2023-12-11 20:43:23
A recent publication In Science announces the discovery made by a very special telescope built in a Utah desert of a cosmic ray that fell on Earth on May 27, 2021.
The Earth is constantly bombarded by radiation coming from the cosmos, but this ray is endowed with an energy never before observed, especially since this macroscopic energy is carried by a probable proton, that is to say that it is concentrated in an infinitesimal volume. Such energy density is found nowhere else on Earth.
It is probably a tiny piece of dust testifying to a cataclysmic event born in the depths of the sky a very long time ago and its provenance poses a problem of interpretation for physicists.
Cosmic rays
In 1911, Victor Hess discovered radiation coming from the cosmos. To do this, he did not hesitate to climb in a balloon to an altitude of five kilometers in order to escape the terrestrial radiation coming from the radioactivity emitted by our planet. He used an “electroscope”, an instrument capable of measuring the flow of ionizing particles passing through it. Thus, he observed that the flow increased with altitude and therefore had its origin in space. Hess received the Nobel Prize in 1936.
The earth’s surface permanently receives around one hundred charged particles per square meter. These particles are “muons”, elementary particles similar to electrons but with a higher mass.
But these particles are not in themselves cosmic rays which come from the depths of the cosmos: they are “secondary” particles, created by interactions initiated in the atmosphere by protons or other heavy nuclei which come from much further away. far. Upon arrival on Earth, only muons and neutrinos remain, because the other particles produced have disappeared (either because they disintegrate or because they interact in turn).
Atmospheric showers
The atmosphere that surrounds the Earth forms a thick skin several tens of kilometers thick. In total, we have the equivalent of 100 meters of water above our heads. This is a lot of matter and a proton arriving at the upper layers will necessarily interact during the crossing. On average, the interaction with molecules in the atmosphere takes place at an altitude of regarding 20 kilometers.
The interactions of elementary particles are studied in detail in laboratory experiments such as at CERN. Thus, we know that a proton passing through matter will produce a first interaction with the creation of a wider range of secondary objects as its energy increases: pions, kaons, etc. But these particles will have the opportunity to interact in turn, and the particles thus produced will interact… In the end, we obtain what we call a “shower” of particles.
We model the passage of protons in the atmosphere up to the energies reached at the accelerators and, for higher energies, we extrapolate using computer simulation programs. Thus, the sheaf can stretch for kilometers with a heart located at an altitude of approximately 10 kilometers. The higher the energy of the cosmic ray, the greater the number of secondary particles and, at the energies we are going to talk regarding, the shower can be rich in billions of secondary particles which will sprinkle several square kilometers of the earth’s surface. Detecting such showers allows us to go back to the particle that gave birth to them.
The “Telescope Array” device
How can we see such showers created in the atmosphere? For Plato, knowledge is deduced from the interpretation of shadows perceived at the bottom of a cave. In the present case, it involves extracting the properties of the cosmic ray responsible for the shower from the imprint left upon arrival on Earth.
Very high energy events are extremely rare. The one we are talking regarding has a reconstructed energy of 244 Exa-eV (244 x 1018 eV), and the corresponding flux is expected at the level of one copy per century and per square kilometer! Here, energies are measured in eV and its multiples, 1 eV being the energy acquired by an electron in a potential difference of 1 volt – a tiny energy which, in conventional units, corresponds to 1.6 10-19 joules.
Consequently, to have a chance of detecting some of these rare phenomena, it is necessary to build a gigantic telescope by instrumenting as large a surface area as possible.
The “Telescope Array” at the origin of this observation is located in a Utah desert in the middle of the United States. It is made up of a square network of 507 stations installed on the ground, each with an area of 3 square meters, constructed of “plastic scintillators” which react to the passage of particles. The stations are distributed with a spacing of 1.2 kilometers between them, giving a total sensitive area of 700 square kilometers. This terrestrial network is supported by fluorescence detectors pointed towards the sky: these are capable of seeing luminous traces associated with the showers which streak the atmosphere during moonless nights.
The intensity of the signals collected provides information on the shower which makes it possible to measure the energy of the cosmic ray responsible, and its direction of arrival is deduced by the time differences measured at the various ground stations. The uncertainty is estimated at 1.5 degrees.
The ultra-energetic event of May 27, 2021
Thus, the published event triggered a total of 23 coincident neighboring detectors in the telescope, covering an area of approximately 30 square kilometers. A large component of muons is observed, which excludes that the original particle is a photon (photons generate electromagnetic showers composed of particles different from those expected for a proton) – but the more in-depth study of the composition of the sheaf did not make it possible to determine whether it is a pure proton or a heavier nucleus.
The reconstructed energy of 244 Exa-eV is affected by an uncertainty of approximately 25%. It’s colossal energy, 30 million times higher than the energy of protons reached at CERN by the accelerator that discovered the Higgs boson. This corresponds to approximately 40 joules in current units, it is the energy transported by a tennis ball sent by the smash of a champion during a major tournament. This is breathtaking energy on a macroscopic scale concentrated in a particle – probably a proton – whose size does not exceed 10-15 meters!
Where does this cosmic ray come from? A mystery still to be solved
For Aristotle, the cosmos was immutable – unlike the Earth, which is perishable. Cosmic rays, which the Greek philosopher might not anticipate, prove very directly that the Universe is in perpetual upheaval. We know today that the sky hides titanic dramas – black holes swallow up their neighboring stars, galaxies telescope, binary stars coalesce… We are far from the harmony that we admire by turning our eyes towards the sky during a beautiful summer night dotted with stars.
The cited publication describes an exceptional event, but its interpretation is not obvious.
At such energies, a proton cannot travel infinite distances in space, because it is above the threshold for interaction with cosmic microwave background photons from the Big Bang. These photons, detected in particular by the Planck satellite, fill all space to the tune of 400 per cubic centimeter, each carrying a tiny energy of 10-4 eV. However, a proton of extreme energy has every chance of interacting with these photons and thus loses its initial energy by converting into other particles; this is what we call coupure de Greisen-Zatsepin-Kuzmin (GZK). We can only receive such energetic rays if they come very close to us. This cutoff was clearly highlighted thanks to a previous experiment, the Auger observatory covering 3000 square kilometers in the middle of the Argentine pampas.
This means that, to survive the crossing of the intergalactic medium, the ray studied must be produced less than 100 Megaparsecs from the Earth, that is to say in our close neighborhood, barely 1% of the Universe.
In total, since 2008, the “Telescope Array” experiment has measured 28 showers of more than 100 Exa-eV. Their distribution in the sky is isotropic, that is to say they come from all directions. We cannot therefore clearly identify their source.
For the record event of 244 Exa-eV, the direction of arrival points towards a void in the large-scale structure of the Universe, which seems a priori surprising, since in this direction we do not find any object likely to for having generated such a ray.
Since the starting particle is charged, perhaps unknown galactic or extragalactic magnetic fields bent the path of the ray during its travel causing it to lose its original direction? The known fields are too weak.
Another more daring escape route is proposed by the publication: such a ray which seems to violate the GZK cutoff might indicate a new effect pointing to a defect in our current knowledge of particle physics. This is the “New Physics” that we invoke every time a result deviates from the beaten track.
To move forward, we would have to greatly increase the current statistics, that is to say, cover increased areas or wait for inordinate amounts of time. More reasonably, we can hope to imagine new detection techniques. In fact, developments are underway to detect showers by the radio waves they emit, for example BIG projector observe them from space, for example the EUSO proposal.
The story is not closed.
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