Sunday, February 12, 2017

Particle Astrophysics at the South Pole!

One of my hopes in doing this blog is to convey how amazing the science is that is taking place down here at the South Pole. So I would finally like to talk about the scientific experiment I am working on.  As I have stated in the past, I am very fortunate to be working here at the South Pole on the IceCube Neutrino Observatory (http://icecube.wisc.edu) . I am also working with another experiment called ARA, which will be discussed in a later post.  IceCube is a high energy neutrino observatory, and I am going to give a small description of the detector below along with some of the science we are working on.  I am trying to be as understandable but as concise as possible, so please forgive me if things seem a little pedantic at times. Also, I apologize in advance for the length of this post, but what can I say? It is literally particle astrophysics we are talking about here...

To understand IceCube, you need a little background in neutrinos. Neutrinos are the lightest (massive) particles we have ever observed.  Many people are familiar with atoms.  Atoms are the smallest particles that we generally interact with from day-to-day.  For a long time, atoms were believed to be the most fundamental particle.  Then, around the turn of the twentieth century, we found out that atoms could be broken down even further to protons, neutrons, and electrons.  It was then believed that these were the most fundamental particles.  After this we began to probe deeper and discovered that protons and neutrons could be broken down even further into particles we call quarks. So far as we now know, these quarks can't be broken down into anything smaller, and similarly, the electron is also a fundamental particle.  Many of you may be familiar with the periodic table:





Particle physicists have a somewhat analogous  table:


This table comprises the most fundamental particles we know about.  All of the particles we know about (excluding perhaps dark matter and dark energy) are made up of these fundamental particles.  The important thing to know for our purposes is that there is this strange particle called the neutrino which is a fundamental particle related to the electron.  In fact, there are a group of these particles called leptons.  Three of these leptons are in a sense bigger and carry a charge: the electron, muon, and tau. For each of these particles there is a corresponding neutral particle called a neutrino: the electron neutrino, muon neutrino, and tau neutrino.  We say these are the three neutrino flavors. These neutrinos are so "small" that we don't actually know how "small" they are.  Where in this case "small" refers to mass.  It is actually still an unanswered question in particle physics: What are the masses of the three neutrino flavors? That being said, IceCube studies these neutrinos in order to do all sorts of astrophysics and particle physics.

In particular, we look for high energy neutrinos with energies from tens of GeV to over a PeV.  This is a huge energy range.  Unfortunately, it is very difficult to put this energy in every day terms.  The only comparison that seems to make any sense at this level is to compare the energies to those of the Large Hadron Collider (LHC) at CERN.  The LHC is currently the most energetic man made particle accelerator ever created. These energies are so extreme that there were a few people back when the LHC was about to turn on that were concerned that it could possibly create tiny black holes that would destroy the earth.  The energies it was going to operate at were on the order of tens of TeV.  For those of you who are totally confused right now, or if you have just forgotten all your prefixes, let me explain:

IceCube observes energies from 10,000,000,000 eV to over 10,000,000,000,000,000,000 eV

The LHC observes energies up to around 10,000,000,000,000 eV

Notice that the highest energies that the LHC creates fall well within the range of the particles IceCube observers. In fact, one of our highest energy observed particles is estimated to have an energy 1,000 times greater than the energies that are seen in the LHC. I think it is safe to say that the LHC won't be destroying the Earth any time soon!  To be clear, our atmosphere is constantly being hit by particles with more energy than the LHC explores, and IceCube is one of a handful of experiments that observes these particles to do physics.  In fact, one thing IceCube hopes to do is to find the astrophysical origin of some of these particles.  This is one of the interesting things about our detector.  It can detect particles with much higher energies than we can explore with any man made particle accelerators.  That is not to say that the LHC is not an essential experiment.  In reality, it is an amazing experiment doing very important particle physics research.  They explore particle interactions in a way that we cannot and are able to do physics that would be impossible with our detector.  Similarly, the same can be said about IceCube.  Our experiment is complimentary and explores physics regimes that would difficult or even impossible with the LHC.

In IceCube we observe these high energy particles to do astrophysics and particle physics.  We do everything from study the most energetic and violent astrophysical events involving exploding starts and colliding neutron stars and black holes to fundamental particle physics studying the properties of neutrinos and finally to the most exotic physics involving dark matter and even searches for magnetic monopoles.  As it turns out, neutrinos are great for studying the universe!  As I said before they are neutral particles, and they also very rarely interact with other particles.  They also pass through the universe without interacting with magnetic fields that would otherwise alter their direction. This means that they can pass through the outer layers of stars and give insight into the processes that cannot be seen with normal telescopes. Unfortunately, this also makes them very difficult to detect.  In fact, this is one reason we are at the South Pole!


The IceCube Neutrino Observatory is located between 1.5km and 2.5km in the clearest ice in the world here at the geographic South Pole.  Not many people realize this, but at the South Pole we are sitting on almost 3km or over 9,000ft of ice.  At the depths of the IceCube detector, the ice is so clear, that it is clearer than any ice that can be made by any person on earth.  It is clearer than glass.  We have instrumented roughly one cubic km of ice with over 5000 very sensitive light detectors called photo-multiplier tubes.  This is over a gigaton of ice that we use!  The ice that is in our detector weighs more than all of the people on Earth combined!  We need all that ice because the neutrinos are so difficult to detect.  In fact, we don't even detect the neutrinos directly.  We detect the results of neutrino interactions.  For those who have heard of a sonic boom, we see something similar in our detector.  Every so often, a neutrino will come through the ice and interact with an ice atom and create a new charged particle.  This charged particle will move through the ice faster than the speed of light (in ice) and create a flash of light called Cherenkov radiation.  This is just like when a plane moves faster than the speed of sound and creates a sonic boom.  Now, I can tell some of you may be concerned because I said the particle was moving faster than the speed of light, and you may have heard that NOTHING moves faster than the speed of light...  and you are correct!  But that is only true in a VACUUM.  In some other media, light slows down!  In air or water or ice, light slows down enough that you can have a charged particle move fast enough to move faster than light in that particular medium, and this is the light we detect with IceCube.  We detect the Cherenkov radiation generated by charged secondary particles created in neutrino matter interactions in the ice to do particle physics and astrophysics to better understand the universe around us... PHEW!

As I said, we can use neutrinos to explore the insides of stars where telescopes cannot see, but we can also use neutrinos to explore more fundamental particle physics.  As it turns out neutrinos are weird!  Neutrinos can actually change flavor from say an electron neutrino to a muon neutrino or a tau neutrino. While it may not sound like much, this is a very interesting phenomenon to particle physicists and tells us something about how the universe works!  IceCube is one of a handful of experiments that can actually measure these "oscillations".  There are also questions about dark matter that we are exploring. The interesting thing about dark matter is that there is far more dark matter in the universe than normal matter.  There is still so much we don't yet understand about dark matter, and since there is roughly 5 times more dark matter than normal matter in the universe, I personally feel it is a very important and interesting area of research.  On top of this there are several other research projects being done using the data collected here at the South Pole using the IceCube Neutrino Observatory.

So as a bit of a summary, we are doing some very interesting scientific research down here with the IceCube Neutrino Observatory. We participate in a diverse array of physics research using the data collected in one of the most isolated and extreme environments in the world located at the geographic South Pole using one of the largest scientific experiments ever constructed in over a gigaton of ice located between 1.5km and 2.5km deep in the clearest ice in the world!  I have glossed over a great number of details in this post, but I wanted to at least get an overview of what is going on.  Please feel free to ask questions in the comments, and I will do my best to answer them!

3 comments:

Zachery Cole said...

Could you point me in the direction of any interesting theories that have been proven or disproven by the research you guys do?

Peter Ginsburg said...

Jim, great blog article on Ice Cube. Is Robert Schwarz back for another season? Someone posted a video of the last Herc departure a few years ago. When I watched it, I went, humm uh-oh. Just watching the fly-over video made me a bit anxious. Best of luck on the upcoming winter over.

James Casey said...

Hey Zach! I will try to get you some, but things have been a little busy here, and with limited internet it is hard to find a few of the papers. The best one might be our Science publication (http://science.sciencemag.org/content/342/6161/1242856) where we talk about the first ever high energy astrophysical neutrino flux ever detected. We also had a big publication in Nature (http://www.nature.com/nature/journal/v484/n7394/full/nature11068.html) about the lack of neutrinos from GRBs. There is a ton of other stuff out there, but it less "sensational". We have set limits on dark matter mass, measured the oscillations of neutrinos, made measurements on various attributes of cosmic rays, and all sorts of other science, but those papers may be a little harder to read as they were directed more towards scientists than the public.

Peter, Robert is indeed back for another season! We just closed the station yesterday, and it was an exciting experience. I posted a couple of pictures from the flyby this time around on my latest blog post. I am really looking forward to this winter!