High purity pion beam at TRIUMF

Abstract

An extension of the TRIUMF M13 low-energy pion channel designed to suppress positrons based on an energy-loss technique is described. A source of beam channel momentum calibration from the decay π + → e + ν is also described.Key words: Beam channel, Particle separation, Pion Decay PACS: 41.85.Ja, 13.20.Cz, 29.27.Eg MotivationThe branching ratio of pion decays [1, 2], R=Γ(π → eν + eνγ)/Γ(π → µν + µνγ), has provided the best test of the hypothesis of electron-muon universality in weak interactions. The new TRIUMF PIENU experiment [3] aiming to improve the precision of the branching ratio measurement by a factor of five or more measures positrons from the π + → e + ν decay (E e + = 69.8 MeV) and the π + → µ + → e + decay chain (π + → µ + ν decay followed by µ + → e + νν decay, E e + < 52.8 MeV). In order to obtain maximum acceptance with minimum uncertainties arising from positron energy-dependent cross sections, the detector system involving a large NaI crystal is placed on the beam axis. Positrons in the beam (1/4 of the rate of pions) severely increase detector rates, trigger rates and background in the π + → e + ν spectrum. The TRIUMF M13 channel [4] has therefore been upgraded to suppress the positron contamination in the pion beam.

Non-unitarity of the leptonic mixing matrix: present bounds and future sensitivities

“…[28] for the analysis in the following. At tree level the MUV prediction is given by 40) where the first factor stems from the fact that a muon-neutrino beam was deployed, and we used the values r i = 0.5 for R ν and r i = 2 for Rν. The NuTeV measurements for R ν and Rν, together with their theory prediction in the SM and in the MUV scheme, are summarized in table 6.…”

Improved search for heavy neutrinos in the decay π→eν

“…The π þ beam was provided by the TRIUMF M13 beam line, modified to deliver 75 AE 1 MeV=c pions with < 2% positron contamination [20]. A detailed description of the PIENU detector (Fig.…”

Improved Measurement of theπ→eνBranching Ratio

“…Figure 1 shows a schematic view of the apparatus [9] in which a 75-MeV/c π + beam from the TRIUMF M13 channel [10] was degraded by two thin plastic scintillators B1 and B2 and stopped in an 8-mm thick scintillator target (B3) at a rate of 5 × 10 4 π + /s. Pion tracking was provided by wire chambers (WC1 and WC2) at the exit of the beam line and two (x,y) sets of single-sided 0.3-mm thick planes of silicon strip detectors, S1 and S2, located immediately upstream of B3.…”