B → K* μ+ μ−

B → K* μ+ μ−
Name	B → K* μ+ μ−
Parent	B meson
Daughters	K* meson; muon; antimuon
Interaction	Flavor-changing neutral current; electroweak penguin; box diagrams
Conservation	Charge; lepton number; energy; momentum

B → K* μ+ μ− B → K* μ+ μ− is a rare flavor-changing neutral current decay of a B meson into an excited kaon K* and a muon pair that provides sensitive probes of the Standard Model and physics beyond the Standard Model. Measurements of branching fractions, angular observables, and lepton-flavor universality tests involving this decay have been central to studies at experiments such as LHCb, ATLAS, CMS, Belle II, and BaBar, and have motivated theoretical work by groups at CERN, Fermilab, KEK, SLAC National Accelerator Laboratory, and numerous universities.

Introduction

The decay proceeds via electroweak loop diagrams including penguin and box topologies mediated by virtual W bosons, Z bosons, and top quarks, with amplitudes computed in the framework developed by theorists at institutions such as Princeton University, University of California, Berkeley, University of Cambridge, University of Oxford, Massachusetts Institute of Technology, and Institute for Advanced Study. Interest in the channel surged after measurements by Belle, BaBar, and CLEO hinted at tensions with predictions by groups led by researchers at INFN, Max Planck Institute for Physics, DESY, Institute of High Energy Physics (IHEP), and the University of Tokyo. The decay ties into wider flavor-physics programs at collaborations including Heavy Flavor Averaging Group, Particle Data Group, and national laboratories like Brookhaven National Laboratory.

Theoretical framework

Calculations rely on effective field theory constructs such as the weak effective Hamiltonian formulated by Gerhard Buchalla and collaborators, with Wilson coefficients C7, C9, and C10 encoding short-distance physics evaluated using techniques from Quantum Chromodynamics groups at CERN Theory Division, SLAC, RIKEN, University of Notre Dame, and University of Hamburg. Nonperturbative hadronic matrix elements require form factors computed by lattice QCD teams at HPQCD, RBC-UKQCD, Fermilab Lattice and MILC Collaborations, and sum-rule practitioners at Institute for Nuclear Theory, University of Barcelona, Universidade de São Paulo, and Universität Mainz. Soft-collinear effective theory methods advanced by researchers at Caltech, Rutgers University, University of Michigan, University of Illinois Urbana-Champaign, and Yale University address factorization and endpoint divergences. Global analyses frequently cite methodologies developed at Joint Institute for Nuclear Research and theoretical reviews from European Organization for Nuclear Research.

Experimental measurements and observables

Experiments report branching ratios, differential decay rates dΓ/dq^2, forward-backward asymmetry A_FB, longitudinal polarization fraction F_L, and optimized angular observables P5' and S_i measured by collaborations at LHCb, ATLAS, CMS, Belle II, BaBar, and CLEO. Detector-level work involves instrument teams such as CERN's LHCb detector group, ATLAS Inner Detector teams, CMS Tracker groups, Belle II's SuperKEKB engineers, KEK accelerator staff, and computing centers at CERN OpenLab and GridPP. Measurements are compared with predictions from theory groups at IPPP Durham, CP3 at Université catholique de Louvain, Perimeter Institute, Institute for Advanced Study, and national analysis consortia including HFAG and PDG.

Anomalies and global fits

Anomalous deviations in P5' and tensions in lepton-flavor universality ratios motivated global fits by collaborations at University of Copenhagen, Weizmann Institute of Science, Technische Universität München, University of Warwick, Scuola Normale Superiore, Universidad Autónoma de Madrid, Niels Bohr Institute, University of Zurich, and Tel Aviv University. Statistical combinations utilize frameworks developed by Gfitter-like teams, Bayesian analyses from groups at CERN Theory Department and frequentist scans from Fermilab and KEK. Results are discussed at conferences such as Moriond, ICHEP, EPS-HEP, Rencontres de Blois, and workshops hosted by Perimeter Institute and KITP.

Interpretation and new physics models

Proposed explanations involve modified Wilson coefficients from scenarios including Z' gauge bosons studied by theorists at Institute for Advanced Study and University College London, leptoquark models developed by researchers at Ohio State University and University of Bern, supersymmetric contributions from groups at CERN, DESY, University of Chicago, and extra-dimensional frameworks associated with Stanford University and Columbia University. Flavor symmetries such as U(2) and Minimal Flavor Violation are invoked by teams at University of California, San Diego, International Centre for Theoretical Physics, Scuola Normale Superiore, and University of Padua. Model discrimination leverages inputs from Kaon physics at NA62, Muon g-2 at Brookhaven National Laboratory and Fermilab Muon g-2, and high-pT searches at ATLAS and CMS.

Experimental techniques and challenges

Precision requires control of backgrounds handled by analysis groups at LHCb and Belle II, particle identification systems like RICH detectors developed by collaborations including CERN and University of Glasgow, muon systems from Institute of High Energy Physics teams, and tracking from CERN and KEK engineers. Systematic uncertainties stem from form-factor inputs from HPQCD and RBC-UKQCD, electromagnetic corrections studied by Budker Institute of Nuclear Physics, and trigger efficiencies managed by LHCb and ATLAS operations. Statistical methods originate in work at CERN Statistical Data Analysis groups, Bayesians and Frequentists communities across Princeton University and Imperial College London.

Future prospects and ongoing experiments

Ongoing data collection at LHCb Upgrade, ATLAS Phase-II, CMS Phase-II, and Belle II promises improved precision, while planned facilities such as SuperKEKB luminosity upgrades, proposed projects at Future Circular Collider, International Linear Collider, and regional initiatives at IHEP Beijing and TRIUMF could extend sensitivity. Theoretical improvements are expected from lattice collaborations Fermilab Lattice, RBC-UKQCD, and perturbative calculations by teams at CERN and KITP, with coordinated efforts through organizations like European Strategy for Particle Physics and conferences at Moriond and ICHEP facilitating progress.

Category:Flavor physics