Surprising Signal in the XENON1T Dark Matter Experiment

Scientists of the PRISMA+ Cluster of Excellence of the Johannes Gutenberg University Mainz significantly involved

University Press release

Scientists from the international XENON collaboration announced today that data from their XENON1T, the world's most sensitive dark matter experiment, show a surprising excess of events. The scientists do not claim to have found dark matter. Instead, they say to have observed an unexpected rate of events, the source of which is not yet fully understood. The signature of the excess is similar to what might result from a tiny residual amount of tritium (super heavy hydrogen), but could also be a sign of something more exciting: the existence of a new particle known as the solar axion or the indication of previously unknown properties of neutrinos.
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From China to the South Pole: Joining forces to solve the neutrino mass puzzle

Study by Mainz physicists indicates that the next generation of neutrino experiments may well find the answer to one of the most pressing issues in neutrino physics

University Press release

Among the most exciting challenges in modern physics is the identification of the neutrino mass ordering. Physicists from the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU) play a leading role in a new study that indicates that the puzzle of neutrino mass ordering may finally be solved in the next few years – thanks to the combined performance of two new neutrino experiments that are in the pipeline, the upgrade of the IceCube experiment at the South Pole and the Jiangmen Underground Neutrino Observatory (JUNO) in China. They will soon give the physicists access to much more sensitive and complementary data on the neutrino mass ordering.
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Signals from inside the Earth: Borexino Experiment releases new data on geoneutrinos

Exclusive insight into processes and conditions in the Earth's interior

University Press release

Scientists involved in the Borexino Collaboration, among them researches from the PRISMA+ Cluster of Excellence of Johannes Gutenberg University Mainz (JGU), have presented new results for the measurement of neutrinos originating from the interior of the Earth, so called geoneutrinos. These elusive "ghost particles" rarely interact with matter, making their detection difficult. With the newly presented analysis, the researchers have now been able to access 53 events in Borexino, which is almost twice as many as in the previous analysis of the data. The results provide an exclusive insight into processes and conditions in the Earth's interior that remain puzzling to this day.

The Earth is shining, even if it is not at all visible to the naked eye. The reason for this is geoneutrinos, which are produced in radioactive decay processes in the interior of the Earth. Every second, about one million of these elusive particles penetrate every square centimeter of our planet's surface.

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Matthias Schott receives ERC Consolidator Grant for searches for axions

Data from the Large Hadron Collider at CERN can help to find axions

University Press release

Axions are hypothetical elementary particles that physicists initially postulated to solve a theoretical inadequacy of the strong interaction, the so-called strong CP problem. In recent years, however, it has emerged that axions or axion-like particles (ALPs) could also solve other puzzles of modern physics: They are considered promising candidates for dark matter and could also cause the theory-experiment discrepancy for the value of the abnormal magnetic moments of the muon.

This search can now begin: The European Research Council (ERC) supports the project "Search for Axion-Like-Particles at the LHC - Light @ LHC" with an ERC Consolidator Grant for Prof. Dr. Matthias Schott in the amount of more than 1.5 million euros. The project will be realized over the next five years at the PRISMA+ Cluster of Excellence at the Johannes Gutenberg University Mainz.

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2019 EPS High Energy and Particle Physics Prizes

The 2019 High Energy and Particle Physics Prize of the European Physical
Society for an outstanding contribution to high energy physics has been
awarded to the CDF and D0 collaborations for the discovery of the top
quark and the detailed measurement of its properties.

EPS News

The CDF and D0 experiments were large multi-purpose particle detectors at the Fermilab proton-antiproton collider Tevatron close to Chicago. Scientists from the ETAP group at the physics department at Mainz University have been long members of the D0 Collaboration and have contributed to the top quark physics programme of the collaboration, in particular to the development of measurement techniques for the top quark mass.

The measurements at the Tevatron have paved the way to the LHC experiments at the European particle physics center CERN in Geneva. Within the Standard Model of particle physics, the mass of the top quark is a key parameter and is directly related to the mass of the Higgs particle which was discovered by the ATLAS and CMS experiments at CERN's LHC collider, again with a prominent contribution by physicists from the ETAP group at Mainz University.

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Astroparticle physicists observe the longest half-life ever directly measured

Detector for dark matter search provides impressive measurement results / Publication in Nature

University Press Release
(Photo: XENON Collaboration)

The universe is almost 14 billion years old, which is an inconceivable length of time by human standards. Compared to some physical processes, however, it is but a moment. There are radioactive nuclei that decay on much longer time scales. An international team of scientists now has directly measured the rarest decay process – or the longest half-life – ever recorded in a detector. Using the XENON1T detector, which mainly searches for dark matter at the Gran Sasso National Laboratory, the researchers were able to observe the decay of Xenon-124 atomic nuclei for the first time. The determined half-life measured for Xenon-124, which is the time after which half of the radioactive nuclei present in a sample have decayed, is about one trillion times longer than the age of the universe.

This makes the observed radioactive decay, the so-called double electron capture of Xenon-124, the rarest process ever seen happening in a detector. "The fact that we managed to observe this process directly demonstrates how powerful our detection method actually is – also for signals which are not from dark matter," said Professor Christian Weinheimer from the University of Münster, whose group lead the study. In addition, the new result provides information for further investigations on neutrinos, the lightest of all elementary particles the nature of which is still not fully understood. XENON1T is a joint experimental project of about 160 scientists from Europe, the United States of America, and the Middle East. German partners in the project are the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg as well as the universities of Münster, Freiburg, and Mainz. The results have been published in Nature.

The XENON1T detector searching for dark matter

The Gran Sasso Laboratory of the National Institute for Nuclear Physics (INFN) in Italy, where scientists are currently searching for dark matter particles, is located about 1,500 meters beneath the Gran Sasso massif, well protected from cosmic rays which can produce false signals. Theoretical considerations predict that dark matter should very rarely "collide" with the atoms of the detector. This assumption is fundamental to the working principle of the XENON1T detector: its central part consists of a cylindrical tank of about one meter in length filled with 3,200 kilograms of liquid xenon at a temperature of minus 95 degrees Celsius. When a dark matter particle interacts with a xenon atom, it transfers energy to the atomic nucleus, which subsequently excites other xenon atoms. This leads to the emission of faint signals of ultraviolet light that sensitive light sensors located in the upper and lower parts of the cylinder detect. The same sensors also detect a minute amount of electrical charge released by the collision process.

The new study shows that the XENON1T detector is also able to measure other rare physical phenomena, such as double electron capture. To understand this process, one should know that an atomic nucleus normally consists of positively charged protons and neutral neutrons, which are surrounded by several atomic shells occupied by negatively charged electrons. The element xenon occurs in nature in different variants, which differ only in the number of neutrons in the nucleus. One of these so-called isotopes, Xenon-124, for example, has 54 protons and 70 neutrons. In double electron capture, two protons in the nucleus simultaneously "catch" two electrons from the innermost atomic shell, transform into two neutrons, and emit two neutrinos. The other atomic electrons reorganize themselves to fill in the two holes in the innermost shell. The energy released in this process is carried away by X-rays and so-called Auger electrons. However, these signals are very hard to detect as double electron capture is a very rare process hidden by signals from the omnipresent natural radioactivity. One of the tasks of the German groups is to develop new methods to reduce interference signals from radioactivity.

Measuring double electron capture

This is how the XENON Collaboration succeeded with this measurement: the X-rays from the double electron capture in the liquid xenon produced an initial light signal as well as free electrons. The electrons were moved towards the gas-filled upper part of the detector where they generated a second light signal. The time difference between the two signals corresponds to the time it takes the electrons to reach the top of the detector. Scientists used this interval and the information provided by the sensors measuring the signals to reconstruct the position of the double electron capture. The energy released in the decay was derived from the strength of the two signals. All signals from the detector were recorded over a period of more than one year, however, without looking at them at all as this was conducted as a blind experiment. This means that the scientists could not access the data in the energy region of interest until the analysis was finalized to ensure that personal expectations did not skew the outcome of the study. Thanks to the detailed understanding of all relevant sources of background signals it became clear that 126 observed events in the data were indeed caused by the double electron capture of Xenon-124.

Using this first-ever measurement, the physicists calculated the enormously long half-life of 1.8×1022 years for the process. This is the slowest process ever measured directly. It is known that Tellurium-128 decays with an even longer half-life, however, its decay has never been observed directly and its half-life was inferred indirectly from another process. The new results show how well the XENON1T detector can detect rare processes and reject background signals. While two neutrinos are emitted in the double electron capture process, scientists can now also search for the so-called neutrino-less double electron capture which could shed light on important questions regarding the nature of neutrinos.

"The analyses developed under the lead of the University of Münster are an extremely valuable contribution to our understanding of the forces in the atomic nucleus and to our search for new physics," said Professor Uwe Oberlack of Johannes Gutenberg University Mainz (JGU). Oberlack is one of the founding members of the XENON Dark Matter Project. Before joining JGU in 2010, he worked for ten years in this field and in high energy astrophysics in the US. "It is the close collaboration of all participating institutions that provides the basis for the excellent work of the detector. We are curious how our search for dark matter will proceed in the next phase of the project."

Status and outlook of the experiment

The XENON1T detector acquired data from summer 2016 until December 2018 and was then switched off. Currently, the XENON Collaboration scientists are upgrading the experiment for the new XENONnT phase, with an active detector mass three times larger than before. Together with further suppression of interference signals, this will boost the detector's sensitivity by an order of magnitude. The German groups will also have a leading role in this phase of the project.


In Germany, the XENON1T experiment received financial support from the Max Planck Society, the German Federal Ministry of Education and Research (BMBF), and the German Research Foundation (DFG). International funding came from the USA, Switzerland, Italy, Israel, Portugal, France, Sweden, the Netherlands, and the European Union.

XENON Collaboration, Observation of two-neutrino double electron capture in 124Xe with XENON1T,
Nature 568, 532-535, 25 April 2019,
DOI: 10.1038/s41586-019-1124-4

Related Links:
The XENON experiment at Mainz University

XENON1T: Enlightening the Dark

Read more:
press release "XENON1T proves to be the most sensitive detector on Earth searching for WIMP dark matter" (18 May 2017)

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BMBF supports Mainz projects in particle physics

Federal Ministry of Research provides 7 million euros for cooperative research work at CERN. ATLAS experiment and development of scintillator-based particle detectors as one of Mainz's main tasks in the coming years.

University Press Release
(Photo: ETAP/JGU)

Physicists from Johannes Gutenberg University Mainz (JGU) are involved in numerous projects of the international large-scale research institution CERN in Geneva. The Federal Ministry of Education and Research (BMBF) will continue to financially support this work in the coming years. For the three-year funding period until mid-2021, the BMBF provides nearly 7 million euros. "Projects at major research institutions are usually accompanied by a long-term commitment. “We therefore thank the Federal Ministry of Research for its continued support of these tasks with long-term, substantial funding,” says Prof. Dr. med. Georg Krausch, President of Johannes Gutenberg University Mainz. "The grants are also an award for our physicists, who make important contributions to the impressive research at CERN in many areas. We are also pleased that the achievements of the Mainz's physicists have been recently rewarded with the approval of the Cluster of Excellence PRISMA+," says the JGU President. The BMBF funds will be particularly used for Mainz's participation in cooperation projects such as the ATLAS experiment or the NA62 experiment.

One focus of the group Experimental Particle and Astroparticle Physics (ETAP) is the research on the Large Hadron Collider (LHC), the largest particle accelerator in the world, which has been in operation at CERN since 2008. "The Mainz group has taken a great deal of responsibility for the ATLAS experiment at the LHC," explains Prof. Dr. Volker Büscher from ETAP. Prof. Büscher is spokesperson of ATLAS Germany since July 2018, in which the 17 institutions working on the experiment are organised. ATLAS discovered the Higgs boson in 2012 together with another LHC experiment.

An important task of the ATLAS experts from Mainz is to analyze the data collected and recorded when the particles collide - more than 1 gigabyte per second. For this purpose, the Mainz supercomputer MOGON II, one of the fastest high-performance computers in the world, is available at the JGU. In addition, the physicists at Mainz University contribute to the upgrade of the ATLAS detector. They can rely on the infrastructure built up by the PRISMA Cluster of Excellence in Mainz, including the PRISMA Detector Laboratory. By granting the follow-up application for PRISMA+, this support will be secured in the future.

Looking to the future, the ETAP Group is developing novel particle detectors with a wide range of potential applications. In the BMBF joint project for research and development on scintillator-based detectors, Prof. Dr. Lucia Masetti, also a physicist of the University of Mainz, is the spokesperson for the joint project. "Scintillator-based particle detectors are ideal for a wide range of applications, the challenges of which we now address together as a community," says the scientist.

"Other activities we are particularly involved in, and funded by the BMBF, include the measurement of extremely rare kaon decays with the NA62 experiment, the search for axion-like particles, and machine learning," adds Büscher. The scientist notes that the LHC accelerator is in a long shutdown for the next two years since early December to carry out major upgrade work. "There is a lot to do for us during this time," says Prof. Büscher. "Then, with the LHC restarting with increased luminosity, we'll go deeper into exploring the smallest particles that make up our matter."

ETAP group part of the network "Innovative Digital Technologies for Universe and Matter Research"

Large-scale experiments in fundamental research require more and more computing and storage resources. In order to gain further scientific insight in the future, physicists from several research institutions have now joined forces in a project funded by the Federal Ministry of Education and Research (BMBF) to develop innovative digital processing methods.

BMBF Press Release

In the context of the framework programme "Erforschung von Universum und Materie – ErUM", the Federal Ministry of Education and Research (BMBF) is funding this network with the name "Innovative Digital Technologies for Research on Universe and Matter" as a pilot project with a total of 3.6 million euros over the next three years. Researchers of the ETAP group are participating in this network together with other researchers from the groups of the Universities of Aachen, Erlangen-Nuremberg, Frankfurt am Main, Freiburg, Hamburg, Munich, Wuppertal and the Karlsruhe Institute of Technology as well as the associated partners DESY (Deutsches Elektronen-Synchrotron), CERN, Forschungszentrum Jülich, Grid Computing Centre Karlsruhe (GridKa), GSI Helmholtzzentrum für Schwerionenforschung and the Universities of Bonn, Göttingen and Münster. Prof. Thomas Kuhr from the LMU coordinates the network.
The participating researchers contribute their diverse experience and knowledge in the fields of distributed computing infrastructures and algorithm development to the project.

Within the next three years, the joined project will develop and test new computing systems. One promising approach is the use of virtualization technologies to tap previously inaccessible resources. The scientists are also thinking about the use of new processor architectures, which are used, for example, in graphics cards and promise better energy efficiency (Green IT). The researchers see an important pillar in the development of improved algorithms and the use of artificial intelligence (AI) for Big Data analyses. Innovative methods of "machine learning" will play an important role here.

"The huge amounts of data are a great challenge for us. Innovative digital methods will be indispensable in the future if fundamental research is to advance decisively," said network coordinator Prof. Thomas Kuhr. However, it is not only physical research that faces the digital challenge. "Sooner or later, other scientific disciplines will also need powerful computing environments and will benefit from the new competences," Kuhr is certain. The joint project offers the participating young scientists an excellent opportunity to acquire comprehensive knowledge in new computing technologies. This means they are well prepared to fill leading positions in science or business in order to drive digital change forward.

The contribution of the ETAP group within this network is the replacement of existing algorithms for the real time detection and reconstruction of physics objects by deep neural networks. The group is targeting a processing rate of up to 40 MHz and a maximum response time of less than a millionth of a second. This will be sufficient to cover a large range of applications within the network. Field Programmable Gate Arrays (FPGAs) are the ideal choice.

The challenge is to optimally adapt the neural networks to the architecture of FPGAs, and the first trigger stage of the ATLAS experiment provides an ideal test environment for this.

PRISMA+ Cluster of Excellence selected by the German Research Funding Agency

PRISMA + is the follow-up application for the Cluster of Excellence "Precision Physics, Fundamental Interactions, and Structure of Matter" (PRISMA), which has been successful in the previous Excellence Initiative and has been funded since 2012.

press release by the Germany Resaerch Funding Agency
press release of the university

Today the German Research Funding Agceny (DGF) has announce the selection of the projects that will be funded under the Excellence Initiative. An international committee of experts and the science ministers of the federal and state goverments has selected 57 out of 88 submitted projects.

The PRISMA+ project for precision physics, fundamental interactions and structure of matter which was submitted by the University of Mainz was among the selected projects.

The total amount of the requested funding is expected to be 64 million euro over the next seven years.