SIDIS-RC EvGen

This is an event generator for polarized semi-inclusive deep inelastic scattering (SIDIS) with QED radiative corrections, following closely work done in [1]. The project is divided into a library component sidis for calculating SIDIS cross-sections, and a binary component sidisgen for generating events.

A detailed description of the generator can be found on the arxiv.

Quick start

Generator

The sidisgen generator uses a parameter file. The allowed options are listed by sidisgen --help. As an example:

mc.num_events        10000
file.gen             gen.root
file.event           event.root
phys.sf_set          prokudin
phys.rc_method       approx
phys.soft_threshold  0.01
phys.mass_threshold  1.073249081
setup.beam           e
setup.target         p
setup.hadron         pi+
setup.beam_energy    10.6
setup.beam_pol       0
setup.target_pol     0 0 0

Then, initialize a FOAM to approximate the differential cross-section in the specified kinematic region.

sidisgen --initialize <param-file>

Finally, use the FOAM for Monte-Carlo event generation.

sidisgen --generate <param-file>

To process the resulting ROOT file, see examples/process_events.cpp.

Library

Many demonstrations of the different features of the sidis library can be found in the examples folder. To get started quickly:

#include <iostream>
#include <sidis/sidis.hpp>
#include <sidis/sf_set/prokudin.hpp>

sidis::Real const PI = sidis::PI;
sidis::Real const M_TH = sidis::MASS_P + sidis::MASS_PI_0;

int main() {
	sidis::part::Particles particles(
		sidis::part::Nucleus::P,   // Target nucleus.
		sidis::part::Lepton::E,    // Beam lepton.
		sidis::part::Hadron::PI_P, // Leading hadron.
		M_TH                       // Threshold mass of undetected part.
	);
	sidis::Real S = 2. * 10.6 * particles.M; // Kinematic variable `S = 2 p k1`.
	sidis::kin::PhaseSpace phase_space {
		0.2,      // Bjorken x.
		0.9,      // Bjorken y.
		0.3,      // Bjorken z.
		2.,       // Transverse momentum of hadron, squared.
		0.5 * PI, // Azimuthal angle of hadron.
		0.,       // Azimuthal angle of transverse target polarization.
	};
	sidis::kin::Kinematics kin(particles, S, phase_space);
	sidis::Real beam_pol = 0.;
	sidis::math::Vec3 target_pol(0., 0., 0.);
	// Compute structure functions with WW-type approximation.
	sidis::sf::set::ProkudinSfSet sf;
	sidis::Real born_xs = sidis::xs::born(kin, sf, beam_pol, target_pol);
	std::cout << "Born unpolarized cross-section is " << born_xs << std::endl;
	return 0;
}

Build

For building the generator, ROOT must be installed on your system. For building the documentation, Doxygen must be installed. For building the tests, Catch2 must be installed.

# Clone the project including submodules.
git clone https://github.com/duanebyer/sidis.git
cd sidis
git submodule init
git submodule update
# Make a build directory.
mkdir build
cd build
# Configure the build.
cmake -DCMAKE_BUILD_TYPE=Release ..
# Build.
make
# Optionally install the library and binary files to your system. Use
# `CMAKE_INSTALL_PREFIX` during the configure to choose the install location.
make install

The following CMake configuration options may be of use:

• Sidis_REAL_TYPE: The floating point type to use for all cross-section calculations. Only the standard C++ floating point types float, double, and long double are supported.
• Sidis_BUILD_TESTS: Whether to build the tests.
• Sidis_BUILD_EXAMPLES: Whether to build the examples.
• Sidis_BUILD_DOXYGEN: Whether to build the Doxygen documentation.
• Sidis_BUILD_APPS: Whether to build the sidisgen binary.
• Sidis_ENABLE_IPO: Whether to build with interprocedural optimization.

Acknowledgements

This software makes use of the following libraries:

• WW-SIDIS: Structure functions for proton using WW-type approximation [2].
• mstwpdf: Parton distribution functions for proton [3].
• ROOT: Plotting and data analysis.
• FOAM: Monte-Carlo event generation using spatial partitioning.
• cubature: Multi-dimensional numerical integration.
• GSL: Multi-dimensional Monte-Carlo integration.

References

[1] I. Akushevich and A. Ilyichev, 2019. Lowest order QED radiative effects in polarized SIDIS. Phys. Rev. D 100(3), 033005.

[2] S. Bastami, H. Avakian, A. V. Efremov, A. Kotzinian, B. U. Musch, B. Parsamyan, A. Prokudin, M. Schlegel, P. Schweitzer. Semi-inclusive deep- inelastic scattering in Wandzura-Wilczek-type approximation. J. High Energy Phys. 1906(2019), 007.

[3] A. D. Martin, W. J. Stirling, R. S. Thorne, G. Watt. Parton distributions for the LHC. Eur. Phys. J. C. 63(2009), 189-285.