### TABLE OF CONTENTS

- 1. /XS_VMD
- 1.1. XS_VMD/vmd
- 1.2. XS_VMD/gvmd
- 1.3. XS_VMD/Eleformf

## /XS_VMD [ Modules ]

NAME

module **XS_VMD**

PURPOSE

This module contains the routines for cross section calculations of VMD and GVMD processes.

INPUTS

(none -- see NOTES)

NOTES

- The routine vmd uses the shadowing scaling by using PARP(161)..PARP(164)
- The routines vmd and gvmd return 0 for a cross section, if srts<0.938+m_V. This could (and should) be improved.

## XS_VMD/vmd [ Subroutines ]

[ Top ] [ XS_VMD ] [ Subroutines ]

NAME

subroutine **vmd**(srts,Q2,eps,sigma,useVM)

PURPOSE

calculate the VMD cross sections for (rho,omega,phi,J/psi) (using the PYTHIA parameters for scaleVMD)

INPUTS

- real :: srts -- the W value
- real :: Q2 -- the Q2 value
- real :: eps -- the epsilon value of the photon
- logical, dimension(4), OPTIONAL :: useVM -- true, if the corresponding meson should be considered; i.e. if false, the XS of the VM is set to 0

OUTPUT

- real, dimension(0:4) :: sigma -- the XS

NOTES

- Attention, we respect the vacuum thresholds, i.e. W>0.938+m_V. This should be improved!
- Ref: C.Friberg, T.Sjöstrand, JHEP 09 (2000) 010 sigma^{Vp} : eq. (2.15) sigma^{\gamma^*p}: eq. (2.24) (for eps=0) : eq. (2.28) (for eps=1)
- uses the interpolation (1+eps*r_i) between eq.(2.24) and (2.28) with r_1: eq.(2.26) [=default] or r_2: eq.(2.27) as set in MSTP(17)=4,5 in /PYPARS/.
- CfV(4) are the coupling constants [f_V^2/4pi] encoded in PARP(160+i).
- PYTHIA uses mV == mrho. Therefore we have the second column in the output array XS. If one wants to compare the output of this routine with the Monte Carlo output, then one realizes: XS(1) from CollectXS_class() <=> XS(0,2)*(W2/(W2+Q2))**3 from here

## XS_VMD/gvmd [ Subroutines ]

[ Top ] [ XS_VMD ] [ Subroutines ]

NAME

subroutine **gvmd**(srts,Q2,eps,siggvmd)

PURPOSE

calculate the GVMD cross sections

## XS_VMD/Eleformf [ Subroutines ]

[ Top ] [ XS_VMD ] [ Subroutines ]

NAME

subroutine eleformf(srts,Q2,eps,formfac)

PURPOSE