HOWTO to generate electromagnetic background
This page explains how to generate MC samples with electromagnetic background. There are several parameters which have to be set in the control.in to handle the simulation:
- BEAM 12. 9.
Electron beam energy and position of the coherent Bremsstrahlung peak.
- BGRATE 11.0
The rate of generated background photons in GHz in the Tagger region before the collimator. This parameter should be set either to 1.1 or 11.0 corresponding to intensities of a tagged photon beam of 10^7 and 10^8 photons/sec on target, respectively. To simulate high-luminosity runs one should use 11.0.
- BGGATE min max
Time interval in the ns, the earliest (min) and the latest (max) time, where the background photons are produced. This parameters is used for simulation of electromagnetic pile-up events.
- EBREMS_MIN Thr
The low energy of generated Bremsstrahlung photons, Thr. If EBREMS_MIN is commented, or set to 0,
the default photon-energy threshold of 0.12 GeV is used. If EBREM_MIN is set, the rate of
generated background photons is automatically scaled as follows:
Rate(E_gamma > Thr) = Rate(E_gamma = 0.12 Gev) *K( E_gamma = Thr),
where K is a calibration coefficient which depends on a photon energy threshold, Thr. The value
of K as function of the energy threshold is shown in Fig. 1. Note, Rate(E_gamma = 0.12 Gev) can be set
by the BGRATE parameter as has been explained in the previous paragraph.
To detector responses for electromagnetic background can be studied using three types of MC events:
Example 1
MC sample containing a single photon per event
The following parameters have to be set in the control.in file
- BEAM 12. 9.
Optional
- EBREMS_MIN THR
THR is a threshold on the min Bremsstrahlung photon energy. The default value is 0.12 GeV.
Note, Bremsstrahlung photons are generated befor the collimator, i.,e., a photon is NOT required to pass throught the collimator into a detector area - in most cases it gets stuck in collimator volumes. Therfore, in most MC generated events, you don't see the photon in the target region (the number of photons which pass throught the collimator is, on average, about a factor of seven less than that produced for the entire energy spectrum).
Example 2
Electromagnetic interaction superimposed with hadronic decays of interest (event pile-up),
i.e., every hadronic event is mixed with electromagnetic interactions. The background beam photons are generated within the time window given by BGGATE parameter. The following parameters should be used in the control.in file
- INFILE 'dir/file_name.hddm'
Input file with hadronic interactions (which will be mixed with the EM background)
- BEAM 12. 9.
- BGGATE min max
- BGRATE rate (use 11 for the luminosity of 10^8 photons/sec)
Optional
- EBREMS_MIN THR ( The default value is 0.12 GeV if this parameter is commented or set to 0)
Example 3
Pile-up of pure electromagnetic interactions
This MC samples contain event pile-up for electromagnetic interactions only. Similar to the Example 2, photons are generated within a time interval given by parameter BGGATE. The same parameters in the control.in file as that from Example 2 should be used except, for the INFILE. For the input events one can use events containig 'dummy' low-energy photons which are not seen in the detector. These events can be generated using 'genphoton' program as follows (this procedure should be changed in the future).
genphoton -M numEvents -Pmax 0.00001 -Pmin 0.000005 -Thetamin 0. -Thetamax 0.001 -o file_out.ascii
Here, 'genphoton' generates a MC sample with the number of events numEvents, each event contains a photon in the energy range Pmax and Pmin (GeV). The output file 'file_out.ascii' should subsequently be converted into an hddm file using
genr8_2_hddm file_out.ascii
Produced hddm file should be used as the input file in INFILE.
If you want to use the EBREMS_MIN parameter, you have to make sure that you are using the latest HDGeant svn version, in particular beamgen.F, uginit.F, and control.in files.
