Enhanced Neutrino Tagging and Energy Reconstruction – ENTER

The neutrino tagging concept can already by tested at the NA62 experiment at CERN. The experiment is primarily dedicated to rare kaon decays and, as the main kaon decay mode is K->mu nu, the intense kaon beam provides as a by-product a neutrino beam. A non negligible amount of the produced neutrino may interact and be detected in the experiment calorimeter. As the beam is instrumented with tracker the decay can be reconstructed and associated with the neutrino interaction thus demonstrating with real data the feasibility of the method.

Scaling this demonstrator into a real neutrino experiment would require dedicated design studies that will be carried out for the P2O project: a long base line neutrino experiment from the U70 accelerator complex in Protvino (Russia) to the KM3NeT-ORCA neutrino telescope installed in the abyss offshore of Toulon (France). The design will address the optimisation of the beam line and the trackers using the sensitivity to measure CP violation as figure of merit. To derive more realistic estimates, the algorithm used to reconstruct the oscillated neutrino flavor at ORCA will be re-optimise to account for the extra information provided by the tagging.

Technologically, the most challenging aspect of the neutrino tagging is the tracking of such a high intensity beam. These trackers have to provide excellent time resolution in order to resolve the track multiplicity. The only tracker featuring time resolved pixel is the NA62 beam spectrometer. Studies will be done to understand better the limitations of this detector and find solution to overpass them.

At mid term of the ENTER project several goals have been achieved.
First the trigger strategy to record the K->mu nu event at NA62 has been optimised. The main goal was to remove the down-scaling factor that was writing only 1 event out of 15. This new trigger line will be used for the 2021 data taking.
Second, studies on the silicon pixel time resolution has progressed using existing data that collected in a past beam test. The analysis of the data set is at the final stage and will give very valuable results on the impact of the pixel geometry. These studies also led to new collaborations with groups at CERN and in TRIUMF carrying R&D on the same topic. New sensors will be produced and tested through this collaboration.
Finally, the potential of a long baseline neutrino experiment from U70 Protvino to ORCA using the technique propose in this project was estimated with simple but conservative hypotheses. The results are very promising. P2O could measure the cp violating phase with an uncertainty up to 4 times smaller than the next generation experiments (DUNE, T2HKK). These results were presented in several international conferences and workshops and, as an outcome, ENTER is now being integrated at the CERN Physics Beyond Collider Study Group. In addition, a valuable synergy was found with the ENUBET project. Collaborations with this group are under discussion. Besides these collaborations, joining the PBC is very beneficial for the project visibility and credibility.

The new trigger strategy developed to record the K->mu nu event at NA62 will be tested in 2021. Data collected with it will be carefully scrutinized as soon as possible and a full analysis of the first fully reconstructed data will be made. For this analysis the simulation of the neutrino interaction will be implemented in the NA62 simulation software (on-going). Hopefully the first year of data will be sufficient to extract a significant handful of neutrino interaction. In this case a publication with a large impact can be expected.

In parallel, the study on the pixel time resolution will be finalized and published. In a second stage this experimental study may be confronted to simulation and/or to other experimental study using new sensors produced though the collaboration we established with CERN and TRIUMF.

The physics potential of tagged P2O will be refined with more precise simulations. In this view, a collaboration is being developed with a Russian colleague – Roman Sinyukov – to check and refine the beam line design initially proposed.
Once the beam line design will be available, the beam tracker design will be optimized based on simulations. These studies will allow us to derive the specifications of the trackers and to seek collaboration to develop the required technology.
Finally the ORCA reconstruction algorithm will be re-optimized to use the information provided by the trackers. At each step the physics potential of ENTER will be evaluated. A final A final publication of the results is envisaged.

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.