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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 tex2html_wrap_inline2568 10 tex2html_wrap_inline2570 and tex2html_wrap_inline2572 10 tex2html_wrap_inline2574 , respectively) [9] 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 tex2html_wrap_inline2618 reconstructed D tex2html_wrap_inline2620 -decays and about 1-2 10 tex2html_wrap_inline2618 D tex2html_wrap_inline2624 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/ tex2html_wrap_inline2626 . At this stage, possible backgrounds are difficult to estimate. Most limits may therefore be regarded as lower bounds on those limits.

tex2html_wrap_inline2250 - tex2html_wrap_inline2630 mixing. Mixing can be observed in different ways. Certainly the cleanest way is flavour tagging by means of full reconstruction of the associated D-meson. We estimate that we may detect mixing if r tex2html_wrap_inline2632 is tex2html_wrap_inline2634 10 tex2html_wrap_inline2636 .

Higher statistics may be obtained if c-flavour tagging is performed by tagging of the decay tex2html_wrap_inline2638 . 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 tex2html_wrap_inline2574 (1/ tex2html_wrap_inline2626 behaviour) - 10 tex2html_wrap_inline2644 (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 ( tex2html_wrap_inline2248 -production). CLEO III and BaBar will have similar discovery potentials with BaBar expecting about 10 tex2html_wrap_inline2648 D's of each kind produced in total. Their estimated sensitivities to mixing are in the order of 10 tex2html_wrap_inline2636 -10 tex2html_wrap_inline2652 . Again, a tex2html_wrap_inline2566 /charm factory would probably do better than COMPASS or any B-factory.

CP-violation in the D-system. With standard model predictions of the order of 10 tex2html_wrap_inline2574 for direct CP-violation (assuming a favourably large FSI in Cabibbo suppressed decays) the D-system is the most unfavourable for standard model tests. Samples of tex2html_wrap_inline2634 10 tex2html_wrap_inline2648 -10 tex2html_wrap_inline2662 reconstructed charmed D-mesons are needed to possibly observe a signal. According to various models, possible FSI giving raise to CP-violation might be enhanced. In particular the Penguin diagram, responsible for a possible effect in Cabibbo suppressed D-decays, might get contributions from exotic meson states like the tex2html_wrap_inline2264 (1800) [8].

Promising decay channels are D tex2html_wrap_inline2666 K tex2html_wrap_inline2668 K tex2html_wrap_inline2624 and D tex2html_wrap_inline2672 K tex2html_wrap_inline2674 , K tex2html_wrap_inline2676 ' [9]. If no background is present, sensitivity in tex2html_wrap_inline2678 of about 0.005-0.01 may be achieved where


where tex2html_wrap_inline2684 (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% [10].

Indirect CP-violation is not discussed here since its observation is even less probable due to the low tex2html_wrap_inline2250 - tex2html_wrap_inline2630 mixing expected.

Rare or forbidden decays. Flavour changing neutral currents (FCNC) may be searched for in D tex2html_wrap_inline2690 . This decay channel has very good acceptance and trigger efficiency in COMPASS. We should thus be sensitive to branching ratios of the order of 10 tex2html_wrap_inline2644 - 10 tex2html_wrap_inline2694 , one to two orders of magnitudes below current limits. A possible feed-through of D tex2html_wrap_inline2696 has to be taken into account and good tex2html_wrap_inline2264 / tex2html_wrap_inline2278 separation has to be achieved.
D tex2html_wrap_inline2702 + cc (lepton number violation) is less favourable due to bremsstrahlung effects and the more difficult task of clean electron identification. Still, one can hope to obtain a similar improvement on upper limits as in D tex2html_wrap_inline2690 .

Competition again comes from E831 which may reach limits of order 10 tex2html_wrap_inline2652 , 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 tex2html_wrap_inline2708 10 tex2html_wrap_inline2652 and 2 tex2html_wrap_inline2708 10 tex2html_wrap_inline2652 on D tex2html_wrap_inline2690 and D tex2html_wrap_inline2718 e tex2html_wrap_inline2720 , respectively. The tenfold increase in statistics expected by CLEO III for the year 2001 will decrease these limits to between 5 tex2html_wrap_inline2708 10 tex2html_wrap_inline2644 and 10 tex2html_wrap_inline2652 , assuming the limits scale as 1/ tex2html_wrap_inline2626 [11].

Other interesting decays which only can be studied with high statistics charm samples are tex2html_wrap_inline2274 S=1 transitions in charmed strange systems. Such decays (e.g. tex2html_wrap_inline2732 ) should have branching ratios of about 10 tex2html_wrap_inline2574 . 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 tex2html_wrap_inline2736 which has a mass only 16 MeV/c tex2html_wrap_inline2256 lower than tex2html_wrap_inline2740 . About 25-50 such events might be observed.
The observation of this decay gives information complementary to tex2html_wrap_inline2274 S=1 decays from hyperons and may shed more light on the tex2html_wrap_inline2274 I=1/2 rule [12].

next up previous contents
Next: What to do with Up: PHYSICS WITH CHARMED HADRONS Previous: Leptonic decays

Lars Schmitt
Wed May 22 16:44:09 METDST 1996