• 24.09.2019
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Callum Wilkinson prepares the DUNE experiment at the University of Bern

A trap for ghost particles

View into the experimental hall of high-energy physics at the University of Bern: Callum Wilkinson stands next to the cryostat (cold unit), which holds four cubes of the later time projection chamber of the DUNE near detector for the tests.
Image: Foto: B. Vogel CHIPP, Switzerland
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View into the experimental hall of high-energy physics at the University of Bern: Callum Wilkinson stands next to the cryostat (cold unit), which holds four cubes of the later time projection chamber of the DUNE near detector for the tests.
View into the experimental hall of high-energy physics at the University of Bern: Callum Wilkinson stands next to the cryostat (cold unit), which holds four cubes of the later time projection chamber of the DUNE near detector for the tests. (Image: Foto: B. Vogel CHIPP, Switzerland)

In 2025, the 'Deep Underground Neutrino Experiment' (DUNE) will be launched in the north of the USA, with which physicists want to learn more about neutrino - a still mysterious elementary particle. An important component of the DUNE experiment is currently being prepared by scientists from the University of Bern.

When physicists today build up a new experiment, it is usually a long way. Often hundreds, even thousands of scientists and technicians are involved in such a scientific project. Many questions of a physical, but also of a technical, organisational and financial nature have to be clarified, from the conception to the detector research and development, to construction, integration and commissioning. This is also the case with the DUNE experiment. In the summer of 2019, another milestone was reached for the experiment, whose roots go back years and which is expected to go into operation in 2025: The 'Technical Design Report' (TDR) was finished. The TDR is a comprehensive document that describes in detail how the technical issues involved in setting up and operating the experiment are solved. The report also shows the way in which the DUNE experiment intends to achieve its scientific goal of describing the neutrino more precisely.

Neutrinos on a 1300 km long journey

The neutrino was postulated by Wolfgang Pauli in the early 1930s and only proved experimentally 26 years later in 1956. It has only been known for about two decades that the neutrino that exists in three species ('muon-neutrino', 'electron-neutrino', 'tau-neutrino') also does change from one species to another. Scientists call this transformation 'oscillation'. The DUNE experiment will measure precisely the oscillation from muon neutrinos to electron neutrinos and the oscillation from anti-muon neutrinos to anti-electron neutrinos. The two oscillations were observed earlier, but with DUNE this observation will become even more precise. Even more: The DUNE experiment wants to test whether the two types of oscillations occur with different probabilities. If such an inequality should exist, it could lead to fundamentally new approaches to explaining the origin of the universe and would hint why there is only matter, but no anti-matter abundant in the universe.

The DUNE experiment is currently being constructed in the north of the USA in an ancient deep underground mine. At the 'Fermi National Laboratory' (Fermilab) near Chicago, physicists produce a neutrino beam from a high-intensity proton beam that is shot on a graphite target, creating pions that then decay into muons and neutrinos. These neutrinos are sent through the Earth's crust to the 'Sanford Underground Research Laboratory' in South Dakota, 1300 km away. Since the neutrinos practically do not interact with matter, they can penetrate the earth largely undisturbed. At the destination, the researchers catch a tiny portion of the neutrinos with the so-called far detector. They can then determine which changes the neutrinos have undergone on their journey.

Argon stops the volatile guys

As with all large-scale experiments in modern elementary particle physics, researchers from numerous countries and universities are involved in the DUNE experiment. From Switzerland, these include the ETH Zurich, the University of Basel, CERN and the University of Berne. A Berne research team led by Prof. Antonio Ereditato and supported by Prof. Michele Weber is working on DUNE there. The Bernese scientists are helping to build the so-called near detector. The near detector is used to determine the purity of the neutrino beam at its starting point - Fermilab. Purity means that the beam of muon neutrinos and antimuon neutrinos should contain as little electronneutrinos or anti-electroneutrinos as possible at the starting point. This is an important prerequisite for the precision of any neutrino measurements.

"Our central contribution is the liquid argon part of the near detector, in this area Bern has many years of expertise," says Dr. Callum Wilkinson, postdoc in Antonio Ereditato's research team. When the language comes to argon, it becomes exciting for neutrino physicists: the neutrinos, which otherwise traverse substances with rare interaction, more often remain in the noble gas (because of its relatively high density). Argon has therefore long been used for the detection of neutrinos. In the case of the DUNE near detector a tank filled with liquefied argon will be used, 5 meters long, 7 meters wide and 3 meters high. Callum Wilkinson and his colleagues from Berne are working to ensure that the particles are reliably detected when they interact within the tank.

From T2K to DUNE

Callum Wilkinson (29) comes from England. He grew up in Stratford-upon-Avon, the birthplace of William Shakespeare. Like Shakespeare, Wilkinson soon became interested in big questions. He studied physics and philosophy at the renowned King’s College in London. In 2011 he moved to the University of Sheffield for his doctoral thesis and worked in the British research group of the T2K experiment, which is used to research neutrinos in Japan. After his doctoral thesis, the Briton moved to the University of Bern in 2015, where a T2K research group also worked. Soon the neutrino expert would also be involved in setting up the DUNE experiment.

Callum Wilkinson is now standing in the large experimental hall of the Bern Physics Institute. In the middle of the room, a ladder leads down into a metal container, which could at first be mistaken for a diving bell. "We built one of our cubes almost in its full design size and are now testing it down there," says Wilkinson. These tests have been taking place since August 2019. The work must be completed by summer 2020. Then the components will be shipped to the USA and later installed in the DUNE experiment.

35 smaller cubes instead of one large tank

The cube Callum Wilkinson talks about is 3 cubic meters (1 m x 1 m x 3 m) and part of the DUNE near detector. Twenty of these cubes are needed for the detector. Together they form a 'time projection chamber', the heart of the near detector. In simple terms, the time projection chamber is a tank filled with liquid argon in which an electric field prevails. If a neutrino in this tank hits an argon atom, electrons are knocked out of its shells - resulting in a current of (positively charged) argon isotopes and a current of (negatively charged) electrons. In the electric field the electrons move to the anode and the argon isotopes to the cathode. With the time projection chamber it is possible to determine the traces of the electrons - and in this way to detect the (very rare) neutrino argon collisions.

This method is used in the far and near detector of the DUNE experiment for the detection of neutrinos. In order to achieve this as accurately as possible, the scientists must determine the electron tracks very reliably and accurately. The scientists at the University of Bern have developed a new design of time projection chamber for the near detector. The basic idea is to divide the large time projection chamber into 35 small cubes measuring 3 cubic meters. Each of these so-called 'cubes' has its own electric field (actually two, since the cathode is placed in the middle of two anodes). With this arrangement, the electron tracks can be determined more precisely, and the detector becomes even more robust. The proximity detector of the DUNE experiment is expected to detect 30 million neutrino events per year, i.e. one neutrino collision per second.

Readout chip with pixels

Time projection chambers have existed since the 1970s. The division of the large chambers into smaller cubes and their handling is an important further development of this physical research instrument. This is not the only innovation used in the proximity detector of the DUNE experiment. The electrons arriving at the anode are no longer captured with a wire mesh, but with a pixel-based readout chip developed at the Lawrence Berkely National Laboratory, USA. The path of the electrons can also be better detected using light flashes. "With the further development of the liquid-argon time projection chamber, Bern is making an important contribution to increasing the precision of neutrino measurements within the DUNE experiment," says Callum Wilkinson.

More information about the DUNE experiment: www.dunescience.org

More information on high-energy physics at the University of Bern: www.lhep.unibe.ch

Author: Benedikt Vogel

The Science of the Deep Underground Neutrino Experiment (DUNE)

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