Higher-order predictions of the Standard Model (SM) [1]
can be tested in an unambiguous way [2] by studying the decay 
which is suppressed in first order by the GIM [3] cancellation of flavor changing
neutral currents.
In the context of the SM assuming three generations, the
branching ratio for 
is predicted to be in the
range 
[4].
Before Experiment 787 at Brookhaven National Laboratory
was undertaken the experimental upper limit for
was 
[5].
Based on early data from E787, an improved limit 
has been reported [6].
New limits for other rare decays including 
[7] ,
[8] and 
[9]
have also been obtained.
The main goal of Experiment 787 is to observe 
if it occurs within
the range of predictions of the SM. A signal in the predicted
range will place constraints on parameters including
the top quark mass and certain of the Cabbibo-Kobayashi-Maskawa
matrix elements which describe the weak mixing among the three
generations of quark mass eigenstates.
Observation of 
well above the prediction would indicate new
physics beyond the SM such as the existence of new
generations of leptons
or quarks, or of one or more new light,
weakly-interacting neutral particles.
The signature of 
is a single unaccompanied 
emitted
by a stopped 
. Fig. 1 shows the expected charged-particle
spectra of the range and momentum for 
along with the
other decay modes which could be sources of background.
Figure 1: Charged-particle spectra in (a) momentum and
(b) range (including finite resolution) for 
and the major sources of backgrounds from Ref [5].
The decays 
(
)
(
) and 
(
) are major potential
background sources with branching ratios of 0.635 and 0.212, respectively.
It is possible to reduce the impact of 
,
at the cost of acceptance, by restricting the search to the
20% of the total spectrum in the kinematic region above the
monoenergetic peak of 
(momentum 
MeV/c)
and having efficient photon detection. In this case only the effects of finite
kinematic resolution could allow 
to contribute events in the
signal region. Backgrounds with muons can
be managed with a high level of 
/
discrimination.
The critical characteristics of the detector, in addition
to high sensitivity, are
unambiguous 
particle identification, efficient photon
detection, and good kinematic resolution.
These features are crucial also for rejection of the
other potential background decay modes appearing in the kinematic
region of interest, such as
(
)
and 
. The detector also must be capable
of rejecting beam-related backgrounds from processes involving kaon
charge exchange or pion scattering.