From 16 to 22 June, Milan hosted the largest international conference on neutrino physics, Neutrino-2024. The event discussed current research results in the field and related scientific directions. Special attention was paid to the projects implemented as part of the Neutrino Programme of the Joint Institute for Nuclear Research.

An important piece of news was the improvement of the upper bound on the effective neutrino mass m_β = √∑_i|V_ei|² m²_i in the KATRIN Experiment studying the kinematics of tritium beta decay. The new value is m_β < 0.45 eV at a 90% confidence level (CL). The upper bound is expected to improve to m_β < 0.3 eV (90% CL) with data collected up to and including 2025.

Cosmological data, such as the anisotropy of the cosmic microwave background and baryon acoustic oscillations, allow limiting the sum of light neutrino masses. The unexpected news was the announcement that the determination of the sum of neutrino masses from cosmological data obtained by the PLANCK and DESI Experiments is inconsistent with the results of oscillation experiments. The statistical significance of this discrepancy does not exceed 2.5σ yet. It is noteworthy that cosmological measurements are highly dependent on models.

The search for sterile neutrinos continues due to the presence of anomalies that the existence of such neutrino state could explain. Experimental data from LSND and MiniBooNE, reactor antineutrino anomaly, gallium anomaly, and observations of Neutrino-4 indicate different values of mixing parameters with sterile conditions. Therefore, there is no single explanation for these anomalies in the form of a sterile neutrino with specific parameters. The results of these experiments are being tested in a series of new projects. Currently, neither new anomalies nor evidence of the existence of sterile neutrinos have been found.

The results of the MicroBooNE Experiment do not confirm the results of the LSND and MiniBooNE Experiments. The reactor anomaly, which arose as a disagreement between the predicted and measured antineutrino flux from reactors, apparently begins to find its explanation in connection with the appearance of new calibration data and measurements carried out in the Daya Bay Experiment, and then confirmed by the RENO, STEREO, NEOS, and DANSS Collaborations. Apparently, the anomaly is associated with inaccuracies in modelling the contributions of various chains of nuclear reactions, especially with the participation of the atomic nucleus ²³⁵U. The final resolution of this anomaly will require a deeper understanding of the processes occurring in a nuclear reactor. Experimental collaborations continue their research.

The Neutrino-4 result in most of the range of valid parameters, including the best fit value, is excluded by the new results of the PROSPECT Experiment at a significance level of more than 5σ. The causes of the gallium anomaly (BEST, SAGE/GALLEX) remain unknown.

It is expected that new results of existing experiments on the search for sterile neutrinos, including DANSS, Neutrino-4, BEST, PROSPECT, and the first data from new projects (SBND at the Fermilab Laboratory, JSNS2 at the J-PARC Proton Accelerator Complex) can shed light on the causes of the anomalies under discussion.

The first results of the search for neutrinoless double beta decay in the LEGEND-200 Experiment, stricter half-life limits from KamLAND-Zen in a configuration with 800 kg of xenon, and recent CUORE results were discussed. Existing experiments have reached the range of sensitivity to neutrinoless double beta decay predicted for the inverted neutrino mass ordering, but the process itself has not yet been observed. This important achievement highlights significant progress in research on neutrinoless double beta decay and brings us closer to a possible answer to the question of the neutrino mass nature.

SuperNEMO and SNO+ are gathering data and preparing to present the first results. LEGEND-1000, KamLAND2-Zen, CUPID, and other next-generation experiments plan to cover the entire parameter space in the next decade for the case of the inverted neutrino mass ordering. The experiments will require either huge matter volumes or a new technique to achieve sensitivity to double neutrinoless beta decay in the case of the normal neutrino mass ordering. A particularly important aspect is the urgent need for theoretical physicists to work on the calculation of nuclear matrix elements, which are the key input parameter for this type of experiments.

Neutrino telescopes have long played an important role in space exploration, along with gamma-ray and radio telescopes. IceCube continues to measure the spectrum of astrophysical neutrinos and catalogue their sources in search of correlations with known space objects. The ongoing projects of the Northern Hemisphere are Baikal-GVD (currently the largest in terms of volume in the hemisphere) and ARCA/KM3NeT Projects. The latter consists of 28 detection units, and their total number is planned to go up to 280. ARCA/KM3NeT was the first to register ultrahigh-energy neutrinos estimated at tens of PeV. In the future, detectors with record active volumes are set to be built in the Northern Hemisphere: P-ONE (Canada), TRIDENT, and HUNT (China).

More updated results were presented by the main experiments with accelerator neutrinos to date, which determine the accuracy of measuring the parameters of neutrino oscillations, NOvA and T2K. The T2K Experiment added 10% of the neutrino data statistics to the previous result and started a dataset with an updated near detector. The NOvA Experiment reported on the first results with doubled statistics with a neutrino beam. Both experiments, repeated with higher statistical significance, indicate the normal ordering of neutrino masses.

As for the values of the CP violation phase δ_CP, their results still differ: NOvA indicates a CP conservation phase, while T2K indicates CP violation. Records for the accuracy of the presented measurements of the remaining oscillation parameters were broken one after another: an accurate Δm²₃₂ measurement with atmospheric neutrinos, done by IceCube, was updated by T2K. So far, the most accurate measurement of this parameter has been made by the NOvA Experiment. The first results of the SNO+ Experiment with reactor antineutrinos confirmed the existing discrepancy between measurements of Δm²₂₁ on oscillations of solar neutrinos and reactor antineutrinos.

The first results on measuring neutrino oscillation parameters in the ORCA/KM3NeT Detector were reported. The facility is gradually increasing its volume. The Super-Kamiokande Detector continues to collect data. The final measurement of the parameters of three-flavour neutrino oscillations is expected to be obtained only with the commissioning of the DUNE and Hyper-Kamiokande Experiments. However, indications for solving one of the flagship tasks, determining neutrino mass ordering, may be found in the upcoming years in the JUNO, IceCube-Upgrade, and ORCA Experiments. The ongoing NOvA and T2K will continue to collect data for several more years and will have a chance to improve modern measurements of the neutrino mass ordering and the CP violation phase.

There is a global trend towards a full-fledged joint analysis of data from various neutrino experiments in order to improve the statistical significance of measurements. Some of the first experiments to do such a joint analysis were NOvA and T2K, as well as Super-Kamiokande and T2K. These results were also presented at the conference. In the future, it is planned to conduct a joint analysis of the data from these experiments with increased statistics. The JUNO, IceCube, and ORCA Experiments plan to conduct a joint data analysis as well, aiming to measure the neutrino mass ordering with high statistical significance. The Daya Bay, Prospect, and Stereo Experiments are analysing their data collaboratively in order to obtain new limits on the sterile neutrino mixing parameters.

In addition, the conference discussed theoretical research in neutrino physics (the origin of neutrino mass, calculation of matrix elements for the search for neutrino-free double beta decay, etc.), registration of neutrinos from proton collisions in experiments at the LHC, measurement of neutrino interaction cross sections, and much more.

The results discussed at the conference confirm significant progress in neutrino physics, demonstrating transition to the era of precision measurements. The Joint Institute for Nuclear Research plays an important role in the development of all key research directions in this field.