Enhanced Neutrino Tagging and Energy Reconstruction – ENTER

The concept of neutrino tagging can already be tested in the NA62 experiment at CERN. This experiment is mainly dedicated to rare kaon decays and, as the main decay mode of kaons is K->mu nu, the intense kaon beam also produces a neutrino beam. A non-negligible quantity of the neutrinos produced can interact and be detected in the experiment's calorimeter. As the beam is equipped with trajectrographs, the decay can be reconstructed and associated with the neutrino interaction, demonstrating the feasibility of the method with real data.

The scaling-up of this demonstrator to a real neutrino experiment will be studied using generic case studies for short-base and long-base experiments.

The design will focus on optimizing the beamline and experimental set-up.

Finally, studies will be carried out to better understand the limitations of the silicon pixel detector technology developed at NA62 for the beam trajectograph, and to find solutions to overcome them.

The studies carried out with the NA62 experiment have yielded the first tagged neutrino candidate in history. This major result proves that the method proposed in ENTER is viable. This result has been enthusiastically welcomed by the community. A publication describing the study is under review.

A preliminary study of the physical potential of a long baseline experiment exploiting the ENTER method has been published, showing record sensitivity for the key measurement of the lpetonic CP violating phase.

A collaboration has been formed with beam engineers at CERN to design beamlines to implement the ENTER method for long and short baseline experiments. Preliminary results are currently being published, showing that such a beamline is feasible for a long baseline experiment.

Moreover, a project for a short baseline experiment at CERN, aimed at improving effective cross-section measurements, is being developed in collaboration with the ENUBET project.

Studies on the limits of silicon pixel detector technology have unfortunately not been conclusive, as the quality of the data exploited has been insufficient. Nevertheless, a collaboration was established with the TIMESPOT/IGNITE project developing a new generation of highly time-resolved silicon pixel detectors. This collaboration has given rise to an ERC SYNERGY project, which is on the complementary list for funding.

A final study of all the data collected by NA62 will be carried out by 2026. This should lead to a more significant number of tagged neutrino candidates.

A project is currently under study with ENUBET and TIMESPOT/IGNITE to propose a short baseline experiment at CERN implementing the ENTER method. In the medium term, this project will enable precise measurement of neutrino interaction cross sections. This measurement is essential for future neutrino experiments.

The works developed in this project have been presented in several international conferences and workshop:

1. NeuTel [Perrin-Terrin]
agenda.infn.it/event/24250/contributions/130081/

2. PBC [Perrin-Terrin]
indico.cern.ch/event/1002356/contributions/4229626/attachments/2201704/3724138/2021-03-04_PBC_NuTAG_PerrinTerrin.pdf

3. VLVnT (plenary talk) [Perrin-Terrin]
indico.ific.uv.es/event/3965/contributions/14787/

4. IRN Neutrino [Perrin-Terrin]
indico.in2p3.fr/event/24095/contributions/95546/

5. ICRC [Perrin-Terrin]
indico.desy.de/event/27991/contributions/101374/

6. CERN EP-RD-WP1.1 [De Martino]
indico.cern.ch/event/959707/

The ENTER project aims at exploring an original experimental technique to provide Enhanced Neutrino Tagging and Energy Reconstruction (ENTER) for beam based neutrinos experiment.

Neutrinos are mysterious elementary particles extremely hard to detect due to their weak interaction with matter. They are always produced and detected in three quantum sates called flavour eigenstates. These states are in fact admixtures of the neutrino mass states and their compositions are determined by neutrino fundamental properties. During propagation, the admixture composition evolves, as if the mass states were travelling at different velocities due to their mass differences. Hence, after propagation, a neutrino can be detected in a flavour different from the one in which it was created.
Measuring the probability of these flavour transitions allows to access the fundamental properties of the neutrinos.

In practice, beam based neutrino experiments use high intensity beams of pions which naturally decay to a neutrino muon pair. The beam is oriented toward a detector where neutrinos interactions are observed.

The next generation of experiments aims at collecting large datasets of precisely reconstructed neutrinos. The solution normally adopted is to use a powerful neutrino beam aimed toward an ultra-granular neutrino detector.

ENTER challenges this paradigm. The innovative approach is to measure the neutrino properties (energy, direction, chirality, flavour and times) at birth, when the pion decays to a neutrino muon pair. Due to energy and momentum conservation, this decay is fully characterised once two of the three particles are reconstructed. Pions and muons being charged particles, they can be reconstructed using magnetic spectrometers allowing to reach unprecedented precision on the neutrino properties. The challenge is to build spectrometers able to operate at such high particle rates. Recent progress were made by the NA62 collaboration in developing such detectors, and devices able to sustain rates involved in neutrino beams will soon be within reach. With such spectrometers, neutrino detectors of coarser granularity could be employed allowing to build much bigger devices and thus collect larger neutrino datasets of unprecedented quality even with beams of moderate intensities.

The ENTER project will test the feasibility of this technique using NA62 benchmark. NA62 is producing a large (10^13) sample of kaon decays where the kaon and its decay products can be reconstructed. As most of these kaons decay to a neutrino muon pair, few hundred of these neutrinos are expected to interact in the NA62 calorimeter. The first goal of ENTER is to look for these neutrino interactions. For the first time ever, the decay of the kaon to muon and neutrino will be observed with all three particles detected. This ground-breaking result would prove that the ENTER way to beam based neutrino experiment is feasible.

The second goal of the project is to understand how to build beam trackers able to operated with very high particle rates. One of the key elements is the pixel tracker time resolution. Tests were made with the NA62 beam tracker to understand the fundamental processes limiting this resolution. The data collected during these tests will be analysed in particular to understand and quantify the impact of the pixel geometry. These analyses will be supported by precise simulations.

Finally, a simulation of a neutrino experiment using pion and muon spectrometers will be developed assuming that a neutrino beam is produced at Protvino (Russia) and oriented toward the KM3NeT-ORCA detector, in construction off-shore Toulon in France. Such a project called P2O is currently under study. These simulations will allow to determine more precisely the specifications for the spectrometers and beam line as well as to estimate the sensitivity of the experiment to measure the neutrino fundamental properties with this technology.