The ANITA experiment has been designed to identify ultra-highly-energetic cosmic neutrinos reaching Earth and to understand better their properties. Ultra-highly energetic cosmic objects have always been puzzling!
[image credits: ANITA]
A couple of years ago, the experiment has recorded two weird events that have triggered some attention within the high-energy physics community.
I am saying weird as such events have to find some explanation beyond the Standard Model of particle physics.
The puzzling fact is that the ANITA records seem to contradict the findings of the Auger and IceCube experiments. And contradictions must be explained (i.e. a lot of coffee is somehow needed).
As a result, these two events triggered some intense activities especially on the theoretical side. I will give one example at the end of this post.
I know, many big words in this introduction: neutrinos, cosmic rays, and so on. Please do not worry. Explanations are coming below!
NEUTRINOS
The Standard Model of particle physics is the theory describing the world of the elementary particles. How they live, how they interact with each other, and how they… well whatever…
Among all the fundamental building blocks of all matter, we have three interesting guys for this specific post, that are known as neutrinos.
[image credits: homemade]
Neutrinos were first introduced a bunch of years ago in order to explain the beta decay mechanism, as illustrated in the sketch on the right.
Around a century ago, physicists were studying the decays of given atomic nuclei each into another when an extra electron were emitted.
By studying the properties of the final state electron, these physicists were trying to understand the theory explaining the properties allowing to change the atomic species. One thing that was clear was that an invisible guy (named later a neutrino) had to be present, in order to have data matching the predictions.
This is how neutrinos were first introduced by Pauli, and a bit later theorized in particular by Fermi.
In the meanwhile, we have observed that there are three different flavors of neutrinos, respectively associated with the electron, muon and tau particles.
These three particles are just heavy copies of each other in the Standard Model. For some fishy reason, we have three copies of everything (please find out why and get a Nobel prize for free).
COSMIC RAYS
On cosmological standpoint, one very important property of the neutrinos is that they are very weakly interacting. This means that they can travel across the galaxy (and even beyond it) almost unaltered, i.e. without interacting with anything else.
This brings us to cosmic rays, or stuff coming from the outer space on Earth. One usually refers to massive particles that undergo a long trip within the universe, starting somewhere very from outside our solar system (or even from outside the Milky Way), and ending up on Earth where they can be detected.
Those particles are mostly atomic nuclei. 90% of the cosmic rays hence consist in protons (or hydrogen nuclei), 9% in helium nuclei, and in a very small quantity, we also have cosmic electrons and antimatter particles (antiprotons and positrons).
But those are not the only things that manage to come to us from outer space: we also have photons and neutrinos (that are usually not accounted for as cosmic rays).
[image credits: Sven Lafebre (CC-SA 3.0)]
The flux of those particles is presented on the figure on the left. The x-axis consists of the cosmic ray energy and the y-axis their flux.
The crucial point not to miss (otherwise, too bad for you) is that we are actually expecting very little cosmics with ultra-high energies.
Billions of billions of billions less ultra-highly energetic particles than the most common ones.
It is therefore a challenge to detect them, and one either needs huge detectors (larger than a city like Paris itself for instance) or collect data during a long time, or both.
And the reason is the one above-mentioned. High-energy particles have a huge probability to interact with anything on their way to Earth, and they will thus loose energy several times before getting to us.
COSMIC NEUTRINOS AND ANITA
In contrast to any other particle, neutrinos originating from the other side of the universe could in principle reach Earth unaffected by anything thanks to their very weak interaction rate.
[image credits: ANITA]
The only missing point is how to detect these ultra-energetic neutrinos once they reach us after having been produced in galaxies far far away (hum…).
As already emphasized, the fact that they are damned weakly interacting renders their detection somewhat complicated.
Neutrinos may indeed just go through any standard detector without leaving any track.
The catch is the following. With such an energy, neutrinos may travel very close to the speed of light. They may actually travel faster than the speed of light in some dense media like ice.
[image credits: Anita]
Under these conditions, our ultra-energetic neutrinos can yield the emission of a bunch of fainted radio-waves. This consists in the Askaryan effect.
This feature is what is exploited with Anita, the experimental device shown in the above picture.
As the signal is rare and fainted, the apparatus is carried by a balloon above the Antartica ice-cap. A very calm place that is perfect to record any signal.
Anita hence listens to neutrinos through several runs that have been carried out since 2006.
OBSERVATIONS AND SUMMARY
To sum up what I have said so far, ultra-energetic neutrinos can reach Earth almost unaffected after having been produced in the other side of the universe.
Thanks to a property called the Askaryan effect, the Anita experiment, an apparatus carried by a balloon above Antartica, has looked for the signature of such neutrinos when they travel through the Antartica ice. Such a trip is indeed associated with radio-waves that can be recorded.
In 2016 and 2018, Anita reported its most recent observations. A bunch of ultra-energetic neutrinos have been detected, but two events were really intriguing. And by really, I mean ‘really’.
The properties of these two observed events do not match the expectation. They are in fact in conflict with other measurements carried by other experiments. Various hypotheses have then been proposed to explain what was going on.
One of these hypotheses is in particular interesting. It relies on the existence of a special class of neutrinos called sterile neutrinos.
Whilst this works pretty well, sterile neutrinos can also explain how to model the neutrino masses and anomalies observed in various neutrino experiments. This is what I like. However, there are some tensions with some results, even if for the moment, no conclusive statement can be drawn.
More data is needed to unravel this mystery, and kill (or not) this hypothesis. As we say: we must stay tuned…
Once again, some stupid stuff is hidden in this post… Can you find it?
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