THURSDAY, 2 MAY 2019The questions asked by physicists today require us to build larger and more precise experiments than were previously thought possible. Just take a look at the Laser Interferometer Gravitational-Wave Observatory and the Large Hadron Collider. We are entering an era where our ability to investigate further will be limited by the cost and expertise required to build the necessary equipment. But what if we could avoid building these detectors from scratch? In the pursuit of understanding mysterious particles called neutrinos, physicists at the South Pole have made use of the Antarctic ice sheets as a core part of their experiments. By using the Earth itself as a laboratory, the IceCube and ANITA collaborations have been able to probe some of the most elusive challenges in particle physics and cosmology today.
The standard model of particle physics provides us with a recipe for the universe. It gives us the ingredients, fundamental particles like the electron and the Higgs boson, and it tells us how to combine them through interactions such as the electromagnetic force. While the properties of electrons have been known for decades and the Higgs boson has been in the public eye since its observation in 2012, there are still particles in the standard model that manage to hold an air of mystery. These are the neutrinos. The 2015 Nobel Prize in physics was awarded for the discovery that proved that some types of neutrinos must have a small, but nonzero, mass. Prior to this they were thought to be completely massless, and even now their masses have not been accurately measured.
So, what is it about the neutrinos that makes their properties so difficult to determine? Unlike most other particles in the standard model, neutrinos do not carry electric charge. This means that if you were to shine a light on a bunch of neutrinos the light would pass straight through, as if they weren’t there. Usually we detect particles by looking at how they interact with other particles. However, of all the forces in the standard model, neutrinos can only interact through the weak force. The weak force allows neutrinos to interact with subatomic particles like protons and neutrons, producing electrons which can then be detected. However, as the name suggests, these interactions are weak: they rarely occur, and create only a faint signal for particle physicists to measure.
It does not take much inspection to notice that the standard model is incomplete. The force of gravity, for example, is completely absent from the recipe. Finding a way to extend the current theory is a challenge. Determining what lies beyond the standard model is one of the most important goals pursued by physicists today. Because we know so little about neutrinos, there is plenty of scope for incorporating them into new theories. The question of why neutrino masses are so tiny has led to interesting new theories postulating the existence of additional heavier neutrinos, called ‘sterile’ neutrinos. Sterile neutrinos have even been suggested as candidates for dark matter, the invisible matter thought to constitute over 20% of the universe. By studying neutrinos and determining their properties, physicists can explore and improve these possible theories.
It is not only particle physicists that are interested in neutrino detection: it is also a valuable tool to astrophysicists. Astrophysicists study cosmic rays, streams of extremely high energy protons and other subatomic particles, and try to determine their sources. However, protons are easily scattered and deflected by magnetic fields in their path. When a cosmic ray signal is detected, tracing the ray back to its origin is extremely challenging because we cannot assume it has travelled its whole journey in a straight line. Neutrinos avoid this problem. They are much less likely to be thrown off course because they rarely interact with other particles, and so they can travel long distances without being disturbed. This means they can provide an important mechanism for probing the sources of high energy cosmic rays in the distant universe.
If neutrinos are capable of travelling these long distances undisturbed, how can scientists stand a chance of finding them? Luckily, their interactions with water and ice provide recognisable signatures. You might have heard that nothing can travel faster than the speed of light. This is true in a vacuum, but when light travels through a medium such as ice, its interactions with other particles slow it down to a fraction of the vacuum speed. A neutrino, however, is not slowed down, and this means that it may actually move faster than light. Like the sonic boom for sound waves, this leads to characteristic forms of radiation called Cherenkov and Askaryan radiation. By analysing these, detectors are capable of reconstructing the neutrino’s speed and direction of motion.
Because of how rare these events are, the experimental setup needs to be large. The bigger the detector, the more likely an interaction is to occur and the better our chances of seeing it.
On top of this, experiments need to be as isolated as possible because such a weak signal is hard to distinguish from background noise. Some laboratories tackle these challenges with huge man-made water tanks, such as Super Kamiokande in Japan, or by building the experiments in underground mines, such as SNOLAB in Canada. Others have made clever use of the isolation and abundance of ice at the Earth’s South Pole.
Antarctica is home to two neutrino experiments: the IceCube Neutrino Observatory and the Antarctic Impulsive Transient Antenna (ANITA). The IceCube detector lives up to its name: it encompasses a cubic kilometre of Antarctic ice, throughout which 5,160 Cherenkov radiation sensors are distributed. A high energy neutrino arriving at the Earth can interact with the ice to produce Cherenkov radiation which then travels through the ice and is detected by the sensors. The distance radiation travels through this ice depends on the ice’s purity: the more pure and transparent the ice, the further radiation will go. Not only does Antarctica provide a large quantity of ice: its ice is some of the purest naturally occurring ice in the world. This is good news for IceCube: Cherenkov radiation travelling a long distance through the detector will pass through many sensors compared to radiation travelling short distances. This allows for more data to be collected, and better measurements to be made.
The ANITA experiment makes use of the Antarctic ice sheets in a slightly different way. Rather than embedding sensors in the ice, it consists of detectors held afloat over 35km above the Earth’s surface by a helium-filled balloon. High energy neutrinos pass through the Earth’s atmosphere and interact with the ice, producing Askaryan radiation, which is then measured by the ANITA detectors above. To maximise the amount of useful data that can be taken during each run, the launches are scheduled to take advantage of the Polar Vortex, an area of low pressure near the South Pole. By launching when the Polar Vortex is strong, the ANITA detectors are transported around the sky above the Eastern Ice Sheet, the largest ice sheet on Earth. This is where the ice is smoothest and the path of radiation can be easily reconstructed.
Both IceCube and ANITA have already succeeded in creating excitement in the world of physics. In 2017, IceCube detected a high energy neutrino which was traced back to an origin 3.7 million light years away. This was the first time the origin of such a high energy neutrino was localized in this way, providing a new insight into distant sources of cosmic rays. In 2018, ANITA announced that it had detected something unusual: signals from very high energy neutrinos that had travelled upwards through the earth. The likelihood of a neutrino with such a high energy passing through the earth is small, and so this measurement suggests the possibility that the neutrino may have been produced by some mysterious new particle. Whether this really is a sign of new physics is yet to be confirmed. Physicists wait in anticipation of further analysis and measurements.
From the Polar Vortex to the purity of the Antarctic ice sheets, the conditions at the South Pole are ideal for neutrino experiments. It is almost as though Antarctica was designed for neutrino detection.
By using the Earth’s resources as part of their detectors, IceCube and ANITA have been able to make unprecedented measurements, and continue to shine light on some of the most pressing issues in physics and cosmology today.
Maeve Madigan is a PhD student in theoretical physics at St John's College.
Banner image credit: NASA Goddard Space Flight Center from Greenbelt, MD, USA. Reused under Creative Commons Attribution 2.0 Generic License.