For decades, the Standard Model has been particle physics’ most resilient masterpiece: dazzlingly precise, yet plainly incomplete. The latest intrigue comes from CERN’s LHCb experiment and a rare “electroweak penguin” decay, in which a neutral B meson decays into a K* meson plus a muon pair. Because this transition, b → sμ+μ−, occurs only through quantum loops rather than at the simplest tree level, it is unusually sensitive to the influence of unknown heavy particles that might exist beyond direct experimental reach. (cds.cern.ch)
In its newest analysis, LHCb studied 8.4 inverse femtobarns of proton-proton collision data collected in 2011, 2012, and 2016–2018, producing its most precise measurement yet of the decay’s angular structure and branching fraction. The familiar troublemaker is the observable called P′5, which again shows localized tension with Standard Model predictions, especially in the 4.0–6.0 and 6.0–8.0 GeV²/c⁴ regions of dimuon mass squared. When the data are interpreted through a fit to the effective coupling Re(C9), the discrepancy rises to 3.6–3.8 standard deviations from the Standard Model, or 4.0–4.1 standard deviations once the branching fraction is included. Intriguingly, the best fit prefers a negative shift of about −0.93 in Re(C9), while the measured CP asymmetries remain consistent with zero. (cds.cern.ch)
That does not yet amount to a discovery. The chief caveat is theoretical: messy long-distance strong-interaction effects, often discussed under the nickname “charming penguins,” can imitate new physics. Even so, LHCb notes that recent analyses of local and nonlocal amplitudes do not point to dramatically enhanced long-distance contributions. Meanwhile, CMS has published its own high-statistics angular analysis at 13 TeV using 140 inverse femtobarns, and it too finds tension in P′5 below the J/ψ resonance, broadly reinforcing the pattern seen by LHCb. With the upgraded LHCb detector now taking data at record pace in 2026, the next wave of measurements should tell us whether this is a statistical mirage, a hadronic misunderstanding, or the first real fissure in the Standard Model’s façade. (cds.cern.ch)
