KARMEN Limits on nu_e -> nu_tau Oscillations in 2-neutrino and 3-neutrino Mixing Schemes

TL;DR

KARMEN conducts a comprehensive short-baseline ν_e disappearance search at ISIS using a 56 t liquid scintillator detector to probe ν_e → ν_x oscillations in 2- and 3-flavor frameworks. Three complementary analyses—absolute ν_e CC cross section, CC/NC cross-section ratio, and spectral-shape ML fits—show no evidence of oscillations and provide robust 90% CL limits on oscillation amplitudes and mixing angles across Δm^2 ranges. The 3-flavor analysis yields global constraints on the mixing parameters Φ and Ψ, illustrating the complementary reach with LSND and other experiments and underscoring the need to test all oscillation channels for a consistent picture. Projected upgrades and increased statistics aim to further tighten these limits, improving sensitivity to ν_e disappearance and short-baseline mixing in the coming years.

Abstract

The 56 tonne high resolution liquid scintillation calorimeter KARMEN at the beam stop neutrino source ISIS has been used to search for neutrino oscillations in the disappearance channel nu_e->x. The nu_e emitted in mu+ decay at rest are detected with spectroscopic quality via the exclusive charged current reaction 12-C(nu_e,e-)12-N_g.s. almost free of background. Analysis of the spectral shape of e- from the nu_e induced reaction as well as a measurement of the absolute nu_e flux allow to investigate oscillations of the type nu_e->nu_tau and nu_e->nu_mu . The flux independent ratio R(CC/NC) of charged current events 12-C(nu_e,e-)12-N_g.s. to neutral current events 12-C(nu,nu')12-C* provides additional information in the oscillation channel nu_e->x . All three analysis methods show no evidence for oscillations. For the nu_e->nu_tau channel 90%CL limits of sin^2(2t)<0.338 for dm^2>100eV^2 and dm^2<0.77eV^2 for maximal mixing in a simple 2 flavor oscillation formalism are derived. A complete 3 flavor analysis of the experimental data from five years of measurement with respect to nu_e<->nu_tau and nu_e<->nu_mu mixing is presented.

Figures (13)

• Figure 1: Neutrino energy spectra (a) and production time (b) at ISIS. The proton double pulses are repeated with a frequency of 50 Hz.
• Figure 2: Signatures of the $\nu_{e}$ detection reaction $^{12}$C ( $\nu_{e}$ , e$^{-}$ ) $^{12}$N$_{\rm g.s.}$ : time and energy distributions of the prompt electron (a;c) and the delayed positron (b;d). Data points are with background subtracted, MC expectations are shown as histograms, total background as shaded histograms.
• Figure 3: Simulated detector response in energy, target distance and time difference including readout efficiency of $^{12}$C ( $\nu_{e}$ , e$^{-}$ ) $^{12}$N$_{\rm g.s.}$ (solid histo-grams) and $^{12}$C ( $\bar{\nu}_{e}$ , e$^{+}$ ) $^{12}$B (dashed histograms) for $\Delta m^2$ = 100 eV$^2$/c$^4$ normalized to the same arbitrary number of events.
• Figure 4: Energy dependence of the cross sections of charged current (CC) reactions $^{12}$C ( $\nu_{e}$ , e$^{-}$ ) $^{12}$N$_{\rm g.s.}$ and $^{12}$C ( $\bar{\nu}_{e}$ , e$^{+}$ ) $^{12}$B and of neutral current (NC) $^{12}$C ( $\nu$ , $\nu'$ ) $^{12}$C$^{*}$ ( 1$^+$ , 1 ; 15.1 MeV ) for different $\nu$ flavors in an elementary particle treatment of the Carbon nucleus fuku .
• Figure 5: Relative detection efficiencies $\epsilon$ as functions of the mass parameter $\Delta m^2$ : a) $\epsilon(\nu)$ for $\nu_{e}$$\leftrightarrow\,$$\nu_{\tau}$ mixing; b) $\nu_{e}$$\leftrightarrow\,$$\nu_{\mu}$ mixing: curve 1 shows $\epsilon(\nu)$ for $\nu_{e}$$\rightarrow\,$$\nu_{\mu}$ disappearance, curve 2 shows $\epsilon(\bar{\nu})$ for $\bar{\nu}_{\mu}$$\rightarrow\,$$\bar{\nu}_{e}$ appearance; curve 1--2 shows the net efficiency $\epsilon(\nu)-\epsilon(\bar{\nu})$ for the combination of $\nu_{e}$$\rightarrow\,$$\nu_{\mu}$ and $\bar{\nu}_{\mu}$$\rightarrow\,$$\bar{\nu}_{e}$ . The efficiencies $\epsilon$ are normalized to the detection efficiency of $^{12}$C ( $\nu_{e}$ , e$^{-}$ ) $^{12}$N$_{\rm g.s.}$ sequences within the applied cuts.