January 28, 2008
Smash! The Search for ‘Sparticles’
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.
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."
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."