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April
23, 2002
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RELEASE: 02-061
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 10^20
eV.
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