A common theme of my past research experience in physics is the use of atomic, molecular, and optical (AMO) physics techniques to perform precision measurements which look for physics beyond the Standard Model.
Measurement of the Electron Electric Dipole Moment in ThO
Despite the immense success of the Standard Model in the last few decades, several crucial questions remain. One of the most puzzling is explaining the current abundance of matter over antimatter, the so-called baryon asymmetry problem. In 1967, Andrei Sakharov proposed the so-called Sakharov conditions for baryogenesis. One of these is the necessity of a sufficient amount of CP (charge-parity) symmetry violation. CP violation (CPV) was first discovered by Cronin and Fitch in neutral kaons (1964). While a few other instances of CPV have since been experimentally detected, the overall amount is insufficient to explain baryogenesis. Various theories have been proposed which predict the existence of beyond Standard Model (BSM) CPV, which by the CPT theorem implies T (time) symmetry violation. Thus searches for electric dipole moments (EDMs) in fundamental particles (which are T-violating) can be used as powerful probes for BSM physics.
The ACME EDM (Advanced Cold Molecule Electron EDM) experiment utilizes the state-of-the-art AMO methods to perform a precise measurement of the electron EDM. The apparatus consists of a cryogenic buffer gas cooled beam (CBGB) of thorium monoxide (ThO). Various techniques are used to transfer as many molecules as possible into the -electronic state, where we perform a spin precession measurement. The molecules are prepared into a superposition of states, which precesses as it goes through applied electric and magnetic fields in the interaction region. The strong internal electric field inside ThO amplifies the effective field experience by the electron, boosting the sensitivity of our measurement by many orders of magnitude. At the end of the precession, the evolved state is then projected and its phase measured. The existence of a non-zero electron EDM would cause a small additional phase.
The ACME collaboration (a collaboration between 3 labs at Harvard and Yale) published its first generation result in 2014, an order of magnitude improvement in the upper limit for the electron EDM.1 In the summer of 2013, I spent a summer at ACME as an undergraduate under the supervision of Prof. Dave DeMille (Yale), designing an electrostatic lens for improving molecule yield. In September 2015 I joined the experiment as a graduate student in the group of Prof. Gerald Gabrielse at Harvard. I have since been heavily involved in building and running the apparatus to perform a second generation measurement which is projected to have an order of magnitude increased statistical sensitivity. I am also working on further upgrades to the planned third generation measurement on the same apparatus.
Search for Long Range Spin-Spin Interactions
The great success of the Standard Model was partially due to the discoveries of
the gauge bosons (W, Z, and gluons) which mediate the weak and strong interactions. Naturally, searches for physics beyond the Standard Model (BSM) have included extensive experimental and theoretical investigations into the existence of additional gauge bosons – both the spin-1 (vector boson) variety as well as their more exotic cousins (scalar and pseudoscalar bosons). The existence of some of these new particles would have potential to explain dark matter or the strong CP problem.
In the Hunter Lab at Amherst College, we use a cesium-mercury co-magnetometer in a Bell-Bloom configuration to look for evidence of certain interactions mediated by BSM vector bosons, specifically long-range spin-spin interactions (LRSSIs).2 The apparatus consists of an Hg vapor cell sandwiched between two Cs vapor cells immersed in a magnetic field. The atoms in the cells are pumped to a coherent spin state by optically pumping them with circularly polarized modulated light. Afterwards, the pump laser is turned off and a weaker laser tuned several GHz off resonance is used to probe the frequency of the spin precession via detection of Faraday rotation. The apparatus is mounted on a rotating table to allow for different orientations of the spin-polarized atoms. If LRSSIs existed, then the spin-polarized electrons inside the Earth would perturb the spins in the lab apparatus.
This technique of using polarized geoelectrons to look for LRSSIs was pioneered by Larry Hunter in 2013.3 This technique also allowed experimental bounds to be set for the first time on a variety of velocity-dependent LRSSIs.4 I spent time during my sophomore and junior years at Amherst assisting Prof. Hunter with these calculations.
An earlier version of this apparatus was used to look for evidence of Local Lorentz Invariance (LLI) violation.5 During my senior year at Amherst, I did work on testing upgrades on the apparatus to reduce systematics from AC Stark shifts. These upgrades are expected to increase sensitivity to LRSSIs by an order of magnitude. Details on this can be found in my senior thesis, In Search of New Geometries for Probing Spin-Spin Interactions. For more information, contact Larry Hunter at Amherst College.
- The ACME Collaboration. Order of Magnitude Smaller Limit on the Electric Dipole Moment of the Electron. Science 343, 269-272 (2014). ↩
- Dobrescu, B. A. & Mocioiu, I. Spin-dependent macroscopic forces from new particle exchange. Journal of High Energy Physics 2006, 005 (2006). ↩
- Hunter, L., Gordon, J., Peck, S., Ang, D. & Lin, J.-F. Using the earth as a polarized electron source to search for long-range spin-spin interactions. Science 339, 928-932 (2013). ↩
- Hunter, L. & Ang, D. Using geoelectrons to search for velocity-dependent spin-spin interactions. Phys. Rev. Lett. 112, 091803 (2014). ↩
- Peck, S. et al. Limits on local lorentz invariance in mercury and cesium. Physical Review A 86, 012109 (2012). ↩