A simulation of a particle collision inside the Large Hadron Collider, the world's largest particle accelerator near Geneva, Switzerland. When two protons collide inside the machine, they create an energetic explosion that gives rise to new and exotic particles.
LONDON — Squarks, selectrons and neutralinos may be lurking in the universe, say physicists who suggest supersymmetry — the idea that every known particle has a yet-to-be-discovered sister particle — is not dead, despite the lack of evidence found in its favor.
The world's most powerful atom smasher, the Large Hadron Collider (LHC), has yet to find evidence of the existence of such sparticles (supersymmetric particles), though perhaps physicists are not interpreting the data in the right way, said particle theorist Ben Allanach of Cambridge University.
Speaking here at the Royal Society conference "Before, behind and beyond the discovery of the Higgs Boson" on Tuesday (Jan. 21), Allanach proposed that the LHC might detect the elusive supersymmetric particles once it is up and running again next year with much higher energies. [Sparticles to Neutrinos: The Coolest Little Particles in the Universe]
The underground accelerator at the CERN laboratory, located near Geneva, is currently switched off until early 2015 for a technical upgrade, which will allow it to smash protons together at the machine's near-maximum energy of 14 teraelectronvolts (TeV).
The first run of the LHC at 7 TeV culminated with the successful detection of what is widely believed to be the Higgs boson, a particle thought to explain how other particles get their mass. The discovery completed the Standard Model of particle physics and earned the two scientists who worked on the theory the Nobel Prize.
But the collider has so far failed to produce any evidence of supersymmetry. Also known as SUSY, it is one of the leading theories physicists have put forward as an extension of the Standard Model of physics.
Such an extension is needed to explain the remaining mysteries in the universe that the Standard Model does not account for, such as the nature of dark matter, the invisible stuff that is thought to make up most of the matter in the universe. So far, it has not been possible to observe it directly.
Mysterious heavy 'partners'
According to the supersymmetry theory, the early universe was filled with very heavy supersymmetric particles — exact copies of the particles that exist today, only much heavier. Over time, these particles disappeared, decaying into dark-matter particles and so-called ordinary particles, such as quarks and leptons.
"Supersymmetric particles are not around today, [except for] perhaps in dark matter," Allanach said. So the only way to find these elusive heavy supersymmetric "partners" to the ones in today's universe is by producing them in the lab, via proton collisions at very high energies. When protons collide with each other at near the speed of light, as they do inside the LHC, they can produce new, exotic particles alongside known particles. [Images: Dark Matter Throughout the Universe]
If sparticles exist, they are expected to appear as jets of hadrons — composite particles made of quarks — streaming out of proton-proton collisions. The momentum of these jets would not be balanced.
This missing momentum would be a signal of a supersymmetric neutralino particle, a hypothetical particle that is the leading candidate for dark matter. The neutralino "acts like a thief, stealing away momentum without leaving any trace in the detector," said Allanach.
So far, neither the neutralino nor any other supersymmetric particle has been found. But Allanach said that to net them, researchers need to account for a loophole in the way they read the collision data.
This loophole is the existence of so-called multiple solutions, or several ways to interpret the results of proton-proton collisions. "We've found out how to find these multiple solutions, and it is now possible to check on a case-by-case basis whether your interpretation is safe or not," Allanach said.
"For instance, one fixes the model details, and thinks the masses and interaction strengths of the supersymmetric particles are set," he said. "But the multiple solutions have different masses and interaction strengths for the supersymmetric particles, meaning that they would look different in the detector."
For example, a researcher may be looking for particles with a certain mass. But there could be another solution — one where the particles would have a slightly different mass, and they would then decay in slightly different ways.
In that case, "the pattern of the collision in the LHC could actually be different," said Allanach.
His team has already applied the multiple-solutions method to check the data from the LHC's first run that lasted from 2010 to 2013, but still hasn't been able to find any evidence of supersymmetry.
Even so, Allanach remains hopeful. "With much more energy, the LHC will be able to produce heavier supersymmetric particles, so hopefully, we'll discover them then," he said. "The real job will be to take the data apart, look at the measurements, try and work out precisely what's going on, not to misinterpret anything."
Physicist Paris Sphicas of the University of Athens, who works at CERN, said there are so many parameters in the supersymmetry theory (SUSY) to explore that "it can never be declared dead."
"We really think that the LHC will see the evidence; we just need more energy," Sphicas told LiveScience. "But SUSY remains a well-motivated, much-anticipated, although-yet-unseen extension to the Standard Model."
Renowned CERN physicist John Ellis agrees with Allanach and Sphicas.
"I think that the physics case for supersymmetry has, if anything, improved with the LHC's first run, in the sense that, for example, supersymmetry predicted that the Higgs [boson particle] should weigh less than 130 gigaelectronvolts, and it does," Ellis said.
"Of course, we haven't seen any direct signs of supersymmetric particles, which is disappointing, but it's not tragic," Ellis added. "The LHC will shortly almost double its energy — we're expecting eventually to get maybe a thousand times more collisions than have been recorded so far. So we should wait and see what happens at least with the next run of the LHC."
And if the LHC's next run does fail to reveal any sparticles, there is still no reason to give up on looking for them, he said. In that case, new colliders with even higher energies should be built, for collisions at energies as high as 100 TeV.
"I'm not giving up on supersymmetry," Ellis told LiveScience. "Individual physicists have to make their own choices, but I am not giving up."