arXiv subjects
Publications and source records attributed to C. Hanhart.
In this manuscript we reply to the criticisms of our paper [Eur. Phys. J. A 24, 437 (2005)] raised in a recent preprint by Achasov and Kiselev [hep-ph/0606268] and demonstrate that all their criticisms are completely irrelevant and misleading.
We estimate the rates for radiative transitions of the lightest scalar mesons f_0(980) and a_0(980) to the vector mesons rho and omega. We argue that measurements of the radiative decays of those scalar mesons can provide important new information on their structure.
The $Λ(1520)$ resonance has been recently studied in a unitarized coupled channel formalism with $πΣ(1385)$, $KΞ(1530)$, $\bar{K}N$ and $πΣ$ as constituents blocks. We provide a theoretical study of the predictions of this model in physical observables of the $pp\to pK^+K^-p$ and $pp\to pK^+π^0π^0Λ$ reactions. In particular, we show that the ratio between the $π^0π^0Λ$ and $K^-p$ mass distributions can provide valuable information on the ratio of the couplings of the $Λ(1520)$ resonance to $πΣ(1385)$ and $\bar{K}N$ that the theory predicts. Calculations are done for energies which are accessible in an experimental facility like COSY at Jülich or the developing CSR facility at Lanzhou.
We use realistic two- and three-nucleon interactions in a hybrid chiral-perturbation-theory calculation of the charge-symmetry-breaking reaction $dd\toαπ^0$ to show that a cross section of the experimentally measured size can be obtained using LO and NNLO pion-production operators. This result supports the validity of our power counting scheme and demonstrates the necessity of using an accurate treatment of ISI and FSI.
The d+d->alpha+eta reaction has been investigated near threshold using the ANKE facility at COSY-Juelich. Both total and differential cross sections have been measured at two excess energies, Q=2.6 MeV and 7.7 MeV, with a subthreshold measurement being undertaken at Q=-2.6 MeV to study the physical background. While consistent with isotropy at the lower energy, the angular distribution reveals a pronounced anisotropy at the higher one, indicating the presence of higher partial waves. Options for the decomposition into partial amplitudes and their consequences for determination of the s-wave eta-alpha scattering length are discussed.
We study the production amplitude for the reaction NN->NNpi up to next--to--leading order in chiral perturbation theory using a counting scheme that takes into account the large scale introduced by the initial momentum. In particular we investigate a subtlety that arises once the leading loop contributions are convoluted with the NN wavefunctions as demanded by the non--perturbative nature of the NN interaction. We show how to properly identify the irreducible contribution of loop diagrams in such type of reaction. The net effect of the inclusion of all next-to-leading order loops is to enhance the leading rescattering amplitude by a factor of 4/3, bringing its contribution to the cross section for pp->dpi+ close to the experimental value.
I review recent insights and open questions connected with the production of eta--mesons from hadrons.
We give a prove of evidence that the original power counting by Weinberg can be applied to estimate the contributions of the operators contributing to the pi-deuteron scattering length. As a consequence, pi-deuteron observables can be used to extract neutron amplitudes--in case of pi-deuteron scattering this means that the pi-neutron scattering length can be extracted with high accuracy. This result is at variance with recent claims. We discuss the origin of this difference.
A recently proposed method based on dispersion theory, that allows to extract the scattering length of a hadronic two-body system from corresponding final-state interactions, is generalized to the situation where the Coulomb interaction is present. The steps required in a concrete practical application are discussed in detail. In addition a thorough examination of the accuracy of the proposed method is presented and a comparison is made with results achieved with other methods like the Jost-function approach based on the effective-range approximation. Deficiencies of the latter method are pointed out. The reliability of the dispersion theory method for extracting also the effective range is investigated.
We investigate the radiative decays of the $ϕ$ meson to the scalar mesons $a_0(980)$ and $f_0(980)$. We demonstrate that, contrary to earlier claims, these decays should be of the same order of magnitude for a molecular state and for a compact state and, therefore, the available experimental information is consistent with both, a molecular as well as a compact structure of the scalars. Thus, the radiative decays of the $ϕ$ meson into scalars establish a sizable $K\bar K$ component of the scalar mesons, but do not allow to discriminate between molecules and compact states.
The reaction $γd\to π^+ nn$ is calculated up to order $χ^{5/2}$ in chiral perturbation theory, where $χ$ denotes the ratio of the pion to the nucleon mass. Special emphasis is put on the role of nucleon--recoil corrections that are the source of contributions with fractional power in $χ$. Using the known near threshold production amplitude for $γp\to π^+ n$ as the only input, the total cross section for $γd\to π^+ nn$ is described very well. A conservative estimate suggests that the theoretical uncertainty for the transition operator amounts to 3 % for the computed amplitude near threshold.
We present a partial-wave analysis of the polarization data for the reaction $\vec{p}\vec{p}\to ppπ^\circ$, based solely on the recent measurements at IUCF for this channel. The fit leads to a $χ^2$ per degree of freedom of 1.7. Methods for an improved analysis are discussed. We compare the extracted values to those from a meson exchange model.
The reaction pp -> dpi+eta has been measured at a beam energy of T=2.65 GeV (p=3.46 GeV/c) using the ANKE spectrometer at COSY-Juelich. The missing mass distribution of the detected dpi+ pairs exhibits a peak around the eta mass on top of a strong background of multi-pion pp -> dpi+(n(pi)) events. The differential cross section d^4(sigma)/d(Omega_d)d(Omega_pi+)d(p_d)d(p_pi+) for the reaction pp -> dpi+eta has been determined model independently for two regions of phase space. Employing a dynamical model for the a0+ production allows one then to deduce a total cross section of sigma(pp -> da0+ -> dpi+eta)=(1.1 +/- 0.3_(stat) +/- 0.7_(syst)) microbarn for the production of pi+eta via the scalar a0+(980) resonance and sigma(pp -> dpi+eta) = (3.5 +/- 0.3_(stat) +/- 1.0_(syst)) microbarn for the non-resonant production. Using the same model as for the interpretation of recent results from ANKE for the reaction pp -> dK+(bar(K0)), the ratio of the total cross sections is sigma(pp -> d(K+(bar(K0)))_(L=0))/sigma(pp -> da0+ -> dpi+eta) = 0.029 +/- 0.008_(stat) +/- 0.009_(syst), which is in agreement with branching ratios in the literature.
We explore the features of Flatte-like parametrizations. In particular, we demonstrate that the large variation in the absolute values of the coupling constants to the pi-eta (or pi-pi) and KKbar channels for the a_0(980) and f_0(980) mesons that one can find in the literature can be explained by a specific scaling behaviour of the Flatte amplitude for energies near the KKbar threshold. We argue that the ratio of the coupling constants can be much better determined from a fit to experimental data.
Recently several proposals were put forward to determine the parity of narrow baryonic resonances, in particular the $Θ^+$. In these proceedings we will briefly comment on the general problems in this task and then discuss in detail the potential of reactions of the type $\vec N\vec N\toΘ^+Y$, where $Y$ denotes a hyperon (either $Σ$ or $Λ$) and the arrows indicate that a polarized initial state is required. Besides reiterating the model--independent properties of this class of reactions we discuss the physics content of some model calculations.
In a recent paper by the TAPS collaboration \cite{exp} a first measurement of a bound system of an $η$ meson and a ${^3}$He nucleus was reported. In this comment we critically reexamine the interpretation of the data and show that the data prefers a solution where there is no bound state present. Given the low statistics of the measurement, however, it does not exclude the existence of a bound state.
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.
The possible existence of $η$-mesic nuclei poses an interesting and still open issue of research. Since the occurence of such $η$-nucleus bound states is reflected in the corresponding $η$-nucleus scattering length, we critically review the present knowledge for the $η$$^3$He system. Specifically, we scrutinize the available experimental information for the reaction $p{+}d \to η{+} ^3{\rm He}$ which is commonly used to extract the $η$$^3$He scattering length. We point out several striking discrepancies between the various measurements. Subject to those inconsistencies we deduce a value of $a{=}|4.3{\pm}0.3|{+}i(0.5{\pm}0.5)$ fm.