My group carries out theoretical research in particle physics. Our work often uses a computational approach. We also develop and leverage techniques from data science.
Our guiding questions include:
Are anomalies in decays of B mesons pointing to new physics?
Will precise CKM metrology reveal a breakdown of unitarity in the CKM matrix, signaling the existence of new particles?
For which inclusive hadronic observables can calculations in lattice QCD make the biggest impact?
Can neutrino-nucleus scattering be understood at the level needed to determine neutrino mixing parameters or the neutrino mass hierarchy?
A complete list of my papers can be found on inspire-hep.
The two main areas of research in my group are:
Lattice Quantum Chromodynamics
Quantum chromodynamics (QCD) is the quantum field theory within the Standard Model of particle physics which describes the strong sub-nuclear force. This force is responsible for the interactions of subatomic particles known as quarks and gluons, which give rise to familiar particles like protons and neutrons as well as panoply of other bound states known as hadrons.
Lattice QCD refers a particular class of theoretical techniques used to compute the properties of hadrons. In a nutshell, we approximate continuous spacetime by a finite lattice of points. Quark and gluon degrees of freedom are defined on this lattice, allowing us to compute the quantum mechanical path integral of the theory. We calculate properties like the masses of hadrons or form factors describing the interaction of hadrons with external weak or electromagnetic probes.
Weak decay of heavy mesons
First-principles calculations using lattice QCD give us access to the Standard-Model predictions of many important physical processes, including weak decays of hadrons. Such calculations, combined with experimental measurements of the decay rates, give a way to determine the CKM matrix elements, which are fundamental constants defining the Standard Model. Like the speed of light or Plank’s constant, the CKM matrix elements are expected to be constant. Determining their values from many different decays furnishes a stringent test of the Standard Model and is a popular method to look for evidence of new particles or interactions beyond the Standard Model.
Tantalizingly, combined analysis of theoretical predictions and experimental data shows evidence for tensions in decays of B mesons. B mesons are quark-antiquark bound states containing a so-called “bottom quark.” The bottom quark is short-lived heavy cousin of the stable quarks that form protons and neutrons which has a mass roughly four times that of a proton. Different experimental measurements, together with a theoretical calculation of the Standard-Model prediction, paint a confusing picture for the rates at which the bottom quark decays into a charm quarks and up quarks. The following figure shows the 2024 summary by the Flavour Lattice Averaging Group:
The essential feature is the disagreement between the black data point (coming from the average of the different shaded bands) and the blue data point (coming from the blue band).
My group is working to shed light on the situation through state-of-the-art lattice QCD calculations. We are members of the Fermilab-Lattice and MILC collaborations and are leading the calculation of the decays of B mesons to D mesons and pions. These calculations are expected to yield precise new values for the CKM matrix elements |Vcb| and |Vub}.
My group is also interested in lattice QCD calculations of the inclusive rate, which have traditionally treated using alternative theoretical methods. My recent work, together with ongoing efforts throughout the lattice community, suggests that robust lattice QCD calculations of inclusive observables may will soon be possible.
Tau decays
The tau lepton is a short-lived particle with properties very similar to the electron and muon. Unique among the charged leptons, the tau can decay via the weak interaction into hadrons. These decay channels offer opportunities for calculations using lattice QCD. In particular, inclusive decays rates give access to the CKM matrix elements |Vud| and |Vus|. Their values are particular interesting in light of recent tensions going by the name of the Cabibbo anomaly. Recent constraints on |Vud| and |Vus| were summarized by Bryman et al. in the following figure:
My group would like to add precise constraints for both quantities from tau decays using lattice QCD.
Theoretical Modeling for Neutrino-Nucleus Scattering
The three neutrinos are perhaps the most elusive particles in the Standard Model. Experiencing neither the strong nor the electromagnetic interactions, they scatter off of regular matter only through the weak interaction. This means that a beam of neutrinos, produced from the decays of more familiar particles like pions, can propagate through long distances (e.g., the entire earth) with almost no attenuation.
But perhaps their most remarkable aspect of neutrinos is the fact that they oscillate, undergoing a spontaneous transformation from one flavor to another as they propagate. The discovery of this property was the subject of the 2015 Nobel Prize in Physics. The existence of neutrino oscillation is now well established experimental fact, but our quantitative understanding of the phenomenon remains incomplete.
Upcoming experiments like the Deep Underground Neutrino Experiment (DUNE) at Fermilab and Hyper-Kamiokande in Japan aim to study neutrino interactions with unprecedented precision. Making sense of high-precision experiments requires high-precision theoretical predictions.
Achilles: A theory-driven event generator
Together with collaborators at Fermilab and Argonne National Laboratory, my group is developing Achilles, an open-source theory-driven scientific software package for predicting experimental cross sections seen in lepton-nucleus scattering experiments. Our focus is on the kinematic range relevant for the accelerator-based neutrino experiments like DUNE and Fermilab’s short-baseline neutrino program.
My group at CSU is especially interested in the physically rich but poorly understood kinematic region known as shallow inelastic scattering. In this region, the scattering probe has a short enough Compton wavelength to resolve the substructure of individual protons and neutrons (and to create a large variety of short-lived resonances) but not enough energy to enter the scaling region of deep inelastic scattering.
Theoretical research in this area is highly computational and sits at the experiment, first-principles calculations (e.g., with lattice QCD), phenomenological modeling.
