Baryon resonance analysis from SAID

CPC(HEP & NP), 2009, 33(12): 1063- 1068Chinese Physics CVol. 33, No.12，Dec, 2009Baryon resonance analysis from SAID*R. A. Arndt W. J. Briscoe M. W. Paris I. I. Strakovsky1) R. L. Workman(Center for Nuclear Studies, Department of Physics,The George Washington University, Washington, D.C. 20052, USA)Abstract We discuss the analysis of data from πN elastic scattering and single pion photo- and electropro-duction. The main focus is a study of low-lying non-strange baryon resonances. Here we concentrate on somedificulties associated with resonance identification, in particular the Roper and higher P1 states.Key words non- strange baryons, N(1440), pion- nucleon elastic scattering, pion photo- and electro productionPACS 14.20.Gk, 13.30.Eg, 13.75.Gx1 Introductionpoles which are not far away from the physical axis.It is important to emphasize that these resonancesMany of the SAID fts to scattering data haveare not put in by hand, contrary to the Breit-Wignerbeen motivated by ongoing studies of the N(BW) parametrization. The poles arise, in a sense,propertiesl4l. Most of these (for instance, EBAC2],dynamically as a result of the enforced (quasi-) twoGiessen'), DMT4), Jilich!l) have used, as input,body unitarity cuts and the ft to the observable onamplitudes extracted from elastic πN scatteringthe real energy axis. We have, however, also given thedatalo, 71. Our pion photoproduction multipoles areresults of a BW parametrization, mapping x2[Wr, r]also determined using a K -matrix formalism basedwhile searching all other partial-wave parameters byupon rπN partial wave amplitudesl8- -101. Further, thefitting data over a relatively narrow energy range,pion-electroproduction analysis is anchored to oursay 100- -200 MeV. Some subjectivity in the BWQ2 = 0 photoproduction results, with additional facstudy is involved, such as: (i) energy binning, (itors intended to account for the Q2 variation!the strength of constraints (such as dispersion reOne of the most convincing ways to study thelations), and (ii) the choice of partial waves to bespectroscopy of non-strange baryons is through πNsearched. We should stress that the standard PWApartial- wave analysis (PWA). The main sources of thereveals resonances with widths of order 100 MeV, butReview of Particle Physics (RPP) N* Listingsl1l arenot too wide (I >500 MeV) or possessing too smallthe PWA of the KH, CMB, and GW/VPI groups.a branching ratio (BR < 4%), tending (by construc-The analysis of πN scattering data remains crucialtion) to miss narrow resonances with r< 30 MeV.in this effort. Double polarization quantities (R andThe partial waves of solution KA8412] and the single-A) measured long after the KH and CMB analysesenergy solutions (SES) associated with our SP06 rewere completed have found discrepancies in these ear-sults agree reasonably well over the full energy rangelier fits, which weakens claims for the existence andof the SP06 (Figs. 4- 7 from Ref. [6). However, thisproperties of some of the weaker (mainly isospin 3/2)does not lead to agreement on the resonance content.resonances.For instance, our studylo) does not support several N*In the GW DAC πN PWA, we determine πN am-and△* reported by PDG4. It is important here toplitudes by the ftting πN elastic data (up to W =remember that during last 25 years, the πN database2.50 GeV) andπp→→ηn data (up to W = 1.63 GeV). has increased by a factor of 3- 4, and these data wereResonances are then found through a search for polesnot available to the KH and CMB groups.in the complex energy plane. We consider mainly中国煤化工Received 7 August 2009[HCNMHG* Supported by U. S. Department of Energy (DE-FG02-99ER41110 and DE-AUUD-84EH40150)1) E-mail:igor@gwu.edu02009 Chinese Physical Society and the Institute of High Energy Physics of the Chinese Acadery of Sciences and the Instituteof Modern Physics of the Chinese Academy of Sciences and IOP Publisbing Ltd.1064Chinese Physics C (HEP & NP)Vol. 33Table 1 (we use the same methodology in all of our2πN featuresPWAs). This renormalization procedure was also ap-plied to the other non-SAID solutions. Here, how-ever, only the normalization constants were searched2.1 Minimization and normalization factorto minimize x2 (no adjustment of the partial wavesAs in previous analyses, we have used the system-vas possible). Clearly, this procedure can signif-atic uncertainty as an overall normalization factor forcantly improve the overall x attributed to a fit (weangular distributions. Renormalization freedom sig-cannot ignore this experimental input), and has beennificantly improves our best-ft results, as shown in| applied in calculating the x values of Table 1.Table 1. Comparison of x /data values (Norm/Unnorm) for normalized (Norm) and unnormalized (Unnorm)data used in the SP0610 and FA02l1 solutions, Karlsruhe KA842, EBAC2, Giessenl3l, and DMT4. Valuesfor SP06 (FA02) correspond to a 2.46 (2.26) GeV energy limit for W in CM. KA84 is evaluated up to 2.9 GeV,EBAC up to 1.91 GeV, Giessen up to 2 GeV, and DMT up to 2.2 GeV.reactionSP06FA02KA84EBACGiessenDMTx2/datax/datax /datax2 /datax /data .πtp→ntp2.0/6.12.1/8.85.0/24.913.1/23.710.5/17.715.4/37.4π p→π~p1.9/6.22.0/6.69.1/51.94.9/16.012.1/34.19.0/23.0π~p→rIn2.0/4.01.9/5.94.4/8.83.5/6.36.3/15.26.5/16.7πp→nn2.5/9.6_2.5/10.52.2 Roperelastic scatteringh", 81 with“masses”closer to the realpart of the pole position!o, ”". These differences couldDiscovered more than 40 years agolal, this res-reflect the complicated structure described above.onance state has remained controversial for manyyears. The prominent N(1440)P1 resonance is clearlyevident in both KH and GW /VPI analyses (Figs. 4-257 from Ref. [6), but occurs very near the π, ηN, and15pN thresholds (Fig. 8 from Ref. [7), makinga BW fitn1questionable. The N(1440) is unique in that its be-havior on the real energy axis is infuenced by poles-200on diferent Riemann sheets (with respect to the π△-1600cut) as was first reported by Arndt et al.14I. Due to500~1400Im EDMeVthe nearby πO threshold, both P1 poles are not farReE[MeV] 1001200 0from physical region (Fig. 1). There is a small shiftbetween pole positions on the two sheets, due to anon-zero jump at the πO-cut. Our conclusion is thata simple BW parametrization cannot account for sucha complicated structure. This point was also empha-，5sized by Hohler!4. Recent studies by the Jjilichl5 andEBACI15 groups have confirmed the two pole deter-mination. An earlier study by Cutkosky and Wangcame to a similar conclusionl'6l.Following the first indications from PWA stud-18001500r 150ies, evidencel171 for the Roper was found through theReE[MeV-lmE IMeV]analysis of hydrogen bubble chamber events. More re-cent evidence for a direct measurement of the N(1440)has been found using electromagnetic interactions (at中国煤化工p: the ro cutBES inete~→J/ψ→pπ n+nπtpl8) and at JLab inCNMHGandrunsfromep→e'Xl9). Hadronic processes (at SATURNE IIarger t ollules vaucs ul ue real part of thein∞p→x'X(20) and at Uppsala in pp→nprt+|21)energy. Bottom: the π△cut is clearly visiblehave also studied. Some of the peaks found have po-running from smaller to larger values of thesitions diferent from the BW interpretation of πNreal part of the energy..No. 12R. A. Arndt et al: Baryon resonance analysis from SAID1065Overall, most of analyses of N(1440) are basedthe existing ft to pion-nucleon elastic scattering data,on its BW parametrization, which implicitly assumesa candidate energy was found at 1680 MeV with athat the resonance is related to an isolated pole. How-IaN <0.5 MeV.ever, given the complicated structure found in our(i) There are several independent suggestions forPWA, the BW description may be only an effectivethe N(1680)25- -27,parametrization, which could be different in different(ii) Its width is much less than any non-strangeprocesses. Some inelastic data indirectly support thisN1l24- -27,point, giving N(1440) BW masses and widths signif-(iv) The Chiral-soliton approach gives support foricantly diferent from the PDG BW valuesl". ThisN(1680) production in both γp and γrnl28,may also cast some doubt on recent Q2 evaluation(心) The GRAAL γn→nηn cross section measure-resultsl22,. 2), since the Q2-dependences for contribu-ments allow one to determine the radiative widthtions of diferent singularities may be different. Thisof N(1680) and transition magnetic momentuml29)problem can be studied in future measurements withwhich is much smaller than for the O case.JLab CLAS12.2.3 Pii beyond 1500 MeV[PDG06Beyond the Roper resonance, the P1 partial wavewraps around the center of the Argand diagram(Fig. 2) and the total elastic cross section is half theσlottotal cross section (Fig. 3). As a result, small changes8tin the amplitude can produce large changes in thephase, though these changes have little infuence onSP06the ft to data. For πN elastic scattering, we con-clude that there is little sensitivity to resonances in108014301780 2130 2480P1 above 1500 MeV except possible states with smallW (MeV)r。124.Fig. 3. Pi contribution to total and total elas-1.0tic cross sections for SP06. Vertical arrowsindicate resonance WR values and horizon-0.8tal bars show full r and partial TrN widths.P11The lower BW resonance symbols are associ-).6ated with the SP06 values; upper symbols givePDG values, which include higher mass states.0.40.22.4 π p -→nn database puzzle.0-0.20.0Most measurements of the π p - →ηn reactionReAcro88 section are rather old and sometimes conflict-Fig.2. Argand plot for the P11 partial-waveing (Fig. 4). There are few cross section (106 data)armplitude from threshold (1080 MeV) to W =measurements above 800 MeV and no polarized mea-2500 MeV. Crosses indicate 50 MeV steps insurements below 1040 MeViso. A detailed analysisw. The solid circle corresponds to the SP06of the older data can be found in the review by Cla-BW WR.jus and Nefkens[31l. Most NIMROD data do not sat-One may speculate about the existence of a veryisfy a consistency requirement (aystematics are notnarrow P1 state which, as mentioned above, wouldunder control, momentum uncertainties up to 50not be clearly evident in a standard PWA. Such a100 MeV/c, and so on]. For this reason, we are notstate was originally motivated by investigations aim-able to use these data in our π p elastic, π~p- + π°n,ing to explain how a very narrow (less than 1 MeV)and中国煤化工-ing data. In par-pentaquark state could exist. Here we can summa-ticulloes not permit arize our knowledge of one such“narrow”candidate,modeMYH. CN M HG→ηn.N(1680)P:The existing data types and energy limits severely(i) Using a modified PWAl24, designed to searchrestrict any attempt to determine resonance parame-for slots where a very narrow state would not destroyters above the first S11 resonance.1066Chinese Physics C (HEP & NP)Vol. 33.3 T Frakhovos.the CLAS appears especially at forward angles (Fig. 8to Debenham75百160deg.from Ref. [9)). The overall systematic uncertainty for|口Brown79合。the CB-ELSA measurements is stated to be 5% below宣.2兢兢“至1300 MeV and 15% above that energy. This compares:with the roughly 5% systematic uncertainty obtained&,at JLab.吕Moreover, the CLAS π'p measurements and SAIDfit do not confirm the existence of weak states re-SP06 .ported by the BoGa group in a ft to the CB-ELSA550600650750800datalBy.T((MeV)Given the smooth behavior exhibited by the ex-citation functions in Figs. 9 and 10 from Ref.[9], theFig.4. Fixed angle excitation functions forCLAS cross sections provide no hint of“missing" res-π p→ηn. We are not able to use data shownonance structure between 2 and 3 GeV. The SAIDby open symbols in our analysis.fits implicitly contain only those resonances found inthe corresponding SAID analysis of elastic πN scat-3 Pion photo- and electroproductiontering data. No change in the form of the SAIDphotoproduction fht was found to be necessary. InIn ftting the electroproduction database, we ex-contrast, the CB-ELSA ft required many additionalresonance contributions, some of which are 1- andtrapolate from the relatively well determined Q2 =2-star rated PDG states, as well as a new N(2070)0 point. The photoproduction multipoles can beresonance. One possible explanation is apparent inparametrized using a form containing the Born termsFig. 10 from间, which shows the CLAS data to be(no free parameters) and phenomenological piecessomewhat smoother than the CB- ELSA excitationmaintaining the correct threshold behavior and Wat-functions. Model-dependence in the separation of res-son's theorem below the two-pion production thresh-onance and background contributions is also a criti-old. The πN T matrix connects each multipole tocal factor. This uncertainty may be reduced throughstructure found in the elastic scattering analysis. Themeasurements of further (polarized) data.parametrization above two-pion production is basedClearly, additional measurements at forward an-on a unitary K-matrix approach, which no stronggles are needed to determine whether the rapid in-constraints on the energy dependence apart from cor-crease suggested by the most forward CB-ELSA datarect threshold properties.Overall, the difference between MAID andis correct, or whether the behavior suggested by theGW/VPI amplitudes tends to be small but resonancemost recent fits properly describes the CFOS8 section atforward angles. That is critical because the forwardcontent may be esentially different (Figs. 7 and 8measurements are sensitivity to highest N*s (most offroml10l)One reason for differences is database de-these are inelastic).pendent. MAID0722 did not use recent CLAS π°pl .and π+n(10} with LEPS πp[32) backward measure-3.2 π p Photoproductionments. Other differences are tied to different as-sumptions regarding the inclusion of resonance andComplementary measurements of π牛photopro-background contributions. Some rather large difer-duction are required for an isospin decomposition ofences are evident in those wave connected to the pion-the multipoles. There are no prior comprehensivenucleon S11 and D13 partial waves.tagged π p measurements. Final-state-interactionsThere are several isues in pion photoproduction(FSI) play a critical role in a state of the- art analysisabove the O(1232) which require resolution. We con-of the γn→πp data. A preliminary study sug-sider them in the remainder of this section.gests FSI (Fermi motion included) varies between 15and 40% for the CLAS energy range (Ey = 1050 -3.1 Forward π°p photoproduction3500中国煤化工rgy and scatteringFor incident photon energies up to 1.3 GeV, the .angliYHleasurements com-π°p data obtained the CLAS Collaboration!l are foringfC N M H Gdata are ailablethe most part in very good agreement with previ-to accomplish a reliable PWA and determine neutronous measurements. At higher energies, a disagreecouplings. A JLab analysis addressed to these datament between the CB-ELSA133) measurements andis coming from the γd -→π pp experiment (g10 run.No. 12R. A. Arndt et al: Baryon resonance analysis from SAID1067period) ( in progress). The diference between previ- is the result of MAMI-B for π°p and Bonn withous and CLAS measurements may result in signifcantMAMI-B πtn activity, and finally, the 5th jumpchanges for the neutron couplings.(2003一2007) depends from MAMI-B for both π°pand n+n.3.3 Pion electroproductionA major pion electroproduction database problemOngoing fits incorporate all available electropro-is that most data are from unpolarized measurements.duction data, with modifications to our fitting pro~There are no π'n data and very few π P data (nocedure implemented as necessary (Table 2). We notepolarized measurements). This does not allow a rig-that the CLAS Collaboration produced 85% of theorous neutron coupling evaluation vs. Q*. The Q2world pion electroproduction data, much of whichdistribution of available data is shown in Fig. 6.was focused on the mapping of the properties of the△(1232) resonance. Useful comparisons will requireof Datapπ°those involved in this effort to make available all am-30000plitudes obtained in any new determination of REMand RsM for the O(1232) which may be comparedwith LQCD calculations35l.10000Table 2. GW N* program.reactiondataQL(Gev可)r°p→π°p55,76681,28440000# of Datanπ*51,31280,004redundant14,77217,375total124,453178,663γp→πN24,88850,68420000all photo159,341229,317πN→πN31,87657,255all πN .191,217286,572Q(Gev40000 .-120# ofDatan71互王甄克堂全强甄A1/z+-1502000PDG-180|: ER/vPI-210-240Fig. 6. Q2 distribution of pion electroproduc-198519911997 2003 2009 2015tion data which are now available.Fig. 5. Time variation of the A1/2 and A3/2proton coulpings for the 0(1232).4 Summary and prospectsOf all resonances, one might asume that theLet us, in the interest of clarity, summarize where△(1232) properties are know to great precision. Un- the analysis of the single meson productions reactionsfortunately, this is not really true (Fig. 5). The PDGaverage values look stable while our determinationi) πN analysis is crucial for the N° program,depends on the database. The first jump (1990-中国煤化工production anal-1993) is a88ociated with the π°p LEGS activity, the yses;MHc N M H Gduction analyes2nd jump (1993 - 1996) is the product of the MAMI-B for π'p and Bonn πtn activity, the 3rd jumpare done up to W = 1640 MeV.(1996- 1997) depends again from MAMI-B for π°pLooking forward, our eforts will be focused on theand Bonn π+n activity, then 4th jump (1997- 2003) following important issues.1068Chinese Physics C (HEP & NP)Vol. 33i) Production measurements on the“neutron" tar-Finally, issues which will receive further attentionget are necessary to determine neutron couplings atare as follows.Q2= 0,i) Q2 evaluation of resonance couplings up to veryi) Future improvement will be possible withlarge Q2,future measurements of spin observables at JLab,ii) The critical question is can we reach an asymp-MAMI-C, LEPS, LNS, and CB-ELSA,totic regime as pQCD predicted?ii) Complete experiments make possible a directii) Neutron electroproduction measurements arereconstruction of helicity amplitudes for pion and etanecessary to determine neutron couplings at Q2> 0.photoproduction.References18 Ablikim M et al (BES Collb.). 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