Neutrino Halo May Change Supernova 'Flavor'


When Carl Sagan said “we’re made of star stuff” he wasn’t kidding; there’s no other way that the elements heavier than lithium could have been formed if it wasn’t for the stellar kilns in the cores of stars or the violent eruption of supernovae.

In the case of a supernova, when a massive star runs out of fuel, it may collapse under gravity and then explode. During the detonation process, the supernova generates heavy elements that go on to seed planets, other stars and, ultimately, the elements that form organic compounds that we find inside our bodies.

But what elements are formed? And how do neutrinos affect the collapse of a massive star? Graduate student John Cherry, of the University of California, San Diego, thinks he’s stumbled across an answer.

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One type of supernova is known as a core-collapse supernova — it occurs when a massive star is starved of fuel. When this happens, the rate of nuclear fusion slows in the core and gravity starts to take over. The core rapidly collapses.

During this process, a dense neutron star can suddenly form, causing a rebound of in-falling stellar material. At this point, neutrinos flood outward from the core, blasting through the shrinking stellar body.

Neutrinos are ghostly particles that zip through space unhindered. They are so weakly interacting that they can pass through the Earth without hitting any other particle. In fact, as I sit here typing at my computer, billions of neutrinos — from the sun and other cosmic sources — are passing through my body every second. (Aside: To detect neutrinos, huge caverns filled with water surrounded by sensors are used to detect the rare “flash” a neutrino will make when it happens to hit a water molecule head-on. These “accidental” collisions are very rare, so in the interest of statistical probability it helps to make your neutrino detector as large as possible.)

Inside a core collapse supernova, the density of the material is very high as it accretes into the core forming the neutron star. It is calculated that this material will interact with the outflowing neutrinos, scattering a small percentage of them. These scattered neutrinos create a “neutrino halo” that can, in turn, interact with the outflowing neutrinos.

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But the fraction of outflowing neutrinos affected was thought to be minuscule and was often ignered during simulations. Cherry’s calculations disagree, however — his model suggests that a correction factor of 14 percent needs to be applied. In the outermost portions of the exploding supernova, this correction factor rose ten-fold. This means the neutrinos streaming from the core interacted with halo neutrinos far more often than previous theories anticipated.

What has this got to do with the price of cheese? Neutrinos are weakly interacting lightweights; a supernova literally shapes galaxies — one has little effect on the other, right?

Actually, this neutrino halo interaction could directly influence the nature of the supernova, literally changing the types of elements that can be spawned from the stellar event. And it all comes down to flavor — the “flavor” of neutrinos.

A strange little fact about neutrinos is that they come in three different varieties, or flavors — electron, muon and tau. Three flavors may seem pretty limited when it comes to an ice cream shop, but to neutrinos, their flavor is critical. What’s more, neutrinos can change their flavor.

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Neutrinos can oscillate naturally between flavors as they travel through space, but in a supernova, as the outflowing neutrinos scatter with the neutrinos in the halo, a flavor change can be forced. “Even though few neutrinos are scattered in funny directions, they can completely dominate how the neutrinos change their favors,” said George Fuller, professor of physics at UCSD, co-author of the study.

“What’s going on with neutrinos sets the entire stage for what’s happening in the explosion,” added Cherry.

As supernovae create new elements, the flavor of neutrinos acting on the matter generated have a strong influence over what elements are produced.

“Those neutrino flavor states allow the neutrinos to change protons to neutrons or neutrons to protons.” Cherry said. “What matter is produced, what kinds of atoms, elements are produced by these supernovas are changed dramatically if you change the flavor content of neutrinos.”

What started out as an insignificant “correction factor” in a simulation could suddenly have profound effects on the elements generated by a supernova and the characteristics of the material inside a galaxy. Neutrinos may be tiny, but they sure are mighty.

This study was published in the journal Physics Review Letters.

Image: Chandra X-ray Observatory image of Cassiopeia A, a 300-year-old supernova remnant. Credit: NASA/CXC/SAO

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