Elegant universe is back in fashion

After s short spell on the rocks, a mathematically elegant view of the universe is back in vogue. Recent hints of the Higgs boson at the Large Hadron Collider (LHC) help explain why we have not seen evidence for the beautiful theory of supersymmetry yet – and point to fresh ways to focus the search.

Supersymmetry, or SUSY, is an extension to the standard model of how particles and forces interact. Via elegant equations, it posits that every fundamental particle – including quarks, electrons, photons and neutrons – has a heavier, as yet unseen “superpartner” with slightly different properties (see diagram). This smooths some embarrassing wrinkles in the standard model. However, not one superpartner has yet shown up at the LHC, the particle smasher at CERN near Geneva, Switzerland, prompting fears that, despite its beauty, SUSY could be wrong.

That changed on 13 December, when LHC physicists reported that they might have found traces of the Higgs boson, the standard-model particle that is thought to give all others mass. The data suggested a mass for the Higgs close to 125 gigaelectronvolts, 133 times that of the proton and too light for a Higgs to survive without a heavier companion particle, which could be a superpartner.

“This is very good news for people who believe in supersymmetry,” says Howard Baer of the University of Oklahoma in Norman. He’s one of several researchers who have calculated what the suspected Higgs mass could mean for SUSY particle, or sparticle, detections at the LHC.

Baer reckons it can explain why sparticles have not yet been seen. Particles get their masses by interacting with the Higgs field; the stronger the interaction, the heavier the particle. So if the Higgs is confirmed at 125 GeV, which is heavy for SUSY models, many superpartners must be on the heavy side too. Baer and colleagues calculated that in several different versions of SUSY, a 125-GeV Higgs means squarks (the SUSY version of quarks) and sleptons (SUSY versions of electrons and neutrinos) must weigh 10,000 GeV or more, far too heavy for the LHC’s detectors to find.

“Even last summer*, people thought that squarks might be quite light and around the corner,” Baer says. “This makes it look like the LHC will have a little bit more difficulty trying to pull out a SUSY signal.”

That’s not to say the LHC won’t find any sparticles, though. Given the new estimated mass of the Higgs, Baer calculates that the gluino – superpartner to the gluon, which carries the force that holds atomic nuclei together – could be as light as 500 to 1000 GeV. The LHC is already probing this range, albeit not for gluinos specifically. Light gluinos won’t be detected directly, but by the particles they decay into.

Another possible super-quarry is the stop, the superpartner of the top quark. In some models of supersymmetry, there are two stops, one monstrously heavy and another relatively light. According to Marcela Carena at Fermilab in Batavia, Illinois, and colleagues, a 125-GeV Higgs could put the light stop between 100 and 130 GeV, easily visible at the LHC.

Of course, all this assumes that the Higgs signals recur in further experiments. Right now, they do not have the statistical significance to count as a discovery. “I very much hope that what we have seen so far finally ends up being the real Higgs,” says Carena.

“Fingers crossed.”

Lisa Grossman

* 2011