aa r X i v : . [ nu c l - t h ] A ug Notes on New Narrow N ∗ Maxim V. Polyakov
Institute for Theoretical Physics II , Ruhr-University Bochum, D-44780 Bochum, GERMANY
We briefly discuss the most recent evidences for narrow nucleon excitation (N ∗ ) with massaround 1680 MeV. The data show that the N ∗ should have much stronger photocoupling tothe neutron than to the proton. That makes it a good candidate for the anti-decuplet member. Theoretical predictions for anti-decuplet N ∗ In this short contribution we discuss fresh evidences for the nucleon from the anti-decuplet[1]. A detailed account for predictions and evidences for new narrow nucleon can be foundin Ref. [2]. Main properties of N ∗ from the anti-deculpet which were predicted theoreticallyin years 1997-2004 are the following:• Quantum numbers are P ( J P = + , isospin= ) [1].• Narrow width of Γ ≤
40 MeV [1, 3, 5].• Mass of M ∼ − π N coupling is suppressed, N ∗ prefers to decay into η N , K Λ and π ∆ [1, 3, 5]. N ∗ in γ n collisions In the γ n collisions (with non-suppressed exit channels such as η n , γ n , K S Λ , etc.) thesignal of the anti-decuplet nucleon should be seen as a prominent narrow peak in thecross section [6]. However, the neutron is bound in a nucleus, hence the narrow resonancesignal is hidden by nuclear effects (by the Fermi motion at the first place ). Four groups- GRAAL [9, 10], CBELSA/TAPS [11], LNS [12], and Crystal Ball/TAPS [13] - managed toovercome this difficulty and reported evidence for a narrow structure at W ∼ η photoproduction on the neutron (neutron anomaly ).In year 2011 more results on the neutron anomaly were obtained. In Ref. [14] the neutronanomaly was also observed in the Compton scattering – the study of quasi-free Comptonscattering on the neutron revealed a narrow ( Γ = ±
12 MeV) peak at ∼ ∼ σ . Such peak is absent in the proton Compton scattering. Observation of the neutron anomaly in the η photoproduction off He [7] excludes other nuclear effects. The name “neutron anomaly" was introduced in Ref. [8] to denote the bump in the quasi-free γ n → η n cross section around W ∼ γ p → η p cross section. IV International Conference on Hadron Spectroscopy (hadron2011), 13-17 June 2011, Munich, Germany s [ m b ] W [ MeV ] s ( h p), free proton s ( h p), quasi-free proton3/2 s ( h n), quasi-free neutron s n / s p W [ MeV ] Figure 1:
Figure from Ref. [15]. Total cross sections as function of final state invariantmass W with cut on spectator momentum p s ≤
100 MeV. (Red) dots: quasi-free neutron,(blue) squares: quasi-free proton, (green) stars: free proton data. Insert: ratio of quasi-freeneutron - proton data.
In Ref. [15] the de-folding of the Fermi motion in quasi-free η photoproduction off neutronhas been performed. As a result the data exhibit pronounced narrow ( Γ = ±
12 MeV)peak at W ∼ γ n → η n shown in Fig. 1.Looking at this figure, the first natural hypothesis is that the peak is due to contribution ofa narrow nucleon resonance. However, due to the very negative attitude of the communityto narrow pentaquarks (see e.g. [16]) one tries to find another explanation for the neutronanomaly first. Detailed discussion of the “conventional explanations" can be found inRef. [2]. Some of them are refuted already by recent experimental data of Refs. [14, 15].Here we touch presently popular results of Ref. [17] only. Ref. [17] attributes the peak inthe neutron channel (see e.g. Fig. 1) to the KY threshold cusp effects.A dedicated experimental search of the KY threshold cusp effects was performed in Ref. [18].A very small effect was found. Our studies (in preparation) showed that if the peak inFig. 1 is due to the cusp effects it would imply that S ( ) resonance must have extraor-dinarily large coupling to KY channels, in acute disagreement with flavour SU(3). More-over several questions to cusp effects of Ref. [17] remain unanswered: 1) Why the neutronanomaly is absent in the pion photoproduction? 2) What is the physics reason for very finecancelation (fine tuning) of the KY threshold cusp effects in the proton channel? N ∗ in γ p collisions The first search of the putative anti-decuplet nucleon in γ p → η p process was performedin Refs. [2, 8]. It was found that the beam asymmetry Σ exhibits a sharp structure around W ∼ IV International Conference on Hadron Spectroscopy (hadron2011), 13-17 June 2011, Munich, Germany observable extracted value refs. (neutron data) refs. (proton data)mass (MeV) 1680 ±
15 [9–15] [3] ⋆ ) [2, 8, 20, 22] [3] ⋆ ) Γ tot (MeV) ≤
40 [9–15] [3] ⋆ ) [2, 8, 20, 22] [3] ⋆ ) Γ π N (MeV) ≤ ⋆ ) [3] ⋆ ) p Br η N A n ( − GeV − ) p Br η N A p ( − GeV − ) Table 1:
Our estimate of properties of the putative narrow N ∗ extracted from the data. ⋆ ) In Ref. [3] the elastic π N scattering data were analyzed and the tolerance limits for N ∗ parameters were obtained. The preferable quantum numbers in this analysis are P . ence of a narrow resonance with a smooth background. The observed structure was iden-tified in Refs. [2, 8] with the contribution of a resonance with mass M ∼ Γ ≤
25 MeV, and small photo-coupling of p Br η N A p ∼ ( − ) · − GeV − .About an year ago the Crystal Ball Collaboration at MAMI published high precision dataon η photoproduction on the free proton [19]. The cross section was measured in fine stepsin photon energy. The measured cross section exhibits an oscillating with energy structurearound 1690 MeV. The best fit to the data was achieved with a new version of SAID (GE09)[19]. However, inspection of this fit reveals a systematic deviation of data from the fitcurves in the 1650 − M ∼ Γ ≤
50 MeV, anda small resonance photo-coupling in the range of p Br η N A p ∼ ( − ) · − GeV − .In this case no PWA of the data was performed as needed to decide whether or not aresonance occurs in a certain partial wave.Such PWA was performed in Ref. [22]. A fit using only known broad resonances and stan-dard background amplitudes can not describe the relatively narrow oscillating structurein the cross section in the mass region of 1660-1750 MeV. An improved description of thedata can be reached by either assuming the existence of a narrow resonance at a mass ofabout 1700 MeV with small photo-coupling or by a threshold effect. In the latter case theobserved structure is explained by a strong (resonant or non-resonant) γ p → ω p couplingin the S partial wave. When the beam asymmetry data of Refs. [2, 8] are included inthe fit, the solution with a narrow P state is preferred. In that fit, mass and width ofthe putative resonance converge to M ∼ Γ ∼
40 MeV, respectively, and thephoto-coupling to p Br η N A p ∼ · − GeV − .In Table 1 we summarize our estimates of the properties of the narrow N ∗ which can beextracted from the present data. The obtained values fit neatly to the predicted propertiesof the anti-decuplet N ∗ . Future experiments, especially on double polarization neutronobservables, will show whether an analogous Table will appear in PDG.3 IV International Conference on Hadron Spectroscopy (hadron2011), 13-17 June 2011, Munich, Germany
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