Research

My research using lattice QCD includes quark flavor physics, hadron spectroscopy, and hadron structure. I am particularly interested in hadrons containing bottom or charm quarks and their decays.

Flavor physics with heavy baryons

A promising way of searching for new fundamental physics is the precision study of processes in which quarks change from one type (flavor) to another. Such processes are sensitive to quantum effects of hitherto undiscovered elementary particles that may be too heavy to be produced directly in high-energy accelerators. An intriguing pattern of deviations between experimental results and Standard-Model predictions has emerged in semileptonic decays of mesons containing heavy quarks. Complementary information on the fundamental interactions involved in these processes can be obtained by studying decays of baryons containing heavy quarks. The nonzero spins of the initial and final-state baryons make the decay amplitudes sensitive to all possible operator structures appearing in the weak effective Hamiltonian, and provide a large number of angular observables that can be used to disentangle these structures. As with meson decays, lattice-QCD calculations of hadronic form factors are needed to relate the decay rates measured in experiments to the underlying fundamental interactions. I have performed such calculations for several types of bottom and charm baryon decays, leading, combined with experimental data by LHCb and BESIII, to novel determinations of CKM matrix elements, tests of lepton-flavor universality, and constraints on flavor-changing neutral-current Wilson coefficients. I am working on next-generation higher-precision calculations and am exploring additional processes.

Please see my Timeline of heavy-baryon semileptonic-decay results for more.

Weak decays with multi-hadron final states

Among B-meson decays, the most interesting tensions between Standard-Model predictions and experimental measurements have emerged in decays involving the rare transition of the bottom quark to a strange quark and a pair of two charged leptons. These hints for physics beyond the Standard Model were first seen in LHCb measurements of the decay B → K^*(→ K π) μ⁺μ^-. The K^* is an unstable spin-1 meson that, within an extremely short time of approximately 10^-23 seconds, decays through the strong interaction into the two spin-0 mesons K and π. My collaborators and I published a lattice-QCD determination of the B → K^*μ⁺μ^- local form factors in 2013, which is still being used in today's global fits of new-physics couplings to the experimental data. However, in this calculation, we treated the K^* as if it is a stable particle, which causes some irreducible systematic errors that will become more relevant in future high-precision calculations. To avoid these errors, B → K π μ⁺μ^- transition matrix elements with the two-hadron final state must be calculated on the lattice. This is challenging because lattice-QCD calculations are done in a finite volume, where the K and π are constantly interacting with each other. The mathematical formalism needed to map the finite-volume matrix elements to the desired infinite-volume matrix elements was pioneered by Lellouch and Lüscher in the year 2000, and has since been generalized by others in several aspects, making it suitable for semileptonic decays. We are now applying these methods to B → K π μ⁺μ^- and several other interesting transitions with two hadrons in the final state, including B→ π π l ν, which is being studied by Belle II and can provide new information on a longstanding puzzle concerning the CKM matrix element V_ub. We have completed the necessary calculations of the K π and π π scattering amplitudes using the Lüscher method, and have completed a full calculation of a simpler 1→2 process: the electromagnetic π γ^* → π π transition.

Radiative leptonic decays

In radiative leptonic decays of mesons, the final state contains no hadron but instead a photon and two leptons (for example, an electron and an antineutrino). These decays are described by nonlocal matrix elements of two currents, and probe both the weak interactions and the internal structure of the decaying meson in interesting ways. The addition of a hard photon in the final state lifts the helicity suppression of purely leptonic decays. At the Lattice 2019 conference, I showed how the hadronic tensor of radiative leptonic decays can be calculated from a Euclidean three-point function computed on the lattice. Since then, we have developed several improvements to the methodology. This includes methods for controlling unwanted exponentials of Euclidean time in the sum over intermediate states and unwanted excited states created by the meson interpolating field, a new way of computing the correlation functions that allows continuous variations in the photon energy, and methods for reducing the statistical noise. We are now working toward realistic calculations of pion, kaon, and heavy-meson radiative leptonic decays.

Exotic mesons

Exotic mesons are mesons (hadrons with integer spin) that cannot be understood in the quark model as a bound state of a single valence quark-antiquark pair. All exotic mesons discovered in experiments to date are unstable under the strong interactions. However, it is expected that QQqq tetraquark mesons containing two heavy antiquarks, Q, and two light quarks, q, become QCD-stable for sufficiently large m_Q. Lattice-QCD calculations by us and by other groups have shown that the physical bottom-quark mass is indeed sufficient, and a bbud bound state with J^P=1⁺ exists about 100 MeV below the lowest potential strong-decay final state, i.e., about 100 MeV below m_B+m_B^*. The bbud tetraquark is likely out of reach of current experiments. In 2021, LHCb discovered the charm-quark partner of this tetraquark, ccud, barely below the DD^* threshold but decaying strongly into DDπ. A natural follow-up question is whether mixed charm-bottom tetraquarks (bcud) are QCD-stable. Answering this question has proven challenging for early lattice-QCD calculations. More recently, by calculating the full energy-dependent B^(*)D scattering amplitudes using lattice QCD with the Lüscher method, we found very shallow bcud bound states for both J^P=0⁺ and J^P=1⁺.

Other projects

I am also working on, or have worked on: lifetimes of hadrons in heavy-quark effective theory, masses and decay constants of positive-parity heavy-strange mesons, nucleon electromagnetic and axial form factors, nucleon parton distribution functions, nucleon-pion scattering, bottomonium spectroscopy in vacuum, in quark-gluon plasma, and at nonzero isospin density, heavy-baryon spectroscopy, axial couplings and strong decays of heavy hadrons in chiral perturbation theory, and lattice actions for heavy quarks.

Publications

Please see my publication list on INSPIRE HEP.