Researchers find possible solution to cosmic ray and muon puzzle

Scientists have a problem with cosmic rays: they produce too many muons on the Earth’s surface. Muon cascades are byproducts of high-energy cosmic rays when they collide with nuclei in the upper atmosphere, and scientists see more muons on Earth’s surface than standard physics models predict .

Explaining these muon excesses has been a challenge for several years. But now, a team of researchers says they can explain this excess by invoking “gluon condensation” during the first cascade collision. Their work was published in The Astrophysics Journal.

Every second, millions of high-energy cosmic rays traveling at near the speed of light bombard Earth’s upper atmosphere, where they collide with atomic nuclei, primarily nitrogen, oxygen and argon. The nuclei explode in a shower of particles, mainly pions, kaons and baryons. These then decay mainly into muons, the heavier copy of the electron. (About 90% of muons come from pion and kaon decays.)

These muons experience only a small loss of energy as they pass through the atmosphere until they reach the surface. (Their ability to reach the surface, although apparently too short-lived to do so, is triumphantly explained by special relativity.) About one muon per square centimeter reaches the Earth’s surface per minute, with an energy average of 4 GeV but a very broad energy spectrum.

Yet, compared to the results of standard physical models derived from experiments with particle accelerators such as those at the Large Hadron Collider and CERN’s Super Proton Synchrotron, too many muons are observed at the surface. For muon energies between 6 and 16 exa-electronvolts (1.0 to 2.5 joules!), the muon flux is 30 to 60% higher than expected.

“Although research on the muon enigma is becoming more and more detailed,” write Bingyang Liu and colleagues at East China Normal University, “the truth remains elusive, requiring further investigation.”

In tackling the excess problem, the team wrote that they “tend to believe” that the main difference between experiments and theory lies in the initial collision in the air showers, when the particle of High-energy cosmic ray interacts for the first time with a nucleus in the atmosphere. (These incoming cosmic rays can have energies as high as the “Oh My God” particle observed in 1991, which had an energy of 320 exa-eV. This is 25 million times the highest energies of accelerators created on Earth, 13 TeV at CERN.)

Twenty-five years ago it was proposed that in very high energy collisions, when the incoming cosmic ray particle (mainly protons and helium nuclei) collide with hadrons in atmospheric nuclei, the distribution of gluons in hadrons could undergo condensation, forming a gluon condensate. The formation of these gluon condensates would then influence the subsequently produced hadrons, which in turn would affect the final muon production.

Hadrons are particles made up of two or more quarks – for example the proton and neutron, with three quarks each, and pions and kaons with two quarks. Just as molecules are composed of atoms held together by the electric force, hadrons are composed of quarks held together by the strong force.

A photon carries electrical force between electrically charged electrons and protons in atoms and molecules and is described by the theory of quantum electrodynamics (QED); gluons carry the strong force between quarks and are described by quantum chromodynamics (QCD).

The difference is that while photons have no electrical charge, gluons are charged, charged with the colors of the strong force. (Quarks, of course, also carry one of three types of colored charges.) This “nonlinearity” makes states governed by the strong force much more complex than atoms and molecules. Even with QCD quantum field theory, strong interaction states are very difficult to analyze, model and calculate.

We know that accelerated electrical charges emit QED (photons) radiation, which will contribute to the hadron cascades resulting from the initial cosmic ray collision. Likewise, accelerated color quarks (and gluons themselves) emit QCD (gluon) radiation. Unlike uncharged photons, gluons have their own charge and therefore emit additional radiation, leading to much larger showers of particles.

Gluon distributions in hadrons can form “gluon condensates”. Such high energy states consist of a large number of gluons at a certain energy level which can generate a certain number of hadron constituent quarks, making hadron cascades more efficient and increasing the number of pions and quarks strange, which make up the kaons.

In a frame of reference centered on the center of mass of the cascades, almost all of the available collision energy is used to produce these pions and kaons, and then produce the final muon that rushes to the surface.

The researchers used the gluon condensation model described by QCD to analyze the initial collision of the cascades in an attempt to resolve the muon excess problem.

They discovered that temporary plasmas of quarks and gluons can be formed by the high collision energies of gluons; in particular, their theoretical analysis revealed that the appearance of a quark-gluon plasma leads to an increase in the number of strange and antistrange quarks.

“The appearance of the gluon condensate requires higher collision energy,” they write, “and we find that this leads to production of stranger quarks (antiquarks) than under quark-gluon plasma conditions.”

After a complicated calculation, the team discovered that the production of strange quark pairs was two to ten times greater, depending on the energy, when starting with a gluon condensate state than when starting with a gluon condensate state. quark-gluon plasma.

They conclude by writing: “In high-energy collision experiments, existing research suggests that QGP (quark-gluon plasma) can occur, but considering only the conventional QGP effect cannot solve the problem of the excess of muons in the air shower. We consider that GC (gluon condensate) can occur during very high energy collisions.

“In the GC state, the production of strange quarks will be greatly enhanced,” resulting in more muons on the Earth’s surface than would be the case without the gluon condensates.

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