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The neutrino pulls its weight

Writer's picture: Mishkat BhattacharyaMishkat Bhattacharya

Today's post is about the neutrino, the lightest particle with mass in the universe [the photon (which makes up light) and gluons (which are responsible for interactions between quarks) are massless].


Why should we care about it? Because neutrinos are abundant and 10 billion of them go through every square centimeter of the Earth surface per second. They also go through us - some readers might remember me quoting John Updike's poem on the topic.


Actually it turns out that the neutrino plays a crucial role in our (understanding of the) universe (in fact there are three varieties of it - the electron neutrino, the muon neutrino and the tau neutrino plus their antiparticles): it is almost massless, but its effect is massive (couldn't resist the pun).


Neutrino physics is a big subject and definitely outside of my expertise, but I find it interesting because there are many open questions in the field. Here is a brief and incomplete account of the particle, including of how I got to meet one of the Nobelists in the field:


Major events in the life of the neutrino


i) The existence of the neutrino was proposed by Wolfgang Pauli in 1930, so that the conservation of energy could be satisfied in radioactive (beta) decay processes (such as when a proton decays into a neutron). He made the suggestion, not in a published paper, but in a famous letter to the attendees at a conference, beginning "Dear radioactive ladies and gentlemen,..".


ii) The electron anti-neutrino was discovered, before the neutrino, in 1956, leading to a Nobel prize. The electron neutrino was first directly observed in 1965.


iii) The discovery of the other types followed: for example, the muon neutrino was discovered in 1962, leading to another Nobel.


iv) An important discovery was that neutrinos were being produced by the Sun, but the number detected was lower than expected from theory. For this observation Ray Davis and Masatoshi Koshiba shared a Nobel.


Koshiba was an alumnus of the University of Rochester (my alma mater) and came by shortly before he won the Nobel, to accept the Wolf Prize. For some reason I was one of the few graduate students selected to have lunch with him at the Faculty Club!


v) The low number of solar neutrinos was shown to be due to the fact the neutrinos spontaneously converted (oscillated) between the three types - electron, muon and tau. The earlier experiments were only sensitive to one type of neutrino. The oscillations established that the neutrino has mass. For this discovery a Nobel was given to Takaaki Kajita (a student of Koshiba's) and Arthur McDonald.


Characteristics of the neutrino


i) The neutrino weighs about a million times less than an electron.

ii) It has no electric charge.

iii) It only responds to gravity and the weak nuclear force (responsible for radioactive decay).

iv) It is a fermion (with spin 1/2).


Sources of neutrinos


i) The Sun: The neutrino plays a crucial role in the reactions in the Sun that provides us with energy. Hints from the solar neutrinos led to detection of neutrino flavor oscillations.

ii) The atmosphere: These are produced by particles coming from sources other than the Sun, such as galaxies.

iii) Particle accelerators: Neutrinos from these sources are human-made.

iii) Nuclear reactors: Reactor neutrinos were the first ones to be detected.

iv) Nuclear bombs: Explosions carried out in secrecy by any country can be monitored by looking for neutrinos coming from these detonations - they will travel through the earth.


Some open questions in neutrino physics


i) No one knows why the neutrino has mass. That's probably a Nobel prize winning answer.

ii) No one knows if the neutrino is its own anti-particle.

iii) No one has detected the Cosmic Neutrino Background (CNB), which is expected to exist (just like the Microwave Background, which accounts for the light produced in the Big Bang) since the neutrinos basically stopped interacting with the matter in the universe. Probably another Nobel prize-in-the-offing.

iv) No one knows if the neutrino responds to magnetic fields.

v) No one knows if the neutrino decays into some other particle after a while.


Bonus fact


Neutrinos can be used for sending and receiving messages.


[1] The Neutrino Story by R. N. Mohapatra.

[2] The Elusive Neutrino by J. Bernstein.

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