MCEGENpiN_radcorr V9b

This V9b version of the newest MC event generator for exclusive single pion electroproduction allows you to generate a massive statistics for the large invariants' scales. Relatively fast and effective procedures in conjunction with model representations make this program a convenient and reliable choice for data analysis in particle physics.

What's it all about?

One of the biggest parts of any experiments in physics is data analysis. Hadron physics with CLAS12 spectrometer gets pretty tricky when one should deal with its efficiency. This is where programs like this generator come quite handy. Not only do they allow you to restore the original cross-section, but they also can be used as an instrument for event selection development.

The elaboration of this generator was carried out based on MAID representations. As a starting point, we use multipole amplitudes for the charged channels. This data is needed to evaluate the differential cross-section that we further use as weights for event generation.

Formalism

Helicity amplitudes were considered the most convenient intermediate stage of the whole data handling. Using the multipole decomposition with Legendre polynomials one can obtain Helicity amplitudes as follows:

These amplitudes are further used for the structure functions evaluation, which one requires for the differential cross-section calculation.

where K and k and q are respectively, the photon equivalent energy and the virtual photon and pion 3-momenta in c.m.s. For unpolarized particles and for a longitudinally polarized electron beam, the φ-dependence of the cross section can be specified in the following way:

The differential cross section of the electroproduction of pions off nucleons in the one-photon approximation was used as a weight for each specific event.

Radiative Corrections

For RE (radiative effects) simulations the Mo and Tsai approach was chosen. Using the peak approximation we were able to implement the following corrections:

1. Weights re-evaluation according to RC
2. The radiative tail simulation
3. Outgoing radiative photon generation

It's advised to use this program configuration with --weight option and for W < 2 GeV

Main idea

The general procedure for the event generation contains 2 parts:

• Weight calculation
• Particles kinematics

In the first case, the program decides whether it's the soft or hard region for the RC and completes the differential cross section calculation. The additional factor for the weight is also calculated here. The program completes the event generation cycle with the kinematics evaluation. Thus it writes all the information about the specific event in the output file using the "Lund" format.

As a result, for each point and (beam polarization) values, this program creates a bunch of outgoing particles with the appropriate kinematic values.

Usage

local machine

1. Install Root Cern
2. git clone of the EG: git clone https://github.com/Maksaska/MCEGENpiN_radcorr
3. Type command: chmod +x compile_gen
4. Set the path to data:

setenv MCEGENpiN_radcorr_path "path to generator directory"

setenv MCEGENpiN_radcorr_path ./Downloads/MCEGENpiN_radcorr

export MCEGENpiN_radcorr_path="path to generator directory"

export MCEGENpiN_radcorr_path="./Downloads/MCEGENpiN_radcorr"

5. Compile with "compile_gen",i.e. ./compile_gen
6. Start the compiled file with ./MCEGENpiN_radcorr command

JLab machine

1. git clone of the EG: git clone https://github.com/Maksaska/MCEGENpiN_radcorr
2. source setup.sh
3. Type command: chmod +x compile_gen
4. Compile with "compile_gen",i.e. ./compile_gen
5. Start the compiled file with ./MCEGENpiN_radcorr command

Requirements: Root Cern

Options for program start:

• --beam_energy - beam energy
• --target_R - target cross-section radius (Keep in mind, the target is a cylinder)
• --target_L - target length
• --W_min - left border for the invariant
• --W_max - right border for the invariant
• --Q2_min - left border for the invariant
• --Q2_max - right border for the invariant
• --hist - , , , and histograms will be created (No input value required)
• --RC - switch for radiative correction procedure (Enabled when entered. No input value required)
• --weight - switch for uniform distributions in kinematically admissible phase space
• -n or (--pi_plus_n) - switch for channel. (Charged pion channel is chosen when entered. No input value required)
• --trig - number of events
• --trunc_out - LUND output will be written in 10k events per file format in the output destination directory
• --extra_Q2 - alternative extrapolation proceedure, where all multipoles extrapolated as 1/Q4 (except M1+ ~ 1/Q6)
• --docker - Sets all the parametes to default values
• --seed - Used to initialize the event generator's RNG. Its value is a 32-bit RNG seed based on system clock with microsecond precision.
• -h - beam polarization ("0" when beam is not polarized)

Example:

./MCEGENpiN_radcorr --beam_energy=10 --target_R=0.5 --target_L=10 --W_min=1.1 --W_max=1.8 --Q2_min=0.5 --Q2_max=2 --RC -n --trig=10000

The program will be started with (h = 0 by default) for W:[1.1, 1.8] and :[0.5, 2] with RC mode enabled for channel. Number of generated events is 10,000. Target radius is 0.5 cm and its length is 10 cm.

Available kinematic range from MAID data W:[1.08, 2] and :[0.05, 5] with extrapolation through axis.

Extrapolation:

• All multipoles extrapolated as 1/Q2 (except M1+ ~ 1/Q6)
• if W > 2.0 we assume W = 2.0 (RC integrals too)

Event sampling proceedure:

Metropolis-Hastings MCMC algorithm with normal distribution as proposal density (or jumping distribution) - default setting Uniform distributions in kinematically admissible phase space. Cross sections are recorded for each event as weights. To use this configuration add --weight to ./MCEGENpiN_radcorr

Default settings:

• --beam_energy = 6.5
• --target_R = 1 cm
• --target_L = 10 cm
• --W_min = 1.08
• --W_max = 2
• --Q2_min = 0.05
• --Q2_max = 5
• --hist Disabled
• --RC Disabled
• -n Disabled ( channel by default)
• --trig = 1,000,000
• --seed = time(NULL)
• -h = 0

Some histograms for default options with --weight