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June 15, 2011 — Physicists on verge of solving neutrino mystery

by Sean Bettam — Wednesday, Jun 15, 2011

Early findings may explain why the Universe contains matter but no anti-matter

June 15, 2011
Sean Bettam

An international experiment with contributions from physicists at the University of Toronto is announcing results that could solve a long-standing puzzle in particle physics and may even turn the Standard Model on its head. The team has detected the first indication of oscillation from muon-type neutrinos to electron-type neutrinos.

"Two other modes of neutrino oscillation between the three known types of neutrino have been established  by the Super-Kamiokande experiment in Japan and the SNO experiment in Canada”, said Professor John Martin of the University of Toronto. “However, the new result is tremendously exciting, since it indicates a third mode of oscillation, which is coupled intimately with an asymmetry between matter and anti-matter called CP violation.”

The findings will be  presented tomorrow at a meeting of the Canadian Association of Physicists (CAP) in St. Johns Newfoundland by Dr. Mark Hartz, a research associate at the University of Toronto and York University.

The T2K experiment, which studies neutrino interactions across a large distance, involves sending a beam of the particles from the J-PARC accelerator in eastern Japan, through the island of Honshu, to the Super-Kamiokande detector 295 km away. A device known as an optical transition radiation (OTR) detector takes an image of the proton beam for each beam pulse (once every 2.5 seconds) right in front of the target.

The OTR detector helps determines the neutrino beam direction by measuring the proton beam position on the target, and also the beam width. If the beam width is too small, the energy from the high intensity beam can damage the target. The OTR detector was a responsibility of U of T and collaborators from York University and Canada's national particle physics laboratory TRIUMF.  It is the only type of detector that can survive in the intense proton beam at T2K.  The  mechanical systems were designed by Department of Physics mechanical engineer Mircea Cadabeschi and built mostly at U of T.

The T2K experiment was interrupted by the earthquake that struck Japan in March. The scientists expect to resume full operations by the end of 2011.  It is early on in the experiment with only two percent of the planned data collected so far.

"We hope to be back to running the experiment again by December after the ongoing recovery process from the earthquake, in order to move our measurement from the realm of indication to discovery. We will then go to the next step comparing the oscillation in neutrino and anti-neutrino beams to study an asymmetry between particle and anti-particle reactions that may explain why we live in a matter-dominated universe."

Data from the experiment is being analyzed at U of T by Hartz and graduate student Patrick de Perio. Both individuals are leading analysis efforts to understand the neutrino beam and neutrino-nucleus interactions. They contributed directly to a paper reporting the findings submitted to the Physical Review of Letters.

"One of the conditions for solving the fundamental mystery of why there is only matter in the universe today, even though equal amounts of matter and anti-matter were created in the Big Bang, is that there was a source of charge-parity symmetry (CP) violation in the particle interactions in the very early universe," says Martin." There seems to be not enough CP violation in the quark sector of the Standard Model for this. So, particle physicists think it may be in the neutrino sector."

"If our indication for muon to electron neutrino oscillation holds up with more data, we will be able to go on to measure the CP violation in the neutrino sector by comparing results for this oscillation using neutrino and anti-neutrino beams. Thus, we may be able to get a handle on why we - human beings made of matter - are here at all to try to understand the universe."