Research in neutrino oscillation physics has been progressing in the last two decades with a long series of outstanding science results and discoveries. The Nobel Prize awarded in 2015 to Kajita and McDonald for the discovery of the oscillations is emblematically confirming this fact. The study of the PMNS mixing matrix, accomplished through a large number of experiments employing atmospheric, solar, reactor and accelerator neutrinos, has contributed to the partial clarification of the neutrino physics landscape, leading to the only evidence so far for new physics beyond the Standard Model of Particles and Interactions and providing intriguing hints for even more complex scenarios.
Among the issues recently settled, there is the important measurement of the third PMNS angle, theta13, through accelerator and reactor experiments. For the first we mention the T2K experiment (see below) and the NOvA detector; for the second the angle was measured by reactor disappearance experiments, such as Daya-Bay in China, RENO in South Korea and Double Chooz in France. While a first indication of a non-vanishing theta13 angle came from T2K, the discovery was made with high significance by the Chinese reactor experiment. Last but (definitely) not least, in 2014 the T2K experiment provided the discovery of oscillation appearance with a muon- to electron-neutrino oscillation search by the observation of an excess of electron events. This was followed by the discovery of tau-appearance in 2015 by OPERA.
The neutrino oscillation projects in progress or planned point to remaining unanswered questions, such as the knowledge of the actual mass eigenstate hierarchy, the possible existence of a not-vanishing CP violating phase in the PMNS matrix, and that of a more complex structure of neutrino mixing, possibly involving yet undiscovered sterile states. While both T2K and NOvA could alone or in combination get first indications for both the mass hierarchy and CP violation, dedicated large-size, high-sensitivity experiments have been proposed for the more distant future (>2025) and are currently under scrutiny of the community and of the funding agencies. DUNE/LBNF in USA and Hyper-Kamiokande in Japan have been designed with these ambitious objectives and, in parallel, will play the role of large underground astroparticle physics neutrino observatories and proton decay detectors with an operation time of more than a decade.
Complementary approaches to the above mentioned experiments, employing artificial neutrinos for the measurement of the still outstanding oscillation parameters, foresee the use of astrophysical neutrinos. In this respect, we mention the excellent results from the IceCube experiment and look forward to the planned upgrades and follow up projects.
Concerning the search for sterile neutrinos triggered by the long standing LSND/MiniBooNE anomaly, several experiments involving different reactions and methods are being attempted and planned, in addition to the SBN project at Fermilab, for which Bern is involved in the MicroBooNE and SBND experiments.
Of particular interest, we mention the measurement of low-energy neutrino cross-section and the related nuclear models. This is being addressed by us given the availability of large data sets both in T2K and in SBN.
An elusive neutrino source has long been collider experiments - building detectors for neutrino physics around proton-proton collision points, such as the one at ATLAS at the LHC, is very difficult. The high-energy collisions at ATLAS provide a large flux of hadrons travelling in the far-forward direction, many of which decay to neutrinos with energies in the TeV-regime. Bern holds a leading role in the FASER Collaboration’s neutrino programme, based at the CERN LHC, 480 metres downstream of the interaction point along the collision axis. Bern focuses on the FASERnu detector - a passive detector made up of 730 interleaved emulsion films and tungsten plates, it provides a 1.1 tonne target mass and the ability to detect all three flavours of neutrinos, distinguished by their interaction topology and kinematics. FASER’s results will extend cross-section measurements for electron and tau neutrinos, and bridge the gap between the current accelerator limits and astrophysical neutrino sources for muon neutrinos The first observation of neutrinos at a collider was announced in 2023 using FASER’s electronic components to detect muon neutrinos, followed by the first cross-section measurements for muon and electron neutrinos using the FASERnu detector, published in July 2024.
