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LHC Experiment Yields No Insight into Post-Higgs Physics


LHC beauty, magnet, B meson

The giant magnets of the LHCb detector. Credit: CERN

A new discovery at the Large Hadron Collider near Geneva casts a shadow across a hypothetical realm of particle physics that many had hoped would be the collider’s next major exploration after the apparently successful hunt for the Higgs boson.

Physicists working with the collider’s LHC beauty, or LHCb, detector have observed a new kind of particle behavior, the researchers announced November 12 at the Hadron Collider Physics Symposium in Kyoto, Japan. The LHCb physicists have identified the very rare decay of a strange beauty particle B0s (a beauty antiquark bound to a strange quark) into two particles called muons. (A muon is a charged particle akin to a heavyweight electron.) And the new data limit the possibilities for many hypothesized extensions of the Standard Model of particle physics, including the concept of supersymmetry.

The short-lived strange beauty particle decays into other particles almost immediately after its creation in LHC collisions, but it almost never decays into a positive muon and a negative muon. The rarity of the decay makes it difficult to observe, and in fact the LHCb physicists claim that their measurements are the first solid evidence for the phenomenon. But the reigning theory of subatomic particles and forces, the Standard Model of particle physics, predicts just how often the effect should occur—about three times in a billion. Any deviation from that number would suggest the existence of new particles or forces unaccounted for by the Standard Model.

Strange beauty particle, decay, LHCb

A candidate event for a strange beauty particle decaying into two muons. Credit: CERN

The LHCb data (pdf) match up well with the Standard Model predictions of roughly three in a billion for this effect, potentially ruling out hypothetical extensions to the Standard Model that would impact the decay rate of the strange beauty particle into muons. “The detailed implications of this result will take a while to work through, but the general implication is easy to state: the Standard Model has survived another test,” physicist Matt Strassler wrote on his blog. “And the constraints from LHC data on speculative ideas that predict particles and forces beyond those of the Standard Model have become tighter.”

One of the most popular ideas that could move physics beyond the Standard Model is supersymmetry—the proposal that every elementary particle has a heavier, as-yet-unseen cousin that rounds out the particle zoo. If the strange beauty particle had decayed at an unexpected rate that the Standard Model had failed to predict, it might have provided some justification for supersymmetry. The LHC has yet to find any evidence for supersymmetric particles of any kind, and indeed some variants of supersymmetry predict a different pattern of particle decays than those now documented by LHCb. “Supersymmetry is not ruled out by our measurement, but it is strongly constrained,” LHCb spokesperson Pierluigi Campana told the CERN Bulletin, meaning that only certain flavors of supersymmetry jibe with the new data.

A BBC article framed the LHCb finding as dealing “a significant blow” to supersymmetry, but Strassler objected to that wording on his blog. “Failure to find one variant of a theory is not evidence against other variants,” he wrote. “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.”

The views expressed are those of the author and are not necessarily those of Scientific American.

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