from the threshold behavior of the TPTOEM trigger.
The TPTOEM trigger is composed of two distinct components: the time-of-flight
(TOF) component of the trigger, and the electromagnetic component (EM) of the
trigger.
In Figure 8.1 (a) and (b), the TOF component of the trigger is
seen to be independent of the polar and azimuthal angles with an average
value of 99.13±0.25%.
In Figure 8.1 (c), the EM component of the trigger is shown
as a function of the threshold energy of the electron.
The average efficiency of the EM component of the trigger is determined
to be 90.87±0.81% for electrons which deposit between 1.25 GeV and
1.5 GeV in the electromagnetic calorimeter, increasing to 98.79±0.41%
for electrons which deposit more than 1.5 GeV.
The total trigger efficiency for electrons depositing between 1.25 GeV and
1.5 GeV in the electromagnetic calorimeter is 90.08±0.24% and for
electrons depositing more than 1.50 GeV in the electromagnetic calorimeter
this becomes 97.93±0.21%.
A summary of the average trigger efficiency is shown in
Table 8.4.
Figure 8.1: TOF and EM Single Electron Trigger Efficiencies.
Of the events which trigger TPTTEM, the efficiency of the TPTTTO trigger
is shown as a function of cosq in (a) and a function of f in (b).
The data are shown by the histogram and by solid dots with statistical error
bars fit to a straight line.
Of the events which trigger TPTTEM or TPTTTO, the efficiency of the TPTOEM
is shown as a function of the threshold energy in (c) for the low energy
threshold near Eg » 1.00 GeV by solid dots and for the high energy
threshold near Eg » 2.50 GeV by open dots.
The data are shown by the histogram and by solid dots with statistical error
bars fit to a second degree polynomial.
8.2 Single Photon Selection Efficiencies
Since this analysis selects single photon events which convert in the coil,
it is important to determine the rate of conversions in the coil from the data.
Related to the process of conversion in the coil is the efficiency of matching
a presampler cluster to the electromagnetic shower of the photon and the rate
of non-conversion of single photons before arriving at the coil.
The efficiencies are first determined from the e+e-® n[`(n)]g Monte Carlo sample as
shown in Table 8.1.
Other sources of single photons have been selected from the 1991 data set to
to check the efficiencies determined from the Monte Carlo against the
simulation of the detector and assign systematic errors.
The radiative lepton pairs (e+e-® l+l-g) have been selected using the following
criteria:
(1) The event must pass the event filter, the FYZ1 criteria, the
l+l- preselection, and the detector trigger and status requirements shown
in Table 6.5.
(2) Only two or three high-momentum tracks with
pt > 0.5 Ebeam
and a quality vertex |d0 | < 5.0 and |z0 | < 50.0
in the fiducial volume |cos(qtrack) | < 0.95.
(3) One electromagnetic cluster with Eraw > 1.0 GeV
in the fiducial volume |cosq| < 0.7 not associated with either
of the two high-momentum tracks.
The photon electromagnetic cluster must not have more one high-momentum
track or five tracks within 200 mrad.
(4) The two high-momentum tracks associated with the two leptons
must be separated by more than 200 mrad and the electromagnetic cluster
associated with the photon must also be separated by more than 200 mrad
from each of the two leptons.
Using this criterion, 1216 events from e+e-® e+e-g, 797 events from e+e-® m+m-g, and
54 events from e+e-® t+t-g have been selected and have been used to study the
single photon selection efficiencies as shown in Table 8.1.
The photon pairs (e+e-® gg) have been selected using the following
criteria:
(1) To ensure the overall quality of accepted event, the event must
pass the event filter, the FYZ1 criteria, and the detector trigger and
status requirements shown in Table 6.5,
(2) Only two electromagnetic clusters with Eraw > 1.0 GeV
can be associated with the photons in the fiducial volume
|cosq| < 0.7.
The two electromagnetic clusters must have qAcol. < 0.05 rad.
All other electromagnetic clusters must have Eraw < 0.3 GeV in the
fiducial volume of the detector.
(3) No more than three tracks can be associated with a single
electromagnetic cluster for a photon in a cone of 45 degrees around the cluster.
Using these criteria, 252 events from e+e-® gg have been selected and have been
used to study the single photon selection efficiencies as shown in
Table 8.1.
Table 8.1 indicates that there is good agreement for photon
conversion in the coil, photon non-conversion in the central detector, and
cluster match between the presampler and electromagnetic calorimeter.
between many different single photon samples.
The relative difference between the data and the Monte Carlo for the different
types of events considered reflects an effective systematic error on the
measurement and will be discussed in Section 10.2.
| Sample | Type | Conversion | Non-Conver.
| EB-PB | Selection |
| | in Coil (%) | in CD (%) | Match (%)
| Total (%) | |
| | Single | MC | 79.81±0.88 | 93.14±0.53
| 96.40±0.46 | 71.66±1.12 | |
| | Tagged | Data | 79.26±2.01 | - | 96.73±1.22 | - |
| Photon | MC | 81.58±1.06 | - | 95.90±0.60 | - |
| Single | Data | - | - | 94.69±0.19 | - |
| Electron | MC | - | - | 95.17±0.18 | - | |
| | Photon | Data | 77.92±1.46 | 92.22±0.91
| 97.45±0.63 | 70.01±1.83 |
| Pair | MC | 76.10±0.99 | 93.79±0.55
| 95.58±0.55 | 68.21±1.26 | |
| | Lepton | Data | 77.92±1.46 | 92.22±0.91
| 97.45±0.63 | 70.01±1.83 |
| Pair | MC | 76.10±0.99 | 93.79±0.55
| 95.58±0.55 | 68.21±1.26 |
|
Table 8.1: Single Photon Selection Efficiencies.
The selection efficiencies used in the analysis are taken from the single
photon Monte Carlo efficiencies.
These efficiencies are checked in detail with several different sources of
single photon and like events.
Among the sample events with similar topology are the tagged photon events
( e+e-® (e+e-) g) and single electron events
( e+e-® (e+) e- (g) ).
Finally, photon pairs ( e+e-® gg) and
the radiative lepton pairs ( e+e-® l+ l- g)
have been used to check the overall consistency of the selection efficiencies.
8.3 Veto Efficiencies
Since a single photon event is selected only if it passes strict selection
criteria regarding non-activity in each of the critical subdetectors for this
analysis, any discrepancies in the simulation from the inherent noise of the
OPAL detector introduce inefficiencies associated with vetoing events based
on such requirements.
The dependence of these simulation inefficiencies is due to the veto
requirements and studied from period to period with random beam events.
After correcting for hot sectors, hot clusters, and bad runs, the vetoing
inefficiencies are determined from a total of 381,904 random events taken during
the run and summarized in Table 8.2.
The vetoing inefficiencies are fairly constant, varying from 2.42±0.42%
to 3.51±0.03% with an total average veto efficiency of 96.94±0.03.
| EFFICIENCIES (%)
| TOTAL |
| Period | TRK | FD | ECAL | HCAL
| Efficiencies (%) | |
| | 20 | 1.78±0.19 | 0.41±0.06 | 0.47±0.09 | 0.21±0.06
| 96.93±0.24 |
| 21 | 1.74±0.09 | 0.34±0.03 | 0.11±0.02 | 0.25±0.04
| 97.58±0.11 |
| 22 | 1.91±0.11 | 0.37±0.04 | 0.06±0.02 | 0.54±0.06
| 97.28±0.14 |
| 23 | 2.08±0.09 | 0.43±0.03 | 0.05±0.01 | 0.21±0.03
| 97.45±0.10 |
| 24 | 2.68±0.14 | 0.28±0.05 | 0.08±0.03 | 0.14±0.04
| 96.61±0.16 |
| 25 | 2.58±0.13 | 0.50±0.04 | 0.41±0.05 | 0.21±0.04
| 96.63±0.15 |
| 26 | 2.54±0.10 | 0.54±0.04 | 0.59±0.05 | 0.18±0.03
| 96.45±0.12 |
| 27 | 1.85±0.11 | 0.33±0.04 | 0.16±0.04 | 0.30±0.05
| 97.39±0.14 |
| 28 | 1.88±0.08 | 1.03±0.05 | 0.23±0.03 | 0.20±0.02
| 96.56±0.10 |
| 29 | 2.24±0.08 | 0.37±0.02 | 0.28±0.04 | 0.27±0.03
| 97.04±0.09 |
| 30 | 2.40±0.08 | 0.41±0.03 | 0.08±0.02 | 0.22±0.03
| 97.02±0.10 |
| 32 | 2.35±0.07 | 0.35±0.02 | 0.12±0.03 | 0.23±0.02
| 97.06±0.08 |
| 33 | 2.92±0.09 | 0.31±0.02 | 0.19±0.02 | 0.25±0.03
| 96.49±0.10 | |
| | Total | 2.39±0.03 | 0.47±0.01
| 0.20±0.01 | 0.24±0.01 | 96.94±0.03 |
|
Table 8.2: Veto Inefficiencies.
Summary of the inefficiencies of the single photon selection showing
separately the various veto cuts from the random beam crossings.
The veto requirements consists of the vertex and jet chambers (TRK), the
forward calorimeter (FD), electromagnetic calorimeter (ECAL), and
the hadron calorimeter (HCAL) (see Selection Criteria (4), (5), (6),
and (7)).
Since the quality of the data taken during a run is related to the
vetoing efficiency, strict requirements on the detector and trigger status
have been applied in analyzing the data as outlined in Table 6.5.
Of all the critical subdetectors to this analysis, only the barrel presampler
has been allowed to have a detector status of either 2 or 3.
This was done in order to retain more than 24% of the total integrated
luminosity for the 1991 run from those events with barrel presampler detector
status of 2.
The detector status of 2 indicates the operation of some of the barrel
presampler chambers are questionable with at least half operational.
This increase in acceptance is corrected through the presampler veto
coefficients determined from the single electron events.
The presampler veto coefficients are defined to be the ratio of the number of
single electron events accepted with a presampler barrel detector status
of 3 divided by the number of single electron events accepted with a
presampler barrel detector status of 2.
The presampler veto coefficients are determined at each center-of-mass energy
as shown in Table 8.3.
| Ös | PRESAMPLER | TOTAL |
| (GeV) | COEFFICIENT (%) | VETO (%) | |
| | 88.45 | 93.06±1.75 | 90.21±1.75 |
| 89.45 | 90.43±1.58 | 87.66±1.58 |
| 90.20 | 92.43±1.43 | 89.60±1.43 |
| 91.20 | 98.30±0.47 | 95.29±0.47 |
| 91.95 | 94.20±1.47 | 91.32±1.47 |
| 92.95 | 100.00+0.0-1.59 | 96.94±1.59 |
| 93.70 | 95.50±1.52 | 92.58±1.52 | |
| | TOTAL | 94.85±0.39 | 91.95±0.39 |
|
Table 8.3: Presampler and Veto Efficiencies.
The effect of the detector status of the barrel presampler is determined
from the single electron events.
The occupancy veto efficiency is taken to be 96.94±0.03 at each
center-of-mass energy point.
The occupancy requirements consist of the vertex and jet chambers (TRK),
the forward calorimeter (FD), electromagnetic calorimeter (ECAL), and
the hadron calorimeter (HCAL) as given the Selection Criteria (4), (5), (6),
and (7)).
The total veto efficiency is determined at each center-of-mass energy point
from the presampler and the occupancy veto efficiencies.
8.4 The Overall Efficiency
A summary of the efficiencies for the single photon selection is shown in
Table 8.4.
The total efficiency is determined from the total of each of the three
different types of efficiencies: the trigger efficiency, the selection
efficiency, and veto efficiencies.
For photons with energies 1.5 GeV £ Eg £ 1.75 GeV, the total
average efficiency is 59.35±1.43%.
However, for photons depositing more than 1.75 GeV in the calorimeter, the total
average efficiency is 64.53±1.20%.
| Ös | TRIGGER | SELECTION | VETO
| TOTAL |
| (GeV) | Efficiency (%) | Efficiency (%)
| Efficiency (%) | Efficiency (%) | |
| | 88.45 | 90.08±0.81 | 71.66±1.12 | 90.21±1.75
| 58.23±2.23 |
| (97.93±0.21) | | | (63.31±2.09) |
| 89.45 | 90.08±0.81 | 71.66±1.12 | 87.66±1.58
| 56.58±2.10 |
| (97.93±0.21) | | | (61.52±1.95) |
| 90.20 | 90.08±0.81 | 71.66±1.12 | 89.60±1.43
| 57.83±1.99 |
| (97.93±0.21) | | | (62.88±1.83) |
| 91.20 | 90.08±0.81 | 71.66±1.12 | 95.29±0.47
| 61.49±1.46 |
| (97.93±0.21) | | | (66.87±1.23) |
| 91.95 | 90.08±0.81 | 71.66±1.12 | 91.32±1.47
| (58.94±2.02) |
| (97.93±0.21) | | | (64.09±1.86) |
| 92.95 | 90.08±0.81 | 71.66±1.12 | 96.94±1.59
| 62.57±2.11 |
| (97.93±0.21) | | | (68.03±1.96) |
| 93.70 | 90.08±0.81 | 71.66±1.12 | 92.58±1.52
| 59.76±2.05 |
| (97.93±0.21) | | | (64.97±1.90) | |
| | Total | 90.08±0.81 | 71.66±1.12
| 91.95±0.39 | 59.35±1.43 |
| (97.93±0.21) | | | (64.53±1.20) |
|
Table 8.4: Total Efficiencies of the Single Photon Selection.
The total efficiencies are determined as the product of each of the
three different efficiencies: the trigger efficiencies, the selection
efficiencies, and veto efficiencies.
The total efficiencies are determined for both first and the second sample
(shown in parenthesis).
The trigger efficiencies refer to averages of the energy bin around 1.75
GeV for the second sample (Egcorr ³ 1.75 GeV) and around 1.5 GeV for the
first sample (1.5 GeV £ Egcorr £ 1.75 GeV).
The selection efficiencies have been averaged over all energy points and
include the conversion in the coil, non-conversion in the central detector,
and matching between the presampler and electromagnetic clusters.
The veto requirements consist of the vertex and jet chambers (TRK),
the forward calorimeter (FD), electromagnetic calorimeter (ECAL),
and the hadron calorimeter (HCAL).
