REAL FUNCTION dimuon()
      REAL
     +nrun    ,nev     ,eventct ,ncmuo   ,ntrks   ,mpdet   ,
     +mpeme   ,mphae   ,mpisol4 ,mpisol7 ,mpcmtw  ,mpcptw  ,
     +mpcxtw  ,mpz0    ,mmdet   ,mmeme   ,mmhae   ,mmisol4 ,
     +mmisol7 ,mmcmtw  ,mmcptw  ,mmcxtw  ,mmz0    ,mass    ,
     +psvxct  ,psvxc2  ,msvxct  ,msvxc2  ,vcode   ,px      ,
     +py      ,pz      ,sx      ,sy      ,sz      ,dl      ,
     +dle     ,upphi   ,upd     ,upz     ,uppt    ,umphi   ,
     +umd     ,umz     ,umpt    ,umass   ,udmass  ,uchi2   ,
     +vpphi   ,vpd     ,vpz     ,vppt    ,vmphi   ,vmd     ,
     +vmz     ,vmpt    ,vmass   ,vdmass  ,vchi2   ,xpphi   ,
     +xpd     ,xpz     ,xppt    ,xmphi   ,xmd     ,xmz     ,
     +xmpt    ,xmass   ,xdmass  ,xchi2   ,ppphi   ,ppd     ,
     +ppz     ,pppt    ,pmphi   ,pmd     ,pmz     ,pmpt    ,
     +pmass   ,pdmass  ,pchi2   ,bmass   ,bpd     ,bpphi   ,
     +bmd     ,bmphi   ,bcmass  ,bcpd    ,bcpphi  ,bcmd    ,
     +bcmphi  ,qmass   ,qpd     ,qpphi   ,qmd     ,qmphi   ,
     +vpsipt  ,vpsip   ,vpsiphi ,vpsieta ,bmagnt  ,upsigd  ,
     +umsigd  ,xpsigd  ,xmsigd  ,updsv   ,umdsv   ,vpdsv   ,
     +vmdsv   ,zsave   ,ngdm                                
*
      LOGICAL         CHAIN
      CHARACTER*128   CFILE
*
      COMMON /PAWCHN/ CHAIN, NCHEVT, ICHEVT
      COMMON /PAWCHC/ CFILE
*
      COMMON/PAWIDN/IDNEVT,OBS(13),
     +nrun    ,nev     ,eventct ,ncmuo   ,ntrks   ,mpdet   ,
     +mpeme   ,mphae   ,mpisol4 ,mpisol7 ,mpcmtw  ,mpcptw  ,
     +mpcxtw  ,mpz0    ,mmdet   ,mmeme   ,mmhae   ,mmisol4 ,
     +mmisol7 ,mmcmtw  ,mmcptw  ,mmcxtw  ,mmz0    ,mass    ,
     +psvxct  ,psvxc2  ,msvxct  ,msvxc2  ,vcode   ,px      ,
     +py      ,pz      ,sx      ,sy      ,sz      ,dl      ,
     +dle     ,upphi   ,upd     ,upz     ,uppt    ,umphi   ,
     +umd     ,umz     ,umpt    ,umass   ,udmass  ,uchi2   ,
     +vpphi   ,vpd     ,vpz     ,vppt    ,vmphi   ,vmd     ,
     +vmz     ,vmpt    ,vmass   ,vdmass  ,vchi2   ,xpphi   ,
     +xpd     ,xpz     ,xppt    ,xmphi   ,xmd     ,xmz     ,
     +xmpt    ,xmass   ,xdmass  ,xchi2   ,ppphi   ,ppd     ,
     +ppz     ,pppt    ,pmphi   ,pmd     ,pmz     ,pmpt    ,   
     +pmass   ,pdmass  ,pchi2   ,bmass   ,bpd     ,bpphi   ,
     +bmd     ,bmphi   ,bcmass  ,bcpd    ,bcpphi  ,bcmd    ,
     +bcmphi  ,qmass   ,qpd     ,qpphi   ,qmd     ,qmphi   ,
     +vpsipt  ,vpsip   ,vpsiphi ,vpsieta ,bmagnt  ,upsigd  ,
     +umsigd  ,xpsigd  ,xmsigd  ,updsv   ,umdsv   ,vpdsv   ,
     +vmdsv   ,zsave   ,ngdm
*
 
c    nrun = run number
c    nev  = event number
c    eventct = event count
c    ncmuo = number of muon candidates (very very loose cuts)
c    ntrks = number of tracks in the event
c
c    muon+ candidate variables
c    -------------------------
c    mpdet = mu+ detector
c    mpeme = mu+ EM calorimeter energy
c    mphae = mu+ Hadron calorimeter energy
c    mpisol4 = mu+ isolation in R = 0.4        
c    mpisol7 = mu+ isolation in R = 0.7
c    mpcmtw  = mu+ trigger tower # (cmu)
c    mpcptw  = mu+ trigger tower # (cmp)
c    mpcxtw  = mu+ trigger tower # (cmxP
c    mpz0    = mu+ Z0
c
c    muon- candidate variables
c    -------------------------
c    mmdet   ,mmeme   ,mmhae   ,mmisol4 ,
c    mmisol7 ,mmcmtw  ,mmcptw  ,mmcxtw  ,mmz0
c
c    dimuon variables
c    ----------------
c    mass     = invariant mass (two CTC tracks)
c
c    silicon hits for each muon
c    psvxct  = mu+ # of SVX hits + 4/0 (CTC-SVX matching good/bad)
c    psvxc2  = mu+ SVX CTC-SVX matching chi2
c    msvxct  = mu- # of SVX hits + 4/0 (CTC-SVX matching good/bad)
c    msvxc2  = mu- SVX CTC-SVX matching chi2    
c    vcode   = where the primary vertex comes from
c    px,py,pz = primary vertex point
c    sx,sy,sz = secondary vertex point (CTVMFT)
c    dl,dle   = decay length, error on decay length (CTVMFT)
c
c    upphi,upd,upz,uppt = unconstrained mu+ track
c                         phi, impact parameter, z, Pt
c    umphi,umd,umz,umpt = unconstrained mu- track
c                         phi, impact parameter, z, Pt
c    umass,udmass,uchi2 = unconstrained mass, error, chi2
c
c    vpphi,vpd,vpz,vppt = vertex constrained mu+ track
c                         phi, impact parameter, z, Pt
c    vmphi,vmd,vmz,vmpt = vertex constrained mu- track
c                         phi, impact parameter, z, Pt
c    vmass,vdmass,vchi2 = vertex constrained mass, error, chi2
c
c    xpphi,xpd,xpz,xppt = beam constrained mu+ track
c                         phi, impact parameter, z, Pt
c    xmphi,xmd,xmz,xmpt = beam constrained mu- track
c                         phi, impact parameter, z, Pt
c    xmass,xdmass,xchi2 = beam constrained mass, error, chi2
c
c    ppphi,ppd,ppz,pppt
c    pmphi,pmd,pmz,pmpt
c    pmass,pdmass,pchi2
c
c    bmass,bpd,bpphi,bmd,bmphi
c
c    bcmass,bcpd,bcpphi,bcmd,bcmphi
c
c    qmass,qpd,qpphi,qmd,qmphi
c
c    vpsipt,vpsip,vpsiphi,vpsieta
c
c    bmagnt,
c    upsigd,umsigd,xpsigd,xmsigd,updsv,umdsv,vpdsv,vmdsv
c    zsave,ngdm

      call hfill(1001,vmass,0.,1.)
      call hfill(1002,vmass,0.,1.)
      call hfill(1003,vmass,0.,1.)
      call hfill(1004,vmass,0.,1.)

      if(psvxct.gt.7. .and. msvxct.gt.7.) then
         call hfill(1101,vmass,0.,1.)
         call hfill(1102,vmass,0.,1.)
         call hfill(1103,vmass,0.,1.)
         call hfill(1104,vmass,0.,1.)
      end if

      END