Differential cross section and photon-beam asymmetry for the $\vec{\gamma }p\to {\pi }^{+}n$ reaction at forward ${\pi }^{+}$ angles at $E_{\gamma }=1.5$–2.95 GeV

Differential cross sections and photon-beam asymmetries for the $\vec{\gamma }p\to {\pi }^{+}n$ reaction have been measured for $0.6<cos{\theta }_{\pi }<1$ and $E_{\gamma }=1.5\text{–}2.95\mathrm{GeV}$ at SPring-8/LEPS. The cross sections monotonically decrease as the photon beam energy increases for $0.6<cos{\theta }_{\pi }<0.9$. However, the energy dependence of the cross sections for $0.9<cos{\theta }_{\pi }<1$ and $E_{\gamma }=1.5\text{–}2.2\mathrm{GeV}$ ($W=1.9\text{–}2.2\mathrm{GeV}$) is different, which may be due to a nucleon or $\mathrm{\Delta }$ resonance. The present cross sections agree well with the previous cross sections measured by other groups and show forward peaking, suggesting significant $t$-channel contributions in this kinematical region. The asymmetries are found to be positive, which can be explained by $\rho$ exchange in the $t$ channel. Large positive asymmetries in the small-$\left|t\right|$ region, where the $\rho$-exchange contribution becomes small, could be explained by introducing $\pi$-exchange interference with the $s$ channel.

Figure 1

Missing mass spectra for the $\gamma p\to {\pi }^{+}X$ reaction for (a) $0.6<cos{\theta }_{\pi }<0.7$, (b) $0.7<cos{\theta }_{\pi }<0.8$, (c) $0.8<cos{\theta }_{\pi }<0.9$, (d) $0.9<cos{\theta }_{\pi }<0.933$, (e) $0.933<cos{\theta }_{\pi }<0.966$, and (f) $0.966<cos{\theta }_{\pi }<1$ with $E_{\gamma }=1.5\text{–}2.95\mathrm{GeV}$. The thick solid curves are the results of the fits, and the dotted and dashed curves are the contributions from the positron background and the $\pi \pi$ production events, respectively.

Figure 2

Differential cross sections $d\sigma /dcos\theta$ for the $\gamma p\to {\pi }^{+}n$ reaction as a function of $cos{\theta }_{\pi }$.

Figure 3

Differential cross sections for the $\gamma p\to {\pi }^{+}n$ reaction for (a) $0.6<cos{\theta }_{\pi }<0.7$, (b) $0.7<cos{\theta }_{\pi }<0.8$, (c) $0.8<cos{\theta }_{\pi }<0.9$, (d) $0.9<cos{\theta }_{\pi }<0.933$, (e) $0.933<cos{\theta }_{\pi }<0.966$, and (f) $0.966<cos{\theta }_{\pi }<1$ with $E_{\gamma }=1.5\text{–}2.95\mathrm{GeV}$. The closed circles are the present LEPS data. The open squares and the closed triangles are the CLAS [2] and DESY data [3, 4], respectively. The solid curves are the results of the said analysis by the George Washington University group [11]. The dashed curves are the results of partial-wave analysis by the Bonn–Gatchina group [39].

Figure 4

(a) Differential cross sections $d\sigma /dt$ for the $\gamma p\to {\pi }^{+}n$ reaction as a function of $\left|t\right|$. (b) Differential cross sections scaled with $s^2$ as a function of $\left|t\right|$.

Figure 5

(a) Differential cross sections $d\sigma /dt$ for the $\gamma p\to {\pi }^{+}n$ reaction as a function of $E_{\gamma }$. The data $E_{\gamma }<3.0\mathrm{GeV}$ were measured by LEPS and the data $E_{\gamma }\ge 5\mathrm{GeV}$ were measured by SLAC. The curves are the results of exclusive fits to the SLAC data. (b) The $\alpha \left(t\right)$ values for the $\gamma p\to {\pi }^{+}n$ reaction are the results from the SLAC (triangle), the LEPS and SLAC (squares), and the LEPS (circles). The $\pi$ and $\rho$ trajectories are represented by using the functions ${\alpha }_{\pi }\left(t\right)=0.7(t-m_{\pi }^2)$ and ${\alpha }_{\rho }\left(t\right)=0.55+0.8t$, respectively [5].

Figure 6

The ratio $(N_V-N_H)/(N_V+N_H)$ as a function of ${\pi }^{+}$ azimuthal angle (${\varphi }_{\pi }$) for the $\vec{\gamma }p\to {\pi }^{+}n$ reaction for (a) $0.6<cos{\theta }_{\pi }<0.7$, (b) $0.7<cos{\theta }_{\pi }<0.8$, (c) $0.8<cos{\theta }_{\pi }<0.9$, (d) $0.9<cos{\theta }_{\pi }<0.933$, (e) $0.933<cos{\theta }_{\pi }<0.966$, and (f) $0.966<cos{\theta }_{\pi }<1$ with $E_{\gamma }=1.5\text{–}2.9\mathrm{GeV}$. The solid curves are the result of the fits with $P_{\gamma }\mathrm{\Sigma }cos2{\varphi }_{\pi }$.

Figure 7

Photon-beam asymmetries for the $\vec{\gamma }p\to {\pi }^{+}n$ reaction for $E_{\gamma }=1.5\text{–}2.95\mathrm{GeV}$. The closed circles are the present LEPS data and the open squares are the CLAS data [12]. The solid curves are the results of the said analysis by the George Washington University group [11]. The dashed curves are the results of partial-wave analysis performed by the Bonn–Gatchina group [39].

Figure 8

Photon-beam asymmetries for the $\vec{\gamma }p\to {\pi }^{+}n$ reaction as a function of $\left|t\right|$.