First measurement of the $\Sigma$ beam asymmetry in $\eta^{\prime}$ photoproduction off the proton near threshold

Abstract

Abstract. The Σ beam asymmetry in η photoproduction off the proton was measured at the GrAAL polarised photon beam with incoming photon energies of 1.461 and 1.480 GeV. For both energies the asymmetry as a function of the meson production angle shows a clear structure, more pronounced at the lowest one, with a change of sign around 90• . The observed behaviour is compatible with P -wave D-wave (or S-wave F -wave) interference, the closer to threshold the stronger. The results are compared to the existing state-of-the-art calculations that fail to account for the data.The experimental study of nucleon excited states is fundamental for the understanding of its internal structure. Important differences are still observed today be- . Recent reports on advances in the experimental studies of the excited nucleon state spectrum can be found in [9,10].Several states predicted by these models have not been observed (missing resonances). The nucleon excited states decay strongly with meson emission; therefore meson photoproduction experiments off the nucleon are an ideal way of searching for missing resonances and complement the information obtained with pion-nucleon scattering experiments.

Study of η and η′ Photoproduction at MAMI

“…Both the exact shape of the cusp in the η photoproduction and the steepness of η 0 photoproduction at threshold are strongly correlated with the properties of Nð1895Þ1=2 − (having only a two-star status [14]), allowing their extraction with good accuracy. The impact of the new data from this Letter on constraining the Nð1895Þ1=2 − properties is illustrated by comparing two fits of the ηMAID2017 model to the data available for η and η 0 photoproduction on protons [1,2,[4][5][6]16], which were made before and after including the new data. As seen in Figs.…”

“…So far, the existing experimental data for η 0 photoproduction, provided by CLAS [4,15] and CBELSA/TAPS [5], are quite scarce, especially in the near-threshold region. Recently, the threshold region attracted more attention after the first results for the beam asymmetry Σ were presented by GRAAL [16], which, although limited in statistics, could not be reproduced by any of the existing models describing η 0 photoproduction [17][18][19][20][21]. The threshold at W ¼ 1896 MeV for the γp → η 0 p reaction is located in a mass region that plays a key role for our understanding of the nucleon spectrum.…”

On the spin-observables in pseudoscalar meson photoproduction

“…Although, in general, we do not expect the higher partial-waves to influence the results at low energies close to threshold, the issue of when the higher partialwaves start to become significant as the energy increases depends, actually, on the particular dynamics of the reaction processes. For example, in η ′ photoproduction the measured beam asymmetry (Σ) exhibits a nearly cos θ sin 2 θ dependence at an excess energy of Q ≡ W − W thr = 7 MeV only [25], which may indicate a possible contribution of even higher partial-waves than the D-wave. Moreover, the observed angular dependence becomes less pronounced at Q = 16 MeV.…”

“…Eqs. (22,24,25,26,29,30,39,49) ] yield (using the commonly adopted notations as given in the previous section)…”

“…Since Ref. [25] reports only the beam asymmetry Σ (at W = 1903 and 1912 MeV) and not the cross section, we have multiplied Σ by the cross section measured by the A2 Collaboration at MAMI [44] (at W = 1904 and 1912 MeV, respectively) to obtain the corresponding unnormalized beam asymmetries. In Fig.1, we (79) to zero, except for a 3 ; for the cross sections, they are the same as the solid blue curves, except for setting a 3 = 0.…”