Squarks,
photinos, selectrons, neutralinos. These are just a few types of supersymmetric
particles, a special brand of particle that may be created when the world's most
powerful atom smasher goes online this spring.
The
Large Hadron Collider (LHC) at a particle physics lab called the European
Organization for Nuclear Research (CERN) in Geneva, Switzerland, will very
likely change our understanding of the universe forever. The 17-mile-long
underground particle accelerator will send protons flying around its circular
track until they smash into each other going faster than 99 percent of the
speed of light. When the particles impact, they will unleash energies similar
to those in the universe shortly after the Big Bang, the theoretical beginning
of time.
Scientists
don't know exactly what to expect from the LHC,
but they anticipate its energetic collisions will create exotic particles that
physicists have so far only dreamed of.
Many
researchers are hoping to see supersymmetric particles, called sparticles for
short. Sparticles are predicted by supersymmetry theory, which posits that for
every particle we know of, there is a sister particle that we have not yet
discovered. For example, the superpartner to the electron is the selectron, the
partner to the quark is the squark and the partner to the photon is the
photino.
Closing
in
Recently,
researchers at Northeastern University have clarified what kind of sparticles
the LHC might find. There are about 10,000 possibilities for the pattern of the
first four lightest sparticles that might be created, said Pran Nath, a Northeastern
theoretical physicist who is working on producing sparticles at the LHC. But
after studying experimental astrophysical data, and
the predictions of certain theoretical models, Nath and his collaborators,
Daniel Feldman and Zuowei Liu, reduced the number of possible patterns down to
16.
"If
these assumptions are correct, we can say in what order these sparticles will
be created," Nath told SPACE.com. "So we tried to look for the
signatures of these sparticles."
If
the LHC produces sparticles, researchers will not be able to observe them
first-hand because they will decay too quickly. The scientists can only hope to
identify the signatures of supersymmetric particles by studying the jets of
regular particles produced when sparticles disintegrate.
"It
is important to know how the sparticles will be ordered in mass because
different theories lead to different patterns," Nath said. "So this
means that if we see those patterns, we may be able to extrapolate back to a theory."
The
LHC will begin testing in April. It will produce the first preliminary data
later this year.
Where
have they gone?
When
sparticles were first imagined, scientists wondered why we don't observe them
in the universe now. The explanation, they think, is that sparticles are much
heavier than their normal sister particles, so they have all disintegrated.
"The
heavier an unstable particle is, the shorter its lifetime," Nath said.
"So as soon as it is produced it begins to decay."
Creating
sparticles requires an extreme amount of energy — the likes of which only
existed shortly after the Big Bang, and perhaps in the LHC.
Physicists
are not sure why sparticles don't have the same mass as particles, but they
speculate that the symmetry could have been broken in some hidden sector of the
universe that we cannot see or touch, but could only feel gravitationally.
Dark
matter and strings
If
supersymmetry truly exists, it could help solve a few nagging problems in
physics.
For
one thing, the theory may offer an explanation for dark
matter — the mysterious stuff in the universe that astronomers can detect
gravitationally, but not see.
"The
most popular supersymmetric theories predict the existence of a stable
supersymmetric particle, the neutralino," said Enrico Lunghi, a
theoretical physicist at the Fermi National Accelerator Laboratory in Chicago.
"This is an excellent candidate for dark matter. The problem is that we
haven't seen any. It's another good reason for hoping to find supersymmetry at
the LHC."
Neutralinos
may be the lightest sparticles, so they might be able to exist in nature
without decaying immediately.
Supersymmetry
also helps resolve the fundamental problems between physics at the very small
scale of particles (quantum physics) and physics at the very large scale, where
Einstein's general relativity takes over.
"It's
a necessary step in solving the discrepancy between the standard model [of
particle physics] and gravity," Lunghi said. "It can be a very
important ingredient in eventually having a theory
of everything."
Additionally,
if supersymmetry is proven correct, it could offer a boost to string theory,
which includes the concept of supersymmetry. However, supersymmetry could still
exist even if string theory is wrong.
"Supersymmetry
can exist with or without string theory," Nath said, "but it would be
very encouraging for string theory if sparticles are observed. If they don't
find any sparticles then it's not good news for supersymmetry or string theory."
Unproven
Some
scientists are skeptical about whether supersymmetry exists and whether LHC
will be able to prove it.
"Supersymmetry
is a very beautiful idea," said Alvaro de Rujula, a theoretical physicist
at CERN, "but it's hard for me to believe that it is not only true in
nature but exists at this energy. It may be true but inaccessible to this
machine."
Even
if the LHC produced sparticles, de Rujula said, it would only create a few of
them and the signatures could be difficult to identify.
"People
will jump to conclusions, but it won't be so easy to tell if they are really
supersymmetric," he said. "It may take some luck to have a convincing
case for supersymmetry at the LHC."
For
many physicists, the possibility of not finding what they are looking for is
exciting, too.
"It's
better when we are wrong than when we are right," de Rujula said.
"Things are really interesting when we don't understand them. That's a
good position for a scientist."
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