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Humboldt-Universität zu Berlin - Mathematisch-Naturwissen­schaft­liche Fakultät - Modern Optics

Precision Experiments on Fundamental Questions in Physics:

 

Matter-antimatter interaction:

Based on the solution of fundamental equations, Dirac predicted the existence of antimatter. This strange form of matter should be a perfect counterpart of the matter surrounding us, but with an opposite charge. Thus every elementary particle should possess its antiparticle counterpart. The later on experimentally proven existence of antimatter has always inspired not only physicists, but also the authors of science-fiction stories. In star trek the space ship is driven by a matter-antimatter machine that gains energy by the annihilation of matter with antimatter. The energy that is set free in this process is immense, since all (anti)matter is converted into energy (according to Einstein's famous E=mc2 equation). In the 1980s the US governmental SDI project was even planned to be run by antimatter fuel. Besides this, there are however also more serious thoughts related to antimatter. If matter and antimatter are perfect mirror images of each other, why is our universe dominated by matter although the Big Bang should have created matter and antimatter in same amounts?

A possible explanation for the missing antimatter could be a (tiny) asymmetry between matter and antimatter. The hunt for this possibly existing asymmetry is, however, tough, since for a long time it was only charged antiparticles that could be produced (or escape from radioactive decays). The charge is masking possible asymmetries, and thus the production of antihydrogen (a neutral antiatom formed from an antiproton and an antielectron (the positron)) would be a big step towards tests of the fundamental symmetries of physics (like the so-called charge-parity-time (CPT) invariance or the weak-equivalence principle (WEP)). The goal is therefore to produce a sufficient amount of cold antihydrogen and use this for high-precison spectroscopy (comparing it to the extremely accurately known hydrogen spectra).

However, producing (cold) antihydrogen is extremely difficult, and it was only in the mid 1990s that a handful antiatoms were produced. About 10 years ago (2002), two groups at CERN managed to produce a much larger number of antihydrogen atoms, but only in very highly excited states and not yet cool enough for high-precision spectroscopy. In the mean time, even a relatively large number of anti-hydrogen atoms could be caught in a trap and kept for some time. Therefore, more sophisticated experiments with antihydrogen including collisions with ordinary matter like hydrogen or helium atoms or hydrogen molecules appear reachable in the near future.

Within an international collaboration we have been studying the interaction of hydrogen atoms with antihydrogen. While the original motivation has been the question whether sympathetic cooling or de-excitation of antihydrogen with ultracold hydrogen atoms is possible, this is evidently also of interest for the fundamental understanding of antimatter-matter interactions. In fact, this collision system turned out to be also a nice model system for quantum mechanics and collision theory. For example, the hadronic constituents (proton and antiproton) attract each other and thus a chemically bound hydrogen-antihydrogen molecule can form. However, this is molecule is metastable, as it can decay via annihilation (of either proton and antiproton or electron and positron). Furthermore, a critical distance exists below which the system can decay into protonium (bound proton-antiproton pair) and positronium (bound electron-positron pair). The proper treatment of the system incorporating this instability is still subject of debates.

Motivated by the fact that slow antiprotons are needed for generating antihydrogen atoms, experiments are also planned (especially within the FLAIR and SPARC projects at the GSI Darmstadt, Germany) that investigate the scattering of slow antiprotons with matter. For this purpose we have developed a new theoretical (time-dependent) approach with which we investigated the interaction of antiprotons with hydrogen molecules. Besides the interest for atomic and molecular physics these cross-sections and their knowledge are important for the design of the corresponding GSI storage ring(s) in which the antiprotons should be decelerated. For practical reasons a compromise between vacuum and beam quality has to be made. In order to determine the beam quality for a certain pressure the cross-sections have to be known. In fact, most of the remaining gas is molecular hydrogen and thus our recent first fully-correlated calculations of antiproton collisions with molecular-hydrogen are of direct technical relevance.

 

Tritium neutrino-mass experiments:

For a long time, the neutrino was declared to possess no rest mass. However, the standard model of physics (and the textbooks) had recently to be revised, since a phenomenon called neutrino oscillations was experimentally shown to occur. This is, however, only possible, if neutrinos have a rest mass. Unfortunately, those experiments do not provide the neutrino rest mass itself, but only mass differences. The experiment that was so far most successful in providing an upper limit to the rest mass of the electronic (anti)neutrino is the tritium neutrino-mass experiment. In this experiment the energy spectrum of the electrons produced in nuclear \beta decay of tritium is measured. The measured spectrum is then fitted to Fermi theory using the neutrino mass as a fit parameter. This fit requires that all other parameters influencing the shape of the \beta spectrum are very accurately known. Since the experiments use molecular tritium instead of atomic one, the so-called molecular final-state distribution (the probability that a certain amount of the nuclear decay energy leads to a rovibronic excitation of the generated daughter molecule 3HeT+) has to be known. This distribution could so far not be measured, and thus it has to be provided by theory. We have been working on the precise calculation of this molecular final-state distribution, including a careful analysis of the underlying approximations. For this purpose, a fully relativistic formalism was developed that allows to calculate the first-order corrections to the usually adopted sudden approximation for arbitrary molecular systems. These corrections are also of interest for the precise determination of nuclear ft values. We have also collaborated with the experimentalists in order to explain the energy loss of the β electrons due to inelastic scattering by neighbour molecules.

 

Optical Tests of Lorentz-Invariance Violating Terms:

One of the goals in physics is to confirm its laws as accurately as possible. In this way the validity of the standard model is thoroughly tested, or "new" physics may be found. One of the corner stones is Lorentz invariance. However, to which extent (accuracy) do we know its validity? Even tiny deviations would have important consequences. Developing models that describe possible deviations, it becomes possible to quantify the accuracy with which Lorentz invariance is fulfilled in nature. It turns out that simple molecules or solids could be used to test Lorentz invariance by spectroscopic means, since some (thinkable) Lorentz-violating terms would modify the energy spectrum. In fact, it could be shown that already existing experiments with optical cavities could be used to determine new upper bounds to some of those possible Lorentz-violating terms.