Shadow Signals: How Dark Matter Could Warp Neutrino Physics

Author: Denis Avetisyan

New research explores the subtle influence of interactions between neutrinos and dark matter on measurements of CP violation, a key to understanding matter-antimatter asymmetry.

The study demonstrates that precision in determining CP violation for the DUNE and T2HK experiments is sensitive to the presence of off-diagonal dark Non-Standard Interactions (NSIs)-specifically, <span class="katex-eq" data-katex-display="false">d_{e\mu}</span>, <span class="katex-eq" data-katex-display="false">d_{e\tau}</span>, and <span class="katex-eq" data-katex-display="false">d_{\mu\tau}</span>-with the sensitivity analysis conducted under the assumption of a normal neutrino mass ordering and a <span class="katex-eq" data-katex-display="false">\delta_{CP} = -{90}^{\circ}</span>, while acknowledging inherent uncertainty reflected in the true value of <span class="katex-eq" data-katex-display="false">\phi_{\alpha\beta}</span> ranging from -180 to 180 degrees. — The study demonstrates that precision in determining CP violation for the DUNE and T2HK experiments is sensitive to the presence of off-diagonal dark Non-Standard Interactions (NSIs)-specifically, $d_{e\mu}$ , $d_{e\tau}$ , and $d_{\mu\tau}$ -with the sensitivity analysis conducted under the assumption of a normal neutrino mass ordering and a $\delta_{CP} = -{90}^{\circ}$ , while acknowledging inherent uncertainty reflected in the true value of $\phi_{\alpha\beta}$ ranging from -180 to 180 degrees.

Synergistic data from the DUNE and T2HK long-baseline neutrino experiments will be crucial to constrain non-standard interactions and enhance sensitivity to CP violation in the presence of scalar dark matter.

Despite the Standard Model’s successes, the nature of dark matter remains elusive, prompting investigations into potential interactions beyond established physics. This paper, ‘Dark NSI & neutrino oscillations : probing via $δ_{CP}$ measurements at DUNE and T2HK’, explores the consequences of neutrinos scattering off a scalar dark matter background – termed ‘dark non-standard interactions’ – and their impact on observed neutrino oscillation probabilities. We demonstrate that these interactions can significantly alter CP-violation sensitivity in upcoming long-baseline experiments, but that combining data from DUNE and T2HK can effectively mitigate parameter degeneracies and potentially restore, or even enhance, CP sensitivity. Could a synergistic approach to neutrino experiments unlock new insights into both dark matter and fundamental neutrino properties?

The Ghostly Neutrino: A Crack in the Standard Model

The Standard Model of particle physics, a remarkably successful framework describing the fundamental constituents of the universe and their interactions, nevertheless encounters a significant challenge when confronted with neutrinos. Initially predicted to be massless, experimental evidence from neutrino oscillations demonstrably proves they possess mass – a property not accommodated within the Standard Model’s original formulation. This discrepancy isn’t merely a minor adjustment; it signals a fundamental incompleteness in our understanding of particle physics. The observed neutrino masses and the associated mixing between neutrino ‘flavors’ – electron, muon, and tau – necessitate the existence of physics beyond the Standard Model, prompting researchers to explore extensions such as sterile neutrinos, extra dimensions, or entirely new interactions. Unraveling the mystery of neutrino mass is therefore considered a crucial step towards a more complete and accurate description of the universe at its most fundamental level.

The seemingly bizarre behavior of neutrinos – their ability to morph between three distinct ‘flavors’ (electron, muon, and tau) as they travel – is known as neutrino oscillation, and it presents a fundamental challenge to the established Standard Model of particle physics. This transformation isn’t a decay, but rather a quantum mechanical phenomenon indicating that neutrinos possess mass – a property the Standard Model originally predicted they lacked. Experiments meticulously tracking neutrinos from sources like the sun, nuclear reactors, and particle accelerators have consistently demonstrated these oscillations, revealing that a neutrino born as one flavor will likely be detected as a mixture of all three. The probability of observing each flavor at a given distance is governed by parameters related to the neutrino masses and mixing, effectively proving that the Standard Model is incomplete and paving the way for explorations into new physics that can account for these elusive particles and their surprising ability to change identity.

The subtle shifts in neutrino identity, known as oscillations, aren’t random; they’re governed by a precise set of parameters that hold the key to understanding these elusive particles. Scientists meticulously measure these parameters – specifically, the differences in the squares of the neutrino masses $\Delta m^2_{ij}$ and the angles dictating how neutrinos change ‘flavor’ (electron, muon, and tau) – to build a comprehensive picture of neutrino behavior. Determining these values isn’t merely an exercise in cataloging; it reveals the underlying structure of the neutrino sector and offers vital clues about physics beyond the Standard Model. Each precisely determined parameter constrains theoretical models, guiding researchers toward a more complete understanding of fundamental symmetries, potential new particles, and the role of neutrinos in the evolution of the universe.

The Feynman diagram illustrates neutrino forward scattering involving the scalar φ from Ge:2019tdi.

The Next Generation of Neutrino Observatories

The Deep Underground Neutrino Experiment (DUNE) and the Hyper-Kamiokande (T2HK) represent the next generation of long-baseline neutrino oscillation experiments. Both projects are designed to achieve high-precision measurements of fundamental neutrino parameters, including mixing angles and mass-squared differences. DUNE will utilize a beam power of 1.2 MW, while T2HK is planned for 1.3 MW, both significantly exceeding the capabilities of previous generation experiments like T2K. These increased beam powers, coupled with large detector masses and long baselines-1300 km for DUNE and 295 km for T2HK-are critical for accumulating sufficient statistics to precisely characterize neutrino oscillations and search for CP violation in the lepton sector.

The Deep Underground Neutrino Experiment (DUNE) utilizes Liquid Argon Time Projection Chamber (LArTPC) technology, where argon atoms are ionized by neutrino interactions, and the resulting electrons are drifted in an electric field, allowing for three-dimensional reconstruction of the event. This provides detailed topological information and enables precise particle identification. The Super-Kamiokande Generation II experiment (T2HK), conversely, employs Water Cherenkov Detectors, which observe the Cherenkov radiation emitted when charged particles travel faster than light in water. While lacking the fine-grained tracking of LArTPCs, Water Cherenkov Detectors offer high efficiency and a large effective detector volume, crucial for observing the relatively rare neutrino interactions. Both technologies aim for high-fidelity reconstruction of neutrino events, but achieve this through fundamentally different physical principles and detector designs.

The Deep Underground Neutrino Experiment (DUNE), with a 1300 km baseline, and the Hyper-Kamiokande experiment (T2HK), at 295 km, are designed with complementary sensitivities to neutrino oscillation parameters. This differing baseline length allows for optimized measurement of the oscillation probability at different oscillation maxima and minima. Specifically, the longer baseline of DUNE enhances sensitivity to the atmospheric mass splitting and $\theta_{23}$ , while T2HK excels in precision measurements of $\theta_{13}$ and the CP-violating phase $\delta_{CP}$ . Combined data from both experiments will significantly reduce systematic uncertainties and resolve parameter degeneracies, ultimately enabling a precise determination of the neutrino mixing matrix and providing insights into CP violation in the leptonic sector.

The difference in muon neutrino energy deposition <span class="katex-eq" data-katex-display="false">\Delta P_{\mu e} = |P_{\mu e}^{DNSI}-P_{\mu e}^{SI}|</span> as a function of <span class="katex-eq" data-katex-display="false">\delta_{CP}</span> for DUNE and T2HK, with a fixed peak energy of approximately 2.5 GeV for DUNE and 0.5 GeV for T2HK, reveals sensitivity to <span class="katex-eq" data-katex-display="false">\phi_{\alpha\beta}</span> when <span class="katex-eq" data-katex-display="false">d_{\alpha\beta} = 0.02</span>. — The difference in muon neutrino energy deposition $\Delta P_{\mu e} = |P_{\mu e}^{DNSI}-P_{\mu e}^{SI}|$ as a function of $\delta_{CP}$ for DUNE and T2HK, with a fixed peak energy of approximately 2.5 GeV for DUNE and 0.5 GeV for T2HK, reveals sensitivity to $\phi_{\alpha\beta}$ when $d_{\alpha\beta} = 0.02$ .

Simulating Reality: Testing Our Understanding

GLoBES (General Long Baseline Experiment Simulator) is a numerical framework utilized for the detailed analysis of neutrino oscillation experiments, notably the Deep Underground Neutrino Experiment (DUNE) and the Tokai to Hyper-Kamiokande (T2HK) experiments. The software allows researchers to model the complete experimental response, including neutrino propagation, detector effects, and data reconstruction. This capability facilitates optimization of experimental parameters – such as detector mass, baseline length, and beam energy – to maximize sensitivity to fundamental neutrino properties. Furthermore, GLoBES enables comprehensive assessments of experimental sensitivity to parameters like the mass hierarchy, $\theta_{23}$ , $\theta_{13}$ , and the CP-violating phase $\delta_{CP}$ .

Accurate modeling of systematic uncertainties is paramount in neutrino oscillation experiments due to their potential to obscure or mimic signals from fundamental parameters. Simulations, such as those performed with GLoBES, allow researchers to quantify the impact of these uncertainties – arising from detector effects, imperfect knowledge of the neutrino beam, and reconstruction algorithms – on oscillation parameter measurements. Furthermore, disentangling the effects of individual oscillation parameters – $\theta_{13}$ , $\theta_{23}$ , $\delta_{CP}$ , and the mass hierarchy – requires detailed simulations that can isolate and quantify their contributions to observed event rates and energy spectra. By propagating systematic uncertainties through the simulation framework, researchers can develop strategies to minimize their impact and improve the precision with which these parameters can be determined.

Comparison of numerical simulation outputs – generated by frameworks like GLoBES – with data acquired from neutrino oscillation experiments provides a rigorous method for testing the predictions of the Standard Model. Discrepancies between simulation and experiment can indicate the presence of new physics beyond the Standard Model, and neutrino oscillation experiments are particularly sensitive to phenomena like CP violation in the leptonic sector. CP violation, if observed, would require an extension to the Standard Model to accommodate the asymmetry between matter and antimatter. Analysis focuses on parameters describing neutrino mixing – the $\theta_{12}, \theta_{13}, \theta_{23}$ angles and the Dirac CP phase $\delta_{CP}$ – to identify deviations from expected values and constrain potential new physics models.

The Shadow of Dark Matter: Unseen Interactions

The universe’s substantial dark matter component, while largely invisible, isn’t necessarily aloof from all other matter. Current theoretical frameworks explore the possibility of interactions between dark matter and Standard Model particles, with neutrinos being a particularly intriguing focus. These interactions, termed Non-Standard Interactions (Dark NSI), could subtly alter the behavior of neutrinos as they travel through space, specifically impacting their oscillation patterns – the process by which neutrinos change ‘flavor’ between electron, muon, and tau types. Because neutrino oscillations are exquisitely sensitive to even minute changes in fundamental parameters, Dark NSI present a potential pathway for detecting dark matter indirectly. Any deviation from predicted oscillation patterns could signal the presence of these interactions, offering a crucial window into the nature of dark matter and its role in the cosmos. The effects of Dark NSI are predicted to modify the neutrino Hamiltonian, creating a measurable signature in long-baseline neutrino experiments.

A compelling explanation for the observed discrepancies in neutrino behavior may lie in the realm of dark matter, specifically through interactions mediated by complex scalar dark matter particles. This theoretical framework proposes that dark matter doesn’t simply exert gravitational influence, but actively participates in fundamental particle interactions, altering the standard model predictions for neutrino oscillations. The introduction of a complex scalar field allows for couplings that modify the neutrino Hamiltonian – the mathematical description of neutrino propagation – introducing new terms that can both enhance and suppress certain oscillation channels. Crucially, these alterations can significantly impact CP violation, a fundamental symmetry of nature, potentially masking or mimicking effects that would otherwise reveal insights into the matter-antimatter asymmetry of the universe. This complex interplay necessitates a refined understanding of neutrino physics, pushing the boundaries of current experimental capabilities and theoretical models to disentangle the contributions of standard oscillations from those induced by dark matter interactions.

Recent investigations reveal a pathway to overcome the challenges posed by potential interactions between dark matter and neutrinos. Simulations indicate that non-standard interactions (NSIs) can significantly diminish the ability to detect CP violation – a crucial asymmetry in the behavior of matter and antimatter – within neutrino oscillation experiments. However, this research demonstrates a compelling solution: combining data from the Deep Underground Neutrino Experiment (DUNE) and the Hyper-Kamiokande (T2HK) facilities effectively counteracts these suppressive effects. Specifically, when the strength of the interaction between muon and tau neutrinos, denoted as $|d_{\mu\tau}|$ , equals 0.02, the combined dataset restores CP violation sensitivity to levels comparable to those predicted by the standard model of neutrino oscillations, offering a promising avenue for unraveling the mysteries of both dark matter and fundamental particle physics.

Simulations for a 295 km baseline reveal that non-standard interactions (NSIs) involving electron and muon neutrinos, parameterized by <span class="katex-eq" data-katex-display="false">\eta_{e\mu}</span>, <span class="katex-eq" data-katex-display="false">\eta_{e\tau}</span>, and <span class="katex-eq" data-katex-display="false">\eta_{\mu\tau}</span>, can significantly alter the appearance probabilities <span class="katex-eq" data-katex-display="false">P_{\mu e}</span> (top) and <span class="katex-eq" data-katex-display="false">P_{\mu \mu}</span> (bottom) for the T2HK experiment, assuming <span class="katex-eq" data-katex-display="false">\delta_{CP} = -{90}^{\circ}</span> and <span class="katex-eq" data-katex-display="false">\phi_{\alpha\beta} = 0^{\circ}</span>. — Simulations for a 295 km baseline reveal that non-standard interactions (NSIs) involving electron and muon neutrinos, parameterized by $\eta_{e\mu}$ , $\eta_{e\tau}$ , and $\eta_{\mu\tau}$ , can significantly alter the appearance probabilities $P_{\mu e}$ (top) and $P_{\mu \mu}$ (bottom) for the T2HK experiment, assuming $\delta_{CP} = -{90}^{\circ}$ and $\phi_{\alpha\beta} = 0^{\circ}$ .

The pursuit of precise CP violation measurements, as detailed in the study of neutrino oscillations, necessitates a rigorous approach to disentangling genuine effects from systematic uncertainties and novel physics. This mirrors a fundamental tenet of scientific inquiry: the necessity of repeated testing and refinement. As René Descartes stated, “Doubt is not a pleasant condition, but it is necessary for a clear understanding.” The research demonstrates that relying on a single experiment, even one as powerful as DUNE or T2HK, is insufficient; parameter degeneracies inherent in the data require the synergistic combination of results. This echoes the spirit of Cartesian doubt-a commitment to questioning assumptions and seeking robust evidence before arriving at conclusions, especially when probing the subtle interactions between neutrinos and potential dark matter candidates.

Where Do We Go From Here?

The pursuit of neutrino CP violation remains, as ever, a search for subtle asymmetry in a universe brimming with symmetry. This work highlights a crucial, if frustrating, truth: every dataset is just an opinion from reality, and the interpretation of those opinions is further complicated by the potential for interactions beyond the Standard Model. The exploration of dark Non-Standard Interactions, while theoretically motivated, introduces a landscape of parameters where degeneracies abound. The devil isn’t in the details-he’s in the outliers, and distinguishing a genuine CP-violating phase from the shadow of dark matter effects will demand exquisite precision.

Future endeavors shouldn’t focus solely on increasing statistics. While DUNE and T2HK, in synergy, offer a path forward, the study rightly points to the necessity of considering multi-messenger approaches. Complementary searches for dark matter signatures – direct detection, astrophysical observations – are not merely parallel pursuits, but vital cross-checks. A consistent picture will only emerge if these seemingly disparate fields speak to each other, and the current framework needs expansion to explicitly incorporate those potential dialogues.

Ultimately, the most valuable outcome may not be a definitive measurement of δ_CP, but a sharper understanding of the limitations inherent in any attempt to map the unseen universe. The search for physics beyond the Standard Model is, at its core, a rigorous exercise in self-doubt – a continuous refinement of our models in the face of persistent uncertainty. The real progress lies not in confirming expectations, but in systematically dismantling them.

Original article: https://arxiv.org/pdf/2601.04802.pdf

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

The Ghostly Neutrino: A Crack in the Standard Model

The Next Generation of Neutrino Observatories

Simulating Reality: Testing Our Understanding

The Shadow of Dark Matter: Unseen Interactions

Where Do We Go From Here?

See also: