Hadronic Decays of Charm

Recent hadronic charm decay results from fixed-target experiments are presented. New measurements of the D0 → K−K+K−π+ branching ratio are shown as are recent results from Dalitz plot fits to D+ → K−K+π+, π+π−π+, K−π+π+, K+π−π+ and Ds → π+π−π+, K+π−π+. These fits include measurements of the masses and widths of several light resonances as well as strong evidence for the existence of two light scalar particles, the ππ resonance σ and the Kπ resonance κ. INTRODUCTION Hadronic decays of charm are rich in information about QCD. For instance, the suppression of D0 → π+π− relative to D0 → K−K+ proved the importance of final state interactions in charm decays. Also, hadronic decays give rise to the 2.5× difference between the D+ and D0 lifetimes. Hadronic decays can provide information on relative strengths of decay diagrams (spectator, exchange, annihilation, etc.). Spectator diagrams are believed to be responsible for the vast majority of the charm decay rate. In a spectator diagram, the charm quark decays while the other quark in the meson is a spectator. By contrast, exchange and annihilation diagrams require a connection between the charm quark and the other quark in the meson and are therefore suppressed. Determining the contributions of these diagrams is an interesting open question in charm physics. More recently, charm has been used to investigate the light resonances which are products of charm decays. Although very high statistics scattering experiments have been performed for many years to investigate these resonances, many parameters are still virtually unknown. Charm offers a unique way to investigate these resonances by nature of its low background and well defined initial state (pseudoscalar meson). Experimentally, hadronic decays can be investigated by comparing branching ratios and by analyzing the resonant structure of multibody decays. The results presented here come from the Fermilab experiments E791 and FOCUS. E791 (FOCUS) ran in 1991 (1996–7) with a 500 GeV/c2 π− beam (180 GeV photon beam) on five (four) targets. BRANCHING RATIO MEASUREMENTS Calculations of charm quark decay rates via the weak interaction have been possible for many years. Unfortunately, only charm hadron decay rates (which are affected by the strong force) can be measured by experiments. The strong force is even more intimately involved when the charm particle decays into hadrons. Thus, deviations from the naïve weak prediction for a given decay can provide insight into the nature of the strong force. 0 1 2 3 4 5 6 7 8 9 10 1.7 1.75 1.8 1.85 1.9 1.95 2 Mass (GeV/c) E n tr ie s p er 0 .0 0 5 G eV /c 2 0 50