Fred Reines Neutrino Discovery

Wolfgang Pauli and Enrico Fermi first conceived of the neutrino in 1931 then it proceeded to tease scientists seeking to verify its existence. It took Clyde Cowan and Fred Reines – the Nobel prize winner and founding Dean of the School of Physical Sciences at the University of California (UC) Irvine - until 1959 to prove its existence mainly because neutrinos hardly interact with matter and so are very difficult to detect.

Neutrinos are ghostly particles interacting only through the weak force carriers – the W & Z bosons (so they don't hit any barn doors). These bosons have some mass so they are heavier and travel more slowly than photons. Billions of neutrinos pass through us every second without our noticing them.

Raymond Davis Jr of UC Irvine used his chemistry and physics skills to build a completely new detector, a gigantic tank filled with about 100,000 gallons (about 378,500 liters) of perchloroethylene - a drycleaning fluid - which was placed in the Homestake mine in South Dakota. This instrument detected chlorine atoms being changed to 37Ar atoms in a reaction characteristic of a neutrino strike.

Neutrinos are created in the nuclear reactions that power stars like our sun. Neutrinos are formed in the proton- proton chain.

p + p → deuteron + positron + neutrino,

where the deuteron is the nucleus of deuterium. In the sun, 4 hydrogen nuclei are being fused into helium by means of the proton-proton chain. Neutrinos are important because they allow scientists to peek into the interior of the sun and learn about the processes there. All other information about the sun is from electromagnetic radiation that has to pass through the many layers of the sun interacting and changing along the way before traveling through space to us. This whole process can take up 10⁵ to 10⁶ years. Conversely, the neutrinos pass cleanly through the sun in a few seconds without interacting and take only 8 minutes to travel from the core where they are created to us. (Dr Lucie Green has a wonderful book describing this).

However the Homestake instrument only detected neutrinos about twice a week. It was predicted however that the detector should find about one of the 1016 solar neutrinos a day. This unexplainable lack of solar neutrinos detected became known as the Solar Neutrino Problem. Not all of the neutrinos were detected by the Homestake device because some had changed into muon and tau neutrinos (see below).

Over 30 years Ray Davis captured 2,000 solar neutrino events and was thus able to prove that fusion provided the energy from the Sun. With another gigantic detector, called Kamiokande, a group of researchers led by Masatoshi Koshiba was able to confirm Davis’s results. They were also able, on 23 February 1987, to detect neutrinos from a distant supernova explosion. They captured twelve of the total of 10¹⁶ neutrinos (10,000,000,000,000,000) that passed through the detector. The work of Davis and Koshiba has led to unexpected discoveries –and a new, intensive field of research, neutrino-astronomy.

Neutrinos are fundamental particles – points in space - but are not sub-atomic particles like protons, neutrons and electrons. It was found for instance that the neutrino oscillates between being a neutrino, muon neutrino and tau neutrino due to the Higgs field – which gives particles mass – interacting with the neutrino differently to the weak force so that while the weak force can ‘see’ the neutrino wave function as one continuous vibration the Higgs field sees them as a light, medium and heavy particle as a distinct electron neutrino (made by stars), muon neutrino and tau neutrino – notwithstanding that all three variants still have very, very little mass.

These three flavors of neutrinos: electron, muon, and tau neutrinos are associated with three different mass neutrinos: ν1, ν2, and ν3, alternately referred to as mass 1, mass 2, and mass 3. If each flavor of neutrino matched up with one specific mass, then life would be simpler but neutrinos are tricky. The mass and flavor neutrinos do not overlap perfectly.

Instead, quantum mechanics comes into play. Each neutrino of a specific flavor is actually a combination of neutrinos of different masses. As a result, each neutrino of a specific mass has a certain chance of interacting as a particular flavour.

This oscillation between the types of neutrino as they travel from the Sun to the Earth helped resolve the Solar Neutrino Problem – the observations reconciled with the predictions of the standard theoretical solar model and showing the scientific method in action per Richard Feynman pithy summation of the scientific method - no matter how smart a person is, no matter how elegant their hypothesis, if it does not agree with experimentation, it is wrong. (The BBC has an amusing Infinite Monkey Cage episode discussing this)

It is also speculated that a tau neutrino is both matter and antimatter and may have shaped the balance of matter and antimatter in the early universe and thus be useful in resolving the charge parity (cp) violation that is so apparent today with the amount of ordinary matter far exceeding that of antimatter.

However we still don’t really understand why the weak force and Higgs field interact with neutrinos differently or whether the neutrinos are their own antiparticles so with a caveat on the word wrong lets remember George Bernanrd Shaw’s saying: “Science is always wrong. It never solves a problem without creating ten more”. The fascinating journey to understand the multiverse continues – recent research suggests that it’s an exciting time to be a neutrino physicist.

Final Results Of The Homestake experiment 1998 https://iopscience.iop.org/article/10.1086/305343

Quote

“Science is the enemy of the certain. To paraphrase the Nobel prize winning physicist Richard Feynman, the scientist has a lot of experience with ignorance and doubt and uncertainty. And this experience is of very great importance. When a scientist doesn't know the answer to a problem, they are ignorant, when they have a hunch as to what the result is they are uncertain and when they are pretty darn sure of what the result is going to be they are still in some doubt. Science, Feynman concludes, is a satisfactory philosophy of ignorance, a way of thinking in which doubt is not to be feared but welcomed and discussed.”

Professor Brian Cox, The Infinite Monkey Cage

Some Podcasts

“The Mystery Of The Missing Neutrinos”

Daniel & Jorge Explain The Universe 28 Nov 2019

“What Is The Weak Force”

Daniel & Jorge Explain The Universe, 18 July 2019

“What Is A Neutrino”

Daniel & Jorge Explain The Universe, 7 Dec 2018

In Our Time with Melvyn Bragg “The Neutrino” BBC 4 14 April 2011

High Energy Neutrinos and The IceCube Observatory 21 November 2013

Neutrino Astronomy Links

cupp.oulu.fi/neutrino