Bronze-type Neutrino Events

• Bronze-type neutrino events are signals with intermediate statistical significance and purity, bridging clear astrophysical detection and ambiguous background events.
• They are identified using advanced algorithms, energy-topology cuts, and machine learning approaches to maximize alert potential in real-time analyses.
• Their study enhances multi-messenger follow-up campaigns, refines atmospheric and flavor composition models, and guides future detector innovations.

Bronze-type neutrino events comprise a class of neutrino signals of intermediate statistical significance, purity, or reconstruction quality in contemporary neutrino observatory analyses, particularly at IceCube and similar kilometer-scale detectors. While the “Bronze-type” designation is not uniformly defined, its usage typically refers to events that are likely astrophysical but not sufficiently stringent in purity for “Gold” or “Golden” classifications. These events play a central role in maximizing astrophysical discovery potential and enabling comprehensive multi-messenger follow-up campaigns. Bronze-type event categorization appears across several domains, including real-time transient alerting, correlation studies, atmospheric neutrino identification, detector simulation, and high-energy flavor composition measurements. This article systematically analyzes Bronze-type neutrino events through their physical origin, detection and classification frameworks, experimental signatures, correlation studies, astrophysical and multi-messenger significance, and ongoing methodological evolutions.

1. Physical Origins and Production Mechanisms

Bronze-type neutrino events arise from diverse physical processes spanning atmospheric and astrophysical regimes. At high energies (10 TeV–PeV), production scenarios include:

• Astrophysical Sources: IceCube’s sensitivity to cosmic-ray interactions near Fermi Bubbles (Lunardini et al., 2014), tidal disruption events (TDEs) (Ji et al., 13 Aug 2025), and extragalactic AGNs (Moharana et al., 2016) increases the Bronze-type event rate as their fluxes approach or modestly exceed background expectations.
• Atmospheric Charm Component: At $E \gtrsim 10$ TeV, the prompt decay of charmed hadrons in cosmic-ray air showers produces neutrinos that can manifest Bronze-type muon bundle–embedded signals (see Eq. $f_{p\nu}(x, E)$ in (Gutiérrez et al., 2021)).
• Glashow Resonance: Electron-antineutrinos at $\sim6.3$ PeV produce PeV-scale showers via resonant W-boson production in IceCube, contributing to event topologies classified as showers or cascades (Barger et al., 2012).
• Flavor Ambiguity and Misclassification: Analysis failures to resolve tau double-cascade vertices or ambiguous energy asymmetry $E_\text{A}$ can result in events classically identified as Bronze-type due to their mixed reconstruction likelihoods (Lad et al., 9 Jul 2025).

This diversity implicates Bronze-type events in both discovery and background-limited regimes, with their statistical prevalence hinging on the underlying physical scenario.

2. Detection, Event Selection, and Alerting Systems

Neutrino detectors such as IceCube implement multi-tiered selection criteria for candidate event classification:

• Signalness Thresholds: “Bronze-type” is strictly defined in IceCube’s realtime alert program (Blaufuss et al., 2019) as events with signalness between $30\%$ and $50\%$:

$$\text{Signalness}(E, \delta) = \frac{N_\text{signal}(E, \delta)}{N_\text{signal}(E, \delta) + N_\text{background}(E, \delta)}$$

with Gold channel thresholds at $>50\%$ signalness and Bronze between $30\%$ and $50\%$.

• Event Reconstruction Algorithms: Selections (GFU, HESE, EHE) use machine learning (boosted decision trees), photoelectron charge–based cuts, track length criteria, and vertex containment to control sample purity (Blaufuss et al., 2019). Ambiguous events (those near selection boundaries) are more likely to populate Bronze-type classifications.
• Energy and Topology-Based Cuts: In analyses focused on atmospheric charm, longitudinal energy-deposition profiles with ratios such as $E_\text{max}/E_-$ and $E_+/E_-$ delineate true neutrino–initiated events within muon bundles (Gutiérrez et al., 2021).

These detection systems balance the need for clean astrophysical event identification against maximizing the total number of actionable alerts for follow-up.

3. Experimental Signatures and Event Topologies

Bronze-type events exhibit topologies and experimental characteristics governed by underlying interactions:

Topology	Event Type (Example)	Purity/Significance
Track	$\nu_\mu$ CC, sometimes $\nu_\tau \to \mu$	Medium (Bronze/Gold)
Cascade	$\nu_e$ CC, NC for all flavors	Medium/Variable
Double Cascade	$\nu_\tau$ CC with resolved tau decay	High/Gold if fully reconstructed
Muon Bundle	Atmospheric charm or cosmic ray shower	Bronze if detected with neutrino

• Cascade Energetics: Glashow resonance (W decay to e or $\tau$) yields cascades with spectra characterized by $y = E_\text{shower}/6.3$ PeV and a $(1-y)^2$ distribution (Barger et al., 2012).
• Flavor Ambiguity: Ambiguities in classification (for instance, borderline $E_\text{A}$, $E_\text{C}$ cuts) or failed double-cascade reconstruction can demote otherwise golden events to Bronze-type status (Lad et al., 9 Jul 2025).
• Temporal and Spatial Coincidence: In multi-messenger contexts (TDE associations), Bronze-type neutrino events are defined by positional coincidence within 90% error contours and short delays relative to MIR flares (a few to 100 days in the SMBH frame) (Ji et al., 13 Aug 2025).

Bronze-type topologies thus encompass a broad dynamic range, from ambiguous, intermediate-quality events to those with modest astrophysical purity but significant follow-up potential.

4. Statistical and Correlation Studies

Systematic statistical approaches underpin the identification and astrophysical utility of Bronze-type events:

• Cross-Correlation with Source Catalogues: Studies using invariants such as $\delta\chi^2$ (Moharana et al., 2016)

$$\delta\chi_i^2 = \min_j\left(\frac{\gamma_{ij}^2}{(\delta\gamma_i)^2}\right)$$

reveal no statistically significant associations with extragalactic sources for the majority of the current Bronze or lower-purity IceCube events (p-values 0.58–0.825). In contrast, spatial–temporal correlations with Fermi Bubbles or MIR dust echoes in TDEs provide circumstantial evidence for astrophysical origins (Lunardini et al., 2014, Ji et al., 13 Aug 2025).

• Significance and Event Rates: Bronze channel alerts expect $\sim$20 events/year with signalness $30\%$–$50\%$, nearly doubling the Gold rate, and contribute the majority of alerts with intermediate discovery potential (Blaufuss et al., 2019).

While high statistical significance is not inherent to the Bronze class, their inclusion is vital in broadening multi-messenger opportunities and constraining source population models.

5. Astrophysical Interpretation and Multi-Messenger Implications

• Transient Associations: The case of AT2022sxl exemplifies a TDE where two Bronze-type IceCube neutrino events coincide in both position and temporal proximity with luminous MIR flares, supporting scenarios where efficient $p\gamma$ interactions are enabled by dust-reprocessed emission fields (Ji et al., 13 Aug 2025). The timing of neutrino arrivals relative to MIR peaks (as short as 10–120 days) is more physically motivated for neutrino production than alignment with the optical peak.
• Fermi Bubble Emission: "Bronze-type" characterization is extended to those events wherein the correlation with the FB region is suggestive but falls shy of "golden" statistical robustness (Lunardini et al., 2014).
• Atmospheric vs Cosmic-Origin Disambiguation: Down-going neutrino events embedded in muon bundles, particularly those from charm decay, are identified as Bronze-type by reconstructible energy-deposition patterns distinct from up-going cosmic neutrino events (Gutiérrez et al., 2021). This enables partial background rejection and enrichment for cosmic or prompt atmospheric neutrinos.

These examples highlight Bronze-type events’ critical role in probing new astrophysical source populations and transient phenomena.

6. Simulation, Detector Physics, and Flavor Composition

Bronze-type events also manifest in the context of detector modeling and flavor measurements:

• Simulated Detector Signals: In Cd-based detectors such as COBRA, intermediate-energy neutrino–nucleus scattering events (classified as Bronze-type) are characterized by simulated event rates obtained via folding realistic QRPA cross sections with astrophysical source spectra (Sinatkas et al., 2019). These are differentiated from both rare golden events (e.g., neutrinoless double beta decay) and low-energy geo-neutrino signals.
• Flavor Composition Measurement: In IceCube’s HESE selection, ambiguous events—those marginally failing double-cascade criteria or lying close to the overlap between track and cascade hypotheses—are treated as Bronze-type and play a role in uncertainty estimation for $f_{\nu_e} : f_{\nu_\mu} : f_{\nu_\tau}$ fits (Lad et al., 9 Jul 2025).

Advanced reconstruction methodologies and boosted decision tree classifiers are iteratively refined to ensure the robust treatment of Bronze-type events, improving astrophysical inference and systematic error control.

7. Future Developments and Ongoing Challenges

Bronze-type event handling continues to evolve as statistics, detector sensitivity, and reconstruction algorithms improve:

• Extended Datasets: Longer running times and multi-detector observations (IceCube Gen2, KM3NeT, ANTARES) will improve statistical significance and enable higher-confidence associations and source discrimination (Lunardini et al., 2014).
• Analytic Refinement: Enhanced multi-dimensional PDFs, additional observable incorporation, and machine learning techniques aim to reduce ambiguities that populate Bronze-type event samples (Lad et al., 9 Jul 2025).
• Simulation and Background Characterization: Improved models for atmospheric charm and cosmic diffuse backgrounds are necessary to minimize misclassification and to isolate true astrophysical Bronze-type signals (Gutiérrez et al., 2021).
• Multi-messenger Integration: Continued development of real-time alert infrastructures will amplify Bronze-type event utility for coordinated follow-up, discovery of new source classes, and probing physics beyond the Standard Model (Blaufuss et al., 2019, Ji et al., 13 Aug 2025).

A plausible implication is that Bronze-type events will remain essential for both population-level studies and transient discovery efforts, particularly as data volumes grow and analysis complexity advances.

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Bronze-type neutrino events occupy a functionally intermediate but scientifically vital niche, bridging the gap between “pure” discovery signals and the broader underlying statistical sample. Their careful identification, analytical treatment, and multi-messenger exploitation underpin much of next-generation neutrino astrophysics and rare event search methodologies.