LHC Results Do Battle with Supersymmetry
There's been a bit of buzz this past week about the latest results from the Large Hadron Collider at CERN, announced at the Hadron Collider Physics Symposium in Kyoto, Japan. Much of the focus has been on the Higgs boson, or the recently discovered particle that looks an awful lot like the Higgs boson — and a fairly conventional Standard Model version, at that.
But there's been just as much buzz about a new measurement by an experiment called LHC beauty (LHCb), designed to study b-hadrons (heavy particles containing a bottom quark) in hopes of explaining the matter-antimatter asymmetry in the universe (CP violation).
LHCb scientists identified an unusual decay signature for a so-called strange beauty particle made up of a beauty antiquark bound to a strange quark because it decays so quickly into various other particles. It's a solid 3.5-sigma result, strong enough to constitute “evidence" but shy of the five-sigma threshold usually required for claims of “discovery."
This analysis looked for one particular rare decay pattern: the production of a positive muon and a negative muon. The Standard Model of particle physics predicts such a decay pattern should only occur roughly three times in a billion, which is precisely what the LHCb data showed. So yay for the Standard Model, which continues to hold up remarkably well.
So why aren't physicists just ecstatic over the smashing success of the Standard Model? Well, they were hoping for something a bit less vanilla, something surprising that hints at new exotic physics — an unexpected particle, a new force, hidden extra dimensions, a Higgs that wasn't quite in line with predictions… and evidence for supersymmetry, which predicts the existence of “shadow particles" — heavy versions of all the particles currently included in the Standard Model.
Physicists — some of them, anyway — would like to find evidence of supersymmetry in part because it offers one explanation for dark matter, namely, the most likely versions of supersymmetry predict the existence of a particle dubbed the “neutralino."
This is not the only candidate for dark matter, of course, but it would be among the more intriguing options beyond the Standard Model.
From a purely theoretical standpoint, supersymmetry also helps resolve physics at the very small scale of particles (quantum physics) and physics at the very large scale (general relativity) by offering ways to fuse the two into a Grand Unified Theory (GUT), whereby, at extremely high energies (above 1016 GeV), the electromagnetic, weak nuclear, and strong nuclear forces are fused into a single unified field.
Mix and Match
It's worth taking a moment to talk about what physicists mean by “symmetry," and why it matters to formulation of a GUT. For instance, quantum chromodynamics (QCD) describes the strong nuclear force and the way various quarks interact with each other. There are quarks of three different “colors" that can be randomly interchanged, just like a shell game, so those quarks share a similar internal symmetry.
Supersymmetry extends this interchangeable shuffling to incorporate all subatomic particles. See, not all potential couplings are feasible in the current standard model. Fermions (the particles that make up matter) and bosons (messenger particles that carry fundamental forces) can't mix at all because they have such vastly different properties.
Supersymmetry allows us to interchange a fermion with a boson. But it requires the existence of hypothetical super “shadow" partners, called sparticles: squarks, sleptons, photinos, selectrons, and neutralinos.
Each fermion is paired with a super-boson partner, and each boson has a super-fermion partner. Now they can be mixed via their super partners, but the price is a doubling of the number of subatomic particles.
The reason physicists haven't yet observed sparticles might be because they are so much heavier than their normal sister particles, so they decay far too quickly. That heavy mass also means it takes even larger amounts of energy to produce them — the kinds of energy only a machine as powerful as the LHC is capable of generating.
It's Not Dead Yet!
That rare decay pattern of the strange beauty particle announced last week is an especially sensitive indicator of the possible existence of unknown particles and forces — like sparticles. Unfortunately, the data from LHCb is not good news for fans of supersymmetry because it fits so neatly within the Standard Model predictions.
So far it doesn't show any unusual effects, and thus contradicts many of the most likely supersymmetry models. Those results should make supersymmetry antagonists happy, even as it disappoints the fans — feelings tend to run strong on either end of the spectrum.
“Supersymmetry is not ruled out by our measurement, but it is strongly constrained," LHCb spokesperson Pierluigi Campana told the CERN Bulletin. LHCb scientist Chris Parkes was a bit less circumspect, telling BBC News, “Supersymmetry may not be dead, but these latest results have certainly put it into hospital."
But it's still a rough measurement, and might change as physicists refine those results.
John Ellis of King's College London told BBC News that the observation is “quite consistent with supersymmetry. In fact, (it) was actually expected in (some) supersymmetric models. I certainly won't lose any sleep over the result."
Physicist Matt Strassler sharply criticized that BBC News article on his blog for claiming the results dealt a crushing blow to supersymmetry. Rather, he said that it rules out specific variants whose predictions don't match the data, not the field as a whole.
“Failure to find one variant of a theory is not evidence against other variants. If you're looking for your lost keys, failing to find them in the kitchen, living room and bedroom is not evidence against their being somewhere else in the house," Strassler wrote. “Nature is what it is — your keys are wherever they are — and the fraction of your search that you've completed is not logically related to how likely your search is to be successful."
Strassler pointed to the search for the Higgs boson as an example, in which experimental results at both Fermilab and the LHC kept ruling out various predicted mass ranges for the elusive particle. But eventually the Higgs, or something very much like it, turned up in one of the last places the LHC physicists looked.
Ironically, the discovery of a Higgs-like particle with a mass of 125 GeV/c2 is more of a blow to supersymmetry than the latest LHCb results, according to Strassler, because it is inconsistent with a large number of variants of supersymmetric theory.
The upshot? “Supersymmetry isn't in the hospital; many of its variants — more of them than last week — are just plain dead, while others are still very much alive and healthy," Strassler wrote. “There's still a long way to go before we'll really have confidence that the Standard Model correctly predicts all of the phenomena at the LHC."
Image credit: CERN, public domain