These Two Researchers Deserve Physics Nobel Prize for Work on Neutrinos

The news that the Nobel Prize in Physics has been awarded to Takaaki Kajita in Japan and Arthur B. McDonald of Canada would probably not have come as a surprise to those involved in neutrino physics.  For those unaware, neutrinos are the second most abundant subatomic particles in the universe, after photons, which carry light. Their existence was predicted in 1930, but for decades they remained some of the most enigmatic elements of astrophysics.

The neutrino carries no electric charge and was assumed for many years to have no mass. But the two Nobel -winning scientists showed that neutrinos, which are found in three “flavors,” could oscillate from one flavor to another — changing identities like a spy on the run, as they traveled through the atmosphere or through space from the Sun — demonstrating that they have mass.

Dr. Kajita was part of a team of researchers who in 1998 discovered that neutrinos from the atmosphere switched between two identities on their way to the Super-Kamiokande detector, nearly two-thirds of a mile below the earth’s surface.  In 1999, Dr. McDonald announced that neutrinos from the Sun were not disappearing, but merely changing disguises, on their way to the Earth. He and his colleagues had captured the neutrinos using a uniquely sensitive new detector 6,800 feet below ground, at the Sudbury Neutrino Observatory, which is part of Queen’s University in Kingston, Ontario.

This is extremely important because neutrino counts factor directly in testing our nuclear fusion models for the Sun. If then neutrino were really "missing in action" or absent from the reactions, our fusion models would have to be scrapped. McDonald's research - as well as Kajita's- showed major model revisions were not needed.

What's the basis of all this?

Contrary to the old notion that there was only one type ("flavor") of neutrino, we now know there are three: electron, muon and tau neutrinos. In effect, there must be three different corresponding neutrino masses which we can call: m1, m2 and m3.

Second we now know that the three "flavors" are really different superpositions of the 3 basic neutrino mass states.  Moreover, and to make it more complex, we know that quantum interference between mass states means a neutrino originating in one "flavor" can transmogrify to another over its transit. (Accounting for Kajita's "changing identities" and McDonald's appeal to "changing disguises".)  Experimental confirmation of this (and over large distances) arrived from MeV neutrinos from the Sun and muon neutrinos from the high atmosphere.

Because of the oscillations and quantum interference we need to reckon in a "misalignment" between flavor and the basic neutrino masses. This is done by reference to three independent "mixing angles": Θ_12 , Θ_23  and Θ_13. To a good approximation, oscillation in any one regime is characterized by just one Θ_ij and a corresponding mass difference, defined:

 D m _ij² = [m _j² - m _i²]

As an example, the probability that a muon neutrino of energy E acquires a different flavor after traversing distance L is:

P = sin² Θ ₂₃  sin² (l₂₃)

where l23 is the energy -dependent oscillation length, given by:

4ħ E c /  (D m ₃₂²)

How well do we know the parameters? Atmospheric neutrino observations yield Θ ₂₃  ~ 45 degrees, while D m ₃₂² = 0.0024 eV². Meanwhile, solar neutrino data yield roughly 33 degrees for Θ₁₂ and
 D m ₂₁² = 0.00008 eV². (Note: ħ is the Planck constant of action divided by 2 π)  If then:

 D m ₃₁²  =  [D m ₂₁²    +  D m ₃₂² ] = 0.00008 eV² + 0.0024 eV²

We know, D m ₃₁²  =  0.00248

which is close to D m ₃₂²
 

Hence, the capacity for the neutrino to alter its identity. The Royal Swedish Academy of Sciences, which awards the prize, said in a statement.
 

“The discovery has changed our understanding of the innermost workings of matter and can prove crucial to our view of the universe,”

Which is putting it mildly!

The Sun, let's be clear, isn't the only generator of neutrinos. The universe is deluged with neutrinos that are left over from the Big Bang, and many more are created in nuclear reactions on Earth.

Once thought to travel at the speed of light, they are able to pass through the Earth and our own bodies like moonlight through a window. Knowing that they can change identities — and that they have mass — has helped cosmologists understand how the universe has evolved and how the Sun works. The very first reaction of the proton-proton cycle can be written, for example:

                                                                                                                       

¹H + ¹H + e^- ®  ² H   + n + 1.44 MeV

Where n  denotes the neutrino given off.

Knowledge such as this applied to the Sun's fusion reactions may help us to create fusion reactors on Earth.

Michael S. Turner, a theoretical cosmologist at the University of Chicago, agreed that the Standard Model, a suite of equations that has dominated physics for the last half-century, was not complete. The reason is that it requires neutrinos to be massless, while the newer observations have clearly shown neutrinos possess mass. Hence,  the Standard Model cannot be the complete theory of the fundamental constituents of the universe Turner also suggested that neutrinos left over from the Big Bang might make up part of the dark matter that dominates the universe.

Turner, clearly excited by the Nobel Physics Prize, wrote in an email:

“Neutrinos contribute about as much to the mass budget of the universe as do stars!”

Which is something seriously mind-boggling to consider.