L'anomalie de 4 sigmes B-meson de LHCb indique Z'

LHCb’s latest analysis of 650 billion B‑meson decays shows a 4‑sigma deviation in the angular distribution of kaon‑plus‑two‑muon final states, a hint that a heavy Z′ boson or leptoquark could be lurking in the quantum loop.

Penguin Decays as New‑Physics Probes

B mesons that decay via a bottom‑to‑strange transition—known as penguin decays—are exceptionally sensitive to virtual particles that can appear in the loop. Because only one in a million such decays occurs, any new particle’s influence is less diluted than in more common channels.

Statistical Weight and Cross‑Checks

The 4‑sigma significance, corresponding to a 1‑in‑16,000 chance of a statistical fluctuation, comes from data collected between 2011 and 2018. William Barter of the University of Edinburgh calls it “among the most significant results of the last few years at the LHC.” CMS has observed a similar, though weaker, discrepancy, lending independent support.

Rival Charm Contributions

A competing charm‑quark decay can produce the same kaon‑plus‑two‑muon final state, a phenomenon dubbed “charming penguins.” Theoretical uncertainty in modeling these contributions tempers enthusiasm, as they could mimic the angular anomaly.

Z′ Boson Hypothesis

Ben Allanach of Cambridge proposes a Z′ boson—heavier than the Standard Model Z and flavor‑dependent—as a candidate. Such a particle would mediate a new force that discriminates between particle families, potentially explaining the observed deviation.

Leptoquark Scenario

Alternatively, a leptoquark could transiently combine lepton and quark properties in the loop, altering the decay’s angular distribution. Both models predict new resonances that could be probed at higher energies.

Theoretical Challenges

Accurately predicting the impact of charming penguins remains difficult, and current calculations suggest they cannot fully account for the 4‑sigma shift. This leaves room for genuine new physics.

Implications for the Standard Model

If the anomaly withstands further scrutiny, it would force a revision of the Standard Model’s flavor sector, possibly revealing a new force carrier or a leptoquark that couples differently to generations.

Next Steps

Future LHC runs, with increased luminosity and refined detector upgrades, will gather more data to tighten the statistical significance. A confirmed signal would open a window onto a new flavor‑dependent force or a leptoquark, reshaping our understanding of the Standard Model’s flavor structure.

Source: These exotic particles could break physics
Domain: scientificamerican.com