The Flavour Anomalies: Fluke, Fallacy or New Physics?

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

The Standard Model of particle physics represents our best current knowledge of the fundamental particles that make up our universe and the interactions (forces) that govern them. Although to date most fundamental particles have been observed to behave as predicted by the Standard Model, cosmological observations tell us that the Standard Model cannot be the complete picture. For example, in the Standard Model, matter and anti-matter are a near-perfect mirror of each other. However, the dominance of matter in our universe tells us that there must be some, as of yet unknown, interaction breaking this matter, anti-matter symmetry. The LHCb experiment is one of the four experiments situated on the Large Hadron Collider (LHC) at CERN. It is designed to make precision measurements of a particle called a beauty (b) quark. This award focuses on the study of b-quark decays which are very suppressed in the Standard Model. By precisely measuring the rates at which these rare processes happen, we can infer the existence of new interactions, not described within the Standard Model. Over recent years, measurements of these rare b decays have shown deviations with Standard Model predictions. This award will further our understanding behind what is causing these anomalies, with the goal of determining whether their origin could be the first sign of a beyond-the-Standard-Model process at the LHC. The PI of this project is the founder of the Laura Bassi initiative within the LHCb collaboration, which brings together early career researchers interested in tackling issues of gender and diversity within particle physics. This grant will support the PI’s continued work in this area. Whereas the tensions observed in rare b-decays can be over 5 standard deviations with respect to the Standard Model, these deviations could be due to poor theoretical descriptions of non-local hadronic effects, which cannot be derived from first principles in the Standard Model. This award performs a novel data-driven measurement of non-local hadronic effects in the decay B^0→K^(*0) μ+μ-, providing a crucial part of the flavour-anomaly puzzle. Similar methods are used to measure non-local tauonic contributions to B^0→K^(*0) μ^+ μ^-, via the decay B^0→K^(*0) [τ^+ τ^-→μ^+μ- ]. The expected sensitivity on B(B^0→K^(*0) τ^+ τ^- ) obtained using this method is world-leading. Measurements of b→sτ^+ τ^- are critical to understanding the anomalies, as favored new physics explanations predict large enhancements in theses modes. This project will also perform the first fit for the imaginary contributions to the underlying effective couplings in B^0→K^(*0) μ+μ- , with the proposed method resulting in an order-of-magnitude improvement on CP-violation constraints in b→sl+l- decays. This measurement is interesting in light of the flavor anomalies, and because constraints on beyond-the-SM CP violation are central to understanding the dominance of matter over anti-matter in the universe. This project will also commission and validate the performance of the LHCb Upgrade I detector, which takes data during the LHC Run 3 (2022-2026) for the first time. This commissioning work will be the cornerstone of subsequent measurements of b→sμ^+ μ^-processes using Run 3 data, which are also performed as part of this project. 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.