all known subatomic particles. • They interact very weakly with matter. • Billions of neutrinos from the sun pass through each square centimeter of the Earth's surface every second!
of conservation of energy, Wolfgang Pauli proposed that a light, neutral particle was emitted along with an electron in beta decay. • This particle was considered to interact too weakly to be detectable, and was called a 'neutrino'.
to produce both negative electrons and positive antielectrons (positrons). • These were observed to produce distinct neutrinos, one of which was labelled the 'antineutrino'.
energy particles that create showers of other particle when they hit the Earth's atmosphere. • We can see the paths of cosmic rays in cloud and bubble chambers.
in cosmic rays. • Two important ones are the π meson (pion) and µ meson (muon). • These particles are intermediate in mass between the electron and the proton.
showed it to be identical to the electron except for being ~200 times heavier. • The incongruity of this twin particle led I.I. Rabi to famously remark “Who ordered that?”
to have its own, separate associated neutrino, named the muon neutrino. • A third neutrino, the tau neutrino, followed the discovery of a third charged lepton, the tau, in the 1970s. • It is now known that all matter particles come in three different 'sizes' called flavours.
are organized into three generations • Each generation contains two pairs of particles: a quark pair and a lepton pair. • Each of these pairs differs in charge by one unit.
four fundamental forces identified in particle physics. • It couples together the particles of each pair, allowing interactions that change one to the other. • Beta decay proceeds by the weak interaction, with a down quark being transformed into an up quark.
the transformation of particles. • The fusion of two protons into a deuterium nucleus is the most significant case, but there are others. • These fusion reactions produce neutrinos. 1 H 1 H 1 H 1 H 2 H 1 H 1 H 2 H 3 He 3 He 1 H 1 H 4 He ν γ ν Gamma Ray Neutrino Proton Neutron Positron ν γ γ
in the dry cleaning fluid, it was found that 1/3 of the predicted number of neutrinos were seen. • The solar model calculations were validated through other means, while the experimental value was refined through 24 years of running the Homestake experiment.
forth where neutrinos, in flight, oscillate between the three different flavours. • Since the experiments that first detected solar neutrinos were only sensitive to electron neutrinos, this would explain the discrepancy. • The quantum mechanics of oscillation require the three flavours to have different masses; this violates the Standard Model assumption that neutrinos are massless.
Super-K can see atmospheric neutrinos from all parts of the earth. • Neutrinos that travelled further were less likely to be muon neutrinos, thus providing evidence for oscillations.
built to answer the solar neutrino problem. • With 1,000 tons of heavy water (worth $300M!) deep underground, it was sensitive to all kinds of neutrinos.
After several years of running, SNO clearly saw a total neutrino flux consistent with solar models, and an electron neutrino flux 1/3 as large. • This was clear confirmation of neutrino oscillation and a resolution of the solar neutrino problem. • This resolution earned the 2015 Nobel Prize in physics.
producing particles that decay to neutrinos, then directing them down a decay pipe. • These neutrinos can be directed at distant detectors to measure the changes in the beam over time.
we want two detectors, one at the source and one distant. • This allows us to compare the flavour content of the beam before and after a long propagation
is called T2K • It has a beam of neutrinos from the J- PARC accelerator in Tokai to the Super-K detector in Kamioka • The total distance is roughly 300 km.
was built to look for is electron neutrino appearance. • The beam from Tokai is mostly muon neutrinos. • They had been observed to disappear into tau neutrinos, but not into electron neutrinos. • This measurement would allow us to determine, for the first time, all three of the parameters that are required in neutrino oscillation theory.
of the T2K near detector: the Fine- Grained Detector or FGD. • The FGD consists of layers of plastic detector bars, in which the neutrinos interact. • This allows us to track the products of the interactions at their source.
occur on an atomic nucleus. • Differences in nuclear structure between different elements can affect the probability (or ‘cross-section’) of the interactions on different elements • For T2K, the far detector is made mostly of water (oxygen), while the near detector is made mostly of plastic (carbon). • My physics goal was to measure the ratio of these two cross-sections, so as to help compare the two detectors correctly.
the FGD units to observe interactions on water. • I was heavily involved in building these targets from off-the- shelf polycarbonate greenhouse panels.
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