The ESSnuSB-plus (ESSnuSB+) Project: Status and Prospects

This paper presents the status and prospects of the ESSnuSB+ project, an extension of the ESSnuSB design study that aims to precisely measure neutrino-nucleus cross-sections in the 200–600 MeV range and explore sterile neutrino physics through the development of new facilities and detectors.

The Gist

Imagine the universe as a giant, complex puzzle. For a long time, scientists have been trying to figure out why our universe is made mostly of matter (the stuff we are made of) instead of antimatter (its mirror image, which should have been created in equal amounts). If they were truly equal, they would have cancelled each other out, and we wouldn't exist.

The ESSnuSB project is a massive, high-tech experiment designed to solve this mystery by studying "ghost particles" called neutrinos. Here is a breakdown of what the paper says, using simple analogies.

1. The Main Experiment: ESSnuSB (The Long-Distance Runner)

Think of the ESSnuSB experiment as a high-speed relay race between two cities in Sweden:

• The Start Line (The Accelerator): Located at the European Spallation Source (ESS) in Lund. This is a giant machine that shoots protons (particles) at a target to create a beam of neutrinos.
• The Finish Line (The Detector): Located 360 kilometers away in a deep mine called Zinkgruvan.

The Special Trick:
Most neutrino experiments watch these particles as they pass their first "peak" of activity. ESSnuSB is unique because it waits for them to hit their second peak.

• Analogy: Imagine listening to a song. The first peak is like hearing the chorus loud and clear, but there's a lot of background noise (systematic errors) that makes it hard to hear the subtle details. The second peak is like the song slowing down; the background noise fades away, and the subtle details (the difference between matter and antimatter) become crystal clear.
• The Goal: By measuring this "second peak" with extreme precision, the scientists hope to prove exactly how neutrinos change their identity (oscillate) and why this creates a difference between matter and antimatter. They aim to measure this with such accuracy that they can pick the correct theory explaining why the universe exists.

2. The Problem: Missing Recipe Cards

While the main experiment is great, the scientists realized they were missing a crucial ingredient: precise data on how neutrinos interact with water.

• The Analogy: Imagine you are a chef trying to bake a perfect cake (the main experiment). You have a great oven and a fancy recipe, but you don't know exactly how much flour (neutrino cross-sections) reacts with water at low temperatures. Without this specific data, your cake might not turn out perfectly, no matter how good your oven is.
• The Gap: Current data on how neutrinos bounce off water nuclei at low energies (0.2–0.6 GeV) is either missing or very fuzzy. This uncertainty is the biggest source of error in their measurements.

3. The Solution: ESSnuSB+ (The New Kitchen)

To fix the "missing recipe card" problem, the team proposed ESSnuSB-plus. This is an extension project that builds three new facilities right next to the main experiment to act as a "test kitchen."

• Facility A: The Muon Racetrack (LEnuSTORM): Imagine a circular racetrack where muons (particles related to neutrinos) run in a perfect circle. When they fall off the track, they decay into neutrinos. Because the racetrack is so controlled, the resulting neutrino beam is incredibly clean and predictable.
• Facility B: The Monitored Tunnel (LEMNB): This is a long tunnel where scientists watch every single step of the process. They tag the particles as they are created, ensuring they know exactly what kind of neutrino beam they are sending out.
• Facility C: The "Near-Near" Detector (LEMMOND): This is a small, super-sensitive water tank placed very close to the new facilities.
  • How it works: They shoot the clean, known beams from the racetrack and tunnel into this small water tank. Because they know exactly what went in, they can measure exactly how the neutrinos hit the water. This gives them the "recipe card" they were missing.

4. The Bonus: Hunting for "Sterile" Neutrinos

While building these new facilities, the scientists realized they could use them for a side quest.

• The Analogy: If you are building a new highway, you might as well check if there are any secret, invisible tunnels underneath it.
• The Science: They can use the new setup to look for sterile neutrinos. These are hypothetical particles that don't interact with anything else in the universe (they are "invisible" to normal detectors). The new short-distance setup could prove if these ghostly particles exist.

5. The Tools: AI and New Tech

To make sense of all the data, the team is using advanced technology:

• Graph Neural Networks (GNN): Think of this as a super-smart AI that looks at the messy patterns of light in the water detectors and instantly figures out exactly where a particle hit and what it was. The paper says this AI is very good at pinpointing the location of the interaction.
• Gadolinium: They are also testing adding a special chemical (Gadolinium) to the water. This acts like a "magnet" for neutrons, helping the detectors see even more details of the particle collisions.

Summary

The paper describes a two-step plan:

1. ESSnuSB: A long-distance experiment to solve the mystery of why the universe is made of matter, using a unique "second peak" strategy to get ultra-precise results.
2. ESSnuSB+: A supporting project that builds new, controlled facilities to measure exactly how neutrinos interact with water, removing the biggest source of error from the main experiment. It also opens the door to discovering new, invisible particles.

The ultimate goal is to move from "guessing" how the universe works to "knowing" with high precision, potentially unlocking the secrets of why we are here.

Technical Summary

Technical Summary of the ESSnuSB-plus Project: Status and Prospects

Problem Statement
The primary scientific objective of the next generation of neutrino experiments is to resolve the neutrino mass ordering, determine the quadrant of the atmospheric mixing angle $\theta_{23}$, and prove the existence of CP violation (CPV) in the leptonic sector. However, current and planned Long-Baseline (LBL) experiments lack the precision required to distinguish between theoretical models explaining the matter-antimatter asymmetry of the Universe. A critical bottleneck in achieving this precision is the uncertainty in (anti)neutrino-nucleus cross-sections, particularly in the energy range below 600 MeV/c. While the proposed ESSnuSB experiment reduces the impact of systematics by operating at the second oscillation maximum, the lack of high-precision experimental data in the 0.2–0.6 GeV range remains a dominant source of systematic uncertainty for oscillation parameter measurements.

Methodology and Experimental Design
The paper outlines the status of the ESSnuSB project and the design study for its extension, ESSnuSB-plus (ESSnuSB+).

• ESSnuSB Core Design: The experiment utilizes the 5 MW, 2.5 GeV proton beam from the European Spallation Source (ESS) linear accelerator in Lund, Sweden. The proton beam is modified (pulses shortened from 2.86 ms to 1.2 $\mu$s via an accumulator) to drive a high-intensity neutrino super beam with an energy distribution of 200–600 MeV. The setup features a 360 km baseline to the Zinkgruvan mine, placing the experiment at the second oscillation maximum. This configuration enhances the matter-antimatter asymmetry ($A_{CPV}$) by a factor of 2.5 compared to the first maximum and minimizes matter effects that could mimic CPV. The detector suite includes a near complex (water Cherenkov, super fine-grained scintillator, and emulsion detectors) and a far detector consisting of two 270 kton water-Cherenkov tanks.
• ESSnuSB+ Infrastructure: To address cross-section uncertainties, ESSnuSB+ proposes three new facilities:
  1. Low Energy nuSTORM (LEnuSTORM): A muon racetrack storage ring to generate clean, equal fluxes of muon neutrinos/electron antineutrinos (from $\mu^-$ decay) or muon antineutrinos/electron neutrinos (from $\mu^+$ decay).
  2. Low Energy Monitored Neutrino Beam (LEMNB): A decay tunnel inspired by the ENUBET project, which tags muons from pion decays to provide precise, well-defined neutrino and antineutrino fluxes.
  3. LEMMOND Detector: A new "near-near" cylindrical water-Cherenkov detector (5 m diameter, 10 m length) situated 50 m from the LEnuSTORM or LEMNB facilities. This detector is designed specifically for precise measurements of low-energy (anti)neutrino cross-sections with water.
• Analysis Techniques: The project employs advanced reconstruction methods, including Graph Neural Networks (GNN) for event classification and vertex reconstruction. Simulation studies using GEANT4 and GNNs have been conducted for muon tracks in the 200–600 MeV range.

Key Results and Performance Metrics

• CP Violation Sensitivity: Based on the Conceptual Design Report (CDR), assuming 5% normalization uncertainty and 10 years of data taking (split equally between neutrinos and antineutrinos), ESSnuSB is projected to achieve a CPV discovery sensitivity of approximately $12\sigma$ at maximal violation ($\delta_{CP} \sim \pm 90^\circ$). The experiment is expected to cover more than 70% of the $\delta_{CP}$ range with a significance greater than $5\sigma$ to reject the no-CPV hypothesis. The precision on the measured $\delta_{CP}$ value is projected to be better than $8^\circ$ across all possible true values.
• Cross-Section and Reconstruction: Preliminary studies for the LEMMOND detector indicate high reconstruction capabilities. For muon tracks, angle ($\theta, \phi$) resolutions of less than $1^\circ$ were achieved. Vertex position ($Z$ coordinate) precision was found to be approximately 6 cm with 1.5 ns sensor time resolution and less than 1 cm with 120 ps resolution. GNN-based reconstruction of charged current interactions shows promising results for event classification.
• Additional Physics Capabilities:
  • Sterile Neutrinos: The combination of LEnuSTORM, LEMNB, and LEMMOND creates a Short Baseline (SBL) setup capable of investigating sterile neutrinos with mass-squared differences between 1 and 10 eV$^2$.
  • Atmospheric Neutrinos: Monte Carlo studies suggest the far detector could determine the mass ordering with $3\sigma$ significance in 4 years and resolve the $\theta_{23}$ octant in 4–7 years depending on the ordering.
  • Beyond Standard Model (BSM): The project is evaluating constraints on scalar Non-Standard Interactions (NSI) and Quantum Decoherence parameters.

Significance and Claims
The paper claims that ESSnuSB represents a "next-to-next generation" facility designed to provide the high-precision measurement of the CPV phase necessary to select the correct theoretical model for the Universe's matter-antimatter asymmetry. The significance of the ESSnuSB+ extension lies in its ability to directly address the dominant systematic uncertainty (cross-sections) through dedicated, novel facilities (LEnuSTORM, LEMNB, LEMMOND) that operate in the energy range where data is currently lacking. By integrating these facilities, the project aims not only to refine oscillation parameter measurements but also to expand the physics reach to include sterile neutrino searches and BSM physics investigations, all while operating in parallel with the ESS's primary mission as a neutron source.