Inclusive Hadronic Decay Rate of the τ Lepton from Lattice QCD: The u[over ¯]s Flavor Channel and the Cabibbo Angle.

We present a lattice determination of the inclusive decay rate of the process τ↦X_{us}ν_{τ} in which the τ lepton decays into a generic hadronic state X_{us} with u[over ¯]s flavor quantum numbers. Our results have been obtained in n_{f}=2+1+1 isosymmetric QCD with full nonperturbative accuracy, without any operator product expansion approximation and, except for the presently missing long-distance isospin-breaking corrections, include a solid estimate of all sources of theoretical uncertainties. This has been possible by using the Hansen-Lupo-Tantalo method [M. Hansen et al., Phys. Rev. D 99, 094508 (2019)PRVDAQ2470-001010.1103/PhysRevD.99.094508] that we have already successfully applied [A. Evangelista et al., Phys. Rev. D 108, 074513 (2023)PRVDAQ2470-001010.1103/PhysRevD.108.074513] to compute the inclusive decay rate of the process τ↦X_{ud}ν_{τ} in the u[over ¯]d flavor channel. By combining our first-principles theoretical results with the presently available experimental data, we extract the Cabibbo-Kobayashi-Maskawa matrix element |V_{us}|, the Cabibbo angle, with a 0.9% accuracy, dominated by the experimental error.

Probing the Energy-Smeared R Ratio Using Lattice QCD.

We present a first-principles lattice QCD investigation of the R ratio between the e^{+}e^{-} cross section into hadrons and into muons. By using the method of Ref. [1], that allows one to extract smeared spectral densities from Euclidean correlators, we compute the R ratio convoluted with Gaussian smearing kernels of widths of about 600 MeV and central energies from 220 MeV up to 2.5 GeV. Our theoretical results are compared with the corresponding quantities obtained by smearing the KNT19 compilation [2] of R-ratio experimental measurements with the same kernels and, by centering the Gaussians in the region around the ρ-resonance peak, a tension of about 3 standard deviations is observed. From the phenomenological perspective, we have not included yet in our calculation QED and strong isospin-breaking corrections, and this might affect the observed tension. From the methodological perspective, our calculation demonstrates that it is possible to study the R ratio in Gaussian energy bins on the lattice at the level of accuracy required in order to perform precision tests of the standard model.

Quark and Gluon Momentum Fractions in the Pion from N_{f}=2+1+1 Lattice QCD.

We perform the first full decomposition of the pion momentum into its gluon and quark contributions. We employ an ensemble generated by the Extended Twisted Mass Collaboration with N_{f}=2+1+1 Wilson twisted mass clover fermions at maximal twist tuned to reproduce the physical pion mass. We present our results in the MS[over ¯] scheme at 2 GeV. We find ⟨x⟩_{u+d}=0.601(28), ⟨x⟩_{s}=0.059(13), ⟨x⟩_{c}=0.019(05), and ⟨x⟩_{g}=0.52(11) for the separate contributions, respectively, whose sum saturates the momentum sum rule.

Ruling Out the Massless Up-Quark Solution to the Strong CP Problem by Computing the Topological Mass Contribution with Lattice QCD.

The infamous strong CP problem in particle physics can in principle be solved by a massless up quark. In particular, it was hypothesized that topological effects could substantially contribute to the observed nonzero up-quark mass without reintroducing CP violation. Alternatively to previous work using fits to chiral perturbation theory, in this Letter, we bound the strength of the topological mass contribution with direct lattice QCD simulations, by computing the dependence of the pion mass on the dynamical strange-quark mass. We find that the size of the topological mass contribution is inconsistent with the massless up-quark solution to the strong CP problem.

Combining rheology and MRI: Imaging healthy and tumorous brains based on mechanical properties.

PURPOSE: It is well known that pathological changes in tissue alter its mechanical properties. This holds also true for brain tissue. In case of the brain, however, obtaining information about these properties is hard due to the surrounding cranial bone. In this paper a novel technique to create an imaging contrast based on the aforementioned properties is presented. METHODS: The method is based on an excitation of the brain induced by a short fall. The response of the brain tissue is measured using a motion sensitive MRI sequence. RESULTS: The new method is tested by measurements on phantom material as well as on healthy volunteers. In a proof of principle experiment the capability of the approach to identify local alterations in the mechanical properties is shown by means of measurements on meningioma patients. CONCLUSION: The presented results show the feasibility of the novel method. Even in this early state of the proposed method, comparisons of measurements on meningioma patients with intraoperative palpation suggest that meningioma tissue responds differently to the excitation depending on their mechanical properties. Magn Reson Med 78:930-940, 2017. © 2016 International Society for Magnetic Resonance in Medicine.