Probing cosmic neutrinos with a giant 200000 km2 detector
Neutrinos are the most elusive particles of the Standard Model of particle physics. Some of them, with enormous energies, are once in a while able to reach Earth. To make it clear from the beginning, by ‘high energies’, I really mean it. We consider almost massless particles that have about 100–10.000 times the energy of the protons collided at the Large Hadron Collider at CERN, the largest and most powerful particle collider in the world (see here).
Until very recently, these super energetic cosmic neutrinos were a real puzzle for us. We did not know much about them, including the mechanism behind their production.
[image credits: WISE @ NASA (public domain)]
The IceCube experiment has pointed out the existence of a cosmic engine capable of producing those neutrinos and accelerate them to crazy energies.
This engine consists in a giant galaxy with a rotating black hole in its center. Such galaxies are known as blazars.
From there, physicists are already thinking about the next steps: trying to detect even more of those highly energetic neutrinos, so that we could catalogue more sources and understand them better.
One potential project as a really great name, and is called GRAND, an acronym standing for the Giant Radio Array for Neutrino Detection. With such a giant detector, a 200.000 km2 array of radio-antennas to be located in China, the physics possibilities are extremely appealing.
But before digging further, let me briefly recapitulate some basic facts about neutrinos and cosmic rays, to make this post somewhat self-consistent.
MORE ABOUT NEUTRINOS
In the picture below, we can see the Sun (yeah no kidding :p), one of the many places where weak interactions are taking place for billions of years. Neutrinos are heavily connected to those weak interactions, and the Sun is one great source of (however low-energetic) neutrinos…
[image credits: NASA/SDO ]
About 100 years ago, radioactivity was the novelty of the moment. Physicists were studying the decays of given atomic species into other ones.
There was an intriguing on-going phenomenon: in many of these decays, the results where showing that some energy (and momentum) was missing.
Of course, this missing component was nothing but an invisible neutrino flying away. But this was not known in the 1910s.
100 years later, this invisible stuff is much more well-known, well embedded into the Standard Model of particle physics. Physicists are moreover able to see the invisible, at colliders or in dedicated neutrino experiments for instance, by relying energy and momentum conservation. From the properties of the visible, one can reconstruct what is missing and measure the properties of the invisible.
COSMIC RAYS AND NEUTRINOS
Cosmic rays are also 100 years old… I promise that I am almost done with the old stuff.
Victor Hess discovered at that time that our good old planet was bombarded by particles originating from space, the so-called cosmic rays.
[image credits: NASA/D. Berry (public domain)]
Cosmic rays can be made of a lot of stuff, like charged particles, photons or even neutrinos. They are traveling through space and those arriving on Earth can be detected (with appropriate detectors of course).
One interesting of their features consists in the energy of these particles. Their energy range spans tens of orders of magnitude, and big questions arise with the very energetic guys.
Where do high-energy cosmics come from? How are they accelerated to crazy energies?
To answer these questions, cosmic neutrinos have a great advantages over all the other cosmic species. They are very weakly interacting, so that they can travel from the source to Earth almost undisturbed, following a straight line. This contrasts with any other particle whose trajectory is bended, for instance, by all magnetic fields present in the Universe.
FROM ICECUBE TO GRAND
Some time ago, the IceCube collaboration discovered a source of highly energetic cosmic rays, this blazar I was mentioning in the introduction of this post that is nothing but a giant galaxy containing a rotating black hole in the middle.
This rotating black hole emits jets of very highly-energetic particles from its poles. This is where the super-energetic neutrino event detected by IceCube was initiated. After this observation, IceCube sent an alert to all telescopes around the world and gamma rays, X-rays, light and radio-waves (i.e. cosmic photons) have been observed as originating from the very same location.
We have thus convincingly discovered a cosmic engine capable of emitting and accelerating very-highly energetic cosmic rays! This was the breakthrough of the year.
[image credits: Greg Goebel (CC BY-SA 2.0)]
But life does not end there. We need to continue the searches and GRAND is a potential next step. The idea is to move on with even more energetic neutrinos.
GRAND, the Giant Radio Array for Neutrino Detection is a huge 200.000 km2 array of about 100.000 cheap radio-antennas (about 500 bucks each) to be located in China.
The core idea is to rely on one specific species of neutrinos, namely tau neutrinos. Tau neutrinos with energies 100–1000 times larger than what has been observed by IceCube emit radio waves that can be detected by simple radio antennas.
This contrasts with all existing experiments that focus on the light emitted by the neutrinos, and that are thus more limited in energy and necessitate more complex and expensive apparatus.
For now, a small set of 35 antennas will be deployed as a test case, to show that it works and that the radio background noise can be controlled. One of the key point is also to demonstrate that the budget can be kept under good control!
TLDR - SUMMARY
In this post, I addressed one option for what concerns the future of neutrino physics. Highly-energetic cosmic rays are observed by some time, and the IceCube collaboration has recently discovered one cosmic object, a distant galaxy with a rotating black hole in its center, capable to generate those very energetic cosmic rays.
It is however the first and so far sole observation of such an object capable to produce very energetic cosmics. The GRAND experiment is one option for the next steps, focusing once again on neutrinos (like IceCube). The idea is to focus on very (very very) energetic tau neutrinos that once in a while produce a huge shower of radio waves that can be recorded. As the shower is huge, we need a huge detector to get it. The proposed solution consists in building a 200.000 km2 array of radio-antennas in China.
A first prototype of 35 antennas will be deployed, and the future of GRAND will be decided from there. Can we make it, and at the same time controlling the budget of the entire experiment? This is the main question to answer by now!
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