Neutrino astronomy at Lake Baikal

High energy neutrino astronomy has seen significant progress in the past few years. This includes the detection of neutrino flux from the Galactic plane, as well as strong evidence for neutrino emission from the active galaxy NGC 1068, both reported by IceCube. New results start coming from the two km$^3$-scale neutrino telescopes under construction in the Northern hemisphere: KM3NeT in the Mediterranean Sea and Baikal-GVD in Lake Baikal. After briefly reviewing the status of the field, we present the current status of the Baikal-GVD neutrino telescope and its recent results, including observations of atmospheric and astrophysical neutrinos.

💡 Research Summary

The paper provides a concise overview of the current state of high‑energy neutrino astronomy and then focuses on the Baikal‑Gigaton Volume Detector (Baikal‑GVD), a cubic‑kilometer‑scale underwater neutrino telescope under construction in Lake Baikal, Russia. After a brief introduction to the scientific motivation for neutrino astronomy—namely the unique ability of neutrinos to escape dense astrophysical environments, travel undeflected by magnetic fields, and point back to their sources—the authors discuss the challenges posed by the dominant atmospheric neutrino and muon backgrounds. They emphasize that the steeply falling atmospheric flux forces experiments to target energies above ~10 TeV, where the expected astrophysical signal becomes comparable to the background, and that detector volumes of order 1 km³ are required to collect sufficient statistics.

The paper then surveys the three major high‑energy neutrino observatories: IceCube at the South Pole, KM3NeT in the Mediterranean Sea, and Baikal‑GVD in Lake Baikal. IceCube, with its 5,160 optical sensors on 86 strings, has already discovered a diffuse astrophysical neutrino flux and reported intriguing point‑source candidates such as TXS 0506+056 and NGC 1068. KM3NeT is being built in two configurations—ARCA for high‑energy astrophysics and ORCA for neutrino mass hierarchy studies—using multi‑PMT optical modules that promise superior angular resolution thanks to the excellent optical properties of deep‑sea water. Baikal‑GVD, the focus of this work, offers a complementary field of view to IceCube and aims for similar sensitivity with a different medium (freshwater) and geographic location.

The detector design is described in detail. Each optical module (OM) houses a 10‑inch high‑quantum‑efficiency Hamamatsu R7081‑100 photomultiplier, high‑voltage supply, front‑end electronics, calibration LEDs, and orientation sensors, all sealed in a pressure‑resistant glass sphere. Strings contain 36 OMs spaced 15 m apart between depths of 750 m and 1,275 m. Eight strings form a cluster with 60 m horizontal spacing; clusters are linked to a central electronics hub and then to shore via a dedicated electro‑optical cable. The water at the Baikal site has an absorption length of ~22 m and a scattering length of 30–50 m (effective scattering length up to 480 m after accounting for the forward‑peaked scattering distribution). Ice cover from February to mid‑April provides a stable platform for deployment and maintenance.

As of the writing, Baikal‑GVD comprises 13 fully instrumented clusters, occupying roughly 0.6 km³ of lake volume, making it the largest neutrino telescope in the Northern Hemisphere. The construction plan foresees at least three additional clusters within the next two years, bringing the effective volume for PeV‑scale cascades close to 1 km³.

The physics program is divided into cascade (shower) and track analyses. Cascades arise from νₑ and ν_τ charged‑current interactions and all‑flavor neutral‑current interactions; they provide good energy resolution and dominate the detector response above ~100 TeV. Using data collected from 2018 to 2021, the collaboration performed an upward‑going cascade analysis. The reconstructed energy and zenith‑angle distributions show a clear excess over the expected atmospheric muon and neutrino backgrounds. The null hypothesis of a pure atmospheric flux is rejected at 3.05 σ. Assuming a single power‑law astrophysical flux with equal flavor composition, the best‑fit spectral index is γ = 2.58 +0.27/‑0.33 and the per‑flavor normalization at 100 TeV is Φ_astro = 3.04 +1.52/‑1.21 × 10⁻¹⁸ GeV⁻¹ cm⁻² s⁻¹ sr⁻¹, consistent with IceCube measurements.

A notable high‑energy event with reconstructed energy 224 ± 75 TeV was recorded in April 2021. Its reconstructed direction lies within the 90 % error circle of the blazar TXS 0506+056, the same source previously associated with a IceCube neutrino, highlighting Baikal‑GVD’s capability to contribute to multimessenger alerts. Additional potential associations were found for the high‑mass X‑ray binary LS I +61 303 and the ultra‑luminous X‑ray source Swift J0243.6+6124, based on a doublet of cascade events.

A dedicated search for a Galactic‑plane excess employed cascades with energies above 200 TeV collected from 2018 to 2023. The test statistic was the median absolute Galactic latitude |b|_med of the event sample. The observed |b|_med lies in the tail of the background distribution, corresponding to a 2.5 σ excess of events near the Galactic plane. This result adds independent evidence to the modest Galactic component hinted at in IceCube data.

Track analyses focus on upward‑going muon tracks to suppress the overwhelming atmospheric muon background. Using a boosted decision tree (BDT) classifier on 2020‑2021 data, 671 neutrino‑candidate events were selected after a BDT cut that yields high purity. The data‑Monte‑Carlo agreement is good for all training variables, including the number of hit OMs. The selected sample is dominated by atmospheric neutrinos; a forthcoming energy‑spectrum study aims to isolate the astrophysical component. The same event set is already being used in point‑source and extended‑source searches, though no statistically significant excess has yet been observed.

The authors conclude that Baikal‑GVD has already demonstrated its ability to detect a diffuse astrophysical neutrino flux, to identify high‑energy events possibly linked to known astrophysical objects, and to probe a Galactic‑plane contribution. With the planned addition of new clusters, the detector’s effective volume will approach 1 km³, improving sensitivity to PeV‑scale neutrinos and enhancing angular resolution. Combined operation with KM3NeT and IceCube will enable full‑sky, real‑time monitoring of transient phenomena, strengthening the global multimessenger network. The paper emphasizes that Baikal‑GVD is poised to become a key component of the next generation of neutrino telescopes, delivering complementary sky coverage, competitive sensitivity, and valuable contributions to the emerging field of high‑energy neutrino astronomy.