ACME: Advanced Cold Molecule Electron Electric Dipole Moment Search

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The goal of this project is to search for a new fundamental property of the electron, one of the main constituents of matter and a charged component of all atoms. This new property, called an electric dipole moment, can be described as a slight bulge on an otherwise perfect sphere of charge. This seemingly abstruse property may hold the key to one of the most fundamental mysteries of nature: why is everything in the universe made of matter rather than of antimatter? In accelerator laboratories, whenever energy is converted into particles (according to E=mc2), equal numbers of matter particles and antimatter particles are created. For example, the electron has a counterpart antimatter particle, the anti-electron, which has identical mass but opposite electric charge. Energy can be converted into an electron/anti-electron pair, and conversely an electron and anti-electron can annihilate each other and turn into energy. Just after the Big Bang, energy was converted into particles and anti-particles. Astronomical observations show that since then, essentially all the antimatter annihilated with matter--but a tiny bit of matter was left over. That small excess makes up all of the objects seen in the Universe today. The current framework that describes all known fundamental forces between elementary particles, known as the "Standard Model", cannot explain how this excess of matter survived. However, many mathematical theories have been devised that can explain this "matter-antimatter asymmetry", by positing new forces and particles not yet discovered in any experiment. These same new forces and particles also often lead, according to the same theories, to an electric dipole moment that is large enough to observe in the experiment supported here. Hence, this project is essentially seeking an answer to the question: how is it that matter was slightly preferred over anti-matter at some time in the past, resulting in the physical Universe seen today? In a general sense, this project also advances the range of techniques for precision measurement science, which in the past has led to unexpected breakthroughs in technology such as GPS (the Global Positioning System), new types of sensors, etc. The electric dipole moment (EDM), if it exists, must lie along the spin axis of the electron. In the presence of a nonzero EDM, an electric field will induce a torque on the electron, resulting in precession of the spin about the field. This spin precession angle is the experimental signal. The huge internal electric field of a polar molecule, ThO, is used to amplify this observable effect. The internal structure of ThO also suppresses possible systematic errors. A cryogenic molecular beam source that delivers an unprecedented high flux of molecules is used. Lasers and optical techniques put the ThO molecules in usable coherent superpositions and then probe the quantum interference that signals the electron's spin precession. Over the previous grant period, an initial version of these methods was used to make by far the most sensitive measurement of the electron's EDM. This result was consistent with a zero value for the EDM, but many theories of what lies beyond the Standard Model of particle physics predict that, with improved sensitivity, detection of the EDM is likely. In this project, methods to greatly improve the sensitivity of the experiment will be introduced; these include focusing of the molecular beam and a use of a more efficient process for state preparation, based on adiabatic passage. Improvements such as the use of low-absorption optical elements will reduce systematic errors observed in the previous experiment.