Physicist at many Germany universities and unstitutes have contributed to the analyses in a recently published overview paper about the searches for Dark Matter at the ATLAS experiment. The paper has been accepted for publication by the Journal of High Energy Physics.
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Detector for dark matter search provides impressive measurement results / Publication in Nature
University Press Release
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,
The XENON experiment at Mainz University
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.
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."
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.
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.
Rosa Simoniello and Eduard Simioni from the ETAP group receive the ATLAS outstanding achievement award for outstanding contributions and dedication in successfully commissioning the Level-1 Topo trigger
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.
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.
It was a sensation in 2012: Scientists at CERN have discovered the Higgs Boson that have been postulated for decades. Now another prediction has been confirmed: The Higgs Boson decays into two bottom quarks.
Even before the concrete proof, theoreticians had long predicted the decay in bottom quarks. Although it is a common pattern of decay - it occurs in 58 percent of cases - the teams have only now succeeded to observe this decay mode. Since this reaction can not be observed directly, the researchers instead reconstructed the decay products of particle collisions in the LHC. For this they use high-precision detection devices the size of an apartment building. The challenge here: The pattern occurring resembles other, much more frequent decays, in which the Higgs Boson is not involved.
(Image ATLAS/CERN: HIGG-2018-04)
Elementary particles originate from three billion light-years distant galaxy / black hole as particle accelerator
(Image Martin Wolf, IceCube/NSF, 2017)
New results from the ATLAS and CMS experiments at the Large Hadron Collider (LHC) reveal how strongly the Higgs boson interacts with the heaviest known elementary particle, the top quark
(Image ATLAS Collaboration)
XENON1T sets new limits for "WIMPs" - Mainz scientists are among the founding members of the XENON program for the search for dark matter
XENON1T continues to collect data until the larger version of the detector currently being prepared is ready for use, for which most components are already designed. With three times more xenon in the time-projection chamber and ten times lower background rate, XENONnT will launch a new phase of dark matter particle search beginning in 2019.
Prof. Dr. Uwe Oberlack from the Johannes Gutenberg University Mainz is one of the founding members of the XENON program on the search for dark matter. The Mainz working group is involved in the XENON experiments in both data analysis and simulations as well as detector technology. Mainz astroparticle physicists are responsible for the muon detector and are involved in the ReStoX xenon storage system and the inner detector. In order to make another leap in sensitivity in the next step with the XENONnT experiment, the group is working on the development of a subdetector for the detection of neutrons. The research is also part of the Cluster of Excellence PRISMA, which is funded by the "Exzellenzinitiative des Bundes und der Länder".
(Plot XENON collaboration)