KM3NeT: The next big thing in neutrino physics

By Colin Adcock on June 24, 2016 • ( 2 )

The next generation square kilometre neutrino telescope known as KM3NeT has just published its Letter of Intent in JPhysG. We speak to Prof. Dr Maarten de Jong (Nikhef) spokesperson of the KM3NeT collaboration to find out what it’s all about.

Why is it important to study neutrinos?

Neutrinos are stable, sub-atomic particles which interact only weakly with normal matter. This weak interaction, despite its seeming irrelevance, plays a crucial role in the evolution of the Universe, the first synthesis of atomic nuclei and eventually life on Earth.To us, the weak interaction is a blessing and a curse. It makes neutrinos ideal messengers from the cosmos (almost nothing can stop them), but makes them also notoriously difficult to detect (hence, you need giant detectors). By detecting neutrinos, we will not only explore the Universe in an unprecedented manner but we will also investigate some of the fundamental properties of these enigmatic particles.

A photograph of a completed digital optical module.

Tell us about KM3NeT. What is it, and how will it work?

KM3NeT is the next generation neutrino telescope that will be built at the bottom of the Mediterranean Sea. To detect neutrinos from the cosmos, one needs giant detectors (with a typical mass of a billion tons). The medium of these detectors actually works as a converter target. First, a neutrino interacts with an atomic nucleus in the medium thereby producing relativistic charged particles. Secondly, the passage of these relativistic charged particles through the medium produces so-called Cherenkov light (the typical blue light on pictures of nuclear reactors). Finally, the Cherenkov light is detected by a 3-dimensional spatial array of ultra-fast photo-sensors.

For a cost-effective solution, the medium must be cheap, transparent (but shielded against daylight) and accessible. It turns out that the deep waters of the Mediterranean Sea are ideal. These natural waters come for free and are very transparent to the Cherenkov light. At a depth of several kilometres, there is no more daylight, so one can operate the neutrino telescope 24h/day and 7d/week. Last but not least, KM3NeT has developed a cost-effective technology for the “civil engineering” needed to build a research infrastructure on the bottom of the sea.

The KM3NeT research infrastructure will consist of three so-called building blocks. A building block comprises 115 vertical strings which are anchored on the seabed. Each string comprises 18 optical modules and each optical module comprises 31 photo-multiplier tubes (PMTs). Each building block thus constitutes a 3-dimensional spatial array of photo-sensors that can be used to detect the Cherenkov light produced by relativistic particles emerging from neutrino interactions. All analogue pulses from the PMTs are digitised offshore and all digital data are sent to shore where they are processed in real time. The signal events are filtered from the background using designated software.

What does KM3NeT add to the existing experiments and what are its ultimate goals?

The main objectives of the KM3NeT Collaboration are i) the discovery and subsequent observation of high-energy neutrino sources in the Universe and ii) the determination of the mass hierarchy of neutrinos.

The design of the KM3NeT optical modules offers a number of improvements compared to previous designs based on a single large area PMT, most notably: Larger photo-sensitive area, digital photon counting and directional information, wider field of view. The KM3NeT neutrino telescope offers the world’s best angular and energy resolution for high-energy neutrino astronomy and covers the part of the sky that complements IceCube and includes the Galactic plane (which may host the most nearby astrophysical sources of high-energy neutrinos). Furthermore, a part of the KM3NeT detector will have a dense configuration of the photo-sensors. This will allow for the first determination of the neutrino mass hierarchy.

Simulated signature of a ∼1 PeV atmospheric neutrino event. See Fig 26 in the manuscripts for more details.

Why is the neutrino mass hierarchy and the discovery of high-energy neutrino sources important to physics?

The neutrino mass hierarchy and a possible CP-violation are the two outstanding fundamental questions in neutrino physics. Probing CP-violation with neutrinos touches on an existential question: What caused the matter anti-matter asymmetry in the Universe?

The discovery of high-energy neutrino sources may answer the 100 year old question: Where are cosmic rays coming from? (cosmic rays are atomic nuclei which continuously bombard the Earth from outer space). In more general terms, neutrino astronomy constitutes one of the pillars in the so-called multi-messenger approach in which different signals from the Universe are jointly studied. With this approach, one can learn how cosmic particle accelerators work and what particles they accelerate. Ultimately, one can study particle physics at energies not accessible on Earth.

What stage is the project at and when could we start seeing results?

We are now in Phase-1 of KM3NeT. In this phase, we will build about 10% of the envisaged KM3NeT 2.0 research infrastructure. The funds for this phase have already been acquired. Recently, the 2016 road map of ESFRI (European Strategy Forum on Research Infrastructures) has been published in which KM3NeT has been selected as a high-priority project.

In the Letter of Intent, we present the main science objectives of KM3NeT, a description of the technology and a summary of the costs. It constitutes an important step towards the realisation of the KM3NeT 2.0 research infrastructure. It is generally considered as the “carte de la visite” in the community and potentially attracts the interest of new groups, policy makers and other persons. The next major milestone is the acquisition of the necessary funds to complete the KM3NeT 2.0 research infrastructure. If all goes according plan, the scientific capitalisation of KM3NeT 2.0 will start in 2020.

Finally, what is your motivation behind researching neutrino physics, and what advice can you give to budding scientists?

The discovery of the first (point) source of high-energy neutrinos in the Universe represents to me a glimpse of Heaven. It will mark the start a whole new field, namely that of neutrino astronomy. The neutrino mass hierarchy is a first step towards a better understanding of particle physics in general and neutrino physics in particular. The idea that neutrinos and the weak interaction are responsible for the imbalance between matter and anti-matter is thrilling.

In everyday life, neutrinos and the weak interaction seem insignificant. But the more we learn about neutrinos, the more significant they appear to be. So, I would advise budding scientists to take up the challenge to elucidate the fundamental properties of the neutrinos and to look at the Universe through the eyes of a neutrino.

Read the full KM3NeT letter of intent (Open Access)