KM3NeT - oscillations

Quantum decoherence with ORCA6

30 October 2024 – Search for quantum decoherence in neutrino oscillations with six detection units of KM3NeT/ORCA

(KM3NeT paper, submitted to JCAP, arXiv: 2410.01388)

In a new paper we search for quantum decoherence in neutrino oscillations by looking for deviations of the standard neutrino oscillation pattern. We report upper limits on the decoherence parameters using data from ORCA6.

Usually, neutrino oscillations are studied in the framework of quantum mechanics assuming that the neutrino system is isolated. In this paper we study neutrino oscillations in the framework of open quantum systems, where the neutrino is coupled to a larger environment.

Several theories of quantum gravity postulate fluctuations in spacetime as a stochastic environment. A neutrino that propagates in such an environment will experience changes to its quantum phase. This will lead to a loss of coherence of the neutrino mass eigenstates during propagation. The phenomenon is referred to as decoherence in neutrino oscillations.

The search for decoherence in neutrino oscillations provides a rare opportunity to investigate quantum gravitational effects which are usually beyond the reach of current experiments.

The ORCA detector of KM3NeT is particularly designed to detect neutrinos generated in collision of cosmic ray particles with the Earth’s atmosphere. The atmospheric neutrinos are good probes to study oscillations and hence to search for quantum decoherence effects.

We used the neutrino data collected by the ORCA6 detector – an early detector configuration with six detection units –  in the period January 2020 to November 2021. In the analysis we focused on atmospheric neutrinos with energies of a few GeV to 100 GeV. We measured the parameters Γ21 and Γ31, that describe decoherence, assuming a power-law dependency on the neutrino energy Γij ∝ (E/E0)n and  explored two cases: with n = -2 and with n = -1.

Results

No significant deviation with respect to the standard oscillation hypothesis is observed. Therefore, 90% CL upper limits for the two cases are estimated as

The decoherence sensitivity of ORCA depends on the neutrino mass ordering. Therefore, we report upper limits for both normal and inverted ordering.

The results are comparable to bounds reported for IceCube/DeepCore and display the same dependency on the mass ordering.

In the figure below the 90% confidence level contours for Γ21 and Γ31 are shown for a decoherence model fitting both normal (NO) and inverted (IO) neutrino mass orderings.

 

New publication: Neutrino Mass Ordering and Oscillation Parameters

05 May 2021 – The potential of KM3NeT to measure key properties of neutrinos – in March 2021, the KM3NeT Collaboration released a publication showing that  KM3NeT with its ORCA detector will be in an excellent position to study the phenomenon of neutrino oscillations!

Three neutrino flavours and oscillation

Neutrinos come in three species called flavours: the electron neutrino, the muon neutrino, and the tau neutrino. In the 1960’s, the first experiment was started to study the sun by measuring the flux of electron neutrinos that the solar nuclear processes copiously produce. The experiment revealed that the flux was inconsistent with the expectations! Many solutions were put forward to explain the discrepancy until a measurement of the flux of neutrinos of all three flavours was made and found compatible with the expectation. This key measurement meant that the expectations for the neutrino flux produced by the sun were correct and that the electron neutrinos were converted into other flavours while traveling to Earth. This phenomenon is called neutrino oscillation, subsequently detected also in other contexts. This phenomenon is only explained by quantum mechanics and requires that the neutrinos, initially thought massless, are actually massive!

Neutrino admixture

The neutrinos with definite masses happen to be different from the neutrinos with definite flavours. In other words, a neutrino of a given flavour is an admixture of the neutrinos of definite mass as shown in the top part of fig:1. Because of the mass difference between the neutrino mass states, these states do not propagate at the same velocity. As a result, the neutrino admixture evolves during the propagation, as shown in the bottom part of fig:1. In other words, while propagating, the neutrino flavour changes.

 

Figure 1: Top:the mass state admixtures corresponding to the flavour (so-called weak) states for 2 neutrinos. Middle: a muon neutrino is produced at t=0. As time goes, the neutrino mixture varies reaching periodically a pure muon neutrino state. The probability for the neutrino to be detected in each flavour is represented at the bottom. Reproduced from Slansky et al. Los Alamos Sci. 25 (1997) pp. 28-63.

Using atmospheric neutrinos

The KM3NeT Collaboration aims to study this oscillation phenomenon using neutrinos produced in the collisions of cosmic rays onto the atmosphere. Using these neutrinos, the KM3NeT Collaboration will be able to measure one of the key parameters ruling the neutrino admixture: the so-called θ23 mixing angle. We will also be able to measure the squared mass difference between two of the neutrino mass states – δm232 – and to tell which of the three mass states is the heaviest, i.e. determining the neutrino mass ordering as shown in fig:2. Finally, we will check if the standard three neutrino oscillation paradigm is valid by measuring the fraction of cosmic-ray induced neutrinos that have oscillated to the tau neutrino.

Figure 2: Sensitivity to neutrino mass ordering as a function of data taking time for both normal (red upward pointing triangles) and inverted ordering (blue downward pointing triangles). See the paper for more details and the values of the oscillation parameters considered to obtain the result.

Unique potential

The publication relies on precise simulations to determine the sensitivity of the KM3NeT/ORCA detector to these parameters. The prospects show that the experiment has a unique ability to make these measurements and that world best results can be obtained in few years of data taking with the full detector.

The publication has been submitted to EPJ-C and is available as a pre-print as arXiv:2103.09885.