Inclusive Semileptonic Decays of Heavy Mesons from Lattice QCD

The Standard Model (SM) of particle physics is the best-known description of subatomic physics, although it is known to be incomplete. Testing the SM against possible new models requires precise knowledge of the SM parameters. The SM can be discretised using lattice Quantum Chromodynamics (QCD), allowing predictions from first principles. The Cabibbo-Kobayashi-Maskawa (CKM) matrix contains information about parameters of the SM that cannot be derived analytically, but can only be determined by comparing theoretical predictions with experimental measurements. CKM-matrix elements can be extracted from inclusive semileptonic decays, i.e. decays into a lepton and any possible hadronic final state. Their calculation is theoretically challenging because it requires the extraction of a spectral density. We use gauge ensembles generated by the Extended Twisted Mass Collaboration with dynamical up, down, strange, and charm quarks at physical masses for our lattice calculations, and the Hansen-Lupo-Tantalo (HLT) method to extract spectral densities. We are able to extract phenomenologically relevant information about CKM-matrix elements from lattice calculations with physical masses for the first time. We analyse the decay Ds → Xℓν, which contains information about Vcd and Vcs, and extract the decay rate and the first two lepton-energy moments. Our results are in good agreement with experimental measurements and have a precision similar to that of the experiments. We analyse the statistical noise and all systematic errors arising from the HLT reconstruction, the continuum limit and finite-volume effects. Our error is dominated by statistical noise and is systematically improvable. The decay Bs → Xscℓν contains information about Vcb, which is needed for precision tests of the SM. We simulate the decay on a limited set of gauge ensembles, and calculate the decay rate with a total error of 7%. We present a simulation strategy to improve the signal-to-noise ratio for heavy meson masses that improves the current state-of-the-art. The physical Bs meson mass cannot be simulated directly on the lattice; we therefore use lighter meson masses and predictions for the infinite-mass limit from the Operator Product Expansion to interpolate to the physical point.