Decisive disappearance search at high $\mathrm{\Delta }{m}^2$ with monoenergetic muon neutrinos

“KPipe” is a proposed experiment which will study muon neutrino disappearance for a sensitive test of the $\mathrm{\Delta }{m}^2\sim 1{\mathrm{eV}}^2$ anomalies, possibly indicative of one or more sterile neutrinos. The experiment is to be located at the J-PARC Materials and Life Science Experimental Facility’s spallation neutron source, which represents the world’s most intense source of charged kaon decay-at-rest monoenergetic (236 MeV) muon neutrinos. The detector vessel, designed to measure the charged-current interactions of these neutrinos, will be 3 m in diameter and 120 m long, extending radially at a distance of 32 to 152 m from the source. This design allows a sensitive search for ${\nu }_{\mu }$ disappearance associated with currently favored light sterile neutrino models and features the ability to reconstruct the neutrino oscillation wave within a single, extended detector. The required detector design, technology, and costs are modest. The KPipe measurements will be robust since they depend on a known energy neutrino source with low expected backgrounds. Further, since the measurements rely only on the measured rate of detected events as a function of distance, with no required knowledge of the initial flux and neutrino interaction cross section, the results will be largely free of systematic errors. The experimental sensitivity to oscillations, based on a shape-only analysis of the $L/E$ distribution, will extend an order of magnitude beyond present experimental limits in the relevant high-$\mathrm{\Delta }{m}^2$ parameter space.

Figure 1

An aerial view from Google Maps (2015) of the Materials and Life Science Experimental Facility layout with a superimposed schematic drawing [30] of the first floor, including the target station. The proposed KPipe location (shown with a dotted contour) is 32 m from the target station and 102° with respect to the incident proton beam direction. The detector extends radially outward from the target station.

Figure 2

The KPipe detector design, featuring a 3 m inner diameter HDPE vessel filled with liquid scintillator. SiPMs are seen mounted on the interior panels in hoops spaced by 10 cm in the longitudinal direction. The CR veto is a 10 cm space between the panels and the outer HDPE wall.

Figure 3

Left: The muon neutrino and antineutrino flux with $-0.25<\cos {\theta }_z<-0.16$, representative of the full detector length, where ${\theta }_z$ is the neutrino angle with respect to the proton direction ($+z$). Right: The neutrino creation time relative to the two beam pulses (dotted lines). This distribution includes neutrinos emitted over all solid angles and energies.

Figure 4

The ${\nu }_{\mu }$ charged current event rate, for neutrinos with $-0.25<\cos {\theta }_z<-0.16$, along with the employed ${\nu }_{\mu }$ CC cross section. The monoenergetic 236 MeV neutrino signal is clearly visible above the background nonmonoenergetic events, mainly coming from kaon decay in flight.

Figure 5

The muon and total kinetic energy (${\mathrm{KE}}_{\text{tot}}={\mathrm{KE}}_{\mu }+\sum {\mathrm{KE}}_{\mathrm{p}}$) for the signal 236 MeV ${\nu }_{\mu }$ charged current events compared to all other ${\nu }_{\mu }$. Only neutrinos with $-0.25<\cos {\theta }_z<-0.16$ are considered. The ratio of integrated signal (black) to background (red) is $66:1$.

Figure 6

The number of photoelectrons in a 236 MeV ${\nu }_{\mu }$ CC event’s prompt signal versus the total kinetic energy (${\mathrm{KE}}_{\text{tot}}={\mathrm{KE}}_{\mu }+\sum {\mathrm{KE}}_{\mathrm{p}}$).

Figure 7

The total kinetic energy (${\mathrm{KE}}_{\text{tot}}={\mathrm{KE}}_{\mu }+\sum {\mathrm{KE}}_{\mathrm{p}}$) versus the energy of neutrinos from CC interactions in KPipe. Only neutrinos with $-0.25<\cos {\theta }_z<-0.16$ are considered. The Z-axis units are arbitrary.

Figure 8

Three sample oscillation probability measurements as a function of $L$ for three years of running. The error bars incorporate statistical uncertainties of both the ${\nu }_{\mu }$ signal and the cosmic ray background. The equivalent range of observable $L/E$ corresponds to 0.14 to $0.64\mathrm{m}/\mathrm{MeV}$.

Figure 9

The projected sensitivity of KPipe to muon neutrino disappearance with three years of running, including the cosmic ray background, signal efficiencies, and reconstruction uncertainties described in the text. The red contours are the global allowed regions given by Collin et al. [57].

Figure 10

The 90% C.L. sensitivity of KPipe with six years of running, compared to the sensitivity from six years of the SBN program. The KPipe sensitivity estimate includes the cosmic ray background, signal efficiencies, and reconstruction uncertainties described in the text.