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C. Hanhart

Publications and source records attributed to C. Hanhart.

At least 127 records · Page 7Linked to original sources
arXiv2007

The effect of isospin violation on scalar meson production

We investigate the isospin-violating mixing of the light scalar mesons a0(980) and f0(980) within the unitarized chiral approach. Isospin-violating effects are considered to leading order in the quark mass difference and electromagnetism. In this approach both resonances are generated through meson-meson dynamics. Our results provide a description of the mixing phenomenon within a framework consistent with chiral symmetry and unitarity, where these resonances are not predominantly quark-antiquark states. We discuss in detail the reactions J/Psi to phi S, where S denotes a suitable pair of pseudo--scalar mesons in the scalar channel, namely pi0 eta, K+K-, and K0 \bar K^0. In this work predictions for the cross section in the kaon channels are given for the first time with isospin violating effects included.

hep-ph↗
arXiv2007

Pion--deuteron scattering length in Chiral Perturbation Theory up to order χ^{3/2}

A complete calculation of the corrections to pion-deuteron scattering length up to order $χ^{3/2}$ with $χ=m_π/M_{N}$ is performed. The calculation includes the dispersive contributions and corrections due to the explicit treatment of the $Δ$ resonance. s-wave pion-nucleon scattering parameters are extracted from a combined analysis of modern experimental data.

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arXiv2007

Progress in NN --> NNpi

We survey the recent developments in the reaction $NN \to NNπ$ in effective field theory. We show that the proper construction of the production operator needs a careful separation of irreducible pieces from reducible ones. The result of this consideration is a complete cancellation of all loops in the production operator at NLO. Moreover, we show that this procedure brings the leading Weinberg-Tomozawa vertex on-shell, thus enhancing the corresponding contribution to the transition amplitude by a factor of 4/3 as compared to the commonly used value. We also discuss the role of the $Δ(1232)$ for the s-wave pion production. Being relatively sizable individually the direct and rescattering mechanisms of the $Δ$ excitation at NLO cancel each other to a large extent. Thus, we conclude that the net effect of the $Δ$ at NLO is very small.

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arXiv2007

The physical meaning of scattering matrix singularities in coupled-channel formalisms

The physical meaning of bare and dressed scattering matrix singularities has been investigated. Special attention has been attributed to the role of well known invariance of scattering matrix with respect to the field transformation of the effective Lagrangian. Examples of evaluating bare and dressed quantities in various models are given.

hep-ph↗
arXiv2007

Reconciling the X(3872) with the near-threshold enhancement in the D^0\bar{D}^{*0} final state

We investigate the enhancement in the D^0\bar{D}^0π^0 final state with the mass M=3875.2\pm 0.7^{+0.3}_{-1.6}\pm 0.8 MeV found recently by the Belle Collaboration in the B\to K D^0\bar{D}^0π^0 decay and test the possibility that this is yet another manifestation of the well-established resonance X(3872). We perform a combined Flatte analysis of the data for the D^0\bar{D}^0π^0 mode, and for the π^+π^- J/ψmode of the X(3872). Only if the X(3872) is a virtual state in the D^0\bar{D}^{*0} channel, the data on the new enhancement comply with those on the X(3872). In our fits, the mass distribution in the D^0\bar{D}^{*0} mode exhibits a peak at 2-3 MeV above the D^0\bar{D}^{*0} threshold, with a distinctive non-Breit-Wigner shape.

hep-ph↗
arXiv2007

Is there an eta-3He quasi--bound state ?

The observed variation of the total cross section for the dp -> 3He eta reaction near threshold means that the magnitude of the s--wave amplitude falls very rapidly with the eta centre--of--mass momentum. It is shown here that recent measurements of the momentum dependence of the angular distribution imply a strong variation also in the phase of this amplitude. Such a behaviour is that expected from a quasi--bound or virtual eta-3He state. The interpretation can be investigated further through measurements of the deuteron or proton analysing powers and/or spin--correlations.

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arXiv2007

Role of the Delta(1232) in pion-deuteron scattering at threshold within chiral effective field theory

We investigate the role of the delta isobar in the reaction $πd\to πd$ at threshold in chiral effective field theory. We discuss the corresponding power counting and argue that this calculation completes the evaluation of diagrams up to the order (m_pi/M_N)^(3/2), with m_pi (M_N) for the pion (nucleon) mass. The net effect of all delta contributions at this order to the pion-deuteron scattering length is (2.4 +/- 0.4)x 10^(-3) m_pi^{-1}.

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arXiv2007

Remarks on N N -> N N pi beyond leading order

In recent years a two-scale expansion was established to study reactions of the type N N -> N N pi within chiral perturbation theory. Then the diagrams of some subclasses that are invariant under the choice of the pion field no longer appear at the same chiral order. In this letter we show that the proposed expansion still leads to well defined results. We also discuss the appropriate choice of the heavy baryon propagator.

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arXiv2007

Two-photon decays of hadronic molecules

In many calculations of the two--photon decay of hadronic molecules, the decay matrix element is estimated using the wave function at the origin prescription, in analogy to the two-photon decay of parapositronium. We question the applicability of this procedure to the two-photon decay of hadronic molecules for it introduces an uncontrolled model dependence into the calculation. As an alternative approach, we propose an explicit evaluation of the hadron loop. For shallow bound states, this can be done as an expansion in powers of the range of the molecule binding force. In the leading order one gets the well-known point-like limit answer. We estimate, in a self-consistent and gauge invariant way, the leading range corrections for the two-photon decay width of weakly bound hadronic molecules emerging from kaon loops. We find them to be small. The role of possible short-ranged operators and of the width of the scalars remains to be investigated.

hep-ph↗
arXiv2007

Pion reactions on two--nucleon systems

We review recent progress in our understanding of elastic and inelastic pion reactions on two nucleon systems from the point of view of effective field theory. The discussion includes pion-deuteron scattering, gamma d to pi-nn, and NN to NNpi. At the end some remarks are made on strangeness production reactions.

nucl-th↗
arXiv2007

Lambda-N scattering length from the reaction gamma d -> K^+ Lambda n

The perspects of utilizing the strangeness-production reaction gamma d -> K^+ Lambda n for the determination of the Lambda n low-energy scattering parameters are investigated. The spin observables that need to be measured in order to isolate the Lambda n singlet (1S0) and triplet (3S1) states are identified. Possible kinematical regions where the extraction of the Lambda n scattering lengths might be feasible are discussed.

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arXiv2006

The piNN system - recent progress

Recent progress towards an understanding of the piNN system within chiral perturbation theory is reported. The focus lies on an effective field theory calculation and its comparison to phenomenological calculations for the reaction NN->d pi. In addition, the resulting absorptive and dispersive corrections to the pi-d scattering length are discussed briefly.

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arXiv2006

Dispersive and absorptive corrections to the pion-deuteron scattering length

We present a parameter--free calculation of the dispersive and absorptive contributions to the pion--deuteron scattering length based on chiral perturbation theory. We show that once all diagrams contributing to leading order to this process are included, their net effect provides a small correction to the real part of the pion--deuteron scattering length. At the same time the sizable imaginary part of the pion--deuteron scattering length is reproduced accurately.

nucl-th↗
arXiv2006

Towards an understanding of the light scalar mesons

Although studied for many years the nature of the light scalar mesons remains controversial. Here we shall present a method, applicable for s-wave states located close to a threshold, that allows one to quantify the molecular part of a given state. When applied to the $f_0(980)$ a dominance of the molecular component is found. In the second part we show that requirements of field theoretic consistency and chiral symmetry, when applied to the scattering of light pseudo--scalars, naturally lead to the appearance of dynamical poles in the scalar sector. A program is proposed on how to further investigate experimentally the mixing between these dynamical states and possible genuine quark states.

hep-ph↗
arXiv2006

Advances in the field-theoretic understanding of pion production in nucleon-nucleon collisions

We study the production amplitude for the reaction $NN\to NNπ$ up to next-to-leading order in chiral perturbation theory. We show that the irreducible chiral loops at this order exactly cancel those terms that arise from the off-shell parts of the $πN$ rescattering vertex. This cancellation is required for formal consistency of the whole scheme. 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 compared to phenomenological studies, bringing its contribution to the cross section for $pp\to dπ^+$ close to the experimental value.

nucl-th↗
arXiv2006

Scalar-isovector K K-bar production close to threshold

The reaction pp -> d K^+ K^0-bar has been investigated at excess energies Q = 47.4 and 104.7 MeV above the K^+ K^0-bar threshold at COSY-Juelich. Coincident dK^+ pairs were detected with the ANKE spectrometer, and events with a missing K^0-bar invariant-mass subsequently identified. The joint analysis of invariant-mass and angular distributions reveals s-wave dominance between the two kaons, in conjunction with a p-wave between the deuteron and the kaon pair, i.e. K K-bar production via the a_0^+(980) channel. Integration of the differential distributions yields total cross sections of sigma(pp -> d K^+ K^0-bar) = (38 +/- 2(stat) +/- 14(syst)) nb and 190 +/- 4(stat) +/- 39(syst)) nb for the low and high Q values, respectively.

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