arXiv subjects

M. Döring

Publications and source records attributed to M. Döring.

49 records · Page 3Linked to original sources

arXiv2010

On eta photoproduction on the neutron and pi^0 eta photoproduction

The recently discovered enhancement of eta photoproduction on the quasi-free neutron at energies around s^1/2 ~ 1.67 GeV is addressed within an SU(3) coupled channels model. The K Sigma threshold plays a significant role and the quasi-free cross sections on proton and neutron, sigma_n and sigma_p, can be quantitatively explained. For the reaction gamma p --> pi^0 eta p, we evaluate the polarization observables I^S and I^C from a chiral unitary amplitude developed earlier. The I^S and I^C observables have been recently measured for the first time by the CBELSA/TAPS collaboration. We show the significance of the Delta(1700)D33 resonance and its S-wave decay into eta Delta(1232) which further confirms the dynamical nature of this resonance.

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arXiv2010

Pion photoproduction in a dynamical coupled-channel model

Pion photoproduction reactions are investigated in a dynamical coupled-channel approach based on the Juelich pi-N model, which presently includes the hadronic pi-N and eta-N stable channels as well as the pi-Delta, sigma-N and rho-N effective channels. This model has been quite successful in the description of pi-N to pi-N scattering for center-of-mass energies up to 1.9 GeV. The full pion photoproduction amplitude is constructed to satisfy the generalized Ward-Takahashi identity and hence, it is fully gauge invariant. The calculated differential cross sections and photon spin asymmetries up to 1.65 GeV center-of-mass energy for the reactions gamma p to pi+ n, gamma p to pi0 p and gamma n to pi- p are in good agreement with the experimental data.

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arXiv2010

Meson-baryon interaction in the meson exchange picture

This is the contribution to the proceedings of the MENU 2010 conference. The recent work of Ref. [7] is summarized. Elastic piN scattering and the reaction pi^+ p --> K^+ Sigma^+ are described simultaneously in a unitary coupled-channels approach which respects analyticity. SU(3) flavor symmetry is used to relate the t- and u- channel exchanges that drive the meson-baryon interaction in the different channels. Angular distributions, polarizations, and spin-rotation parameters are compared with available experimental data. The pole structure of the amplitudes is extracted from the analytic continuation.

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arXiv2010

Evaluation of the polarization observables I^S and I^C in the reaction gamma p --> pi^0 eta p

We evaluate the polarization observables I^S and I^C for the reaction gamma p --> pi^0 eta p, using a chiral unitary framework developed earlier. The I^S and I^C observables have been recently measured for the first time by the CBELSA/TAPS collaboration. The theoretical predictions of I^S and I^C, given for altogether 18 angle dependent functions, are in good agreement with the measurements. Also, the asymmetry dSigma/dcos(theta) evaluated here agrees with the data. We show the importance of the Delta(1700)D33 resonance and its S-wave decay into eta Delta(1232). The result can be considered as a further confirmation of the dynamical nature of this resonance. At the highest energies, deviations of the predictions from the data start to become noticeable, which leaves room for additional processes and resonances such as a Delta(1940)D33. We also point out how to further improve the calculation.

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arXiv2009

On the cross section ratio sigma_n/sigma_p in eta photoproduction

The recently discovered enhancement of eta photoproduction on the quasi-free neutron at energies around sqrt(s)~1.67 GeV is addressed within a SU(3) coupled channel model. The quasi-free cross sections on proton and neutron, sigma_n and sigma_p, can be quantitatively explained. In this study, the main source for the peak in sigma_n/sigma_p is a coupled channel effect in S-wave that explains the dip-bump structure in gamma n --> eta n. In particular, the photon coupling to the intermediate meson-baryon states is important. The stability of the result is extensively tested and consistency with several pion- and photon-induced reactions is ensured.

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arXiv2009

Analytic properties of the scattering amplitude and resonances parameters in a meson exchange model

The analytic properties of scattering amplitudes provide important information. Besides the cuts, the poles and zeros on the different Riemann sheets determine the global behavior of the amplitude on the physical axis. Pole positions and residues allow for a parameterization of resonances in a well-defined way, free of assumptions for the background and energy dependence of the resonance part. This is a necessary condition to relate resonance contributions in different reactions. In the present study, we determine the pole structure of pion-nucleon scattering in an analytic model based on meson exchange. For this, the sheet structure of the amplitude is determined. To show the precision of the resonance extraction and discuss phenomena such as resonance interference, we discuss the S11 amplitude in greater detail.

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arXiv2009

Role of the $N^*$(1535) in the $J/ψ\to \bar{p}ηp$ and $J/ψ\to \bar{p}K^+Λ$ reactions

We study the $J/ψ\to \bar{p}ηp$ and $J/ψ\to \bar{p}K^+Λ$ reactions with a unitary chiral approach. We find that the unitary chiral approach, which generates the $N^*(1535)$ dynamically, can describe the data reasonably well, particularly the ratio of the integrated cross sections. This study provides further support for the unitary chiral description of the $N^*(1535)$. We also discuss some subtle differences between the coupling constants determined from the unitary chiral approach and those determined from phenomenological studies.

hep-ph↗

arXiv2009

Meson and Baryon resonances

In this talk I review recent advances on the structure of the meson and baryon resonances which can be dynamically generated from the interaction of mesons or mesons and baryons. Particular emphasis is put on results involving vector mesons, which bring new light into the nature of some of the observed higher mass mesons and baryons and make predictions for new states.

hep-ph↗

arXiv2008

The role of the N*(1535) resonance and the pi^- p --> KY amplitudes in the OZI forbidden pi N --> phi N reaction

We study the pi N --> phi N reaction close to the phi N threshold within the chiral unitary approach, by combining the pi^- p --> K^+ Sigma^-, pi^- p --> K^0 Sigma^0 and pi^- p --> K^0 Lambda amplitudes with the coupling of the phi to the K components of the final states of these reactions via quantum loops. We obtain a good agreement with experiment when the dominant pi^- p --> K^0 Lambda amplitude is constrained with its experimental cross section. We also evaluate the coupling of the N*(1535) to phi N and find a moderate coupling as a consequence of partial cancellation of the large KY components of the N*(1535). We also show that the N*(1535) pole approximation is too small to reproduce the measured cross section for the pi N --> phi N reaction.

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arXiv2008

Photo- and Electron-Production of Mesons on Nucleons and Nuclei

In these lectures I will show some results obtained with the chiral unitary approach applied to the photo and electroproduction of mesons. The results for photoproduction of $ηπ^0 p$ and $K^0 π^0 Σ^+$, together with related reactions will be shown, having with common denominator the excitation of the $Δ(1700)$ resonance which is one of those dynamically generated in the chiral unitary approach. Then I will show results obtained for the $e^+ e^- \to ϕf_0(980)$ reaction which reproduce the bulk of the data except for a pronounced peak, giving support to a new mesonic resonance, X(2175). Results will also be shown for the electromagnetic form factors of the $N^*(1535)$ resonance, also dynamically generated in this approach. Finally, I will show some results on the photoproduction of the $ω$ in nuclei, showing that present experimental results claiming a shift of the $ω$ mass in the medium are tied to a particular choice of background and are not conclusive. One the other hand, the same experimental results show unambiguously a huge increase of the $ω$ width in the nuclear medium.

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arXiv2007

The nature of the Lambda(1405)

We present here some results supporting the nature of the $Λ(1405)$ resonance as dynamically generated from the meson baryon interaction in coupled channels and resulting from the superposition of two close-by poles. We find support for this picture in the $K^- p \to π^0 π^0 Σ^0 $ reaction, which shows a different shape than the one obtained from the $π^- p \to K^0 πΣ$ reaction. We also call the attention to the $K^- p \to γπΣ$ with $πΣ$ in the $Λ(1405)$ region, which shows a narrow peak in the calculations around 1420 MeV. We also report on recent calculations of the radiative decay of the two $Λ(1405)$ states and on reactions to obtain information on these decay modes. Finally, we present results for the $pp\to p K^+Λ(1405)$ reaction recently measured at ANKE/COSY and compare them with theoretical results.

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arXiv2007

Color Screening and Quark-Quark Interactions in Finite Temperature QCD

We analyze the screening of static diquark sources in 2-flavor QCD and compare results with the screening of static quark-antiquark pairs. We show that a two quark system in a fixed color representations is screened at short distances like a single quark source in the same color representation whereas at large distances the two quarks are screened independently. At high temperatures we observe that the relative strength of the interaction in diquark and quark-antiquark systems, respectively, obeys Casimir scaling. We use this result to examine the possible existence of heavy quark-quark bound states in the high temperature phase of QCD. We find support for the existence of $bb$ states up to about $2T_c$ while $cc$ states are unlikely to be formed above $T_c$.

hep-lat↗

arXiv2005

The QCD equation of state for two flavours at non-zero chemical potential

We present results of a simulation of 2 flavour QCD on a $16^3\times4$ lattice using p4-improved staggered fermions with bare quark mass $m/T=0.4$. Derivatives of the thermodynamic grand canonical partition function $Z(V,T,μ_u,μ_d)$ with respect to chemical potentials $μ_{u,d}$ for different quark flavours are calculated up to sixth order, enabling estimates of the pressure and the quark number density as well as the chiral condensate and various susceptibilities as functions of $μ_{u,d}$ via Taylor series expansion. Results are compared to high temperature perturbation theory as well as a hadron resonance gas model. We also analyze baryon as well as isospin fluctuations and discuss the relation to the chiral critical point in the QCD phase diagram. We moreover discuss the dependence of the heavy quark free energy on the chemical potential.

hep-ph↗