_{0}(500) and

_{0}(980) off the nucleon

^{3}

We investigate photoproduction of scalar mesons off the nucleon, using the effective Lagrangians and the Regge approach. We first study

Understanding the structure of low-lying scalar mesons has been one of the most challenging issues in hadronic physics. Their internal structure is still under debate. That the

The scalar mesons have also been extensively studied phenomenologically. There are two scalar–isoscalar mesons (

While there has been a great deal of theoretical work on the structure of the

However, before we carry out the investigation on the

Upon computing the transition amplitude for the

In addition to the Roper resonance, we want to consider other

The present work is structured as follows: In

We start with the tree-level Feynman diagrams relevant to the

Feynman diagrams for

As for the

As we have already briefly mentioned in the introduction, there are ten excited nucleon resonances that can decay into

To compute the Feynman-invariant amplitudes for

Based on the effective Lagrangians in Eq. (

The effective Lagrangian approach has been successfully used to describe hadronic reactions in the low-energy region. However, when the photon energy increases, the results from the effective Lagrangians start to deviate from the data and do not even satisfy the unitarity [

Here,

We adopt a degenerate propagator [

In addition, we introduce the scaling factor for the

While

Spin-1/2 resonances [

Particle | Mass [GeV] | Width [GeV] | Status | |
---|---|---|---|---|

1.440 | 0.300 | **** | ||

1.535 | 0.150 | **** | ||

1.655 | 0.150 | **** | ||

1.710 | 0.100 | *** |

In Model II, we additionally consider the

The excited nucleon resonances [

Particle | Mass [GeV] | Width [GeV] | Status | |
---|---|---|---|---|

1.525 | 0.115 | **** | ||

1.675 | 0.150 | **** | ||

1.685 | 0.130 | **** |

Here,

The coupling constant for the

The

The mass of the

In order to determine the strong coupling constants for the excited baryons, we assume that the

The results of the strong coupling constants are shown in

The strong coupling constants for the

Concerning the photocoupling constants for

The photoncoupling constants for the

0.47 | 0.81 | 0.28 | |

4.63 | |||

15.24 |

A hadron has a spatial size, which can be characterized by phenomenological form factors. Hence, one has to introduce them at each baryon–baryon–meson vertex. Note that each amplitude of

Here,

We are now in a position to present the numerical results of our work. Since there exist experimental data on the differential cross section of the

In order to describe the experimental data on

Differential cross section of the

In

Total cross section of the

The parameters for

We need to delve into the physical reasons for Model I. The

Total cross sections for the

The right panel of

As mentioned briefly in the introduction, the uncertainty in the mass of

In

Total cross sections for the

Thus, if one further increases

In the upper panel of

Differential cross section for the

Differential cross section for the

We aimed in this work to investigate

Though it is very difficult to study

H.-Ch.K. is grateful to M. V. Polyakov and the members of TPII in Ruhr-Universität Bochum for valuable discussions and hospitality during his visit, where part of this work was carried out. The present work was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (Grant Number: NRF-2015R1D1A1A01060707). S.H.K. acknowledges support from the Young Scientist Training Program at the Asia Pacific Center for Theoretical Physics by the Korea Ministry of Education, Science, and Technology, Gyeongsangbuk-Do and Pohang City.

Open Access funding: SCOAP