Neutrino hunting underwater telescopes probe origins

The Baikal project in Russia One of the new telescopes will replace existing equipment already installed in a deep Russian lake

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The number of "eyes" searching for a particle that could shed light on our universe's formation is about to multiply.

High-energy cosmic neutrinos are only able to be detected by a few existing detectors hidden in what may seem bizarre places - inside mountains, underground, underwater and even in solid ice.

Operators use them to unravel the mysteries of cosmos, aiming to provide insights into the nature of dark matter, the evolution of stars and the origin of cosmic rays.

They may also be able to test the results of recent experiments that suggested neutrinos were faster than light which were carried out a Cern, the world's biggest physics laboratory.

Soon two more telescopes will join the network.

The first, a 1 cubic km (0.24 cubic miles) detector, is set to replace an existing small octopus-like device floating more than 1km (0.6 miles) below Russia's Lake Baikal.

The second is destined for the bottom of the Mediterranean and will dwarf its counterpart.

KM3NeT - an acronym of "kilometre-cubed neutrino telescope" - will sit at depths of 3 to 5km and is planned to have a volume of some 5 cu km.

It will consist of a number of vertical strings with spherical modules attached to them. These glass "balls" contain sensors which search for the neutrinos.

Each string will be almost 1km long - so once the entire structure "stands" at the bottom of the Mediterranean, it will be taller than the highest building in the world, the 830-metre Burj Khalifa in Dubai.

The thousands of pressure-resistant optical sensors will register rare and faint flashes of the so-called Cherenkov light - electromagnetic radiation emitted by charged particles originating from collisions of high-energy neutrinos and the Earth.

Neutrinos

Neutrinos are among the most basic building blocks of the Universe - tiny "elementary" particles.

They are produced in certain types of radioactive decay and in nuclear reactions - including those that occur within stars.

The particles are also generated when cosmic rays interact with the other matter in our Universe.

They have no electric charge and negligible mass (a neutrino has less than a billionth the mass of a single hydrogen atom).

The particles come in several different versions, and can flip between these different neutrino "flavours".

Neutrinos interact weakly with other types of matter, which has led to their nickname: "ghost particles".

In fact, a neutrino can pass through about six trillion miles of lead without hitting a single atom.

Researchers in Italy say the particles seem to travel faster than the speed of light.

But this result is still unexplained and may be due to an error.

Like all the other neutrino telescopes, KM3NeT needs to be in as deep and dark a place as possible, to screen out the other particles which bombard our planet from above.

This European effort involves 40 institutes or university groups from 10 countries.

At the moment, there are several neutrino detectors, but only three are searching for the high-energy elusive particle. They are NT-200 in Baikal; Antares, located 2.5km under the Mediterranean; and IceCube in the ice at the South Pole.

To embrace the entire Earth, neutrino telescopes must be located in both northern and southern hemispheres, peering in opposite directions.

'Ghost particles'

Our universe hosts many violent processes including supernovae stellar explosions, star collisions and huge cosmic blasts known as gamma-ray bursts.

These phenomena accelerate charged particles to extremely high energies, far exceeding those reached in laboratory experiments on Earth, creating what is known as high-energy cosmic rays.

The rays propagate through the universe, and rain down on the Earth's atmosphere.

Although astronomers have registered cosmic rays for years, they have been unable to pinpoint their sources.

High-energy neutrinos, think scientists, may be able to help.

Graphic showing KM3Net telescope The KM3NeT telescope is set to be made up of hundreds of slender strings, each supporting dozens of sensors

These subatomic particles are born from cosmic rays' reaction with matter in the universe, so they are believed to originate at the heart of the same violent processes as the rays.

But unlike cosmic rays, neutrinos have no electric charge and almost zero mass.

They have such little interaction with normal matter that they travel unhindered through space, covering great distances. That includes passing through every one of us and our planet, in a straight line.

Being able to speed through the universe without deviation or absorption means they should be able to point back to their origin, making them unique cosmic messengers.

"Registering a high-energy neutrino could be our opportunity to see its source - and it would also guarantee that high energy cosmic rays come from there too, helping us learn more about them and the universe," says Dr Oleg Kalekin, one of the researchers working on the project at the University of Erlangen in Germany.

But detecting a high-energy neutrino is very tricky.

They are so difficult to spot that scientists have nicknamed them "ghost particles".

Lower-energy cosmic neutrinos originating in our Sun, in the Earth's atmosphere, and even in a supernova from a nearby dwarf galaxy have been registered.

However, we have yet to "catch" a high-energy astrophysical neutrino from far away, and there has only been indirect proof of its existence.

From big to bigger

Stumped by continuous failures to spot a real distant traveller, researchers now believe they need to act big.

Hidden telescopes

The telescopes searching for high-energy astrophysical neutrinos are nothing like normal telescopes that astronomers point at the sky.

Not only do these devices look very different, they also have to be either deep underwater, underground or in solid ice. This is done to filter out low-energy atmospheric neutrinos that are created when cosmic rays pass through the atmosphere.

These low-energy particles constantly rain at us from above, and make it extremely difficult to register the neutrinos that come from faraway space.

But once deep underwater, in the darkness, the telescopes manage to screen out most of this "background noise" - in the hope of spotting the particles that originated from distant parts of the universe.

"The neutrino observational window at low energies has been opened," says Dr Christian Spiering of DESY, a German research centre for particle physics, who has been involved in the KM3NeT project.

"We want to open it at high energies and see how this terra incognita looks.

"To do that, we need bigger detectors."

Bigger, he explains, is at least 1cu km. This is why the IceCube detector was built. It started working at full capacity in 2010 and may get even larger in future.

Hence the plans for the lake telescope - Baikal-GVD (Gigaton Volume Detector) - and the colossal KM3NeT.

Although no one has been able to register a high-energy neutrino, the race is on to get there first, says astrophysicist Bair Shaibonov, from the Joint Institute of Nuclear Research in Dubna - one of the researchers participating in the Baikal project.

This is why, he says, the detector in Russia is being upgraded.

Sinking the sensors into icy Lake Baikal helps filter out the "noise" of other neutrino particles

Organisers plan to submerge the first 350m-long vertical string with attached spherical modules during next year's annual expedition out on Baikal's thick ice in March and April.

Conditions at Baikal, the world's deepest lake, are ideal for a neutrino telescope, he adds.

"We have a metre-thick ice, a natural platform for upgrades and repairs. There are no storms, and the water is fresh, so the equipment doesn't rust as quickly.

"And building a huge telescope here would be only a fraction of the cost of KM3NeT or IceCube."

For now, the ambitious project is backed by only a few Russian institutes.

Neutrino telescope Computer artwork of a neutrino telescope detecting Cherenkov radiation (blue light)

Whether the effort will make it beyond the first string, to the full 1 cu km, will depend on funding. The project relies both on private grants and the support of the Russian government.

The scientist overseeing Baikal-GVD, professor Grigorii Domogatskii of the Institute for Nuclear Research, Russian Academy of Sciences, is certain the venture will take off, calling it one of the most ambitious projects in the area of astroparticle physics in Russia.

In any case, building Baikal-GVD should not be a waste of resources: together with KM3NeT, these two big, powerful detectors in the northern hemisphere will complement IceCube - increasing the chances of homing on an elusive high-energy astrophysical neutrino.

And when that happens, a door may open to a new era of uncovering deep space enigmas.

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