Unlocking Neutrino Secrets: The ESSnuSB+ Vision

Author: Denis Avetisyan

A new generation of neutrino experiments, spearheaded by the ESSnuSB+ project, is poised to probe the fundamental properties of these elusive particles and address some of the biggest mysteries in physics.

The ESSnuSB+ project aims to precisely measure CP violation in the neutrino sector, improve our understanding of neutrino interactions, and search for evidence of sterile neutrinos using the European Spallation Source.

Despite persistent ambiguities in the leptonic sector and limitations in current neutrino interaction models, the ν landscape may soon be clarified through ambitious experimental programs like the one detailed in ‘The ESSnuSB-plus (ESSnuSB+) Project: Status and Prospects’. This design study outlines advancements to a long-baseline neutrino experiment leveraging the uniquely powerful European Spallation Source (ESS), with ESSnuSB+ specifically focusing on novel facilities to precisely measure neutrino-nucleus cross-sections between 200-600 MeV and explore the potential for sterile neutrino discovery. By combining enhanced cross-section measurements with a water-Cherenkov detector and a low-energy neutrino beam, the project aims to significantly reduce systematic uncertainties in CP violation studies. Will these advancements unlock a deeper understanding of neutrino properties and finally resolve the matter-antimatter asymmetry puzzle?

The Echo of Asymmetry: A Universe Out of Balance

The universe, as currently understood, should contain equal amounts of matter and antimatter – yet a glaring imbalance exists. This asymmetry represents one of the most profound mysteries in modern physics, as the Big Bang is theorized to have produced both in equal measure. If they were truly equal, matter and antimatter would annihilate each other, leaving a universe filled only with energy. The persistence of matter – everything from stars and galaxies to planets and life itself – implies a slight excess of matter over antimatter in the early universe. Determining the origin of this tiny disparity – estimated to be only about one extra particle of matter for every billion particle-antiparticle pairs – requires unraveling the subtle differences in the behavior of matter and antimatter, a pursuit at the forefront of particle physics research.

The observed prevalence of matter over antimatter in the universe demands an explanation rooted in a fundamental asymmetry in their behavior, specifically through a phenomenon called CP violation. This violation, a subtle distinction in how particles and their antiparticles interact, isn’t a complete prohibition against antimatter creation, but rather a slight preference for matter. Physicists theorize that if CP violation occurs during the decay of certain particles, it could account for the minuscule imbalance necessary to explain the existence of everything around us. Measuring this effect, however, is exceptionally challenging, requiring experiments capable of detecting incredibly rare events and differentiating between particle and antiparticle interactions with unprecedented accuracy. The search for CP violation therefore remains a central focus in particle physics, promising to unlock a deeper understanding of the universe’s origins and its ultimate fate.

Despite significant advancements in neutrino physics, current oscillation experiments – designed to study the transformations between different neutrino flavors – are proving insufficient to fully unravel the mystery of matter-antimatter asymmetry. These experiments, while providing valuable insights into neutrino properties, simply lack the necessary precision to detect the subtle differences in particle behavior required to explain why matter dominates the universe. Consequently, researchers are actively pursuing innovative approaches, including next-generation long-baseline experiments and investigations into charged-lepton flavor violation, hoping to amplify signals and achieve the sensitivity needed to definitively probe $CP$ violation in the lepton sector and, ultimately, address this fundamental imbalance.

The study of atmospheric neutrinos, created when cosmic rays collide with Earth’s atmosphere, offers a crucial first glimpse into the matter-antimatter imbalance, establishing preliminary boundaries for potential explanations. However, these observations are inherently limited by the energy and direction of naturally occurring neutrinos. To truly dissect the subtle differences between matter and antimatter – specifically, CP violation in the lepton sector – physicists are developing dedicated long-baseline neutrino experiments. These experiments, such as the Deep Underground Neutrino Experiment (DUNE), will generate intense beams of neutrinos and track their oscillations over hundreds of kilometers. This precise control and long observation distance allow for detailed measurements of neutrino properties, providing the sensitivity needed to either confirm or rule out current theoretical models and ultimately unravel the mystery of why matter dominates the universe.

A Long-Baseline Vision: Mapping CP Violation

ESSnuSB is designed to investigate Charge-Parity (CP) violation in the leptonic sector through the observation of neutrino oscillations within a long-baseline experimental framework. This involves generating a beam of neutrinos at the European Spallation Source and directing it towards a distant detector, typically several hundred kilometers away. By precisely measuring the probabilities of neutrino flavor transitions – the oscillation – as a function of distance, the experiment aims to determine the $\delta_{CP}$ parameter, which describes the CP-violating phase in the lepton mixing matrix. A long baseline is crucial to maximize the oscillation probability, and the experiment is specifically optimized to operate at the second oscillation maximum, enhancing sensitivity to $\delta_{CP}$ and providing a statistically significant measurement to distinguish between matter and antimatter neutrino behavior.

ESSnuSB’s neutrino beam originates from the European Spallation Source (ESS), a proton accelerator currently under construction in Lund, Sweden. The ESS will deliver a high-intensity proton beam, $5 \text{ MW}$ , impacting a target to produce pions and kaons. These mesons subsequently decay, generating a beam rich in muon neutrinos and, crucially, their associated antineutrinos. The increased beam intensity directly translates to a higher event rate in the detectors, improving statistical precision. Furthermore, a precisely characterized, high-intensity beam allows for a reduction in systematic uncertainties related to flux normalization and energy reconstruction, both critical for accurate neutrino oscillation measurements and CP violation studies.

Water Cherenkov Detectors are employed in ESSnuSB due to their ability to precisely reconstruct neutrino interactions and measure the energy of the resulting leptons. These detectors function by detecting the Cherenkov radiation – a faint cone of light emitted when a charged particle travels through a dielectric medium, such as water, at a speed faster than the speed of light in that medium. The angle of the Cherenkov emission is directly related to the particle’s velocity, allowing for particle identification and energy determination. The large volume of water provides a sufficient target mass for neutrino interactions, while the detectors’ sensitivity to charged particles produced in these interactions enables the measurement of neutrino flavor and oscillation parameters. Specifically, the detectors will identify muons and electrons produced from neutrino interactions, distinguishing between $\nu_e$ , $\nu_\mu$ , and $\nu_\tau$ events and allowing for the reconstruction of the neutrino energy spectrum.

ESSnuSB is designed to operate at the second oscillation maximum, a point in the neutrino oscillation cycle where the probability of neutrino flavor appearance is enhanced, thereby maximizing sensitivity to CP-violating parameters. This operating point allows for a significantly improved measurement of the CP-violation phase $δ_{CP}$ . The experiment’s design is projected to achieve a discovery sensitivity of 12σ for CP violation assuming maximal violation-that is, when $δ_{CP}$ is approximately ±90°. This high level of statistical significance indicates a strong ability to confirm or refute CP violation in the leptonic sector.

Expanding the Horizon: Sterile Neutrinos and Beyond

ESSnuSB+ represents an evolution of the existing ESSnuSB program by incorporating a short-baseline neutrino detection component. This addition is specifically designed to facilitate the search for sterile neutrinos, hypothetical particles not included in the Standard Model. The short baseline-a detector positioned relatively close to the neutrino source-allows for the observation of neutrino oscillations occurring over shorter distances, a key signature expected in models incorporating sterile neutrinos. By analyzing the oscillation patterns at this short baseline, researchers aim to either confirm or refute the existence of these particles and characterize their properties, such as mass and mixing angles.

Precise determination of Neutrino-Nucleus Cross-Sections is a primary goal of ESSnuSB+ due to their significant impact on systematic uncertainties in neutrino oscillation parameter measurements. Current oscillation analyses rely on theoretical models of neutrino interactions with atomic nuclei, which introduce substantial uncertainties, particularly in energy reconstruction and event selection. ESSnuSB+ aims to reduce these uncertainties by directly measuring these cross-sections with increased statistical and systematic precision. This improved knowledge will enable more accurate determination of fundamental neutrino parameters such as the mass hierarchy, $\delta_{CP}$ , and mixing angles, ultimately enhancing the reliability and interpretability of neutrino oscillation results from long-baseline and reactor neutrino experiments.

ESSnuSB+ proposes utilizing novel beam generation technologies to create low-energy neutrino beams, specifically Low Energy nuSTORM and the Low Energy Monitored Neutrino Beam. Low Energy nuSTORM leverages a muon storage ring to produce pions, which subsequently decay into neutrinos; this approach aims for an intense, low-energy neutrino source with a precisely known flux. The Low Energy Monitored Neutrino Beam employs a conventional proton beam interacting with a target, but incorporates advanced monitoring systems to characterize the neutrino flux at the detector with improved accuracy. These technologies are designed to provide neutrino beams peaking below 1 GeV, facilitating studies of neutrino-nucleus interactions and searches for sterile neutrinos with enhanced sensitivity.

The LEMMOND detector, a near-detector Water Cherenkov detector positioned close to the neutrino source, is designed to precisely measure neutrino-nucleus cross-sections with the goal of reducing systematic uncertainties in oscillation analyses. Utilizing advanced data analysis techniques, specifically Graph Neural Networks, LEMMOND aims to achieve a precision of better than 8° on the $δ_{CP}$ parameter across its entire possible range. This enhanced precision will be achieved through detailed reconstruction of neutrino interactions and the application of machine learning algorithms to improve event categorization and background rejection, ultimately contributing to more accurate determinations of fundamental neutrino properties.

Beyond the Standard Model: A Universe Unveiled

The European Spallation Source Neutrino Super Beam (ESSnuSB) and its upgraded iteration, ESSnuSB+, represent a powerful synergistic approach to unraveling some of the most perplexing mysteries in particle physics. These facilities are uniquely positioned to conduct a comprehensive search for CP violation in the lepton sector – a crucial ingredient for explaining the observed matter-antimatter asymmetry in the universe. Simultaneously, the experimental design allows for rigorous investigation into the potential existence of sterile neutrinos, hypothetical particles beyond the Standard Model that could explain anomalies observed in short-baseline neutrino experiments. By combining high statistics with sensitivity to both neutrino and antineutrino oscillations, ESSnuSB and ESSnuSB+ aim to map the CP-violating phase $δ_{CP}$ and constrain the properties of sterile neutrinos, offering a complementary pathway to discoveries at other neutrino facilities and potentially reshaping the understanding of fundamental particles and forces.

The pursuit of highly precise neutrino oscillation parameters serves as a powerful probe for physics beyond the Standard Model, particularly in the search for Non-Standard Interactions (NSIs). These NSIs propose that neutrinos interact with matter in ways not predicted by the Standard Model, potentially altering oscillation probabilities and introducing observable discrepancies. By meticulously measuring parameters like mixing angles, mass-squared differences, and $\delta_{CP}$ , scientists can establish stringent constraints on the strength and form of these novel interactions. Deviations from Standard Model predictions in these parameters would provide compelling evidence for NSIs, offering a window into new physics and potentially resolving long-standing mysteries in particle physics, such as the matter-antimatter asymmetry in the universe. The sensitivity of future experiments, like ESSnuSB+, will be crucial in either confirming or refuting these theoretical extensions and refining our understanding of the fundamental forces governing the cosmos.

Accurate interpretation of neutrino data hinges on a detailed understanding of how these elusive particles interact with the nuclei of atoms. Neutrino experiments, whether designed to probe fundamental properties or cosmological phenomena, rely on predicting the rate and characteristics of these interactions. Uncertainties in nuclear models used to simulate these processes introduce systematic errors that can obscure subtle signals or mimic new physics. Consequently, refining these models – detailing how neutrinos scatter off protons, neutrons, and heavier nuclei – is paramount. Improvements in this area not only enhance the sensitivity of current and future neutrino experiments, but also directly impact cosmological measurements derived from the Cosmic Microwave Background and large-scale structure, where neutrinos play a significant role in shaping the universe’s evolution. By minimizing these uncertainties, scientists can more confidently extract meaningful insights into the nature of neutrinos and their role in the cosmos.

The proposed ESSnuSB+ experiment is designed to achieve unprecedented precision in neutrino measurements, specifically targeting the $δ_{CP}$ parameter which governs CP violation in the leptonic sector. This next-generation facility anticipates exploring over 70% of the possible $δ_{CP}$ values with a statistically significant confidence level exceeding 5σ. Central to this capability is a remarkable angular resolution of less than 1° for reconstructing neutrino interaction vertices, achieved through both 1.5 nanosecond and 120 picosecond timing technologies. Complementing this is a precise determination of the interaction’s Z-coordinate – 6 centimeters for the 1.5 ns sensors and under 1 centimeter for the 120 ps sensors – enabling a 3σ determination of the neutrino mass ordering utilizing atmospheric neutrinos within a four-year timeframe. These combined capabilities promise a substantial leap forward in understanding fundamental neutrino properties and their role in the universe.

The ESSnuSB+ project, with its focus on both CP violation measurements and neutrino cross-section determination, embodies a principle resonant with complex systems thinking. Rather than imposing a rigid, hierarchical structure for understanding neutrino behavior, the experiment encourages the emergence of knowledge from detailed local measurements. As John Dewey observed, “Education is not preparation for life; education is life itself.” Similarly, this experiment doesn’t merely prepare for understanding fundamental physics; the act of precise measurement, the careful consideration of local interactions, is the path toward uncovering deeper truths about the universe. The facility’s design implicitly acknowledges that system outcomes are unpredictable but resilient, and that a comprehensive understanding arises from observing interactions at multiple scales.

Where the Current Flows

The designs detailed within suggest a trajectory not toward control of neutrino behavior, but toward a more nuanced understanding of the rules governing it. The pursuit of CP violation, while framed as a search for asymmetry, may reveal not a deliberate imbalance, but an inevitable consequence of the system’s inherent dynamics. The forest does not choose to grow unevenly; it responds to the distribution of light and resources. Similarly, neutrino oscillations are not a message to be decoded, but a pattern emerging from fundamental interactions.

The addition of dedicated cross-section measurements within ESSnuSB+ is particularly telling. It acknowledges the limitations of imposing theoretical frameworks onto observed phenomena. Precise measurements, while never complete, offer a more grounded approach – a mapping of the terrain, rather than a construction of a pre-defined landscape. The search for sterile neutrinos, a subtle disruption to the established order, highlights a willingness to accept the possibility that the current model is, at best, an approximation.

The future of neutrino physics lies not in finding the answer, but in refining the questions. Each iteration of experiment and analysis will not bring the field closer to a final, definitive truth, but reveal ever more intricate layers of complexity. It is a process of continual adaptation, mirroring the evolution of the system itself. The current flows where resistance is least, and the path forward will be revealed not by design, but by observation.

Original article: https://arxiv.org/pdf/2601.15826.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

The Echo of Asymmetry: A Universe Out of Balance

A Long-Baseline Vision: Mapping CP Violation

Expanding the Horizon: Sterile Neutrinos and Beyond

Beyond the Standard Model: A Universe Unveiled

Where the Current Flows

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