Search results for "Vacuum Polarization"
showing 10 items of 44 documents
Measurements of the Hadronic Cross Section and of Meson Transition Form Factors at BESIII for an improved Standard Model Prediction of (g−2)μ
2015
Abstract The BESIII experiment at the Beijing tau-charm factory BEPCII recently has embarked on a series of form factor measurements with the goal to improve the hadronic vacuum polarization as well as the hadronic light-by-light contributions to ( g − 2 ) μ . The status of the flagship measurements and preliminary results are presented.
Next-to-next-to-leading order τ+τ- production cross section close to threshold
2001
13 páginas, 5 figuras.-- PACS number(s): 12.20.Ds, 14.60.Fg
Electroweak Effective Charges and their relation to Physical Cross Sections
1997
In quantum electrodynamics with fermions f = e,\mu..., knowledge of the vacuum polarization spectral function determined from the tree level e^+e^- -> f^+f^- cross sections, together with a single low energy measurement of the fine structure constant \alpha, enables the construction of the one-loop effective charge \alpha_eff(q^2) for all q^2. It is shown how an identical procedure can be followed in the electroweak sector of the Standard Model to construct three gauge-, scale- and scheme-independent one-loop electroweak effective charges and an effective weak mixing angle from the tree level e^+e^- -> W^+W^-, ZH and e^+\nu_e -> W^+Z, W^+\gamma, W^+H differential cross sections, together wi…
Initial State Radiation Measurements at BESIII
2019
Precision measurements of the hadronic cross sections e+ e−→ π+ π− , π+π−π0, π+π−2π0, ωπ0 and π+π−3π0 are performed at the BESIII experiment located in Beijing, China. These cross section measurements are needed as input for the Standard Model prediction of the anomalous magnetic moment of the muon, (g − 2)µ.The initial state radiation method is used to access the energy regions of interest for (g − 2)µ. Our results will improve the precision of the Standard Model prediction of (g − 2)µ .
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.
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.
Nucleon-nucleon effective potential in dense matter including rho-meson exchange
1998
We obtain the RPA summed one-meson exchange potential between nucleons in symmetric nuclear matter at zero temperature, from a model which includes $\rho $, $\sigma $, $\omega $ and $\pi $ mesons. The behavior of rho mesons inside the medium is first discussed using different schemes to extract a finite contribution from the vacuum polarization. These schemes give qualitatively different results for the in-medium rho mass. The results are discussed in connection with the non-renormalizability of the model. We next study the modified potential as density increases. In the intermediate distance range, it is qualitatively modified by matter and vacuum effects. In the long-distance range ($r>2$…
Can power corrections be reliably computed in models with extra dimensions?
2003
We critically revisit the issue of power-law running in models with extra dimensions. The analysis is carried out in the context of a higher-dimensional extension of QED, with the extra dimensions compactified on a torus. It is shown that a naive $\beta$ function, which simply counts the number of modes, depends crucially on the way the thresholds of the Kaluza-Klein modes are crossed. To solve these ambiguities we turn to the vacuum polarization, which, due to its special unitarity properties, guarantees the physical decoupling of the heavy modes. This latter quantity, calculated in the context of dimensional regularization, is used for connecting the low energy gauge coupling with the cou…
An interpolation of the vacuum polarization function for the evaluation of hadronic contributions to the muon anomalous magnetic moment
2001
We propose a simple parameterization of the two-point correlator of hadronic electromagnetic currents for the evaluation of the hadronic contributions to the muon anomalous magnetic moment. The parameterization is explicitly done in the Euclidean domain. The model function contains a phenomenological parameter which provides an infrared cutoff to guarantee the smooth behavior of the correlator at the origin in accordance with experimental data in e+ e- annihilation. After fixing a numerical value for this parameter from the leading order hadronic contribution to the muon anomalous magnetic moment the next-to-leading order results related to the vacuum polarization function are accurately re…