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| NA31 experiment | |
|---|---|
| Name | NA31 |
| Location | CERN Super Proton Synchrotron |
| Collaboration | CERN |
| Period | 1982–1992 |
| Spokesperson | Günter Della Negra; Nicola Cabibbo |
| Primary goal | Measurement of direct CP violation in neutral kaon decays |
| Detector type | Calorimetry, spectrometry, neutral beam |
NA31 experiment The NA31 experiment was a fixed-target particle physics experiment at the CERN Super Proton Synchrotron designed to measure direct CP violation in the neutral kaon system. Operated by an international collaboration including groups from United Kingdom, Italy, Switzerland, France, and Germany, NA31 produced precision measurements of the parameter Re(ε′/ε), confronting theoretical predictions from the Standard Model and alternative frameworks. The experiment ran in several data-taking periods through the 1980s and early 1990s and influenced later kaon experiments at CERN and Fermilab.
NA31 was motivated by earlier discoveries of CP violation in neutral kaons, first observed in the Cronin and Fitch experiment at Brookhaven National Laboratory and interpreted via the parameter ε describing indirect CP violation. The primary objective was to search for and quantify direct CP violation, parametrized as Re(ε′/ε), predicted within the Kobayashi–Maskawa theory extension of the Cabibbo–Kobayashi–Maskawa matrix and related to quark mixing in the Standard Model. Competing theoretical approaches including Superweak theory and various Left–Right symmetric models offered differing expectations for ε′/ε, making an unambiguous experimental determination decisive for particle physics. NA31 aimed to compare decay rates of neutral kaons into two pions—both π0π0 and π+π−—to extract the double ratio sensitive to direct CP violation.
The detector layout combined electromagnetic calorimetry, charged-particle tracking, and veto systems optimized for neutral and charged pion final states. A large lead-glass electromagnetic calorimeter provided photon energy and position measurements for π0→γγ decays, while multiwire proportional chambers and drift chambers measured charged tracks from π+π− decays. Scintillator counters and hadron calorimeters were used for triggering and particle identification; iron-core muon filters identified muons to suppress backgrounds from semileptonic kaon decays. The apparatus incorporated precision timing derived from fast photomultiplier tubes and trigger electronics developed in collaboration with groups experienced from experiments at DESY and SLAC. Data acquisition systems interfaced to computing centers at CERN and national laboratories for event storage and reconstruction.
NA31 utilized a high-intensity proton beam extracted from the CERN Super Proton Synchrotron striking a beryllium or platinum target to produce secondary beams rich in charged and neutral kaons. A system of magnets and collimators selected a neutral beam with a significant KL component by sweeping away charged particles and focusing neutrons and neutral kaons. To produce a well-defined mixture of KL and KS at the detector, NA31 employed a regenerator in the neutral beam, allowing coherent regeneration of KS via strong interactions in a material block, an approach similar to earlier techniques used at Fermilab and Brookhaven. The beamline featured sweeping magnets, lead absorbers, and precise alignment to minimize background from hyperon and pion decays and to control the kaon momentum spectrum.
NA31 collected large datasets of two-pion decays over multiple running periods, using hardware triggers for π0π0 and π+π− topologies and software filters to reduce backgrounds. Calibration procedures for the lead-glass calorimeter used electron beams and reconstructed π0 masses from two-photon events; alignment of tracking chambers employed cosmic-ray data and straight-through beam runs. Event reconstruction combined kinematic fits, vertex determination, and particle identification to classify decay modes. The analysis extracted the double ratio R = (Γ(KL→π0π0)/Γ(KS→π0π0)) / (Γ(KL→π+π−)/Γ(KS→π+π−)) and derived Re(ε′/ε) from R−1/6, applying corrections for detector acceptance, regeneration effects, and radiative decays. Systematic uncertainties were evaluated through Monte Carlo simulations developed with packages adopted from CERN computing environments and cross-checked with control samples from dedicated calibration runs.
NA31 reported a nonzero positive value of Re(ε′/ε), providing evidence for direct CP violation in neutral kaon decays. The measured value, obtained after accounting for statistical and systematic errors, was inconsistent with the null prediction of pure superweak models, favoring Standard Model expectations within the theoretical uncertainties of hadronic matrix elements. NA31's measurements were compared with contemporaneous results from experiments at Fermilab (notably E731) and later complemented by high-precision determinations from NA48 and KTeV, contributing to an emergent consensus on the presence of direct CP violation.
The NA31 result stimulated intensive theoretical work on hadronic matrix elements, chiral perturbation theory calculations, and lattice QCD evaluations relevant to ε′/ε. It constrained models of CP violation beyond the Cabibbo–Kobayashi–Maskawa matrix such as Supersymmetry scenarios and Left–Right symmetric models, and informed global fits of flavor parameters used in the Particle Data Group compilations. By establishing a nonzero direct CP-violating contribution, NA31 influenced the direction of theoretical research on baryogenesis mechanisms tied to CP violation and supported the broader consistency of the Standard Model flavor sector, while highlighting limitations in computing nonperturbative strong-interaction effects.
NA31's pioneering measurement shaped the design of successor experiments including NA48 at CERN and KTeV at Fermilab, which achieved higher precision and cross-checked the NA31 findings. Techniques developed in NA31—neutral beam regeneration, lead-glass calorimetry, and sophisticated Monte Carlo treatments—were adopted and refined in later kaon and rare-decay searches such as NA62 and studies at J-PARC. The experiment's results remain a cornerstone in the history of CP violation studies and continue to be cited in reviews and textbooks on particle physics and flavor physics.
Category:Particle physics experiments at CERN