With the installation of two more detection units at the French site of KM3NeT, the first phase of building the ORCA detector of the KM3NeT neutrino telescope is now complete. Since 27 January 2020, the detector is taking data with six detection units. The first candidate neutrino events are already seen.

During a sea campaign in January, two new detection units were connected to the seafloor network at the KM3NeT/ORCA deep sea site, 40 km offshore from Toulon, France (Figure 1). The detection units were successfully positioned twenty metres apart to within a metre of their target position 2.5 km below the sea surface. This highlights the skills of the staff on board the deployment ship, the precision of the custom acoustic positioning system and the maturity of the deployment method based on an innovative launching vehicle. Using a robot, remotely operated from a second ship, the deployed units were connected to the seafloor network of the ORCA site. After a visual inspection of the detection units by the robot, the power was switched on and data taking with ORCA6 started immediately.

The ORCA6 detector

The ORCA detector has now six detection units - hence ORCA6. These are six vertical lines each with 18 optical modules (Figure 2), high-pressure resistant glass spheres housing 31 3-inch diameter photo-multiplier tubes to record the faint Cherenkov light generated by charged particles in the sea water. Each photo-multiplier records the intensity of the light flash, i.e. the number or photons, and - with a nanosecond precision - when it arrives. A compass, tilt meter and acoustic receiver record the position of the module in the sea water within a few centimetres. With these measurements the path the charged particle took through the detector is precisely reconstructed.

Searching for neutrino events

Physicists are  on shift around the clock, 7 days a week, to remotely operate the deep sea detector. The recorded data is stored in the computer centres of the KM3NeT Collaboration for further analysis. The first step is to reconstruct from the recorded light flashes the path of charged particles through the ORCA6 detector.  Most of them are muon particles generated in the Earth's atmosphere and travel downwards through the detector (Figure 3). A few others travel upwards (Figures 4 and 5); this is an indication that they are neutrinos that have passed through the Earth and interacted in the vicinity of the detector.

Important milestone for ORCA

Operating six detection units is an important milestone for KM3NeT as it marks the completion of the so-called 'Phase 1' of ORCA. In the next phase of KM3NeT/ORCA, the detector will be extended to 115 detection units. With the full ORCA detector, the KM3NeT researchers aim to determine the neutrino mass ordering using atmospheric neutrinos.

Figure 1: Deployment of a detection unit at the ORCA site of KM3NeT. © KM3NeT

Figure 2: The KM3NeT optical module. © KM3NeT

Figure 3: Example of a reconstructed down-going atmospheric muon observed by ORCA6.
Optical modules hit by the Cherenkov light are shown enlarged. © KM3NeT

Figure 4: Example of a reconstructed candidate neutrino event observed in ORCA6.
Optical modules hit by the Cherenkov light are shown enlarged. © KM3NeT


 

Figure 5: A so-called z-t plot of a candidate neutrino event measured in ORCA6.
The six plots correspond to the detection units. On the y-axis the height in the detector and the on the x-axis the time of arrival of the light. Each point represents a detected photon. The red points are the hits that triggered the event and the red line is the projection of the fitted Cherenkov light cone. © KM3NeT

author: Els de Wolf, Nikhef, Amsterdam