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C. Hanhart

Publications and source records attributed to C. Hanhart.

At least 163 records · Page 9Linked to original sources
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.

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arXiv2004

Meson production in nucleon--nucleon collisions close to the threshold

The last decade has witnessed great experimental progress that has led to measurements of cross sections--polarized as well as unpolarized--of of high accuracy in several $NN \to NNx$ reaction channels. These data, naturally, pose challenges to theorists to develop methods by which they can be understood and explained in commensurate detail. In this work we review the status of the theory with special emphasis on model--independent methods. We discuss in detail not only the many observables involved in the reactions, but also the physical questions that can be addressed by studying them in the various reaction channels. The special advantages of nucleon--nucleon induced reactions are stressed. Foremost among these is the use of the initial and final states as a spin/isospin filter. This opens, for example, a window into the spin dependence of the hyperon--nucleon interaction and the dynamics of the light scalar mesons.

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.

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arXiv2004

Survey of charge symmetry breaking operators for dd -> alpha pi0

The charge-symmetry-breaking amplitudes for the recently observed d d -> alpha pi0 reaction are investigated. Chiral perturbation theory is used to classify and identify the leading-order terms. Specific forms of the related one- and two-body tree level diagrams are derived. As a first step toward a full calculation, a few tree-level two-body diagrams are evaluated at each considered order, using a simplified set of d and alpha wave functions and a plane-wave approximation for the initial dd state. The leading-order pion-exchange term is shown to be suppressed in this model because of poor overlap of the initial and final states. The higher-order one-body and short-range (heavy-meson-exchange) amplitudes provide better matching between the initial and final states and therefore contribute significantly and coherently to the cross section. The consequences this might have for a full calculation, with realistic wave functions and a more complete set of amplitudes, are discussed.

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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

An analysis of the reaction pp --> ppηnear threshold

It is shown that most of the available data on the $pp\to ppη$ reaction, including the invariant mass distributions in the $pp\to ppη$ reaction recently measured at COSY, can be understood in terms of the partial-wave amplitudes involving final $pp$ $S$ and $P$ states and the $η$ meson s-wave. This finding, together with the fact that results within a meson--exchange model are especially sensitive to the details of the excitation mechanism of the $S_{11}(1535)$ resonance, demonstrates the possibility of investigating the properties of this resonance in $NN$ collisions. The spin correlation function $C_{xx}$ is shown to disentangle the $S$- and $P$-wave contributions. It is also argued that spin correlations may be used to help constrain the contributions of the amplitudes corresponding to the final $pp$ $^3P_0$

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arXiv2003

How to extract the $ΛN$ scattering length from production reactions

A dispersion integral is derived that allows one to relate directly (spin dependent) $ΛN$ invariant mass spectra, measured in a large-momentum transfer reaction such as $pp\to K^+pΛ$ or $γd\to K^+nΛ$, to the scattering length for elastic $ΛN$ scattering. The involved systematic uncertainties are estimated to be smaller than 0.3 fm. This estimate is confirmed by comparing results of the proposed formalism with those of microscopic model calculations. We also show, for the specific reaction $pp\to K^+Λp$, how polarization observables can be used to separate the two spin states of the $ΛN$ system.

hep-ph↗
arXiv2003

Hadron Physics at COSY

The workshop 'Hadron Physics at COSY' held from July 7th to July 10th 2003 in Bad Honnef brought together experimentalists and theoreticians from various fields of hadron physics to identify the key physics questions, which can be addressed with (un-) polarized proton and deuteron induced reactions at COSY. Topics discussed include charge symmetry breaking, elastic and inelastic nucleon-nucleon and nucleon-deuteron interactions, hadrons in the medium, mesonic bound states, meson and baryon resonances, and the hyperon-nucleon system. In addition prospects of the planned GSI facility were discussed. These Miniproceedings contain besides short summaries of the presentations the most relevant references for the field.

hep-ph↗
arXiv2003

a0+(980)-resonance production in pp->dK+Kbar0 reactions close to threshold

The reaction pp->d K+ Kbar0 has been investigated at an excess energy of Q=46 MeV above the (K+ Kbar0) threshold with ANKE at COSY-Juelich. From the detected coincident dK+ pairs about 1000 events with a missing Kbar0 were identified, corresponding to a total cross section of sigma(pp->d K+ Kbar0)=(38 +/- 2(stat) +/- 14(syst)) nb. Invariant-mass and angular distributions have been jointly analyzed and reveal s-wave dominance between the two kaons, accompanied by a p-wave between the deuteron and the kaon system. This is interpreted in terms of a0+(980)-resonance production.

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arXiv2003

Evidence that the a0(980) and f0(980) are not elementary particles

We study the interesting problem of whether it is possible to distinguish composite from elementary particles. In particular we generalize a model-independent approach of S. Weinberg to the case of unstable particles. This allows us to apply our formalism to the case of the a0(980) and f0(980) resonances and to address the question whether these particles are predominantly genuine, confined quark states (of $\bar q q$ or $qq\bar q\bar q$ structure) or governed by mesonic components.

hep-ph↗
arXiv2003

Pion-nucleon scattering in a meson-exchange model

The pi-N interaction is studied within a meson-exchange model and in a coupled-channels approach which includes the channels pi-N, eta-N, as well as three effective pi-pi-N channels namely rho-N, pi-Delta, and sigma-N. Starting out from an earlier model of the Julich group systematic improvements in the dynamics and in some technical aspects are introduced. With the new model an excellent quantitative reproduction of the pi-N phase shifts and inelasticity parameters in the energy region up to 1.9 GeV and for total angular momenta J leq 3/2 is achieved. Simultaneously, good agreement with data for the total and differential pi-N -> eta-N transition cross sections is obtained. The connection of the pi_N dynamics in the S_{11} partial wave with the reaction pi-N -> eta-N is discussed.

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arXiv2003

Supernovae and Light Neutralinos: SN1987A Bounds on Supersymmetry Revisited

For non-universal gaugino masses, collider experiments do not provide any lower bound on the mass of the lightest neutralino. We review the supersymmetric parameter space which leads to light neutralinos, $M_\lsp \lsim {\cal O}(1\gev)$, and find that such neutralinos are almost pure bino. In light of this, we examine the neutralino lower mass bound obtained from supernova 1987A (SN1987A). We consider the production of binos in both electron-positron annihilation and nucleon-nucleon binostrahlung. For electron-positron annihilation, we take into account the radial and temporal dependence of the temperature and degeneracy of the supernova core. We also separately consider the Raffelt criterion and show that the two lead to consistent results. For the case of bino production in $NN$ collisions, we use the Raffelt criterion and incorporate recent advances in the understanding of the strong-interaction part of the calculation in order to estimate the impact of bino radiation on the SN1987A neutrino signal. Considering these two bino production channels allows us to determine separate and combined limits on the neutralino mass as a function of the selectron and squark masses. For $M_\lsp \sim 100 \mev$ values of the selectron mass between 300 and 900 GeV are inconsistent with the supernova neutrino signal. On the other hand, in contrast to previous works, we find that SN1987A provides almost no bound on the squark masses: only a small window of values around 300 GeV can be excluded, and even then this window closes once $M_\lsp \gsim 20 \mev$.

hep-ph↗
arXiv2003

Angular asymmetries in the reactions pp \to dπ^+ηand pn \to dπ^0ηand a_0-f_0 mixing

The reactions pp\to dπ^+ηand pn\to dπ^0ηare of special interest for investigating the a_0(980) (J^P=0^+) resonance in the process NN \to da_0 \to dπη. We study some aspects of those reactions within a general formalism and also in a concrete phenomenological model. In particular, it is shown that the presence of nonresonant (i.e. without excitation of the a_0 resonance) contributions to these reactions yields nonvanishing values for specific polarization observables, i.e. to effects like those generated by a_0-f_0 mixing. An experimental determination of these observables for the reaction pp\to dπ^+ηwould provide concrete information on the magnitude of those nonresonant contributions to πηproduction. We discuss also the possibility of extracting information about a_0-f_0 mixing from the reaction pn \to dπ^0ηwith polarized proton beam.

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arXiv2003

Charge symmetry breaking as a probe for the real part of eta--nucleus scattering lengths

We demonstrate that one can use the occurrence of charge symmetry breaking as a tool to explore the eta--nucleus interaction near the eta threshold. Based on indications that the cross section ratio of pi+ and pi0 production on nuclei deviates from the isotopic value in the vicinity of the eta production threshold, due to, e.g., pi0-eta mixing, we argue that a systematic study of this ratio as a function of the energy would allow to pin down the sign of the real part of the eta-nucleus scattering length. This sign plays an important role in the context of the possible existence of eta-nucleus bound states.

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arXiv2002

Production of eta mesons in nucleon-nucleon collisions

A microscopic calculation of near-threshold eta-meson production in the reaction NN -> NNeta is presented. It is assumed that the eta meson is produced via direct emission and via elementary rescattering processes MN -> etaN of various mesons M = pi, rho, etc. As a novel feature the amplitudes for the latter production mechanism are taken from a multi-channel meson-exchange model of the pi N system developed by the Juelich group which contains explicitly the channels pi N, rho N, eta N, sigma N, and pi Delta. Furthermore, effects of the NN interaction in the final as well as in the initial state are taken into account microscopically. Our results are compared with recent data from the COSY and CELSIUS accelerator facilities. Reasonable agreement with available near-threshold cross section data for the reactions pp -> ppeta, pn -> pneta, and pn -> deta is found.

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arXiv2002

Positive pion absorption on 3He using modern trinucleon wave functions

We study pion absorption on 3He employing trinucleon wave functions calculated from modern realistic NN interactions (Paris, CD Bonn). Even though the use of the new wave functions leads to a significant improvement over older calculations with regard to both cross section and polarization data, there are hints that polarization data with quasifree kinematics cannot be described by just two-nucleon absorption mechanisms.

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