Photoproduction background MC for GLUEX

Introduction

The goal is to simulate hadronic photoproduction in the full range of the beam energies - from the thereshold of 0.15 GeV to the maximum of 12 GeV. The procedure is as follows:

The photon energy is simulated, in a given range, accordingly to the coherent Bremsstrahlung spectrum, times the total photoproduction cross section at the given energy.
If the photon energy is greater than some limit, 3 GeV by default, the interaction is simulated using PYTHIA.
Otherwise, for lower energies, the type of interaction is randomly chosen from a list of 10 exclusive reactions, dominant at these energies. The differential cross sections of these reaction are parametrized using the existing experimental data.

Beam

The coherent Bremsstrahlung spectrum is simulated using a routine cobrems.F extracted from the HDGeant code. The settings were as follows:

76 m distance from the diamond crystal to the collimator;
3.4 mm diameter collimator;
20 μm crystal thickness;
11.7 cm - the radiation length of the crystal.

From this, I would expect an incoherent photon flux without collimation, in the energy range 2-11.8 GeV, of 20·10^-6/0.117·log(11.8/2.)=0.00030 per one incoming electron. The code gives a value of 0.00024, about 25% lower, which seems reasonable.

With a collimator 3.4mm diameter, the code gives the incoherent photon flux of 3.8·10^-5 per electron, in the same energy range.

The beam spectrum and the photoproduction hadronic rate are presented on the next figure.

Coherent Bremsstrahlung beam

Photoproduction cross section

The full photoproduction cross section along with the partial cross sections important below 2 GeV are presented on the next picture.

Photoproduction cross sections

Simulation code

The code is available in the GLUEX repository src/programs/Simulation/bggen
This directory contains a README file with a manual.

Beam

The coherent Bremsstrahlung code from R.Jones is used.

Processes

The following processes are included:

Processes considered
The processes 1-10 are chosen randomly for each event, depending on their cross sections. The processes 4, 5 and 11 produce the same final state. The process 3 is artificially added, as a difference between the full process 11 and the resonant processes 4 and 5.
# Process Experimental data Source of Simulation Comment

σ(E) dσ/dΩ Rate distribution

1 p π^o 0.15 - 2.00 GeV dσ/d(cosθ_CM) SAID SAID

2 n π⁺ 0.15 - 2.00 GeV dσ/d(cosθ_CM) SAID SAID

3 p π⁺ π^- #11-#4-#5 phase space Non-resonant contribution, #11-#4-#5

4 p ρ° 1.00 - 2.50 GeV dσ/dt exper, extrapolated exper dσ/dt Resonance ρ°→π⁺ π^- : sinθ_CM

5 Δ⁺⁺ π^- 0.40 - 3.00 GeV dσ/dt exper, extrapolated exper dσ/dt Resonance Δ⁺⁺ → p π⁺

6 p π° π° 0.40 - 0.80 GeV exper, extrapolated phase space

7 n π⁺ π° 0.40 - 0.80 GeV exper, extrapolated phase space

8 p η 0.70 - 2.50 GeV dσ/d(cosθ_CM) exper, extrapolated exper dσ/d(cosθ_CM) dσ/d(cosθ_CM)(E) - polynomial fit

9 p π⁺ π^- π° 1.50 - 10.0 GeV exper, extrapolated phase space

10 n π⁺ π⁺ π^- 1.50 - 10.0 GeV exper, extrapolated phase space

11 p π⁺ π^- 0.40 - 12.0 GeV dσ/dt Used for σ(E) shape, and for #3

**Processes considered**
The processes 1-10 are chosen randomly for each event, depending on their cross sections. The processes 4, 5 and 11 produce the same final state. The process 3 is artificially added, as a difference between the full process 11 and the resonant processes 4 and 5.
#	Process	Experimental data	Source of Simulation	Comment
σ(E)	dσ/dΩ	Rate	distribution
1	p π^o	0.15 - 2.00 GeV	dσ/d(cosθ_CM)	SAID	SAID
2	n π⁺	0.15 - 2.00 GeV	dσ/d(cosθ_CM)	SAID	SAID
3	p π⁺ π^-			#11-#4-#5	phase space	Non-resonant contribution, #11-#4-#5
4	p ρ°	1.00 - 2.50 GeV	dσ/dt	exper, extrapolated	exper dσ/dt	Resonance ρ°→π⁺ π^- : sinθ_CM
5	Δ⁺⁺ π^-	0.40 - 3.00 GeV	dσ/dt	exper, extrapolated	exper dσ/dt	Resonance Δ⁺⁺ → p π⁺
6	p π° π°	0.40 - 0.80 GeV		exper, extrapolated	phase space
7	n π⁺ π°	0.40 - 0.80 GeV		exper, extrapolated	phase space
8	p η	0.70 - 2.50 GeV	dσ/d(cosθ_CM)	exper, extrapolated	exper dσ/d(cosθ_CM)	dσ/d(cosθ_CM)(E) - polynomial fit
9	p π⁺ π^- π°	1.50 - 10.0 GeV		exper, extrapolated	phase space
10	n π⁺ π⁺ π^-	1.50 - 10.0 GeV		exper, extrapolated	phase space
11	p π⁺ π^-	0.40 - 12.0 GeV	dσ/dt			Used for σ(E) shape, and for #3

The process 11 is used to evaluate the non-resonant process 3. The measured t-dependence for the non-resonant process happens to be close to the t-dependence for the iniform phase space production, therefore no additional correction to the "phase-space simulation" was done.

E-Mail : gen@jlab.org
Last updated: Oct 30, 2007