Discovery of the Muon Neutrino

Transcription

1 Discovery of the Muon Neutrino Tony Thompson 1

2 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. 2

3 Continued History 1955: (electron) antineutrino directly observed 1962: direct observation of muon neutrino 2000: direct observation of tau neutrino 3

4 Mysterious Decays 4

5 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) 5

6 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? 6

7 The AGS Neutrino Experiment at Brookhaven Pions produced Spark Chamber to detect neutrinos? Steel shield stops strongly interacting particles 7

8 Beryllium Steel Shield Spark Chamber Protons Pions some decay to muons+(muon?) neutrino few muons + (muon?) neutrinos 8

9 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 Some pions decay into muon + (muon?) neutrino Classic particle physics problem: why not electron+ neutrino? Exercise left to reader. Spoiler : Chirality 9

10 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? 10

11 Spark Chambers: Cosmic Muons 11

12 12

13 Leon Lederman: Spark Chamber Model 13

14 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? 14

15 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? 15

16 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 16

17 Electron Showers Muon Events 17

18 Multi Muon Events 18

19 113 Events? The number of events found matching this criteria was 113 when 3.48 x 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 19

20 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 20

21 Additional Sources DiscoveringParticles/detection/spark-chamber/