More mysteries in cosmic rays, and a proposed solution

The highest energetic particle collisions that we observe on our planet are created by particles from outer space that hit atomic nuclei in Earth’s upper atmosphere. The initial particle produces a large number of secondary particles which decay or scatter again, creating what is called a cosmic ray shower. The shower rains down on the surface where it is measured in large arrays of detectors. The challenge for the theoretical physicist is to reconstruct the cosmic ray shower so that it is compatible with all data. In practice this is done with numerical simulations in which enters our knowledge about particle physics that we have from collider experiments.

Cosmic ray shower, artist's impression. Source: ASPERA

One of the detectors is the Pierre Auger Observatory whose recent data has presented some mysteries.

One mystery we already discussed previously. The “penetration depth” of the shower, ie the location where the maximal number of secondary particles are generated, doesn’t match expectation. It doesn’t match when one assumes that the primary particle is a proton, and Shaham and Piran argued that it can’t be matched either by assuming that the primary is some nuclei or a composite of protons and nuclei. The problem is that using heavier nuclei as primaries would change the penetration depth to fit the data, but on the expenses that the width of the distribution would no longer fit the data. Back then, I asked the authors of the paper if they can give me a confidence level so I’d know how seriously to take this discrepancy between data and simulation. They never came back to me with a number though.

Now here’s an interesting new paper on the arXiv that adds another mystery. Pierre Auger sees too many muons

A new physical phenomenon in ultra-high energy collisions
Glennys R. Farrar, Jeffrey D. Allen
arXiv:1307.2322 [hep-ph]

In the paper the authors go through possible explanations for this mismatch between data and our understanding of particle physics. They discuss the influence of several parameters on the shower simulation and eventually identify one that has the potential to influence both, the penetration depth and the number of muons. This parameter is the total energy in neutral pions.

Pions are the lightest mesons, that is particles composed of a quark and an anti-quark. They get produced abundantly in highly energetic particle collisions. Neutral pions have a very short lifetime and decay almost immediately into photons. This means essentially all energy that goes into neutral pions is lost for the production of muons. Besides the neutral pions there are two charged pions and the more energy is left for these and other hadrons, the more muons are produced in the end. Reducing the fraction of energy in neutral pions also changes the rate at which secondary particles are produced and with it the penetration depth.

This begs the question of course why the total energy in neutral pions should be smaller than present shower simulations predict. In their paper, the authors suggest that a possible explanation might be chiral symmetry restoration.

The breaking of chiral symmetry is what accounts for the biggest part of the masses of nucleons. The pions are the (pseudo) Goldstone bosons of that broken symmetry, which is why they are so light and ultimately why they are produced so abundantly. Pions are not exactly massless, and thus “pseudo”, because chiral symmetry is only approximate. The chiral phase transition is believed to be close by the confinement transition, that being the transition from a medium of quarks and gluons to color-neutral hadrons. For all we know, it takes place at a temperature of approximately 150 MeV. Above that temperature chiral symmetry is “restored”.

In their paper, the authors assume that the cosmic ray shower produces a phase with chiral symmetry restoration which suppresses the production of pions relative to baryons. They demonstrate that this can be used to fit the existing data, and it fits well. They also make a prediction that could be used to test this model, which is a correlation between the number of muons and the penetration depth in individual events.

They make it very clear that they have constructed a “toy model” that is quite ad-hoc and mainly meant to demonstrate that the energy fraction in neutral pions is a promising parameter to focus on. Their model raises some immediate questions. For example it isn’t clear to me in which sense a cosmic ray shower produces a “phase” in any meaningful sense, and they also don’t discuss to what extent their assumption about the chirally restored phase is compatible with data we have from heavy ion physics.

But be that as it may, it seems that they’re onto something and that cosmic rays are about to teach us new lessons about the structure of elementary matter.