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M. Kirchbach

Publications and source records attributed to M. Kirchbach.

70 records · Page 4Linked to original sources
arXiv1999

Comment on ``Mixing and Decay Constants of Pseudoscalar Mesons''

The key assumption used recently by Feldmann, Kroll and Stich [Phys.Rev. D58, 114006 (1998)] that the decay constants f_η, and f_η' of the respective eta and eta ' mesons in the quark flavor basis follow the pattern of strange and non--strange quarkonia mixing in their wave functions, is reproduced in identifying the non-isotriplet part of the strong neutral axial current with the genuine axial hypercharge current J_{μ, 5}^Y =\bar q γ_μγ_5 Y/2 q, where Y=C+S+B is defined by the Gell-Mann-Nakano-Nishijima relation as the sum of charm (C), strangeness (S), and baryon (B) quark quantum numbers. The inequivalence between octet and hypercharge axial currents is pointed out.

hep-ph↗
arXiv1999

Baryons As Hyperspherical O(4) Partial Waves -- Is This The Message From The Spectra?

It is argued that the baryon excitations group to four-dimensional partial waves described by means of the three Rarita-Schwinger (RS) fields [(σ-1 )/2 ,(σ-1)/2]* [(1/2, 0)+(0,1/2)] with σ=2,4 and 6, where all components happen to be occupied. In the O(4) decomposition of the πN scattering amplitudes, the RS spin- and parity clusters appear as poles on the complex energy plane, socalled H"ohler poles. This phenomenon indicates that the symmetry of the πN scattering amplitude is O(4) and thereby the space-time version of chiral symmetry, rather than O(3). Accordingly, the baryon spectrum generating algebra is su(2)_I*su(3)_c*o(1,3)_{ls} rather than su(6)_{sf} * su(3)_c * o(3)_l. The nucleon and Δspectra below \sim 2500 MeV are complete up to only 5 `missing' resonances. The three O(4) poles are distributed over two distinct Fock spaces of opposite vacuum parities thus defining the energy scale of the chiral phase transition for baryons. Within this new symmetry scenario, the covariant description of the RS baryon clusters is straightforward and their averaged masses are fitted by a Balmer-series like formula emerging from a simple quark-diquark model in the O(4) basis with Coulomb potential and a four-dimensional rigid rotator.

hep-ph↗
arXiv1998

Anomalous Hypercharge Axial Current And The Couplings Of The eta And f_1 (1420) Mesons To The Nucleon

It is argued that the precise three flavor symmetry of hadrons is not SU(3)_F but rather U(4)_F restricted to SU(2)_{ud}*SU(2)_{cs}*U(1) and considered in the limit of frozen charm degree of freedom. Within this scheme the only hypercharge generator which is consistent with the Gell-Mann--Nishijima relation appears to be an element of the su(2)_{cs}*u(1) subalgebra as it contains the unit matrix generating the baryon number current. In considering now hypercharge axial transformations, the u(1) generator will give rise to the anomalous U(1)_A axial current and the resulting hypercharge axial current will be anomalous, too. The only anomaly free neutral strong axial current having a well defined chiral limit turns out to be identical (up to a constant factor) with the weak axial current, and the eta meson acquires features of a `masked' strange Goldstone boson. The eta N and f_1(1420)N couplings will now proceed via a purely strange isosinglet axial current and appear strongly suppressed relative to the predictions of the octet quark model. For this reason, loop corrections like the a_0πN and KK^*Y triangles to the respective eta NN and f_1(1420)NN vertices acquire importance. Here we calculate them in using effective lagrangians and show that the model developed is convenient for data description beyond the limits of applicability of chiral perturbation theory.

hep-ph↗
arXiv1998

Strange Isosinglet Versus Octet Scalar Axial Current

The flavor SU(3)_f group symmetry of QCD is systematically considered as the heavy c quark limit of SU(4)_f. Within that scheme we argue that the U(1)_A anomaly prevents Gell-Mann's choice for the realization of the su(4)_f (and thereby the su(3)_f) algebra for the particular case of the neutral axial currents. Rather, Weyl's choice for the SU(4)_f generators has to be used which leads to a neutral strong axial current having same structure (up to a constant factor) as the weak one.

nucl-th↗
arXiv1998

Lorentz Covariant Spin-Grouping of Baryon Resonances

A well pronounced spin--grouping of baryon resonances to O(4) partial waves is found in baryon spectra and shown to be well interpreted in terms of Lorentz group representations of the type (1/2 +l', 1/2 +l')* [(1/2, 0)+(0,1/2)] with l' integer. In this way the relativistic description of finite dimensional resonance towers containing higher-spin states becomes possible. It is further argued that the nucleon excitations into the l'=1 and l'=2 multiplets are chiral phase transitions.

hep-ph↗
arXiv1997

On the Parity Degeneracy of Baryons

The gross features of the observed baryon excitation spectrum below 2 GeV are well explained if the spectrum generating algebra of its intrinsic orbital angular momentum states is o(4)*su(2)_I. The spins of the resonances are obtained through the coupling of a Lorentz bi-spinor (1/2,0)+ (0,1/ 2) to a multiplet of the type (j,j) in its O(4)/O(3) reduction. The parities of the resonances follow from those of the O(3) members of the (j,j) multiplets. In this way relativistic SL(2,C) representations are constructed. For example, the first S11, P11, and D13 states with masses around 1500 MeV fit into the (1/2, 1/2)* [(1/2,0)+(0,1/2)] representation. The observed parities of the resonances correspond to natural parities of the (1/2,1/2) states. The second P11, S11, D13- together with the first P13, F15, D15, and (a predicted) F17 -resonances, centered around 1700 MeV, are organized into the (3/2,3/2)*[(1/2,0)+(0,1/2)] representation. I argue that the members of the (3/2,3/2) multiplet carry unnatural parities and that in this region chiral symmetry is restored. In the N(939)- N(1650) transition the chiral symmetry mode is changed, and therefore, a chiral phase transition is predicted to take place.

hep-ph↗
arXiv1997

Lorentz Multiplet Structure of Baryon Spectra and Relativistic Description

The pole positions of the various baryon resonances are known to reveal well-pronounced clustering, so-called Hoehler clusters. For nonstrange baryons the Hoehler clusters are shown to be identical to Lorentz multiplets of the type (j,j)*[(1/2,0)+(0,1/2)] with j being a half-integer. For the Lambda hyperons below 1800 MeV these clusters are shown to be of the type [(1,0)+ (0,1)]*[(1/2,0)+(0,1/2)] while above 1800 MeV they are parity duplicated (J,0)+(0,J) (Weinberg-Ahluwalia) states. Therefore, for Lambda hyperons the restoration of chiral symmetry takes place above 1800 MeV. Finally, it is demonstrated that the description of spin-3/2 particles in terms of a 2nd rank antisymmetric Lorentz tensor with Dirac spinor components does not contain any off-shell parameters and avoids the main difficulties of the Rarita-Schwinger description based upon a 4-vector with Dirac spinor components.

nucl-th↗
arXiv1997

Is the eta Meson a Goldstone Boson?

The decoupling of the eta meson from the nucleon, as recently deduced from analyzing \bar p p collisions and eta photoproduction off the proton at threshold, is shown to provide an argument against the octet Goldstone boson nature of the eta meson. This argument concerns the structure of the strong isoscalar axial vector current. A vanishing eta-N coupling means a vanishing contribution of the eta-pole term to the hypercharge nucleon axial vector current. Therefore, no partial conservation of the latter can be achieved. This new situation invalidates the octet Goldberger--Treiman relation for the eta-N coupling constant that is indicative of the octet Goldstone boson nature of the eta meson. In such a case there is no longer a compelling reason for the standard belief that the eta meson should be coupled to the hadronic vacuum through the hypercharge axial vector current. Rather, it will be coupled to it as any ordinary neutral pseudoscalar meson via the neutral axial vector current of the electroweak theory. As a result of the suggested universal structure of the weak and strong neutral axial vector currents the eta meson satisfies the strange analog of the Goldberger-Treiman relation so that it acquires features of a strange Goldstone boson. Hence the eta-NN vertex constant appears proportional to $Δs$, the fraction of nucleon spin carried by the strange quark sea, thus explaining its suppression naturally.

nucl-th↗
arXiv1996

In-Medium Pion Properties from Chiral Perturbation Theory

Two-point functions related to the pion weak decay constant $f_π$ are calculated from the generating functional of chiral perturbation theory in the mean field approximation and the heavy-baryon limit. The aim is to demonstrate that Lorentz invariance is violated in the presence of background matter. This fact manifests itself in the splitting of both $f_π$ and the pion mass into uncorrelated time- and space-like parts. We emphasize the different in-medium renormalizations of the correlation functions, show the inequivalence between the in-medium values of $f_π$ deduced from Walecka-type models, on the one hand, and QCD sum rules, on the other hand, and elaborate on the importance for some nuclear physics observables.

hep-ph↗
arXiv1996

New Insights into the Coupling of $η$ and $f_1$ Mesons to the Nucleon

We show that the contact couplings of neutral pseudoscalar and axial mesons to the nucleon are proportional to $Δs$, the fraction of nucleon spin carried by the strange quark sea, and thus are strongly suppressed relative to the couplings of charged mesons to the nucleon. On the other side, recent high accuracy data on $η$ photoproduction at threshold reveal the need for non--negligibile $ηNN$ vertices, while fitting $\bar N N$ phase shifts by means of effective meson exchange potentials requires a substantial presence of $f_1$ mesons there. We here advocate the idea to attribute the couplings of neutral pseudoscalar and axial vector mesons to effective triangular diagrams containig non--strange mesons and demonstrate its usefulness in describing available data.

nucl-th↗
arXiv1996

On the Coupling of the $η$ Meson to the Nucleon

The pseudoscalar and pseudovector $ηN$ coupling constants are calculated from an effective vertex associated with the $a_0(980)πN$ triangle diagram. The predicted values are in agreement with the ones concluded from fitting $η$ photoproduction amplitudes. In this context we stress the importance of the properties of the scalar meson octet for $η$ meson physics.

nucl-th↗
arXiv1995

Linear Wave Equations and Effective Lagrangians for Wigner Supermultiplets

The relevance of the contracted SU(4) group as a symmetry group of the pion nucleon scattering amplitudes in the large $N_c$ limit of QCD raises the problem on the construction of effective Lagrangians for SU(4) supermultiplets. In the present study we suggest effective Lagrangians for selfconjugate representations of SU(4) in exploiting isomorphism between so(6) and ist universal covering su(4). The model can be viewed as an extension of the linear $σ$ model with SO(6) symmetry in place of SO(4) and generalizes the concept of the linear wave equations for particles with arbitrary spin. We show that the vector representation of SU(4) reduces on the SO(4) level to a complexified quaternion. Its real part gives rise to the standard linear $σ$ model with a hedgehog configuration for the pion field, whereas the imaginary part describes vector meson degrees of freedom via purely transversal $ρ$ mesons for which a helical field configuration is predicted. As a minimal model, baryonic states are suggested to appear as solitons of that quaternion.

hep-ph↗
arXiv1995

Proton Spin in Chiral Quark Models

The spin and flavor fractions of constituent quarks in the proton are obtained from their chiral fluctuations involving Goldstone bosons. SU(3) flavor symmetry breaking suggested by the mass difference between the strange and up, down quarks is included, and this improves the agreement with the data markedly.

hep-ph↗
arXiv1995

THE COUPLING OF THE $f_1(1285)$ MESON TO THE ISOSCALAR AXIAL CURRENT OF THE NUCLEON

The weak decay as well as the strong nucleon coupling constants of the isoscalar axial vector meson $f_1 (1285)$ are estimated within the octet quark model. It is shown that the empirical value for the coupling of $\overline{s}s$ quarkonium to nucleons can be understood by attributing the $f_1(1285)$-nucleon coupling to the $a_0πN$ triangle diagram.

nucl-th↗
arXiv1994

Higher Resonance Contributions to the Adler-Weisberger Sum Rule in the Large N_c Limit

We determine the $N_c$--dependence of the resonance contributions to the Adler--Weisberger sum rule for the inverse square $1/g_A^2$ of the axial charge coupling constant and show that in the large $N_c$ limit the contributions of the Roper-like excitations scale as $O(1/N_c)$. Consistency with the $1/N_c^2$ scaling of the $1/g_A^2$ term in the sum rule requires these contributions to cancel against each other.

hep-ph↗