5) Absolute Gain Calibration with Electrons


Electron calibration

The absolute gain calibration is done by identifying electrons and finding the E/p peak for small groupings of towers.  It is desirable to find the E/p peak for as small groupings as possible.  In 2006 the calibration was done in rings in η, while in 2009 it was done for "crate-slices", which are groups of 8 towers in the same crate in the same η ring. 

The procedure is as follows...

7) Make the electron trees with runElecJobs.sh which will execute electron_tree_maker.C  Ensure that the correct output filenames are specified in the submit script.

The macro electron_tree_maker.C makes slimmer trees of electron candidates which satisfy the following criteria:
-- event vertex |vz| < 60
-- track must come from reconstructed vertex (ranking >= 0)
-- 1.5 < p < 20 GeV/c
-- nhits >= 10
-- matched tower status = 1
-- dE/dx > 3e-6
-- ADC-ped > 1.5*pedRMS
In this macro, if the electron track points towards a HT trigger tower then it is assigned htTrig = 2

In the old calibration: Make a list of the output files from step (5) called electrons.list.  Run electron_master.C.
---- OR ----
In the new calibration: Run runFinalElec.sh, which executes electron_master_alt.C (make sure that the input/output filenames and directories are correct).  Hadd the resulting output files, and use this file as the input to electron_drawfits.C

In this macro even more stringent cuts are placed on the electron candidates:
-- ADC-ped > 2.5*pedRMS
-- track must enter and exit the same tower
-- p < 6 GeV/c
-- track does not point towards a tower which fired the HT trigger (htTrig != 2)
-- dR < 0.025 (distance from the center of the tower)
-- dE/dx > 3.4e-6
-- the maximum Et in the 3x3 cluster of towers must be in the central tower
-- there are no other tracks pointing to the central tower

The resulting histograms of E/p in each crate-slice are drawn and fit with a Gaussian plus a first-order polynomial.  If the calibration is already correct, then the E/p peak should be at 1.  The deviation from unity establishes the absolute gain calibration which, combined with the relative gain calibration from the MIP procedure, defines the overall BEMC gain calibration.