Transcript Part I
Neutrinos 2. Neutrino scattering Super Kamiokande located 1000m underground in Kamioka mine in Japan is a water Cherenkov detector made up of 50,000 m3 of ultra-pure water. Target contains 32,000 m3 of water viewed by 11,000 51cm diameter PMTs which is surrounded by a veto containing the rest of the water viewed by 1,900 PMTs. Building was completed in 1985. Advantage: Provides information on energy and direction albeit at higher complexity and cost. Neutrinos 2. Neutrino scattering Super Kamiokande http://www.youtube.com/watch?v=PlVIoD4jHKc&feature=related Neutrinos 2. Neutrino scattering Scattered electrons create a cone of Cherenkov light, projected as a ring onto the wall of the detector and recorded by the PMTs. Using timing information from each PMT, the path of the incoming neutrino can be determined. The figure shows a typical neutrino event. Super Kamiokande was designed to study solar and atmospheric neutrinos, and keep watch for supernovas in the Milky Way Galaxy. Neutrinos 2. Neutrino scattering The detector succeeded in detecting neutrinos from a supernova explosion which was observed in the Large Magellanic Cloud in 1987. SuperKamiokande was also first to announce evidence of neutrino oscillations in 1998, which means a neutrino has non-zero mass. 11 points at 0 seconds show events of SN1987A neutrinos. Neutrinos 2. Neutrino scattering In 2001, several thousand PMTs imploded, apparently in a chain reaction as the pressure waves from each imploding tube cracked its neighbours. The detector has been partially restored with about 5000 photomultiplier tubes with protective shells that will prevent the chain reaction from recurring. Neutrinos The Solar Neutrino Problem When Homestake began taking data in 1968 it recorded only a third as many neutrinos as expected from the Sun. Experiment bad? Nuclear physics wrong? Solar model was wrong? However, other experiments saw only 40% and Super Kamiokande saw only 50% of the expected signal. The probability that something more fundamental was wrong began to be taken seriously. Neutrinos The Solar Neutrino Problem Accepted theory at the time was that neutrinos were massless meaning type of neutrino would be fixed when it was produced. Sun emits only electron neutrinos so all solar neutrinos were expected to be electron neutrinos. All neutrino detectors at the time were only sensitive to electron neutrinos. Sun makes electron neutrinos Massless case Earth sees 100% electron neutrinos Neutrinos The Solar Neutrino Problem But if neutrinos have mass, they could change flavour. Thus, "missing" solar neutrinos could be electron neutrinos which changed into other types along the way to Earth and therefore escaped detection. Sun makes electron neutrinos Non zero mass case Earth sees 33% electron neutrinos Neutrinos The Solar Neutrino Problem In 2001 Sudbury Neutrino Observatory (SNO) in Canada detected all types of neutrinos coming from the Sun, and was able to distinguish between electron neutrinos and the other two flavours. It was found that about 35% of the arriving solar neutrinos are electron neutrinos, with the others being muon or tau neutrinos. Neutrinos Sudbury Neutrino Observatory (SNO) http://www.youtube.com/watch?v=WE565jXuVuM 1000 tonnes of heavy water in a 12 metre diameter transparent acrylic sphere viewed by approximately 9,600 PMTs 2 km underground in Ontario, Canada. Detection rate is about one neutrino per hour. Turned on in 1999 and was turned off on in 2006 although analysis of the data recorded still continues. Neutrinos Sudbury Neutrino Observatory (SNO) Neutrinos Sudbury Neutrino Observatory (SNO) Because SNO uses heavy water, it is able to detect not only electronneutrinos through the scattering interaction (which Super- Kamiokande relies on), but also the other neutrino flavours through different interaction processes, namely: 3. Charged current interaction Neutrino is absorbed, converts a neutron in deuteron to a proton and an electron is produced. Solar neutrinos have energies smaller than the mass of muons and tau leptons, so only electron neutrinos can participate in this reaction. Neutrinos Sudbury Neutrino Observatory (SNO) 4. Neutral current interaction Neutrino breaks deuteron into its constituent neutron and proton. The neutrino continues on with slightly less energy. All three neutrino flavours are equally likely to participate in this interaction. The neutron and proton go on to deposit their energy in the target. Neutrinos Sudbury Neutrino Observatory (SNO) With this ability to register interaction of all neutrino flavours with the target, SNO became the first observatory to see the expected neutrino flux from the Sun. Next step: liquid scintillator will replace heavy water as an interaction will produce several times more light and so the energy threshold for the detection of neutrinos will be lower. Gravitational waves http://www.youtube.com/watch?v=v1tkM_f5B9s General Relativity describes how the fabric of space-time bends and stretches when a massive object is placed in it. Distortion becomes critical around objects of very high mass, black holes for example forming a singularity (or very sharp spike) in the space-time continuum. Gravitational waves are "ripples in spacetime." A more massive moving object will produce more powerful waves. Analogous to movement of electric charge in an aerial. No accepted theory exists and no one has yet measured this effect. 2 dimensional model Gravitational waves http://www.youtube.com/watch?v=v1tkM_f5B9s Gravitational waves arent absorbed by dust like EM waves. Gravitational waves ARE the fabric of space and so are absorbed by nothing. They can therefore tell us much about the far reaches of the Universe. But is this just a maths theory or is there proof ? Gravitational waves Only evidence found so far is Hulse-Taylor Pulsar (PSR 1913+16) - a binary star system. Like EM radiation, gravitational waves carry energy away from their sources and, in the case of orbiting bodies, this is associated with a decrease in the orbit. Since 1974 the period of this pulsar has been measured. Figure shows decrease in orbital period of pulsar over time. (Only 40 seconds over 30 years but it agrees with General Relativity). Gravitational waves are incredibly weak Best detector is based on interferometry.