[Frost] missing mass of pion

[dugger at jlab.org]

Hideko,

You are right that a missing mass for pi0 of 168 MeV is too high and must
be dealt with.

A few thoughts:
For a reaction coming from the Carbon you should expect that the photon
energy will be greater than for a Hydrogen target by an amount greater
than ~16 MeV. This extra photon energy coming from the reaction being
analyzed as coming from Hydrogen (when it actually comes from Carbon) will
factor into your pi0 mass.

Assuming that you have been able to subtract the carbon influence from you
pi0 mass, then the most likely suspects are Eloss, and CLAS momentum
corrections. (The CLAS momentum corrections should be too small to account
for a missing mass of pi0 being so far from the PDG value.)

You might want to look for z-vertex dependence of your pi0 mass. For a
fixed proton lab-angle you should expect that the Eloss corrections to
contribute more for lesser values of z-vertex. (This only holds for the
Butanol target.) Since the effect is greater for small theta-lab angles,
it might be good to look at pi0 missing mass for a fixed "small" theta-lab
angle as a function of z-vertex. A z-vertex dependence in your pi0 mass
would be a strong indication that Eloss might be giving you problems.

Take care,
Michael

> Dear Eugene,
>
> I applied the newest version of eloss correction for proton, and got the
> missing mass of pion. The mean value of missing mass (pi 0) is around 168
> MeV, which is much better than using the old version of eloss correction.
> But, it is still away from the rest mass of pi 0, ~135 MeV. I would like
> to
> know the reason(s) which causes this difference.
>
> Hideko
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