Search results for "dipole"
showing 10 items of 982 documents
Position-space approach to hadronic light-by-light scattering in the muon $g-2$ on the lattice
2016
The anomalous magnetic moment of the muon currently exhibits a discrepancy of about three standard deviations between the experimental value and recent Standard Model predictions. The theoretical uncertainty is dominated by the hadronic vacuum polarization and the hadronic light-by-light (HLbL) scattering contributions, where the latter has so far only been fully evaluated using different models. To pave the way for a lattice calculation of HLbL, we present an expression for the HLbL contribution to $g-2$ that involves a multidimensional integral over a position-space QED kernel function in the continuum and a lattice QCD four-point correlator. We describe our semi-analytic calculation of t…
Working group on goldstone boson production and decay
2008
This is the summary of the working group on Goldstone Boson Production and Decay of the Chiral Dynamics Workshop in Mainz, September 1–5, 1997. For the production aspects we discuss π0 and η production in nucleon-nucleon collisions and the behaviour of hadrons in various sum rules. For the decays we present a discussion on various K, η and π decay channels. Other aspects discussed are a new treatment of meson-meson scattering, the light-by-light contribution to the muon anomalous magnetic moment and progress in various aspects of the p 6 generating functional in the mesonic sector.
Pseudoscalar Transition Form Factors from Rational Approximants
2014
The $\pi^0$, $\eta$, and $\eta^\prime$ transition form factors in the space-like region are analyzed at low and intermediate energies in a model-independent way through the use of rational approximants. Slope and curvature parameters as well as their values at infinity are extracted from experimental data. These results are suited for constraining hadronic models such as the ones used for the hadronic light-by-light scattering piece of the anomalous magnetic moment of the muon, and for the mixing parameters of the $\eta - \eta^\prime$ system.
Recent Results on Meson Decays from A2
2018
Light meson decays are used to investigate topics related to fundamental aspects of particle physics. Precision measurements of meson Dalitz decays give input to theoretical evaluations of the Hadronic Light-by-Light contribution (HLbL) to the anomalous magnetic moment of the muon. The pseudoscalar η' decays allow for studies of topics like ππ scattering lengths, effective field theories and fundamental symmetries. The A2 collaboration, using the Crystal Ball/TAPS setup at MAMI, has recently published several high precision results on transition form factors which are related to HLbL. The value obtained for the slope parameter of the π0 electromagnetic (e/m) transition form factor (TFF) is …
Light-by-light scattering sum rules in light of new data
2016
We evaluate the light-quark meson contributions to three exact light-by-light scattering sum rules in light of new data by the Belle Collaboration, which recently has extracted the transition form factors of the tensor meson $f_2(1270)$ as well as of the scalar meson $f_0(980)$. We confirm a previous finding that the $\eta, \eta^\prime$ and helicity-2 $f_2(1270)$ contributions saturate one of these sum rules up to photon virtualities around 1 GeV$^2$. At larger virtualities, our sum rule analysis shows an important contribution of the $f_2(1565)$ meson and provides a first empirical extraction of its helicity-2 transition form factor. Two further sum rules allow us to predict the helicity-0…
The leading hadronic contribution to the running of the Weinberg angle using covariant coordinate-space methods
2018
We present a preliminary study of the leading hadronic contribution to the running of the Weinberg angle $\theta_{\mathrm{W}}$. The running is extracted from the correlation function of the electromagnetic current with the vector part of the weak neutral current using both the standard time-momentum representation method and the Lorentz-covariant coordinate-space method recently introduced by Meyer. Both connected and disconnected contributions have been computed on $N_{\mathrm{f}}=2+1$ non-perturbatively $O(a)$-improved Wilson fermions configurations from the CLS initiative. Similar covariant coordinate-space methods can be used to compute the leading hadronic contribution to the anomalous…
Search for a Dark Photon ine+e−Collisions atBaBar
2014
Dark sectors charged under a new Abelian interaction have recently received much attention in the context of dark matter models. These models introduce a light new mediator, the so-called dark photon (A^{'}), connecting the dark sector to the standard model. We present a search for a dark photon in the reaction e^{+}e^{-}→γA^{'}, A^{'}→e^{+}e^{-}, μ^{+}μ^{-} using 514 fb^{-1} of data collected with the BABAR detector. We observe no statistically significant deviations from the standard model predictions, and we set 90% confidence level upper limits on the mixing strength between the photon and dark photon at the level of 10^{-4}-10^{-3} for dark photon masses in the range 0.02-10.2 GeV. W…
New sum rule for the nuclear magnetic polarizability
2015
I extend the well-known photonuclear sum rule that relates the strength of the photoexcitation of the giant dipole resonance in a nucleus to the number of elementary scatterers-nucleons to the case of virtual photons. The new sum rule relates the size of the magnetic polarizability of a nucleus to the slope of the transverse virtual photoabsorption cross section integrated over the energy in the nuclear range. I check this sum rule for the deuteron where necessary data is available, discuss possible applications and connection with other sum rules postulated in the literature.
The anomalous magnetic moment of positive and negative muons
1977
Abstract The anomalous g -factor a ≡ ( g −2)/2 has been measured for muons of both charges in the Muon Storage Ring at CERN. The two results, a μ + = 1165910(12) × 10 −9 and a μ − = 1165936(12) × 10 −9 , are in good agreement with each other, and combine to give a mean a μ = 1165922(9) × 10 −9 , which is very close to the most recent theoretical prediction 1165921(10) × 10 −9 . For the experimental results, the total statistical and systematic error is given. The measurements thus confirm the remarkable QED calculation plus hadronic contribution, and serve as a precise verification of the CPT theorem for muons.
The GDH experiment at MAMI
1999
Abstract The Gerasimov-Drell-Hearn (GDH) sum rule connects the helicity dependent photoabsorption cross section with the anomalous magnetic moment of the nucleon. The GDH-collaboration is measuring the total cross section of circularly polarized photons with longitudinally polarized protons to check this sum rule experimentally to do a further progress in the investigation of the spin structure of the nucleon. The experiment has been started using the polarized electron beam of the Mainz electron accelerator MAMI in the energy range 200 – 800 MeV and will be continued at the Bonn accelerator ELSA up to an energy of about 3 GeV. We have finished the datav taking period at MAMI for the proton…