A Widening Anomaly Strains the Standard Model

Slavomira Stefkova

Viewpoint

A Widening Anomaly Strains the Standard Model

Slavomira Stefkova

Institute of Physics, University of Bonn, Bonn, Germany

July 8, 2026• Physics 19, 94

Does a new measurement of a rare decay of the neutral B meson portend new physics?

Figure caption — **Figure 1:** The beam pipe of the LHCb experiment at CERN. The beam pipe’s main function is to sustain an ultrahigh vacuum for the colliding proton beams while enabling particles produced in the collisions to pass through its walls.

In particle physics, ten years is a long time to sit with a puzzle. Since 2013, measurements of a rare decay—a neutral B meson (B⁰) transforming into an excited kaon (K^*0) and a muon–antimuon pair (µ⁺µ ^–)—have stubbornly refused to match the predictions of the standard model, the theory that describes all known particles and forces [1]. Small enough to be dismissed at first as a statistical fluctuation, the pattern of discrepancies has grown with each new dataset into one of the most tantalizing hints of new physics in experimental particle physics. Now the LHCb Collaboration at CERN in Switzerland has published its most comprehensive analysis of the decay to date [2]. The result is clear: The anomaly persists. Encouragingly, the theoretical and experimental tools to understand it have never been sharper.

Within the mathematical framework of the standard model, the decay in question, B⁰ $\to$ K^*0µ⁺µ^–, can occur only through so-called higher-order electroweak loop diagrams in which a bottom, or b, quark transforms into a strange, or s, quark [3]. As a result, the decay is extraordinarily rare. In every million B-meson decays of all kinds, you can expect to find only one. That rarity makes the decay valuable: It could bear measurable imprints of particles beyond the standard model that contribute to the same loop processes but have so far escaped detection because they are too heavy.

The LHCb experiment (Fig. 1) records the energies and directions of pairs of muons produced in collisions. To infer the influence of undiscovered particles, particle physicists look at various observable quantities [4]. In this and many other rare-decay processes, standard-model and potential new physics contributions to the decay rate depend sensitively on the squared invariant mass of the muon pair q². This is why the most direct probe of physics beyond the standard model is the differential branching fraction dℬ/dq², the decay rate as a function of q². Unfortunately, strong-interaction effects, which are difficult to calculate from first principles, complicate the interpretation of dℬ/dq². However, certain properties of the decay itself offer a way around this limitation. Because K^*0 decays into K⁺ $𝜋$ ^– (where K⁺ is a positively charged kaon), the four-body, final state K⁺ $𝜋$ ^–µ⁺µ⁻ carries angular information that can be used to build observables far less contaminated by the strong-interaction effects. In particular, the forward–backward asymmetry A_FB captures whether muons tend to emerge parallel or antiparallel to the B-meson flight direction; A_FB is predicted to change sign at a value of q² where hadronic uncertainties are particularly small. Another observable, called P $_{5}^{'}$ , goes further. Derived from the angular distributions of collision products, P $_{5}^{'}$ was formulated to be insensitive to leading hadronic contributions. To complete the picture, CP-asymmetry observables compare B⁰ decays with those of its antiparticle to probe matter–antimatter asymmetries in the transition.

The LHCb Collaboration’s newly published analysis is based on the largest dataset amassed to date, corresponding to roughly 17,000 selected signal events and 8.4 fb^–¹ of proton–proton collision data. Even so, the increase in statistics is only part of the story.

The real progress is methodological. A first advance concerns the angular structure of the K⁺ $𝜋$ ^– pair. The kaon and pion can be produced in different orbital configurations. The dominant one, called the P wave, proceeds through the K^*0(892) resonance, but a broader, nonresonant contribution, the S wave, feeds into the same final state and interferes with it. Compared with previous analyses, the sensitivity to the S-wave effects has been improved by explicitly including the K⁺ $𝜋$ ^– mass as a fit variable and by extending the analysis to a much broader K⁺ $𝜋$ ^– mass range. A second major advance concerns the treatment of the muon mass. At low dimuon invariant mass—the region most sensitive to new physics—the muon’s finite value introduces corrections to the angular distributions that had previously been neglected. The LHCb Collaboration included these mass-dependent corrections for the first time.

After more than a decade of measurements, the picture that emerges is the following. The differential branching fraction dℬ/dq² is consistently below the standard-model prediction across the relevant range of dimuon invariant masses. The angular observable P $_{5}^{'}$ shows deviations in the same region where previous analyses also saw them. The forward–backward asymmetry A_FB also started to deviate from standard-model expectations. When the angular observables are combined in global fits, they exhibit a tension with standard-model predictions at the level of a few standard deviations, depending on the treatment of theoretical (in particular hadronic) uncertainties. By contrast, the CP-asymmetry observables agree with the standard model.

That pattern is consistent with a specific kind of new physics: a shift in the effective coupling between the b quark, the s quark, and the muon pair. Candidate new particles that could produce such a shift include a heavy Z $^{'}$ boson and a leptoquark [5].

There is, however, an alternative interpretation that doesn’t imply new physics: Charm-loop contributions, a particular subset of hadronic effects, may be larger than currently estimated . Recent attempts to quantify these effects from LHCb data have found no evidence for an enhancement sufficiently large to relieve the tension [6].

I find it exciting that independent measurements of these observables from other experiments at the Large Hadron Collider, CMS [7] or ATLAS [8], show a broadly coherent pattern, as do measurements from related channels involving the same b-to-s transitions at the LHCb itself [9]. However, given the current experimental uncertainties, these independent results are also mostly consistent with the standard model.

The upcoming data taking around the globe for this process is highly awaited. The LHCb, CMS, and ATLAS collaborations are expected to collect significantly larger datasets in the coming years. Meanwhile, the Belle II experiment in Japan continues to operate in the cleaner environment of an electron–positron collider. It will provide complementary measurements with different systematic uncertainties.

The concurrent development of sophisticated analysis methods has been remarkable, providing unprecedented control over both statistical and systematic uncertainties and, with it, the prospect of clarifying whether the tension observed by the LHCb Collaboration reflects physics beyond the standard model or residual hadronic effects. In parallel, the theoretical community is also continuously refining its tools, leading to improved descriptions of hadronic effects. One of the few remaining candles for illuminating possible new physics continues to burn.

References

R. Aaij et al. (LHCb Collaboration), “Measurement of form-factor-independent observables in the decay B⁰ $\to$ K^*0µ⁺µ^–,” Phys. Rev. Lett. 111, 191801 (2013).
R. Aaij et al. (LHCb Collaboration), “Comprehensive analysis of the B⁰ $\to$ K^*0µ⁺µ^– decay,” Phys. Rev. Lett. 137, 021802 (2026).
N. Gubernari et al., “Improved theory predictions and global analysis of exclusive b $\to$ sµ⁺µ^– processes,” J. High Energ. Phys. 2022, 133 (2022).
S. Descotes-Genon, J. Matias, and J. Virto, “Understanding the B⁰ $\to$ K^*0µ⁺µ^– anomaly,” Phys. Rev. D 88, 074002 (2013).
B. Capdevila et al., “Patterns of new physics in b $\to$ s $ℓ^{+} ℓ^{-}$ transitions in the light of recent data,” J. High Energ. Phys. 2018, 93 (2018).
R. Aaij et al. (LHCb Collaboration), “Amplitude analysis of the B⁰ $\to$ K^*0µ⁺µ^–decay,” Phys. Rev. Lett. 132, 131801 (2024).
A. Hayrapetyan et al. (CMS Collaboration), “Angular analysis of the B⁰ $\to$ K^*(892)⁰µ⁺µ^–decay in proton–proton collisions at √s = 13 TeV,” Phys. Lett. B 864, 139406 (2025).
M. Aaboud et al. (ATLAS Collaboration), “Angular analysis of B $_{d}^{0}$ $\to$ K^*µ⁺µ^– decays in pp collisions at √s = 8 TeV with the ATLAS detector,” J. High Energ. Phys. 2018, 47 (2018).
R. Aaij et al. (LHCb Collaboration), “Branching fraction measurements of the rare B $_{s}^{0}$ $\to$ $𝜑$ µ⁺µ^– and B $_{s}^{0} \to$ f $_{2}^{'}$ (1525)µ⁺µ^– decays,” Phys. Rev. Lett. 127, 151801 (2021).

About the Author

Slavomira Stefkova is a substitute professor in experimental physics at the University of Bonn in Germany. She received her PhD from Imperial College London, where her research focused on analysis of rare B decays with the LHCb experiment at CERN. After earning her PhD, she joined the Belle II experiment at the High Energy Accelerator Research Organization (KEK) in Japan, where she continues to work on rare B decays and serves as convener of the Electroweak and Radiative Penguin Physics Working Group.