The Advanced Cold Molecule Electric Dipole Moment Search (ACME) is a new collaborative effort to detect the electric dipole moment (EDM) of the electron. The most fundamental of physics theories, the Standard Model of particle physics, predicts an extremely small electron EDM, while essentially all proposed extensions to the Standard Model predict an electron EDM that is within the projected sensitivity for this experiment. The ThO molecule that ACME will use provides in one species all of the advantages identified by past molecular EDM studies, with the added benefit of a higher internal electric field which will further increase the sensitivity of the experiment. Moreover, the prototype ThO molecular beam source that ACME is developing, using laser ablation and cryogenic buffer gas cooling, already demonstrates the possibility of a usable flux of molecules that is a thousand times that of previous experiments. The initial ACME goal is to measure the electron EDM with a sensitivity that greatly exceeds the current limit.
Many students and postdocs will learn their craft working on this experiment, and many of the techniques developed will have general application to precision measurements. There will be a broad impact on physics, whatever the ACME result. The lack of an electron EDM at the attainable precision would eliminate the most studied and motivated extensions to the Standard Model. The discovery of an electron EDM would be one of the great breakthroughs in physics, signifying the long anticipated breakdown of the Standard Model of particle physics, and heralding a new understanding of the imbalance between antimatter and matter in the universe
(ACME) is an ongoing collaborative effort to detect the electric dipole moment (EDM) of the electron. The most fundamental of theories, the Standard Model of particle physics, predicts an extremely small electron EDM (eEDM), while nearly all proposed extensions to the Standard Model predict an eEDM within the ultimate projected sensitivity for ACME. The first generation of ACME, supported by this grant, achieved sensitivity to the eEDM 10 times better than any previous experiment. This made it possible to set an improved upper limit on the size of the eEDM, smaller by a factor of 10 than before, significantly constraining particle theory and our knowledge of new particles in the 1-10 TeV range. This result was achieved within one 5-year grant period; by comparison, the previous improvement of a factor of 10 took nearly 20 years. Several key features enabled ACME to extend the sensitivity to the eEDM farther and faster than previous experiments—a rate of progress we aim to continue through future generations of the experiment. The eEDM was probed using the enormous effective electric field felt by the electron within a polar molecule. The thorium monoxide (ThO) molecules used in ACME were produced as a cold molecular beam whose useful flux was more than 2 orders of magnitude greater than other methods. ThO has structural properties that greatly reduce the sensitivity to key systematic errors that could potentially mimic the eEDM. A final key ingredient to the success of ACME is the ACME team, which is larger than has been traditional in the field. This made it possible to combine expertise and resources in an efficient way to tackle this difficult and important problem. The ACME program bridges a gap between two disciplines within the field of physics, with particle physics providing the motivation while atomic, molecular and optical (AMO) physics provides the methods. The scale of resources required for ACME is also between what is typical for these communities, being tiny on the scale of accelerators such as the Large Hadron Collider (LHC) but a factor of about three larger than a major AMO experiment. Intellectual Merit: The new limit on the eEDM provided by ACME implies strong constraints on theoretically favored extensions to the Standard Model, such as theories that involve Supersymmetry with new particles of mass similar to that of the Higgs boson. The importance of this eEDM measurement is complementary to direct searches for new particles at accelerators such as the LHC. EDM measurements such as this one are the most promising way to determine whether any such new particles are related to time-reversal violation. This could be crucial for understanding the observed asymmetry between matter and antimatter in the universe. In addition, the ACME result is, in some theoretical models, already ruling out the existence of certain particles that would otherwise be expected to be eventually observable at the LHC. In the closing months of the grant, the ACME team also developed detailed plans for a next generation of the experiment, which projects another factor of 10 improvement in sensitivity. This lays the groundwork for future measurements of the eEDM, which many theoretical models predict could lead to the first evidence for new particles that lie outside the framework of the Standard Model. Broader Impacts: The ACME project provided the primary training for 2 postdocs, 9 Ph.D. students, and 13 undergrads. In the course of the project, new experimental methods were developed for producing molecular beams and measuring and manipulating quantum states of molecules. These methods may be useful in other areas of AMO physics. Finally, the extremely fundamental nature of the scientific goal has its own intrinsic impact. The ACME result was widely reported in the popular media, helping to engage the public in the fundamental science of particle physics and AMO physics. The lack of an eEDM at the attained level of sensitivity has eliminated some versions of the most studied and motivated extensions to the Standard Model. The discovery of an eEDM would be one of the great breakthroughs in physics, signifying the long anticipated breakdown of the Standard Model of particle physics and heralding a possible new understanding of the imbalance between antimatter and matter in the universe.