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Template/LO/Source/PDF/PhotonFlux.f

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c/* ********************************************************* */

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c/*  Equivalent photon approximation structure function.   * */

 

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c/*   V.M.Budnev et al., Phys.Rep. 15C (1975) 181          * */

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c/*     Improved Weizsaecker-Williams formula              * */

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c/*   V.M.Budnev et al., Phys.Rep. 15C (1975) 181          * */

 

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c/*     http://inspirehep.net/record/359425                * */  

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c/* ********************************************************* */

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c   provided by Tomasz Pierzchala - UCL

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      real*8 function epa_electron(x,q2max)

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      integer i

 

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      real*8 function epa_lepton(x,q2max, mode)

 

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      implicit none

 

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      integer i, mode, imode

 

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c     mode is +3/-3  for electron and +4/-4 for muon      

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      real*8 x,phi_f

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      real*8 xin

 

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      real*8 xin(3:4)

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      real*8 alpha

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      real*8 f, q2min,q2max

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      real*8 PI

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      data PI/3.14159265358979323846/

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      data xin/0.511d-3/ !electron mass in GeV

 

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      data xin/0.511d-3, 0.105658d0/ !electron mass in GeV

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      alpha = .0072992701

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      imode = abs(mode)

 

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C     // x = omega/E = (E-E')/E

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      if (x.lt.1) then

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         q2min= xin*xin*x*x/(1-x)

 

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         q2min= xin(imode)*xin(imode)*x*x/(1-x)

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         if(q2min.lt.q2max) then 

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            f = alpha/2d0/PI*

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     &           (2d0*xin*xin*x*(-1/q2min+1/q2max)+

 

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             f = alpha/2d0/PI*

 

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     &           (2d0*xin(imode)*xin(imode)*x*(-1/q2min+1/q2max)+

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     &           (2-2d0*x+x*x)/x*dlog(q2max/q2min))

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         else

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      endif

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c      write (*,*) x,dsqrt(q2min),dsqrt(q2max),f

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      if (f .lt. 0) f = 0

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      epa_electron= f

 

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      epa_lepton = f

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      end

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