Observational strategies for ultrahigh-energy neutrinos: the importance of deep sensitivity for detection and astronomy

TL;DR

This work analyzes how to optimize observational strategies for ultrahigh-energy neutrinos (E > 10^{17} eV) by weighing wide/shallow versus deep/narrow detectors in the context of time-domain and multi-messenger astronomy. It updates realistic diffuse-flux targets using Auger constraints and IceCube extrapolations, and systematically assesses transient populations and their detectability with different instrument classes, emphasizing the importance of sub-degree angular resolution for source identification. The authors demonstrate that deep, narrow instruments are particularly powerful for rare, bright short bursts and advocate stacking and follow-up programs, while wide-field surveys complement by probing local, serendipitous sources. The paper concludes with concrete design guidance for next-generation UHE neutrino observatories, stressing global coordination, a Local Universe source catalog, and combined deep and wide approaches to maximize discovery potential and multi-messenger science.

Abstract

Detecting ultrahigh-energy neutrinos can take two complementary approaches with different trade-offs. 1)~Wide and shallow: aim for the largest effective volume, and to be cost-effective, go for wide field-of-view but at the cost of a shallow instantaneous sensitivity -- this is less complex conceptually, and has strong discovery potential for serendipitous events. However, it is unclear if any source can be identified, following detection. And 2)~Deep and narrow: here one uses astrophysical and multi-messenger information to target the most likely sources and populations that could emit neutrinos -- these instruments have deep instantaneous sensitivity albeit a narrow field of view. Such an astrophysically-motivated approach provides higher chances for detection of known/observed source classes, and ensures multi-messenger astronomy. However, it has less potential for serendipitous discoveries. In light of the recent progress in multi-messenger and time-domain astronomy, we assess the power of the deep and narrow instruments, and contrast the strengths and complementarities of the two detection strategies. We update the science goals and associated instrumental performances that envisioned projects can include in their design in order to optimize discovery potential.

Figures (8)

• Figure 1: What diffuse flux level should we aim for at ultrahigh energies? Theoretically predicted diffuse UHE neutrino fluxes from astrophysical (blue dotted lines, see text) Kotera09PhysRevD.90.103005Fang:2017zjfRodrigues21lundquist2024 and cosmogenic origins, obtained with a comprehensive fit to the Auger UHECR data: 99% C.L. fit (light+darker blue band), corresponding to the standard source parameters and 90% C.L. fit (darker blue band), for pessimistic parameters Alves_2019. Overlayed is a theoretical extension to UHE energies of the measured IceCube flux Stettner:2019tokIceCube21 (navy blue band) and the Murase-Beacom (or effective nucleus-survival) line (thick navy blue line): the astrophysical neutrino flux level expected for iron UHECRs, assuming an effective photodisintegration optical depth of 1, and strong source evolution history Murase_2010. The pink solid lines indicate the projected 10-year differential sensitivities of several projects (for BEACON, the 1000 station limit is quoted Wissel:2020secBEACON:2025qcq). Black solid lines mark the upper limits on UHE neutrinos from IceCube IceCubeCollaborationSS:2025jbi and Auger Aab_2019. The gray cross corresponds to the flux needed to achieve the detection of the event KM3-230213A. The horizontal span corresponds to the central 90% neutrino energy range associated with the event, and the vertical bars represents the $3\sigma$ Feldman–Cousins confidence interval on this estimate KM3NeT:2025npi. Note that the IceCube limits are derived assuming an $E^{-1}$ differential flux, whereas the Auger limits assume an $E^{-2}$ flux, and are therefore not directly comparable.
• Figure 2: Top: Acceptance, defined as the product of the average effective area and the day-averaged field-of-view (FOV) in steradians, shown for GRAND GRAND:2018iaj (solid), IceCube-Gen2 Radio IceCube-Gen2:2021rkfGen2_TDRGlaser:2019cws (dashed), and RNO-G RNO-G:2020rmc (dot-dashed), and BEACON BEACON:2025qcq (dotted). Bottom: The instantaneous effective area for each detector in the most sensitive band, where $\theta_z$ denotes zenith angle and $\delta$ denotes declination. Note that the legend is the same as above with the most sensitive band indicated for each detector.
• Figure 3: Significance of detection of point sources, within a diffuse UHE neutrinos flux, by experiments with given angular resolutions and number of detected events. Here, we present the specific case of a source population density $n_{\rm s}= 10^{-9}\,{\rm Mpc}^{-3}$, following the star formation rate evolution, up to redshift $z=6$. With this source number density, $\sim 100$ events and $\sim 0.1^\circ$ angular resolution are needed to reach a $4\sigma$ detection of point sources within a diffuse flux. The angular resolution of GRAND is taken from DecoenePhD20 while that for IceCube-Gen2 Radio from 2022NatRP...4..697G. The observation time assumed is $10$ years for both the detectors. (Adapted from Fang:2016hop.)
• Figure 4: Detection possibility of short (top) and long (bottom) transients within a given distance, within 10 years of observation. The regions spanned by each source populations in the ${\cal E}_{\rm bol}-D_{\rm L,10\,yr}$ (bolometric energy, typical distance for a 10 yr observation) parameter space are presented, following Table \ref{['tab:transients']}, assuming conservatively $f_\nu^{\rm UHE}=1$. Maximal experimental sensitivity bands (for $E_\nu\gtrsim 10^{17.5}\,$eV) are overlaid. The most promising short transients are distant and bright, while the long ones are local. Detection strategies will be different for both populations.
• Figure 5: All-flavor neutrino fluence sensitivities per decade in energy for an assumed $E_\nu^{-2}$ neutrino spectrum for various upcoming experiments, as indicated. For Auger and IceCube 2017ApJ...850L..35A, the upper-limit sensitivity to neutrino transients at the location of GW170817A is shown. Overlaid are theoretical neutrino fluences for (top) a short GRB/binary neutron star merger at 40 Mpc Kimura:2017kan, an early GRB afterglow at 200 Mpc Murase:2007yt, (bottom) a SN Ibc supernova with magnetar 2 weeks after birth Murase:2009pg, located at the indicated distances, inside the Galaxy, or galaxies of the Local Group (Leo IV and M31). Adapted from 2022NatRP...4..697G.