JUNO-TAO
Neutrino physics has gone through a revolution in the previous 25 years. The discovery of neutrino oscillations demonstrated that at least two of the three neutrino states carry mass, which is the first and by now the only direct observation of particle physics beyond the standard model. The flavor eigenstates turned out to be non-orthogonal to the mass eigenstates, leading to mixing and oscillations. Oscillations have been detected with neutrinos from the sun, neutrinos created in the atmosphere, from reactor neutrinos, and from long-baseline neutrino beams. The observed phenomena are described consistently by the PMNS matrix (Pontecorvo–Maki–Nakagawa–Sakata matrix). Today most of the parameters of the matrix have been measured with a precision better than 5%. The most important issue in neutrino physics is the measurement of the remaining parameters, i.e. the sign of ∆m322 and the CP-violating phase δ in the PMNS matrix, the determination of the absolute mass scale, the question of Majorana neutrinos and the search for new sterile neutrinos. The sign of ∆m322 is connected to the mass ordering (see Figure 1). JUNO in combination with its satellite detector TAO (Taishan Antineutrino Observatory) might become the decisive experiment for several of these open questions.
Scientific Motivation for the TAO Experiment
JUNO aims at simultaneously probing the two main frequencies of three-flavor neutrino oscillations, as well as their interference related to the mass ordering, at a distance of ~53 km from two powerful nuclear reactor complexes in China [1-3]. The present information on the reactor spectra is not meeting the requirements of an experiment like JUNO, with a design resolution of 3 % at 1 MeV. Unknown fine structures in the reactor spectrum might cause severe uncertainties, which could even make the interpretation of JUNO’s reactor neutrino data ambiguous. TAO is aiming for a measurement of the reactor neutrino spectrum at very low distances (< 30 m) to the 4.6 GWth core with a ground-breaking resolution better than 2 % at 1 MeV [4, 5]. Furthermore, TAO will make a major contribution in the investigation of the so-called reactor anomaly [6]. Present calculations of the reactor neutrino spectrum indicate a deficit of approx. 3 % in the measured reactor fluxes. Currently, these anomalies can be interpreted as indications for the existence of right-handed sterile neutrinos. Beyond that, the reactor neutrino spectra recorded by Double Chooz [7], Reno [8] and Daya Bay [9] show an excess in the neutrino flux from 5 MeV to 6 MeV of unknown origin. This can be considered as one of the most-puzzling questions in the physics of reactor neutrinos today. Beyond these studies, TAO will search for signatures of sterile neutrinos in the mass range of 1 eV, which have just regained importance in light of the recently reconfirmed gallium anomaly by the BEST Experiment [10]. An additional goal of the TAO experiment is the verification of the detector technology for reactor monitoring and safeguard applications for the future effective fight against the proliferation of nuclear weapons material.
The JUNO-TAO Detector Design
The TAO experiment (see Figure 2) will realize a neutrino detection rate via the inverse beta decay (IBD) of about 2x103 per day, which is approximately 30 times the rate in the JUNO main detector [4, 5, 11]. In order to achieve its goals, TAO is relying on cutting-edge technology, both in photosensor and liquid scintillator (LS) development which is expected to have an impact on future neutrino and Dark Matter detectors. The experiment will realize an optical coverage of the 2.8 tons of Gd-loaded LS close to 95 % with novel silicon photomultipliers (SiPMs), with a photon detection efficiency (PDE) well above 50 % [12]. To efficiently reduce the dark count rate of these light sensors, the entire detector will be cooled down to -50 °C. The combination of SiPMs with a cold high-performance LS will lead to an increase in the photo electron yield by a factor of 4.5 compared to the JUNO central detector [11].
Publications and References
[1] Fengpeng An et al., Neutrino physics with JUNO, J. Phys. G: Nucl. Part. Phys. 43, 03040, 2016
[2] H. Th. J. Steiger on behalf of the JUNO Collaboration, Design and Status of JUNO, J. Phys.: Conf. Ser. 1468, 012187, 2020.
[3] H. Th. J. Steiger, Design, Status and Physics Potential of JUNO, PoS DISCRETE 2020-2021, 086, 2022
[4] JUNO Collaboration, A. Abusleme et al., TAO Conceptual Design Report: A Precision Measurement of the Reactor Antineutrino Spectrum with Subpercent Energy Resolution, arXiv:2005.08745, 2020
[5] H. Th. J. Steiger on behalf of the JUNO Collaboration, TAO – The Taishan Antineutrino Observatory, Instruments 6(4), 50, 2022.
[6] G. Mention et al., The Reactor Antineutrino Anomaly, Phys. Rev. D 83, 073006, 2011.
[7] H. de Kerret et al., First Double Chooz θ13 Measurement via Total Neutron Capture Detection, Nature Physics 16, 558–564, 2020.
[8] G. Bak et al., Measurement of Reactor Antineutrino Oscillation Amplitude and Frequency at RENO, Phys. Rev. Lett. 121, 201801, 2018.
[9] Daya Bay Collaboration, F. An et al., Measurement of the Reactor Antineutrino Flux and Spectrum at Daya Bay, Phys. Rev. Lett. 116, 061801, 2016 [Erratum: Phys. Rev. Lett. 118, 099902, 2017].
[10] V. V. Barniov et al., Results from the Baksan Experiment on Sterile Transitions (BEST), Phys. Rev. Lett. 128, 232501, 2022.
[11] JUNO Collaboration, JUNO physics and detector, Prog. Part. Nucl. Phys. 123, 103927, 2022.
[12] Xu H. et al., Calibration strategy of the JUNO-TAO experiment, Eur. Phys. J. C 82, 1112, 2022.
Contact
For collaboration, thesis opportunities, or further inquiries, please contact:
Dr. rer. nat. Hans Steiger
- Tel.: +49 (89) 289 - 51320
- hans.steiger@tum.de