“The Two Neutrino Experiment”

Observation of High-Energy

Neutrino Reactions and the Existence of Two Kinds of Neutrinos

Trip Adler

June 24, 1962

Background on Neutrinos

• Fermions with half-integer spin

• Left-handed chirality

• Electrically neutral

• Interacts only by weak force

• Very small cross section (for neutrinos produced in the sun, it would take about one light-year of lead to block half of them)

History of Neutrinos

• First postulated in 1931 by Pauli to explain the energy spectrum of beta decays

• Postulated again in 1947 by Powell to explain the “kink” observed in pion decay

• Experimentally observed in 1956 by Cowan and Reines by setting up a large tank of water and watching the “inverse” beta-decay reaction

• Davis and Harmer established that there are neutrinos and antineutrinos in 1959

• Concept of conservation of electron number and muon number has been introduced to explain why some reactions occur and some do not

Motivation for Experiment

• The reaction µ- —> e- + is never observed (why?)• Recently was postulated that if there were two kinds of neutrino,

one associated with the electron (ve) and one with the muon (vµ), then all allowed and forbidden processes can be accounted for:

n —> p+ + e- + ve

π + —> µ+ + vµ

µ+ —> e+ + ve + vµ

• But until now there has never been any experimental confirmation of this

• At the same time, there are no methods of studying the weak force at high energies

The Experiment• If only one type of neutrino, the following two reactions

should take place at the same rate:v + p+ —> n + µ+ v + p+ —> n + e+

• To test this, neutrino beam generated as follows:– Pions were produced by 15-BeV protons striking Be target– Pions decayed according to the following reactions:

π+ —> µ+ + vπ- —> µ- + v

• Lots and lots of shielding used so that only neutrinos from this reaction and no cosmic rays interact with detector

• Spark chamber used to detect neutrino interactions

The Accelerator• The Alternating Gradient Synchrotron (AGS) in Brookhaven,

New York• Particles are stored in a storage ring• 240 magnets used to focus the particles and curve their tracks• Microwave cavities accelerate the particles

The shielding

• Additional filtering factor:– Beam would hit spark chamber for only 2 microseconds– This short time window allowed for the exclusion of events

that took place outside this window, which were caused by cosmic radiation

• 13.5 meters of steel from a dismantled warship used to filter out unwanted particles

• Large amounts of concrete to filter cosmic rays

The Detector

• Used a spark chamber– Consisted of 90 aluminum plates,

weighing a total of 10 tons– Neon gas sandwiched between plates

• If any neutrino interact with aluminum nuclei to produce charged particles, the particles would ionize the gas

• Then, under the influence of an electric field, the gas would spark where ionized, revealing the path of the newly created particles

• Photos of such paths were taken

The Data

• In an exposure of 3.48 x 1017 protons, a total of 113 events observed– 34 “single muon” events– 22 “vertex” events– 49 “short single track” events– 8 “shower” events

• The 56 “single muon” and “vertex” events will be called “events,” and resulted in muon production

• The “short single track” events have too little momentum to be muons, and the “shower” events are not muons

The Data (Cont.)

Single muon events Vertex events Shower events

Interpretation of Data

• We know the events are not produced by cosmic rays– Ran experiment with AGS off, and without gating requirements– Determined that 1 in 90 cosmic ray events are neutrino events– The actual experiment ran for a total of 5.5 sec, with gating,

and in this case 4-6 cosmic neutrino events (of the 56 events observed) would be expected

• We know that neutrinos produced were the decay products of pions and K mesons:– Ran experiment with 4 feet less of steel– This reduces path available for pions to decay– Reduced rate of events, confirming the source of neutrinos

Interpretation of Data (cont.)• Events are not neutron produced

– Are uniformly distributed throughout volume of spark chamber

– Second background run (with less steel) did not increase events rate, which shows that shield is sufficient

• The single particles produced are presumed to be muons, because they show little or no nuclear interaction– No large angle or charge exchange scattering observed– The mean free path for nuclear interactions is less than 100

cm of aluminum, so the mean free path for the observed single tracks is more than 8 times this length

Interpretation of Data (Cont.)

• If only one type of neutrino, the following two reactions should take place at the same rate:

v + p+ —> n + µ+ v + p+ —> n + e+

• This means that there should have been on the order of 29 electron showers

• However, only 6 showers were observed, and these were qualitatively different from normal electron showers (probably neutron produced stars or from the decays of K+ or K2

0)• In conclusion, ve ≠ vµ

Other Conclusions/Questions

• The intermediate boson– If the mass of the intermediate boson is less than that of

the proton, it could have been produced in the process v + p+ —> w+ + p+ + µ-

– This is consistent with 5 of our vertex events– More experiments should be conducted on this using a

chamber with higher resolution

• Other questions about neutrinos– Could there be a third neutrino?– Could there be neutrino flavor oscillations?

Bibliography• Observation of High-Energy Neutrino Reactions and the Existence of Two

Kinds of Neutrinos. Phys. Rev. Lett. 9, 36 (1962)

• Griffiths, David. Introduction to Elementary Particles. Wiley, 1987

• Essays of an Information Scientist: Creativity, Delayed Recognition, and other Essays, Vol:12, p.216, 1989. Current Contents, #32, p.3-9, 1989

• Press Release: The 1988 Nobel Prize in Physics: http://nobelprize.org/physics/laureates/1988/press.html

• Brookhaven National Laboratory Website:

http://www.bnl.gov/bnlweb/facilities/AGS.asp Leon Lederman, Melvin Schwartz, and Jack Steinberger won the Nobel Prize in 1988 for their discovery