Charting the Future of Neutrino Experiments

Kristiane Bernhard-Novotny (CERN) 15th Sep 2024

The development of next-generation neutrino experiments is underway at CERN, which will shed light on the nature of these elusive particles. While the development of far and near detectors is advancing well, novel identification techniques for neutrino beams are vital to strongly reduce systematic uncertainties on key observables – such as the flavour-dependent neutrino flux. These techniques are being conceived within Physics Beyond Colliders.

Neutrinos are the least interacting particles in the Standard Model and many of their characteristics still d to be fully determined. During the past few decades, physicists discovered that neutrinos have non-zero masses through observing their oscillations when travelling from one place to another. Now, physicists want to understand their properties to a high precision to scrutinise the Standard Model of particle physics, which does not foresee them to be massive. Measurements typically feature a determination of energy, flavour, and direction, based on the signals neutrinos produce when they interact with the detector. These characteristics are then used to extrapolate the total neutrino flux, a key ingredient to studying neutrino oscillations by comparing the absolute number of neutrinos of a given flavour at different locations.

What might sound like a trivial problem proves to be very complicated: Since their discovery in 1956, the probability of their interaction with matter, which is referred to as neutrino cross-section, is only known to the 30% to 10% level. This precision does not match the need of the latest high-precision measurements, requiring this knowledge rather at the 1% level. This imprecision is already limiting the latest results of T2K and will be dominant for the next-generation neutrino experiments, such as DUNE and Hyper-K.

To overcome these limitations, neutrino oscillation experiments typically use a near detector, such as ND280 [1] or the Water Cherenkov Detector [2], which are tested at CERN. In a complementary approach, the ENUBET and NuTAG collaborations proposed instrumented decay tunnels in order to identify neutrinos and their flavours from characteristic decays of their parent particles. Both projects are supported by the Physics Beyond Colliders (PBC) initiative at CERN.

The ENUBET collaboration wants to exploit the fact that every time a neutrino is produced, it is accompanied by a charged lepton that can be detected and identified in a calorimeter with a very good resolution. Thus, the number and the flavour of the neutrinos is known via the multiplicity and flavour of the charged leptons. This principle is called a monitored neutrino beam. ENUBET wants to exploit mainly the Ke3 decay, which is the decay of a charged kaon to a neutral pion, a positron or electron, and an (anti-)electron neutrino.

The demonstrator of the EUNBET experiment, which just concluded their test beam campaign in the T9 beamline in the East Area. Credit: ENUBET collaboration.

Physicists envision a fully instrumented decay tunnel of 40 m length [3]. For this, two alternate beam line designs have been developed. Elisabetta Parozzi, who designed one of the preliminary beamlines during her PhD thesis at CERN [4], highlights, “ENUBET is very versatile. It is possible to monitor the neutrino energy in the area of interest of experiments such as HyperK, T2K, and DUNE through the same beam line without the need of physically changing the configuration.”

The challenge is, however, to obtain a very precise detector response as many charged leptons are produced at the same time. Therefore, the ENUBET collaboration developed a demonstrator setup to validate the concept in the CERN East Area T9 experimental area. They just completed successfully a first test of their demonstrator’s full data acquisition system.

The NuTAG collaboration proposed to follow the concept of a tagged neutrino beam to study neutrino oscillations [5], initially proposed by Pontecorvo in 1979 [6]. Until now, instrumentation directly exposed to the hadron source beam was unable to withstand the required high intensities and to individually measure each particle. Thanks to progress in the silicon detector technology, this concept becomes now feasible and would drastically reduce the statistical uncertainties. NuTAG is based on the NA62 Gigatracker technology and new developments for the originally suggested HIKE experiment [7].

A proof-of-principle of the NuTAG technique has been recently demonstrated with the NA62 experiment at CERN, where two tagged neutrino candidates have been found for the first time [8]. The physicists aim to kinematically reconstruct neutrinos from the decays of pions and kaons into a muon and a neutrino by measuring the momentum of the charged particles in each individual decay. Anna Baratto Roldan, a beam physicist with the BE-EA group, explains: “The real challenge in designing the NuTAG beam line was ensuring the simultaneous transport of both charges to enhance the neutrino rate and to be able to produce neutrinos and antineutrinos at the same time.”

The proposed beamline design for NuTAG’s tagged neutrino beam, where the four-momentum of the parent meson decaying into a muon and the corresponding neutrino are measured. Source: arXiv:2401.17068

While both projects were studied within the PBC Conventional Beams Working Group framework at CERN, the NuTAG and ENUBET collaborations joined forces and created the concept of a short-base neutrino beamline (SBN). Combining these two concepts would result in a tagged neutrino beam with full particle identification. SBN foresees a double-bend achromat to select and measure the momentum of the source beam in front of a fully instrumented, 40m long decay tunnel. Such a novel beam line is supposed to measure the respective ν_e and ν_μ cross-sections to the 1% level, which would be a world’s first and which would open an avenue for exploring many physics cases. “We are using a state-of-the-art multi-objective genetic algorithm for optimising our design,” says Marc Jebramcik, a beam physicist in the BE-EA group working on the SBN project. “We improved the theoretical beam transmission by a factor of four compared to earlier designs.” At the moment, a feasibility study is progressing on finding a suitable site for a possible first implementation. At CERN, such a beam line could potentially feed the protoDUNE detectors in the North Area, while other locations are also under consideration.

Simulation of a hadron beam including kaons and pions, which decay into charged leptons and a (anti-)neutrino in the proposed SBN beamline. Credit: M Jebramcik.

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