CKMfitter Group

CKMfitter Group
Name	CKMfitter Group
Field	High-energy physics
Founded	1990s
Headquarters	Orsay, France
Leaders	Sébastien Descotes-Genon; Jérôme Charles (examples)
Members	international collaboration of theorists and experimentalists
Affiliations	CNRS; Université Paris-Saclay; CERN

CKMfitter Group is an international collaboration of theoretical and experimental physicists focused on global analyses of the Cabibbo–Kobayashi–Maskawa (CKM) matrix and flavour physics. The group develops statistical frameworks and software to combine results from particle physics experiments, lattice gauge theory, and phenomenology to constrain parameters associated with quark mixing and CP violation. Its work interfaces with experiments, theory networks, and funding agencies across Europe, North America, and Asia.

History

The origins trace to the 1990s when analyses of the Cabibbo angle and the Kobayashi–Maskawa theory became central after results from LEP and SLC. Early contributions responded to measurements from the CLEO and Belle collaborations and inputs from Lattice QCD efforts at institutions like Jülich Research Centre and Riken. As results from the Tevatron and later the Large Hadron Collider matured, the group expanded to include experts connected to CDF, DØ, BaBar, LHCb, and ATLAS. The team engaged with global initiatives such as the Flavour Lattice Averaging Group and the Heavy Flavor Averaging Group to harmonize inputs and standards. Over decades the group adapted to theoretical developments like Operator Product Expansion refinements and experimental milestones including the discovery of the Higgs boson.

Mission and Methods

The group's mission emphasizes rigorous determination of quark-mixing parameters, tests of CP violation within the Standard Model, and searches for indications of physics beyond the Standard Model by comparing global fits to theoretical predictions. Methodologically, it integrates experimental likelihoods from collaborations such as Belle II, SuperKEKB, BESIII, and CMS with theoretical inputs from Lattice QCD computations at centers like Fermilab and Brookhaven National Laboratory. Statistical approaches draw on techniques related to Bayesian statistics, frequentist constructions developed in particle physics, and tools inspired by the RooFit/RooStats ecosystem. The group places emphasis on treatment of systematic uncertainties, correlations among measurements, and incorporation of constraints from processes like K–Kbar mixing, B–Bbar mixing, and rare decays such as B → K* μ+ μ−.

Global CKM Fit Framework

The core product is a global fit combining observables sensitive to the elements of the CKM matrix including determinations of |V_ud|, |V_us|, |V_cb|, and |V_ub|, the Unitarity Triangle parameters (α, β, γ), and constraints from meson oscillations and rare processes. Inputs derive from measurements at SuperKEKB, LHCb, Belle, BaBar, CLEO-c, KLOE, NA62, and KOTO, alongside theoretical calculations from collaborations such as HPQCD, ETM Collaboration, and RBC-UKQCD. The framework accounts for electroweak corrections computed in contexts like MS-bar scheme treatments and leverages perturbative calculations performed by groups at DESY, SLAC, and CERN. Outputs are used to test scenarios including Minimal Flavour Violation, contributions from Supersymmetry, Z' models, and generic effective field theory operators catalogued in the Warsaw basis.

Collaborations and Membership

Membership spans researchers affiliated with institutions such as CNRS, Université Paris-Saclay, CEA Saclay, University of Cincinnati, University of Michigan, University of Edinburgh, Kavli Institute for Theoretical Physics, Perimeter Institute, KEK, Nagoya University, Tata Institute of Fundamental Research, Institute for Advanced Study, Yale University, University of Oxford, Imperial College London, Max Planck Society, ETH Zurich, Princeton University, and Caltech. The group collaborates with experimental consortia including LHCb collaboration, ATLAS collaboration, CMS collaboration, and flavor-focused experiments at CERN SPS. It engages with theory networks like the Flavour Lattice Averaging Group, HEPData, and working groups organized by the Particle Data Group and European Strategy for Particle Physics panels.

Key Results and Impact

The collaboration produced precise constraints on the Unitarity Triangle angles and sides, clarified tensions in determinations of |V_ub| and |V_cb| from inclusive versus exclusive methods, and quantified allowed windows for new-physics contributions to ΔF=2 and ΔF=1 transitions. Results influenced the interpretation of anomalies reported in B-physics anomalies such as lepton-flavour universality tests exemplified by comparisons involving R_K and R_{K*} measurements from LHCb and Belle. The fits have provided inputs to global electroweak fits performed by groups associated with LEP Electroweak Working Group and informed constraints used in searches for supersymmetric particles at CMS and ATLAS. The group's outputs are widely cited in papers from collaborations like Belle II and theory reviews appearing in Reviews of Modern Physics and Physics Reports.

Software and Data Releases

The group maintains public codebases implementing its statistical framework, data tables, and likelihood decompositions compatible with repositories like HEPData and formats used by Rivet and Gnu Scientific Library. Releases include detailed covariance matrices, treatment of theoretical uncertainties, and modular interfaces for testing new-physics parameterizations such as those in the Standard Model Effective Field Theory. Software development draws on toolchains used across particle physics including Python (programming language), C++, ROOT (software), and GitLab/GitHub for version control. The datasets and releases facilitate reproducibility for analyses by teams at University of Barcelona, University of Padua, IHEP (China), and other global groups.

Category:Particle physics