Saturday, January 5, 2013

Viewing exploding stars through the polar ice cap

As we waited for cameras to cool in the lab on a slow day, I took an excursion to IceCube and the Askarian Radio Array (ARA), where they are using water jets to drill deep holes into the ice.  Here's a video they made while checking if one is wide enough:

The scientists on these two projects drop sensors into holes like that to search for astrophysical neutrinos, using the Antarctic Ice shelf as a giant target.

Neutrinos are basic particles that are nearly massless and only interact with other particles by a "weak" nuclear force.  As a consequence, they can sail through dusty regions of our galaxy and universe and, in principle, let us see things that we never could with light that would get scattered away.  This is particularly true with Supernovae- stars that exhaust their fuel, collapse under their own weight, and then explode.  The electrons, protons, and photons in collapsing stars get so hot that they effectively bind together and cannot escape; only the neutrinos can get out, and as a result carry away 99% of the star's radiant energy.  This makes neutrinos an alluring way to study stellar collapse.

Picture of the 1987A supernova (with photons, not neutrinos), from Hubble Space Telescope
But just as neutrinos can sneak through hot and dusty regions outside Earth, they also sneak through most matter on Earth, which makes them very difficult to see.  They occasionally collide with atomic nuclei in something dense like water or ice.  These collisions produce a shower of charged particles that can emit light, and light is something that we're actually good at measuring.  If only we had a large chunk of ice…

Oh wait!  I've spent the past two weeks living on one that holds 60% of the Earth's fresh-water!  The idea behind the Ice-Cube Observatory is to drop strings of the sensors (photo-multiplier tubes) in the picture below into holes that are 2km deep and look for evidence of neutrinos scattering off a cubic kilometer section of ice.  In principle, one can even use the multiple detectors to infer where the neutrino came from.  They haven't seen anything yet, although the project only finished construction a year ago.

Detector for Ice-Cube (on display in the station)
Since then, the team has moved onto constructing the next project- the Askaryan Radio Array (ARA).  When even hotter neutrinos ("ultra-high-energy neutrinos") scatter off atomic nuclei in the ice and produce charged particles, those particles should also produce faint radio pulses (known as the Askaryan Effect).  So the team is also dropping an array of antennas down similar holes to listen for these events.

An antenna for the Askaryan Radio Array, being dropped into a hole.
Ultra-high energy neutrinos are interesting for different reasons than the slower ones for Ice Cube.  Black holes and supernovae blasts can accelerate protons to super-high speeds- far faster than we can in the lab- and fling them off into extragalactic space.  We say that these are high energy because their speed is so high and for historical reasons call them "cosmic rays."  The catch is that we can only detect them up to a threshold energy level because hotter cosmic rays scatter off the CMB (remember, the radio waves my telescope looks at) and get destroyed.  But as they are destroyed, they can produce ultra-high energy neutrinos and the ARA is attempting to use those neutrinos to understand particle collisions at energies far higher than we can make at accelerators. It's an attempt to discover new physics that we can't produce in a lab.

There's reason for optimism for the ARA folks- a balloon borne version called ANITA-2, which flew from McMurdo a few years back, saw evidence for one ultra-high-energy neutrino event.  With time on their side, ARA may have the chance to see more than one.

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