Neutrinos: the nanoscopic messengers from space

The IceCube South Pole Neutrino Observatory detects a wide range of cosmic particles. Photo by UW-Madison/NSF.

One of the most prevalent particles in the universe is also one of the most difficult to detect. Neutrinos can travel billions of light years — or even from the beginning of time itself — unimpeded by any of the matter they come into contact with. Their near masslessness and neutral charge make neutrinos almost ghostlike in that they are unaffected by both physical barriers as well as magnetic fields. This makes detecting the phantom particles all the more challenging.

UW-Madison is the lead institution working on the IceCube South Pole Neutrino Observatory. IceCube uses the neutrino’s unobstructable nature to their advantage. It is the only detector of its kind — it covers a cubic kilometer of Antarctic ice — and seeks to answer some of the most puzzling mysteries of the universe. Three hundred scientists from all over the world (many of whom hail from UW) are working at the observatory. In order to detect these high-energy neutrinos, optical sensors are buried a mile beneath the Antarctic ice. This mile of ice acts as a barrier to cosmic particles which occupy our atmosphere, and yet neutrinos continue freely. This removes the need to sift through the multitudes of other particles.

Detecting high-energy neutrinos, while a feat in itself, is not the only goal of the observatory. IceCube doesn’t just work with neutrinos, they also study dark matter, neutrino oscillations, and even things like glaciology.

In many ways, neutrinos act as nanoscopic messengers which deliver valuable information about the nature of our universe. For example, the origin of cosmic neutrinos give scientists insight into the origin of cosmic rays. Cosmic rays are composed of mostly protons and electrons which are accelerated through the universe by the source from which they emanate.

This is analogous to the way that particle accelerators on earth work. However, the particles of a cosmic ray collide with forty million times the energy of the Large Hadron Collider in Geneva. The collision of protons can create pions, which decay to create the neutrino. Since cosmic rays are composed of charged particles they cannot be traced. Their trajectory is warped by magnetic fields throughout the galaxy.

Neutrinos, while difficult to initially detect, can be easily traced, as they travel in a straight line directly from their sources, impeded by practically nothing. A high-energy neutrino that was detected by the IceCube Observatory in September of 2017 was traced back to a source.

The first concrete evidence was published this July and indicated that the neutrino detected in 2017 originated from a blazar adjacent to the Orion constellation. A blazar is a supermassive black hole at the center of a galaxy which expels a stream of ionized matter from both axes, one of which is in the direction of earth.

The galaxy that the high energy neutrino was traced back to is estimated to be 4 billion light years away from earth. The significance of this discovery of the neutrino and the blazar is that other observatories saw high energy photons coming from the blazar at the same time as the September neutrino, which is what helped to make the connection.

The researchers also looked into older data after detecting this really high-energy neutrino and discovered an excess of neutrinos from the same blazar in 2014-2015, giving even more evidence that the blazar is a neutrino emitter. If the blazar is the origin of the high-energy neutrino detected by the IceCube Observatory then the blazar is also the source of high-energy cosmic rays. This revelation has shocked scientists who have been, thus far, uncertain of the origin of cosmic rays.

In fact, cosmic rays, while studied by many, remain somewhat of a mystery. This is in large part due to their untraceability. Any information about the nature of cosmic rays could be valuable.

Outside of the protection of earth’s atmosphere, cosmic rays pose a very real threat. Not only do they supply a dose of about 400-900 millisieverts of radiation a year (that’s the same as the peak amount of radiation emitted per hour by the Fukishima nuclear disaster), they also have the capacity to alter electronics in space by shifting bits inside circuits.

Intel even patented a cosmic ray detector to assimilate into their electronics in an attempt to alleviate this issue. Discovering where cosmic rays come from is the first step in better protecting ourselves and our equipment from their ravages. As the lead institution working on the observatory, UW-Madison’s astronomists are making big strides in this area.

The story of the neutrino is one as old as time (literally). There is still so much to learn about the universe, and these miniscule particles are one of the keys to the vast unknown. IceCube used the path of one tiny, inconsequential neutrino from 4 billion light years away to lead scientists to this discovery. That underscores the idea that every part of the world we live in, regardless of how negligible it may seem, helps to paint an even clearer picture of the universe as a whole.

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