Neutrino oscillations with ORCA6
29 October 2024 – Measurement of neutrino oscillation parameters with the first six detection units of KM3NeT/ORCA
(KM3NeT paper, accepted by JHEP, arXiv: 2408.07015)
In a new paper we show the potential of the ORCA detector: the measurements of neutrino oscillation parameters – with only 5% of ORCA’s final volume and during limited observation time – align with those from other experiments and are already becoming competitive.
Since the discovery of neutrino oscillations, the three-flavour neutrino model with non-zero neutrino masses has become well-established. The oscillation parameters are being measured with improving precision by several experiments world-wide. However, several questions persist. Among them the question what the value of the neutrino mixing angle θ23 is and the question whether the ordering of the three neutrino masses is “normal” (NO) with m1<m2≪m3 or “inverted” (IO) with m3≪m1<m2. KM3NeT has joined the effort to answering these questions using its ORCA detector.
Neutrinos created in collisions of cosmic ray particles with the Earth’s atmosphere are good probes to study the neutrino oscillations. The ORCA detector in the Mediterranean Sea is particularly designed to detect atmospheric neutrinos. Therefore, it is an optimal detector for the measurement of neutrino oscillation parameters.
However, the ORCA detector is still under construction. The paper reports on the measurements with ORCA6 – an initial detector configuration that comprises six out of the foreseen 115 detection units. We extracted a high-purity neutrino sample, corresponding to an exposure of 433 kton-years. The sample of 5828 neutrino candidates was analysed following a binned log-likelihood method in the reconstructed neutrino energy and the cosine of the zenith angle.
Results
The atmospheric oscillation parameters measured with ORCA6 for both normal (NO) and inverted (IO) neutrino mass ordering are:
The inverted neutrino mass ordering hypothesis is disfavoured with a p-value of 0.25.
The 90% confidence level contours for the fitted oscillation parameters are shown in the left figure below with a solid line assuming normal ordering (NO) and a dashed line assuming inverted ordering (IO).
In the figure at the right you find a comparison of the contour – assuming NO – with the corresponding contours measured by other experiments. The ORCA contour aligns with those other measurements and, moreover, it shows that ORCA is becoming competitive.
