CH-5232 Villigen PSI, Switzerland
PSI - Virginia
K. Assamagan², Ch. Brönnimann¹, M. Daum¹, R. Frosch¹, R. Horisberger¹, P.-R. Kettle¹, C. Wigger¹
¹ | PSI, Paul-Scherrer-Institut, CH-5232 Villigen-PSI, Switzerland |
² | Institute of Nuclear and Particle Physics, University of Virginia, Charlottesville, Virginia 22901,USA. |
If one considers the possibility of non-zero neutrino masses, one must also consider leptonic mixing which could occur in analogy to quark mixing. The weak eigenstates nul would then consist of a mixture of mass eigenstates
Here nui are the mass eigenstates, and Uli the corresponding transformation matrix elements. Thus, if neutrinos are massive and nondegenerate then the momentum spectrum of muons from stopped pions in the decay
if kinematically allowed would consist of monochromatic lines at
We measured this muon momentum spectrum by analyzing a surface muon beam in a magnetic spectrometer equipped with a silicon microstrip detector. The experimental set-up is described in detail in reference [1].
Since the muons loose energy on their way out of the target, we do not observe a line spectrum but a approximately flat spectrum with a sharp cut-off at the line position (for details see [1]). The admixture of a heavy neutrino in pion decay would lead to a second edge in the spectrum at lower momentum, as shown in Figure 1 (11 kB).
From our measured momentum spectrum [1] we are sensitive to neutrino masses between 0.4 MeV and 2.5 MeV. In this range we found no evidence for the existence of heavy neutrinos. The upper limits for the decay
where nuµ denotes the lightest neutrino in the decay (2) and nux a heavier neutrino, is shown in Figure 2 (8 kB). In the mass range 0.4 MeV < mnu < 1.5 MeV our upper limit of the branching ratio is lower than that of all previous experiments [2].
[2] M. Daum et al., Phys. Rev. D 36 (1987) 2624, and references therein.
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