Neutrinos are the ninjas of the universe. They don't interact with other particles very often, but when they do, they obliterate them. Until now. Scientists have observed a new way that neutrinos interact with the world.

Neutrinos in the Universe

Neutrinos have, roughly, the size of an electron, but without the electric charge of an electron. This makes them elusive. While electrons can be ensnared by nearby protons, or pushed away by nearby electrons, neutrinos can drift through pretty much anything. Neutrinos can pass through light-years of lead without interacting with a single atom.

If it seems odd that we even know neutrinos exist, consider how many of them there are in the universe. These particles are produced during the decay of radioactive material. Every time we set off a nuclear bomb, or a nuclear reactor, or when we intentionally collide particles at high speeds, we create neutrinos - but we aren't the primary source of neutrinos. The primary source of neutrinos is the Big Bang. It gave them their start, and they've been traveling through the universe ever since. About ten trillion of them are going through your hand right now. And now. And now.

When the neutrinos do hit an atom, they deliver a wallop. A neutrino striking a carbon nucleus can blast the nucleus apart. Which makes it strange that scientists have recently observed neutrinos giving the gentlest of pats to carbon atoms before moving peacefully on.

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A Bubble, a Bullet, and a Plum Pudding

In 1909, Ernest Rutherford and his associates, Ernest Marsden and Hans Geiger, were studying the density of an atom. Not a nucleus, or a material, but an atom. At the time, scientists believed in the "plum pudding" model of the atom. The nucleus was considered a big lump of proton goo, with electrons stuck inside it. The three physicists thought that firing alpha particles at the goo might help them find out the internal workings of the pudding. Alpha particles are two protons and two neutrons stuck together. Compact and hard, they had very high energy when fired. Geiger and Marsden were meant to fire alpha particles at thin gold foil, to see how the particles were deflected. Because the foil was so thin, and the atom was meant to be so soft and yielding, nobody expected much of a change in the direction of the alpha particles.

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Much to everyone's surprise, Marsden and Geiger got huge angles of deflection. Sometimes the alpha particles they fired into the foil came zooming right back at them. The scientists realized there was something very dense and hard inside the supposed pudding turning the alpha particles back. Rutherford later said of the experiment, "It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

Professor Kevin McFarland uses a similar analogy to describe a neutrino hitting a carbon nucleus. The carbon nucleus is a fragile bubble, and the neutrino is a bullet. We can guess how interactions between the two should go. But sometimes something very strange happens. According to McFarland, "The bubble - a carbon nucleus in the experiment - deflects the neutrino 'bullet' by creating a particle from the vacuum. This effectively shields the bubble from getting blasted apart and instead the bullet only delivers a gentle bump to the bubble."

At the Fermilab, McFarland was able to confirm that the carbon nucleus remained whole and undisturbed while charged pions suddenly went whizzing around, blocking neutrinos. This interaction, by the way, takes more energy than simply obliterating a carbon nucleus. So particle physics has just been confirmed as getting a little bit weirder.

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Fermilab Top Image: Renzo Borgatti

Neutrino Observation: Argonne National Laboratory

[Via Reanalysis of bubble chamber measurements of muon-neutrino induced single pion production,Neutrino Penetration Depth, The Rutherford Scattering Experiment, The University of Rochester.]

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