Hiding New Physics in Top Quarks Fails to Solve a Stubborn B Meson Puzzle

For more than a decade, one of the most stubborn discrepancies in particle physics has been quietly building in the decays of B mesons, unstable particles containing a bottom quark. When these particles transform into charmed mesons accompanied by a light kaon or pion, experiments at the LHCb detector and elsewhere measure branching ratios that stubbornly refuse to match the clean predictions of the Standard Model. In some channels, the tension between theory and experiment now exceeds five standard deviations, a level of disagreement that in other contexts would already have been celebrated as a discovery. A new theoretical study published in The European Physical Journal C by Jack Araz, Christoph Englert, Matthew Kirk and Gilberto Tetlalmatzi-Xolocotzi has now systematically tested the most popular escape routes that model-builders might use to explain the anomaly, and the verdict is sobering: nearly all of them fail.

The decays in question, written in shorthand as b to c anti-u q transitions, occupy a special place in flavour physics because they are unusually clean theoretically. In the framework known as QCD factorisation, the decay amplitudes can be computed perturbatively, and crucially, the final states contain no identical quark-antiquark pairs. That absence eliminates the so-called weak annihilation topologies, which are the dominant source of uncalculable infrared divergences and typically inflate the error budget of non-leptonic B decays. The residual power corrections, suppressed by powers of the QCD scale divided by the bottom quark mass, are expected to sit at the level of roughly one per cent. With experimental precisions now pushing below three per cent, these channels should be among the best-understood processes in the entire flavour sector, which makes the observed deviations all the more perplexing.

Theorists have proposed two broad families of explanations. The first is that the hadronic uncertainties have simply been underestimated, and that quantum chromodynamics, the theory of the strong force, is not as well controlled in these decays as the formalism suggests. The second is more tantalising: the deviations are indirect fingerprints of new particles that mediate the quark-level transition at tree level, modifying the decay amplitude before the Standard Model contribution even enters. Previous work had already shown that the simplest realisation of the new physics idea, an additional Higgs doublet coupling to quarks, runs into immediate trouble with LHC dijet searches, because the same mediator that shifts the B meson decay also produces spectacular resonances decaying to pairs of jets that the experiments have not observed.

The new study examines three plausible ways around that impasse. The first is that the new physics is top-philic, meaning the neutral component of the extra Higgs doublet preferentially decays to pairs of top quarks rather than light jets. Top-pair resonance searches are notoriously difficult, suffering from large QCD backgrounds and significant signal-background interference that limits their sensitivity. If the neutral scalar dumps most of its branching fraction into top quarks, one might reasonably hope that the dijet bounds would weaken dramatically, opening a viable window for the model. The authors extended the minimal coupling structure with a new coupling to right-handed up-type quarks, allowing the neutral scalar to decay to top pairs with branching ratios approaching unity, and then carefully recast the relevant ATLAS and CMS analyses.

The result defied expectations. Even with a large branching ratio to top quark pairs, the dijet constraints on the neutral component did not weaken in any systematic way, and in some regions of parameter space they actually became stronger. More importantly, the charged component of the doublet, which cannot hide in top final states, provides bounds that are roughly equally strong across most of the scanned parameter range. The authors used the full suite of modern dijet searches, including the CMS-EXO-21-010 analysis, which postdates and supersedes the constraints used in earlier studies, and found that the updated limits are even more restrictive than those previously reported. Hiding the new scalar in top quarks, in other words, does not reconcile the collider data with the flavour anomaly.

The second escape route involves questioning QCD itself. The authors asked how large the power corrections in the clean decays would need to be to explain the discrepancy without any new physics. To calibrate expectations, they turned to related B decays into pairs of light mesons, such as B to pi pi and B to K K, where non-factorisable contributions are known to be prominent and where recent data-driven analyses have bounded the power-suppressed annihilation terms at roughly twenty per cent at most. Since the clean channels studied here should be far less sensitive to such effects, power corrections anywhere near that size would already be surprising. Yet the analysis shows that a shift of around minus fifteen per cent would be needed to fully absorb the anomaly, and even a more moderate correction of minus ten per cent, which halves the required new physics coupling, still leaves the favoured parameter region in substantial tension with the dijet limits.

The third strategy is brute force: add more particles. If the model contains n copies of the additional Higgs doublet, each with identical couplings, their contributions to the low-energy decay amplitude add constructively while the collider signal for each individual resonance is diluted. The authors considered extensions with up to four additional doublets, a five-Higgs-doublet model, and found that even this extreme scenario remains excluded by current collider bounds when no power corrections are assumed. Only when all three mechanisms are combined, multiple doublets, top-philic neutral components and a ten per cent power correction, does a narrow viable window emerge, with degenerate doublets near a mass of 600 GeV. Even then, the model cannot fully reach the coupling strength maximally preferred by the non-leptonic data, and the authors note that a hypothetical fourteen-doublet model would be needed to evade the collider constraints entirely without power corrections.

The study also checked whether the surviving parameter space conflicts with other precision measurements. Extended Higgs sectors contribute to the decay of the Z boson to bottom quarks, to B meson mixing, to rare radiative decays, and to the lifetime ratio of charged to neutral B mesons. Using the global likelihood tool smelli, the authors found that the parameter space not excluded by the LHC is generally compatible with the Z pole and low-energy flavour observables, with one notable exception: the mass difference in the B_s mixing system, which can be in tension at up to three standard deviations depending on the relative sign of the Yukawa couplings. That tension could partially negate the fit improvement gained from explaining the non-leptonic anomalies, adding yet another obstacle for would-be model-builders.

The broader message is that the b to c anti-u q puzzle has grown more, not less, mysterious. The anomalies persist in channels where the theory is cleanest, and the new physics explanations that fit the flavour data most naturally predict collider signals that simply are not there. The authors conclude that these non-leptonic discrepancies will remain among the most puzzling deviations from the Standard Model for the foreseeable future, resolvable only in finely tuned scenarios that combine multiple Higgs doublets with unexpectedly large QCD power corrections. For experimentalists, the message is equally clear: the dijet and top-pair resonance searches at the LHC, often overshadowed by direct discovery channels, are doing quiet but decisive work in constraining the space of possible new physics, and future improvements in their systematics may ultimately settle whether the B meson anomaly is a window into new particles or simply a reminder that the strong force still holds secrets of its own.

Subject of Research: Non-leptonic B meson decay anomalies and constraints on new physics models from LHC collider searches

Article Title: New insights into the (b\rightarrow c \bar{u}q) puzzle through Top–Bottom synergies

Article References: Araz, J. Y., Englert, C., Kirk, M., & Tetlalmatzi-Xolocotzi, G. (2026). New insights into the $$b\rightarrow c \bar{u}q$$ puzzle through Top–Bottom synergies. The European Physical Journal C, 86(9), Article 1045. https://doi.org/10.1140/epjc/s10052-026-16297-5

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16297-5

Keywords: B meson decays, flavour physics, QCD factorisation, two-Higgs-doublet model, LHC, dijet searches, top quark, new physics, Standard Model, power corrections, collider constraints, particle physics