MuCap

MuCap
Name	MuCap
Field	Particle physics
Goal	Measurement of the singlet muon capture rate in hydrogen
Start	2000s
Location	Paul Scherrer Institute
Collaborators	Paul Scherrer Institute, Brookhaven National Laboratory, Fermilab, TRIUMF, Argonne National Laboratory, University of Chicago, Princeton University, University of Illinois Urbana–Champaign, University of Michigan, Columbia University, Yale University, University of Washington, University of California, Berkeley, Massachusetts Institute of Technology, Stanford University, Los Alamos National Laboratory, CERN, DESY, KEK, Rutherford Appleton Laboratory, Imperial College London, Oxford University, Cambridge University, University of Liverpool, University of Oxford, University of Amsterdam, Nikhef, Max Planck Society, ETH Zurich, Eidgenössische Technische Hochschule Zürich, University of Zurich, University of Geneva, University of Manchester, University of Edinburgh, University of Glasgow, University of Birmingham, University of Southampton, Lund University, Uppsala University, Karolinska Institute, Weizmann Institute of Science, Tel Aviv University, Seoul National University, Peking University, Tsinghua University, University of Tokyo, Tohoku University, Osaka University, Australian National University, Monash University, University of Melbourne, University of Sydney, McGill University, University of Toronto, University of British Columbia"

MuCap MuCap was a precision experimental program designed to determine the singlet muon capture rate on the proton by measuring the lifetime of negative muons in hydrogen. It sought to extract the pseudoscalar coupling and test predictions from chiral effective field theory and lattice Quantum Chromodynamics by combining high-statistics muon decay measurements with strict control of atomic and molecular chemistry in low-density hydrogen. The collaboration conducted measurements at facilities that host intense muon beams and engaged groups from major laboratories and universities worldwide.

Introduction

The experiment focused on the process μ^- + p → n + ν_μ, connecting to weak interactions studied at James Clerk Maxwell Telescope, Paul Scherrer Institute, Brookhaven National Laboratory, Fermilab, TRIUMF, and comparable muon sources. The singlet capture rate provides input for calculations in Chiral perturbation theory, Lattice QCD, Effective field theory, and tests of radiative corrections computed in frameworks used by Particle Data Group, Institute of Physics, American Physical Society, and collaborations that include CERN and DESY. Results bear on interpretations relevant to Wolfgang Pauli-era weak interaction phenomenology, constraints used in analyses performed at Los Alamos National Laboratory, Argonne National Laboratory, and theoretical work connected to Steven Weinberg, Gerard 't Hooft, Ken Wilson, and contemporary particle physics groups.

Experimental Setup

The apparatus was installed at the muon beamline of the Paul Scherrer Institute and integrated technologies developed at TRIUMF, Fermilab, and CERN. The central element was a time projection chamber optimized for low-density hydrogen, while surrounding systems included plastic scintillator arrays, wire chambers, and muon beam monitoring inherited from experiments at Brookhaven National Laboratory and KEK. Cryogenic controls, gas handling, and pressure systems drew on expertise from Max Planck Society laboratories and university groups at ETH Zurich and University of Chicago. Beam optics and transport components shared heritage with designs used at Rutherford Appleton Laboratory and DESY, and computing infrastructure used frameworks common at SLAC National Accelerator Laboratory and Lawrence Berkeley National Laboratory.

Measurement Technique and Data Analysis

The technique measured the disappearance rate of negative muons stopped in protium using lifetime spectroscopy, with background rejection via coincidence systems developed along lines used in Muon g−2 and SINDRUM experiments. Event reconstruction used time projection chamber data reduction algorithms similarly employed in ALICE, CMS, and ATLAS collaborations for tracking and particle identification. Systematic uncertainties were constrained through calibrations referencing standards from National Institute of Standards and Technology, simulations using packages akin to those used by Geant4-based collaborations at CERN, and cross-checks relying on beam-dump studies from Fermilab and TRIUMF. Statistical analyses adopted likelihood-based fits comparable to those used by Belle, BaBar, KLOE, and combined fits methodologies familiar to groups like Particle Data Group and Heavy Flavor Averaging Group.

Results and Interpretation

MuCap reported a singlet capture rate that provided a precise determination of the proton’s induced pseudoscalar coupling, enabling direct comparison with predictions from Chiral perturbation theory and high-precision Lattice QCD calculations. These results influenced theoretical analyses by researchers associated with Institute for Advanced Study, Perimeter Institute for Theoretical Physics, and university groups across United States, Europe, and Asia. The measurement constrained corrections relevant to muon capture in nuclei studied in experiments at TRIUMF and RIKEN, and impacted interpretations of weak-interaction processes referenced in reviews by American Physical Society and textbooks by authors like John Preskill, Steven Weinberg, and Frank Wilczek.

Theoretical Implications

The extracted pseudoscalar coupling served as a benchmark for calculations in Chiral perturbation theory, influenced inputs for Lattice QCD collaborations at CERN and RIKEN, and provided constraints on radiative corrections evaluated in the context of electroweak theory developed by Sheldon Glashow, Abdus Salam, and Steven Weinberg. Comparisons engaged theorists at Princeton University, MIT, Caltech, Harvard University, Yale University, University of Cambridge, and University of Oxford and fed into global fits that intersect with efforts at Particle Data Group and phenomenology programs at SLAC and DESY.

Instrumentation and Upgrades

Instrumentation included a low-mass time projection chamber, silicon-based veto detectors, high-granularity scintillator arrays, and precision gas-handling systems. Upgrades pursued by collaborating institutions paralleled developments in detector technology at CERN experiments such as LHCb and ALICE, and electronics advances seen in Belle II and DUNE collaborations. Future techniques considered for follow-up programs included higher-rate muon beams at facilities like PSI, J-PARC, and Fermilab, advanced cryogenic systems inspired by developments at SNOLAB and Gran Sasso National Laboratory, and enhanced readout schemes influenced by projects at Brookhaven National Laboratory and Lawrence Berkeley National Laboratory.

Category:Particle physics experiments