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| B_s → μ+ μ− | |
|---|---|
| Name | B_s → μ+ μ− |
| Decay particles | B_s meson; muon; antimuon |
| First observed | 2013 (evidence); 2017 (observation) |
| Studied by | CERN; LHCb Collaboration; CMS Collaboration; ATLAS Collaboration; Fermilab; Belle II |
| Significance | Precision test of flavor physics; probe of Higgs-sector and supersymmetry |
B_s → μ+ μ− The decay of the B_s meson to a muon pair is a rare flavor-changing neutral current transition that provides a sensitive test of the Standard Model and probes for effects from extensions such as supersymmetry, extra dimensions, and extended Higgs sectors. Experimental study of the decay involves collaborations at major facilities and experiments and has informed global fits performed by theory groups and experimental consortia. Measurements compare branching fraction and effective lifetime to predictions from lattice quantum chromodynamics and perturbative calculations, constraining models developed by research institutions and funding agencies.
The B_s → μ+ μ− channel is a loop- and helicity-suppressed decay of the strange-bottom meson first sought by experiments operating at colliders and fixed-target facilities, including teams from CERN, Fermilab, and KEK. Historically motivated by work at DESY and early results from the Tevatron collaborations, the signal gained prominence through searches by the LHC experiments LHCb, CMS, and ATLAS and via interpretive efforts by theorists at institutions such as Princeton, MIT, and CERN theory groups. The mode sits alongside related processes like B_d → μ+ μ− and K → πνν̄ in global flavor physics programs coordinated by the Particle Data Group and advisory panels.
Within the Standard Model calculations performed by groups at INFN, SLAC, and IPPP predict the branching fraction at next-to-next-to-leading order using electroweak and QCD corrections, with input from lattice QCD determinations of decay constants from collaborations such as HPQCD, ETM, and FNAL/MILC. The decay proceeds through Cabibbo–Kobayashi–Maskawa-suppressed box and penguin diagrams involving top-quark loops and electroweak bosons, and its amplitude is sensitive to scalar and pseudoscalar operators that appear in models proposed by groups at CERN, DESY, and the University of Cambridge. Effective field theory approaches used by theorists at Harvard, Yale, and Stanford map high-scale new-physics scenarios—such as minimal supersymmetric standard model constructions by groups at SLAC and the Max Planck Institute, Randall–Sundrum frameworks developed at Columbia and Johns Hopkins, and two-Higgs-doublet models considered by researchers at Fermilab—onto low-energy Wilson coefficients constrained by the decay. Lattice determinations of the B_s decay constant, studied by collaborations at the University of Adelaide and the University of Glasgow, reduce theoretical uncertainty, while global fits by UTfit and CKMfitter incorporate inputs from KEK, TRIUMF, and the National Institute for Nuclear Physics.
Searches and measurements have been performed by the LHCb, CMS, and ATLAS collaborations at CERN, with complementary analyses by CDF and DØ at Fermilab and prospective studies at Belle II in Japan. Techniques developed by the experimental groups build on vertexing algorithms from the Silicon Vertex Detector communities, particle-identification systems used by LHCb and Belle, and muon systems designed by collaborations affiliated with universities such as Oxford, Imperial College London, and the University of California. Results announced at conferences organized by ICHEP, Moriond, and EPSHEP were combined by joint working groups involving representatives from CERN, FNAL, and KEK. Measurements report branching fractions and effective lifetimes, exploiting normalization modes studied by collaborations at SLAC and DESY and calibration samples provided by the ATLAS inner-detector and CMS tracker groups.
Combined analyses by LHCb and CMS yielded an observed branching fraction consistent with Standard Model expectations within uncertainties, following earlier evidence reported by LHCb and limits set by CDF and DØ. Statistical techniques used in the analyses derive from methods promoted by statisticians at Imperial College, Oxford, and Carnegie Mellon and employ likelihood fits, multivariate discriminants developed by groups at the University of Wisconsin and the University of Manchester, and background estimates cross-checked by teams from Rutherford Appleton Laboratory. The effective lifetime measurement by LHCb provides additional constraints relevant to CP-violating phases studied in collaborations at CERN and Caltech. These results feed into global electroweak and flavor fits carried out by groups at CERN, SLAC, and the University of Geneva, and affect parameter spaces in supersymmetric scans produced by researchers at DESY and the University of Hamburg.
Because the decay rate is suppressed in the Standard Model, small deviations can signal contributions from new heavy particles hypothesized in frameworks developed at institutions such as Harvard, MIT, and the Max Planck Institute. Constraints from the branching fraction and lifetime exclude regions of parameter space in minimal supersymmetric extensions studied by groups at Fermilab and SLAC, limit scalar operators in two-Higgs-doublet models investigated at KEK and Kyoto University, and bound flavor-changing interactions in composite Higgs and extra-dimensional scenarios proposed by research teams at Columbia and Johns Hopkins. Combined with other observables from Belle II, BaBar, and kaon experiments at J-PARC, the measurements inform global fits by UTfit and CKMfitter and influence search strategies at CERN and the Large Hadron Collider upgrade programs.
Planned upgrades to LHCb, CMS, and ATLAS at CERN, together with increased luminosity at the High-Luminosity LHC and upcoming data from Belle II and proposed facilities at KEK and Fermilab, will improve precision on branching fraction and effective lifetime measurements. Detector R&D led by collaborations at CERN, DESY, and SLAC aims to enhance vertex resolution and muon identification, while theoretical improvements from lattice collaborations at FNAL/MILC and HPQCD and perturbative calculations from groups at the University of Durham and the Niels Bohr Institute will reduce systematic uncertainties. Combined efforts by international consortia and advisory bodies such as the European Strategy Group and national funding agencies will determine timelines for sensitivity to small deviations predicted by models from Princeton, the University of Tokyo, and other centers of particle physics research.