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# Changes

,  09:39, 28 March 2017
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The divergence is calculated from the beam parameters obtained by the machine designers, specifically the (un-normalized) beam emittance, &epsilon;, and &beta;<sup>*</sup>, via the equation $\sigma_\theta = \sqrt{\varepsilon/\beta^*}$.  This formula is used to generate the angle which the outgoing proton will be rotated.  The angle is chosen randomly from a Gaussian distribution with a width equal to the calculated &epsilon;<sub>&theta;</sub>.  The script /direct/eic+u/rmpetti/workarea/roman_pots/MCafterburners/addDivergence.C will take in as input, the ASCII file generated by running MILOU in the previous step and will output a modified event record with the proton rotated by the specified angle.  The script also takes in the (un-normalized) emittance and &beta;<sup>*</sup> in both the x and y direction to calculate the magnitude of the divergence in each direction separately.

The divergence is calculated from the beam parameters obtained by the machine designers, specifically the (un-normalized) beam emittance, &epsilon;, and &beta;<sup>*</sup>, via the equation $\sigma_\theta = \sqrt{\varepsilon/\beta^*}$.  This formula is used to generate the angle which the outgoing proton will be rotated.  The angle is chosen randomly from a Gaussian distribution with a width equal to the calculated &epsilon;<sub>&theta;</sub>.  The script /direct/eic+u/rmpetti/workarea/roman_pots/MCafterburners/addDivergence.C will take in as input, the ASCII file generated by running MILOU in the previous step and will output a modified event record with the proton rotated by the specified angle.  The script also takes in the (un-normalized) emittance and &beta;<sup>*</sup> in both the x and y direction to calculate the magnitude of the divergence in each direction separately.
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== Roman Pot setup ==
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The next described step is the construction of the Roman Pot systems.  This can be done by running the script /direct/eic+u/rmpetti/workarea/roman_pots/detectorSetup/tracker.C.  This takes many arguments as detailed in the defined function in the script.  The script allows for the construction of the Roman Pot through all the arguments supplied, including the position along the lattice in x and z, the rotation of the pot so that the beam comes in perpendicular to the sensors, the denoting of how many layers of sensors should be included in the station and the size of the rectangular "hole" that represents the gap in acceptance due to how close the pots can be placed to the beam (currently assumed to be 10&sigma; of the beam width from the core of the beam and determined from the emittance and &beta; function).  Each station is also given an index through the arguments.  This script needs to be run inside ROOT.  Some more general information of constructing detector geometries inside EicRoot can be found [https://wiki.bnl.gov/eic/index.php/Eicroot#Tutorials_and_Examples_to_Get_Started here].
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== EicRoot simulation ==
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The next step is to run the GEANT tracking of the events through the magnet setup and the Roman Pots within the EicRoot package.  An example script to handle this is found at /direct/eic+u/rmpetti/workarea/roman_pots/sim/simulation.C.  This script will use the events generated above in step 1, as well as the detector constructs from step 3.  Additionally, the magnetic setup in the IR needs to be defined.  Some general information on the importing of magnetic fields into EicRoot can be found [https://wiki.bnl.gov/eic/index.php/Eicroot#Realistic_IR_Magnet_Setup here] or [https://wiki.bnl.gov/eic/index.php/Eicroot#Alternate_Magnet_Lattice_Import here].
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If one wants to study the effect of the angular divergence, then this step needs to be performed twice.  The simulation.C macro will be run once on the event record without divergence and run again on the event record with divergence.  The output of simulation.C WITHOUT divergence is simply used to gain access to the original, un-smeared, proton p<sub>T</sub> and to pass this value along the simulation chain.  The actual detector hits that will be analyzed later for acceptance and momentum reconstruction studies will utilize the output of simulation.C WITH divergence applied.
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In both cases, the output of simulation.C is a ROOT file containing the standardized EICTree format used by the BNL EIC Task Force group.
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== Digitizing hits ==
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The next step is performed to add an additional layer of reality to the simulation.  Rather than use the raw exact hits of the protons in the sensors, a digitized hit is used, which takes into account the pixelation of the detector sensor.  This step is applied by running the macro /direct/eic+u/rmpetti/workarea/roman_pots/sim/digitization.C.  The pixel size in x and y can be adjusted within this script.  This macro takes in the output of the previous simulation step and outputs another ROOT file containing TTrees with the hit information in each Roman Pot station.  If it is desired to study the effect of divergence, this macro needs to be run with the simulated file which includes the divergence.  For more general information on the digitization step in EicRoot, please consult this [https://wiki.bnl.gov/eic/index.php/Digitization_details page].
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