Minimising Systematic Uncertainties in the Determination of Theta_13 at T2K

Neutrinos are the most mysterious of the fundamental particles. These electrically neutral leptons are nearly massless and only interact via the weak nuclear force, so they are exceedingly hard to detect. However, they are created copiously in the earth, in the atmosphere, in the sun, and out in the cosmos. Neutrinos are key players in the processes that heat the earth, make the sun shine, and make supernovae explode. Despite the fact that neutrinos are everywhere, their intrinsic properties have puzzled us for years. Thanks to a series of exciting discoveries over the past decade, we now know that the elusive behavior of neutrinos probes the deepest questions in particle physics. Studying the most basic constituents of matter, and how they interact with each other, is the primary challenge of particle physics. The current sum of knowledge in the field is called the Standard Model, and it has been remarkably successful in describing the microscopic universe. The model contains two major classes of building blocks: quarks and leptons. For example, protons are made of three quarks, while electrons and neutrinos are types of leptons. However, we know that the Standard Model is incomplete because there must be a more fundamental theory that explains how the two major classes of building blocks are related to each other. Precise measurements of the properties of these two sectors provide our best clues to the form of the underlying theory. But it's not a matter of dotting the i's and crossing the t's; much work remains to be done. In 1998 there was a major surprise in the lepton sector: the three kinds of neutrinos were observed to change from one into another. This kind of oscillation is only possible if neutrinos have mass and mix with each other, thanks to an interesting twist of quantum mechanics. This recent discovery was the first experimental evidence for neutrino mass, and is theoretically attractive because it has been known for decades that the quarks can mix with each other. Uncovering the similarities between the quark and lepton sectors will help to reveal the deeper symmetries of the fundamental theory. The physics of mixing between neutrinos---or between quarks---can be described in terms of six independent parameters. The first generation of neutrino oscillation experiments measured four of the six. To capture the remaining two parameters will require measurements with unprecedented accuracy. The next generation, led by the T2K experiment in Japan, will improve upon the precision of the existing four parameters, and, starting in 2009, T2K will make the world's best attempt to measure the remaining unknowns. Understanding the mysterious properties of neutrinos may illuminate the way to the fundamental theory beyond the Standard Model. We are now embarking on a bold programme to improve the accuracy of neutrino mixing measurements. This is a necessary next step toward answering the key questions of particle physics.