Search for electron neutrino appearance in a 250 km long-baseline experiment

We present a search for electron neutrinoappearance from accelerator produced muon neutrinosin the K2K long baseline neutrino experiment. One candidate event is found in the data corresponding to an exposure of 4.8*10^19 protons on target. The expected background in the absence of neutrino oscillationsis estimated to be 2.4+-0.6 events and is dominated by mis-identification of events from neutral currentpi^0 production. We exclude the \nu_\mu to \nu_e oscillations at 90% C.L. for the effective mixing angle in 2-flavor approximation of sin^2(2theta_\mu_e) (~= 1/2 sin^2 2 th_13) > 0.15 at Delta m^2_\mu_e = 2.8*10^{-3} eV^2, the best fit value of the \nu_\mu disappearance analysis in K2K. The most stringent limit of sin^2(2theta_\mu_e) < 0.09 is obtained at Delta m^2_\mu_e = 6*10^{-3} eV^2.