physics

Two topics at the frontier of physics are decays and penguin decays, both of which occur in the decays of mesons to two light mesons. The decay is an example of a two-body decay, and it is most likely the mode in which CP violation in such decays will eventually be seen. The decay is an example of a hadronic penguin decay which should be observed in the near future. For each of these modes, the other one is a serious background, depending on the relative branching ratios and the ability to separate pions from kaons. Measuring both decays will also give some estimate of the effect of penguins in the decay. A statistically significant signal has been observed in CLEO II data in the sum of these two modes and limits on the individual branching ratios are found to be less than . At the present level of statistics the separation of the two channels is not possible. With the increased luminosity of the upgrade, these two modes will both be measurable. Unfortunately as the expected branching ratios decrease, the background suppression becomes more difficult. The existing analyses show that good momentum resolution is critical. It will be necessary to separate the two modes cleanly, especially if one turns out to be much larger than the other. This requires identification at momenta of 2.8 GeV/c. Furthermore, an attempt to measure CP violation would involve telling from . In this case there is no help from kinematic constraints and the particle identification system must provide full discrimination.

One of the centerpieces of the physics program at the upgraded CESR will be measuring the CKM matrix elements as precisely as possible. These are important not only as fundamental parameters of the Standard Model, but also as the information needed to predict the CP asymmetries to be measured later. will be measured using the exclusive decay channels because they are the likely dominant modes beyond the endpoint in the lepton energy spectrum, and because the form factors will be reasonably well understood. Although the recent CLEO analysis sets upper limits on the branching ratios for these decay modes, the size of the signal in the inclusive spectrum implies that they will be observed very soon. The analysis requires a large background suppression, which relies on almost every aspect of the CLEO detector. In the present analysis, the dominant background is due to continuum leptons combined with other pions in the event, which happen to pass the strict kinematic cuts. Information from the silicon vertex detector will provide an extra handle to remove this background. Real leptons coming from charm decays will tend to have a three-dimensional impact parameter removed from the event vertex, while those from decay will not. Light quark events which include fake leptons will be consistent with having only one vertex, while events will have multiple vertices. Thus suppressing these backgrounds will give a good measure of performance for the measurement of vertices.

The familiar decay chain , although not as interesting a physics mode in the future, serves as a useful measure of performance for many other decays, such as . The comparison of efficiency and background suppression with the present detector tests many performance characteristics. In particular, the acceptance for the slow pion and the resolution in mass difference must be studied carefully. We already know from CLEO II how important, but difficult, it is to measure this pion's parameters well. The mass difference is one case in which the angle measurement dominates the resolution, so it tests the measurement of particularly well. It has already been shown that the event vertex constraint is needed to get full use of the silicon detector in measuring this angle.

We have studied in some detail the practicality of the suggestion of Gronau and Wyler[3] to measure CP violation with self-tagging modes. Although it is unclear if such an effect could be seen with the luminosity available with the CESR upgrade, the decay modes involved are interesting in their own right, and provide a good test of the proposed detector. It also makes it possible to look for the CP violation, in case the relevant branching ratios turn out to be larger than expected. In particular, we have examined the decay mode . This is a decay and color suppressed, so that the branching ratio is expected to be in the range of . The decay , which is a Cabibbo-suppressed decay, has a branching ratio in the range of and represents a more accessible mode with the same experimental requirements. Even measuring this mode is well beyond the capability of CLEO II.

Studying these modes will give a good measure of the background suppression for a wide range of rare decays. We will need high efficiency for detecting 's with small background. We will use many decay modes, most of which have higher multiplicities than the familiar decays. The experimental requirements are large acceptance for tracks, including those at small angles and low momentum, and good mass resolution. To separate the decays from decays will require good hadron identification at momenta around GeV/c. Finally, it will be necessary to observe these decays with the decaying to CP eigenstates such as and before one can attempt the CP measurement discussed above.