Search results for "Expansion"
showing 10 items of 630 documents
Recent developments in the evaluation of Mössbauer line intensities
1985
The angular dependence of the intensities of pure or mixed multipole transitions is formulated in a new fashion, which takes into account arbitrary texture of the Mossbauer absorber ranging from complete orientation (single crystals) over preferred orientation to random powders and an anisotropic absorption fraction of the nuclei. The application of the theory is demonstrated by means of typical examples from the literature.
Analysis of the Intermediate-State Contributions to Neutrinoless Double β− Decays
2016
A comprehensive analysis of the structure of the nuclear matrix elements (NMEs) of neutrinoless double beta-minus (0νβ-β-) decays to the0+ground and first excited states is performed in terms of the contributing multipole states in the intermediate nuclei of0νβ-β-transitions. We concentrate on the transitions mediated by the light (l-NMEs) Majorana neutrinos. As nuclear model we use the proton-neutron quasiparticle random-phase approximation (pnQRPA) with a realistic two-nucleon interaction based on the Bonn one-boson-exchangeGmatrix. In the computations we include the appropriate short-range correlations, nucleon form factors, and higher-order nucleonic weak currents and restore the isospi…
Cosmology with a very light Lμ − Lτ gauge boson
2019
In this paper, we explore in detail the cosmological implications of an abelian L − L gauge extension of the Standard Model featuring a light and weakly coupled Z′. Such a scenario is motivated by the longstanding ∼ 4σ discrepancy between the measured and predicted values of the muon’s anomalous magnetic moment, (g − 2) , as well as the tension between late and early time determinations of the Hubble constant. If sufficiently light, the Z′ population will decay to neutrinos, increasing the overall energy density of radiation and altering the expansion history of the early universe. We identify two distinct regions of parameter space in this model in which the Hubble tension can be significa…
Compton Scattering by the Proton using a Large-Acceptance Arrangement
2001
Compton scattering by the proton has been measured over a wide range covering photon energies 250 MeV lt or approximately=E/sub gamma / lt or approximately=800 MeV and photon scattering angles 30 degrees lt or approximately= theta /sub gamma //sup lab/ lt or approximately=150 degrees , using the tagged-photon facility at MAMI (Mainz) and the large-acceptance arrangement LARA. The previously existing data base on proton Compton scattering is greatly enlarged by more than 700 new data points. The new data are interpreted in terms of dispersion theory based on the SAID-SM99K parametrization of photo-meson amplitudes. It is found that two-pion exchange in the t-channel is needed for a descripti…
$W^{+}W^{-}$ production at the LHC: fiducial cross sections and distributions in NNLO QCD
2016
We consider QCD radiative corrections to $W^+W^-$ production at the LHC and present the first fully differential predictions for this process at next-to-next-to-leading order (NNLO) in perturbation theory. Our computation consistently includes the leptonic decays of the $W$ bosons, taking into account spin correlations, off-shell effects and non-resonant contributions. Detailed predictions are presented for the different-flavour channel $pp\to\mu^+e^-\nu_\mu {\bar \nu}_e+X$ at $\sqrt{s}=8$ and $13$ TeV. In particular, we discuss fiducial cross sections and distributions in the presence of standard selection cuts used in experimental $W^+W^-$ and $H\to W^+W^-$ analyses at the LHC. The inclus…
Nuclear response functions in homogeneous matter with finite range effective interactions
2005
The question of nuclear response functions in a homogeneous medium is examined. A general method for calculating response functions in the random phase approximation (RPA) with exchange is presented. The method is applicable for finite-range nuclear interactions. Examples are shown in the case of symmetric nuclear matter described by a Gogny interaction. It is found that the convergence of the results with respect to the multipole truncation is quite fast. Various approximation schemes such as the Landau approximation, or the Landau approximation for the exchange terms only, are discussed in comparison with the exact results.
Mirror energy differences above the 0f7/2 shell: First γ-ray spectroscopy of the Tz = −2 nucleus 56Zn
2021
5 pags., 4 figs.
The collective structure of106,108Ru
1984
Levels in106,108Ru have been studied by measuringγγ coincidences andγγ directional correlations of theγ-rays following theβ−-decay of 36 s106Tc and 5 s108Tc. Tc activities were separated chemically from the fission products of thermal neutron induced fission of239Pu by the continuously running system “SISAK 2” and theγ-radiation was measured on-line. An extended level scheme for108Ru, spin assignments,E2/M1 multipole mixing ratios andB(E2) ratios for106Ru and108Ru have been obtained. The collective structure of the two nuclei is discussed in the framework of different nuclear models.
Double Polarization Observables in η Electroproduction
2009
Abstract A new measurement of beam-recoil double-polarization observables in η electroproduction at the A1 collaboration at MAMI is presented. The energy region of the S 11 ( 1525 ) resonance was covered at a four-momentum transfer of Q 2 = 0.1 GeV 2 / c 2 . It turned out, that the usual interpretation of the reaction mechanism via S 11 and D 13 resonance fails to explain the relative phase between the corresponding multipole amplitudes tested by this experiment, while the new data are still in good agreement with previous measurements.
Theory of double beta decay
2005
Basic features of the two-neutrino double beta ( 2 ν β β ) decay and neutrinoless double beta ( 0 ν β β ) decay are discussed. A short review of the most important nuclear models, used to describe the ββ decay, is given. A deeper survey of the nuclear matrix elements, related to the above-mentioned decay modes, is performed using the proton-neutron quasiparticle random-phase approximation (pnQRPA). In particular, the extraction of a proper value of the proton-neutron particle-particle interaction parameter in this theory is addressed. The relevance of different multipoles in the multipole decomposition of the 0 ν β β matrix element is also discussed in the pnQRPA framework.