The Neutrino

Charlton LuMathematics of the Universe – Math 89S

Professor Bray27 September 2016

Introduction

Developing our knowledge about the neutrino, whose existence was first predicted by

Wolfgang Pauli in 1930, has been a long and arduous process. Due to its elusive properties—its

sub-microscopic size and miniscule mass in addition to its tendency to travel at near-light speeds

—twelve additional years passed after Pauli’s initial prediction before scientists figured out how

they could potentially detect a neutrino and an additional fourteen years elapsed before they

finally detected the existence of the particle. Despite the difficulty of researching neutrinos,

scientists persisted.

Further discoveries proved the existence of different types or “flavors” of neutrinos and

neutrino oscillation—the idea that travelling neutrinos change between different flavors. More

recent discoveries showed neutrinos have mass, contrary to the popular opinion before the 21st

century. Currently, scientists are continuing efforts to learn about the peculiar qualities of the

neutrino.

But the extensive amount of research in neutrinos leads to the overarching question: Why

do we care so much about this tiny, mysterious particle? John Conway, a physics professor at UC

Davis paradoxically seems to offer a dismissive comment towards neutrinos, saying, “They’re

almost nothing at all, because they have almost no mass and no electric charge…They’re just

little whisps of almost nothing” (Marder). In reality, he believes that neutrinos are in fact a

fundamental particle of the universe; knowledge about the neutrino is vital for the understanding

of properties of the universe.

Scientists have invested substantial amounts of time and effort into neutrino research

because the neutrino offers a potential pathway towards a new comprehension of the world. The

neutrino is at the rare intersection of history and innovation—it offers insight into theories and

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questions that explain the history of the universe such as the Big Bang and the Asymmetry

between matter and antimatter (Baryon Asymmetry) and could potentially lead to new theories

about physics and astrology. This paper will discuss the development of our current knowledge

about neutrinos in addition to the ongoing efforts to understand more about the neutrino and to

apply its unique characteristics to new technology.

Beta Decay

Beta decay is a natural process that occurs when a proton or neutron transforms into the

respective other, releasing a pair of subatomic particles. In beta plus decay (β+), a proton decays

into a neutron while simultaneously creating a positron and an electron anti-neutrino1.

Conversely, beta minus decay (β−) involves the conversion of a neutron into a proton, which

leads to the release of an electron and an electron anti-neutrino.

Prior to the current understanding of the neutrino, scientists thought that beta decay

simply involved the emission of an electron or positron. However, they began to doubt their

understanding when they discovered two inconsistencies (Close).

First, when measuring the energy released from beta decay, scientists found that the beta

decay emission spectrum was continuous rather than discrete. This contrasted the emission

spectrum of alpha decay—scientists knew that atoms undergoing alpha decay emitted a constant,

discrete amount of energy because an atom that undergoes alpha decay releases a single alpha

particle whose energy could be calculated. They were confused by the emission spectrum of beta

decay (Pictured below, Fig 1.) which was not fixed at a certain value, but rather was a

distribution of possible values (Jensen).

1 Scientists did not know that they were electron antineutrinos at the time

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Furthermore, beta decay seemed to violate the law of conservation of angular momentum.

After measuring the angular spin of the particles before and after beta decay, scientists found a

discrepancy of ½ spin—the spin angular momentum of the reactants was different from that of

the products (Brown).

The strange emission spectrum and deviation from the law of momentum conservation

seemed to imply that scientists were missing something from their current model of beta decay.

It was Wolfgang Pauli who boldly proposed the existence of a new particle—the neutrino. He

suggested that the new particle emitted during beta decay can carry a range of possible energies.

Therefore, the emission spectrum, which only measured the energy of the electron or positron

released, was continuous because it did not account for the energy carried by the neutrino.

Additionally, Pauli predicted that the neutrino had a spin angular momentum of ½, which would

resolve the violation of the law of momentum conservation (Pauli). His theory seemed to be

plausible, but Pauli had no way to detect the neutrino.

Electron Capture

In 1942, Wang GangChang proposed the use of electron capture (also known as beta

capture) to detect neutrinos. Electron capture is the process by which a proton in an electrically

Figure 1. Beta Particle Emission Spectrum121

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neutral atom absorbs an inner electron, leading to the emission of a neutron, neutrino, and a

gamma ray (Fig 2.). The nuclear reaction is given by equation

p + e− → n + νe

where ve was later named the electron anti-neutrino (GangChang).

Scientists Clyde L. Cowan and Frederick Reines built on top of Wang GangChang’s idea

and conducted an experiment to detect neutrinos. Cowan and Reines used a variation of

Gangchang’s proposed experiment by inducing inverse beta decay. Inverse beta decay, unlike

beta capture, had an electron anti-neutrino as a reactant rather than a product. It involved the

combination of an electron anti-neutrino and a proton to produce a neutron and a positron:

νe+ p → n + e+

Cowan and Reines knew that the positron released from inverse beta decay would immediately

annihilate itself with an electron (cancel both particles out), releasing a detectable gamma ray.

Instead of measuring the neutrino directly, as Gangchang suggested, Cowan and Reines instead

attempted to detect the emitted gamma rays that would prove the existence of the neutrino

(Cowan, Reines).

Their first experiment involved the use of a nuclear reactor as a source of neutrinos;

Cowan and Reines shot the neutrinos into a tank of water surrounded by a scintillator—a

Figure 2. Carbon-11 transforming in to boron-11 through electron capture

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material that absorbs gamma rays and gives off light. By detecting the light in the scintillator,

they would have effectively proved the existence of inverse beta decay and thus neutrinos.

However, even though Cowan and Reines detected some light in the scintillators, their results

were not significant enough to conclusively prove the existence of the neutrino (Los Alamos

Science).

Their solution involved a second experiment. Cowan and Reines decided to shoot

neutrinos into a solution of aqueous cadmium chloride. Cadmium has a high propensity for

absorbing neutrons and releases a gamma ray when it absorbs each neutron. Therefore, Cowan

and Reines could use the cadmium to detect not only the annihilation of an electron and positron,

but also the emission of a neutron. Their results in 1956 were conclusive: they detected an

average of roughly three neutrinos per hour over months of data collection, which they further

confirmed was a significant result by shutting the nuclear reactor off and showing a consistent

difference in the number of neutrinos detected (Cowan, Reines). In 1995, Cowan and Reines

won the Nobel Prize in Physics for confirming Pauli’s prediction.

Neutrino Flavors

When scientists discovered the neutrino in 1956, they did not realize that there were

different types of neutrinos. Scientists later found that the electron anti-neutrino was actually

only one of three known neutrino types, or “flavors.” In fact, each lepton (an elementary particle

with ½ spin that does undergo strong interaction) had an associated neutrino. The three leptons—

the electron, muon, and tau—all had an accompanying neutrino.

In 1962, Leon M. Lederman, Melvin Schwartz, and Jack Steinberger hypothesized the

existence of a muon neutrino. Their experiment was based around the equation

νl + n → p + l−

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(scientists knew that the l− was either an electron or muon). If the reaction resulted in a muon,

Lederman, Schwartz, and Steinberger would have known that the reactant neutrino must be

distinct from the electron neutrino, which in contrast would have produced an electron. Indeed,

scientists found that the muon was created, proving the existence of the muon neutrino (Anicin).

Lederman, Schwartz, and Steinberger later received the second Nobel Prize in Physics dedicated

towards neutrino research.

When the tau particle was discovered as the third lepton between 1974 and 1977,

scientists strongly believed in the existence of an accompanying neutrino. Just like the emission

spectrum of beta decay, scientists found a continuous spectrum associated with tau decay,

providing the first hint at the presence of a tau neutrino. But it was not until 1997 that scientists

discovered the first direct evidence for the tau neutrino. Using Fermilab's Tevatron accelerator,

scientists aimed a neutrino beam suspected to be comprised of tau neutrinos at target of iron

plates. When a tau neutrino interacted with an iron nucleus, scientists expected a tau particle to

form (Anicin). Though it took scientists three years (one out of one million million tau neutrinos

interacted with an iron nucleus and produced a tau particle), the experiment finally confirmed

their theory—the tau neutrino was indeed a distinct particle (Fermilab).

Neutrino Oscillation

In the late 1960’s, scientists Raymond Davis, Jr. and John N. Bahcall conducted an

experiment known as the Homestake Experiment to detect neutrinos being emitted from nuclear

fusion taking place in the sun. Their results only showed one-third of the theoretical number of

neutrinos that should have been detected, leading to the solar neutrino problem—why are so few

neutrinos coming from the sun (Anicin)? The solar neutrino problem confused scientists for

decades until the SuperKamiokande Collaboration in 1998 discovered the first convincing

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evidence for neutrino oscillation—the idea that neutrinos change between different flavors. The

Homestake Experiment could only sense muon neutrinos, which accounted for the lack of solar

neutrinos being detected.

The SuperKamiokande team filled a tank with 50,000 tons of water to detect atmospheric

muon neutrinos which were produced when atmospheric particles decay. Since neutrinos are so

small that they can pass through the earth virtually unopposed, scientists measured the neutrino

flux (rate at which neutrinos pass through) of the tank from both sides (they measured neutrinos

coming from the opposite side of the earth as well as from the side closer to the surface). They

found that roughly half of the neutrinos from the other side of the earth were lost. Since the

SuperKamiokande neutrino detector could not detect tau neutrinos, scientists concluded that the

detectable muon neutrinos must have oscillated and transformed into the tau neutrino (Kamioka

Observatory, University of Tokyo).

In 2004, definitive evidence of neutrino oscillation was produced, when the K2K

experiment in Japan measured a real occurrence of neutrino oscillation. Scientists at The High

Energy Accelerator Research Organization in Japan, also known as KEK, shot a beam of muon-

neutrinos 300 km towards a neutrino detector similar to SuperKamiokande. Since scientists shot

a measurable amount of muon-neutrinos from KEK, they were able to compare the amount of

muon-neutrinos detected at SuperKamiokande. They found, with a 99.9985% confidence (4.3 σ),

that there had been a disappearance of muon neutrinos and an increase in electron and tau

neutrinos. The K2K experiment conclusively proved that neutrinos do in fact oscillate (CERN

Courier).

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Neutrino Mass

Scientists used the K2K experiment to prove that neutrinos have mass. Though their

reasoning requires an understanding of quantum physics beyond the scope of this paper, the

significance of their conclusion can still be appreciated. The Standard Model, which had endured

every experimental challenge since its inception, was finally proven incomplete. The discovery

that neutrinos had mass was inconsistent with the Standard Model, proving the necessity for a

dramatic change in our understanding of particle physics. (Berkeley Center for Theoretical

Physics)

CUORE

At the cutting edge of neutrino research today is the question of how to incorporate

neutrino mass into the Standard Model. Scientists are currently trying to answer this question by

figuring out whether or not the neutrino is its own antiparticle i.e., are the electron, muon, and

tau neutrinos the same as their respective antiparticles? If proven true, the discovery would have

profound impacts on our understanding of how the antimatter is formed, how neutrinos gain their

mass, and how the neutrino can fit into the Standard Model.

The current efforts to answer this question involve the detection of the theoretical process

neutrinoless double beta decay. In this decay, two protons would be transformed into two

neutrons, two positrons, and two anti-neutrinos. As in single beta decay, double beta decay also

has the analogous process in which two neutrons transform into two protons, two electrons, and

two anti-neutrinos. If the neutrino is its own antiparticle, the two anti-neutrinos would annihilate

one another, leading to neutrinoless double beta decay (Kayser).

The Cryogenic Underground Observatory for Rare Events (CUORE) is the largest and

newest facility to search for neutrinoless double beta decay. Their strategy is to measure the

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energy of the two released electrons. If the two electrons have a combined energy equal to the

total decay energy, scientists can conclusively prove that the electrons carry all of the energy in

the decay—that is, neutrinos account for no energy in the decay, which means that they must

have annihilated one another as antiparticles (CUORE Collaboration).

CUORE Design

As with all neutrino experiments, the logistics of setting up such an experiment is

exceedingly difficult. The bolometer (device that measures the energy of the electron) must be

cooled to cryogenic temperatures in order to accurately detect the miniscule energies released in

double beta decay. In addition, the experiment must be well shielded from background radiation

that may skew results; a plethora of backgrounds could hide a decay signal—cosmic muons,

surrounding hardware, and radioactive decays of contaminants within the detector are among the

many sources of interference scientists at CUORE are concerned about.

CUORE’s impressive technology alleviates these problems. Scientists in CUORE cool

down the TeO2 crystals used in the bolometer down to 10mK, making its heat capacity so small

that a single electron can produce a measurable rise in its temperature. As for the background

radiation, scientists surround the detectors with a 73-ton shield with three layers that individually

combat a specific type of background ray (Fig 3.). With the shield, they are able to reduce the

background radiation to several orders of magnitude less than the energy expected to be detected

within the apparatus. Finally, scientists clean the detectors using a process consisting of abrasive

tumbling, electropolishing, chemical etching, and magnetron plasma etching, which reduces the

background radiation of contaminants to a lower, predictable level (CUORE Collaboration).

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Potential Impacts of the Study

If scientists do prove that neutrinos are in fact Majorana fermions (they are their own

antiparticle), then further explanations of neutrinos are necessary. The discovery would mean

that neutrinos violate the law of conservation of lepton number, necessitating even more changes

in the Standard Model (Anicin). On a more positive note, such a discovery could also imply the

existence of a new particle to account for the inconsistency. Scientists have already postulated

the existence of a “sterile neutrino”, a more massive type of neutrino that can account for some

of the dark matter in the universe. The discovery could also give insight into the origin of matter,

offering new evidence about the matter-antimatter asymmetry and the Big Bang. But regardless

of whether or not scientists prove that neutrinos are Majorana fermions, the current research will

undoubtedly result in further studies that unravel the mysterious characteristics of the neutrino.

Figure 3. The design of the CUORE Detector Shield

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Conclusion

The properties of the neutrino are a great unknown within the scientific community, and

yet they provide huge potential for discovery. Learning more about the neutrino could quite

literally reveal some secrets of the universe; our understanding of the Standard Model and dark

matter among many other areas in physics could be significantly altered as the properties behind

the neutrinos become more clear. Perhaps the reason why so many scientists are interested in

neutrinos is its far-reaching impacts and unpredictability; who knows what interesting properties

of the neutrino will be discovered next?

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Works Cited

Anicin, Ivan V. THE NEUTRINO Its past, present and future. Belgrade, 22 March 2005.

Berkeley Center for Theoretical Physics. Neutrino Physics: Implications of neutrino mass. Berkeley.

Brown, Laurie. "The Idea of the Neutrino." Physics Today (1978): 23-28.

CERN Courier. Synthetic neutrinos appear to disappear. Geneva, 18 August 2000.

Close, Frank. Neutrino. Oxford, February 2012.

CUORE Collaboration. Searching for neutrinoless double-beta decay of 130Te with CUORE. L'Aquila, 15 February 2015.

Fermilab. Physicists Find First Direct Evidence for Tau Neutrino at Fermilab. Warrenville, 20 July 2000.

GangChang, Wang. "A Suggestion on the Detection of the Neutrino." Physical Review (1942): 97.

Jensen, Carsten. Controversy and Consensus: Nuclear Beta Decay 1911–1934. Bern: Birkhäuse, 2000.

Jr., C. L Cowan, et al. "Detection of the Free Neutrino: a Confirmation." Science (1956): 103-104.

Kamioka Observatory, University of Tokyo. What is Atmospheric Neutrino? Tokyo, 17 March 2010.

Kayser, Boris. Are Neutrinos Their Own Antiparticles? Batavia, 5 March 2008.

Los Alamos Science. The Reines-Cowan Experiments: Detecting the Poltergeist. 1997.

Marder, Jenny. "PBS ." 25 January 2011. What is a Neutrino…And Why Do They Matter? 21 September 2016 <http://www.pbs.org/newshour/rundown/what-is-a-neutrino-and-why-should-anyone-but-a-particle-physicist-care/>.

Pauli, Wolfgang. "Royal Holloway University of London Department of Physics." 4 December 1930. Pauli's letter of the 4th of December 1930. 24 September 2016 <http://www.pp.rhul.ac.uk/~ptd/TEACHING/PH2510/pauli-letter.html>.

Quigg, Chris. Cosmic Neutrinos. Botavia, 8 February 2008.

T2K Collaboration. The T2K Experiment. Hida, 10 June 2011.

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