Study of $\eta$ and $\eta \prime$ Photoproduction at MAMI

The reactions $\gamma p\to \eta p$ and $\gamma p\to {\eta }^{\prime }p$ are measured from their thresholds up to the center-of-mass energy $W=1.96\mathrm{GeV}$ with the tagged-photon facilities at the Mainz Microtron, MAMI. Differential cross sections are obtained with unprecedented statistical accuracy, providing fine energy binning and full production-angle coverage. A strong cusp is observed in the total cross section for $\eta$ photoproduction at the energies in the vicinity of the ${\eta }^{\prime }$ threshold, $W=1896\mathrm{MeV}$ ($E_{\gamma }=1447\mathrm{MeV}$). Within the framework of a revised $\eta \mathrm{MAID}$ isobar model, the cusp, in connection with a steep rise of the ${\eta }^{\prime }$ total cross section from its threshold, can only be explained by a strong coupling of the poorly known $N(1895)1/2^{-}$ state to both $\eta p$ and ${\eta }^{\prime }p$. Including the new high-accuracy results in the $\eta \mathrm{MAID}$ fit to available $\eta$ and ${\eta }^{\prime }$ photoproduction data allows the determination of the $N(1895)1/2^{-}$ properties.

Figure 1

$m({\pi }^0{\pi }^0\eta \to 6\gamma )$ distributions for $E_{\gamma }=1558\mathrm{MeV}$ and $\cos \theta =0.1$. (a) MC simulation of $\gamma p\to {\eta }^{\prime }p\to {\pi }^0{\pi }^0\eta p\to 6\gamma p$ with a Gaussian fit. (b) Experimental spectrum fitted with the sum of a Gaussian and a polynomial of order 4.

Figure 2

$\gamma p\to \eta p$ differential cross sections from the present analysis of Run I obtained from $\eta \to \gamma \gamma$ (the red triangles) and from $\eta \to 3{\pi }^0$ (the blue triangles) are shown for two energy bins and compared to the previous analysis from Ref. [3] (the black circles) based on $\eta \to 3{\pi }^0$ only. The error bars of all data points represent statistical uncertainties combined with the angular-dependent systematic uncertainties (which were added for $W>1796\mathrm{MeV}$ only). The new $\eta \mathrm{MAID}$2017 solution is shown by the black solid line.

Figure 3

Combined $\gamma p\to \eta p$ differential cross sections from Run I (the red triangles) compared to the results from Run II (the blue triangles) for the two energy bins in which the largest discrepancies were observed. The meaning of the displayed line and error bars is the same as in Fig. 2.

Figure 4

Combined $\gamma p\to \eta p$ differential cross sections from Run III (the magenta squares) compared to the results (a) from Run II (the blue triangles) and (b)–(d) from previous measurements by CLAS [4] (the black crosses) and by CBELSA/TAPS [5] (the black open circles). The meaning of the displayed line and error bars for Run II and Run III is the same as in Fig. 2. The error bars for the previous measurement represent their statistical uncertainties.

Figure 5

Combined $\gamma p\to {\eta }^{\prime }p$ differential cross sections from Run III (the magenta squares) compared to the previous measurements by CLAS [4] (the black crosses) and CBELSA/TAPS [5] (the black open circles) at four overlapping energy bins. The meaning of the displayed line and error bars is the same as in Fig. 4.

Figure 6

$\gamma p\to \eta p$ total cross sections from this Letter compared to the previous measurements by CBELSA/TAPS [5] and to model calculations by $\eta \mathrm{MAID}$-2003 [13] (the black dotted line), SAID-GE09 [3] (the blue long-dashed-dotted line), BG2014-2 [8] (the magenta long-dashed line). The notation for the new $\eta \mathrm{MAID}$2017 solution and the meaning of the error bars are the same as in Fig. 4. The Regge background and its sum with the contributions from $N1/2^{-}$ resonances are shown by green and black dashed-dotted lines, respectively. The $\eta \mathrm{MAID}$2017 solution from the fit only to the previous $\eta$ and ${\eta }^{\prime }$ data is shown by the black dashed line. The vertical line corresponds to the ${\eta }^{\prime }$ threshold.

Figure 7

$\gamma p\to {\eta }^{\prime }p$ total cross sections from this Letter compared to the previous measurements by CBELSA/TAPS [5], SAPHIR [32], ABBHHM [33], and AHHM [34], and to the model calculations by the $\eta \mathrm{MAID}$-Regge-2003 [19] and $\eta \mathrm{MAID}$2017 solutions (with the same notations as in Fig. 6 and statistical uncertainties shown for the previous measurements). The fit to the CBELSA/TAPS data from Ref. [35] is shown by the cyan solid line. For the final $\eta \mathrm{MAID}$2017 solution, the individual contributions from $N(1895)1/2^{-}$, $N(1900)3/2^{+}$, and $N(2120)3/2^{-}$ are shown by the red dashed-dotted, green long-dashed, and blue long-dashed-dotted lines, respectively.