Measurement of the form factor ratios in semileptonic decays of charm mesons

Abstract

I have measured the form factor ratios r2 = A2 (0)/A1 (0) and rV = V (0)/A1 (0) in the semileptonic charm meson decay D+ → $\bar{K}$*0 e+ve from data collected by the Fermilab E791 collaboration. Form factors are introduced in the calculation of the hadronic current in semileptonic decays of strange, charm, or bottom mesons, such as D+ → $\bar{K}$*0 e+ ve . Semileptonic decays provide insight into quark coupling to the W boson since the leptonic and hadronic amplitudes in the Feynman diagram for the decay are completely separate. There are no strong interactions between the final state leptons and quarks. A number of theoretical models predict the values of the form factors for D+ → $\bar{K}$*0 e+ ve , though there is a large range of predictions. E791 is a hadroproduction experiment that recorded over 20 billion interactions with a 500 GeV π- beam incident on five thin targets during the 1991-92 Fermilab fixed-target run. Approximately 3000 D+ → $\bar{K}$*0 e+ ve decays are fully reconstructed. In order to extract the form factor ratios from the data, I implement a multidimensional unbinned maximum likelihood fit with a large sample of simulated (Monte Carlo) D+ → $\bar{K}$*0 e+ve events. The large E791 data sample provides the most precise measurement of the form factor ratios to date. The measured values for the form factor ratios are r2 = 0.71 ± 0.08 ± 0.09 and rV = 1.84 ± 0.11 ±} 0.08. These results are in good agreement with some Lattice Gauge calculations. However the agreement with quark model predictions is not as good.