Complementarity between atmospheric and super-beam neutrinos at ESSnuSB

Abstract

The ESSnuSB experiment aims to measure the leptonic CP phase $δ_{CP}$ with an unprecedented resolution by probing neutrino oscillations at the second oscillation maximum. In the present work, the complementarity between the long-baseline neutrino program and atmospheric neutrinos is investigated for ESSnuSB. By simulating atmospheric neutrino events equivalent of 5.4 Mt$\cdot$year exposure, the resolution for $δ_{\rm CP}^{}$ is found to improve from $7.5^\circ$ ($6.7^\circ$) to $7.1^\circ$ ($6.5^\circ$) at $1σ$~CL for $δ_{\rm CP}^{} = -90^\circ$ ($+90^\circ$) with respect to super-beam neutrinos, resolving also the degeneracies arising from neutrino mass ordering. These findings highlight the synergies that exist between super-beam neutrinos and atmospheric neutrinos in ESSnuSB.

Figures (7)

• Figure 1: Neutrino oscillation probability $P_{\nu_{\mu}^{} \rightarrow \nu_{e}^{}}$ as a function of neutrino energy $E_\nu^{}$ for the ESSnuSB setup. The first and second oscillation maxima are found around $E_\nu^{} \sim 0.65$ GeV and $0.25$ GeV, respectively. The probability is shown for $\delta_{\rm CP}^{} = 0$ (solid curve)$,\delta_{\rm CP}^{} = \pi/2$ (dashed curve)$, \delta_{\rm CP}^{} = \pi$ (dot-dashed curve) and $\delta_{\rm CP}^{} = 3\pi/2$ (dotted curve). Neutrino mass ordering is assumed to be NO. The shaded histogram represents the $\nu_\mu^{}$ fluxes that have been obtained at $100$ km distance. The neutrino fluxes have been obtained from Ref. Alekou:2022emd.
• Figure 2: Correlation between the neutrino oscillation probabilities $P(\nu_\mu^{} \rightarrow \nu_e^{})$ and $P(\overline{\nu}_\mu^{} \rightarrow \overline{\nu}_e^{})$ for super-beam neutrinos with baseline length $L = 360$ km. The probabilities are obtained for neutrino energies $E_\nu = 0.25$ GeV (left panel) and $0.65$ GeV (right panel), which coincide with the second oscillation maximum and the first oscillation maximum, respectively. The correlations are shown for NO (black curves) and IO (red curves) at $\sin_{}^2 \theta_{23}^{} = 0.470$ (solid curves) and $\sin_{}^2 \theta_{23}^{} = 0.423$ (dashed curves). Note that the probabilities are shown up to $0.10$ ($0.07$) for the second (first) oscillation maximum.
• Figure 3: Atmospheric neutrino events expected for ESSnuSB FD. The expected $e$-like event (upper left) and $\mu$-like event (lower left) samples are shown for an equivalent of 5.4 Mt$\cdot$year exposure taking into account the detector effects. The relative differences in events computed for $\sin_{}^2 \theta_{23}^{} = 0.470$ and $\sin_{}^2 \theta_{23}^{} = 0.517$ are displayed for $e$-like events (upper right) and $\mu$-like events (lower right), where $\sin_{}^2 \theta_{23}^{} = 0.470$ is the central value. Neutrino mass ordering is assumed to be NO.
• Figure 4: Allowed values for $\sin_{}^{2}\theta_{23}^{}$ and $\Delta m_{31}^2$ (left panel) and for $\sin_{}^{2}\theta_{23}^{}$ and $\delta_{\rm CP}^{}$ (right panel) by analyzing data for atmospheric neutrinos (blue regions) and super-beam neutrinos (yellow regions) at ESSnuSB FD (2 dof). The allowed values are shown at $2\sigma$ CL (darker colors) and $3\sigma$ CL (lighter colors), while the true values for $\sin_{}^{2} \theta_{23}^{}$ and $\Delta m_{31}^2$ are indicated by the black stars. The true neutrino mass ordering is assumed to be NO. Note that no priors are used for $\sin_{}^2 \theta_{23}^{}, \Delta m_{31}^2$ or $\delta_{\rm CP}^{}$.
• Figure 5: Effect of neutrino zenith angle resolution on the precisions at which $\sin^2_{} \theta_{23}^{}$ (left panel) and $|\Delta m_{31}^2|$ (right panel) could be measured by analyzing atmospheric neutrino data for ESSnuSB FD (2 dof). The neutrino zenith angle resolution is varied for the MC events in the sub-GeV neutrino energy bins. The results are indicated by the blue rectangles, which have been fitted with cubic splines. The true neutrino mass ordering is assumed to be NO.