Search for active-sterile neutrino transitions using Pierre Auger Observatory data

Technical Summary: Search for Active–Sterile Neutrino Transitions using Pierre Auger Observatory Data

Problem Statement
Ultrahigh-energy neutrinos (UHE$\nu$s) are expected to originate from interactions of ultrahigh-energy cosmic rays (UHECRs), yet their flux remains uncertain due to unknowns in UHECR composition and source environments. Current observatories, including the Pierre Auger Observatory, have not detected UHE neutrinos above 1 EeV, with predicted fluxes in mixed-composition scenarios lying below current sensitivities. However, an observed excess of events could stem from either a higher-than-expected astrophysical flux or new physics beyond the Standard Model (BSM). Specifically, the existence of a transition magnetic moment ($\mu_{\nu N}$) coupling active neutrinos to heavy sterile states ($N$) could enhance the neutrino–nucleon cross section ($\sigma_{\nu n}$) above a kinematic threshold. This enhancement would alter neutrino detection rates at extreme energies, potentially mimicking a flux increase. Distinguishing between a genuine flux enhancement and a modified interaction cross section is a critical challenge in UHE neutrino astronomy.

Methodology
The authors investigate the sensitivity of the Pierre Auger Observatory to active–sterile neutrino transitions mediated by a dipole portal. The study focuses on sterile neutrino masses ($m_N$) in the range of 1 TeV to 100 TeV.

1. Cross-Section Modeling: The paper models the upscattering process $\nu n \to N X$ (where $X$ is hadronic debris) and the subsequent prompt decay of the heavy neutrino ($N \to \gamma\nu$ or $Z\nu$). The differential cross section is calculated using deep-inelastic scattering (DIS) formalism, dominated by sea quarks at small Bjorken-$x$. The interaction is governed by the transition magnetic moment $\mu_{\nu N}$.
2. Detection Channels: The analysis evaluates two distinct detection modes:
   • Down-going Channel: Neutrinos interacting in the atmosphere. The study calculates the exposure ($E^{DG}_{\nu}$), accounting for the fact that the upscattering process produces final states resembling Standard Model (SM) interactions. A conservative efficiency cut is applied to ensure neutrinos do not interact too high in the atmosphere to be distinguishable from cosmic-ray backgrounds.
   • Earth-skimming Channel: Tau neutrinos ($\nu_\tau$) interacting in the Earth's crust to produce $\tau$ leptons that decay in the atmosphere. The study calculates the exposure ($E^{ES}_{\nu_\tau}$), noting that BSM interactions effectively enhance neutral-current-like processes. This increases the probability of energy loss without producing the charged lepton necessary to initiate an observable air shower, thereby suppressing the Earth-skimming event rate.
3. Flux Scenarios: To derive constraints, the authors evaluate expected event numbers ($n_{exp}$) against three benchmark neutrino flux scenarios ($\Phi_\nu$-low, $\Phi_\nu$-high, and $\Phi_\nu$-with-UHE-p), which span the range of current astrophysical uncertainties regarding UHECR composition and source evolution.
4. Statistical Analysis: Using the non-observation of UHE neutrino candidates, the authors exclude regions in the ($m_N$, $\mu_{\nu N}$) parameter space where the expected number of events exceeds 2.44 at 90% confidence level.

Key Contributions

• Diagnostic Discrimination: The paper demonstrates a unique diagnostic capability of the Pierre Auger Observatory: the contrasting responses of down-going and Earth-skimming channels. While an enhanced flux would increase rates in both channels, a magnetic-moment-induced cross-section enhancement increases down-going rates while suppressing Earth-skimming rates.
• Exposure Calculations: The authors provide detailed calculations of the Observatory's exposure to UHE neutrinos in the presence of BSM interactions, incorporating the specific kinematics of heavy neutrino production and decay.
• Flavor-Independent Limits: The derived constraints are flavor-independent, offering coverage for electron and tau neutrino flavors, complementing existing limits that often focus on muon neutrinos.

Results

• Exclusion Regions: The study derives 90% C.L. exclusion limits in the ($m_N$, $\mu_{\nu N}$) plane for sterile neutrino masses between 1 TeV and 100 TeV.
• Parameter Space: The constraints extend into the region $m_N > 1$ TeV, a mass range accessible only to UHE neutrino observatories.
• Comparison: The resulting limits are compared with existing bounds from IceCube, LHC, and the KM3NeT event KM3-230213A. While other experiments provide constraints, the Pierre Auger limits probe a complementary region of parameter space, particularly for higher masses and different neutrino flavors.
• Sensitivity: Under the assumed flux scenarios, the limits extend existing bounds into previously unexplored parameter space. The lower bounds of the exclusion contours are shaped by the fall-off of incoming fluxes, while the upper bounds are determined by the requirement that the interaction probability remains valid for down-going detection (i.e., neutrinos must not interact too high in the atmosphere).

Significance
The paper claims that the Pierre Auger Observatory offers sensitivity to BSM interactions at energy scales beyond the reach of laboratory experiments. By utilizing the non-observation of UHE neutrinos, the study provides a model-dependent yet robust assessment of the sensitivity to magnetic-moment-mediated transitions. The work highlights the Observatory's ability to discriminate between astrophysical flux uncertainties and new physics effects through the differential behavior of detection channels. These results provide the first constraints on active–sterile transitions for sterile neutrino masses in the multi-TeV regime using UHE neutrino data, effectively testing the Standard Model description of neutrino scattering at extreme energies.