howto.txt

How to calibrate HMS calorimeter with real electrons.

The calibration code resides in hallc_replay/CALIBRATION/hms_cal_calib
directory. It consists of 3 header files called
THcShHit.h,THcShTrack.h, THcShowerCalib.h, and a steering script
hcal_calib.cpp. There is also an input file input.dat containing
threshold parameters and initial gain constants necessary for
calibration.

The scripts work on root files from hcana analysis and make use of
quantities pertained to tracking, gas Cherenkov, and TOF from
hodoscopes. Hence it is convenient to calibrate on root files from
full HMS analysis. The root files are assumed to be in a linked
ROOTfiles directory.

Th input file contains thresholds on the quantities used in the
calibration, such as HMS: delta range, the hodoscope beta range,
threshold on the gas Cherenkov signals, required minimum number of
hits in the calorimeter channels in order to be calibrated, initial
gain constants, and parameters for an auxiliary histogram of energy
deposition calculated with initial gain constants. The gas Cherenkov
signals and beta are used to select good electron events for
calibration.  Additional clean up of the electron sample from hadron
contamination (if any) left after PID with gas Cherenkov and beta is
done by a rough localization of the electron peak in the auxiliary
histogram. Note that code does not iterate the gain constants. The
user is free to modify cuts in the input file as needed. Caution must
be exercised not to alter the format of the file.

The steering script hcal_calib.cpp takes 3 parameters: prefix of the
root file to be calibrated, last (-1 by default) and the first (0 by
default) events to be used for calibration. The -1 value for last
event number defaults to the number of entries in the root file.

Once your hcana, hallc_replay and Root are set up, you can compile and
run hcal_calib under hcana, by issuing command

.x hcal_calib.cpp+("Prefix"),

where Prefix is the prefix part in the name of root file.

For instance, for calibration with hms_replay_303.root file in
ROOTfiles, the correct command would be

.x hcal_calib.cpp+("hms_replay_303") .

Upon calibration, a canvas with representative plots will pop up. The
calibration constants will be written in output file
hcal.param.<Prefix>, in a format suitable for plugging them into your
hcal.param file for subsequent use. The representative canvas is saved
in a pdf file <Prefix>.pdf.