LHCb 'Penguin Decays' Now Show 4-Sigma Crack in the Standard Model — and Charming Penguins Can't Explain It

The most precise measurement yet of a class of rare particle decays at CERN's Large Hadron Collider has produced a four-standard-deviation discrepancy from the predictions of the Standard Model, the framework that has described the behavior of matter and forces with extraordinary accuracy for half a century. The result, drawn from the LHCb experiment's analysis of roughly 650 billion B-meson decays collected between 2011 and 2018, was published in late April and is being described by senior physicists as one of the most credible hints in years that physics beyond the Standard Model may finally be within reach.

The specific process at the heart of the result is known as an "electroweak penguin decay" — a deceptively whimsical name for a class of transformations in which a B meson, a particle containing a heavy bottom quark, decays through a quantum-loop process into four lighter particles: a kaon, a pion, and two muons. The name comes from the loop diagram physicists draw when they sketch the process on a blackboard, which a creative graduate student in the 1980s thought looked vaguely like a cartoon penguin. The decays are extremely rare — occurring less than once in a million B-meson disintegrations — but precisely because they are rare, they are exquisitely sensitive to the influence of any new heavy particles that might exist beyond the Standard Model.

What the LHCb team measured is a small but persistent discrepancy between the rate at which the decays occur in nature and the rate predicted by the Standard Model. The discrepancy, calculated at roughly four standard deviations after a careful accounting of contributions from a notoriously difficult class of background processes known as "charming penguins," is below the conventional five-sigma threshold required to claim a discovery — but it is more than enough to take seriously. "The evidence is starting to mount," Marie-Helene Schune, a senior LHCb physicist and member of the analysis team, told CERN Courier. "Each new measurement narrows the range of possible explanations, and what's left increasingly looks like new physics."

The most popular candidate explanation involves a hypothetical new particle called a Z-prime, an electrically neutral cousin of the Standard Model's Z boson that would mediate a previously unknown force coupling preferentially to certain quarks and leptons. Other models invoke supersymmetric partners of known particles, or a fourth generation of quarks beyond the three that have been observed since the 1970s. None of those candidates have been directly produced at the LHC, but penguin decays could in principle reveal them indirectly even if the new particles are too massive to be created head-on in proton-proton collisions.

The physics community is now waiting for two things. First, the LHCb collaboration is in the middle of analyzing a much larger dataset collected since 2018 — about three times the volume of the dataset behind the current result — that should be able to push the discrepancy to or past the discovery threshold within the next two years if the effect is real. Second, the planned high-luminosity upgrade of the LHC, scheduled to begin operations in the early 2030s, will give experiments access to a dataset roughly 15 times larger again. "We've been living with hints like this since the early 2010s," said Patrick Koppenburg, an LHCb physicist at Nikhef in the Netherlands. "This time it really feels like we are on the threshold of something."