Neutrinos are electrically neutral and very light elementary particles, which interact only weakly with other matter and are therefore difficult to observe. From 2025, a new neutrino experiment in the US aims contributing to a better understanding of the neutrinos. At the University of Bern physicists are currently working on the prototype for a detector to be used in the upcoming experiment.
At 9am we have an appointment with Dr. James Sinclair at the Albert Einstein Center for Fundamental Physics' (AEC). The AEC is located in a functional building next to the main building of the University of Bern. James Sinclair sits in an office room at a desk with four screens in front of him. What is going on there is very remote from University of Bern. On the screens flicker the data of an experiment that runs 7000 km away at Fermilab. The Fermi national laboratory, or short Fermilab, one of the world's leading particle physics research organizations, is based near Chicago. The MicroBooNE experiment is currently underway there, and James Sinclair today has the task to do an eight-hour shift in Bern to ensure that the experiment runs smoothly.
"At Fermilab it is currently 20 past 3am at night," says the 32 year old particle physicist from Malvern (UK), who did his PhD at University of Sussex. "I monitor various subsystems of the experiment from here - here electric fields, there the cooling system and over there the incoming cosmic radiation. If anything went wrong in the experiment, I would contact an expert at Fermilab, at the MIT in Boston, or here in Bern; they would then do the necessary to get the experiment back on track, "says Sinclair. Bern is one of four control centers, besides Manchester, Cambridge and Virginia, where scientists monitor the MicroBooNE experiment around the clock, without the need for real night-shifts as one can profit from all the time zones involved.
From MicroBooNE to DUNE
MicroBooNE (short for: micro booster neutrino experiment) was set up under the scientific direction of Prof. Michele Weber, particle physics professor at the University of Bern, and has been in operation since 2015. Its centerpiece is the 'Liquid Argon Time Projection Chamber', a detector for elementary particles. The Time Projection Chamber is a tank filled with liquid argon cooled down to -200 ° C. The liquid noble gas is used to observe and analyze the neutrinos produced in a neutrino source 500 meters away. Among other things, the researchers want to use the experiment to find out whether, in addition to the three known neutrinos - tau neutrino, muon neutrino, electron neutrino - there is still a fourth type. Physicists call this presumed fourth type 'sterile neutrino'. The term 'sterile' refers to the fact that this hitherto hypothetical elementary particle does not interact with matter in the same way as the three known neutrinos do. The experiment involves 120 physicists from the US, UK, Italy, Russia, and Switzerland.
The researchers at University of Bern have a wealth of experience in detector technology based on the liquid noble gas argon. They are now bringing this knowledge to a new neutrino experiment, which will start operating in 2025 under the name DUNE (for: Deep Underground Neutrino Experiment). DUNE is also built at Fermilab. From here, a high energy neutrino beam (generated from a 2MW proton accelerator) is sent on a 1300km journey through the earth before hitting a detector at the Sanford Underground Research Laboratory in South Dakota. Of particular interest to physicists is how neutrinos change their flavor from one neutrinos type to another along the path of their journey. To be able to detect this, the neutrino beam is analyzed at the beginning and at the end of its journey, and the results of the measurements are then compared. DUNE consists essentially of the so-called 'near detector' at Fermilab and the 'far detector' in South Dakota, 1300 km away at SURF, the Sanford Underground Research Facility.
Big questions of neutrino research
The DUNE experiment is intended to enable a deeper understanding of the still insufficiently understood neutrinos. The aim, among other things, is a more precise measurement of the neutrino oscillation parameters; so the mixing of the three previously known neutrino species is specified. The physicists also hope that the experiment will answer to great unresolved questions of modern physics: Why is there more matter in the universe than antimatter? Can the four known forces acting between elementary particles be traced back to a common fundamental force? What leads to the formation of black holes?
The far detector of DUNE has the same structure as the above-mentioned argon detector of the MicoBooNE experiment. The detector is based on the proven argon technology, which was developed significantly at the University of Bern, but has 400 times more volume than the MicroBooNE detector. A major challenge is the near detector of the DUNE experiment. "The near detector is designed to record seven times more neutrino events than MicroBooNEs near detector," James Sinclair outlines the requirement. "By the end of 2019, the concept for the near-detector should be established. Then we want to build a prototype at the University of Bern. From 2026 on, the construction of the near detector at the DUNE experiment will start. That means, the experiment will start step by step and operate without near detector in the first step. "
„This detector’s principal role is to characterize the neutrino beam in its initial state — before it travels 800 miles (1,300 km) to the far site — in order to better understand the signals collected at the Far Detector, and thus to maximize the neutrino oscillation physics potential of the DUNE experiment“, says the website of the DUNE collaboration. „In addition, the Near Detector will enable a rich program of particle physics measurements independent of the Far Detector and provide the data for many PhD theses.“
The Near Detector design is not yet finalized so far. But James Sinclair and his fellow scientists already have a very concrete idea of how to make the near detector faster compared to the MicroBooNE detector: „To make the a Liquid Argon Time Projection Chamber robust to the high rate of the DUNE near detector, we propose two main modifications“, Sinclair says. Firstly, the Liquid Argon detection volume will be segmented into a number of small, self-contained, Time Projection Chambers. Secondly, we propose to move from a traditional charge-collection wires to a pixelated charge collection scheme. "The latter modification is the biggest advance since the invention of the 'Liquid Argon Time Projection Chamber' in the early 1970s by the US American physicist David Nygren," says James Sinclair.
Author: Benedikt Vogel