Tikalon Blog by Dev Gualtieri

Neutrino Mass

March 5, 2014

Beta decay , in which an atomic nucleus is bumped up one atomic number by the emission of an electron (or positron ), was an early crisis in elementary particle physics . The problem was that this radioactive decay process violated the well-known conservation laws of physics (energy , momentum , and angular momentum ).

Physicists weren't about to abandon their beloved, and useful, conservation laws that easily, so Wolfgang Pauli had the idea that there was another, unobserved, neutral particle emitted in beta decay. He called this particle the neutron, but this name was usurped by the nuclear neutron , which was discovered just two years later. Enrico Fermi later named the presumably smaller Pauli particle the neutrino , or little neutron.

Pauli proposed the neutrino in 1930, but this weakly-interacting particle wasn't detected experimentally until 1956. It's also interesting that the experiment itself was not detected by the Nobel Prize committee until 1995, when the Nobel Prize in Physics was awarded to Frederick Reines (1918-1998) for this experiment. Unfortunately, his collaborator on the experiment, Clyde Cowan Jr (1919-1974), had died before that time, and the Nobel Prize is never awarded posthumously.

"But, Niels, how is the angular momentum conserved ?"

Wolfgang Pauli (1900-1958), left, and Niels Bohr (1885-1962), right, examining the operation of a tippe-top toy in Lund, Sweden , in 1954.

(Photo by Erik Gustafson, via Wikimedia Commons .)

When the neutrino was discovered, it was thought to have zero mass , but such an elusive particle could hardly be detected, let alone allowing itself to be weighed. Massless neutrinos were never thought to be a problem, until the Homestake experiment of Raymond Davis, Jr..

As I wrote in a previous article (Bacterial Iron Isotopes, July 22, 2013), this experiment, located nearly a mile underground in the Homestake Mine in Lead, South Dakota , was designed to detect solar neutrinos .[1-2] When the data were in, Davis had detected just a third of the neutrinos predicted by Hans Bethe in 1939.[3] This experiment gave birth to the "Solar neutrino problem ."

The reason for the discrepancy was that Davis' experiment was designed to detect just electron neutrinos , but these neutrinos had transformed into a mixture of electron, mu , and tau neutrinos on their travel from the Sun to Earth . This neutrino "flavor" oscillation is only possible if neutrinos have mass; and, additionally, if they have different masses.

At this time, the best guess is that the sum of neutrino masses should be less than about 0.3 eV , as derived from various cosmological constraints.[4] For comparison, the electron mass is 0.510998928 MeV, and the proton mass is 938.272046 MeV.

Richard A. Battye of the Jodrell Bank Centre for Astrophysics , University of Manchester (Manchester, UK), and Adam Moss of the Centre for Astronomy & Particle Theory , University of Nottingham (Nottingham, UK), have recently refined this estimate using measurements of the cosmic microwave background radiation from the Planck spacecraft , along with gravitational lensing observations.[5-7]

Battye and Moss sought an explanation for the observed distribution of galaxies in the universe . They show that the large-scale structure of the universe can be explained if the sum of masses of neutrinos is 0.320 ± 0.081 eV.[7] This is the closest estimate yet for neutrino mass.

The Lovell Telescope at the Jodrell Bank Observatory .

Richard Battye is at Jodrell Bank.

(Photograph by Mike Peel, Jodrell Bank Centre for Astrophysics, University of Manchester, via Wikimedia Commons .)

Says Richard Battye,

"We observe fewer galaxy clusters than we would expect from the Planck results and there is a weaker signal from gravitational lensing of galaxies than the CMB would suggest. A possible way of resolving this discrepancy is for neutrinos to have mass. The effect of these massive neutrinos would be to suppress the growth of dense structures that lead to the formation of clusters of galaxies."[7]

Neutrino Mass

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