Chiral constituent quark model study of the process γp → ηp

Abstract

Within a chiral constituent quark model approach, η-meson production on the proton via electromagnetic and hadron probes are studied. With few parameters, differential crosssection and polarized beam asymmetry for γp → ηp and differential cross section for π − p → ηn processes are calculated and successfully compared with the data in the centerof-mass energy range from threshold up to 2 GeV. The five known resonances S 11 (1535), S 11 (1650), P 13 (1720), D 13 (1520), and F 15 (1680) are found to be dominant in the reaction mechanisms in both channels. Possible roles plaied by new resonances are also investigated and in the photoproduction channel, significant contribution from S 11 -and D 15 -resonances, with masses around 1715 MeV and 2090 MeV, respectively, are deduced. For the so-called missing resonances, no evidence is found within the investigated reactions. The helicity amplitudes and decay widths of N * → πN, ηN are also presented, and found consistent with the PDG values.both differential cross section [2, 3,4,5], and polarized beam asymmetry [5,6]. The situation is very different for the π − p → ηn reaction. Actually, the data come mainly from measurements performed in 70's [7,8,9,10,11,12] and suffer from some inconsistencies [13]. A recent experiment, performed at BNL using the Crystal Ball spectrometer [14], offers a high quality data set, though limited to the close to threshold kinematics. Consequently, a combined data base embodying experimental results for both electromagnetic and strong channels turns out to be highly heterogeneous. In spite of that uncomfortable situation, recent intensive theoretical investigations interpreting both channels within a single approach has proven to be fruitful in revealing various aspects of the relevant reaction mechanisms, as discussed, e.g., in Refs. [1,15].In the photoproduction sector, a significant progress has been performed in recent years within coupled-channels formalisms [16,17,18,19] allowing to investigate a large number of intermediate and/or final meson-baryon (MB) states: γN → M B, with M B ≡ πN, ηN, ρN, σN, π∆, KΛ, KΣ.Those approaches have been reviewed in our recent paper [1]. Also advanced coupled-channels approaches are being developed [15,20,21,22] for the strong channels: πN → M B. However, fewer studies embody both electromagnetic and strong production processes. Moreover, those works are based on the effective Lagrangian approaches (ELA), where meson-baryon degrees of freedom are implemented, (see e.g. Refs. [17,18,23,24]). Investigations based on subnucleonic degrees of freedom, via constituent quark models (CQM) have been successful [1,25,26,27,28,29] in the interpretation of photoproduction data on the proton, namely, γp → πN, ηp, KΛ, and a recent work [30] has considered the π − p → ηn reaction.At the present stage, the ELA and the CQM approaches are complementary. However, the QCDinspired CQM developments deal on the one hand with more fundamental degrees of freedom and on the other hand require a much smaller number of adjustable p...