VOLUME 35, NUMBER 9 PHYSICAL REVIEW D MAY 1987 Rapid Communications The Rapid Communications section is intended for the accelerated publication of important new results. Since manuscripts submitted to this section are given priority treatment both in the editorial once and in production, authors should explain in printed their submittal letter why the work justijies this special handling. A Rapid Communication should be no longer than pages and must be accompanied by an abstract. Page proofs are sent to authors, but, because of the accelerated schedule, publication is not delayed for receipt of corrections unless requested by the author or noted by the editor Limit on the decay D o e —p ~ K. Riles, J. M. Dorfan ' G. S. Abrams, ~ D. Amidei, ~ ' A. R. Baden, ~ T. Barklow, A. M. Boyarski, ' J. Boyer, ~ D. L. Burke, F. Butler, ~ G. J. Feldman, G. Gidal, ~ L. Gladney, '(') M. S. Gold, t' P. R. Burchat, J. Haggerty, ~ ' G. Hanson, K. Hayes, D. Herrup, T. Himel, G. Goldhaber, L. J. Golding, ~ W. R. Innes, J. A. Jaros, I. Juricic, ~ J. A. Kadyk, ~ D. Karlen, S. R. Klein, R. J. Hollebeek, A. J. Lankford, ' R. R. Larsen, B. W. LeClaire, M. E. Levi, N. S. Lockyer, '(') V. Liith, ' C. Matteuzzi, ' M. E. Nelson, ~' ~ R. A. Ong, M. L. Perl, B. Richter, P. C. Rowson, t' T. Schaad, t' H. Schellman, ~ (' W. B. Schmidke, ~ P. D. Sheldon, p G. H. Trilling, & C. de la Vaissiere, ' D. R. Wood, ~ and J. M. Yelton' l Stanford Linear Accelerator Center, Stanford University, Stanford, California 94305 Lawrence Berkeley Laboratory and Department of Physics, University of California, Berkeley, California 94720 Department of Physics, Harvard University, Cambridge, Massachusetts 02138 (Received 30 December 19g6) an- e — p in 204 pb ' of e+e We have searched for the lepton-Aavor-violating decay D nihilation data at E, . =29 GeV from the Mark II detector. No candidates were found; we Dv, estimate an upper limit on the cross section times branching ratio of o(e+e inclusive)B(D e — p ) &0. 35 pb at the 90% confidence level. Simple assumptions yield the e — p ~) & 2. 1x10 rough limit B(DO D; Recent theoretical ideas' suggest it may be possible to observe the liavor-changiny reaction D e — p despite and B current limits on K e p e — p . Typi- cal schemes involve an isoscalar pair of scalar leptoquarks with charge ~ —, G and G„which couple charge 3 quarks to charged leptons and charge 3 quarks to neutral e — p is allowed (see Fig. 1) leptons. The decay D without introducing diquark couplings that are incon- sistent with lower limits on the proton lifetime. Since the leptoquark couplings are Yukawa couplings, strong flavor dependence is expected, favoring the observation of e — p heavy-quark processes such as D We have searched for D e — p (throughout this paper the charge-conjugate reaction is also implied) in 204 pb ' of data taken with the Mark II detector at the SLAC e+e storage ring PEP (E, ~ =29 GeV). A de- tailed description of the Mark II detector can be found in Ref. 5. A brief description of those elements important to FIG. 1. A possible mechanism for Do isoscalar charge — —, ' scalar leptoquark G. e+p involving an this analysis is given here. Two cylindrical drift chambers concentric with the beam line provide charged-particle tracking in a 2. 35-kG solenoidal magnetic field. The inner vertex chamber contains several axial sense-wire layers; the outer chamber has ten stereo and six axial layers. To- gether they yield a momentum resolution Sp/p = [(0. 025 ) + (0. 01p ) ] '/2 (p in GeV/c) in the plane transverse to the beam direc- tion. Immediately surrounding the magnetic coil are eight lead-liquid argon calorimeter modules which cover 64% of the solid angle and have an energy resolution for photons the 0. 14/ JE (E in GeV). Surrounding of bE/E calorimeter are four layers of steel and proportional tubes, providing in this analysis good muon identification over 45% of the solid angle for tracks with p 2 GeV/c. Ha- dronic events were selected to have five or more charged tracks and a total visible energy (charged and neutral) greater than 4 E, ~. These charged tracks were required to have a measured momentum less than 16 GeV/c and a momentum transverse to the beam direction greater than 100 MeV/c. In addition, they were required to pass within 8 cm of the interaction point along the beam direc- tion and within 4 cm in the transverse plane. These cri- teria were satisfied by 82000 events. A further require- ment was that the thrust axis, as calculated from the 1987 The American Physical Society