Evidence of massive neutrinos was sought in the .rr+-+e+v decay spectrum with the background from the .rr-+p-+e decay chain highly suppressed. Upper limits (90% C.L.) were set on the neutrino mixing parameter / U,, / < lo-' in the mass region 50 < m, < 130 M e~/ c *.PACS number(s): 14.60. Gh, 12.15.Ff, 13.20.C~ Based on the Z O width, the experiments [ l ] at the CERN e + e -collider LEP have demonstrated that there are only three light neutrinos. The existence of conventional neutrinos with masses between the upper bound of the r neutrino mass m, I 35 M~V / C [2] and mZo/2 -45 G~V / C has therefore become unlikely. Nevertheless, some extensions of the standard model such as ones involving left-handed neutrino singlets (vX,,vX,, . . . , vXk )[3], in addition to the three generations bf left-handed lepton doublets and corresponding right-handed neutrinos, lead to neutrino mixing without affecting the Z O width. The weak eigenstates vxk of such neutrinos are related to the mass eigenstates vi by a unitary matrix, v, where I =~, , L~, I -, X~, X~,. . . , x k . Such mixings would produce additional peaks in the positron energy spectrum from two-body meson decays such as a + -+ e + v 7 as discussed by Shrock [4]. The contribution to the a + -e + v decay from the mixing of a massive neutrino vi can be written as where v, is the conventional massless neutrino and p, is a kinematic factor, with 6, =m:/m : , and Si = m t, / m 2,. The sensitivity to I U,, l2 (or p,) tends to increase as the helicity suppression effect relaxes with the increase of m v i 7 but tends to decrease above m,, -80 MeV/c2 due to the diminishing phase space. Previous direct searches for the production of massive neutrinos coupling to the electron in the mass region above the r neutrino mass limit have been carried out using the decays a + +e + v [5,6] and K +-e + v [7]. There have also been indirect searches for such mixing using neutrino beams [8,9]. Furthermore, limits on neutrinoless double-B decay can be used to constrain the mass and *present address: the mixing parameters of possible Majorana neutrinos [lo].The present experiment is a refinement of the previous work performed at TRIUMF [5]. Figure 1 shows a schematic diagram of the experimental setup. Positive pions of momentum Pv+ =83 MeV/c from the TRIUMF M13 channel were stopped at a rate of lo5 s-I in a 12.7-mm-thick scintillator target B3 sandwiched between two 1.6-mm-thick scintillation counters B2 and B4. Positrons from the decay n-+-e+v ( Te+=69.3 MeV with the branching ratio -lop4) and from the decay ,~i+-e'va following the decay a + -,~i + v (the ~+ -~+ -e + chain, Te + =0-52.3 MeV) were detected by counters T1 -T4 (1.6-3.2 mm thick) and energy-analyzed by a 51-cmlong X 46-cm-diameter NaI (TI) crystal TINA placed 25 cm from the target. The detection solid angle defined by T4 was -3%. Figure 2(a) shows the energy spectrum of positrons in an early time window 5-30 ns after the pion stop time. The pedestal peak at channel 850 ( T e + = 4 . 0 MeV after a correction for the energy loss of...
