Search for nu(mu)-->nu(e) Oscillations in the NOMAD Experiment

TL;DR

The NOMAD experiment conducts a blind search for ${\nu_\mu}\rightarrow{\nu_e}$ oscillations using a ${\nu_\mu}$-dominant wide-band beam from CERN, comparing observed ${\nu_e}$ CC events to a detailed beam-flux prediction via the ratio ${R_{e\mu}}$. No oscillation signal is found, yielding 90% C.L. limits of ${\Delta m^2} < 0.4\ \mathrm{eV^2}$ for maximal mixing and ${\sin^2(2\theta)} < 1.4\times10^{-3}$ for large ${\Delta m^2}$, thereby excluding the LSND favored high-${\Delta m^2}$ region. The analysis emphasizes careful treatment of beam-flux systematics and uses a robust, blind approach with control samples to validate predictions. Collectively, NOMAD provides a stringent short-baseline constraint on ${\nu_\mu}\rightarrow{\nu_e}$ oscillations and complements reactor and accelerator searches.

Abstract

We present the results of a search for nu(mu)-->nu(e) oscillations in the NOMAD experiment at CERN. The experiment looked for the appearance of nu(e) in a predominantly nu(mu) wide-band neutrino beam at the CERN SPS. No evidence for oscillations was found. The 90% confidence limits obtained are delta m^2 < 0.4 eV^2 for maximal mixing and sin^2(2theta) < 1.4x10^{-3} for large delta m^2. This result excludes the LSND allowed region of oscillation parameters with delta m^2 > 10 eV^2.

Figures (8)

• Figure 1: Composition of the ${\nu_\mu}$ and ${\nu_e}$ energy spectra at NOMAD, within a transverse fiducial area of $260 \times 260$ cm$^2$, as predicted by the NOMAD simulation of the neutrino beam line.
• Figure 2: The two-dimensional distributions $\phi_{\rm eh}$-$Q_{\rm lep}$ (defined in the text) for Monte Carlo ${\nu_\mu}~{\rm CC}$ and ${\nu_\mu}~{\rm NC}$ (background) events and for ${\nu_e}~{\rm CC}$ (signal) events, as well as for the NOMAD data. The events to the right of the curve were selected.
• Figure 3: The distributions of $z$ (defined in the text) for ${\nu_\mu}~{\rm CC}$ (left) and ${\nu_e}~{\rm CC}$ (right) candidates in the data (points with error bars) and in the Monte Carlo (histogram). The Monte Carlo distribution of ${\nu_\mu}~{\rm CC}$ events is normalized to the number of ${\nu_\mu}~{\rm CC}$ events in the data; the Monte Carlo distribution of ${\nu_e}~{\rm CC}$ events is normalized using the relative ${\nu_e}~{\rm CC}/{\nu_\mu}~{\rm CC}$ abundance predicted by our simulation. The background contribution to the ${\nu_e}~{\rm CC}$ sample is shown in the hatched histogram.
• Figure 4: Neutrino energy spectra for the data (points with error bars) and the Monte Carlo (histogram), for ${\nu_\mu}~{\rm CC}$ (left) and ${\nu_e}~{\rm CC}$ (right) candidates. The normalization of the Monte Carlo distributions is the same as in Fig. \ref{['fig:cc_zvtx']}. The background contribution to the ${\nu_e}~{\rm CC}$ sample is shown in the hatched histogram.
• Figure 5: The distribution of $r^2$, the square of the radial position of the neutrino interaction vertex with respect to the nominal beam axis, for the data (points with error bars) and the Monte Carlo (histogram), for ${\nu_\mu}~{\rm CC}$ (left) and ${\nu_e}~{\rm CC}$ (right) candidates. The normalization of the Monte Carlo distributions is the same as in the two previous plots. The background contribution to the ${\nu_e}~{\rm CC}$ sample is shown in the hatched histogram.