eic_elastic

Generator and analysis for elastic electron-proton scattering at the EIC

Running ROOT-based generator

The ROOT-based generator in this repository -- elas_gen_hepmc.C -- will generate elastic e-p events at EIC energies and write those events to a HepMC3 file. The events are generated using the Kelly Form-Factor parameterization at the BORN level (no QED radiation).

The generator can be run by doing:

root -l -b -q elas_gen_hepmc.C

The code will prompt the user to select the EIC beam energy combination, the upper and lower Q² limits, and the number of events to generate. The user will also be asked to select a generation option. There are two options:

Uniform generation. This will generate the scattered electron uniformly in solid angle in the proton beam rest frame. For each event, a cross-section weight is calculated and saved to the 'weight' attribute in the HepMC file.
FOAM generation. This will generate the e-p events according the generator's cross-section model. In this case, no event-by-event weight needs to be applied. (In the output HepMC file, the 'weight' attribute is set to 1 for all events.)

The FOAM generation option is preferred for most studies.

Running DJANGOH generator

DJANGOH version 4.6.21 can generate elastic e-p events including QED radiation. (Note that the generation of these radiative elastic events will not work correctly in earlier versions of DJANGOH.)

To run DJANGOH v.4.6.21, clone and follow the instructions in this external repository. In this repository, the standard text output format has been modified so that the generated events can be converted into the eic-smear and HepMC3 formats.

While you can compile DJANGOH locally, I compile and run using the eic-shell environment, since this provides pre-installed LHAPDF libraries and grids as well as the libraries needed for the eic-smear and HepMC3 file conversion.

Although the LHAPDF grids are not used for the elastic e-p running, you still need to set the location of the grids. When working in the eic-shell, you can link to the existing LHAPDF6 grids:

ln -s /cvmfs/sft.cern.ch/lcg/external/lhapdfsets/current
export LHAPDF_DATA_PATH=/cvmfs/sft.cern.ch/lcg/external/lhapdfsets/current

To run an elastic simulation with DJANGOH, use the run_ep_rad_elas_5_41.sh file in this local repository and do the following:

./run_ep_rad_elas_5_41.sh

This script runs DJANGOH, using the ep_elas_rad_5x41.in input card to define the simulation parameters. The generated events are saved to a text file; a LOG file is also created, which contains information about the simulation. The script will then run the make_tree.C code to save the events to a ROOT file in the eic-smear format. Lastly, the script will run the make_hepmc.C code to save the events in the HepMC3 format, which can then be used as input for the detector simulation.

Afterburner for beam-crossing and beam-smearing effects

To apply EIC beam effects to the events created with the above generators, use this afterburner utility.

If running in the eic-shell environment, this utility is included by default. To run the converter, do the following (for example):

abconv -o elas_gen_5_41_beameffects elas_gen_5_41.hepmc

This will create a file called elas_gen_5_41_beameffects.hepmc, which can then be used as an input to the ePIC detector simulation.

Summary of generated data

Data Set	Generator	Beam Energies	Run Information	QED effects	Events	Cross Section (fb)	Int. Lumin. (fb^-1)
1	ROOT FOAM	5x41 GeV	5 < Q² < 200 GeV²	No	100k	13709	7.29
2	ROOT FOAM	5x100 GeV	5 < Q² < 200 GeV²	No	100k	13772	7.26
3	ROOT FOAM	10x100 GeV	5 < Q² < 200 GeV²	No	100k	13794	7.25
4	ROOT FOAM	10x275 GeV	5 < Q² < 200 GeV²	No	100k	13808	7.24
5	ROOT FOAM	18x275 GeV	5 < Q² < 200 GeV²	No	100k	13811	7.24
6	DJANGOH	5x41 GeV	1 < Q²_e < 1000 GeV²	Yes	100k	$4.47 \times 10^7$	$2.24 \times 10^{-3}$

The cross sections in the table above are reported by the event generator. The corresponding integrated luminosity is calculated using that cross section and the number of generated events.

The generated files can be found here. The ROOT FOAM events are stored in HepMC3 format, both before and after applying the beam-effects afterburner. The DJANGOH events are stored in the eic-smear and HepMC3 formats before applying the beam-effects afterburner. They are stored in the HepMC3 formate after applying the beam-effects afterburner.

Running generated events through the ePIC simulation

To run the 100 generated events through the detector simulation, access the eic-shell environment.

Then simply do the following:

source /opt/detector/epic-main/bin/thisepic.sh

#Run DIS events through npsim
ln -s elas_gen_5_41_beameffects.hepmc input.hepmc #Just to make npsim command easier to read
npsim --compactFile $DETECTOR_PATH/epic_craterlake.xml --numberOfEvents 100 --inputFiles input.hepmc --outputFile output.edm4hep.root

#Run reconstruction
eicrecon -Ppodio:output_file=eicrecon_out.root -Pjana:nevents=100 -Pdd4hep:xml_files=epic_craterlake.xml output.edm4hep.root

The output ROOT file from this simulation can then be analyzed as discussed below.

To run events on a HTCondor-based batch farm, use the code in the condor subdirectory.

Analysis

Example analyses can be found in the analysis subdirectory.

bschmookler / eic_elastic