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B. Golli

Publications and source records attributed to B. Golli.

At least 19 recordsLinked to original sources
arXiv2022

The $Λ(1405)$ resonance as a genuine three-quark or molecular state

The mechanism for the formation of the $Λ(1405)$ resonance is studied in a chiral quark model that includes quark-meson as well as contact (four point) interactions. The negative-parity $S$-wave scattering amplitudes for strangeness $-1$ and $1$ are calculated within a unified coupled-channel framework that includes the $KN$, $\bar{K}N$, $πΣ$, $ηΛ$, $KΞ$, $πΛ$, and $ηΣ$ channels and possible genuine three-quark bare singlet and octet states corresponding to $\frac12^-$ resonances. We show that in order to reproduce the scattering amplitudes in the $S_{01}$ partial wave it is important to include the pertinent three-quark octet states as well as the singlet state, while the inclusion of the contact term is not mandatory. The Laurent-Pietarinen expansion is used to determine the $S$-matrix poles. Following their evolution as a function of increasing interaction strength, the mass of the singlet state is strongly reduced due to the attractive self-energy in the $πΣ$ and $\bar{K}N$ channels; when it drops below the $KN$ threshold, the state acquires a dominant $\bar{K}N$ component which can be identified with a molecular state. The attraction between the kaon and the nucleon is generated through the $\bar{K}NΛ^*$ interaction rather than by meson-nucleon forces.

nucl-th↗
arXiv2019

The enigmatic $Δ(1600)$ resonance

Our recently proposed model of the $Δ(1600)$ resonance, in which the dominant component is a quasi-bound state of the $Δ(1232)$ and the pion, is confronted with a similar model of the $N^*(1440)$ resonance as its counterpart in the P11 partial wave. We stress an essentially different mechanism responsible for generating the two resonances.

hep-ph↗
arXiv2019

Dynamical generation of resonances in the P33 partial wave

We investigate the formation of resonances in the P33 partial wave with the emphasis on possible emergence of dynamically generated quasi-bound states as a consequence of a strong $p$-wave pion attractive interaction in this partial wave, as well as their possible interaction with the genuine quark excited states. By using the Laurent-Pietarinen expansion we follow the evolution of the $S$-matrix poles in the complex energy plane as a function of the interaction strength. Already without introducing a genuine quark resonant state, two physically interesting resonances emerge with pole masses around 1200 MeV and 1400 MeV, with the dominant $πN$ and $πΔ$ component, respectively. The added genuine resonant state in the $(1s)^3$ quark configuration mixes with the lower dynamically generated resonance forming the physical $Δ(1232)$ resonance, and pushes the second dynamical resonance to around 1500 MeV, which allows it to be identified with the $Δ(1600)$ resonance. Adding a second resonant state with one quark promoted to the $2s$ orbit generates another pole whose evolution remains well separated from the lower two poles. We calculate the helicity amplitudes at the pole and suggest that their $Q^2$ dependence could be a decisive test to discriminate between different models of the $Δ(1600)$ resonance.

hep-ph↗
arXiv2018

The Roper resonance as a meson-baryon molecular state

The recently proposed mechanism for the formation of the Roper resonance, in which a dynamically generated state as well as a genuine three-quark resonant state play an equally important role, is confronted with the model proposed almost twenty years ago in which the Roper is pictured as a molecular state of the nucleon and the $σ$ meson.

hep-ph↗
arXiv2018

The Roper resonance: a genuine quark state or a dynamically generated structure?

In view of the recent results of lattice QCD simulation in the P11 partial wave that has found no clear signal for the three-quark Roper state we investigate a different mechanism for the formation of the Roper resonance in a coupled channel approach including the $πN$, $πΔ$ and $σN$ channels. We fix the pion-baryon vertices in the underlying quark model while the $s$-wave sigma-baryon interaction is introduced phenomenologically with the coupling strength, the mass and the width of the $σ$ meson as free parameters. The Laurent-Pietarinen expansion is used to extract the information about the $S$-matrix pole. The Lippmann-Schwinger equation for the $K$ matrix with a separable kernel is solved to all orders. For sufficiently strong $σNN$ coupling the kernel becomes singular and a quasi-bound state emerges at around 1.4~GeV, dominated by the $σN$ component and reflecting itself in a pole of the $S$-matrix. The alternative mechanism involving a $(1s)^22s$ quark resonant state is added to the model and the interplay of the dynamically generated state and the three-quark resonant state is studied. It turns out that for the mass of the three-quark resonant state above 1.6~GeV the mass of the resonance is determined solely by the dynamically generated state, nonetheless, the inclusion of the three-quark resonant state is imperative to reproduce the experimental width and the modulus of the resonance pole.

hep-ph↗
arXiv2009

Pion electro-production in the Roper region in chiral quark models

We present a method to calculate pion electro-production amplitudes in a coupled-channel framework incorporating quasi-bound quark-model states. The method offers a clear prescription how to extract the resonant part of the amplitudes, even in the presence of different decay channels and a strong mixing of neighbouring resonances. The method is applied to the calculation of the M1- and the S1- amplitudes in the P11 partial wave in a simple chiral quark model. A good agreement with the observed M1- amplitude is found with a significant contribution from the pion cloud. The same effect is also prominent in the S1- amplitude but a rather uncertain data prevent us to draw a definitive conclusion.

nucl-th↗
arXiv2005

Direct calculation of the K-matrix for pion electro-production in the Delta channel

We present a method to calculate directly the K-matrices for the pion electro-production processes in the framework of chiral quark models which allows for a clean separation of the resonant amplitudes from the background. The method is applied to the calculation of the multipole amplitudes M_{1+}, E_{1+}, and S_{1+} in the Delta channel within the Cloudy Bag Model. A good overall description is found in a broad energy range.

hep-ph↗
arXiv2002

Baryons as Solitons in Chiral Quark Models

We describe the formation of solitons in NJL-type models and discuss the influence of the regularization scheme on the stability of the solution. We concentrate on models with non-local regulators in which stable solutions exist without introducing additional constraints.

hep-ph↗
arXiv2002

Axial Excitation of the Delta in Chiral Quark Models

We discuss electro-magnetic and weak axial N-Delta transition amplitudes in the linear sigma-model and the Cloudy-Bag Model as typical representatives of chiral quark models. We show that good qualitative understanding of the transition can be obtained in models which, in addition to the pion cloud, incorporate a fluctuating sigma-field inside the baryon.

hep-ph↗
arXiv2002

N-Delta axial transition form factors

We review some basic properties of the N-Delta transition axial amplitudes and relate them to the strong pi-N-Delta form-factor. In models with the pion cloud we derive a set of constraints on the pion wave function which guaranty the correct behaviour of the amplitudes in the vicinity of the pion pole. Corrections due to the spurious center-of-mass motion are calculated to the leading order in the inverse baryon mass. We give explicit expressions for the amplitudes in the Cloudy Bag Model and show that they rather strongly underestimate the experimental values.

hep-ph↗
arXiv2002

Axial amplitudes for Delta excitation in chiral quark models

We study the axial amplitudes for the N-Delta transition in models with quarks and chiral mesons. A set of constraints on the pion field is imposed which enforces PCAC and the off-diagonal Goldberger-Treiman relation. The quark contribution to the amplitudes in general strongly underestimates the C^A_5 amplitude as well as the piNDelta strong coupling constant. We show that the results are considerably improved in models that, in addition to the pion cloud, incorporate a fluctuating sigma field inside the baryon.

hep-ph↗
arXiv2002

Radial excited states of the nucleon in quark models with dynamical confinement

The Roper state is described in the framework of the chiral chromodielectric model as a single quark excitation in self-consistent meson potentials. The interplay between quark and meson excitations is discussed and we emphasise the role of non-quark degrees of freedom in the electroproduction of nucleon excitations.

hep-ph↗
arXiv2002

Roper Electroproduction Amplitudes in a Chiral Confinement Model

A description of the Roper using the chiral chromodielectric model is presented and the transverse $A_{1/2}$ and the scalar $S_{1/2}$ helicity amplitudes for the electromagnetic Nucleon--Roper transition are obtained for small and moderate $Q^2$. The sign of the amplitudes is correct but the model predictions underestimate the data at the photon point. Our results do not indicate a change of sign in any amplitudes up to $Q^2\sim1$ GeV$^2$. The contribution of the scalar meson excitations to the Roper electroproduction is taken into account but it turns out to be small in comparison with the quark contribution. However, it is argued that mesonic excitations may play a more prominent role in higher excited states.

hep-ph↗
arXiv2001

Excitation of non-quark degrees of freedom in N*

We study the contribution of glueball and sigma-meson degrees of freedom to nucleon excited states in the framework of a chiral version of the chromodielectric model. We have found that these degrees of freedom considerably lower the energy of the Roper and may substantially weaken the electroexcitation amplitudes for the N(1440) and in particular for the N(1710).

hep-ph↗
arXiv2001

N^* electroproduction amplitudes in a model with dynamical confinement

The Roper resonance is described in a chiral version of the chromodielectric model as a cluster of three quarks in radial-orbital configuration (1s)$^2$(2s)$^1$, surrounded by $π$ and $σ$-meson clouds and by a chromodielectric field which assures quark dynamical confinement. Radial profiles for all fields are determined self-consistently for each baryon. Transverse $A_{1/2}$ and scalar $S_{1/2}$ helicity amplitudes for the nucleon-Roper transition are calculated. The contribution of glueball and $σ$-meson vibrations is estimated; although small for N(1440), the $σ$ contribution can be large for N(1710).

hep-ph↗
arXiv1996

The constituent quark as a soliton in a linear sigma-model

We study a stable, soliton-like solution in the linear $σ$-model where the chiral fields are coupled to a single massless nonstrange quark. We investigate a possibility that this solution represents the constituent quark of Georgi and Manohar. We show that its properties are indeed consistent with nucleon observables. Furthermore, we derive chiral meson exchange potentials between such objects which are similar to recently used potentials in the constituent quark models.

hep-ph↗
arXiv1996

The role of the pion cloud in electroproduction of the $Δ$(1232)

We calculate the ratios $E2/M1$ and $C2/M1$ of the multipole amplitudes for electroproduction of the $Δ$(1232) in the range of photon virtuality $0<-K^2<1$~GeV$^2$ in a chiral chromodielectric model and a linear $σ$-model. We find that relatively large experimental values can be explained in terms of the pion contribution alone; the contribution arising from d-state quark admixture remains below 10\%. We describe the pion cloud as a coherent state and use spin and isospin projection to obtain the physical nucleon and the $Δ$. The $A_{1/2}$ and $A_{3/2}$ amplitudes are reasonably well reproduced in the $σ$-model; in the chromodielectric model, however, they are a factor of two too small.

hep-ph↗