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The Online Data Filter for the KM3NeT Neutrino Telescopes

10 Juni 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 filters 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)

KM3NeT has met in Les Houches for its Third Town Hall Meeting

18 April 2025 – The third KM3NeT Town Hall Meeting took place in the School of Physics of Les Houches, close to Chamonix (France), in front of the Mont Blanc, from 13th to 18th April 2025.

The meeting brought together a vibrant community of scientists to exchange ideas, present recent progress, and build new external collaborations in the field of neutrino astronomy.

Held in person, the agenda featured plenary sessions in the mornings and interactive working groups in the afternoons, fostering deep discussions between experimentalists and theorists. This meeting structure proved highly effective in encouraging meaningful collaboration and idea-sharing across the astroparticle physics community.

Highlights of the meeting included updates on the status and performance of KM3NeT’s ORCA and ARCA detectors, as well as discussions on ultra-high-energy events (among which, of course, KM3-230213A), Galactic neutrino sources and neutrino follow-up strategies. Additionally, poster sessions showcased ongoing analyses and new research directions.

With its promising angular resolution, broad energy range and sky-coverage, KM3NeT is positioned at the forefront of the multi-messenger astronomy era. The Town Hall Meeting reinforced the groundwork for exciting future collaborations.

On a lighter note, the event also featured networking sessions on mountain trails, where participants enjoyed scenic trekking routes, and a surprising April snowfall, which added both charm and challenge to the experience.

A sincere thank you to all participants, speakers, and organizers for making this event a success. We look forward to continuing the momentum and welcoming you to future KM3NeT events!

A new PMT for the KM3NeT detectors

7 April 2025 – We present a new paper with the title ‘Evaluation of the upgraded 3-inch Hamamatsu photomultiplier for the KM3NeT neutrino telescope’

In the paper we report the characteristics of a new photomultiplier tube for the KM3NeT detectors.

Our tests of more than 200 new Hamamatsu R14374-02 photomultiplier tubes (PMTs) show better quantum efficiency and more uniform response across the photocathode surface compared to the previous model.

We also measured a significantly better time performance of the new photomultiplier with cleaner, more precise signals and fewer delayed pulses and afterpulses.

This means sharper detection of the faint Cherenkov light generated by relativistic charged particles induced by neutrino interactions.  In particular it means a lower noise level and overall a better performance of the KM3NeT neutrino telescope.

The upgrade of the optical modules of our detectors with this new 3-inch PMT represents yet another step forward for deep-sea neutrino astronomy!

The paper has been submitted to the Journal of Instrumentation and is available as a preprint at arXiv 2504.02989.

In the picture the instrumental setup in the CAPACITY lab with PMTs for two optical modules ready for tests in a dark box. In the inlay the distributions of the percentage of measured afterpulses and delayed pulses for the new R14374-2 PMTs (blue) and the old R12199-2 PMTs (green). The distributions of the new PMTs peak at lower values and are significantly narrower.

A set of dedicated studies on KM3-230213A

20 February 2025 – Recently the KM3NeT Collaboration has published evidence for the cosmic neutrino with the highest energy ever detected (the article on Nature can be accessed from here). This event is identified as KM3-230213A.

In a set of dedicated studies, the Collaboration has investigated the possible sources of the event and the implications that may be derived from it.

These studies are included in a set of articles which have recently been released:

The ultra-high-energy event KM3-230213A within the global neutrino landscape

The compatibility of the occurrence of KM3-230213A with the constraints placed by other experiments is explored.

Read the full article here: https://arxiv.org/abs/2502.08173

 

On the Potential Galactic Origin of the Ultra-High-Energy Event KM3-230213A 

The possibility that KM3-230213A may have originated in our Galaxy is discussed. The study did not allow to identify plausible mechanisms and sources which could sustain such hypothesis, leading to the conclusion that the neutrino is most likely of extra-Galactic origin.

Read the full article here: https://arxiv.org/abs/2502.08387

Characterising Candidate Blazar Counterparts of the Ultra-High-Energy Event KM3-230213A

In this paper the possibility is explored that KM3-230213A may have originated in a distant blazar. The study concerned a set of 17 blazars, which were identified as plausible sources of high-energy neutrinos due to their multiwavelength properties, highlighting in particular three of them. This work involved plenty of facilities: KM3NeT, VLA, VLBA, RATAN-600, OVRO, Swift, Fermi, SRG/eROSITA, Gaia, CRTS, ATLAS, ZTF, WISE/NEOWISE, Chandra and ROSAT!

Read the full article here: https://arxiv.org/abs/2502.08484

On the potential cosmogenic origin of the ultra-high-energy event KM3-230213A

The intriguing possibility that KM3-230213A may be of cosmogenic origin, i.e. it was originated from the interaction of ultra-high-energy cosmic rays with ambient photon and matter fields, is discussed in this paper, leading to hypotheses for reconciling the occurrence of this event with the latest measurements of cosmic rays of extreme energy.

Read the full article here: https://arxiv.org/abs/2502.08508

KM3NeT Constraint on Lorentz-Violating Superluminal Neutrino Velocity

The Lorentz symmetry, the fundamental principle which states that nothing can go faster than the speed of light in vacuum, is tested in this study. By looking at the energy of KM3-230213A and the distance travelled, the difference between neutrino and light speed was constrained to less than 1 part in 1000 billion billion, which represents the most stringent limit ever set using this method of analyzing high-energy neutrinos.

Read the full article here: https://arxiv.org/abs/2502.08508

Limits on the violation of the Lorentz principle (see the article for full details)

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.