Energy Disappears High in Atmosphere, Scientists Say

When cosmic rays smack into the Earth’s atmosphere, part of the energy released seemingly disappears, entering a realm not measurable by current detectors. This energy possibly forms miniature black holes or is transferred to “particles” of gravity, called gravitons, which might leak into other dimensions, according to scientists at NASA and the University of Thessaloniki in Greece.

Demos Kazanas of NASA’s Goddard Space Flight Center in Greenbelt, Md., and Argyris Nicolaidis of the department of theoretical physics at Thessaloniki describe this notion at the joint meeting of the American Physical Society and the High Energy Astrophysics Division of the American Astronomical Society in Albuquerque, N.M.

New physics, which scientists hope to discover with the next generation of particle accelerators, may have already given us hints of its existence in the cosmic rays, essentially atomic particles hurling through space at nearly light speed, the scientists say.

“In the study of cosmic rays we may have a glimpse of novel structures in the physics of high-energy interactions beyond that of the so-called Standard Model of weak-electromagnetic interactions,” said Kazanas. “We may be seeing a ‘chink in the armor’ that might even lead to an understanding of the physics that unifies gravity and quantum mechanics, a direction sought for many years now by both theorists and experimentalists.”

Cosmic rays are the fastest-moving bits of matter in the Universe; some of these particles are almost a billion times more energetic than those produced in particle accelerators.

Kazanas and Nicolaidis set out to explain the abrupt change in the sampling rate of cosmic rays collected above a specific energy range, known as the “cosmic ray spectrum knee.”

Higher-energy cosmic rays are much rarer than their lower-energy cousins. In fact, for every tenfold increase in their energy, they become roughly 60 times rarer. Yet above a very specific energy level, 10^15.5 electron volts, cosmic rays suddenly become even rarer, roughly 100 times for a tenfold increase in energy.

It is above this energy level, the scientists suggest, that the effects of the new physics occur leading to the “disappearance” of some energy into forms or realms beyond detection by the cosmic ray techniques. When cosmic rays collide with atoms in the atmosphere, they break apart, forming electrons and muons that in turn collide with other nuclei in the atmosphere. (A muon is a negatively charged elementary particle about 200 times heavier than an electron, and ultimately decays into an electron and a neutrino.) A high-energy cosmic ray particle creates what scientists call an “air shower” of electrons, photons, muons and neutrinos. Scientists estimate the energy of the initial cosmic ray by adding up the energies of the secondary collisions.

If part of the energy of these collisions is channeled to a form other than photons, electrons or muons — e.g. into a form which cosmic-ray detectors cannot register — then the inferred energy of a given cosmic ray will seem to be lower than it really is.

The well-recorded break in the cosmic-ray sampling rate with energy at 10^15.5 eV corresponds to a particle collision energy of 1 TeV. This turns out to also be the energy at which, according to theory, the signatures of new physics beyond the so-called Standard Model should be revealed. This seems to be a little more than a coincidence, Kazanas claims.

What is the precise nature of new physics? Kazanas and Nicolaidis prefer not to be specific in the absence of further evidence. They nonetheless cite a number of the possible alternatives: supersymmetry (a symmetry that relates fermions and bosons), technicolor (a new strong force at high energies) or even the possibility that space has additional spatial dimensions into which the energy is channeled in the form of gravitons. All these sound quite exotic, but this is the stuff theorists have been playing with for the past several years.

The Kazanas-Nicolaidis proposal of missing energy in high-energy collisions should be testable in accelerators to come in line in the near future. The new particle accelerator being built at CERN in Switzerland, called the Large Hadron Collider, to be completed in 2006, will be the first accelerator capable of producing energies above the TeV level.

A proposed space-based mission called OWL could detect the highest-energy cosmic rays, extremely rare particles (perhaps only a few thousand to be sampled per year) above 10²⁰ eV.