Friday, December 19, 2014

The world's largest particle detector

The other large balloon in the works out at McMurdo was ANITA-3, which flew earlier this week.  When we were in McMurdo, my colleague Abby Vieregg gave us a tour.  ANITA (Antarctic Impulsive Transient Antenna) is flying over Antarctica and looking for radio signals from two sources:
  1. Ultra high-energy cosmic neutrinos hitting the ice below.
  2. Cosmic rays crashing into the atmosphere above the ice
Neutrinos are very light neutral particles that are notoriously difficult to detect, and no one has ever seen the ultra high-energy variety before.  Cosmic Rays are an old fashioned name for high-energy protons of still unknown origin.

The picture above shows the ANITA balloon payload (taken from an article run in the Economist), and it looks like a Christmas tree of antennas.  Each of those square fixtures is an antenna designed to look at 300MHz-1GHz frequencies-- near where your cell phone operates.  In the picture, the payload is going through a “hang test” to check both weight and mechanical integrity prior to launch.

When neutrinos crash into the Antarctic ice cap, they collide with atoms to form a shower of charged particles, which then emit a cone of radio waves.  The polarization (direction the electric fields shake on the radio wave) is vertical and that's why those antennas have two fins.  The vertical one is meant to look for these radio waves generated by neutrinos whereas the horizontal fins should not see effects from neutrinos.

On the other had, cosmic rays seldom make it all the way to the surface of the Earth.  Instead, they hit gas in the atmosphere and produce their own shower of air born charged particles.  Those particles then spin around the Earth’s magnetic field and emit their own radio waves known as synchrotron.  Remember that at the South Pole, those fields are vertical – perpendicular to the Earth’s surface—which means the synchrotron radio waves are horizontally polarized.  So the ANITA team uses the other fin in each antenna to watch for that.

The instrument will fly for a 4-5 weeks over Antarctica, but by comparing all the antennas and rejecting data with no interesting events, they expect to record only 5-10 seconds of data over the entire time.  Each event will span only a few milliseconds, so this flight will hopefully provide thousands of events to consider.

Why bother doing this experiment?  One reason is that we don't know where cosmic rays come from, in part because those particles themselves are charged and can get whipped around by our galaxy’s magnetic fields, obscuring their origin.  But scientists strongly suspect that the ultra-high-energy neutrinos have a similar source.  Since neutrinos are neutral, they will ignore our galaxy's magnetic fields and their paths may point directly to their origin.  Scientists certainly haven’t found any nearby sources of these cosmic rays or neutrinos and we suspect that they should scatter off the microwave background if they come from really far away.  So we really don’t have a good guess of where they come from or what accelerated them to such high energies.

But there’s an even bigger long-term reason to do this.  Each of these neutrinos have nearly a Joule of energy – comparable to the energy of a 60mph baseball—packed into one of the lightest things in the universe.  That’s several hundred times more energy than the world most powerful particle accelerator can squeeze into each particle.  If these neutrinos interact with the ice or Earth and the ANITA team sees radio wave from that, they will be exploring the rules of particle physics far beyond what can be done in the lab.  And if they see enough events, then they may be able to use their data to test for deviations from those expected rules.  We’re all hoping that they someday find such a discrepancy because such a discovery might inspire the next generation of accelerators, experiments.  But for now, we have to wait and see if they can even detect these particles.

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