Detection of Neutrinos and Tau Leptons

NOBEL PRIZE 1995

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Awarded to:

Frederick Reines and Martin L. Perl

"for pioneering experimental contributions to lepton physics, specifically for the detection of the neutrino"

FREDERICK REINES PHOTOS

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FREDERICK REINES HISTORY

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Frederick Reins was an American physicist born on the 16th of March 1918, in Paterson, New Jersey.

During highschool, one of his teachers gave him a key to the school laboratory and gave him permission to work whenever he wanted. This cultivated a love of science in Reines by his senior year, and led him in the direction of a career in science.

In the yearbook, Reines' quote was,

"TO BE A PHYSICIST EXTRAORDINAIRE"

He graduated from highschool in 1935.

Reines attended Stevens Institute of Technology in Hoboken, where he earned his M.E. and M.S. degrees, before receiving his Ph.D. from New York University. In 1940 he married Sylvia Samuels, to whom he remained married for the rest of his life.

DISCOVERY OF THE NEUTRINO

In 1956, Reines and his colleague, Clyde Cowan, first detected neutrinos. Neutrinos had been first proposed theoretically by Wolfgang Pauli 20 years earlier to explain undetected energy that escaped when a neutron decayed into a proton and an electron.

On the basis of his work in first detecting the neutrino, Reines became the head of the physics department of Case University from 1959 to 1966.

In 1966, Reines took most of his neutrino research team with him when he left for California to become the founding dean of physical sciences at University of California, Irvine (UCI).

Reines had prepared for the possibility of a supernova explosion, so that he could catch the neutrinos streaming from it in his specially-designed detectors. After Supernova 1987A exploded, results showed that when a supermassive star collapses and then explodes, the resulting jets of neutrinos bombard the escaping masses to create the elements up through uranium that are heavier than iron.

Researchers have concluded that without these natural neutrino processes in exploding supermassive stars, the elements like copper, silver, platinum, and gold that are heavier than iron would not exist.

In 1995, Reines was honored, along with Martin L. Perl with the Nobel Prize in Physics, and his work with Clyde Cowan in first detecting the neutrino was recognized by the National Academy of Sciences. Reines also received many other awards, including the National Medal of Science.

Reines remained on UCI's faculty until his death of natural causes in 1998, aged 80

MARTIN LEWIS PERL PHOTO

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MARTIN LEWIS PERL HISTORY

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Martin Lewis Perl was born on the 24th of June, 1927 in New York. He was an American physicist who received the 1995 Nobel Prize for Physics for discovering a subatomic particle that he named the tau, a massive lepton with a negative charge. Perl was jointly awarded the Nobel Prize with physicist Frederick Reines, who discovered another subatomic particle, the neutrino, in the 1950s.

In 1948 Perl graduated from the Brooklyn Polytechnic Institute with a degree in chemical engineering. After working as a chemical engineer for two years, he studied nuclear physics at Columbia University (Ph.D., 1955). He was an instructor and associate professor at the University of Michigan (1955–63) before joining the faculty of Stanford University in 1963.

In 1966 Perl was part of a research team that made an unsuccessful attempt to discover new charged leptons by colliding electrons at the Stanford Linear Accelerator Center (SLAC).

A new particle accelerator that began operation at SLAC in the early 1970s had the capacity to reach high energy levels that were previously inaccessible. With this new machine, Perl recorded frontal collisions between electrons and their antiparticles, positrons.

In a series of experiments conducted between 1974 and 1977, he found that the collisions formed heavy leptons, later called tau particles, that decay in less than a trillionth of a second into neutrinos and either an electron or a muon. He also discovered the antitau, which decays into neutrinos and either a positron or an antimuon.

THE NEUTRINO

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The Neutrino is a Chargeless, nearly massless, lepton. It was first proposed as a theoretical particle in the 1930's by Wolfgang Pauli. Pauli's work on negative Beta decay is what lead to the theorising of the Neutrino. Beta decay at this point was assumed to be the decay of a Neutron to a Proton and an Electron. However, when the energy of Beta decay was found and the momentum of the decay was found, there was a difference between expected values and experimental values. This meant that the equation:

N → P + e-

Is incomplete. Pauli proposed that a particle of no charge and almost no mass existed, which carried all the energy and momentum of the reaction. This was the Neutrino:

Despite being "found" in the 1930's, it took until 1956 when the neutrino was first observed.

USES AND TYPES OF NEUTRINOS

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uses and types of neutrinos

Tau Lepton

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The tau lepton (often called the tau or occasionally the tauon) is a negatively charged elementary particle with a lifetime of 3×10−13 seconds and a mass of 1777 MeV (compared to 939 MeV for protons and 0.511 MeV for electrons). It has an associated antiparticle (the anti-tau) and neutrinos (the tau neutrino and tau antineutrino).

Classification

The tau lepton belongs to the 3rd generation of leptons. It is the third generation counterpart of the electron (1st generation) and the muon (2nd generation). Like the electron and muon, the tau lepton appears to be pointlike; no structure has been detected, and if there is any, it would have to be on a scale of less than 10−18 meters. Also, like the electron and muon, the tau has a spin of 1/2. The tau lepton and its antiparticle carry the same electric charges as the electron and positron, respectively.

Decay

Feynman diagram of the common decays of the tau lepton by emission of a W boson. The tau is the only lepton that can decay into hadrons—the other leptons do not have the necessary mass. Like the other decay modes of the tau lepton, the hadronic decay is through the weak interaction. Since tau-like lepton number is conserved in weak decays, a tau neutrino is created when a tau lepton decays to a muon or electron.

The branching ratio of the common tau decays are:

a. 17.84% for decay into a tau neutrino, electron and electron neutrino

b. 17.36% for decay into a tau neutrino, muon and muon neutrino

Discovery

The tau lepton was detected through a series of experiments between 1974 and 1977 by Martin Lewis Perl with his colleagues at the SLAC-LBL group. Their equipment consisted of SLAC's new e+-e− colliding ring, called SPEAR, and the LBL magnetic detector. They could detect and distinguish between leptons, hadrons and photons. They did not detect the tau lepton directly, rather they discovered anomalous events:

There must have been undetected particles because not all energy from the initial collision could be accounted for in the final state. However, they did not detect any other muons or electrons, or any hadrons or photons. It was proposed that this event was the production and subsequent decay of a new particle pair:

This was difficult to verify because the energy to produce the τ+τ− pair is similar to the threshold for D meson production. Work done at DESY-Heidelberg, and with the Direct Electron Counter (DELCO) at SPEAR, subsequently established the mass and spin of the tauon.

Martin Perl shared the 1995 Nobel Prize for physics with Frederick Reines. The latter was awarded his share of the prize for detecting the neutrino.

ref: www.wikipedia.org

http://en.wikipedia.org/wiki/Tau_lepton

COWAN AND REINES NEUTRINO EXPERIMENT

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Cowan and Reines came up with a way of detecting the Neutrino by creating a chain of events which would give off a signal using gamma rays. This is based on positive Beta Decay, where an anti-electron neutrino was used to cause a Proton to decay, instead of gamma rays or other forms of energy:
The Proton decays into a Neutron and a Positron. A Positron is the anti-particle of an Electron, and when the Positron eventually interacts with an Electron, the two particles will eliminate each other which sets of a blast of gamma radiation. If this radiation can be detected within the sample it is can be assumed that the neutrino set off the reaction, ad in an ambiguous way has been observed.

THE EXPERIMENT

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The experiment was carried out at Hanford in the preliminary stages, however, it was moved to the Savannah Plant near Augusta in Georgia. The plant had was chosen as it had shielding against cosmic radiation. The shielded location was 11m from the reactor and 12m underground.

A nuclear reactor with with a neutrino flux of 12-13 per second per centimeters squared was used to send neutrinos into a tank of water. The tank held 200 litres of water, and also had 40 kg of dissolved cadmium chloride in it. There were three lines of scintillators with 110 5 inch or 27 cm photomultiplier tubes on it. These tubes were used to collect gamma rays released from an annihilation reaction.

The theory is that the neutrino's will react with protons in the water, which causes it to decay to a neutron and a positron. This positron will react with an electron causing an annihilation which release a pair of gamma rays, each going in a opposite directions. These gamma rays cause the scintillator to visibly flash.
Cowan and Reines thought that this was not conclusive enough and decided that the neutrons should be measured as well. This was done with the cadmium. The neutrons cause the cadmium to become unstable and give off gamma rays of there own. The experiment was set out in a way that the gamma rays were released from the annihilation were detected first, then 5 microseconds later the gamma rays from the cadmium are seen on the scintillator. This created a distinctive signature for the detection of the neutrino reaction, the gamma pair hits, then 5 microseconds later the neutron gamma ray hits.

RESULTS OF THE EXPERIMENT

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The results of the experiment was that the neutrino was detected in a form. The experiment was conducted over months, and as the experiment was being conducted, information of the neutrinos was recorded. At the end of the experiment, they had accumulated data on about 3 neutrinos an hour. To confirm these were neutrinos that where being detected, they shut the reactor down to ensure that there was a difference in the number of detected events. They also calculated the reaction cross-section as 6.3 x 10-44 centimeters squared, close to their original prediction of 6 x 10-44 centimeters squared.