Fifty years ago, in 1972, the first neutrino conference was held in Hungary, because the organisers were not satisfied with the subdued position of neutrino interactions at the international conferences at that time. Nowadays, the neutrino conference is one of the major conferences in neutrino (astro)particle physics. Neutrino2022 took place in virtual Seoul, 30 May-4 June, 2022 and of course KM3NeT was there to show the results of data taking with the first detection units of ARCA and ORCA.

Aart Heijboer, physics coordinator of KM3NeT, showed the results of one year of data taking with ORCA6 and 100 days with ARCA6. The ‘6’  refers to the number of detection units in a detector used in the data analysis.

The ORCA detector is optimised to measure the oscillation parameters of neutrinos travelling through the Earth. Neutrino oscillation is a quantum mechanical phenomenon in which a neutrino created with a particular flavour – electron, muon or tau neutrino – can be later measured to have changed its flavour. In figure 1 below, it is evident that the data does NOT follow the flat blue horizontal line indicating the absence of neutrino oscillations. In other words, already with only six detection units, the ORCA6 detector ‘sees’ oscillations. In the second figure two oscillation parameters are plotted against each other. Clearly, the contour of ORCA6 is still wider than that of other experiments. More data with more detection units will make it narrower.


Also the ARCA detector, optimised for the search of high energy neutrinos from sources in the Universe, is well underway pushing the limits of the potential to discover sources of neutrinos down towards the expected limits of the full detector.


In the poster sessions KM3NeT physicists presented the details of many analyses being performed with the ARCA and ORCA detectors.

Aart Heijboer concluded at the plenary session that ARCA and ORCA will span eight decades in energy, that there is a rich variety of data analyses going on in the collaboration and that construction of the detectors is ramping up. Promising conclusions.

Very nice to have been invited to share the progress of KM3NeT with the community of neutrino (astro)physicists!

The KM3NeT Collaboration thanks the organisers of Neutrino2022 for an excellent edition of the conference. See you in two years time in Milano.


Welcome, ORCA-10!

22 November 2021 – Four new detection units have been added to ORCA with a 5-day intense and successful sea operation.

The operation has been performed with two ships: the Castor, equipped for deployment and precise installation of the detection units on the sea floor, and Janus, equipped with a remotely operated vehicle used for inspecting and for connecting the new units to the submarine infrastructure.

You can read all details of the operation in the daily reports published in our ORCA-blog.

Data taking with 10 detection units has been started!

One of the new detection units of ORCA outboard of Castor, preparing to its journey to the seafloor, 2,500 m below.

Sea campaign completed: 10 detection units in operation at ORCA

22 November 2021 – Excellent progress today, with the work finalized on the two detection units lately deployed: after a confirmation from the tests that both were ok, the unfurling was started of the two units. The launcher vehicles were safely recovered from the Castor; the ROV inspected both units after unfurling, confirming that both are in nominal configuration. A new functionality test was then performed from the shore station to ensure that both detection units work nominally.

As the sea condition rapidly degraded during the day, it was decided to quit the campaign. The remaining detection units on deck will be saved for next time.

The final outcome of the campaign is: ORCA enlarged from 6 to 10 detection units!

The KM3NeT Collaboration is grateful to the heroic teams who have made such great effort onboard the two ships and in the shore station, as well as to all institutes which contributed to the construction of the detection units. We look forward to the next deployment campaign. Having said that: it’s time to resume data taking with an enlarged ORCA-10 now!

Approaching the ORCA submarine junction box at 2,500 m depth – in the foreground is the connector that the ROV is preparing to plug on it.
The team onboard the Janus – on one of the screens is the sonar image of the ORCA telescope, showing the nodes and the detection units on the sea floor. Note that the operation was performed respecting all applicable rules for protection against COVID-19 – the team got so close together only for the purpose of the picture.
The team onboard the Castor showing the ’10’ sign in front of a recovered launcher vehicle.
The onshore team. The featured instrument is an Optical Spectrum Analyser, showing the signals of the 10 detection units of ORCA. Note that the operation was performed respecting all applicable rules for protection against COVID-19 – the team got so close together only for the purpose of the picture.

News from the ORCA campaign – hard work ongoing, bad weather approaching

21 November 2021 – The weather forecast does not look promising for next days. The main effort today was spent on some tuning of the equipment and preparation to next steps.

The situation at the sea bottom is: two new detection units have been already added to the ORCA detector; a third one, after installation, connection and test, is awaiting unfurling, and a fourth one is also on the sea floor awaiting to be connected – next step will be to connnect this one.

The video shows the overboarding of the ROV from the Janus – this takes place by means of a so-called A-frame (the structure at the stern of the ship which, rotating, reaches outboard).

Two new detection units added to ORCA… and counting

20 November 2021 – It was night and day again. The work continues around the clock when you are at sea. The second detection unit was also deployed, connected and tested last night. After that, unfurling was performed for the two detection units. A final test was then made from the shore to check that the two units are ok even after unfurling, et… voilà:  ORCA is enlarged to 8 detection units.

The campaign is now continuing with deployment of the next detection units; a third one is already on the sea bottom, connected and tested. A fourth one has been deployed in the evening: we will keep going until the deck is empty or the weather changes (the forecast is not ideal for next week): which of the two will happen first?

After unfurling of a detection unit, the launcher vehicle on which the unit is installed for deployment reaches the surface and is recovered for next campaigns.
Inspection of a detection unit, after touchdown on the sea floor (~2,500 m depth). The unit featured here is the fourth one which has been deployed in this campaign.

First day of activity at the ORCA site

19 November 2021 – The first day of activity has gone at the ORCA site. Most efforts today were aimed at getting everything well prepared for the action. The first detection unit was then deployed to the sea bottom, precisely placed on the sea floor and connected to the submarine infrastructure. The tests performed from shore confirm that the dection unit is nicely working, and it now awaits to be “unfurled” to its work configuration. Deployment of the second detection unit has been started in the evening.

Today was also the last day of our joint ANTARES-KM3NeT online Collaboration meeting, which ended with a cheerful activity to enjoy each other’s company even from a distance – you can read more about the meeting here.

The tho ships in action at the ORCA site: Castor (in the foreground) is equipped for deploying and accurately placing the detection units on the sea floor, while the Janus (in the background) is equipped with a ROV for submarine connections.

The movie shows the overboarding of one of the new detection units of ORCA.



Yet another virtual spring meeting

A few weeks ago, KM3NeT held its two-week long spring meeting, once again virtually, like almost all meetings nowadays.

With twelve detection units operating in the ARCA and ORCA detectors, it was a joy to discuss the progress of the data analysis groups and prepare for the reports at the summer conferences. With our smooth network of almost twenty production sites new detection units are being prepared at the maximum speed that the COVID-19 restrictions allow. New deployment campaigns are in preparation.

Although at a distance, we felt close to each other thanks to the virtual coffee breaks in the gather town set up by our colleagues of Laboratoire de Physique Corpusculaire de Caen (many thanks!). We concluded the meeting with an exciting quest to fix the unexpected problems found in a virtual shift room: this was a run against the clock to find out the password to get free from the locked room and reconvene for a final party at the bottom of the sea – real shifts won’t ever be so hilarious!

As usual, the meeting was also the occasion to welcome the many newcomers and to remind the accomplishments of those that are leaving the collaboration for a next step in their professional career. Thanks a lot for your work for KM3NeT. We wish you all the best and success in your  new working environment!

We sincerely congratulate Diego Real whose PhD thesis was recently awarded an important prize of the Spanish Society of Astronomy!

We were pleased to welcome new teams from the University of Toulon and Institut de Ciències del Mar in Barcelona – both aim at new investigations in the deep-sea environment and have already collaborated with the ANTARES telescope and the NEMO pilot project in the Mediterranean.

Among the new activities announced at the meeting: an Open Science Committee has been established, while the representatives of our early-career-scientists put forward a plan for making the life of our youngest collaborators easier even in these difficult times.

It was a fruitful and pleasant meeting!

The call for an institute to organise the next Collaboration meeting in the fall has been opened – hopefully the next meeting will be in person?

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.


Data taking with KM3NeT

19 August 2019 – Since this spring, the KM3NeT telescopes are routinely operating with five detection units: four at the ORCA site, one at the ARCA site. First data results have been reported on the international conferences and workshops.

For the ORCA detector, off shore the French Provencal coast, four units were installed and connected to the seabed network. An earlier deployed unit was damaged during inspection and had to be recovered for repair in the labs of the Collaboration. It will be re-deployed in a next sea campaign. Also during the Spring-campaigns, three autonomous acoustic beacons were deployed at the seabed in the vicinity of the ORCA array. They are used for acoustic positioning of the optical modules in the detection units that move with the slowly varying deep sea currents. Sea campaigns for further expansion of the ORCA detector are scheduled after the summer break.

Offshore Sicily at the site of ARCA, after a fix of the seabed network, a detection unit that was deployed three years ago, could be revived and is again taking data since. Currently, the seabed network is being re-designed to allow for the extension of the ARCA detector to more than 200 detector units. The successful though temporary fix of the existing network makes connection of more detection units possible, while waiting for the implementation of the upgraded network.


Differences between ARCA and ORCA

The technology used for the ARCA and ORCA detectors is  almost identical, but the difference in volume and height of the detectors and the density of optical modules in the detectors are strikingly different.  When finished, the volume of ARCA  will be more than 100 times larger then that of ORCA. ARCA will have a volume of about 1 Gton and ORCA ‘only’ about 8 Mton, while the number of optical modules in ARCA will only be twice that of ORCA: about 4000 vs about 2000. Consequently,  module density in ORCA will be about five times larger than that in ARCA. How is that achieved? In both detectors, eighteen optical modules are attached to each vertical detection unit. In ARCA, the distance between the lowest and the highest module is about 600. In ORCA this is about 150 m. Also the horizontal spacing between detection units is different: about 90 m in ARCA vs about 20 m in ORCA. Although, ARCA will have only twice the number of detection units, its foot print on the sea bed is much larger  that that of ORCA.

The geometrical differences reflect the main scientific purpose for which the detectors will be used. These are also visible in the first character of their names: ARCA stands for ‘Astroparticle Research with Cosmics in the Abyss’. The sparsely instrumented detector is optimised for the detection of high-energy cosmic neutrinos from distant sources in the Universe. ORCA is the acronym for ‘Oscillation Research with Cosmics in the Abyss’. The more densily instrumented detector is optimised to measure lower energy neutrinos,  thus providing data for the study of neutrinos oscillating between the three known neutrino flavours. The words ‘in the Abyss’ refer to the locations of the detectors several kilometres deep in the Mediterranean Sea.


KM3NeT: Comparison of the physical size of the ARCA and ORCA detectors.


ORCA is operational

8 March 2019 – Last month, the KM3NeT team of CPPM, Marseille together with the ship crews successfully installed an ORCA detection unit. It was the first unit connected to the refurbished main electro-optical cable to shore. After a few weeks of technology tests, the unit is given free for physics runs. ORCA is operational!

Unfortunately, after the deployment of one unit, the winch of the heavy lift line failed and three other units could not be deployed. They will be deployed during the next sea campaign.

In the mean time, KM3NeT researchers have taken up the duty of 24/7 shifts overlooking proper functioning of the detection units at both the ORCA and ARCA site. It is a pleasure to watch good quality data streaming to shore.

Pictures below: Four detection units in their deployment mode on deck of RV Castor (left), the package with the detection unit hanging on the heavy weight lift line just above the water surface (middle) and a plot of the signals that a down-going muon particle leaves in the detection unit: height vs the time of the recorded light signals (right).