The sample is selected by requiring that at least one pair of
photons have invariant mass within the (0.08, 0.2) GeV window.
This corresponds to the case of selecting the
as opposed
to rejecting the
for
selection.
In the case of
, no
veto is needed.
The
mass is small compared to the
and in order to form
, the third photon should have a large separation
so that invariant mass formed with one of the remaining photons will
often be in the
region. Thus cutting on the
significantly decreases our
signal.
Invariant mass distributions, obtained with the above criteria,
for 4 total energy thresholds are shown in Fig. 2.
They are fitted by the sum of two Gaussians, with the one corresponding
to the background represented by the dashed line.
Similar to the case, the signal to background ratio is enlarged when the
energy threshold in the LGD is increased.
The
mass, width and yield for the corresponding energy cut
are given in Table 2.
In order to show the influence of
selection on our
sample, by dotted lines in Fig. 2 are shown
invariant mass distributions obtained with a narrower window
around the
(0.1
0.18
).
It seems that the signal to background ratio is not affected
by narrowing the
window since the reduced events are almost
uniformly distributed over the mass range, especially for the higher energy cuts.