arXiv subjects

U. -G. Meißner

Publications and source records attributed to U. -G. Meißner.

137 records · Page 8Linked to original sources

arXiv2004

Radiative tritium beta-decay and the neutrino mass

The shape of the electron energy spectrum in 3H beta-decay permits a direct assay of the absolute scale of the neutrino mass; a highly accurate theoretical description of the electron energy spectrum is necessary to the empirical task. We update Sirlin's calculation of the outer radiative correction to nuclear beta-decay to take into account the non-zero energy resolution of the electron detector. In previous 3H beta-decay studies the outer radiative corrections were neglected all together; only Coulomb corrections to the spectrum were included. This neglect artificially pushes m_nu^2 < 0 in a potentially significant way. We present a computation of the theoretical spectrum appropriate to the extraction of the neutrino mass in the sub-eV regime.

hep-ph↗

arXiv2004

Universal Properties of the Four-Boson System in Two Dimensions

We consider the nonrelativistic four-boson system in two dimensions interacting via a short-range attractive potential. For a weakly attractive potential with one shallow two-body bound state with binding energy B_2, the binding energies B_N of shallow N-body bound states are universal and thus do not depend on the details of the interaction potential. We compute the four-body binding energies in an effective quantum mechanics approach. There are exactly two bound states: the ground state with B_4^(0)=197.3(1)B_2 and one excited state with B_4^(1)=25.5(1)B_2. We compare our results to recent predictions for N-body bound states with large N>>1.

cond-mat.other↗

arXiv2004

On the determination of the parity of the $Θ^+$

We critically examine the possibility of determining the parity of the $Θ^+(1540)$ from the reactions $NN\to Θ^+Y$ ($Y$ = $Λ$, $Σ$) recently discussed in the literature. Specifically, we study the energy dependence of those observables that were suggested to be the most promising ones to unravel the parity of the $Θ^+$, namely the spin correlation coefficient $A_{xx}$, and the spin transfer coefficient $D_{xx}$. We show that the energy dependence of $σ_0(1+A_{xx})$, corresponding to the spin-triplet production cross section, guarantees unambiguous information on the parity of the $Θ^+$. Here, $σ_0$ denotes the unpolarized cross section. Also, the possibility to determine the parity of the $Θ^+$ through the energy dependence of $σ_0D_{xx}$ is discussed.

hep-ph↗

arXiv2004

Determination of the \bar K^0 d scattering length from the reaction pp \to d \bar K^0 K^+

The real and imaginary parts of the \bar K^0 d scattering length are extracted from the \bar K^0 d mass spectrum obtained from the reaction pp \to d \bar K^0 K^+ measured recently at the Cooler Synchrotron COSY at Jülich. We extract a new limit on the K^- d scattering length, namely Im a \le 1.3 fm and |Re a| \le 1.3 fm. The limit for the imaginary part of the K^- d scattering length is supported by data on the total K^- d cross sections.

nucl-th↗

arXiv2004

Radiative neutron beta-decay in effective field theory

We consider radiative β--decay of the neutron in heavy baryon chiral perturbation theory, with an extension including explicit Δdegrees of freedom. We compute the photon energy spectrum as well as the photon polarization; both observables are dominated by the electron bremsstrahlung contribution. Nucleon-structure effects not encoded in the weak coupling constants g_A and g_V are determined at next-to-leading order in the chiral expansion, and enter at the {\cal O}(0.5%)-level, making a sensitive test of the Dirac structure of the weak currents possible.

hep-ph↗

arXiv2004

The role of the nucleon recoil in low--energy meson--nucleus reactions

The role of the nucleon recoil corrections in low--energy meson--nucleus interactions is examined. We demonstrate explicitly when calculations within the static approximation are justified and when the recoil terms need to be kept explictly in the propagators, depending on whether the S-wave two--nucleon intermediate state is Pauli blocked or not, while the meson is in flight. While the effect is studied in detail for $πd$ scattering only, a large class of other reactions is discussed for which the findings are relevant as well.

nucl-th↗

arXiv2004

How to measure the parity of the $Θ^+$ in $\vec p\vec p$ collisions

Triggered by a recent paper by Thomas, Hicks and Hosaka, we investigate which observables can be used to determine the parity of the $Θ^+$ from the reaction $\vec p\vec p \to Σ^+Θ^+$ near its production threshold. In particular, we show that the sign of the spin correlation coefficient $A_{xx}$ for small excess energies yields the negative of the parity of the $Θ^+$. The argument relies solely on the Pauli principle and parity conservation and is therefore model--independent.

hep-ph↗

arXiv2003

Chiral Dynamics of the Two Λ(1405) States

Using a chiral unitary approach for the meson--baryon interactions, we show that two octets of J^π=1/2^- baryon states, which are degenerate in the limit of exact SU(3) symmetry, and a singlet are generated dynamically. The SU(3) breaking produces the splitting of the two octets, resulting in the case of strangeness S=-1 in two poles of the scattering matrix close to the nominal Λ(1405) resonance. These poles are combinations of the singlet state and the octets. We show how actual experiments see just one effective resonance shape, but with properties which change from one reaction to another.

nucl-th↗

arXiv1998

$η^{\prime}$ production in proton-proton scattering close to threshold}

The $pp \to pp η^{\prime}$ (958) reaction has been measured at COSY using the internal beam and the COSY-11 facility. The total cross sections at the four different excess energies \mbox{$ Q = ~1.5 ~MeV, ~1.7 ~MeV, ~2.9 ~MeV,$ and $ ~4.1 MeV$} have been evaluated to be \mbox{$ σ= 2.5 \pm 0.5~nb$, $~~~ 2.9 \pm 1.1~nb$, $~~~ 12.7 \pm 3.2~nb$, ~ and $~~~ 25.2 \pm 3.6 ~nb $}, respectively. In this region of excess energy the $η^{\prime}$ (958) cross sections are much lower compared to those of the $π^0$ and $η$ production.

nucl-ex↗

arXiv1996

The Standard Model at Low Energies

These are short proceedings of the workshop on ``The Standard Model at Low Energies'' held at ECT* in Trento, Italy, from April 29 to May 10, 1996. The workshop concentrated on Chiral Perturbation Theory in its various settings. Included are a one page contribution with references per speaker and a listing of some review papers of relevance to the field.

hep-ph↗

arXiv1993

Strong and Radiative Meson Decays in a Generalized Nambu--Jona-Lasinio Model

We investigate strong and radiative meson decays in a generalized Nambu--Jona-Lasinio model. The one loop order calculation provides a satisfactory agreement with the data for the mesonic spectrum and for radiative decays. Higher order effects for strong decays of $ρ$ and $K^*$ are estimated to be large. We also discuss the role of the flavour mixing determinantal interaction.

hep-ph↗