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Project Overview

Alex George

Neutrinos and anti-neutrinos are among the most interesting topics in physics. They are everywhere – the sun, cosmic rays, and even nuclear power plants produce billions upon billions of these each minute. Yet, they almost never interact with matter. They are uncharged, so they do not interact electromagnetically. There are no known strong interactions. They are so light (until very recently, it was postulated that they were massless) that they have no real interaction gravitationally. In fact, the only known interaction with anything else is via the weak nuclear force: other things decay into neutrinos.

There are three types of neutrinos: the electron neutrino, the muon neutrino, and the tau neutrino. The electron neutrino is tiny even by atomic standards: it is no greater than approximately four one-millionths the mass of the electron. The muon neutrino is probably less than 2/5 the mass of an electron, while the tau neutrino might be as much as thirty times the electron mass.

Detectors for these neutrinos have been built, and there was immediately a “problem.” Solar structure is well understood, so the expected neutrino flux is known with relatively little uncertainty. However, the measured neutrino flux was about one-third of what it should be. Many explanations were proposed, the most likely of which involves neutrino “oscillations” – that neutrinos can oscillate from being muon neutrinos to being tau neutrinos to being electron neutrinos as they move.

Experiments to test this (or any high-energy) process are very expensive, so they are carried out internationally, with hundreds of physicists needed to collaborate on each one. The KamLAND process, at the Superkamiokande facility in Japan, was set up to measure the mixing angle and the change in mass of the particle, to see what could explain these oscillations. That experiment was successful; oscillations are now understood. KamLAND is now shifting an emphasis on creating an “ultralow background,” free from noise, that can be used to measure neutrinos created at the sun, which will have a much lower energy than those neutrinos originally measured, which were created at a power plant quite nearby.

The other project that the collaboration is working on is Double Chooz, in France, which could simply be described as a search for the mixing angle between electron and tau neutrinos. It is believed that neutrinos are not always in a definitive state (where they would self-identify as an electron or tau neutrino), but might also be in some linear combination of those states, where they would truly be a combination of both simultaneously. But what is the ratio? Are neutrinos usually 99% electron and 1% tau, or are they 50% electron and 50% tau? That’s the (primary) goal of Double Chooz.

But while these experiments are very important, our goal will always be to understand the physics well enough so that what happens on paper is exactly what happens in real life. These experiments have so many variables that they don’t normally happen on paper, but rather as the result of some very complicated software, based on c++.

So at last I can define my project. I will be working with the c++ simulations, and the software, to predict what “should” happen in the reactors in Japan and France, if our understanding of neutrino structure is correct. We can then compare this to what actually happens, so we know how to adjust our theories, should there be differences.

For me, this project has two components:

· Run simulations in Geant 4, a c++-based programming language, that will allow me to understand, describe, and report on the effect of “spill-in/spill-out” – the chance that a particle created outside the detector would capture on the gadolinium inside it, or alternative that a particle created inside the detector would capture on the hydrogen outside the detector.

· Go to KamLAND, the project in Japan, to take data there and ensure that the reactor does not malfunction.