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| CP violation in the B meson system | |
|---|---|
| Name | CP violation in the B meson system |
| Field | Particle physics |
| Discovered | 1999–2001 |
| Discoverers | KEK; SLAC National Accelerator Laboratory; Belle (experiment); BaBar (experiment) |
| Related | Charge conjugation symmetry; Parity; CP symmetry; Cabibbo–Kobayashi–Maskawa matrix; B meson; Neutral meson mixing |
CP violation in the B meson system
CP violation in the B meson system refers to asymmetries between processes involving B mesons and their antiparticles that violate the combined charge conjugation and parity symmetries. Measurements of these asymmetries by collaborations at facilities such as KEK, SLAC National Accelerator Laboratory, CERN, and Fermilab have provided critical tests of the Cabibbo–Kobayashi–Maskawa matrix within the Standard Model and probes for physics beyond the Standard Model involving hypothetical sectors like supersymmetry and flavour-changing neutral current extensions.
The phenomenon was anticipated after the formulation of the Cabibbo–Kobayashi–Maskawa matrix by Nicola Cabibbo and Makoto Kobayashi and Toshihide Maskawa to explain quark mixing and CP violation in weak interactions. Experimentally, CP-violating effects in the B system were first conclusively observed by the BaBar (experiment) collaboration at SLAC National Accelerator Laboratory and the Belle (experiment) collaboration at KEK in the early 2000s, with subsequent precision studies by LHCb at CERN. These efforts built on earlier discoveries of CP violation in the K meson system by James Cronin and Val Fitch and have motivated work at detectors including ATLAS, CMS, CDF (experiment), DØ (experiment), and future projects like Belle II.
The theoretical description uses the Cabibbo–Kobayashi–Maskawa matrix formalism introduced in the context of electroweak theory developed at institutions like CERN and articulated by theorists such as Sheldon Glashow, Steven Weinberg, and Abdus Salam. In the quark sector, CP violation stems from a complex phase in the CKM matrix that appears in charged-current weak interactions mediated by the W boson. The unitarity of the CKM matrix yields triangles such as the bd unitarity triangle studied by collaborations at KEK, SLAC National Accelerator Laboratory, and CERN. Theoretical tools include Operator product expansion methods developed in part by researchers at Princeton University and Harvard University, heavy-quark effective theory advanced by groups at Brookhaven National Laboratory and DESY, and lattice quantum chromodynamics calculations performed by consortia including Fermilab Lattice Collaboration and HPQCD. Phenomenological analyses incorporate inputs from papers by Lincoln Wolfenstein and later refinements by A. J. Buras and Gerard 't Hooft.
Key observables are time-dependent CP asymmetries measured as functions of proper time in neutral B decays, direct CP asymmetries determined by rate differences in charged and neutral B decays, and mixing-induced asymmetries associated with interference between decay and oscillation amplitudes. Experiments at KEK's Belle (experiment), SLAC National Accelerator Laboratory's BaBar (experiment), and CERN's LHCb exploit asymmetric-energy colliders and precision vertex detectors such as those developed by Vertex detectors groups at KEK and CERN. Flavor tagging uses algorithms and techniques pioneered at Fermilab and in analyses by collaborations at Brookhaven National Laboratory; time resolution and background suppression depend on subsystems designed by teams from Stanford University, University of Tokyo, University of Geneva, and Imperial College London. Statistical methods for likelihood fits and profile likelihoods draw on expertise from CERN statisticians and collaborations with Harvard University and Columbia University.
Neutral B mesons, including B0 (bd) and B_s^0 (bs) systems, exhibit oscillations between particle and antiparticle states via box diagrams involving top quark exchange, studied originally by theorists at CERN and Fermilab. Mixing is quantified by mass and width differences Δm and ΔΓ measured by LHCb, CDF (experiment), and DØ (experiment). CP violation in mixing is probed via semileptonic asymmetries and is constrained by measurements from BaBar (experiment), Belle (experiment), and LHCb, with theoretical expectations set by calculations from groups at Massachusetts Institute of Technology and University of California, Berkeley. The B_s^0 system, explored extensively at Tevatron (particle accelerator) experiments CDF (experiment) and DØ (experiment), and later by LHCb, provides sensitivity to new CP-violating phases predicted in models like supersymmetry and left–right symmetric models researched at University of Chicago and CERN.
Decay modes such as B0 → J/ψ K_S, B0 → π+π-, B → Kπ, and B_s^0 → J/ψ φ are canonical channels for observing direct and mixing-induced CP violation. The "golden mode" B0 → J/ψ K_S provided the first clean measurement of sin2β by BaBar (experiment) and Belle (experiment), testing predictions by Makoto Kobayashi and Toshihide Maskawa. Measurements of angles α, β, and γ of the unitarity triangle involve inputs from experiments at Belle II and LHCb and theoretical constraints from lattice collaborations such as RBC/UKQCD. Rare decay modes, including B → K* μ+μ- and B_s^0 → μ+μ-, measured by LHCb, CMS, and ATLAS, also probe CP-violating phases and potential contributions from Z' boson models and leptoquark scenarios developed at CERN and SLAC National Accelerator Laboratory.
Global fits to CKM parameters combine measurements from BaBar (experiment), Belle (experiment), LHCb, CDF (experiment), and DØ (experiment) and are performed by collaborations including the CKMfitter Group and UTFit teams with inputs from lattice results by Fermilab Lattice Collaboration and HPQCD. These fits test the consistency of the Standard Model as formulated by Gerard 't Hooft and Martinus Veltman; current results show general agreement with small tensions in observables like the B → K* μ+μ- angular distributions identified by LHCb and followed up by CMS and ATLAS. Combined constraints on the unitarity triangle vertices rely on data from KEK, SLAC National Accelerator Laboratory, and CERN and on phenomenological frameworks advanced at institutions including University of Oxford and University of Cambridge.
Precision studies constrain extensions of the Standard Model proposed by groups at CERN, SLAC National Accelerator Laboratory, Brookhaven National Laboratory, and Fermilab, including supersymmetry, additional Higgs doublet models studied at DESY, and flavour-changing scenarios motivated by anomalies reported by LHCb. Any confirmed deviation from CKM expectations would have implications for baryogenesis scenarios first discussed by Andrei Sakharov and for mechanisms of matter–antimatter asymmetry explored by researchers at Princeton University and University of Chicago. Ongoing and planned upgrades at Belle II and LHCb and prospective facilities such as SuperKEKB aim to improve sensitivity to CP-violating phases and rare processes, further constraining theoretical proposals from groups at Stanford University, Harvard University, and MIT.