In analysing the performance of the CTD SLT we have made
extensive use of MC simulated data.
Using the LEPTO event generator, 1000 NC and 1000 CC events
were generated.
Default values were taken for the LEPTO parameters but a
greater than 100 GeV
was required and the parton
cascade model for the QCD evolution was used.
In addition, BG background was simulated by generating 2000
pp interactions using the FRITIOF generator.
To simulate the effects of materials and the responses of the CTD we have tracked the generated events using the ZEUS trigger MC [4]. All ZEUS components in the MC were turned on and the shower terminators were used. To simulate the BG background, pp events were generated uniformly between -9 m and +1 m along the beam line. This beam-line region gave a rate for at least one hit in the CTD of about 60 kHz [2].
The simulation of the CTD responses uses a planar drift approximation. Planar drift lines (PDL) are lines through each sense wire parallel to the tangent to the circle which is centred on the axis of the CTD and through the cell centre. Tracks crossing these lines are considered to give hits on the corresponding sense wire in the planar drift approximation.
The information recorded for each hit is as follows.
The spatial co-ordinates, track pointer, and drift sign are used as truth for efficiency studies. The track pointer can be used to form a link back to the generated track, vertex, and event kinematics. The final drift distance is obtained by calculating the drift time from the drift distance along the PDL and the drift velocity of the gas, adding the particle time-of-flight, and subtracting the reference time-of-flight. This total time is then converted back to a drift distance using the drift velocity of the gas. The energy loss was not used in the studies presented in this note.
The output of the trigger MC is written to a disk file in ASCII format for further processing. This data includes the response information from all ZEUS components in the MC and the above responses from the CTD. We will refer to the above information as hit data. The hit data does not include a simulation of the CTD electronics, inefficiencies, noise, or resolutions.
The CTD hits are further processed by the ZGANA package [5]. The processing steps include
The first four steps are described below, while the last two steps are described in the next section.
The detector hits and tracks are read into common-blocks
using the ZGANA-reading package.
Noise hits from the uranium surrounding the CTD are
distributed in the CTD according to the predicted
distribution and rate [6].
No synchrotron-radiation noise is simulated.
The drift distances are smeared with a gaussian distribution
of 130 m width and signal propagation delays along the
wires are added to the drift times.
The z-by-timing and energy loss are also smeared but are
not of importance here.
Next, 1% of the hits are deleted representing dead or
inefficient channels.
All inefficiencies, noise, and resolutions are parameters of
the simulation and can be varied.
Also, the CTD layer structure of the hits are mapped onto
the readout channels in the FADC crates, and dead boards or
crates can be simulated.
This was not done in the studies presented here.
All the above information is placed in an additional CTD
common-block and is referred to as the CTD
digitisation data.