Dear colleagues,
This message contains a short theoretical review of the pi^0 photoproduction
kinematics for the 12C target specifically and also a brief comment on the
Dustinīs report (Timing and TAGM selection) dated Sep-9-2005.

The calculations presented here were performed under the following conditions:
(1) Primakoff cross section: PWIA without distortion effects due to FSI of the
outgoing pion. This cross section is calculated only for the sake of
completeness and should be improved in various ways: accurate determination of
12C electromagnetic FF and pion-nucleus FSI;
(2) Nuclear Coherent cross section: calculated within the MCMC cascade model,
taking into account the FSI effects of the outgoing pion. The peak of the cross
section was normalized using the Cornell parameterization. The final results
may need more inputs;
(3) Nuclear Incoherent cross section: calculated within the MCMC cascade model,
taking into account the FSI effects of the outgoing pion. The normalization of
the cross section fits the proton data (strong part). The momentum distribution
of the bounded nucleons of 12C is calculated using an appropriate WS potential
(shell model).

Some additional remarks also apply for all the three contributions:
a) The nuclear recoil is explicitly taken into account for the Primakoff and
Nuclear coherent mechanisms in a relativistic kinematics;
b) The dependence of the differential cross sections on the pion energy is
investigated very carefully for all the processes;
c) No geometrical constraints were applied in the final results in such a way
that ALL the produced pions are detectable. 

The figures are organized as follows:
i) Dependence of the differential cross sections on the pi^0 energy for the
Primakoff, coherent and incoherent mechanisms for 5.5 GeV:
12C_Primakoff_5.5GeV.ps, 12C_Coherent_5.5GeV.ps, 12C_Incoherent_5.5GeV.ps. The
value of Delta_E is the incident photon energy (tagger energy) minus the
outgoing pi^0 energy (cluster energy). It is interesting to point out that even
for the elastic channels the pion energy is not equal the photon energy due to
the nuclear recoil.
ii) Differential cross sections pertaining to the elastic peak kinematics for
three different energies: 12C_elastic_peak_5.0GeV.ps,
12C_elastic_peak_5.5GeV.ps, 12C_elastic_peak_6.0GeV.ps. The elastic peak
considered in this calculation was taken to be within approximately 5 MeV of
the incident energy. I really do not know exactly the energy resolution of the
experiment and I took approximately 0.1% for this calculation. So, the pions
pertaining to the elastic peak may have a total energy equal to the incident
energy +/- energy resolution. 
iii) Pi^0 yield with and without the small incoherent contribution pertaining
to the elastic kinematics defined on (ii): pi^0_yield_5.0GeV.ps,
pi^0_yield_5.5GeV.ps, pi^0_yield_6.0GeV.ps.

Finally, I would like to make a brief comment on the Dustinīs report:
When looking at the Carbon data on pages 12 and 13 of Dustinīs report, either
using the complicated or simple algorithm to extract the yields, I noticed a
small shoulder around 0.45 degrees (is that real?). I know that this is a
preliminary result but if this shoulder is confirmed with later analysis, we do
have a nice interpretation for this: the small incoherent contribution within
the elastic kinematics (the pi^0 yield is approximately 15% higher around 0.45
degrees for 5.5 GeV when we include the incoherent part). Also note that this
small contribution in the pion yield is much more prominent in the differential
cross sections and should be taken into account in future analysis.
Furthermore, the incoherent cross section within this kinematical region peaks
at approximately 0.25 degrees and may contaminate the information of the
Primakoff peak (see figures 12C_elastic_peak_5.0GeV.ps and
12C_elastic_peak_5.5GeV.ps)
It seems strange to have a nuclear excitation (incoherent pi^0 production)
pertaining to the “elastic” peak but we have to take into account that the
energy resolution of the experiment is approximately 5 MeV and the energy gap
between the relevant shells of 12C is ~2 MeV!

In conclusion, I personally consider it impossible to completely separate the
incoherent part from the other mechanisms even looking only at the elastic
peak. Such possibility would happen if the energy resolution of the experiment
was reduced by a factor of two. In order words, if we know the pion energy
within a precision of 2 MeV, them we can guarantee that no incoherent processes
would pertain to the elastic kinematics.
As far as we have all the conditions well established I can make the
calculations for the Lead target.
Suggestions or comments are very welcome. 
With my best regards,
-Tulio.