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Discovery of the Muon Neutrino Tony Thompson

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Discovery of the Muon Neutrino Tony Thompson Discovery of the Muon Neutrino Tony Thompson T. Thompson (UPenn) 1 Selected History • 1930: Neutrino Predicted (energy not conserved in beta decays) • 1937: Muon Discovered • 1941: Muon shown to decay into electron + neutrino(s) • 1948: Energy spectrum of Muon decay shown to be continuous, meaning there must be 2 neutrinos in the decay. • Most people felt that the neutrinos should be different, and they named them so (neutrino and neutretto), but the name didn’t stick. T. Thompson (UPenn) 2 Continued History • 1955: (electron) antineutrino directly observed • 1962: direct observation of muon neutrino • 2000: direct observation of tau neutrino T. Thompson (UPenn) 3 Mysterious Decays T. Thompson (UPenn) 4 Fermi’s Theory of Weak Interactions • Theory to explain beta decay, muon decay • direct coupling of 4 fermions • Successful at low energies • Problem: • Breaks down at energies > 100 GeV • No reason for different neutrino flavors (though lepton number conservation was proposed in 1952) T. Thompson (UPenn) 5 Enter: W Boson • The breaking down of the Fermi theory can be avoided if a mediating boson is introduced (W boson) • Problem is that this ratio of cross sections is much lower than predicted by the theory (<10-8 from experiment vs 10-4 from theory)! • This can be saved if the two neutrinos are different, a muon neutrino and an anti electron neutrino • How to test if neutrinos are different? T. Thompson (UPenn) 6 The AGS Neutrino Experiment at Brookhaven Spark Chamber to detect neutrinos Pions produced ? Steel shield stops strongly interacting particles T. Thompson (UPenn) 7 Beryllium Steel Shield Spark Chamber Protons Pions some decay to few muons + muons+(muon?) (muon?) neutrinos neutrino T. Thompson (UPenn) 8 The Experiment • 15GeV beam of protons strikes Beryllium target, producing pions • Pions hit 13.5m think iron shield, 21m from the target • absorbs strongly interacting particles, attenuation of order 10-24 • Some pions decay into muon + (muon?) neutrino • Classic particle physics problem: why not electron+ neutrino? Exercise left to reader. Spoiler : Chirality T. Thompson (UPenn) 9 The Experiment++ • 5.5m of concrete on floor and roof to reduce cosmic muons • Interactions observed in a 10-ton aluminum spark chamber behind steel shield • If these neutrinos are muon neutrinos, they should only produce muons, not electrons • electrons produce distinct shower, muons produce nice tracks • But how does spark chamber work? T. Thompson (UPenn) 10 Spark Chambers: Cosmic Muons T. Thompson (UPenn) 11 T. Thompson (UPenn) 12 Leon Lederman: Spark Chamber Model T. Thompson (UPenn) 13 Brookhaven Experiment:Spark Chamber • Same idea as before, turned on its side • Use outer slabs to veto cosmic and accelerator produced muons • Want to capture interactions from neutrinos interacting inside the chamber • Inner slabs used to trigger • Question for audience: how do you use this to measure energy of the particle? T. Thompson (UPenn) 14 Brookhaven Experiment: Triggering • 40 coincidence pairs in anti-coincidence with the outer shield. • Calibrated by increasing the energy of the proton beam enough that many muons go through shield • ~10 triggers an hour • But how do you take data in 1962? T. Thompson (UPenn) 15 Analog Data: Photographs! • Use trigger to take photos, about half of photos usually blank • Then look for events that meet the following criteria • more than 4 inches from sides, 2 inches from top/bottom • first 2 gaps must not fire • For single tracks: • extrapolation of track backwards for 2 gaps must remain in fiducial volume • production angle relative to beam must be < 60 degrees T. Thompson (UPenn) 16 Electron Showers Muon Events T. Thompson (UPenn) 17 Multi Muon Events T. Thompson (UPenn) 18 113 Events? • The number of events found matching this criteria was 113 when 3.48 x 1017 protons were fired at the target • Of these, 34 were single muon events (and originated inside the detector) • If there was no difference between muon neutrinos and electron neutrinos, we would expect a similar number of electron showers • only 6 showers observed T. Thompson (UPenn) 19 Conclusion • This experiment definitively showed that the neutrinos from beta decay and the neutrinos from muon decay were different • First direct observation of muon neutrinos • Important part of developing our current theory of weak interactions • 1988 Nobel Prize in Physics was given for this experiment to Leon (God Particle) Lederman, Melvin Schwartz, and Jack Steinberger T. Thompson (UPenn) 20 Additional Sources • http://www.ep.ph.bham.ac.uk/general/outreach/ DiscoveringParticles/detection/spark-chamber/ • http://arxiv.org/pdf/physics/0503172v1.pdf • https://en.wikipedia.org/wiki/Fermi's_interaction T. Thompson (UPenn) 21.
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