This experimental research program will continue a series of relatively small-scale experiments using lasers and atoms, which will address fundamental questions about physics of the electroweak interaction as well as other possible symmetry-violating interactions. Experiments will be undertaken in atomic systems of potential importance to electroweak tests such as lead. These experiments make use of diode laser systems and thallium samples in heated vapor cells as well as an atomic beam apparatus. In an extension of these measurements, the thallium atomic beam apparatus, high-voltage field plates, and existing laser system are being used to pursue a new experimental search for a possible time-reversal-violating (but parity conserving) long-range interaction. Proof-of-principle tests of this new experimental apparatus have demonstrated sufficient sensitivity to set important new limits on this class of potential symmetry-violating interactions. The broader impact of the program involves the commitment to a quality undergraduate research experience.
1. Introduction Precise, ab initio determination of atomic wavefunctions in multi-electron atoms present a significant challenge to state-of-the-art atomic theory calculations. Yet any atomic test of elementary particle physics rely critically on accurate atomic theory, which is essential to distinguish the complicated quantum mechanics from the more ‘fundamental’ physics that is being targeted. There is a long history of ever-improving atomic theory and corresponding precise measurements of atomic properties in the single-valence alkali systems. In our lab we focus on the somewhat more complicated three-valence-electron systems in Group IIIA (thallium, indium, gallium, aluminum), for which there is significantly less experimental data. Thus, at Williams, we are pursuing a series of related high-precision experiments in which semiconductor diode lasers are used to probe Group IIIA atoms (indium, thallium...). Atomic samples are contained in heated quartz vapor cells and also in an atomic beam vacuum chamber/apparatus. First, these experiments test the accuracy and guide the refinement of the cutting-edge atomic theory work. Second, we can use similar techniques to design experiments that explicitly search for physics of (and beyond) the Standard Model of elementary particle physics. Key aspects of these ‘precision measurements’ include careful experimental design, laser stabilization techniques, and various sophisticated signal processing and analysis techniques. These measurements have been completed in collaboration with nearly three dozen Williams undergraduate students as well as four postdoctoral research associates over the past decade. 2. Research Results and Broader Impact from most recent NSF Award In this grant cycle we completed the first ever measurement of the hyperfine structure of a particular indium excited state. Hyperfine structure reveals information concerning the nuclear charge distribution and the shape of the electron wavefunction near the nucleus. For this work, we employed two semiconductor diode lasers directed into a quartz vapor cell heated 800 C. A blue (410 nm) diode laser is used to excite the atoms from the ground state of indium to the first excited state. By using the technology recently developed to create GaN semiconductor lasers for the "Blue-Ray", we can obtain this blue laser radiation much more cheaply and conveniently than ever before. This blue laser is frequency ‘locked’ to the relevant atomic transition using a technique developed by the PI, our postdoc, and several undergraduate students. This was published as: " A frequency stabilization technique for diode lasers based on frequency-shifted beams from an acousto-optic modulator ", Mevan Gunawardena, Paul W. Hess '08, Jared Strait '07, and P.K. Majumder, Rev. Sci. Instrum. 79, 103110 (2008). The second laser, operating at 1291 nm in the infrared, then is scanned over the hyperfine structure of an excited state of indium that had not ever before been observed. Through the use of a lock-in amplifier and other signal processing electronics, and careful frequency calibration of the scans, we were able to measure the hyperfine splittings of this indium state to better than 1 MHz accuracy. Our results confirm existing atomic theory work on the hyperfine structure of indium. These results were published, again with two student co-authors, as: " A frequency stabilization technique for diode lasers based on frequency-shifted beams from an acoust0-optic modulator ", Mevan Gunawardena, Paul W. Hess '08, Jared Strait '07, and P.K. Majumder, Rev. Sci. Instrum. 79, 103110 (2008). Research Training At Williams College, these experiments serve as ideal research training for undergraduate physics majors, the large majority of whom attend graduate school in the physical sciences. Indeed, nine Williams students involved in experiments in this laboratory during the most recent NSF grant period are now enrolled in, or are presently applying to, Ph.D. programs in physics or related fields (attending schools including Cornell, Stanford, MIT, and Princeton(3)). Students take active, central roles in completing laboratory projects through which they develop expertise with optics and lasers, electronics and control systems, as well as sophisticated data analysis procedures. Because of the relative small physical scale of the research, they can appreciate the entirety of the experimental effort, gain valuable, specific experimental skills, and yet participate in an exciting cutting-edge research field. During the most recent grant period, eight first or second year undergraduates have worked in this laboratory, two of whom were women. A continuing effort will be made to include students in this research effort at this crucial early point in their undergraduate years, as they begin to make longer-term career choices. This laboratory has also served as a space for mentoring and developing the careers of NSF-funded postdoctoral research associates. These individuals both help with setting and reaching experimental goals in the laboratory, as well as with mentoring the undergraduate students. The postdoctoral associate funded with this most recent NSF grant has begun a tenure-track teaching/research career at Stonehill College in Easton, MA.
