KM3NeT - KM3NeT

Atmospheric neutrino flux measured with ORCA6

15 July 2025 – The European Physical Journal C has accepted for publication a new KM3NeT paper with the title ‘Measurement of the atmospheric νμ flux with six detection units of KM3NeT/ORCA‘.

For the study we used data recorded by the ORCA6 detector between January 2020 and November 2021. Using machine learning classification, we selected 3894 neutrino candidates and estimated the background from atmospheric muons less that 1%. We derived the atmospheric muon-neutrino energy spectrum using an unfolding procedure and found that the measured flux is in agreement with values measured by other experiments and with theoretical predictions.

To date, the ORCA detector has 28 detection units (ORCA28). We expect that our measurement of the atmospheric muon-neutrino flux will improve with more and more ORCA detection units installed.

The paper is available as a preprint at arXiv 2504.09119.

In the figures below the atmospheric muon-neutrino flux measured with ORCA6 as a function of  neutrino energy: (a) in comparison with the HKKM14 model; (b) in comparison with the measurements of other experiments.

(a)

(b)

 

KM3NeT AISBL is born!

20 June 2025 – A very important step forward for KM3NeT has been accomplished this week by establishing the KM3NeT AISBL.

An Association Internationale Sans But Lucratif (AISBL) is a non-profit legal entity established according to Belgian law and recognised at international level. This is the form of legal entity which had been identified as the most appropriate one for KM3NeT after a detailed evaluation conducted through the KM3NeT-Infradev2 project, supported by the EU (Grant Agreement no. 101079679). Setting up a legal entity for KM3NeT was in particular the main objective of the work package 2 of the project, coordinated by Piera Sapienza of INFN.

This new entity will allow for an efficient and legally binding reorganisation of the KM3NeT activities for construction, installation, operation, maintenance, scientific exploitation and decommissioning of the infrastructure.

On 18 June 2025, in Brussels, the KM3NeT AISBL was signed by Nicolas Leroy, Ekaterini Tzamariudaki, Juan de Dios Zornoza, Marco Pallavicini and Jorgen D’Hondt, representing the AISBL founding members, respectively: CNRS (France), INPP/NCSRD (Greece), University of Valencia (Spain), INFN (Italy) and NWO-I (the Netherlands). The other institutes of KM3NeT are expected to join soon, either as full members or observers.

The Online Data Filter for the KM3NeT Neutrino Telescopes

10 June 2025 – Recently, we submitted a new paper with the title ‘The Online Data Filter for the KM3NeT Neutrino Telescopes’

In this paper, we present the design and performance of the software that is used to filter the data recorded by the photo-sensors of the KM3NeT detectors.

The KM3NeT telescopes

The KM3NeT collaboration is constructing a large research infrastructure at the bottom of the Mediterranean Sea comprising two telescopes named ARCA and ORCA. Unlike conventional telescopes, the KM3NeT telescopes detect neutrinos and not light from the cosmos.

Neutrinos are notoriously difficult to detect. To overcome this difficulty, KM3NeT uses water in the deep sea. Neutrinos are detected indirectly using three-dimensional arrays of photo-sensors which detect the Cherenkov light that is produced when relativistic charged particles emerge from a neutrino interaction. The density of the water in the deep sea provides for the necessary mass for neutrinos to interact and its transparency for a sufficiently large detection volume.

The online data filter

To filter the data recorded by the photo-sensors in the deep sea, we have implemented a custom designed software system. First, the analogue pulses from the photo-sensors are digitised offshore in the deep sea. Then, all digital data are sent to a control station onshore where they are processed in real time using a farm of commodity servers and custom software.

The filter quality

We have evaluated the performance of the data filter in three terms: its purity, its capacity and its efficiency. The purity – or signal-to-noise ratio – is measured by a comparison of the event rate caused by muons produced by cosmic ray interactions in the Earth’s atmosphere with the event rate caused by the background from decays of radioactive elements in the sea water and bioluminescence. The capacity of the filter is measured by the minimal number of computer servers that is needed to sustain the rate of incoming data. The efficiency is measured by the effective detection volumes of the sensor arrays.

Different event topologies

In nature three different flavours of neutrinos exist, namely electron, muon and tau. They are named after the charged particle that emerges from the neutrino interaction with matter. In the KM3NeT detectors these charged particles are recognised by the different topologies of photo-sensors hit by the Cherenkov light. In particular, a muon produces a long linear track of sensors hit, while the electron reveals itself as a ‘shower’ of sensors hit in multiple directions. Each type of neutrino yields a different effective detection volume.

The effective detection volumes in the figures

As an example, we present in the figures below the effective detection volumes of the ARCA telescope as a function of the neutrino energy for muon- and electron-neutrinos using two designated software algorithms. They show that in both cases the largest volume is obtained with the algorithm that matches the neutrino flavour.

They also show that for neutrinos of about 1 TeV the effective volume reaches the geometrial volume of the detector (dashed line). Below this threshold, the effective volume is smaller due to limited visible energy. Beyond the threshold, the growth of the effective volume can be attributed to neutrino interactions in the vicinity of the detector.

The paper has been submitted to section A of the journal on Nuclear Instruments and Methods in Physics Research – (NIM-A) and is available as a preprint at arXiv 2506.05881.

2nd DOM integration workshop

27 May 2025 – Do you know how the optical modules of KM3NeT are built? This has been shown and practised in the second edition of the Digital Optical Module (DOM) integration workshop which took place last week.

In total 30 people participated in the workshop. They came from the eight KM3NeT integration labs, including the new lab in Salerno, Italy. Experts from the KM3NeT steering committee and the central and local quality management were also present.

The workshop was purely hands-on! Each step in mounting a DOM was scrutinised, while the participants shared their experience and the procedures were discussed.

The workshop took place in the CAPACITY laboratory in Caserta, Italy, taking advantage of the new facilities which were recently inaugurated.

The construction of the KM3NeT optical modules consists of many steps, comprising several delicate operations. The final product is a pressure-resistant glass sphere which contains 31 photomultipliers and various electronics devices for the power supply and acquisition and transmission of data.

In addition, the optical modules contain important calibration devices, such as a compass, a piezoacoustic sensor for positioning the modules and a fast LED pulser, the nanobeacon, for calibrating the photomultipliers. They are fundamental for pushing the performance of the KM3NeT neutrino detectors.

The two hemispheres which compose an optical module are assembled and tested separately. When everything is installed and all functional tests are passed, it is time to proceed to non-reversible steps of integration, such as pouring optical gel in the interface between the photomultipliers and the glass of the hemispheres. Finally, the optical module can be closed and sealed. After undergoing a last acceptance test, the module is ready for being integrated in a detection line.

All integration and test procedures strictly comply with the high quality standards of KM3NeT.

And what if a problem occurs in a completed and sealed optical module? Is it possible to open it? The answer is yes! But with a very very delicate procedure.

Below are some pictures taken during the workshop.

4 new detection units installed in ORCA

16 May 2025 – This week at the ORCA site, a sea operation was performed with a twofold purpose: the recovery from the sea bottom of some oceanographic instruments which required some maintenance and the installation of a set of 4 new detection units. The number of detection units in ORCA has thus been increased to 28.

The field is getting crowded!
This is a sonar map of the ORCA site after the installation of the new detection units. Also marked in the image are the various components of the submarine infrastructure, comprising a junction box (“JB1”), a module for interface with oceanographic instrumentation (Module Interface Instrumented – MII) and a calibration structure (Calibration Base – CB).

As usual, the operation was performed with two ships: the Castor of Foselev, for deployment of the detection units, and the Janus II of SAAS (formerly Comex), equipped with the Apache deep-sea remotely operated vehicle, for submarine operations.

Everything worked very smoothly – many thanks to the crews offshore as well as to the team who performed the functional tests of the new detection units from the shore station!

 

The Castor at the end of the sea campaign.

Inauguration of the new facilities of the CAPACITY Laboratory in Caserta

14  May 2025 – Today the new facilities of the CAPACITY laboratory in Caserta have been inaugurated.

The extension of the CAPACITY laboratory will allow for building, testing and integrating a large fraction of the KM3NeT digital optical modules, base modules and detection units, thus considerably speeding up the construction of the KM3NeT detectors. The laboratory is in fact equipped to facilitate all the integration steps which are necessary for building complete detection units and for preparing them in the packed configuration used for deployment.

In addition, the CAPACITY laboratory hosts sophisticated test facilities, including a state-of-the-art laboratory dedicated to the characterisation of optical sensors, a large tank for tests of digital optical modules in water, a large thermal chamber and more, allowing for extensive tests of different components to be carried out at the site. The CAPACITY laboratory also hosts the European Logistics Center of the Collaboration, where the components needed for detector construction are collected for distribution to the integration sites.

CAPACITY (Campania AstroPArtiCle InfrastrucTure facilitY) is a laboratory created in 2019 thanks to the joint action of the Italian National Institute of Nuclear Physics (INFN) and the University of Campania “L. Vanvitelli”, within the Research Laboratories Centre (POLAR) of the Department of Mathematics and Physics of the University of Campania.

The new CAPACITY facilities have been made possible by the efforts of the institutions, the University of Campania “L. Vanvitelli” and the Italian National Institute of Nuclear Physics, in the framework of the NextGenerationEU Italian PNRR KM3NeT4RR project. With KM3NeT4RR, crucial actions towards the expansion of the KM3NeT Italian site off the coast of Capo Passero in Sicily have been funded. These include the extension of the submarine infrastructure and the strengthening of the detector integration laboratories and of its testing facilities.

Lucio Gialanella (left) and Pasquale Migliozzi (right), respectively the representative of University of Campania “L. Vanvitelli” and the CAPACITY director, at the inauguration time

 

Paul de Jong, KM3NeT Spokesperson

 

One of the new integration halls at CAPACITY (for DOM integration)

Observation of an ultra-high-energy cosmic neutrino with KM3NeT

On February 12, 2025, The KM3NeT Collaboration has announced the detection from the abyss of the Mediterranean Sea of a cosmic neutrino with a record-breaking energy of about 220 PeV.

More information can be found in the:

Additionally, 4 companion papers have been published on arXiv:

Tau neutrinos and neutrino mixing with ORCA6

6 February 2025 – We present a new paper with the title ‘A study of tau neutrinos and non-unitary neutrino mixing with the first six detection units of KM3NeT/ORCA‘.

Oscillations of atmospheric muon and electron neutrinos produce low-energy tau neutrinos, which can be observed by the ORCA detector of the KM3NeT neutrino telescope. For a first measurement we used the ORCA6 configuration, an early subarray corresponding to about 5% of the final detector. For the study we selected a sample of 5,828 neutrino candidates.

The measured ντ normalisation, defined as the ratio between the number of observed and expected tau neutrino events, is

This translates into a ντ charged-current cross section of

at a median ντ energy of 20.3 GeV. The result is consistent with the measurements of other experiments. In addition, we could improve the current limit on the non-unitarity parameter affecting the τ-row of the neutrino mixing matrix with α33 > 0.95 at 95% confidence level.

The paper is submitted to the Journal of High Energy Physics.

A preprint is available at arXiv 2502.01443

In the figures a comparison of our results with those from other experiments.

A fruitful Collaboration Meeting in Louvain-la-Neuve!

4 February 2025 – Last week, KM3NeT has met, both in person and online, for its winter Collaboration Meeting , in Louvain-la-Neuve, Belgium, hosted by UCLouvain.

Various discussions and presentations highlighted progresses and activities related to the ARCA and ORCA detectors, including updates on construction, simulation, calibration and data analysis efforts. The meeting also featured talks on the latest scientific results, including…plans for very exciting results to be announced soon!

Beyond the scientific sessions, the event fostered community engagement through social activities and networking opportunities.

It was also the occasion to welcome our new Management Team: Paul De Jong (Nikhef and University of Amsterdam, The Netherlands) serves as Spokesperson, Damien Dornic (CPPM/CNRS, France) as Deputy Spokesperson, Rosa Coniglione (INFN-LNS, Italy) as the Physics and Software Manager, and Antonio D’Amico (Nikhef, The Netherlands) holds the position of Technical Project Manager. The entire Collaboration extends its congratulations to the outgoing Management Team and wishes the best of luck to its newly elected members.

The KM3Net Collaboration is pleased to welcome a new team, from INFN and University of Florence, Italy, coordinated by Nicola Mori: we are happy to have you as part of our Collaboration and look forward to your valuable contributions.

Thanks a lot to the whole local team for the wonderful organization.

The next Collaboration Meeting is scheduled for the coming summer, in France, at Caen.

The KM3NeT Management team (from left to right): Damien Dornic, Paul De Jong, Rosa Coniglione and Antonio D’Amico.

 

 

 

 

Sneak peek into the KM3NeT labs: Nikhef

17 January 2025 – Today we start a series of items highlighting the work of our technical staff in the labs of KM3NeT. Numerous technicians and engineers are working on the construction of the ARCA and ORCA detectors of the KM3NeT neutrino telescope. Together, but in distributed labs, they design and build the many detector components, assemble them into thousands of optical modules and integrate them into hundreds of deployment-ready detection units. It requires high standards of quality control and logistics between the labs.

Spotlights on the Nikhef lab

In this first item we set the spotlights on the KM3NeT production lab of the Nikhef institute in Amsterdam, The Netherlands. It is the lab where our multi-PMT optical module was born and it is one of the first production labs in KM3NeT.

Video’s in this item: Courtesy Nikhef – Marco Kraan. He has filmed the full process of assembling an optical module at Nikhef. Below we use his collection of short video clips.

“You just have to get the hang of it” according to Menno de Graaff, electronics engineer featuring in the clips.

The KM3NeT multi-PMT optical module is the key component of the neutrino telescope. It is a complex sensor module that registers the Cherenkov light from the charged particles from neutrino interactions with the seawater and monitors the position of the module in the deep sea. All sensors and the electronic boards for their power and readout are densely packed in a pressure resistant glass sphere. Therefore, assembly of the module requires highly skilled technicians.

(Compilation video (3min))

Step 1 – The assembly of an optical module starts with the empty pressure resistant glass hemispheres.

Step 2 – In the top hemisphere an aluminum cooling ‘mushroom’ is glued to the glass using a gel, which is applied in layers to avoid bubble forming in the gel. Once in the deep sea the cooling mushroom will keep the temperature in the optical module below 30 degrees.

Step 3 – In the bottom hemisphere an acoustic sensor is glued to the glass using a hard adhesive for good contact with the glass. The sensor is part of the acoustic system used to monitor the position of the optical module in the deep sea.

Step 4 – Two electronic boards are installed in the cooling mushroom in the top hemisphere. One provides the electrical power to the sensors in the module. The other is the heart of the module – the central logic board that collects and digitises the signals of all sensors in the module and transmit the digitised data via optical fibres to the backbone cable of the detection unit that is connected to the electro-optical cable network toward the control station on the shore. The connection is made via a feedthrough in a hole in the glass hemisphere. It is carefully checked that the hole is leaktight after installation of the feedthrough.

Step 5 – On top of the central logic board a fibre tray is installed with the fibres that connect the board with the backbone cable of the detection unit.

Step 6 – Switch from the glass spheres to stacking the 31 photomultiplier tubes (PMTs). In the structure that later fills the top glass hemisphere, 12 PMTs are stacked leaving room for the cooling mushroom to pass. A reflective metal ring is put around each PMT to improve their light collection.

Step 7 – In the structure that later will be placed in the bottom glass hemisphere, 19 PMTs are stacked. Together the 31 PMTs form a ‘fly’s eye’ looking in almost all directions. The PMTs will ‘see’ the faint Cherenkov light of charged particles induced by neutrino interactions in the deep sea.

Step 8 – The structures filled with PMTs are placed in the prepared glass hemispheres and connected to the ‘octopus’ electronics boards which are placed in the space left between the PMTs. The hemispheres are ready for a functional test in the test room.

The two structures are connected through a test cable to verify the functional behaviour of the optical module before closing it. During the functional test, the PMTs, acoustic piezo sensor and a LED are verified to work according to specification. In addition, fibre and power connections are checked, as well as the temperature of the electronic boards.

Step 9 – Once the functional test is passed, the PMTs are fixed on their position in the glass sphere and the space between the PMTs and the glass is filled with a special highly transparent gel. This is necessary in order not to loose light during operation in the deep sea. The filling must be done with care to avoid forming of air bubbles in the gel. Air bubbles would distort the path of the light.

Step 10 – Time to close the two hemispheres. The trick is that the readout electronics boards of the PMTs in the lower hemisphere of the module must be mounted on the stem of the cooling ‘mushroom’ before closing the optical module. When everything is connected correctly, the module is closed and some air is removed from it to create a small underpression. Then the module is sealed with a sticky strip to prevent the two hemispheres getting loose during transportation or during deployment to the bottom of the Mediterranean Sea. Once in operation the two remain tightly close due to the high pressure in the deep sea. Finally, a collar for attachment to the supporting ropes of the detection unit is mounted.

Step 11 – The module is ready for connection to the electro-optical backbone cable of a detection unit. The backbone cable runs the full length of hundreds of metres along the detection unit. It comprises copper wires for electrical power and optical fibres for data transmission. The video clip presents a birds-eye view of two sets of the final product of 18 optical modules connected with a backbone cable.

Step 12 –  The 18 optical modules connected to a backbone cable are shipped to the labs in France or Italy, which will build a ready-to-deploy detection unit and load it onto a launching vehicle for deployment in the deep sea.

This marks the end of our highlight of the work of technicians and engineers constructing KM3NeT multi-PMT optical modules and detection units at Nikhef.

In a next edition we will report on the work of technical staff in other KM3NeT labs.

Interested in the technical details of the KM3NeT multi-PMT optical module? We published a paper here.