Smash! The Search for 'Sparticles'

Squarks,photinos, selectrons, neutralinos. These are just a few types of supersymmetricparticles, a special brand of particle that may be created when the world's mostpowerful atom smasher goes online this spring.

TheLarge Hadron Collider (LHC) at a particle physics lab called the EuropeanOrganization for Nuclear Research (CERN) in Geneva, Switzerland, will verylikely change our understanding of the universe forever. The 17-mile-longunderground particle accelerator will send protons flying around its circulartrack until they smash into each other going faster than 99 percent of thespeed of light. When the particles impact, they will unleash energies similarto those in the universe shortly after the Big Bang, the theoretical beginningof time.

Scientistsdon't know exactly what to expect from the LHC,but they anticipate its energetic collisions will create exotic particles thatphysicists have so far only dreamed of.

Manyresearchers are hoping to see supersymmetric particles, called sparticles forshort. Sparticles are predicted by supersymmetry theory, which posits that forevery particle we know of, there is a sister particle that we have not yetdiscovered. For example, the superpartner to the electron is the selectron, thepartner to the quark is the squark and the partner to the photon is thephotino.

Closingin

Recently,researchers at Northeastern University have clarified what kind of sparticlesthe LHC might find. There are about 10,000 possibilities for the pattern of thefirst four lightest sparticles that might be created, said Pran Nath, a Northeasterntheoretical physicist who is working on producing sparticles at the LHC. Butafter studying experimental astrophysical data, andthe predictions of certain theoretical models, Nath and his collaborators,Daniel Feldman and Zuowei Liu, reduced the number of possible patterns down to16.

"Ifthese assumptions are correct, we can say in what order these sparticles willbe created," Nath told SPACE.com. "So we tried to look for thesignatures of these sparticles."

Ifthe LHC produces sparticles, researchers will not be able to observe themfirst-hand because they will decay too quickly. The scientists can only hope toidentify the signatures of supersymmetric particles by studying the jets ofregular particles produced when sparticles disintegrate.

"Itis important to know how the sparticles will be ordered in mass becausedifferent theories lead to different patterns," Nath said. "So thismeans that if we see those patterns, we may be able to extrapolate back to a theory."

TheLHC will begin testing in April. It will produce the first preliminary datalater this year.

Wherehave they gone?

Whensparticles were first imagined, scientists wondered why we don't observe themin the universe now. The explanation, they think, is that sparticles are muchheavier than their normal sister particles, so they have all disintegrated.

"Theheavier an unstable particle is, the shorter its lifetime," Nath said."So as soon as it is produced it begins to decay."

Creatingsparticles requires an extreme amount of energy — the likes of which onlyexisted shortly after the Big Bang, and perhaps in the LHC.

Physicistsare not sure why sparticles don't have the same mass as particles, but theyspeculate that the symmetry could have been broken in some hidden sector of theuniverse that we cannot see or touch, but could only feel gravitationally.

Darkmatter and strings

Ifsupersymmetry truly exists, it could help solve a few nagging problems inphysics.

Forone thing, the theory may offer an explanation for darkmatter — the mysterious stuff in the universe that astronomers can detectgravitationally, but not see.

"Themost popular supersymmetric theories predict the existence of a stablesupersymmetric particle, the neutralino," said Enrico Lunghi, atheoretical physicist at the Fermi National Accelerator Laboratory in Chicago."This is an excellent candidate for dark matter. The problem is that wehaven?t seen any. It's another good reason for hoping to find supersymmetry atthe LHC."

Neutralinosmay be the lightest sparticles, so they might be able to exist in naturewithout decaying immediately.

Supersymmetryalso helps resolve the fundamental problems between physics at the very smallscale of particles (quantum physics) and physics at the very large scale, whereEinstein's general relativity takes over.

"It'sa necessary step in solving the discrepancy between the standard model [ofparticle physics] and gravity," Lunghi said. "It can be a veryimportant ingredient in eventually having a theoryof everything."

Additionally,if supersymmetry is proven correct, it could offer a boost to string theory,which includes the concept of supersymmetry. However, supersymmetry could stillexist even if string theory is wrong.

"Supersymmetrycan exist with or without string theory," Nath said, "but it would bevery encouraging for string theory if sparticles are observed. If they don?tfind any sparticles then it's not good news for supersymmetry or string theory."

Unproven

Somescientists are skeptical about whether supersymmetry exists and whether LHCwill be able to prove it.

"Supersymmetryis a very beautiful idea," said Alvaro de Rujula, a theoretical physicistat CERN, "but it's hard for me to believe that it is not only true innature but exists at this energy. It may be true but inaccessible to thismachine."

Evenif the LHC produced sparticles, de Rujula said, it would only create a few ofthem and the signatures could be difficult to identify.

"Peoplewill jump to conclusions, but it won't be so easy to tell if they are reallysupersymmetric," he said. "It may take some luck to have a convincingcase for supersymmetry at the LHC."

Formany physicists, the possibility of not finding what they are looking for isexciting, too.

"It'sbetter when we are wrong than when we are right," de Rujula said."Things are really interesting when we don't understand them. That's agood position for a scientist."

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