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The European Spallation Source (ESS), currently under construction in Lund, Sweden, is a research center that will provide, by 2023, the world's most powerful neutron source. The average power of the proton linac will be 5 MW. Pulsing this linac at higher frequency will make it possible to raise the average total beam power to 10 MW to produce, in parallel with the spallation neutron production, a very intense neutrino Super Beam of about 0.4 GeV mean neutrino energy. This will allow searching for leptonic CP violation at the second oscillation maximum where the sensitivity is about 3 times higher than at the first. The ESS neutrino Super Beam, ESSnuSB operated with a 2.0 GeV linac proton beam, together with a large underground Water Cherenkov detector located at 540 km from Lund, will make it possible to discover leptonic CP violation at 5 significance level in 56% (65% for an upgrade to 2.5 GeV beam energy) of the leptonic CP-violating phase range after 10 years of data taking, assuming a 5% systematic error in the neutrino flux and 10% in the neutrino cross section. The paper presents the outstanding physics reach possible for CP violation with ESSnuSB obtainable under these assumptions for the systematic errors. It also describes the upgrade of the ESS accelerator complex required for ESSnuSB.

In this paper, we present the physics performance of the ESSnuSB experiment in the standard three flavor scenario using the updated neutrino flux calculated specifically for the ESSnuSB configuration and updated migration matrices for the far detector. Taking conservative systematic uncertainties corresponding to a normalization error of $$5\%$$ 5 % for signal and $$10\%$$ 10 % for background, we find that there is $$10\sigma $$ 10 σ $$(13\sigma )$$ ( 13 σ ) CP violation discovery sensitivity for the baseline option of 540 km (360 km) at $$\delta _\mathrm{CP} = \pm 90^\circ $$ δ CP = ± 90 ∘ . The corresponding fraction of $$\delta _\mathrm{CP}$$ δ CP for which CP violation can be discovered at more than $$5 \sigma $$ 5 σ is $$70\%$$ 70 % . Regarding CP precision measurements, the $$1\sigma $$ 1 σ error associated with $$\delta _\mathrm{CP} = 0^\circ $$ δ CP = 0 ∘ is around $$5^\circ $$ 5 ∘ and with $$\delta _\mathrm{CP} = -90^\circ $$ δ CP = - 90 ∘ is around $$14^\circ $$ 14 ∘ $$(7^\circ )$$ ( 7 ∘ ) for the baseline option of 540 km (360 km). For hierarchy sensitivity, one can have $$3\sigma $$ 3 σ sensitivity for 540 km baseline except $$\delta _\mathrm{CP} = \pm 90^\circ $$ δ CP = ± 90 ∘ and $$5\sigma $$ 5 σ sensitivity for 360 km baseline for all values of $$\delta _\mathrm{CP}$$ δ CP . The octant of $$\theta _{23}$$ θ 23 can be determined at $$3 \sigma $$ 3 σ for the values of: $$\theta _{23} > 51^\circ $$ θ 23 > 51 ∘ ($$\theta _{23} < 42^\circ $$ θ 23 < 42 ∘ and $$\theta _{23} > 49^\circ $$ θ 23 > 49 ∘ ) for baseline of 540 km (360 km). Regarding measurement precision of the atmospheric mixing parameters, the allowed values at $$3 \sigma $$ 3 σ are: $$40^\circ< \theta _{23} < 52^\circ $$ 40 ∘ < θ 23 < 52 ∘ ($$42^\circ< \theta _{23} < 51.5^\circ $$ 42 ∘ < θ 23 < 51 . 5 ∘ ) and $$2.485 \times 10^{-3}$$ 2.485 × 10 - 3 eV$$^2< \varDelta m^2_{31} < 2.545 \times 10^{-3}$$ 2 < Δ m 31 2 < 2.545 × 10 - 3 eV$$^2$$ 2 ($$2.49 \times 10^{-3}$$ 2.49 × 10 - 3 eV$$^2< \varDelta m^2_{31} < 2.54 \times 10^{-3}$$ 2 < Δ m 31 2 < 2.54 × 10 - 3 eV$$^2$$ 2 ) for the baseline of 540 km (360 km).

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