New narrow nucleon N* (1685)

Abstract

We argue that the existence of a new narrow (Γ ≤ 25 MeV) nucleon resonance N * (1685) is strongly supported by recent data on η photoproduction off the nucleon. The resonance has much stronger photo-coupling to the neutron than to the proton. This nucleon resonance is a good candidate for the non-strange member of the exotic anti-decouplet of baryons -the partner of the pentaquark Θ + . All up to date known properties of new N * (1685) are summarized. PACS: 13.60.Le, 14.20.Gk 1) For brevity we call this enhancement "neutron anomaly".

Partial wave analysis

“…The "discovery" in 2003 of Θ + (1540), an exotic baryon with positive strangeness stimulated further studies [142][143][144][145] and the search for its non-strange partner. Indeed, a narrow structure was observed in the reaction γn → ηn studied at GRAAL [146].…”

Photoproduction of Mesons

“…Around 1680 MeV at the closely spacedKΣ and ωp thresholds, a narrow peak structure is observed in η-photoproduction off the neutron [19][20][21]. This seems associated with a dip structure off the proton at the same energy, in particular at forward η angles [22], and a pronounced structure in the photon beam asymmetry off the proton at central angles [23]. In the φp photoproduction channel, an unexpected narrow peak is observed in the vicinity of the almost degenerate K * Λ g.s.…”

Present status on experimental search for pentaquarks

“…Whereas, accounting for the SU (3) flavor symmetry violation, the photoexcitation can also occur on the proton, even if it may be highly suppressed. With the data collected at the GRAAL facility in 1998-1999, a narrow resonance in the region from 1550 to 1750 MeV/c 2 was searched for through the analysis on the beam asymmetry for the η meson photoproduction on the free protons [104][105][106]. The photon polarization degree varied from 0.5 to 0.85 depending on the energy.…”