Neutrino mass hierarchy in JUNO

With the neutrino mixing angles and absolute values of the mass squared differences discovered, the focus of oscillation experiments is shifting towards the determination of the neutrino mass hierarchy: Is the ordering of the neutrino masses normal, i.e. m1 < m2 < m3 as for quarks and charged leptons? Or is it inverted, m3 < m1 < m2?
The answer to this question will have interesting consequences for model building and neutrino-less double beta decay experiments. Maybe most significantly, it is an integral step towards the discovery of CP violation in neutrino oscillation experiments.

Figure 1: The two possible orderings of the neutrino mass eigenstates.

 

The JUNO experiment

The Jiangmen Underground Neutrino Observatory (JUNO) is but one of several proposed next-generation experiments aiming for a discovery of the mass hierarchy. This will be achieved by a precise measurement of subdominant effects in the oscillations of reactor antineutrinos. The detector will be located near the city of Jiangmen (so. China) at the optimum oscillation baselines of ~55km from the Yangjiang and Taishan nuclear power stations. Laboratory construction has started in January 2015, while the start of data taking is expected for 2020.

Figure 2: Map of the experimental layout.

 

Detector concept

With 20 kilotons of target mass, JUNO will be far larger than any present-day liquid-scintillator detector. Moreover, an energy resolution of at least 3% at 1MeV is a prerequisite for resolving the small wiggles that are induced by the mass hierarchy (see below). This sets challenging requirements in terms of scintillator transparency, optical coverage and the understanding of detector systematics. As a result, though, JUNO will provide unprecedented sensitivity to low-energy neutrinos, allowing for a precision measurement of oscillation parameters and astrophysical neutrino fluxes.

Figure 3: Layout of the JUNO detector.

 

Determination of the neutrino mass hierarchy

As illustrated in figure 4, JUNO's sensitivity to the neutrino mass hierarchy arises from the small phase shift in the oscillation terms depending on the two large mass-squared differences Δm231 and Δm232. The corresponding interference in the electron antineutrino survival probability Pee becomes largest at a baseline of ~55km that coincides with the first solar oscillation maximum (governed by Δm221). Depending on the mass hierarchy, the position of maxima and minima of the subdominant oscillations undergo a shift of 180°. Resolving the position of the wiggles in the L/E spectrum requires an energy resolution of 3% at 1MeV, and an excellent understanding of the linearity of the energy response.
Based on 6 years of data-taking, the sensitivity to the mass hierarchy will reach the level of 3-4σ.
 

Figure 4: Reactor electron antineutrino survival probability as a
function of L/E. The mass hierarchy can be determined by the position of maxima and minima of the sub-dominant oscillation wiggles.

 

Precision measurement of oscillation parameters

In course of the hierarchy measurement, JUNO will as well perform a precise determination of the solar mixing angle θ12 as well as all mass squared differences. The aspired precision is better than 1%. This will be an important step towards tests of the unitarity of the PMNS mixing matrix and the neutrino masses sum rule, and allow to search for signs of other neutrino generations.

parameter
today
w/ JUNO
sin22θ12
6%
0.7%
Δm221
3%
0.6%
Δm232
5%
0.6%

 

Astrophysical neutrinos

Beyond JUNO’s core program of oscillation physics, JUNO will act as an observatory for low-energy astrophysical neutrinos: With a target mass of 20 kiloton and excellent energy resolution, JUNO will be able to perform high-resolution measurements of the fluxes and spectra of Supernova, solar and geoneutrinos. Moreover, large target mass and inherent background suppression capabilities will provide the opportunity to search for very rare events, greatly enhancing present-day experimental limits and sensitivities for the detection of the Diffuse Supernova Neutrino Background, dark-matter annihilation neutrinos, proton decay into kaon and antineutrino and other rare processes. Given the diversity of this program, JUNO will be a true multi-purpose facility.

 

Contribution by the working group at JGU Mainz

The group performs laboratory-scale experiments for the R&D and characterization of the LAB-based liquid scintillator that will be a crucial component of the JUNO detector. We are also involved in sensitivity studies and the development of the Monte-Carlo simulation and data analysis tools.

 

Interesting Links

  • JUNO homepage at IHEP Beijing.
  • Press release: Particle physicists from Mainz University participate in the JUNO neutrino experiment

     

    Theses

    Heike Enzmann, M.Sc. thesis (2017)

    Victoria Schuy, B.Sc. thesis (2016)

    Wilfried Depnering, M.Sc. thesis (2016)

    Related publications

    JUNO collaboration (2015)
    JUNO Conceptual Design Report

    JUNO collaboration (2015)
    Neutrino physics with JUNO

    M. Smirnov et al. (2015)
    A search for neutrino-antineutrino mass inequality by means of sterile neutrino oscillometry
    Nucl. Phys. B900 (2015) 104-114, arXiv:1505.02550

    Xiang Zhou, Qian Liu, Michael Wurm, et al. (2015)
    Rayleigh scattering of linear alkylbenzene in large liquid scintillator detectors
    Rev. Scient. Instrum. 86 (2015) 7, 073310, , arXiv:1504.00987