Search results for "vacuum polarization"
showing 10 items of 44 documents
Muon Anomaly from Lepton Vacuum Polarization and The Mellin--Barnes Representation
2008
We evaluate, analytically, a specific class of eighth--order and tenth--order QED contributions to the anomalous magnetic moment of the muon. They are generated by Feynman diagrams involving lowest order vacuum polarization insertions of leptons $l=e,\mu$, and $\tau$. The results are given in the form of analytic expansions in terms of the mass ratios $m_e/m_\mu$ and $m_\mu/m_\tau$. We compute as many terms as required by the error induced by the present experimental uncertainty on the lepton masses. We show how the Mellin--Barnes integral representation of Feynman parametric integrals allows for an easy analytic evaluation of as many terms as wanted in these expansions and how its underlyi…
Improving the kinetic couplings in lattice nonrelativistic QCD
2019
We improve the non-relativistic QCD (NRQCD) action by comparing the dispersion relation to that of the continuum through $\mathcal{O}(p^6)$ in perturbation theory. The one-loop matching coefficients of the $\mathcal{O}(p^4)$ kinetic operators are determined, as well as the scale at which to evaluate $\alpha_s$ in the $V$-scheme for each quantity. We utilise automated lattice perturbation theory using twisted boundary conditions as an infrared regulator. The one-loop radiative corrections to the mass renormalisation, zero-point energy and overall energy-shift of an NRQCD $b$-quark are also found. We also explore how a Fat$3$-smeared NRQCD action and changes of the stability parameter $n$ aff…
The hadronic contribution to the running of the electromagnetic coupling and the electroweak mixing angle
2019
37th International Symposium on Lattice Field Theory, Wuhan, China, 16 Jun 2019 - 22 Jun 2019; PoS(LATTICE 2019)010 (2019).
Towards extracting the timelike pion form factor on CLS two-flavour ensembles
2017
35th International Symposium on Lattice Field Theory, Lattice 2017, Granada, Spain, 18 Jun 2017 - 24 Jun 2017; The European physical journal / Web of Conferences 175, 05027 (2018). doi:10.1051/epjconf/201817505027
Space-like transition form factors from BESIII
2018
Abstract The anomalous magnetic momentum of muon, a μ , has been measured in experiment and calculated in theory with a precision up to ∼0.5 ppm. But there is a long standing 3 to 4 standard deviations between these two accurate values. The dominant contribution to the uncertainty in the theoretical calculation comes from the hadroninc contribution, including contributions from the hadronic vacuum polarization and the hadronic light-by-light. The meson transition form factors measured in two photon process at BESIII can be used as input or constrain for the calculation of the hadronic light-by-light contribution to a μ . Recent experimental activities, including the measurements of the tran…
Calculation of the two-loop heavy-flavor contribution to Bhabha scattering
2008
We describe in detail the calculation of the two-loop corrections to the QED Bhabha scattering cross section due to the vacuum polarization by heavy fermions. Our approach eliminates one mass scale from the most challenging part of the calculation and allows us to obtain the corrections in a closed analytical form. The result is valid for arbitrary values of the heavy fermion mass and the Mandelstam invariants, as long as s,t,u >> m_e^2.
Generalized Virasoro anomaly and stress tensor for dilaton coupled theories
2003
We derive the anomalous transformation law of the quantum stress tensor for a 2D massless scalar field coupled to an external dilaton. This provides a generalization of the Virasoro anomaly which turns out to be consistent with the trace anomaly. We apply it together with the equivalence principle to compute the expectation values of the covariant quantum stress tensor on a curved background. Finally we briefly illustrate how to evaluate vacuum polarization and Hawking radiation effects from these results.
Study of the anomalous magnetic moment of the muon computed from the Adler function
2014
We compute the Adler function on the lattice from vacuum polarization data with twisted boundary conditions using numerical derivatives. The study is based on CLS ensembles with two flavours of $O(a)$ improved Wilson fermions. We extrapolate the lattice data for the Adler function to the continuum limit and to the physical pion mass and analyze its dependence on the momentum transfer. We discuss the application of this method to the extraction of the $u,d$ contribution to $a_\mu^{\mathrm{HLO}}$.
Vacuum polarization around stars: Nonlocal approximation
2004
We compute the vacuum polarization associated with quantum massless fields around stars with spherical symmetry. The nonlocal contribution to the vacuum polarization is dominant in the weak field limit, and induces quantum corrections to the exterior metric that depend on the inner structure of the star. It also violates the null energy conditions. We argue that similar results also hold in the low energy limit of quantum gravity. Previous calculations of the vacuum polarization in spherically symmetric spacetimes, based on local approximations, are not adequate for newtonian stars.
Global fits of the SM parameters
2019
I present a global survey of weak mixing angle measurements and other precision experiments and discuss the issue of correlations of the theoretical uncertainties in precision observables. Hadronic vacuum polarization effects complicate global fits in various ways and are also covered alongside some representative fit results.