Simple Example of Using KWFIT

In this section I give a brief example (of anal1, anal2, anal3 routines) of how kwfit is used, from initialization to fitting, for building D0 --> K pi combinations. All calls to kwfit routines are hyperlinked to the documentation for those routines.

      subroutine anal1

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*  Called at beginning of job
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c     Initialization of kwfit
      call kset_init


      subroutine anal2

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*  Called once per run
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      implicit none

#include "seq/clinc/"

      integer I
      logical ltesla
      double precision beam_pos(3), beam_sig(3)
      double precision bfield, bdir(3)
      real bscale

      data bdir/0., 0., 1./
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c     Set beam position & width
c     You need to do this in case you make vertices weighted by the
c     beam spot
c     All quantities must be double precision
      do I=1,3
        beam_pos(I) = dble(bmpos(I,1))  !dble function actually not needed
        beam_sig(I) = dble(bmpos(I,2))
      call kset_beam_position(beam_pos, beam_sig)

c     Set B field (including bfudge correction) and direction
c     Remember that bfie is in kGauss & set ltesla accordingly
      call bfudge(runn, bscale)
      bfield = dble(bfie * bscale)  !dble function actually not needed
      ltesla = .FALSE.
      call kset_bfield(bfield, bdir, ltesla)


      subroutine anal3

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*  Called once per event
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      implicit none

#include "seq/clinc/"

c     Externals
      double precision kget_track_mass
      external         kget_track_mass

c     Local variables
      integer I, type, error, update, list_d0(2), option_d0(2)
      integer ipi, iK, npi, nK, list_pi(100), list_K(100)
      logical lcovar, ldedx, lvtx
      double precision w_K(10), w_pi(10), w_d0(10), chisq
      double precision chisq_d0, chisq_mass, mass_d0
      double precision vtx(3), Vvtx(3,3)

c     Clear the kwfit track list every event
      call kset_clear

c     Get CD tracks
      call getcd

c     Make list of pions and kaons from CD tracks. We want
c     the covariance matrix built but no dE/dx correction is
c     necessary because the tracks have already been Kalman fit.
c     The list of kwfit tracks is returned in list_pi and list_K
      lcovar = .TRUE.
      ldedx = .FALSE.
      type = 3
      call kfil_track_cd_all(type, ldedx, lcovar, npi, list_pi, error)
      type = 4
      call kfil_track_cd_all(typr, lcovar, lcovar, nK, list_K, error)

c     Now loop over K, pi lists and build D0's
      do iK=1,nK
	call kget_track_param(list_K(iK), w_K)
        do ipi=1,npi
          call kget_track_param(list_pi(ipi), w_pi)
          mass_d0 = kutl_mass2(w_k, w_pi)

c     Find the vertex of the K-pi pair. Do not update the tracks at this time.
          update = 0
          list_d0(1) = list_K(iK)
          list_d0(2) = list_pi(ipi)
          lvtx = .FALSE.
          call kvtx_unknown(2, list_d0, update, lvtx, vtx, Vvtx, chisq, error)

c     If the chisq is OK, update the input tracks. The update will cause the
c     track parameters to be adjusted in such a way as to make them pass through
c     the new vertex point (However, the (x, y, z) location of the tracks
c     will not necessarily be at the vertex point. To make that happen, you
c     must call ktrk_move_point_bend to move the tracks and covariance matrices
c     to the new vertex point). The covariance matrices of the tracks are not
c     changed.
         if(chisq .lt. 10.) call kfit_update_tracks(1)

c     Alternatively, you can build a D0 particle with a vertex constraint
c     (to a previously unknown position). The chisq_d0 is from the vertex
c     constraint. The D0 track parameters and covariance matrix are evaluated
c     at the vertex point.
          option_d0(1) = 2
          option_d0(2) = 2
          update = 0
          lvtx = .FALSE.

c     First create a new track entry then fill it with the virtual particle
          call kfil_track_create(kd0)
          call kvir_vertex_unknown(2, list_d0, option_d0, update, lvtx, vtx,
     *                             chisq_d0, kd0, error)

c     Check the Kpi mass. If OK, apply a mass constraint forcing the D0 to
c     have the correct mass. The idea is to improve the D0 track parameters
c     so that the D0 can be used in subsequent fits.
          mass_d0 = kget_track_mass(kd0)
          if(abs(mass_d0 - 1.8654) .lt. 0.010) then
            update = 2
            call kfit_mass(kd0, update, 1.865, chisq_mass, error)