T. Thompson (UPenn)

Discovery of the Muon NeutrinoTony Thompson

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T. Thompson (UPenn)

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.

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Continued History

• 1955: (electron) antineutrino directly observed

• 1962: direct observation of muon neutrino

• 2000: direct observation of tau neutrino

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Mysterious Decays

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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)

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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?

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The AGS Neutrino Experiment at Brookhaven

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Pions produced

Steel shield stops strongly interacting particles

Spark Chamber to detect neutrinos

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Pions

Steel ShieldBeryllium

Protons

some decay to muons+(muon?)

neutrino

few muons + (muon?) neutrinos

Spark Chamber

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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

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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?

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Spark Chambers: Cosmic Muons

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Leon Lederman: Spark Chamber Model

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Brookhaven Experiment:Spark Chamber

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• 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?

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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?

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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

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Muon EventsElectron Showers

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Multi Muon Events

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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

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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

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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

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