SINDRUM

SINDRUM was a series of precision experiments at the Paul Scherrer Institute designed to search for rare processes involving muons, most notably neutrinoless muon-to-electron conversion and charged lepton flavor violating decays. The collaboration brought together instrumentation expertise from CERN, ETH Zurich, University of Zurich, and other European institutions to probe physics beyond the Standard Model using intense low-energy muon beams produced at the Swiss Institute for Nuclear Research. The program complemented contemporary efforts at TRIUMF, Brookhaven National Laboratory, and Fermilab to test symmetries such as lepton flavor conservation and to constrain models like Supersymmetry, Grand Unified Theory, and Left–Right symmetry.

Background and Purpose

SINDRUM was conceived amid increasing interest in charged lepton flavor violation as a sensitive probe of new physics beyond the Standard Model, following theoretical developments associated with PMNS mixing and model-building in Supersymmetry, Seesaw mechanism, and Grand Unified Theory frameworks. The program responded to experimental milestones at facilities including Paul Scherrer Institute, TRIUMF, and CERN ISR and to theoretical predictions from authors such as Steven Weinberg, Howard Georgi, and Goran Senjanović. Goals included setting stringent upper limits on processes like muon-to-electron conversion in the field of a nucleus, muon decay to three electrons, and other neutrinoless transitions proposed in work by Mikhail Shifman and Glashow. Funding and coordination involved agencies and laboratories such as CERN, the Swiss National Science Foundation, and national programs in Germany, France, and Italy.

Experimental Apparatus and Techniques

SINDRUM experiments used beamlines and detector technologies developed in collaboration with groups from PSI, CERN, ETH Zurich, University of Geneva, and University of Mainz. The apparatus integrated magnetic spectrometers inspired by designs at CERN SPS and Brookhaven National Laboratory, cylindrical wire chambers and drift chambers similar to those used at LEP experiments, and plastic scintillator arrays comparable to systems at KEK. Muon stopping targets were chosen from materials studied in nuclear physics by groups at Max Planck Institute for Nuclear Physics and Ludwig Maximilian University of Munich; targets such as aluminium and gold were used to exploit theoretical capture rates computed by researchers at Saclay and Oak Ridge National Laboratory. Particle identification combined time-of-flight measurements, calorimetry adapted from MARK III developments, and tracking in solenoidal fields analogous to Magnet Systems at PSI. Triggering and data acquisition borrowed architectures from digital systems at DESY and modular electronics developed at ETH Zurich, enabling high-rate operation and real-time rejection of background processes like radiative muon decay measured at Los Alamos National Laboratory.

Major Results and Measurements

SINDRUM produced leading limits on charged lepton flavor violating processes, reporting constraints that shaped theoretical model building. The collaboration set competitive upper bounds on muon-to-electron conversion rates in several target materials, improving upon earlier searches at TRIUMF and Los Alamos National Laboratory and informing proposals at Fermilab and J-PARC. SINDRUM also delivered limits on the branching ratio for muon decay to three electrons, constraining parameter space for Supersymmetry models discussed by Dimopoulos and Georgi and affecting interpretations of Left–Right symmetry models by groups including Mohapatra and Pati. Ancillary measurements included precise determinations of muon capture rates and radiative muon decay spectra, which intersected with nuclear theory work by Migdal and Walecka. Experimental papers from SINDRUM appeared in high-impact journals and were cited by theoretical analyses from groups working on Seesaw mechanism implementations and Lepton flavor violation phenomenology.

Data Analysis and Systematics

Analysis strategies combined techniques developed at CERN and Brookhaven National Laboratory for rare-event searches, including blind analysis methods used in experiments like MEG and KOTO and background estimation strategies employed at Super-Kamiokande. Systematic uncertainties were dominated by detector acceptance, particle-identification efficiencies, and muon stopping distributions modeled with inputs from GEANT simulations and nuclear capture calculations from Saclay theorists. The collaboration deployed Monte Carlo frameworks influenced by software at CERN and validation studies against calibration data collected with beams characterized by instrumentation teams from PSI and TRIUMF. Statistical treatments followed frequentist and Bayesian prescriptions common to searches at LEP and Tevatron, leading to confidence limits that constrained couplings in phenomenological models by groups at Harvard University, Princeton University, and INR (Moscow).

Legacy and Impact on Particle Physics

SINDRUM's null results tightened bounds on charged lepton flavor violation, guiding the design and motivation for successor experiments including SINDRUM II, Mu2e, and COMET, and influencing proposals at J-PARC and Fermilab. The experimental techniques and analysis methods propagated to projects at CERN, KEK, and TRIUMF, shaping detector concepts in searches for rare decays and precision muon physics pursued by collaborations such as MEG II and Mu3e. SINDRUM's constraints played a role in narrowing viable parameter space for Supersymmetry, Grand Unified Theory scenarios, and heavy-neutrino models studied by theorists at CERN Theory Department, Harvard, and KEK Theory Center. Many collaborators from SINDRUM later joined international efforts at Fermilab and J-PARC, contributing institutional expertise to the global program probing physics beyond the Standard Model.

Category:Particle physics experiments Category:Paul Scherrer Institute experiments