Goals of decay studies

Does the Standard Model explain quark mixing? There are presently two fundamental questions in the forefront of high energy physics. One concerns the origin of mass which revolves around the scalar sector. This is being pursued in Higgs particle searches at super-high energies at Fermilab and CERN. The other equally deep and probably related question concerns the origin of quark mixing and, thus, CP violation. CLEO III with its unique ability to measure rare processes is in a position to pursue this question by performing redundant measurements of the unitary triangle shown in Fig. . These measurements couple our studies with the important question of baryon-antibaryon asymmetry in the universe and thus with the very reasons that we may exist.

The unitary triangle is depicted in a two dimensional space where the Wolfenstein[4] parameters and form the horizontal and vertical axes. The sides and angles of the triangle are directly related to physical measurements. One side is proportional to the absolute value of the ratio of CKM elements . This can be measured by observing the relative yields of versus in specific processes, such as semileptonic decay. Another side is proportional to the ratio which can be inferred by measuring the ratio of mixing rates for and or by measuring the ratio of widths . The three angles of the triangle can be found by measuring CP violation in three different types of decay. Typical modes are shown in Fig. . Whereas the angles and can be determined at the with an asymmetric collider, the angle can be discovered with a symmetric machine using charged decays. Additional information constraining the unitary triangle is available from other measurements including and in kaon decay and mixing, but these typically have two problems associated with their interpretation: they are functions of the unknown top quark mass and the hadronic matrix elements that relate the CKM parameters to the measurements need to be calculated theoretically. The measurements of the sides of the triangle have the latter problem, although to a lesser extent.

Physics beyond the Standard Model mesons can decay via loop diagrams which are sensitive to new physics, as well as physics predicted by the Standard Model. The existence of one such class of diagrams, called ``penguins,'' was predicted to explain the I = 1/2 rule in kaon decays[5].

In Fig. we show the Feynman diagram for the penguin process. Such decays have recently been established in CLEO II with evidence for the exclusive channel [1]. The study of radiative penguin processes, in particular measurements of the inclusive radiative penguin rate, , are important for setting constraints on models beyond the Standard Model. As the accumulated luminosity increases, the study of the exclusive penguin process will become viable as will the search for other non-standard processes such as .

QCD physics Although the study of weak decays of -quarks is the primary motivation for CLEO III, the -quark is probably the only laboratory to study Quantum Chromodynamics (QCD) in heavy quark systems. The charm quark is probably too light to show the asymptotic behavior expected in Heavy Quark Effective Theory[6] (HQET) and the top quark is too heavy to form mesons prior to its decay. Moreover, a thorough understanding of QCD effects is necessary to extract the fundamental CKM parameters and with precision.