Rare Eta-Meson Decays
Contact Person: B. Nefkens
310-825-4970
Nefkens@physics.ucla.edu
The observation of CP violation is limited to the neutral
K-meson system, that is to say to a flavor-changing weak interaction.
here are no tests of CP invariance in a flavor
non-changing reaction that have the needed sensitivity. Consider the decay
, listed in the PDT as a test of CP.
It is a CP test of Category I (P and T odd and C even). At the present level
of sensitivity, BR (
< 1.5
and
BR(
) <
2.8 x 
, the absence of the two pion
decay of the 
is a consequence of
parity conservation of the strong and electromagnetic interaction which is
well established experimentally. At the level
,
decays start to probe the
P-violating weak interaction and the search for
becomes a test of CP instead of P.
Unfortunately, 
production is generally accompanied by background reactions with 2 pions
and the needed level
of precision cannot be experimentally obtained with present technology.
Fortunately, 
is a test of CP invariance as well and it has no known
background even at
the level 
. The search must reach this level to compensate for
the small available
phase space of the 
. A Crystal Ball-type detector is ideal for
searching for
at the AGS. This decay is
highly
overconstrained; 8 gammas that pair-wise reconstruct to 
's provide
a unique signature.
CP tests of Category II (C and T odd, P even) are of special interest for
two reasons:
- A new kind of CP violation, which is also a C violation, could solve
the cosmological
problem of the known baryon excess over antibaryons by ten orders of
magnitude. The known CP
violation in
decay is not sufficient for this purpose.
- A new kind of C and CP violation could help to explain a peculiar
asymmetry of the
Standard Model, SM, namely, that the basic constituents of the SM are
left-handed
doublets and right-handed singlets quarks and leptons.
The 
-meson is an eigenstate of the C and CP operators, which is a
rarity in nature.
decays can thus be used to test C and CP invariance. All additive
quantum numbers of
are zero and there are no restrictions in 
decay that may come
from the flavor
change that is important in 
decay.
Most tests of C invariance, such as the ones listed in the PDT, are various
decays. The
best is the absence of 
with BR (
) 
. This places only a limit of 10 percent
on a possible C-violating
amplitude. The decay 
violates C if it occurs via a single photon
intermediate
state. The two photon intermediate state is allowed, making it a
process with an
expected branching ratio around 
. A search for 
is a no-lose proposition: if no candidates are found, one has a strong
test of C from the absence
of the single photon intermediate state; if the decay is finally seen, one
can test
C-invariance even more carefully in the search for charge asymmetry of the
.
Another interesting test is the search for a transverse 
polarization in 
which is a direct test of T invariance, or
in the radiative
decay 
which has a BR of 
.
Finally, CP invariance does not allow the muons in 
to be
longitudinally polarized. If there is an extra Higgs boson, it has been
estimated that the
polarization could be of order . The BR (
) is 
which allows a precise measurement at the AGS
in the C or D
lines where 
production by 
is large and the background is small.
References
- B.M.K. Nefkens in ``Future Directions in Particle and Nuclear Physics
at Multi-GeV
Hadron Beam Facilities,'' D. Geeseman, ed., BNL-5238a, p. 191, 1993.
- B.M.K. Nefkens in ``The Fermilab Meeting,'' DPF 1992, C. Albright,
et al., eds.,
World Scientific, Vol. 1, p. 824, 1993.
Wed Feb 28 14:15:19 EST 1996