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The European Spallation Source (ESS) linear accelerator is designed to accelerate a 62.5 mA, 2.86 ms, 14 Hz proton beam up to 2 GeV at the entrance of a rotating tungsten (W) target. Normal Conducting Linac (NCL) and Superconducting Linac (SCL) parts of the accelerator contain over 200 quadrupole, dipole and corrector magnets of different types and parameters for beam envelope and trajectory control. All magnets have been provided to ESS by in-kind collaborators and research institutes across Europe. Following the delivery of these components and their respective power converters to ESS, this proceeding presents the status of the installation together with the methodology and first obtained results from system local and integrated testing phase.

The innovative FEL design by the CompactLight Collaboration is briefly presented. The major contribution of our team, mainly from Greece cooperating with the team of ESS-ERIC, is emphasised to the material selection of the proper photocathode illuminated with the relevant laser, the magnet and solenoid design of the injector for optimum beam conditions, the 3D CAD model design of the baseline FEL facility including the machine, the tunnel, the beam dump and the experimental hall. Additionally, the exploitation assets of the project with the transfer technology are presented.

A bstract This study provides an analysis of atmospheric neutrino oscillations at the ESSnuSB far detector facility. The prospects of the two cylindrical Water Cherenkov detectors with a total fiducial mass of 540 kt are investigated over 10 years of data taking in the standard three-flavor oscillation scenario. We present the confidence intervals for the determination of mass ordering, θ 23 octant as well as for the precisions on sin 2 θ 23 and Δ m 31 2 . It is shown that mass ordering can be resolved by 3 σ CL (5 σ CL) after 4 years (10 years) regardless of the true neutrino mass ordering. Correspondingly, the wrong θ 23 octant could be excluded by 3 σ CL after 4 years (8 years) in the case where the true neutrino mass ordering is normal ordering (inverted ordering). The results presented in this work are complementary to the accelerator neutrino program in the ESSnuSB project.

ESSnuSB is a design study for an experiment to measure the CP violation in the leptonic sector at the second neutrino oscillation maximum using a neutrino beam driven by the uniquely powerful ESS linear accelerator. The reduced impact of systematic errors on sensitivity at the second maximum allows for a very precise measurement of the CP violating parameter. This review describes the fundamental advantages of measurement at the second maximum, the necessary upgrades to the ESS linac in order to produce a neutrino beam, the near and far detector complexes, and the expected physics reach of the proposed ESSnuSB experiment, concluding with the near future developments aimed at the project realization.

ESSnuSB is a design study for an experiment to measure the CP violation in the leptonic sector at the second neutrino oscillation maximum using a neutrino beam driven by the uniquely powerful ESS linear accelerator. The reduced impact of systematic errors on sensitivity at the second maximum allows for a very precise measurement of the CP violating parameter. This review describes the fundamental advantages of measurement at the 2nd maximum, the necessary upgrades to the ESS linac in order to produce a neutrino beam, the near and far detector complexes, the expected physics reach of the proposed ESSnuSB experiment, concluding with the near future developments aimed at the project realization.

This conceptual design report provides a detailed account of the European Spallation Source neutrino Super Beam (ESS\\(\\nu\\)SB) feasibility study. This facility has been proposed after the measurements reported in 2012 of a relatively large value of the neutrino mixing angle \\(\\theta_{13}\\), which raised the possibility of observing potential CP violation in the leptonic sector with conventional neutrino beams. The measured value of \\(\\theta_{13}\\) also privileges the \\(2^{nd}\\) oscillation maximum for the discovery of CP violation instead of the more typically studied \\(1^{st}\\) maximum. The sensitivity at this \\(2^{nd}\\) oscillation maximum is about three times higher than at the \\(1^{st}\\) one, which implies a reduced influence of systematic errors. Working at the \\(2^{nd}\\) oscillation maximum requires a very intense neutrino beam with an appropriate energy. The world's most intense pulsed spallation neutron source, the European Spallation Source (ESS), will have a proton linac operating at 5\\,MW power, 2\\,GeV kinetic energy and 14~Hz repetition rate (3~ms pulse duration, 4\\% duty cycle) for neutron production. In this design study it is proposed to double the repetition rate and compress the beam pulses to the level of microseconds in order to provide an additional 5~MW proton beam for neutrino production. The physics performance has been evaluated for such a neutrino super beam, in conjunction with a megaton-scale underground water Cherenkov neutrino detector installed at a distance of 360--550\\,km from ESS. The ESS proton linac upgrades, the accumulator ring required for proton-pulse compression, the target station design and optimisation, the near and far detector complexes, and the physics potential of the facility are all described in this report. The ESS linac will be operational by 2025, at which point the implementation of upgrades for the neutrino facility could begin.

The ESSnuSB experiment aims to measure the leptonic CP phase \\(\\delta_{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 \\(\\delta_{\\rm CP}^{}\\) is found to improve from \\(7.5^\\circ\\) (\\(6.7^\\circ\\)) to \\(7.1^\\circ\\) (\\(6.5^\\circ\\)) at \\(1\\sigma\\)~CL for \\(\\delta_{\\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.

Atmospheric neutrinos provide a unique avenue to study neutrino interactions in matter. In this work, the prospects of constraining non-standard neutrino interactions with atmospheric neutrino oscillations are investigated for the proposed ESSnuSB far detector. By analyzing atmospheric neutrino samples equivalent to 5.4 Mt\\(\\cdot\\)year exposure, it is found that ESSnuSB could be able to set the upper bounds \\(|\\epsilon_{e\\mu}^m| < 0.053, |\\epsilon_{e\\tau}^m| < 0.057, |\\epsilon_{\\mu\\tau}^m| < 0.021, \\epsilon_{ee}^m - \\epsilon_{\\mu\\mu}^m < 0.075\\) and \\(|\\epsilon_{\\tau\\tau}^m - \\epsilon_{\\mu\\mu}^m| < 0.031\\) at \\(90\\%\\) CL, when the results are minimized for \\(\\phi_{e\\mu}^m, \\phi_{e\\tau}^m\\) and \\(\\phi_{\\mu\\tau}^m\\) and normal ordering is assumed for neutrino masses. It is also shown that the presence of non-standard interactions could affect the sensitivities to neutrino mass ordering and \\(\\theta_{23}^{}\\) octant in comparison to the standard interaction scheme. The results of this work highlight the complementarity between atmospheric and accelerator neutrino programs in ESSnuSB.

Neutrino oscillations constitute an excellent tool to probe physics beyond the Standard Model. In this paper, we investigate the potential of the ESSnuSB experiment to constrain the effects of flavour-dependent long-range forces (LRFs) in neutrino oscillations, which may arise due to the extension of the Standard Model gauge group by introducing new \\(U(1)\\) symmetries. Focusing on three specific \\(U(1)\\) symmetries -- \\(L_e - L_\\), \\(L_e - L_\\), and \\(L_ - L_\\), we demonstrate that ESSnuSB offers a favourable environment to search for LRF effects. Our analyses reveal that ESSnuSB can set \\(90\\%\\) confidence level bounds of \\(V_e < 2.99 10^-14 \\, eV\\), \\(V_e < 2.05 10^-14 \\, eV\\), and \\(V_ < 1.81 10^-14 \\, eV\\), which are competitive to the upcoming Deep Underground Neutrino Experiment (DUNE). It is also observed that reducing the systematic uncertainties from \\(5\\%\\) to \\(2\\%\\) improves the ESSnuSB limits on \\(V_\\). Interestingly, we find limited correlations between LRF parameters and the less constrained lepton mixing parameters \\(_23\\) and \\(_CP\\), preserving the robustness of ESSnuSB's sensitivity to CP violation. Even under extreme LRF potentials (\\(V_ 10^-13 \\, eV\\)), the CP-violation sensitivity and \\(_CP\\) precision remain largely unaffected. These results establish ESSnuSB as a competitive experimental setup for probing LRF effects, complementing constraints from other neutrino sources and offering critical insights into the physics of long-range forces.

In the effort to obtain a precise measurement of leptonic CP-violation with the ESS\\(\\nu\\)SB experiment, accurate and fast reconstruction of detector events plays a pivotal role. In this work, we examine the possibility of replacing the currently proposed likelihood-based reconstruction method with an approach based on Graph Neural Networks (GNNs). As the likelihood-based reconstruction method is reasonably accurate but computationally expensive, one of the benefits of a Machine Learning (ML) based method is enabling fast event reconstruction in the detector development phase, allowing for easier investigation of the effects of changes to the detector design. Focusing on classification of flavour and interaction type in muon and electron events and muon- and electron neutrino interaction events, we demonstrate that the GNN reconstructs events with greater accuracy than the likelihood method for events with greater complexity, and with increased speed for all events. Additionally, we investigate the key factors impacting reconstruction performance, and demonstrate how separation of events by pion production using another GNN classifier can benefit flavour classification.