CE31 - Physique subatomique, sciences de l'Univers, structure et histoire de la Terre

Systematics UNcertainties in Combinations of neutrino Oscillation Results – SUNCORE

SUNCORE : Incertitudes systématiques dans les combinaisons de résultats d'oscillations de neutrinos

Neutrino oscillations are, so far, the only example of physics beyond the Standard Model. Such oscillations were first discovered by Super-Kamiokande in 1998 and have been confirmed by a large amount of experiments exploiting neutrinos produced by the Sun, by the Earth atmosphere, by nuclear reactors and by accelerators. <br />Accelerator are particularly powerful nowadays because they allow to observe oscillation of neutrinos and antineutrinos giving access to possible CP violating effects.

The goal of SUNCORE is to reduce flux and cross-section uncertainties in long baseline neutrino oscillation experiments and to combine different experiments in order to increase sensitivity to CP

The goal of the SUNCORE project is to reduce flux and cross-section systematics in long-baseline neutrino oscillations experiments towards the possible discovery of CP violation in the leptonic sector.<br />This reduction is possible thanks to the use of Near Detector complex in which neutrino beam is measured before the oscillations. Measurements of the Near Detectors allow to predict the expected spectra at the far detector and, comparing with the expected ones, to extract the neutrino oscillations parameters.<br />In T2K we are now working on an upgrade of the Near Detector complex that will allow more precise measurements of neutrino cross-section, by exploiting the hadronic part of the neutrino interactions.<br />In addition, the goal of SUNCORE is to work towards a combination of the T2K data with two other experiments, NOvA and SK, in order to improve the measurements of neutrino oscillations parameters. These combinations, in fact, allow to break some degeneracies between different oscillation effects (for example CP violation and mass hierarchy).<br />A pre-requisite for these analyses is a deep understanding of neutrino cross-sections at different energy range that is what is tackled within the SUNCORE project.

Most of the work of the SUNCORE project has been done within the Upgrade of the Near Detector of T2K. In 2019 the PI of SUNCORE has been named coordinator of this upgrade by the T2K collaboration.
This upgrade is a key ingredient for the success of SUNCORE because it will allow to further reduce cross-section and flux uncertainties in the T2K oscillation analyses and in the combinations with SK and NOvA. The total cost of the upgrade is of ~6 Meuro and, the initial goal was to install it in Japan by the beginning of 2022.
Delays mostly due to COVID have pushed back the installation of the upgrade. The current plan is to install the detectors in Summer/Fall 2022 and be able to start taking data at the beginning of 2023.
Within the SUNCORE project, and more in general in the LPNHE group, we are working on two aspects: the sensitivity studies of the upgrade that have recently been published in arxiv.org/abs/2108.11779 and the evaluation of the performances of the High Angle Time Projection Chambers, one of the new detectors of the upgrade. These latter results have been published here: arxiv.org/abs/2106.12634.
For both these papers, one of the corresponding authors is a member of the LPNHE group.

In addition the LPNHE group has worked on the conception and production of the TPC Front-End electronic cards and, one of the postdocs of SUNCORE, is in charge of developing the DAQ for the HA-TPCs.

Another deliverable of SUNCORE was the inclusion of the NA61/SHINE replica target data in the T2K flux prediction. These data are now included in the T2K Oscillation analyses and have allowed to reduce the flux uncertainties to 5%.

Concerning the combination of the oscillation analyses, we have decided to concentrate on the T2K/SK combination. One of the postdocs hired with SUNCORE is currently in charge of the combination of the detector systematics between T2K and SK that is the main needed ingredient for the combination of T2K and SK. This requires a careful study of correlations between SK detector systematics for the T2K samples and the SK atmospheric samples.

Members of SUNCORE are part of the T2K collaboration that recently published on the Nature journal first hints of CP violation in the leptonic sector. Such hints, if confirmed, could be an important brick in our understanding of the Universe and in particular on the reasons for which we leave in a matter dominated Universe.

In order to confirm this hints, more statistics and a better understanding of systematics uncertainties is required. This lead to the approval of the so-called T2K-II phase with upgrades of beam line and of the Near Detector complex. The PI of SUNCORE is leading the Near Detector upgrade and we are fully committed to install this detector within the year 2022.

This upgrade is expected to reduce systematics uncertainties by performing a full reconstruction of the final state of the neutrino interactions, including the hadronic part that is currently not used in T2K analyses.

In addition, member of the consortium have lead the inclusion of the NA61/SHINE data into T2K flux prediction, allowing for a reduction of uncertainties on the flux from 10 to 5%.

The next big milestone of SUNCORE is the installation of the Near Detector Upgrade expected by the end of 2022 and the exploitation of its first data.
In order to be ready for that we are working on the characterisation of the performances of the Time Projection Chambers as well as the overall expected performances of the Upgrade.

At the same time we are working for the combination of the existing T2K data with SK and NOvA. In particular, the combination with SK is being lead by members of SUNCORE project and Is expected to provide first results by the end of 2022.

Main papers published within the SUNCORE project;
1. Sensitivity of the Upgraded T2K Near Detector to constrain neutrino and anti-neutrino interactions with no mesons in the final state by exploiting nucleon-lepton correlations, submitted to PRD
2. Characterization of resistive Micromegas detectors for the upgrade of the T2K Near Detector Time Projection Chambers, submitted to NIM
3. Improved constraints on neutrino mixing from the T2K experiment with 3.13×1021 protons on target, Phys.Rev.D 103 (2021) 11, 112008
4. Constraint on the matter–antimatter symmetry-violating phase in neutrino oscillations, Nature 580 (2020) 7803, 339-344

The field of neutrino oscillations is entering the precision era: the three mixing angles are known and the community is trying to measure CP violation in the leptonic sector. Such measurement can only be done by observing the appearance of neutrinos of different flavors in long baseline experiments in which muon neutrinos are produced by an accelerator and the transition to electron neutrinos is observed with far detectors hundreds of km away. In order to observe CP violation the appearance probabilities for neutrinos and antineutrinos have to be carefully compared. The growing statistics accumulated by running experiments such as T2K calls for a better understanding of the systematic uncertainties that are dominated by uncertainties on neutrino fluxes and neutrino cross-sections. The goal of SUNCORE is to reduce these two sources of uncertainties in order to do early measurements of CP violation with T2K, with the combination between T2K, NOVA and SK and pave the way to precision measurements with future experiments (HK and DUNE).

Project coordination

Claudio Giganti (Laboratoire physique nucléaire et hautes énergies)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

LPNHE Laboratoire physique nucléaire et hautes énergies

Help of the ANR 296,960 euros
Beginning and duration of the scientific project: September 2019 - 48 Months

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