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| B → K(*) ℓ+ℓ− | |
|---|---|
| Name | B → K(*) ℓ+ℓ− |
| Parent | B meson |
| Products | K meson, K* meson, charged leptons |
| Interaction | Electroweak interaction |
| Importance | Flavor physics, CP violation, beyond Standard Model searches |
B → K(*) ℓ+ℓ−
B → K(*) ℓ+ℓ− decays are flavor-changing neutral current transitions of B mesons that proceed via loop-level amplitudes in the Standard Model and provide sensitive probes for physics beyond the Standard Model. These processes connect experimental programs at LHC, KEK, and SLAC collaborations and have motivated theoretical developments involving the CKM matrix, Operator product expansion, and effective field theories such as Heavy Quark Effective Theory.
These rare decays occur when a bottom quark inside a B meson transmutes to a strange quark with emission of a lepton pair, mediated by electroweak penguin and box diagrams involving virtual top quark, W boson, and photon exchanges. Experimental signatures involve final states with a charged or neutral Kaon or a vector K* resonance and an oppositely charged lepton pair (electrons or muons), enabling comparisons among collaborations including LHCb, Belle, and BaBar. The sensitivity of branching fractions, angular distributions, and lepton-flavor universality tests directly impacts searches related to hypotheses from Supersymmetry, Z′ boson models, and other extensions considered at CERN and in global fits.
The decay is described by an effective Hamiltonian built from local operators such as electromagnetic dipole, semileptonic vector and axial-vector, and scalar/pseudoscalar operators. The Wilson coefficients are computed in perturbative Quantum Chromodynamics and electroweak theory with matching at the Electroweak scale and running via the Renormalization group. Calculations invoke frameworks like Soft-Collinear Effective Theory, Heavy Quark Effective Theory, and lattice QCD inputs from collaborations such as Fermilab Lattice and MILC for nonperturbative form factors. Global analyses often reference constraints from CKM matrix fits, Unitarity Triangle, and rare-decay measurements reported by experiments at Fermilab and DESY.
Measurements include branching fractions, differential distributions in dilepton invariant mass q^2, and angular observables extracted by LHCb, Belle, BaBar, and earlier searches at CLEO. Detector technologies from ATLAS and CMS complement flavor-dedicated triggers to collect muonic final states. Statistical procedures involve likelihood fits, unfolding, and control samples such as J/ψ and ψ(2S) resonances to validate efficiencies, while collaborations coordinate with theory groups including Flavour Lattice Averaging Group for input.
Key observables are the differential branching ratio dB/dq^2, the forward–backward asymmetry A_FB, the longitudinal polarization fraction F_L of the K*, and optimized observables like P5′ designed to reduce form-factor sensitivity. Angular analyses employ transversity amplitudes and measure CP-averaged and CP-violating combinations, with implications for fits of Wilson coefficients C7, C9, and C10. Deviations in observables such as P5′ reported by LHCb spurred intensive theoretical and experimental scrutiny involving collaborations across IHEP and INFN.
Reliable prediction of rates and angular distributions requires knowledge of B→K and B→K* form factors computed via lattice QCD, light-cone sum rules, and QCD factorization. Nonlocal hadronic effects from charm-loop contributions and long-distance resonance tails introduce uncertainties that are modeled using dispersion relations and analytic continuation techniques developed by groups at CERN, IPPP Durham, and Perimeter Institute. Systematic treatment of uncertainties is crucial for distinguishing new-physics signals from Standard Model hadronic effects.
Global fits of Wilson coefficients including results from LHCb, Belle, and BaBar have indicated tensions with the Standard Model in muonic channels, motivating interpretations in terms of lepton-flavor nonuniversality, Z′ models, leptoquarks, and loop-level contributions in Supersymmetry or composite Higgs scenarios. Correlated signals are sought in complementary processes such as B_s→φℓ+ℓ−, B→K νν̄, and charged-current anomalies measured by BaBar and Belle II.
LHCb has provided high-statistics angular measurements and lepton-universality ratios R_K and R_K*, while Belle and BaBar contribute complementary electron-channel information and independent systematic control. Comparisons among experiments consider differing acceptances, trigger efficiencies, and analysis strategies; combined fits often involve inputs from Heavy Flavor Averaging Group and theory priors from Flavour Lattice Averaging Group to assess global significance.
Ongoing upgrades at LHCb Upgrade, the start of data taking at Belle II, and planned enhancements at HL-LHC will increase precision on branching fractions, angular observables, and lepton-flavor-universality ratios. Improved lattice-QCD computations from groups at Fermilab, RBC and UKQCD collaborations and refined theoretical frameworks from Institute for Advanced Study-affiliated researchers are expected to reduce hadronic uncertainties and sharpen tests for Z′ boson and leptoquark explanations. Continued interplay among CERN, KEK, and international theory collaborations will guide searches for physics beyond the Standard Model.