arXiv subjects

Matthias F. M. Lutz

Publications and source records attributed to Matthias F. M. Lutz.

At least 37 records · Page 2Linked to original sources

arXiv2019

On the convergence of chiral expansions for charmed meson masses in the up, down and strange quark masses

We discuss the convergence properties of chiral expansions for the pseudoscalar and vector charmed meson masses based on the chiral SU(3) Lagrangian. Conventional expansion strategies as formulated in terms of bare meson masses are shown to suffer from poor convergence properties. This changes once the expansion is set up in terms of on-shell masses. We find a rapid convergence of the chiral expansion from vanishing quark masses up to physical values of the strange quark mass in this case. Detailed results are presented at the one-loop level for the D-meson and D^*-meson masses. It is emphasized that our results do not depend on the renormalization scale. An approximation hierarchy for the chiral Ward identities of QCD is obtained that keeps the proper form of low-energy branch points and cuts as they are implied by the use of on-shell masses. Given such a scheme we analyzed the charmed meson masses as available on various QCD lattice ensembles. In terms of the determined low-energy constants we consider the coupled-channel interactions of the Goldstone bosons with open-charm mesons. For the isospin violating hadronic decay width of the D_{s0}^*(2317) we predict the range (104-116) keV.

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arXiv2019

On light vector mesons and chiral SU(3) extrapolations

A chiral extrapolation of the light vector meson masses in the up, down and strange quark masses of QCD is presented. We apply an effective chiral Lagrangian based on the hadrogenesis conjecture to QCD lattice ensembles of PACS-CS, QCDSF-UKQCD and HSC in the strict isospin limit. The leading orders low-energy constants are determined upon a global fit to the lattice data set. We use the pion and kaon masses as well as the size of the finite volume as lattice ensemble parameters only. The quark mass ratio on the various ensembles are then predicted in terms of our set of low-energy constants. An accurate reproduction of the vector meson masses and quark-mass ratios as provided by the lattice collaborations and the Particle Data Group (PDG) is achieved. Particular attention is paid to the ω-ϕmixing phenomenon, which is demonstrated to show a strong quark mass dependence.

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arXiv2019

On pion and kaon decay constants and chiral SU(3) extrapolations

We consider the dependence of the pion and kaon decay constants on the up, down and strange quark masses in QCD with strict isospin symmetry. The role of dynamical vector meson degrees of freedom is scrutinized in terms of an effective chiral Lagrangian for vector mesons. Applying a set of low-energy parameters as determined previously from QCD lattice data on the masses of the light vector mesons from PACS-CS, QCDSF-UKQCD and HSC we compute its implications on the pion and kaon decay constants for QCD lattice ensembles of HPQCD, CLS and ETMC. It is shown that with Gasser-Leutwyler L_4 and L_5 parameters fixed to the empirical decay constants an accurate reproduction of their values at unphysical quark masses as computed by HPQCD, CLS and ETMC is achieved. Results for the masses of the light vector meson, the ω-ϕmixing angles and the quark mass ratios for the ensembles used by HPQCD, CLS and ETMC are discussed.

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arXiv2019

Large-N_c sum rules for charmed baryons at subleading orders

Sum rules for the low-energy constants of the chiral SU(3) Lagrangian with charmed baryons of spin J^P=1/2^+ and J^P=3/2^+ baryons are derived from large-N_c QCD. We consider the large-$N_c$ operator expansion at subleading orders for current-current correlation functions in the charmed baryon-ground states for two scalar and two axial-vector currents.

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arXiv2019

A framework for the chiral extrapolation of the charmed baryon ground-state masses

We consider the chiral Lagrangian for charmed baryon fields with $J^P =\frac{1}{2}^+$ or $J^P =\frac{3}{2}^+$ quantum numbers. A chiral expansion framework for the baryon ground state masses is worked out to N$^3$LO as to compute their dependence on the up, down and strange quark masses for finite box QCD lattice simulations. It is formulated in terms of on-shell meson and baryon masses. The convergence of such a scheme is illustrated with physical masses as taken from the PDG. The counter terms relevant at N$^3$LO are correlated systematically by large-$N_c$ sum rules to leading and subleading order in a manner that keeps the renormalization scale invariance of the approach.

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arXiv2019

On chiral excitations with exotic quantum numbers

We consider the flavour sextet of charmed meson resonances with J^P= 1^+ quantum numbers that is predicted by the leading order chiral Lagrangian with up, down and strange quarks. The effect of chiral correction terms as determined previously from QCD lattice data is worked out. Pole masses in the complex energy plane are derived. The most promising signal from such states accessible in experiments like Belle, LHCb and PANDA are foreseen in the s-wave pi D* phase shift and the eta D* invariant mass distribution. For physical quark masses a rapid variation of the phase shift in between the eta D* and the bar K Ds* thresholds is predicted.

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arXiv2018

On chiral extrapolations of charmed meson masses and coupled-channel reaction dynamics

We perform an analysis of QCD lattice data on charmed meson masses. The quark-mass dependence of the data set is used to gain information on the size of counter terms of the chiral Lagrangian formulated with open-charm states with J^P= 0^- and J^P =1^- quantum numbers. Of particular interest are those counter terms that are active in the exotic flavour sextet channel. A chiral expansion scheme where physical masses enter the extrapolation formulae is developed and applied to the lattice data set. Good convergence properties are demonstrated and an accurate reproduction of the lattice data based on ensembles of PACS-CS, MILC, ETMC and HSC with pion and kaon masses smaller than 600 MeV is achieved. It is argued that a unique set of low-energy parameters is obtainable only if additional information from HSC on some scattering observables is included in our global fits. The elastic and inelastic s-wave pi D and eta D scattering as considered by HSC is reproduced faithfully. Based on such low-energy parameters we predict 15 phase shifts and in-elasticities at physical quark masses but also for an additional HSC ensemble at smaller pion mass. In addition we find a clear signal for a member of the exotic flavour sextet states in the eta D channel, below the bar K D_s threshold. For the isospin violating strong decay width of the D_{s0}(2317) we obtain the range (104-116) keV.

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arXiv2018

On the chiral expansion of vector meson masses

We study the chiral expansion of meson masses and decay constants using a chiral Lagrangian that was constructed previously based on the hadrogenesis conjecture. The one-loop self energies of the Goldstone bosons and vector mesons are evaluated. It is illustrated that a renormalizeable effective field theory arises once specific conditions on the low-energy constants are imposed. For the case where the hadrogenesis mass gap scale Lambda_{HG} is substantially larger than the chiral symmetry breaking scale Lambda_chi a partial summation scheme is required. All terms proportional to (M/Λ_χ)^n can be summed by a suitable renormalization, where M is the chiral and large-N_c limit of the vector meson masses in QCD. The size of loop effects from vector meson degrees of freedom is illustrated for physical quarks masses. Naturally sized effects are observed that have significant impact on the chiral structure of low-energy QCD with three light flavours.

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arXiv2015

Resonances in QCD

We report on the EMMI Rapid Reaction Task Force meeting 'Resonances in QCD', which took place at GSI October 12-14, 2015. A group of 26 people met to discuss the physics of resonances in QCD. The aim of the meeting was defined by the following three key questions: What is needed to understand the physics of resonances in QCD? Where does QCD lead us to expect resonances with exotic quantum numbers? What experimental efforts are required to arrive at a coherent picture? For light mesons and baryons only those with ${\it up}$, ${\it down}$ and ${\it strange}$ quark content were considered. For heavy-light and heavy-heavy meson systems, those with ${\it charm}$ quarks were the focus. This document summarizes the discussions by the participants, which in turn led to the coherent conclusions we present here.

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arXiv2014

On kinematical constraints in the hadrogenesis conjecture for the baryon resonance spectrum

We consider the reaction dynamics of bosons with negative parity and spin $0$ or $1$ and fermions with positive parity and spin $\frac{1}{2}$ or $\frac{3}{2}$. Such systems are of central importance for the computation of the baryon resonance spectrum in the hadrogenesis conjecture. Based on a chiral Lagrangian the coupled-channel partial-wave scattering amplitudes have to be computed. We study the generic properties of such amplitudes. A decomposition of the various scattering amplitudes into suitable sets of invariant functions expected to satisfy Mandelstam's dispersion-integral representation is presented. Sets are identified that are free from kinematical constraints and that can be computed efficiently in terms of a novel projection algebra. From such a representation one can deduce the analytic structure of the partial-wave amplitudes. The helicity and the conventional angular-momentum partial-wave amplitudes are kinematically constrained at the Kibble conditions. Therefore an application of a dispersion-integral representation is prohibitively cumbersome. We derive covariant partial-wave amplitudes that are free from kinematical constraints at the Kibble conditions. They correspond to specific polynomials in the 4-momenta and Dirac matrices that solve the various Bethe-Salpeter equations in the presence of short-range interactions analytically.

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arXiv2009

Dynamics of strong and radiative decays of Ds-mesons in the hadrogenesis conjecture

The positive parity scalar D$_{s0}^*$(2317) and axial-vector D$_{s1}^*$(2460) charmed strange mesons are generated by coupled-channel dynamics through the s-wave scattering of Goldstone bosons off the pseudoscalar and vector D(D$_s$)-meson ground states. The specific masses of these states are obtained as a consequence of the attraction arising from the Weinberg-Tomozawa interaction in the chiral Lagrangian. Chiral corrections to order Q$_χ^2$ are calculated and found to be small. The D$_{s0}^*$(2317) and D$_{s1}^*$(2460) mesons decay either strongly into the isospin-violating $π^0$D$_s$ and $π^0$D$_s^*$ channels or electromagnetically. We show that the $π^0$-$η$ and (K$^0$D$^+$-K$^+$D$^0$) mixings act constructively to generate strong widths of the order of 140 keV and emphasize the sensitivity of this value to the $KD$ component of the states. The one-loop contribution to the radiative decay amplitudes of scalar and axial-vector states is calculated using the electromagnetic Lagrangian to chiral order Q$_χ^2$. We show the importance of taking into account processes involving light vector mesons explicitly in the dynamics of electromagnetic decays. The radiative width are sensitive to both $ηD_s$ and $KD$ components, hence providing information complementary to the strong widths on the positive parity $D_s$-meson structure.

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arXiv2008

Dynamics of strong and radiative decays of Ds mesons

The positive parity scalar D$_{s0}^*$(2317) and axial-vector D$_{s1}^*$(2460) charmed strange mesons are generated by coupled-channel dynamics through the s-wave scattering of Goldstone bosons off the pseudoscalar and vector D(D$_s$)-meson ground states. The attraction leading to the specific masses of these states reflects the chiral symmetry breaking scale which characterizes the Weinberg-Tomozawa interaction in the chiral Lagrangian. Chiral corrections to order Q$_χ^2$ are calculated and found to be small. The D$_{s0}^*$(2317) and D$_{s1}^*$(2460) mesons decay either strongly into the isospin-violating $π^0$D$_s$ and $π^0$D$_s^*$ channels or electromagnetically. We show that the $π^0$-$η$ and (K$^0$D$^+$-K$^+$D$^0$) mixings act constructively to generate strong widths of the order of 140 keV. The one-loop contribution to the radiative decay amplitudes of scalar and axial-vector states is calculated using the electromagnetic Lagrangian to chiral order Q$_χ^2$. We show the importance of taking into account processes involving light vector mesons explicitly in the dynamics of electromagnetic decays to obtain a satisfactory description of the available data.

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arXiv2006

Study of the threshold behavior of the $η$N scattering amplitude through the associated photoproduction of $ϕ$- and $η$-mesons

We suggest that the $γp \to ϕηp$ reaction cross section, in the kinematics where the $ηp$ invariant mass in the final state lies between the threshold value (m$_p$+m$_η$) and the N*(1535) resonance mass, is largely determined by the $ηN$ scattering amplitude close to threshold. The initial photon energy is chosen in the range $4 < E_γ^{Lab} < 5$ GeV, in order to reach low (absolute) values of the squared 4-momentum transfer from the initial photon to the final $ϕ$-meson. In these conditions, we expect the t-channel $π^0$- and $η$-meson exchanges to drive the dynamics underlying the $γp \to ϕηp$ process. We show that the $η$-exchange is the dominating contribution to the cross section while the $π^0$-exchange is negligible. The $η$-$π^0$ interference is of the order of $20 - 30 %$. The sign of this term is not known and alters significantly our results. Data on the $γp \to ϕηp$ process would be therefore very useful to help unravelling the behavior of the $ηp$ scattering amplitude close to threshold and assessing the possibility of producing $η$-nucleus bound states.

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arXiv2006

Study of the $η$N scattering amplitude through the associated photoproduction of $ϕ$- and $η$-mesons in the region of the N*(1535) resonance

The $γp \to ϕηp$ reaction is studied in the kinematic region where the $ηp$ final state originates dominantly from the decay of the N*(1535) resonance. The threshold laboratory photon energy for this reaction (at the peak of the S11 resonance) is $E_γ^{Lab} = $3 GeV. We will discuss it somewhat above threshold, at $E_γ^{Lab}\simeq 4-5$ GeV, in order to reach lower (absolute) values of the squared 4-momentum transfer from the initial photon to the final $ϕ$-meson. In these conditions, we expect the t-channel $π^0$- and $η$-meson exchanges to drive the dynamics underlying the $γp \to ϕηp$ process. The initial photon dissociates into the final $ϕ$-meson and a virtual pseudoscalar meson ($π^0$ or $η$). The virtual pseudoscalar meson scatters from the proton target to produce the final $ηp$ state. The $π^0 p \to ηp$ and $ηp \to ηp$ amplitudes are derived in the framework of a coupled-channel effective field theory of meson-baryon scattering. We found the $η$-meson exchange to be largely dominant. The $η$-$π^0$ interference is of the order of $20 - 30 %$. The sign of this term is not known and has a significant influence on the results. The $πN\to ηN$ amplitude being largely determined by data on the $π^- p \to ηn$ reaction, we found that the $γp \to ϕηp$ reaction cross section is rather directly related to the $η$-nucleon scattering amplitude in the N*(1535) resonance region. Accurate data on the $γp \to ϕηp$ process would therefore put additional constraints on this still poorly known amplitude.

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arXiv2005

Coupled-channel study of baryon resonances with charm

Identifying a zero-range exchange of vector mesons as the driving force for the s-wave scattering of pseudo-scalar mesons off the baryon ground states, a rich spectrum of molecules is formed. We argue that chiral symmetry and large-$N_c$ considerations determine that part of the interaction which generates the spectrum. A bound state with exotic quantum numbers is predicted at mass 2.78 GeV. It couples strongly to the $(\bar D_s N),(\bar D Λ),(\bar D Σ)$ channels. A further charm minus-one system is predicted at mass 2.84 GeV as a result of $(\bar D_s Λ), (\bar D Ξ)$ interactions. The two so far observed s-wave baryons with charm one are recovered. We argue that the $Λ_c(2880)$ is not a s-wave state. In addition to those states we predict the existence of about ten narrow s-wave baryon states with masses below 3 GeV. A triplet of crypto-exotic states decaying dominantly into channels with an $η'$ is obtained with masses 4.24 GeV and 4.44 GeV.

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arXiv2005

e+e- pair production from nucleon targets in the resonance region

We discuss two processes, the $πN \to e^+e^- N$ and the $γN \to e^+e^- N$ reactions, below and close to the vector meson production threshold ($1.40<\sqrt s <1.75$ GeV). The aim is to gain understanding of the coupling of vector fields (associated with $ρ^0$- and $ω$-mesons) to low-lying baryon resonances. These couplings are not well-known and important for baryon structure studies and for dynamical descriptions of vector meson propagation in the nuclear medium. The $e^+e^-$ pair production amplitudes are determined by the $πN \to ρ^0 N$, $πN \to ωN$, $γN\to ρ^0 N$ and $γN \to ωN$ amplitudes supplemented by the Vector Meson Dominance assumption. The vector meson production amplitudes are calculated consistently using a relativistic and unitary coupled-channel approach to meson-nucleon scattering. We display results showing the importance of the quantum interference between $ρ^0$- and $ω$-mesons in the $e^+e^-$ channel to unravel the strength of the coupling of $ρ^0$- and $ω$-mesons to specific baryon excitations. The $πN \to e^+e^- N$ and $γN \to e^+e^- N$ reactions underlie the more complex $πA \to e^+e^- X$ and $γA \to e^+e^- X$ nuclear processes whose measurement is planned at GSI (with the HADES detector) and under analysis at JLab (with the CLAS detector).

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arXiv2005

Quantum interferences in the $γN \to e^+e^- N$ reaction close to the vector meson production threshold

The exclusive photoproduction of $e^+e^-$ pairs from nucleons close to the vector meson production threshold ($1.4<\sqrt s <1.8$ GeV) results from two main processes: the emission of Bethe-Heitler pairs and the photoproduction of $ρ^0$- and $ω$-mesons decaying into $e^+e^-$ pairs. The Bethe-Heitler amplitudes are purely electromagnetic and reflect mostly the nucleon magnetic structure. The $γN\to e^+e^- N$ amplitudes arising from vector meson production and decay are derived from $γN\to ρ^0 N$ and $γN \to ωN$ amplitudes supplemented by the Vector Meson Dominance assumption. The vector meson photoproduction amplitudes are calculated using a relativistic and unitary coupled-channel approach to meson-nucleon scattering. They depend sensitively on the coupling of vector fields to baryon resonances. The $γN \to e^+e^- N$ differential cross sections display interference patterns. The interference of Bethe-Heitler pair production with vector meson $e^+e^-$ decay is quite small in the domain of validity of our model for all angles of the emitted $e^+e^-$ pair. The interference of $ρ^0$- and $ω$-mesons in the $e^+e^-$ channel can be large. It is constructive for the $γp \to e^+e^- p$ reaction and destructive for the $γn \to e^+e^- n$ reaction. We discuss the shape and magnitude of the $e^+e^-$ pair spectra produced in the $γp \to e^+e^- p$ and $γn \to e^+e^- n$ reactions as functions of the pair emission angle and of the total center of mass energy $\sqrt s$.

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arXiv2004

Study of the $Σ$(1385) and $Λ$(1405) resonances in K$^+$ photoproduction processes

The $γp \to K^+ π^0 Λ$ and $γp \to K^+ πΣ$ reactions are studied in the kinematic region where the $π^0 Λ$(1116) and $πΣ$(1192) pairs originate dominantly from the decay of the $Σ$(1385) and $Λ$(1405) resonances. We consider laboratory photon energies around 2 GeV, i.e. total center of mass energies above the known resonance region. We compute the t-channel kaon-exchange contribution to these reactions using $K^- p \to π^0 Λ$ and $K^- p \to πΣ$ amplitudes calculated in the framework of a chiral coupled-channel effective field theory of meson-baryon scattering. We extract from the calculated cross section the gauge-invariant double kaon pole term. We find this term to be large and likely to drive significantly the $γp \to K^+ π^0 Λ$ and $γp \to K^+ πΣ$ reactions in the kinematics under investigation. Accurate measurements of t-distributions for these processes, in progress or planned at ELSA and at SPring-8, are needed to confirm this expectation and assess the possibility of studying antikaon-nucleon dynamics just below threshold through these reactions.

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