# Angular Dependence ofη Photoproduction in Photon-Induced Reaction

Authors: Jun-Zhen Wang; Bao-Chun Li
Journal: Advances in High Energy Physics
 (2018)

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## Abstract

Photoproduction ofη mesons from nucleons can provide valuable information about the excitation spectrum of the nucleons. The angular dependence of η photoproduction in the photon-induced reaction is investigated in the multisource thermal model. The results are compared with experimental data from the η
→
3
π
0
→
6
γ decay mode. They are in good agreement with the experimental data. It is shown that the movement factor increases linearly with the photon beam energies. And the deformation and translation of emission sources are visually given in the formalism.


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## Body

## 1. Introduction

The excitation spectrum of nucleons is important to understanding the nonperturbative behavior of the fundamental theory of strong interactions, Quantum Chromodynamics (QCD) [1–4]. The photon-induced meson production off nucleons is mainly used to achieve more information from the excitation spectrum of nucleons. It is very important for missing resonances that the η meson production in photon-induced and hadron-induced reactions on free (and quasi-free) nucleons and on nuclei [5–8]. The advantage of photon-induced reactions is that the electromagnetic couplings can provide valuable information related to the details of the model wave functions. Because the electromagnetic excitations are isospin dependent, we need perform meson-production reactions off the neutron.Recently, the photoproduction ofη mesons from quasi-free protons and neutrons was measured in η
→
3
π
0
→
6
γ decay mode by the CBELSA/TAPS detector at the electron accelerator ELSA in Bonn [9]. At different incident photon energies, the experiments are performed by the incident photon beam on a liquid deuterium target. A great number of η mesons are produced in the photon-induced reaction. The experimental data are regarded as a multiparticle system. And, their angular distributions represent an obvious regularity at different incident photon energies. In order to explain the abundant experimental results, some statistical methods are proposed and developed [10–16]. In this work, we will extend a multi-source thermal model to the statistical investigation of the angular distributions in the photon-induced reaction and try to understand the η photoproduction in the reaction. In our previous wok [17–21], the model was focused on the investigation of the particle production in intermediate-energy and high-energy collisions.


## 2.η Meson Distribution in the Multi-Source Thermal Model

In the multi-source thermal model [17–21], many emission sources are expected to be formed at the final stage of the photon-induced reaction. Every source emits particles isotropically in the source rest frame. The observed η mesons are from different emission sources. The incident beam direction is defined as an o
z axis and the reaction plane is defined as y
o
z plane. In the source rest frame, the meson momentum p
x, p
y, and p
z obeys a normal distribution. The corresponding transverse momentum p
T
=
p
x
2
+
p
y
2 obeys a Rayleigh distribution:(1)
f
p
T
p
T
=
p
T
σ
2
e
-
p
T
2
/
2
σ
2
,where σ represents a distribution width. The distribution function of the polar angle θ is(2)
f
θ
θ
=
1
2
sin
⁡
θBecause of the interactions with other emission sources, the considered source deforms and translates along theo
z axis. Then, the momentum component is revised to(3)
p
z
′
=
a
z
p
z
+
b
z
σwhere a
z and b
z represent the coefficients of the source deformation and translation along the o
z axis, respectively. The mathematical description of the deformable translational source is formulized simply as a linear relationship between p
z
′ and p
z, which reflects the mean result of the source interaction. For a
z
≠
1 or b
z
≠
0, the p
z
′ distribution of η mesons is anisotropic along the o
z axis.By using Monte Carlo method,p
T and p
z
′ are given by(4)
p
T
=
σ
-
2
ln
⁡
r
1
,
(5)
p
z
′
=
a
z
σ
-
2
ln
⁡
r
2
cos
⁡
2
π
r
3
+
b
z
σ
,where r
1, r
2, and r
3 are random numbers from 0 to 1. The polar angle θ is revised to(6)
θ
′
=
a
r
c
t
a
n
p
T
p
z
′
=
a
r
c
t
a
n
-
2
ln
⁡
r
1
a
z
-
2
ln
⁡
r
2
cos
⁡
2
π
r
3
+
b
z
.We can calculate a new distribution function of the polar angle by this formula.


## 3. Angular Dependencies ofη Photoproduction in the Photon-Induced Reaction

Figures1(a)–1(p) show the angular distributions of η mesons for different bins of incident photon energy 698 MeV ≤
Ε
γ
≤
1005 MeV as a function of cos
⁡
θ
η. θ
η is the polar angle of η meson in the beam-target cm system assuming the initial state nucleon at rest. The symbols represent the experimental data from the CBELSA/TAPS detector at the electron accelerator ELSA in Bonn [9]. The results obtained by using the multi-source thermal model are shown with the curves, which behave in the same way as the experimental data in the 16 bins of incident photon energy. By minimizing χ
2 per degree of freedom (χ
2
/
d
o
f), we determine the corresponding parameters a
z and b
z, which are presented in Table 1. It is found that there is an almost linear relationship between the b
z and Ε
γ. As representative energies of Figure 1, we give a schematic sketch of these emission sources at the four different energies in Figure 7(a). The deformations and translations can be seen intuitively in the figure.Table 1
Values ofa
z and b
z taken in Figure 1 model results.
Figure1
E
γ (MeV)
a
z
b
z
χ
2
/
d
o
f
(a)
698
1.050
-0.170
0.118
(b)
712
1.020
-0.210
0.105
(c)
730
1.050
-0.170
0.090
(d)
750
1.040
-0.155
0.134
(e)
770
0.985
-0.105
0.182
(f)
790
0.970
-0.125
0.179
(g)
810
0.979
-0.110
0.194
(h)
830
0.960
-0.120
0.192
(i)
850
0.970
-0.110
0.200
(j)
870
0.980
-0.130
0.211
(k)
890
0.950
-0.120
0.175
(l)
910
0.970
-0.140
0.261
(m)
930
0.940
-0.110
0.179
(n)
955
0.950
-0.120
0.154
(o)
980
0.950
-0.090
0.138
(p)
1005
0.920
-0.110
0.150Figure 1
Angular distributions for different bins of incident photon energy 698 MeV ≤E
γ ≤ 1005MeV as a function of cos
⁡
θ
η in the beam-target cm system assuming the initial state nucleon at rest. The symbols represent the experimental data from the CBELSA/TAPS detector at the electron accelerator ELSA in Bonn [9]. The results in the multi-source thermal model are shown with the curves.In Figures2(a)–2(p) and Figures 3(a)–3(p), we present the angular distributions of η mesons for different bins of incident photon energy 1035 MeV ≤
Ε
γ
≤ 1835 MeV as a function of cos
⁡
θ
η. θ
η is the polar angle of η meson in the beam-target cm system assuming the initial state nucleon at rest. Same as Figure 1, the symbols represent the experimental data from the CBELSA/TAPS detector at the electron accelerator ELSA in Bonn [9]. The results obtained by using the multisource thermal model are shown with the curves, which behave in the same way as the experimental data in the 28 bins of incident photon energy. Parameters a
zand b
z are presented in Tables 2 and 3. As the representative energies of Figures 1 and 2, the schematic sketches of the emission sources are given at different energies in Figures 7(b) and 7(c).Table 2
Values ofa
z and b
z taken in Figure 2 model results.
Figure2
E
γ (MeV)
a
z
b
z
χ
2
/
d
o
f
(a)
1035
0.920
-0.080
0.124
(b)
1065
0.940
-0.090
0.118
(c)
1095
0.970
-0.030
0.132
(d)
1125
0.870
0.030
0.165
(e)
1155
0.850
0.050
0.170
(f)
1185
0.850
0.030
0.168
(g)
1215
0.860
0.070
0.173
(h)
1245
0.860
0.050
0.122
(i)
1270
0.850
0.110
0.121
(j)
1295
0.830
0.150
0.145
(k)
1325
0.910
0.120
0.147
(l)
1355
0.910
0.160
0.142
(m)
1385
0.890
0.110
0.115
(n)
1415
0.930
0.130
0.106
(o)
1445
0.830
0.250
0.122
(p)
1475
0.810
0.210
0.091Table 3
Values ofa
z and b
z taken in Figure 3 model results.
Figure3
E
γ (MeV)
a
z
b
z
χ
2
/
d
o
f
(a)
1505
0.915
0.260
0.243
(b)
1535
0.920
0.260
0.190
(c)
1565
0.920
0.240
0.158
(d)
1595
0.800
0.360
0.315
(e)
1625
0.890
0.310
0.179
(f)
1655
0.930
0.340
0.190
(g)
1685
0.910
0.360
0.206
(h)
1715
0.940
0.400
0.235
(i)
1745
0.960
0.460
0.085
(j)
1775
0.960
0.410
0.144
(k)
1805
1.090
0.400
0.132
(l)
1835
1.060
0.390
0.140Figure 2
Same as Figure1, but showing angular distributions for different bins of incident photon energy 1035 MeV ≤ E
γ ≤ 1475MeV.Figure 3
Same as Figure1, but showing angular distributions for different bins of incident photon energy 1505 MeV ≤ E
γ ≤ 1835MeV.In Figures4, 5, and 6, we show angular distributions in the η-nucleon cm system for γ
p
→
p
η reaction for the different bins of final state energy 1488 MeV ≤
W
≤ 1625 MeV, 1635 MeV ≤
W
≤ 1830 MeV and 1850 MeV ≤
W
≤ 2070 MeV, respectively. Same as Figure 1, the model results and experimental data are indicated by the curves and symbols, respectively. The model results can also agree with the experimental data. In the same way, the deformations and translations of these emission sources are given in Tables 4–6 and Figures 7(d)–7(f). All the parameter values taken in the above calculations are also given in Figures 8 and 9. It can be found that a
z keeps almost invariable and fluctuates around 1.0 with the increasing Ε
γ. The parameter b
z increases linearly with the increasing Ε
γ and their relationship can be expressed by a linearly function, b
z
=
(
0.541
±
0.005
)
×
1
0
-
3
Ε
γ
-
(
0.622
±
0.011
). There are similar relationships between the parameters and different final state energies W in Figure 9, where the fitting function of b
z is b
z
=
(
0.808
±
0.003
)
×
1
0
-
3
W
-
(
1.322
±
0.007
).Table 4
Values ofa
z and b
z taken in Figure 4 model results.
Figure4
W (MeV)
a
z
b
z
χ
2
/
d
o
f
(a)
1488
0.956
-0.098
0.086
(b)
1492
0.968
-0.069
0.125
(c)
1498
0.966
-0.058
0.101
(d)
1505
0.959
-0.063
0.114
(e)
1515
0.965
-0.052
0.205
(f)
1525
0.952
-0.048
0.192
(g)
1535
0.960
-0.063
0.143
(h)
1545
0.967
-0.066
0.206
(i)
1555
0.948
-0.060
0.174
(j)
1565
0.941
-0.072
0.168
(k)
1575
0.934
-0.059
0.170
(l)
1585
0.955
-0.068
0.235
(m)
1595
0.925
-0.051
0.260
(n)
1605
0.934
-0.039
0.251
(o)
1615
0.942
-0.045
0.307
(p)
1625
0.961
-0.058
0.293Table 5
Values ofa
z and b
z taken in Figure 5 model results.
Figure5
W (MeV)
a
z
b
z
χ
2
/
d
o
f
(a)
1635
0.958
-0.042
0.305
(b)
1645
0.940
-0.028
0.350
(c)
1655
0.920
-0.021
0.263
(d)
1670
0.925
-0.009
0.187
(e)
1690
0.902
0.022
0.240
(f)
1710
0.899
0.047
0.209
(g)
1730
0.868
0.095
0.181
(h)
1750
0.843
0.102
0.194
(i)
1770
0.853
0.115
0.235
(j)
1790
0.879
0.125
0.170
(k)
1810
0.855
0.142
0.152
(l)
1830
0.867
0.159
0.138Table 6
Values ofa
z and b
z taken in Figure 6 model results.
Figure6
W (MeV)
a
z
b
z
χ
2
/
d
o
f
(a)
1850
0.881
0.168
0.127
(b)
1870
0.872
0.221
0.132
(c)
1890
0.883
0.241
0.135
(d)
1910
0.856
0.252
0.153
(e)
1930
0.867
0.269
0.218
(f)
1950
0.853
0.253
0.295
(g)
1970
0.860
0.249
0.321
(h)
1990
0.875
0.261
0.184
(i)
2010
0.895
0.278
0.359
(j)
2030
0.906
0.299
0.337
(k)
2050
0.932
0.348
0.285
(l)
2070
0.945
0.337
0.304Figure 4
Angular distributions in theη-nucleon cm system for the reaction γ
p
→
p
η for the different bins of final state energy 1488 MeV ≤ W ≤ 1625 MeV. The symbols represent the experimental data from the CBELSA/TAPS detector at the electron accelerator ELSA in Bonn [9]. The results in the multisource thermal model are shown with the curves.Figure 5
Same as Figure4, but showing angular distributions for the different bins of final state energy 1635 MeV ≤ W ≤ 1830 MeV.Figure 6
Same as Figure4, but showing angular distributions for the different bins of final state energy 1850 MeV ≤ W ≤ 2070 MeV.Figure 7
The deformable and translational source in the reaction plane for different bins of incident photon energyE
γ or final state energy W.
(a)
(b)
(c)
(d)
(e)
(f)Figure 8
a
Z and b
Z for different bins of incident photon energy E
γ. The symbols are the values taken in Figures 1–3. The straight line is a fitted curve.Figure 9
Thea
Z and b
Z values taken for different bins of final state energy W. The symbols are the values taken in Figures 4–6. The straight line is a fitted curve.


## 4. Discussion and Conclusions

The excitation spectrum of nucleons can especially help us to understand the strong interaction in the nonperturbative regime. Before, the hadron induced reactions is a main experimental method in the investigation. In the last two decades, the photon-induced reaction and electron scattering experiment are applied to study the electromagnetic excitation of baryons. Recently, the photoproduction ofη mesons from quasi-free protons and neutrons are measured by the CBELSA/TAPS detector. In the paper, we theoretically study the angular distribution of η mesons for different incident photon energies Ε
γ and for different final state energies W. Then, the results are compared with the experimental data in detail. The deformation coefficient a
z and translation coefficient b
z are extracted by the comparison. a
z is almost independent of incident photon energies and final state energies. b
z is linearly dependent on incident photon energies and final state energies. In particular, we visually give the deformation and translation of the emission sources by schematic sketches. From the patterns, it is intuitive and easy to better understand the motion and configuration of the emission sources.A great number ofη mesons are produced in the photon-induced reaction. These η mesons are regarded as a multiparticle system, which can be analyzed by the statistical method. In recent years, we develop such a model, which is called multisource thermal model. Some emission sources of final-state particles are formed in the reaction. Each emission source emits particles isotropically in the rest frame of the emission source. Due to the source interaction, the sources emit particles anisotropically. The η mesons are emitted from these sources. In our previous work, the model can successfully describe transverse momentum spectra and pseudorapidity spectra of final-state particles produced in proton-proton (p
p) collisions, proton-nucleus (p
A) collisions, and nucleus -nucleus (A
A) collisions at intermediate energy and at high energy [17–21]. In this work, we extend the multisource thermal model to the statistical investigation of final-state particles produced in the photon-induced reaction. The model is improved to describe the angular dependence of the η photoproduction from quasi-free protons and neutrons. The information of the source deformation and translation is obtained with different beam energies. It is helpful for us to understand the η photoproduction.



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