学术报告（10月21日下午）

摘要：Conventional wisdom is that "ordinary" experiments with neutrinos cannot distinguish whether they are Dirac or Majorana states (where in the latter case neutrinos are in some sense their own antiparticles).  It is claimed that only a ``neutrinoless double-beta decay" experiment could do so. To date, no such decay has been observed in experiments with various nuclei.   However, the diagram for a "neutrinoless" double-beta decay includes a virtual neutrino, which also occurs in the diagrams for reactor-neutrinos experiments, for solar-neutrino experiments (as noted in 1946 by Pontecorvo), and also in neutrino-nucleon scattering experiments using neutrinos from charged pion decay.   All of these experiments have results consistent with Dirac neutrinos, and (in my view) inconsistent with the hypothesis of Majorana neutrinos.

报告人简历：Prof McDonald, as a PhD of Caltech in 1972 ,focused on a test of time-reversal invariance in the electromagnetic interactions p d He3 gamma.1972-75,as a postdoc at CERN, worked at the p-p ISR (intersecting storage ring) studying the strong interaction, first via a streamer chamber, and later with a lead-glass Cerenkov counter array (built earlier by the Rubbia group) at the split-field magnet.1972-76,as a postdoc at University Chicago, worked on mu+mu_ pair production via pion beams at Fermilab. First results were on production or the J/psi particle.1976-, at Princeton University,started as Assistant prof. Around 2006 joined the Day Bay reactor antineutrino experiment, mainly working on the hardware for the RPC gas system.