Charm decays (pictured) are a particularly insidious background to DONUT neutrino interacton identification. The charm (now indicated in red) has a decay length comparable to that of the tau and is produced in %8.1 percent of electron and muon neutrino interactions. Additionally, it often has similar decay topologies. Donut physicists determine whether or not a decay is charm by looking at the primary interaction vertex. If the vertex has more than one charged lepton product it is dismissed from being a tau event because, due to lepton number conservation, tau neutrinos do not interact to form other leptons. However,if the lepton products are not identified, the event must be classified as background. Physicists were able to estimate the approximate charm background based on various interaction, detection and calculation parameters and determine the probability of mistaking a tau deacy for a charm particle and use those numbers for the overall data uncertainty. In this particular event, DONUT analysts were able to identify an electron from the primary interaction vertex, indicated by the green track, and thereby identify the red track as a tau (the colors are purely used for convention's sake). For more event analysis and information you can visit our collaborators in japan at

Last updated: 6/29/01 comments