Search for free decay of negative pions in water and light materials

Abstract

We report on a search for the free decay component of π − stopped in water and light materials. A non-zero value of this would be an indication of anomalous νe contamination to the νe and νµ production at stopped-pion neutrino facilities. No free decay component of π − was observed in water, Beryllium, and Aluminum, for which upper limits were established at 8.2 × 10 −4 , 3.2 × 10 −3 , and 7.7 × 10 −3 , respectively.

High purity pion beam at TRIUMF

“…By flipping the beam channel polarity to negative, we also searched for delayed 50 MeV/c electrons from µ − → e − νν decays in the production target as a potential high-intensity source of stopped negative muons. The ratio of delayed and prompt electrons was measured to be (3.5 ± 0.4) × 10 −3 , which is consistent with an estimate of 6 × 10 −3 based on the product of the ratio of delayed and prompt positrons (2.9), the π − /π + production ratio in this energy region (1/5), and the fraction of decay-in-flight of pions in which muons stop in the target (1 %) [10].…”

π−nuclear capture ratio on hydrogen and oxygen in water

“…A detailed description of the experimental setup and conditions can be found in Ref. [15]. Incoming pions were identified by two wire chambers (WC1 and WC2) and two plastic scintillators (B1 and B2), and stopped at a rate of 1-10 kHz in an 8-mm thick purified-water target, tilted by 45 degrees.…”

“…The trigger required a beam particle stopping in the target (B1·B2·B3) with the presence of an outgoing particle into the M2 counter that enabled to accept events containing electrons, γ-rays, and π 0 's (two γ-rays from π 0 → γγ decay). Beam pions were preferentially Monte Carlo (MC) calculations [15] were used to simulate the pion beam and π 0 detection.…”

“…Pions were generated 110 cm upstream of the target, according to the beam profile, and transported to the target. π − 's were assumed to be captured where they stopped with emission of protons or deuterons according to their respective probability [15].…”

“…The stability of the beam was also monitored using the accidental peaks for different particle types in the TOF spectrum. The π 0 acceptance was estimated to be 2.16 ± 0.13 % by MC calculations [15] in which the π − was "converted" where it stopped to a 2.9 MeV π 0 emitted isotropically. The error in the acceptance derives from the uncertainties in the positions of the counters and the Pb converter.…”