Contact:
Bill Steigerwald
Goddard Space Flight Center, Greenbelt, Md.
William.A.Steigerwald@nasa.gov
301-286-5017
April 23, 2002
Albuquerque, N. Mex. -- 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 Goddard Space Flight Center in Greenbelt, Md., and Argyris Nicolaidis of the department of theoretical physics at Thessaloniki describe this notion today at the joint meeting of the American Physical Society and the High Energy Astrophysics Division of the American Astronomical Society in Albuquerque, N. Mex.
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 (about 3.16 quadrillion) 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 theories of the physics of high-energy interactions of elementary particles, the signatures of new physics beyond the so-called Standard Model of fundamental interactions 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. This latter possibility, in particular, predicts that the force of gravity increases much faster than predicted by Newton at distances smaller than 1 mm and becomes as strong as the "strong force" by 10^-17 cm -- that is, for energies of roughly 1 TeV.
The conversion of energy into gravitons (and also the conjectured production of mini black holes in cosmic ray collisions) at such energies is precisely the result of the increase of the strength of gravity. 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.
Although cosmic rays give us energies much higher than particle accelerators for free, they process their products through a thick layer of atmosphere, which makes unequivocal interpretation difficult. Uncovering the details of the new physics is thus hampered by our understanding of the cosmic ray composition near the energy of the knee -- that is, what fraction of cosmic rays at this energy are made of protons, helium, carbon or iron. Sorting out the cosmic ray composition at these energies will help in sorting out the physics of high energy interactions of elementary particles.
The importance of knowing the cosmic ray composition has led to a proposed space-based cosmic-ray mission called ACCESS (Advanced Cosmic Ray Composition Experiment for the Space Station). This mission could collect a wide sample of cosmic ray nuclei at high energies to more thoroughly investigate their composition near the so-called "knee." Aiming at even higher energies, a different 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 10^20 (that's 100 quintillion) eV.