A joint Fermilab/SLAC publication

Pierre Auger Observatory tests particle knowledge beyond reach of LHC


Scientists at the Pierre Auger Observatory recently tested the theory that governs the behavior of protons at energies inaccessible at the Large Hadron Collider.

Before accelerator physicists can declare the discovery of a Higgs boson or any other new addition to the particle zoo, they need to prove they understand the particles they’re colliding.

Scientists at the Pierre Auger Cosmic Ray Observatory in Argentina recently tested the theory that governs the behavior of protons, the particles that collide in the Large Hadron Collider. They did it at energies much higher than manmade accelerators can reach.

Good news: The theory checks out.

On the subject of understanding protons, “Auger picks up where the LHC leaves off,” said Fermilab physicist Eun-Joo Ahn. Ahn presented the result, which was published this month in Physical Review Letters, at a seminar on Aug. 24 at Fermilab.

At the LHC, scientists steer two beams of protons around a ring on a collision course. When two protons interact, the energy in the collision converts into mass, briefly popping into existence particles that weren’t present before the proton crash. Those particles then decay.

In order for scientists to predict the outcome of these collisions, they need to understand quantum chromodynamics, or QCD, the theory of interactions between the quarks and gluons that make up a proton.

“QCD normalizes the data that will tell us if we expect to observe 10 or 10 million of a particle,” said Fermilab physicist Brendan Casey, a member of the Muon g-2 experiment. “If we expect 10 Higgs bosons and see 10 million of a new particle instead, we know it’s not the Higgs.”

Theorists have come up with several models of QCD. The best way to test these models is to collide protons and measure the rates at which they interact with one another. The rate of interaction depends on the energy; higher-energy protons interact more frequently.

“It’s as if the faster they move, the more they puff up, and it’s more probable they will interact,” said Fermilab physicist Paul Lebrun of the Pierre Auger collaboration.

Scientists want to measure proton-proton interactions at a variety of energies because, sometimes, particles do unexpected things. If you crash a beam of neutrinos into a target at a short distance, you will get a very different outcome than you would if you placed your target at a long distance. This is because neutrinos oscillate, or change types, as they travel.

If protons interacted differently than expected at higher energies, it could be a sign of supersymmetry or extra dimensions.

On Earth, scientists can test proton-proton interaction rates up to an energy of 7 TeV, the highest energy achieved thus far at the LHC. When it reaches its design energy, the LHC will raise that limit to 14 TeV.

None of this matters, however, to protons from space. Cosmic rays crash into Earth’s atmosphere at energies accelerator physicists can barely imagine. Scientists can extrapolate higher energy proton-proton interaction measurements from those proton-air collisions.

The Pierre Auger Observatory recently studied cosmic rays to make the world’s most precise proton-proton interaction measurement at an energy inaccessible at the LHC, 57 TeV.

The measurement lined up well with current models of QCD.

“It is not likely that any much larger energy measurement will ever be made,” said Martin Block, professor of physics and astronomy at Northwestern University, whose paper correctly predicting the Pierre Auger measurement was recently accepted to the journal Physical Review D. “It’s effectively the end of the line.”


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