Comments on "Interference phenomena in the $J^{p} = 1/2^-$ - wave in $\eta$ photoproduction" by A.V. Anisovich et al. [arXiv:1501.02093]

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by  Viacheslav Kuznetsov

Comments on “Interference phenomena in the Jp = 1/2− - wave in η photoproduction”
by A.V. Anisovich, E. Klempt, B. Krusche, V.A. Nikonov, A.V. Sarantsev, U. Thoma,
D. Werthmuller, arXiv:1501.02093v1 [nucl-ex].
Viacheslav Kuznetsov
1Petersburg Nuclear Physics Institute, Gatchina, 188300, St. Petersburg, Russia
The authors of Ref. [1] claimed that “ ... narrow structure observed in the excitation function
of γn → ηn can be reproduced fully with a particular interference pattern in the Jp = 1/2−
partial wave...” while anarrow structurein Compton scattering off theneutronis “...a stand-alone
5
observationunrelatedtothestructureobservedinγn→ηn...”. Thesourceforthesecondstatement
1
may be a simple numerical error. If so, the interpretation of the narrow structure in γn → ηn as
0
interference effects in theJp =1/2−-waveand some conclusions from Ref. [1] are questionable.
2

n
a TheobservationofanarrowenhancementatW ∼1.68 is σ =10.8 nb (i.e. 1000 times larger).
res
J
GeV in η photoproduction [2–5] and Compton scatter- Eventhisnumbermaybepessimistic. Theresultsfrom
6
ingoffthe neutron[6](the so-called“neutronanomaly”) GRAAL and CBELSA/TAPS suggestΓ ≤25 MeV. If
1 tot
raised intensive debates about its nature. One possible to set Γtot =20 MeV then σres =24.3 nb. If in addition
] explanation is a signal of a nucleon resonance with un- toassumethatN∗(1685)isahigher-spinresonance,σres
x usual properties: the mass near M ∼1.68GeV, the nar- may be significantly larger.
e row (Γ≤25 MeV) width, the strong photoexcitation on
- The peak in γn → γn at GRAAL was observed at
l the neutron, andthe suppresseddecayto πN finalstate. ◦
uc A new one-star N∗(1685) resonance was included into 1sc5a7tt.erTinhgemoneatshuerepdrodtiffonereantti1a6l0c◦roasnsdseEction=o1f.C02o5mpGteoVn
the listing of the Particle Data Group [7]. γ
n is 27.1±5.4 nb/str [8]. In accordance with dispersion-
[ On the other hand, severalgroups tried to explain the
1 bteurmfeprenincethoef γwnell→-knηonwncrowsisdseecrteisoonnainncteesr.msTohfetrheeceinnt- rneeluattrioonn (cwalicthuolauttionnasrr[o9w] Creosmonpatnonce)craotss16s0e◦ctainond Eonγ =th1e
v GeV may be significantly (∼ 5 times) smaller than that
1 attempt was done in Ref. [1]. The authors concluded on the proton. The peak of N∗(1685) on the top of the
96 tkhnaotwint creasnonbaenfcuelsl wexhpillaeinthede ibnycltuhseioinntoefrfNer∗e(n1c6e8o5f)wonellly- flat γn → γn cross section could and, if N∗(1685) does
exist, should be seen at GRAAL.
3 deteriorates the data fit.
0 One major challenge for this interpretation is the ob- The observationof the peak in Compton scattering at
1. servationof a narrowpeak at the same energy in Comp- the same energy as in γn → ηn challenges the expla-
0 ton scattering on the neutron at GRAAL [6]. The nation of the neutron anomaly in terms of interference
5 authors of Ref. [1] estimated the total cross section of effects. The specific interference of wide resonances can-
1 N∗(1685) in γn → γn assuming the mass M = 1670 notgenerateanarrowpeakinηphotoproductionwhichis
v: MeV,the widthΓtot =30MeV,and(a priori) the quan- governedby only isospin-1/2 resonances, simultaneously
Xi tum numbers P11. The result σres = 10.8 pb led to a generate a peak at the same energy in Compton scatter-
conclusion that “...this value is far below the sensitivity ing which governed by both isospin-1/2 and isospin-3/2
r of the GRAAL experiment. If it is not a statistical fluc- resonances, and generate neither of peak in γn → π0n
a
tuation, ... it is a stand-alone observation unrelated to which is governed by the same resonances as Compton
the structure observed in γn→ηn...”. scattering.
Unfortunately, there might be a simple numerical er- At present, the only available explanation is the exis-
ror: if to check Eq.(1) from Ref. [1], the correct number tence of N∗(1685).
[1] byA.V.Anisovich,E.Klempt,B.Krusche,V.A.Nikonov, 232001; Phys.Rev.C90, 015205 (2014).
A.V. Sarantsev, U. Thoma, D. Werthmuller, [6] V. Kuznetsovet al., Phys. Rev.C83, 022201 (2011).
arXiv:1501.02093v1 [nucl-ex]. [7] K.A.Olive et al., [the Particle Data Group Collboration],
[2] V. Kuznetsov et al.,Phys. Lett.B 647, 23 (2007). Chin. Phys. C38,090001 (2014).
[3] I.Jaegleetal.,Phys.Rev.Lett.100,252002(2008);I.Jae- [8] Y. Wadaet al., Nucl.Phys. b247, 313 (1984).
gle et al.,Eur.Phys.J. A47, 89 (2011). [9] A. Lvov, V. Petrun’kin, and M. Shumacher, Phys. Rev.
[4] F. Miyahara et al., Prog. Theor. Phys. Suppl. 168, 90 C55, 355, (1997), and A.L’vov, Privatecommunication.
(2007).
[5] D. Werthmulleret al., Phys.Rev.Lett. 111 (2013) 23,