Neutrinos: What we ve learned and what we still want to find out. Jessica Clayton Astronomy Club November 10, 2008

Transcription

1 Neutrinos: What we ve learned and what we still want to find out Jessica Clayton Astronomy Club November 10, 2008

2 Neutrinos, they are very small, they have no charge and have no mass, and do not interact at all. John Updike (2003) Almost no mass.

3 Back to the Basics

4 The Standard Model

5 Something s missing Beta decay? Neutron proton electron Conservation of energy, momentum and angular momentum require that something else exists.

6 Birth of a Particle 1930: Wolfgang Pauli predicts that there is another particle involved in beta decay First theories about neutrinos were soon after written by Enrico Fermi Fermi coined the term neutrino - meaning little neutral one

7 Discovery! In 1956, Fred Reines and Clyde Cowan detected the neutrino via inverse beta decay ν + p n + e+ e - 2 photons in opposite directions Cd Another photon, 5 x 10-6 sec later

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9 Predictions for neutrinos from the sun 4p He + 2e ν e + energy Protons in the sun fuse to form helium In the process, neutrinos and energy are released. Ray Davis and John Bahcall formed a team to study this prediction in 1964.

10 Underground in South Dakota Ray Davis built a neutrino detector one mile underground in the Homestake Mine Large tank of cleaning fluid, C 2 Cl 4 Cl + ν -> Ar + e - Count the number of Ar atoms to find the number of neutrinos

11 The Solar Neutrino Problem The number of neutrinos measured by Davis was only 1/3 of what Bahcall predicted. It was Three possibilities: 1) problem with detector 2) problem with solar theory of fusion and neutrino production 3) something is wrong with the Standard Model. Davis and Bahcall at Homestake. Photo from nobelprize.org.

12 Searching for answers Kamiokande detector was built in Japan and detects about half of the neutrinos that Bahcall predicted. GALLEX, SAGE and Super- Kamiokande confirmed the deficit in neutrinos over different energy ranges but still, the theory doesn t match the observations

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14 SNO breakthrough in 2001 The Sudbury Neutrino Observatory could only measure one flavor of neutrinos, ν e. Kamiokande was sensitive mostly to ν e, but also to ν µ and ν τ. Results were combined to come up with the total number of solar neutrinos and the number of solar ν e.

15 Neutrinos change flavors! 1/3 of solar neutrinos are electron flavor by the time they get to Earth The missing electron neutrinos oscillate into ν µ or ν τ. In order to change flavors, neutrinos must have a non-zero mass. That doesn t fit into the Standard Model as we know it!

16 Vindicated after 40 years. Bahcall made his first predictions about the number of neutrinos produced by the sun in the mid-1960s. The Solar Neutrino Problem was born with Davis first results in Neutrinos were studied by several experiments - and were measured from a supernova in 1987 In 2001, SNO results confirmed that neutrino oscillations occur.

17 Supernova 1987a Neutrinos were detected from Supernova 1987a by Kamiokande and IMB Credit: C Burrows (ESA/STScI), HST, NASA Within 12 seconds, Kamiokande saw 12 events (6-35 MeV) and IMB saw 8 events (19-39 MeV). First optical observations were the next day.

18 Neutrinos in our midst A trillion neutrinos pass harmlessly through your body every second! sun Big Bang Human body atmosphere Supernova 1987a Nuclear reactors Accelerators Earth s radioactivity

19 A New Window on the Universe NRAO Radio image of the Crab Nebula Anglo-Australian Obs. Optical image of the Crab Nebula Ultraviolet Imaging Tel. Ultraviolet image of the Crab Nebula Gravitational waves? Neutrinos? Chandra X-ray Obs. X-ray image of the Crab Nebula

20 STAR OR GALAXY? PHOTON (LIGHT) Neutrinos travel in a straight lines. Because they have no electric charge, they are not deflected by magnetic fields in space. NEUTRINO INTERSTELLAR DUST COSMIC RAY PROTON

21 Neutrinos: many open questions What s accelerating neutrinos? Gamma-ray bursts (GRBs)? Active Galactic Nuclei (AGN)? What s the mass of each flavor of neutrino? What s the value of the oscillation parameters? Are neutrinos and anti-neutrinos the same thing?

22 Neutrinos: what we think now Neutral (no charge) Tiny, non-zero mass 3 flavors, which oscillate Very tiny cross-section, meaning that they don t like to interact with matter Promising new way of studying the Universe

23 Why do we study neutrinos? A particle that is almost nothing may tell us everything about the Universe. Christine Sutton