CAREER: Quantifying Uncertainties of Ab initio Nuclear Structure Calculations for New Physics Searches

The Standard Model of particle physics describes nearly everything we can observe in the universe, but we know that it must be incomplete, because it does not explain gravitation, the dark matter that makes up 85% of the mass of the universe, or why there is more matter than anti-matter. Proposed theories that extend the Standard Model to explain these effects typically predict the existence of new particles. In principle, these new particles can be directly searched for with particle colliders. An alternative is to perform very precise measurements to find signatures of the new physics encoded in tiny deviations from the predictions of the Standard Model. These experiments can often be performed on a table-top device. The challenge is that, beyond exquisite experimental control, these searches require very precise predictions of the Standard Model. Often this requires calculating properties of the atomic nucleus, which is a strongly-interacting quantum mechanical many-body system. Historically, such a problem was intractable. But recent developments in nuclear theory have placed precise calculations within reach. This project aims to improve the nuclear theory to the precision needed for searches of new physics. The goal of ab initio nuclear theory is to begin with the force between protons and neutrons and directly solve the quantum many-body problem. Exact solutions are not possible, and so the approach is to formulate an approximation scheme in which tractable calculations systematically approach the exact solution. A crucial feature is the ability to estimate the effect of what has been left out. This project will focus on a many-body method called the in-medium similarity renormalization group (IMSRG). The systematic approximation employed in the IMSRG is that interactions between pairs of nucleons are retained while effects acting on three or more particles are neglected. Including three-particle interactions while neglecting four-particle interactions would yield an improved approximation at greater cost which is just beyond the capabilities of current supercomputers. The goal of this project is to develop a tractable approximation to keeping three-particle interactions, and a framework for estimating the size of what is left out. This will enable the precise calculations with theoretical error bars needed for precision searches for new physics. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.