Hunt for the WIMP
A project based in the Gran Sasso Massif aims to find the constituents of Dark Matter / Focus on the WIMP particle
03.02.2011The spectrum of research in the field of physics at Johannes Gutenberg University Mainz (JGU), Germany has been extended to include one of the most important endeavors of our age: the empirical search for Dark Matter. With the appointment of Professor Dr. Uwe Oberlack in summer 2010, the JGU Institute of Physics acquired one of the leading experts in this field. He will be able to base his work in Mainz on the results of theoretical explorations of the nature of Dark Matter. Mainz University has thus joined a top group of institutes worldwide leading the hunt for Dark Matter.
Dark Matter has been the driving force for structure formation in the universe. We can see its impact on scales ranging from galaxies, galaxy clusters, to the largest natural structures known to us - the colossal superclusters and filaments that surround vast cosmic voids like bubbles in a foam bath. This matter was the cradle in which the very earliest galaxies were able to take shape. Dark matter still surrounds and permeates our and other galaxies and holds these together - but for us it is completely invisible. We know little about Dark Matter to date, although it constitutes nearly one quarter of the material making up our universe. "What we primarily know is what Dark Matter is not," explains Uwe Oberlack, who had been in the USA conducting research in this field and in that of high-energy astrophysics for ten years before his return to Germany. "Dark matter is not just transparent, but is also completely different from all other forms of material that we know." Professor Oberlack participated in the setup of the XENON international Dark Matter research project, which aims at understanding the nature of Dark Matter through its direct detection. The current XENON100 experiment, located in the Gran Sasso underground laboratory (LNGS) in central Italy, is one of the most sensitive ongoing searches for Dark Matter.
Research into Dark Matter will constitute one of the foremost scientific endeavors of the next decade. Dark matter makes up 23 percent of the universe, while normal, visible matter represents a mere 4.6 percent. The greater proportion of our universe, 72 percent, is made up of so-called "Dark Energy". We know even less about Dark Energy than we do about Dark Matter, but it is Dark Energy that is accelerating the expansion of the universe.
It was because of the observed motion of galaxies in galaxy clusters that the existence of Dark Matter was first proposed in the early 1930s. Their orbital velocity is so rapid that these galaxy clusters would fly apart if the gravitational force of visible matter were the only thing holding them together. A similar effect was subsequently observed in the case of spiral galaxies. There had to be some additional force allowing galaxies to rotate at such high rates, and the gravitational force of some unseen matter was a possible explanation. We now know that there is indeed Dark Matter, but this matter cannot consist of the quarks and electrons that make up the atoms we are familiar with. Other candidate particles, such as neutrinos, have also been eliminated from the search. "Our theory at present is that Dark Matter was formed fairly soon after the Big Bang," states Oberlack. "It probably consists of neutral, massive particles that only weakly interact with other particles." These WIMPs (weakly interacting massive particles) have yet to be discovered.
Oberlack is looking for them deep under the surface of the earth as part of a team of researchers from 12 institutes using a xenon detector that has been very carefully shielded against background cosmic radiation. It is hoped that the detector of the Laboratori Nazionali del Gran Sasso that uses liquid xenon at a temperature of -95 degrees Celsius will be able to capture the signature of WIMPs. Following initial trials using smaller detectors, the current XENON100 experiment will be searching for Dark Matter using a detector mass of 62 kilogramms and a 100-fold decrease in background exposure in comparison with its forerunners. This device should increase sensitivity by a magnitude of 15 and be capable of directly detecting a large proportion of the hypothetical particles called "neutralinos", a type of WIMP predicted by the supersymmetry theory. Supersymmetry (often abbreviated to SUSY) is a hypothetical concept of a new symmetry of nature. It is associated only with very high energy particles, which existed in the early universe or can be created in large particle accelerators, such as CERN's LHC.
The successful data taken with XENON100 has spurred the XENON collaboration on to plan an improved detector with a mass of one ton and a sensitivity enhanced by a magnitude of 50 to 100, which they hope will be ready for use within the next three years. If the assumptions are correct and neutralinos are indeed the basic constituent of Dark Matter, it may be possible to demonstrate their existence in the laboratory in the next few years, and then even determine some of their physical properties.
The research work is being financed to the tune of €100,000 by the Alfried Krupp von Bohlen und Halbach Foundation under its special program "Rückkehr deutscher Wissenschaftler aus dem Ausland," designed to encourage German scientists to return to Germany.