Many speculations have been done concerning hints for physics beyond the standard model. One of the promising areas, where such physics could be studied, is the decay of charmed mesons. D-mixing or CP-violations are predicted within the standard model to be very low (r 10 and 10 , respectively)  and will most likely remain unobserved. However, this drawback could be turned into a virtue since no standard model background is expected for measurable unexpected effects.
Table 2.1 shows some of the best current limits on rare decays or other phenomena.
Table 2.1: Current limits on some rare decays channels
For COMPASS, we estimate the limits attainable on the basis of 3 10 reconstructed D -decays and about 1-2 10 D decays (the same number of CP conjugate states should be reconstructed). It should be noted that the limits on the rare decays decrease with 1/N only in the case of no background. When background becomes important upper limits only decrease with 1/ . At this stage, possible backgrounds are difficult to estimate. Most limits may therefore be regarded as lower bounds on those limits.
Higher statistics may be obtained if c-flavour tagging is performed by tagging of the decay . However, this method is limited by backgrounds from double Cabibbo suppressed decays (DCSD) which may in part be reduced by observing the time evolution of the D-decays. Limits which may be achieved are of the order 10 (1/ behaviour) - 10 (1/N behaviour).
Competing experiments are E831 with about 1/5-1/10 of the expected COMPASS statistics but with the advantage of cleaner events ( -production). CLEO III and BaBar will have similar discovery potentials with BaBar expecting about 10 D's of each kind produced in total. Their estimated sensitivities to mixing are in the order of 10 -10 . Again, a /charm factory would probably do better than COMPASS or any B-factory.
Promising decay channels are D K K and
D K , K ' .
If no background is present, sensitivity in of about
0.005-0.01 may be achieved where
where (D) is the decay rate for a given decay channel.
Competition again comes from E831 and B-factories. E831 expects a sensitivity of of about 2-3%, B-factories may be sensitive to asymmetries below 1% .
Indirect CP-violation is not discussed here since its observation is even less probable due to the low - mixing expected.
Competition again comes from E831 which may reach limits of order 10 , about a factor 10 above COMPASS, and from the B-factories, which, however, do not quote any numbers. CLEO II has reported upper limits of 3 10 and 2 10 on D and D e , respectively. The tenfold increase in statistics expected by CLEO III for the year 2001 will decrease these limits to between 5 10 and 10 , assuming the limits scale as 1/ .
Other interesting decays which only can be studied with high statistics
charm samples are S=1 transitions in charmed strange systems.
Such decays (e.g. ) should have branching
ratios of about 10 . The topology of such a cascade decay resembles the decay
of doubly charmed baryons with the exception that only one additional D-meson
is present in the event.
Again, excellent vertex resolution will be needed to sort out this decay
and distinguish it from which has a
mass only 16 MeV/c lower than . About 25-50 such events might be
The observation of this decay gives information complementary to S=1 decays from hyperons and may shed more light on the I=1/2 rule .