Conferences

News from Milan at NEUTRINO-2024

On 16-22 June, the largest international conference on neutrino physics Neutrino-2024 was held in Milan, where the latest achievements in this and related fields of science were presented. In a brief review of the most striking results presented at the Conference, focus was on the projects implemented with the participation of JINR within the framework of the Neutrino Programme.

Significant news from the Conference was the update of the limit on the efficient mass of neutrinos

in the KATRIN experiment studying the kinematics of tritium beta decay. The new value is m_β<0,45 eV at the 90% confidence level (CL). This limit is expected to improve to m_β<0,3 eV (90% CL) with data accumulated up to and including 2025.

Cosmological data, such as the anisotropy of the cosmic microwave background and baryon acoustic oscillations, allow us to bound the sum of the masses of light types of neutrinos. Unexpected news was the announcement of the inconsistency of the cosmological estimate of the sum of neutrino masses obtained in the PLANCK and DESI experiments with the results of oscillation experiments. The statistical significance of this discrepancy does not yet exceed 2,5σ. It is worth noting that cosmological measurements are highly dependent on prototypes.

The search for sterile neutrinos is currently implemented due to the availability of anomalies that can be explained by the occurrence of such a neutrino state. Experimental data from LSND and MiniBooNE, the reactor antineutrino anomaly, the gallium anomaly and Neutrino-4 observations indicate different values of mixing parameters with sterile states. 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 tested in a series of new projects. At present, neither new anomalies nor evidence of the occurrence of sterile neutrinos have been found.

The results of the LSND and MiniBooNE experiments are not confirmed by the results of the MicroBooNE experiment. The reactor anomaly that occurred as a discrepancy between the predicted and measured antineutrino flux from the reactors has started to be explained concerning the emergence of new calibration data and measurements carried out in the DayaBay experiment and afterwards, confirmed by the RENO, STEREO, NEOS and DANSS collaborations. Apparently, the anomaly is related to inaccuracies in simulating the contributions of various chains of nuclear reactions, especially those involving the ²³⁵U atomic nucleus. The final resolution of this anomaly will require a deeper understanding of the processes occurring in a nuclear reactor, according to further research of experimental collaborations.

The Neutrino-4 result in most of the acceptable parameter range, including the best fit value, is excluded by the new results from the PROSPECT experiment at a significance level greater than 5σ. The causes of gallium anomaly (BEST, SAGE/GALLEX) are still unknown.

It is expected that new results from the current experiments to search for sterile neutrinos, including DANSS, Neutrino-4, BEST, PROSPECT and the first data from new projects (SBND at Fermilab, JSNS2 at the J-PARC proton accelerator complex) may shed light on the causes of the discussed anomalies.

The first results of the search for neutrinoless double beta decay in the LEGEND-200 experiment, more stringent half-life constraints from KamLAND Zen in a configuration with 800 kg of xenon, recent results from CUORE are presented. Current experiments have reached the range of sensitivity to double neutrinoless beta decay predicted for the reverse order of neutrino masses, yet the process itself has not been observed. This important achievement highlights significant progress in research into neutrinoless double beta decay and brings us closer to a possible answer to the issue of the nature of neutrino mass.

SuperNEMO and SNO+ currently accumulate data and prepare to present the first results. The next generation of experiments: LEGEND-1000, KamLAND2-Zen, CUPID and others are going to close the entire parameter space for the case of reverse order of neutrino masses in the next decade. To achieve sensitivity to double neutrinoless beta decay in the case of regular order of neutrino masses, either huge volumes of matter or a new technique will be required. Separately, it is worth noting the urgent need for the work of theoretical physicists to calculate nuclear matrix elements that are the key input parameter for this type of experiment.

Neutrino telescopes have long been an important tool in space exploration, along with gamma and radio telescopes. IceCube still measures the spectrum of astrophysical neutrinos and catalogs their sources in search of correlations with known space objects. In the Northern Hemisphere, the Baikal-GVD (at present, the largest in volume in this hemisphere) and ARCA/KM3NeT projects are currently developed. The latter consists of 28 strings and a total of 280 will be installed. ARCA/KM3NeT was the first to register ultra-high-energy neutrinos, estimated at tens of PeV. In the future, detectors with record active volumes will be developed in the Northern Hemisphere: P-ONE (Canada), TRIDENT and HUNT (China).

The main experiments to date with accelerator neutrinos that determine the accuracy of measuring the parameters of neutrino oscillations NOvA and T2K presented updated results as well. The T2K experiment added 10% of the neutrino data statistics to the previous result and started data acquisition with the updated near detector. The NOvA experiment presented the first results with doubled statistics with a neutrino beam. Both experiments, as before, but with increased statistical significance, indicate a regular order of neutrino masses.

Regarding the values of the CP parity violation phase δ_СР, their results are still different: NOvA indicates a CP parity preserving phase, while T2K indicates a CP parity violation. The accuracy records for the presented measurements of the left oscillation parameters were broken one after another: IceCube presented the accurate measurement Δm²₃₂ with atmospheric neutrinos, T2K updated its result for this parameter. At the moment, the most accurate measurement of it was provided by the NOvA experiment. The first results of the SNO+ experiment with reactor antineutrinos confirmed the current discrepancy between measurements Δm²₂₁of solar neutrino oscillations and reactor antineutrinos.

The first results on measuring the parameters of neutrino oscillations in the ORCA/KM3NeT detector are presented; at the moment, it still increases its volume. The Super-Kamiokande detector currently accumulates data. The final measurement of the parameters of three-flavor neutrino oscillations is expected to be obtained only with the implementation of the DUNE and Hyper-Kamiokande experiments. However, indications of meeting one of the flagship issues, determining the order of neutrino masses in the next few years may be found in the JUNO, IceCube Upgrade and ORCA experiments. The currently operating NOvA and T2K will still acquire data for several more years and will have the chance to improve current measurements of neutrino mass order and CP parity violation phase.

There has been 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 carry out this kind of joint analysis were NOvA and T2K, as well as Super-Kamiokande and T2K. These results were also presented at the Conference. In the future, a joint analysis of data from these experiments with increased statistics will be implemented. The JUNO, IceCube and ORCA experiments are also going to carry out joint data analyzes to measure neutrino mass order with high statistical significance. Work on jointly analyzing data from the DayaBay, Prospect and Stereo experiments to obtain new restrictions on the mixing parameters of sterile neutrinos is underway.

Beyond this review there were theoretical investigations in neutrino physics (the origin of neutrino masses, calculation of matrix elements to search for neutrinoless double beta decay, others), registration of neutrinos from proton collisions in experiments at the LHC, measurement of cross sections for neutrino interactions, other investigations were not mentioned in searching for physics beyond the Standard Model R&D of new detectors and projects and much more.

The results presented at the Conference once again confirmed the unique potential of neutrino physics that has entered the era of precision measurements. JINR is significantly involved in all key areas of this research area.

Reference. Within the framework of the JINR neutrino programme that involves about two hundred employees, work is currently carried out in the following experiments: Baikal-GVD, NOvA, T2K, DayaBay, JUNO, DANSS, vGeN, RICOCHET, SuperNEMO, LEGEND, TGV, MONUMENT, Edelweiss, DarkSide, NA65/DsTau, FASER, Borexino, as well as methodological and theoretical research.

As reported by dlnp.jinr.ru