CERN Accelerating science

Interview with Stefan Söldner-Rembold, DUNE's Co-spokesperson

by Panagiotis Charitos

Stefan Söldner-Rembold, has been recently elected co-spokesperson of the DUNE collaboration. In his long-standing career he worked both in accelerator and non-accelerator experiments addressing some of the most fundamental questions about matter and the structure of our universe. The same type of questions that motivated him as a student to move into physics informed his career: ‘During my career, I have tried to address these questions using different tools. I worked for many years in collider experiments including the OPAL experiment at LEP and the DZero experiment at Fermilab before moving to neutrino physics” and he adds ‘’It is a fascinating to see how these two slightly different communities are trying to address these question from different perspectives”. For Söldner-Rembold, complementarity is the key word in describing the paths that collider and neutrino physics have followed in the last two decades.

Neutrino physics has a very rich history dating to the middle of the 20th century when these unique particles were discovered. Their masses are several orders of magnitude lighter compared to all other fermions which makes them one of the most “elusive” particles as it interacts very feebly with ordinary matter. “The discovery of neutrinos and the measurement of their tiny masses has been a scientific tour de force for many years”. Perhaps one of the most spectacular implications of neutrino masses and mixings is the macroscopic quantum phenomenon of neutrino oscillations, first introduced by Bruno  Pontecorvo. The experimental observation of neutrino oscillations, awarded the Nobel prize in 2015 to T. Kajita (from the SuperKakiokande collaboration) and Arthur McDonald (from the SNO collaboration); a discovery that largely boosted the interest of physicists in neutrino physics.

Today the global particle physics community evaluates the different proposals for experiments and facilities, through a broad bottom-up consultation that will result in the next update of the European Strategy for Particle Physics. “We live in exciting times as collider and neutrino physics are moving to the next generation of experiments; a move that reflects the evolution of the two fields based on the lessons from previous experiments and the capabilities offered by the technologies that we have developed over the years”.

Deepening our understanding of neutrino physics is the main scopes of the DUNE experiment. The DUNE detector will be both a “microscope,” used to observe elementary particles, and also a “telescope” for observing the Sun and supernovas, using neutrinos. “The parameters that nature has chosen for the neutrino sector enable us to do this type of experiments and address the role that neutrinos may have in the observed imbalance between matter and antimatter and the so-called neutrino mass hierarchy problem- two of the most fundamental problems in modern physics”.

For DUNE the world’s most intense neutrino beam will be sent through the Earth all the way from Fermilab to South Dakota. To be able to do the experiment you actually need two detectors: one will record particle interactions near the source of the beam, at Fermilab. A second, much larger, detector will be installed more than a kilometer underground at the SURF laboratory, in South Dakota — 1,300 kilometers downstream of the source. “These detectors will enable us to search for new subatomic phenomena and potentially transform our understanding of neutrinos and their role in the universe”. In principle scientists would like to have two identical detectors in both locations to cancel all the systematic effects. However as neutrinos do not interact very often we need gigantic detectors to record any significant statistics, a fact that complicates the task of making them identical. In the case of DUNE, four far detector modules will use the novel Liquid Argon TPC technology, which allows very big detectors — 70.000 tons of Liquid Argon at 87 K. For the near side you would probably build also a liquid argon TPC but then complement this with other detector components.

Dune dual phase in 2019 (Copyright@CERN, Credits: Brice, Maximilien; Ordan, Julien Marius)       

“These three components, the beam, the near detector and the far detector make DUNE a complicated and complex project. Diverse in terms of the technology but also in terms of the communities that contribute to it including particle physicists but also nuclear physicists, astrophysicists and many others. Seeing a coherent, albeit diverse collaboration succeeding to set up such a challenging experiment is a very rewarding result.”

Daily billions of solar or atmospheric neutrinos pass through the Earth and studying them could give more information about neutrinos, their sources and their production. Neutrinos can also be seen as messengers for studying very distant objects in the universe. In this direction, DUNE has the potential to observe neutrinos from Supernovae in our galaxy. Depending on the distance of the source, we could see thousands of neutrinos interactions within a pulse of about 10 seconds. Measurements of the time, flavour and energy structure of the neutrino burst will be critical for understanding the dynamics of this important astrophysical phenomenon, as well as bringing information on neutrino properties and other particle physics. A pivotal moment for neutrino physics was the first observation of neutrinos from a Supernovae in 1987. “In total 24 neutrinos were detected, combining data from different experiments while with DUNE we aim for a few hundreds neutrinos captured in a single experiment.” explains Söldner-Rembold. “This effort is one of the pillars of DUNEs physics programme” notes Söldner-Rembold “ results from DUNE will be highly complementary with neutrino burst information from other neutrino detectors, which contribute to an era of multi-messenger astronomy combined with results from gravitational waves and a broad range of telescopes looking at different parts of the electromagnetic spectrum”.

Overview of the single-phase ProtoDUNE cryostat final structure. (Copyright@CERN, Credits: Brice, Maximilien; Ordan, Julien Marius)       

Interestingly, Söldner-Rembold also points out that DUNE will not cover all aspects of neutrino physics and that there is room for collaboration with other facilities including neutrino telescopes reactor and other non-accelerator experiments to explore in depth the available parameter space. “Such topics include  sterile neutrinos and the existence of Majorana neutrinos, the discovery of which will demonstrate whether neutrinos are their own antiparticle”.

The project dates to 2015 when two independent groups, LAGUNA-LBNO in Europe and LBNE in the US explored the use of liquid argon technology in neutrino detectors. Based on these efforts, in 2014 the US P5 prioritization panel and the previous update of the European Strategy for Particle Physics gave their impetus to create a global programme for neutrino physics and merge these two core activities into what is today known as DUNE. “It was a unique moment for the neutrino physics community that had to collaborate at a global scale, but also as for the first time CERN participates directly in an experiment not physically linked to the laboratory”. Currently a lot of the hardware activity is ongoing at CERN’s North Area to explore different technologies for the detectors and in 2018, the collaboration shows phenomenal success in a number of fronts that gave a lot of momentum to the project. “The fact that within only two and a half years we moved from a construction site to recording a number of beautiful tracks from the first beam is an amazing step that testifies to the success of our approach and promises a bright future for DUNE”.

The successful results also confirm the value of international collaboration for the success of future large-scale experimental facilities. Already from the LHC, the particle physics community learned to collaborate at an international scale to deliver projects that go beyond a national scale. Today DUNE is the biggest international science project in the USA bringing together different stakeholders from all over the world to define a common roadmap for the project. This was not a trivial task and sets an example that other fields of science are looking into for lessons they can adopt in their approach.

But DUNE is not the only player in neutrino physics. Another big experiment in the field is Hyper Kamiokande that will use the next generation of large-scale water Cherenkov detectors instead of liquid argon. “Competition is a good thing. Though we are convinced by our approach, we always welcome complementary approaches that make the field more interesting” says Söldner-Rembold and continues: “HyperK has a good chance to observe CP violation in a different and complementary way to DUNE”.

Finally, further theoretical developments are needed also in neutrino physics. “You may start a new experimental programme based on some theoretical guidance but then, as the experimental programme progresses, questions arise that theorists can tackle by excluding or further developing their models. The way physics often works is not by theorists telling you where to look and then you build an experiment to confirm the predictions. During the course of the experiments, new challenges and questions arise. We have a long history in the field where detectors discovered phenomena that went beyond the original predictions. DUNE and LBNF are the facilities that allow us to make such unexpected discoveries” and concludes “DUNE and LBNF will run for a decade or more and we hope that some of the questions that we will be able to answer in ten or twenty years haven’t been asked yet.” 

The collaboration is now preparing to publish its Technical Design Report later this year that will inform the CD2/CD3 review in the US and help define the baseline of DUNE, build a certain structure for the project - including budget lines - and guarantee the future of this fascinating project.