QPI0DO: $\pi^{0}_{}$ finding routine

The $\pi^{0}_{}$ finding routine QPI0DO is described in the report ALEPH 93-095 / PHYSIC 93-078. It uses the photons from the ALPHA GAT section (8.3.2 on p. ).

To use QPI0DO, one has simply to put in QUEVNT:

Input argument: none

Output arguments:

All output arguments are returned in the ALPHA common deck GAMPI0:

      PARAMETER (MXPI0=200)
      COMMON / GAMPI0 / IQPI0, NTPI0, PI0MOM(4,MXPI0),ITYPI0(MXPI0),
     +                  IPI0GAM(2,MXPI0),CHIPI0(MXPI0)

IQPI0

Return code

• = 0 : All OK
• = 1 : No $\pi^{0}_{}$ found
• > 1 : More than MXPI0 $\pi^{0}_{}$ found
NTPI0
Number of $\pi^{0}_{}$ found
PI0MOM
Refitted 4 momentum of the NTPI0 $\pi^{0}_{}$
IPI0GAM
Number of photons 1 and 2 giving the $\pi^{0}_{}$ , in the GAT section
CHIPI0
Chi2 value after refit. Set to -999. if no convergence.
ITYPI0
$\pi^{0}_{}$ type, see below:

• =1 : 2 photons in same PECO, with N=2 photons in the PECO
• =2 : 2 photons in same PECO, with N>2 photons in the PECO
• =3 : 2 photons in different PECO, with N=1 photons in each PECO
• =4 : 2 photons in different PECO, with N>1 photons in one PECO

QPI0DO uses automatically the best photons from the GAT section. For POTs, DSTs and MINIs $\geq$ 10 the PGAC photons (sec 8.3.3) are used. For MINIs $\leq$ 9, these are the PGPC photons (sec 8.3.2).

Beware that the mass cut used to find the $\pi^{0}_{}$ candidates in QPI0DO photons is quite wide: this mass window is $\pm$2$\sigma$ around the mean GAMPEX reconstructed $\pi^{0}_{}$ mass and is parametrised in the internal subroutine PI0LIM. For some specific analyses, a tighter cut might be needed, in that case one has to come back to the original GAMPEX photons and check their invariant mass.

Five control histograms 9001 to 9005 can optionally be filled. They give a display of the $\gamma$ - $\gamma$ invariant mass versus $\gamma$ energy for all photon pair types ITYPI0 (see above), in scatter plot 9000+ITYPI0 +1 , plot 9001 giving the sum over all types.

To get these histograms , you need simply to put:

CALL QPI0BK

in your QUINIT subroutine.

A debug printout routine is also provided to print all $\pi^{0}_{}$s found in an event; to call it, put:

CALL PI0DEB

in your QUEVNT subroutine.