Photoproduction off the nucleon revisited: Evidence for a narrow N(1688) resonance?

0.16 MB·English

by  V. Kuznetsov

EPJ manuscript No.
(will be inserted by the editor)
η Photoproduction off the nucleon revisited: Evidence for a
narrow N(1688) resonance?
8 V. Kuznetsov1,2, M.V. Polyakov3,4, T. Boiko5, J. Jang1, A. Kim1, W. Kim1, A. Ni1, and G. Yang3
0
0 1 Kyungpook National University,702-701, Daegu, Republicof Korea,
2 2 Institutefor Nuclear Research, 117312, Moscow, Russia,
3 Institutefu¨r Theoretische Physik II,Ruhr-Universit¨atBochum, D - 44780 Bochum, Germany,
n 4 St.Petersburg Institutefor Nuclear Physics, Gatchina, 188300, St.Petersburg, Russia,
a
J 5 Belarussian State University,220030, Minsk, Republicof Belarus.

5
the dateof receipt and acceptance should beinserted later

]
x
Abstract. Revised analysis of Σ beam asymmetry for the η photoproduction on the free proton reveals
e
a structure at W ∼ 1.69 GeV. Fit of the experimental data based on the E429 solution of the SAID
-
p partialwaveanalysissuggests anarrow(Γ ≤25MeV)resonance.Possible candidatesare P11,P13,orD13
e resonances. The result is considered in conjunction with the recent evidence for a bump-like structure at
h W ∼1.67−1.68 GeV in thequasi-free η photoproduction on theneutron.
[

1 PACS. 1 3.60.Le – 1 4.20.Gk
v
8
7 The evidence for a narrow resonant structure at W ∼ M(η,n) (Fig. 1). Such bump is not (or poorly) seen in
7 1.67−1.68inthequasi-freeηnphotoproductionatGRAAL the cross-section data for η photoproduction on the free
0 [1,2], CB/TAPS@ELSA[3], and LNS-Tohoku [4] facilities proton [5].
1. isoneofthemostimportantrecentfindingsinthedomain Thewidthofthebumpintheγn→ηncrosssectionis
0 of the physics of nucleon resonances. The structure has close to that expected due to Fermi motion of the target
8 been observed as a relatively narrow bump in the quasi-
neutronboundinthedeuteron.Anarrowresonancewhich
0 free cross section and in the ηn invariant mass spectrum
would manifest itself as a peak in the free neutron cross
:
v section, would appear in the quasi-free cross section as
i a bump of ∼ 50 MeV(FWHM) width [1] (Fig. 1). The
X
M(η,n) spectrum is much less affected by Fermi motion
ar b/str 00..78 1 Tbuhteionbcsluerdveesdlawrgidetrhu,nocfe∼rta5i0ntMieesVd(uFeWtoHdMet)e,citsocrlorseesptoontshee.
m, 0.6 0.8 instrumental resolution [1].
W/d 0.5
sd 0.4 0.6 Several attempts to explain the observed bump have
0.3 0.4 beenrecentlydone.Theauthorsof[6,7,8]suggestedanar-
0.2 rowP11(1675)resonance.Alternatively,theauthorsof [9,
0.2
0.1 10]explainedthe observedbumpinterms ofphotoexcita-
b/str 01 1.5 1.6 1.7 1.8 1.91000 1.5 1.6 1.7 1W.8, G1e.V9 toironS1a1n(1d53in5t)erafnedreSnc11e(1o6f5t0h)e[1S01]1r(e1s6o5n0a)ncaensd. P11(1710) [9]
m,
Wd 0.8 80 The quasi-free cross section is smeared by Fermi mo-
/
sd 0.6 60 tion of the target neutron and is affected by re-scattering
andfinal-stateinteraction(FSI).Thoseeventswhosekine-
0.4 40
matics is relatively strongerdistortedby Fermi motion or
0.2 20
those which originate from the re-scattering and FSI, are
0 0 in part eliminated in the data analysis. This procedure
1.5 1.6 1.7 1.8 1.9 1.5 1.6 1.7 1.8 1.9
W,GeV Mh n, GeV necessarily depends on experimental setup and on cuts
used in data analysis. Therefore the quasi-free cross sec-
Fig. 1. Quasi-free cross sections and ηn invariant mass spec-
tion measured in experiment may deviate from the cross
trum for the γn → ηn reaction obtained at GRAAL (data
section calculated for the free neutron and then smeared
from Ref.[1]). Solid lines fit the data by the sum of a 3-order
by Fermi motion. M(η,n) is almost unaffected by Fermi
polynomialandanarrowstate. Dashedlinesare3-orderpoly-
motion.ItsspectrumexhibitsanarrowpeakatM(η,n)=
nomialsonly.Darkareasshowthesimulatedsignalsofanarrow
(Γ =10 MeV) resonance.
2 V.Kuznetsov et al.: η Photoproduction off the nucleon revisited: Evidence for a narrow N(1688) resonance?
1.678GeV. This peak seems not to be reproducedby cal-
culations [9,10].
Thus the bump in the η photoproduction on the neu-
tronmaysignalanucleonresonancewithunusualproper-
ties: a possibly narrowwidth and a much stronger photo-
coupling to the neutronthan to the proton.Its identifica-
tion is now a challenge for both theory and experiment.
Ifphotoexcitationofanyresonanceoccursontheneu-
tronitshouldgenerallyoccuralsoontheproton,evenbe-
ing suppressed by any reason. The η photoproduction on
theprotonbelowW ∼1.7GeVisdominatedbyexcitation
oftheS11(1535)resonance.Anarrowweakly-photoexcited
state with the mass below 1.7 GeV would appear in the
crosssectionasasmallpeak/dipstructureontheslopeof
the dominant S11(1535) resonance. This structure would
be smeared in experiment by resolution of a tagging sys-
tem (for example, the resolution of the GRAAL tagging
systemis 16MeVFWHM [11]), andmightbe hidden due
to inappropriate binning.
Polarization observable - the polarized photon beam
asymmetryΣ,ismuchlessaffectedbytheS11(1535)reso-
nance.ThisobservableisthemeasureofazimuthalanisotropyFig. 2. Beam asymmetryΣ fortheη photoproductiononthe
ofthe reactionyieldwhentheincomingphotonislinearly free proton obtained with narrow energy bins (black circles).
polarized. Σ beam asymmetry is much more sensitive to Open circles are data from Ref.[15]. Stars are our results at
116◦ obtained using the same angular binning as in Ref.[15].
signalsofnon-dominantresonancesthanthecrosssection.
Solid lines show our calculations based on the SAID multi-
For the η photoproduction on the proton, the beam
asymmetry Σ has been twice measured at GRAAL. The poles only, dotted lines include the P11(1688) resonance with
the width Γ = 8 MeV; dashed lines are calculations with the
first results[12] covered the energy range from threshold
P13(1688) (Γ = 8 MeV), while the dash-dotted lines use the
to1.05GeV.Twostatistically-independentbutconsistent
resonance D13(1688), also with Γ =8 MeV.
data sets have been reported. The data sets were based
on two different samples of events: i) Events with both
60 MeV wide energy bins. Such wide bins do not allow
photons from η → 2γ decays detected in the BGO Ball;
to reveal narrow peculiarities in the energy dependence
ii) Events in which one of the photons, being emitted at
the angles θ ≤25◦, was detected in the forwardshower of Σ. An ultimate goal of this work is to produce beam
lab asymmetries using narrow bins in energy, in order to re-
wall,andtheotherwasdetectedintheBGOball.Thesec-
trieve in detail the photon energy dependence of Σ for
ond type of events was found to be particularly efficient
E = 0.85−1.15 GeV (or W = 1.55−1.75 GeV) and to
atforwardanglesandenergiesabove0.9GeV.Theresults γ
haveshownamarkedpeakingatforward(∼40−50◦)an- search for a signal of a narrow resonance.
gles and E ∼1.05 GeV. An extension to higher energies In this contribution we present the revised analysis of
γ
upto1.5GeVhasbeenreportedinRef.[13].Twosamples data collected at the GRAAL facility in 1998 - 1999. In
of events were merged and analyzed together. This made general,the data analysisis the same as in Ref. [13]. Two
it possible to significantly reduce error bars at forward types of events, as described above, are merged and used
angles and to retrieve a maximum in the angular depen- together to extract beam asymmetries.
dence at 50◦ and Eγ ∼ 1.05 GeV. A new measurement The results are shown in Fig. 2. Data points are ob-
has been done by the CB/TAPS Collaboration using a tainedusingnarrowenergybins∆E ∼16MeV.Angular
γ
different technique of the photon-beam polarization, the bins are chosen to be rather wide, about (20−40)◦, to
coherent bremsstrahlung from diamond radiator[14]. Re- gain statistics and hence to reduce error bars.
sults are in good agreement with Refs. [12,13]. Atforwardangles(θ =43◦)andE =1.04GeVthe
Very recently a new data attributed to the GRAAL cm γ
data points form a sharp peak with Σ in its maximum as
facility,hasbeen publishedin Ref.[15].This data is quite ◦
large as 0.94. The peak becomes less pronounced at 65 .
similar that presented in Ref. [13] but, despite the triple ◦ ◦
It is replaced by an oscillating structure at 85 and 106 .
increase of statistics, is less accurate at forward angles.
Atmorebackwardangles,theasymmetryabove1.05GeV
The reason is that the second type of events described
above, has not been used in the data analysis1. dropsdownalmostto 0(Fig.2)while its statisticalerrors
grow up.
In Refs. [12,13,14] the main focus has been done on
the angular dependencies of Σ. Data points have been The peak at forward angles and the oscillating struc-
producedusingrelativelynarrowangularbins, butnearly tureatcentralanglestogetherexhibitaninterferencepat-
tern which may signal a narrow nucleon resonance. To
1 ThecomparisonofΣbeamasymmetriesfromRef.[13]and examine such assumption, we employ the multipoles of
Ref. [15] can befound in the slide 18 of Ref. [16]. the recent E429 solution of the SAID partial-wave analy-
V.Kuznetsov et al.: η Photoproduction off the nucleon revisited: Evidence for a narrow N(1688) resonance? 3
sis (PWA) [17] for η photoproduction, adding to them a in the cross section of the η photoproduction on the neu-
narrow Breit-Wigner resonance (as in Ref.[7]). tron, where its photoexcitation is much stronger.
ThenarrowS11,P11,P13,andD13resonancesaretried It is a pleasure to thank the staff of the European
one by one. Each resonance contribution is parametrized SynchrotronRadiationFacility(Grenoble,France)forsta-
by the mass, width, photocouplings (multiplied by the ble beam operation during the experimental run. Special
square root of the ηN branching), and the phase. These thanks to I. Strakovsky for many discussions, encourage-
parameters are varied to achieve the best agreementwith ment and help with SAID data base. We are thankful to
experimental data. The curves with the original SAID Y. Azimov, A. Fix, K. Goeke, and L. Tiator for many
multipoles are smooth and do not exhibit any structure valuable discussions. P. Druck is thanked for support in
(Fig. 2). The inclusion of either P11, or P13, or D13 al- data processing. This work has been supported in part
lows to improve agreement between the data and cal- by the Sofja Kowalewskaja Programme of Alexander von
culations and to reproduce the peak/dip structure. The Humboldt Foundation and in part by Korean Research
mass of the included resonance is strongly constrained Foundation.
by experimental data. Its value belongs to the range of
M =1.682−1.690 GeV. The best agreement with data
R
corresponds to the width Γ ∼ 8 MeV. However, reason- References
able curves may be obtained with Γ up to 25 MeV.
The S11 resonance generatesa dip at43◦ in the entire 1. V. Kuznetsov et al., Phys. Lett. B647, 23 (2007),
range of variation of its photocoupling and phase. This [hep-ex/0606065]; hep-ex/0409032; hep-ex/0601002.
indicatesthattheobservedstructures,mostprobably,can 2. P. Levi Sandriet al.,Int.J. Mod. Phys. A22, 341 (2007).
not be attributed to a narrow S11 resonance. 3. H. Schmieden, Talk at at the International
The calculated cross section is weakly affected by the Workshop on the Physics of Excited Baryons
addedresonances:theP11generatesasmallpeak/dipstruc- NSTAR2007, Bonn, Germany, September 5 - 8 2007,
ture near W ∼1.69 GeV while the P13 and the D13 reso- http://nstar2007.uni-bonn.de.
nances produce almost no effect. This explains while the 4. H.Shimizu,Talkat attheInternationalWorkshoponthe
possible underlying resonance is not (or poorly) seen in PhysicsofExcitedBaryonsNSTAR2007,Bonn,Germany,
the free-proton cross section data. September5 - 8 2007, http://nstar2007.uni-bonn.de.
Themassestimatefortheunderlyingresonanceisabout 5. F.Renardet al.,Phys. Lett.B528,215(2002);M.Dugger
5-10 MeV higher than the value obtained in η photopro- et al.,Phys. Rev. Lett. 89,222002 (2002); V.Credeet al.,
Phys. Rev. Lett. 94, 012004 (2004).
duction on the neutron. This could be explained by the
6. L. Tiator, Int. J. Mod. Phys. A22, 297 (2007),
nuclear shift of the narrow resonance mass M:
[nucl-th/0610114].
<P2 > 7. A.Fix,L.Tiator,andM.Polyakov,Eur.Phys.J.A32311
∆M =− F ∼−5 MeV, (1)
(2007),[nucl-th/0702034].
2M
8. K.- S. Choi, S. Nam., A. Hosaka, and H.- C. Kim, Phys.
where PF is the Fermi momentum in the deuteron. Lett. B636, 253 (2006), [hep-ph/0512136].
ItisworthtonotingthattheauthorsofRef.[15]found 9. V.Shklyar,H.LenskeandU.Mosel,Phys.Lett.B650,172
“... no evidence for a narrow P11(1670) state...” in the (2007), [nucl-th/0611036].
beam asymmetry data. In Fig. 2 our data and the data 10. A.Anisovich,TalkatattheInternationalWorkshoponthe
from Ref. [15] are plotted together. Both data sets are PhysicsofExcitedBaryonsNSTAR2007,Bonn,Germany,
consistent. The major difference is that we observe a dip September5 - 8 2007, http://nstar2007.uni-bonn.de.
structure at 103◦. The authors of Ref. [15] show the data 11. General description of the GRAAL facility is available in
at116◦wheretheydonotobserveanydipstructure.How- V. Bellini et al.,Eur. Phys. J. A26, 299 (2006).
◦ 12. J. Ajakaet al., Phys. Rev. Lett. 81, 1797 (1998).
evernoreliabledatacanbe producedinthis(116 )angu-
13. V. Kuznetsov et al., πN NewsLetters 16, 160
larbin. At the photonenergy1.05GeV recoilprotonsare
(2002); Data are available in the SAID data base at
emitted into a gap between the forward and the central
http://gwdac.phys.gwu.edu.
part of the GRAAL detector where they cannot be prop-
14. D. Elsner et al., Eur. Phys. J. A33, 147 (2007),
erly detected. A question to be addressed to the authors
[nucl-ex/0702032].
of Ref. [15]: whether do they observea dip structure near
15. O. Bartalini et al., Eur. Phys. J. A33, 169 (2007), [nucl-
100◦? 2
ex:0707.1385].
In summary, we report an evidence for a narrow reso-
16. V. Kuznetsov et al., Talk at at the International
nance structure in the Σ beam asymmetry data for the η
Workshop on the Physics of Excited Baryons
photoproduction on the free proton. This structure may NSTAR2007, Bonn, Germany, September 5 - 8 2007,
manifestanarrowresonancewiththemassM ∼1.688GeV http://nstar2007.uni-bonn.de.
andthewidth Γ ≤25 MeV.Ascandidates,narrowS11,P11,17. R. A. Arndt, W. J. Briscoe, I. I. Strakovsky, and
P13,andD13resonancesaretried.Amongthem,eitherthe R. L. Workman,in progress, http://gwdac.phys.gwu.edu.
P11, or P13, or D13 resonance improves the description of
the data. Most probably, the same resonance is observed
2 Some consideration of this issue is given in the slide 19 of
Ref. [16].