Two years ago, an ultra-energetic event focused on Earth

On May 27, 2021, an American telescope detected cosmic rays with the second highest energy in the history of their detection. The energy requirement was so enormous that it challenges our ability to understand particle physics.

A recent publication in Science announces the discovery of a cosmic ray that hit Earth on May 27, 2021, captured by a very special telescope set up in the Utah desert.

The Earth is constantly bombarded with radiation coming from space, but this beam is endowed with energy that has not been observed before, mainly because this macroscopic energy is carried by a probable proton, that is, it is concentrated in an infinitesimally small volume. We cannot find such energy density anywhere else on Earth.

It is probably a tiny fragment of dust indicative of a cataclysmic event born in the depths of the sky a very long time ago, and its origin presents a problem of interpretation for physicists.

Discovery of cosmic rays

In 1911, Victor Hess discovered radiation coming from space. To do this, he did not hesitate to ascend in a balloon to a height of five kilometers to escape the terrestrial radiation coming from the radioactivity emitted by our planet. He used an “electroscope”, an instrument capable of measuring the flux of ionizing particles passing through it. So he observed that the flow increased with height and therefore had its origin in space. Hess received the Nobel Prize in 1936.

The Earth’s surface continuously receives approximately one hundred charged particles per square meter per second. These particles are “muons”, elementary particles similar to electrons but with greater mass.

However, these particles are not themselves cosmic rays that come from the depths of space: they are “secondary” particles, created by interactions initiated in the atmosphere by protons or other heavy nuclei that come from much further away. Upon arrival at Earth, only muons and neutrinos remain, as the other particles produced have disappeared (either decaying or interacting with each other).

A shower of particles in the atmosphere

The atmosphere that surrounds the Earth forms a thick layer several tens of kilometers thick. In total, we have the equivalent of 10 meters of water above our heads. That’s a lot of mass, and a proton coming into the upper layers will necessarily interact during the crossover. On average, the interaction with molecules in the atmosphere takes place at an altitude of about 20 kilometers.

The interactions of elementary particles are studied in detail in laboratory experiments, e.g. at CERN. So we know that a proton passing through matter will cause the first interaction to create a wider variety of secondary objects as its energy increases: pions, kaons, etc. However, these particles will have the opportunity to interact with each other, and the particles thus created will interact… Finally, we get 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 extrapolate for higher energies using computer simulation programs. The beam can thus stretch for kilometers with the heart located at an altitude of approximately 10 kilometers. The higher the energy of the cosmic rays, the greater the number of secondary particles, and at the energies we will discuss, the shower can be rich in billions of secondary particles that sprinkle several square kilometers of the Earth’s surface. Detecting such showers allows us to trace back to the particle that gave birth to them.

Schematic of the atmospheric cascade produced by the proton. // Source: Beetjedwars, Lacosmo, ComputerHotline

A gigantic telescope in the middle of the desert

How can we see such showers formed in the atmosphere? For Plato, knowledge is derived from the interpretation of shadows perceived at the bottom of the cave. In this case, it involves extracting the properties of the cosmic rays responsible for the shower from the footprint left after arriving on Earth.

Very high energy events are extremely rare. The one we are talking about 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! Energy here is measured in eV and multiples thereof, with 1 eV being the energy gained by an electron in a potential difference of 1 volt – a tiny energy that in conventional units corresponds to 1.6 10-19 joule.

As a result, to have a chance to detect some of these rare phenomena, it is necessary to build a gigantic telescope with instrumentation of the largest possible surface area.

The “Telescope Array” at the start of this observation is located in the Utah desert in the middle of the United States. It consists of a square network of 507 stations installed on the ground, each with an area of ​​3 square meters, constructed of “plastic scintillators” that react to the passage of particles. The stations are spaced 1.2 km apart, giving a total sensitive area of ​​700 square kilometers. This terrestrial network is supported by fluorescent detectors aimed at the sky: these are able to see light trails associated with showers that sweep the atmosphere during moonless nights.

The intensity of the collected signals provides information about the shower, which makes it possible to measure the energy of the responsible cosmic radiation, and the direction of its arrival is derived from the time differences measured at various ground stations. The uncertainty is estimated at 1.5 degrees.

For the next

SKA observatory.  // Source: SKA Observatory

Ultra Energetic Event May 27, 2021

Thus, the published event triggered a total of 23 identical neighboring detectors in the telescope, covering an area of ​​approximately 30 square kilometers. A large component of muons is observed, which rules out that the original particle was a photon (photons generate electromagnetic showers composed of particles different from those expected for a proton) – but a deeper study of the composition of the beam did not allow one 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%. This is a colossal energy, 30 million times higher than the energy of protons achieved at CERN by the accelerator that discovered the Higgs boson. This equates to approximately 40 joules in current units, the energy transported by a tennis ball sent smashing a champion during a major tournament. That’s breathtaking macroscopic-scale energy concentrated in a particle—probably a proton—with a size no larger than 10-15 meters!

The mystery of the origin of this cosmic ray

For Aristotle, the cosmos was immutable – unlike the perishable earth. Cosmic rays, which the Greek philosopher could not foresee, prove very directly that the universe is in constant upheaval. Today we know that the sky hides titanic dramas – black holes swallow neighboring stars, galaxies telescope, binaries merge… We are far from the harmony that we admire by turning our eyes to the sky during a beautiful star-studded summer night.

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 interacting with the cosmic microwave background photons from the Big Bang. Detected in particular by the Planck satellite, these photons fill all space at the rate of 400 per cubic centimeter, each carrying a tiny energy 10-4 eV. However, a proton of extreme energy has every chance of interacting with these photons and thus loses its initial energy by transforming into other particles; this is called the Greisen-Zatsepin-Kuzmin cutoff (GZK). We can receive such energy beams only when they come close to us. This limitation was clearly demonstrated by a previous experiment, the Auger observatory covering 3,000 square kilometers in the middle of the Argentine pampas.

This means that to survive the transition of the intergalactic medium, the beam under study must be produced less than 100 megaparsecs from Earth, 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, they come from all directions. So we cannot clearly determine their source.

Group of galaxies.  // Source: Good Free Photos (cropped photo)
The true origin and causes of these cosmic rays are unknown. // Source: Good Free Photos (cropped photo)

For the record event, 244 Exa-eV shows the direction of arrival to a void in the large-scale structure of the universe, which seems surprising a priori, since no object likely to generate such a beam has been found in that direction. .

Since the initial particle is charged, perhaps unknown galactic or extragalactic magnetic fields bent the beam’s trajectory during its journey and caused it to lose its original direction? Known fields are too weak.

The publication suggests another, more daring escape route: such a beam that appears to violate the GZK limit could indicate a new effect pointing to a defect in our current knowledge of particle physics. This is the “new physics” we invoke every time the result deviates from the established track.

In order to move forward, we would have to significantly increase the current stats, that is, cover larger areas or wait inordinately long. Rather, we can hope to imagine new detection techniques. In fact, developments are underway to detect showers using the radio waves they emit, such as the GRAND project, or to observe them from space, such as the EUSO proposal.

The story is not closed.

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François Vannucci, professor emeritus, particle physics researcher, neutrino specialist, University of the City of Paris

This article is republished from The Conversation under a Creative Commons license. Read the original article.


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