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