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EPJ manuscript No. (will be inserted by the editor) Probing the nucleon structure with CLAS Highlights of recent results. 8 0 Volker D. Burkert, for the CLAS collaboration. 0 Jefferson Lab, Newport News, Virginia, USA 2 n February 2, 2008 a J Abstract. An overview of recent results with CLAS is presented with emphasis on the nucleon resonance 5 program and related topics. 2 ] PACS. 1 1.55.Fv, 13.60.Le, 13.40.Gp, 14.20.Gk x e - l 1 Introduction c u n [ The beauty of the electromagnetic probe is that it allows us to efficiently address the central question of hadron 2 physics:Whataretherelevantdegreesoffreedomatvary- v 3 ing distance scales? Using electron beams we can vary 0 the space-time resolution and momentum transfer to the 7 nucleon independently. In doing so we probe the effec- 1 tive degrees of freedom in the nucleon from hadrons,con- 1. stituent quarks, to elementary quarks and gluons. The 1 study of nucleon resonance transitions, which is the fo- 7 cus of this workshop, provides a testing ground for our 0 understanding of these effective degrees of freedom. Us- : ing the SU(6)⊗ O(3) classification scheme of the sym- v i metricconstituentquarkmodel(CQM),theknownstates X can be sorted into supermultiplets of energy and orbital r angular momentum of the 3-quark system. In this talk a I will highlight some of the new CLAS results for the N∆(1232) transition, and for some of the higher excited statesofthenucleon.Thesedataallowustoaddressques- Fig. 1. Magnetic form factor for theN∆ transition. tions about the underlying degrees of freedom of some of the well known states such as the Roper P11(1440), and S11(1535), both of which have also been presented can learn from them about the generalized parton distri- using non-quark degrees of freedom. Studying the reso- butions (GPDs). nancetransitionswillallowustomakemoredefinitestate- ments about the nature of these states. Then I will discuss the well known problem of the ”missing states”, i.e. 2 The N∆(1232) transition resonances predicted within the SU(6)⊗O(3) symmetry. Specific mass ranges are predicted in explicit models that The N∆(1232)transitionhas been studied for more than break degeneracies through spin-spin interactions. How- 50yearswith various probes.But only in the pastdecade ever, many of these states have not been identified in ex- havethe experimentaltools inelectronscatteringbecome perimental analysis. Going somewhat above the nucleon available that enabled precise determinations of the mag- resonance region, there is also new information on the netictransitionformfactorinπ0 productionfromprotons spin structure of the nucleon and the resulting effects on with photon virtualities up to Q2 = 6 GeV2. Benchmark the parton distribution function from recent very precise resultsfromJLab[1,2,3,4],MIT-Bates[5],andMAMI[6] CLAS data. Finally, I will briefly discuss the first DVCS are shown in Fig. 1 relative to the dipole form which ap- resultsthatcoverabroadkinematicsregime,andwhatwe proximately describes the elastic magnetic form factor of 2 VolkerD.Burkert, for the CLAS collaboration.: Probing thenucleon structurewith CLAS 80 2 1/ A 60 40 20 0 -20 -40 -60 -80 0 1 2 3 4 2 2 Q (GeV ) Fig. 3. The transverse transition amplitude A in units of 1/2 10−3GeV−1/2 for the Roper resonance, clearly showing the change in sign. The black triangle is the PDG average, the full squares are the results of single pion analysis, the open square represents the combined single and double pion analysis. The full circles are preliminary results from CLAS. The Fig. 2. The electric and scalar quadrupole ratios R and curvesarepredictionsofquarkmodelcalculationsdiscussedin EM R for theN∆ transition. ref. [8] with the exception of thethin dashed line which is the SM prediction of a hybridbaryon model. theproton.ThetheoreticaldescriptionintheSato-Leedy- namicalmodelincludesdynamicalpioncontributionsthat the D13(1520).Eachofthese resonanceshasfeaturesthat ∗ are needed to explain the magnitude of G especially at makes their investigation particularly interesting. M lowerQ2.It is foundthat the pioncontributions make up morethan30%ofthetotalamplitudeatthephotonpoint, and remain sizeable even at the highest Q2. 3.1 The Roper resonance, P11(1440) The electric and scalar quadrupole contributions, ex- pressed as fractions of the magnetic dipole transition and The P11(1440) is not a well understood state in the stan- givenbytheratioREM =Im(E1+)/Im(M1+)andthera- dard CQM. The mass is more than 100 MeV lower and tio RSM = Im(S1+)/Im(M1+), which are both shown in the photocoupling amplitude has the wrong sign. Alter- Fig.2.REM remainssmallandnegativeevenatthe high- native models have been developed and make predictions estQ2,intherangefrom-2%to-4%,andshowsnoindica- for transition form factors, e.g. models using light cone tionofatrendtowardsthepredictedasymptoticbehavior dynamics[8]kinematics,ormodelsdescribingthestateas of REM → +100% at Q2 → ∞. Although RSM shows a a hybrid baryon[9]. Other models that describe the state differentbehavior,andrisesinmagnitudewithQ2,italso as a nucleon-meson molecule have been proposed but no shows no indication of approaching the predicted asymp- transitionformfactorshavebeencomputed.Thefirstsys- totic behavior, RSM → constant for Q2 → ∞. Both of tematic analyses of the Roper transition form factors was theseresultspresentseriouschallengestotheory.Onemay accomplishedinacombinedanalysisofnπ+ andpπ0,and expectthatLatticeQCD(LQCD)willsoonbeabletocal- of pπ+π− electroproduction data from CLAS [10,11,12] culate these ratios accurately up to high Q2. First com- that showed a rapid drop of the magnitude of the A 1/2 putations in quenched LQCD [7] have produced results amplitude followed by a zero-crossing, while the longitu- at lower Q2 that compare favorably with the measured dinal coupling S is large and positive [13,14]. 1/2 REM values. However, they also reveal shortcomings for The non-relativistic QCM predicts an incorrect sign RSM atthelowestQ2 valueswherepioncontributionsare at the photon point and has no zero crossing, the light expected to be important and may be underestimated in cone quark models give the correct sign at the photon quenched QCD. point and predict the zero crossing, but lack strength at photon point. This is possibly related to contributions of the meson cloud which are not included in the light cone 3 The second resonance region (LC) quark model calculations. Meson effects should be less important at higher Q2, and better agreement is in- In the mass region above the ∆(1232) there are 3 excited deed seen at higher Q2. The analysis of new nπ+ data at nucleon states, the Roper P11(1440), the S11(1535) and highQ2 [15,16]usingtheunitaryisobarmodel(UIM)and VolkerD.Burkert, for the CLAS collaboration.: Probing the nucleon structurewith CLAS 3 1 4 New photocoupling amplitudes from π0 data analysis in full resonance region. 0.75 New π0 photoproductiondata from CLAS have just been 0.5 published [23] that cover a large angle and energy range 0.25 with highstatistics. The SAID analysispacakgewas used to determine new photocoupling amplitudes from these 0 data. The S11(1535) amplitude determined from the pπ0 data set now agrees very well with the analysis of pη -0.25 data. This result is also consistent with the agreement found between these two channels in low Q2 electropro- -0.5 A duction [13], and will hopefully lead to a revision of the hel large uncertainties given in the Review of Particle Prop- -0.75 erties (RPP) for the S11(1535) photocoupling amplitude. -1 Another result of the GWU analysis is that the A1/2 amplitude for the transition to the P13(1720) resonance was 0 1 2 3 4 found as AGWU(0) = 96.6±3.4, while the RPP average 2 2 1/2 Q (GeV ) is listed as ARPP(0) = 18±30, i.e. consistent with zero. 1/2 Fig. 4. The helicity asymmetry for the D13(1520) state. The The new value of A1/2 for the P13(1720) is qualitatively fullredsymbolsarepreliminaryresultsfromthenπ+ analysis, consistent with the strong excitation of this state found whilethebluepointsincludepπ0andnπ+datasets.Thecurve earlier in pπ+π− electroproduction data from CLAS [12]. ′ represents a relativistic quark model calculations [17]. The firstprecisiondata on the pη exclusive channelfrom CLAS have been published recently [24] in the hadronic invariantmass range fromW=1.95 - 2.25 GeV, and cover the mass range of ”missing baryons”. While there are no dispersion relations (DR) approachesresult in the behav- clearsignalsofnews-channelresonances,evidenceforcon- ior shown in Fig. 3. A large positive amplitude A is peaking near Q2 = 2 GeV2, followed by a smooth fa1/ll2off. tributions from the high energy tails of S11(1535) and P11(1710)areseeninthe data.TheanalysisofNakayama Both results are quite close and give a consistent behav- and Haberzettl [25] also shows sensitivity to higher mass ior, indicating that the model-dependence is reasonably well under control. At large Q2 the A (Q2) amplitudes candidate states P11(2100)andD13(2080),that may con- 1/2 tribute to a predicted bump structure in the total cross dropssomewhatfasterthantheLCmodelspredict,which section near an invariant mass of 2.09 GeV. might indicate that the point-like coupling to the quarks is not yet realized at these Q2, and (constituent) quark form factors are be needed to describe this transition. 5 Search for other excited baryon states. A major focus of the CLAS effort is dedicated to clarify- ing some of the ambiguous signals of baryon states, and 3.2 The D13(1520) resonance to the searchfor new states that are predicted within the SU(6)⊗O(3) symmetry group of the symmetric 3-quark system. While there are states predicted that represent non-quark degrees of freedom, it is important to system- TheD13(1520)ispredictedintheCQMtorapidlychange atically search for predicted 3-quark states. Other con- its helicity structure from helicity 3/2 dominance at the real photon point to helicity 1/2 dominance when Q2 in- tributions, e.g. gluonic excitations (hybrid baryons), and nucleon-meson molecule type states will complicate the creases. Indications of such behavior have been seen in picture, and may require special measurements and anal- previousanalyses,but nosystematic study hasbeendone inalargeQ2range.Figure4showsthehelicityasymmetry ysesapproachestoseparatethemfromthe3-quarkstates. The search with CLAS aims at complete or nearly complete measurements of a number of final states and using A21/2−A23/2 linearly and circularly polarized photon beams, in combi- A = hel A2 +A2 nationwith longitudinally andtransverselypolarizedtar- 1/2 3/2 gets. extracted from the nπ+ electroproduction data at high Q2. The lower Q2 data come from the analysis of pπ0 5.1 A new P-wave resonance? and nπ+ data in [13]. A (Q2) shows the rapid switch in hel helicitydominance.Thetransitionappearstooccurinthe TheCLAScollaborationhasrecentlypublisheddataonη range Q2 = 0.5−1.0 GeV2, and the asymptotic value is electroproductioninthe massrangefromthresholdto 2.2 approached at Q2 >3 GeV2. GeV[22].Theintegratedcrosssectionshowsasmallpeak 4 VolkerD.Burkert, for the CLAS collaboration.: Probing thenucleon structurewith CLAS 3 Q2=0.165 Q2=0.250 Q2=0.350 s [m b] (a) 12.0 tot 8.0 2.5 L + 4.0 K CLAS 0.0 12.0 Q2=0.700 Q2=0.900 Q2=1.100 2 8.0 b) 4.0 1.5 0.0 ( tot 12.0 Q2=1.300 Q2=1.500 Q2=1.900 1 8.0 4.0 0.0 0.5 Q2=2.300 Q2=2.700 Q2=3.100 12.0 8.0 0 1600 1800 2000 2200 2400 4.0 g [ ] M( p) MeV 0.0 1.6 1.8 2.0 2.2 1.6 1.8 2.0 2.2 1.6 1.8 2.0 2.2 W (GeV) Fig. 7. Integrated cross section for γp → K+Λ. The curves are from the Bonn-Gatchina analysis and show contributions Fig. 5. Theintegratedcrosssectionforη productionatvaries from the P11(1710) (dashed-dotted), the P13(1900) (dashed), Q2. The large peak is due to the S11(1535) resonance. In the and from non-resonant K-exchangecontributions(dotted). mass range 1.65 to 1.7 GeV a small dip followed by a peak appearsindicatingas-pinterferenceofamplitudesfromneigh- boring resonances. structure near W=1.7 GeV and a dip near W=1.65 GeV. This pattern is shown in Fig. 5 and appears at all Q2. To better understand this behavior we expand the response functions in a Legendre polynomial series: ∞ dσ dσ T L ∗ dΩ +ǫdΩ =XAlPl(cosθη) η η l=0 Inlowestorderthe ratioA0/A1 canbe expressedinterms of the multipoles E0+ and M1− corresponding to s- and p-waves only, and reads ∗ A1 2Re(E0+M1−) = A0 |E0+|2+|M1−|2 Figure6showstheenergydependenceoftheratioA1/A0. It changes sign near W = 1.65 GeV. The observation is consistentwitharapidchangeintherelativephaseofthe E0+ and M1− multipoles because one of them is passing Fig. 6. Legendre coefficients A0, and ratios of higher partial through resonance. A reasonable fit to the CLAS data in wave terms for η electroproduction data from CLAS plotted thatmassrangeisobtainedwiththeS11(1535),S11(1650), versus hadronic mass W. The most significant is the ratio P11(1710) and D13(1520), with a width for the P11(1710) A1/A0 that clearly shows an interference of s- and p-waves of 100 MeV. Similar structures, even more pronounced where one of the waves goes through resonance generating a have been observed in η photoproduction off neutrons, zero-crossing. and have been discussed at this conference [26,27] as a possiblenewresonance.Couldtheobservedstructurebea newresonance?Ithinkitismorelikely,thatthenewdata will merely confirm the existence of the 3-star P11(1710) state, and better define its poorly determined properties such as mass, width, and photocoupling. VolkerD.Burkert, for the CLAS collaboration.: Probing the nucleon structurewith CLAS 5 2)35 c V/ 2)s/(5 MeV/c2500 2)Counts/(5Mev/c 112233405050500000000 Counts/(10 Me223050 GMNc2:::/0 1N7..01d51 f3–:59 2 2–36 0–0. .000/.0 035011.04 t2000 50 un 0 1.41.451.51.551.61.651.71.75 15 o MM(K+K+) (GeV/c2) C 10 1500 M1:1.3223 – 0.0001 s 1:0.0067 – 0.0001 5 N1: 7678 – 173 1000 Ms 22:: 10..50317085 –– 00..00000191 0 1.5 1.55 1.6 1.65 1.7 1.75 1.8 1.85 1.9 1.95 N2: 658 – 91 M(X 0p -) (GeV/c2) 500 Fig.9.ThemassspectrumforΞ(1320π−inthereactionγp→ π−K+K+Ξ(1320). 0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 MM(K+K+) (GeV/c2) )7 x Fig. 8. Missing mass for the γp → K+K+X− showing the C(6 ground state Ξ−(1320) and the first excited state Ξ−(1530). +p 5 The inset highlights the mass range around the 1530 MeV g14 state. 3 2 5.2 Photo- and electroproduction of K-Hyperons E80 AlargenumberofcrosssectiondataonKΛandKΣ production have been published and are now being used by 1 E130 0.9 various groups for phenomenological analyses. The inte- 0.8 EMC 0.7 grated cross section from CLAS [28,29] is shown in Fig. HERMES 0.6 7. These data were used in a fit by the Bonn-Gatchina 0.5 E143 group[31,32]whofoundsignificantcontributionsfromthe 0.4 SMC P13(1900), a two star state candidate in the RPP. The E155 strongest constraints come from the polarization trans- 0.3 JLab EG1B fer data using a circularly polarized photon beam [30]. -1 2 If the existence of the state can be confirmed, it will be 10 1 10 Q2 (G1e0V2) strong evidence against a diquark-quark model that has no room for such a state [35]. Also, unpolarized and po- Fig.10. Theworlddataonstructurefunctiong1p(x,Q2).The larized response functions have been measured [33,34] in CLAS data are shown in the full red triangles. ep→eK+Λandep→eK+Σ thatshowsignificantstruc- turesinthe hadronicmassspectrum,whichareindicative of resonance excitations. where the excited Λ∗ decays through Λ∗ → K+Ξ∗. Fig- ure8fromCLAS[36]showsthatonecanidentifythelow- 5.3 Search for new Ξ∗ cascade baryons. est two cascade states using the missing mass technique. Athigherenergiesotherstatesmaybecomevisibleaswell. ∗ ∗ Production of Ξ cascade baryons with strangeness S = Another way to search for Ξ states is by measuring the -2 provides another avenue in the search for new baryon Ξ0 with an additional pion. Forming the invariant mass states.Thecascadespectrumshouldreflectthesamemass of the Ξ0(1320) with the π− shows in Fig. 9 again the − splitting due to spin-spin interaction as the S=0 states. Ξ(1530) state. No other structure is clearly identified. The advantages are due to the expected (and observed) Should a state at 1.62 GeV emerge at higher statistics, morenarrowwidthsofthesesstates.Thedisadvantagesfor it could be the one star candidate in RPP. Such a state using photon beams are the low cross section for the pro- wouldhowevernotbepartofthe3-quarksymmetrygroup ductionoftwokaonsinthefinalstate.Apossibleproduc- butcouldbeadynamicallygeneratedΞ−πstateprediced tionmechanismisthrought-channelproductionofK+Λ∗, in dynamical models [37]. 6 VolkerD.Burkert, for the CLAS collaboration.: Probing thenucleon structurewith CLAS 0.3 Q2 = 20.8.3 Q2 = 30.3.3 x = 0.45 x = 0.46 B B 0.2 0.2 0.2 0.15 x G errors 0.1 0.1 0.1 Q2 = 3.7 x = 0.46 B 0.10 2 2 0 0 0 Q = 2.5 GeV 0.30 0.5 1Q2 =1 20.5.3.30 0.5 1Q2 = 12.0.57.30 0.5 1Q2 = 13..50 0.05 x = 0.35 x = 0.36 x = 0.36 B B B 0.2 0.2 0.2 0.00 0.1 0.1 0.1 0 0 0 -0.05 0.30 0.5 1Q2 =1 10.5.7.30 0.5 1Q2 = 11.0.59.30 0.5 1Q2 = 12..52 x = 0.25 x = 0.25 x = 0.25 -0.10 0.2 B 0.2 B 0.2 B LSS’05 LSS’06 (EG1 data incl.) 0.1 0.1 0.1 -0.15 CLAS12 0 0 0 0.0 0.2 0.4 0.6 0.8 1.0 x a(0t.)30 0.5 1Q2 = 110..52.30 0.5 1Q2 = 11.0.54.30 0.5 1Q2 = 11..56 x = 0.13 x = 0.17 x = 0.18 0.2 B 0.2 B 0.2 B Fig. 11. Impact of the CLAS data on the uncertainties in 0.1 0.1 0.1 the in the parton distribution functions from the LSS QCD analysis. The uncertainty in the polarized gluon distribution 0 0 0 is reduced by a factor of 3 at a modest xB = 0.4 (change 0 0.5 1-t 1(G.5e0V2)0.5 1 1.5 00 0.5 1 1.5 from dashed-dotted to dashed lines) giving new constraints on the polarized gluon distribution. The uncertainties in the Fig. 12. The beam spin asymmetry showing the DVCS-BH seaquarkdistributionfunctionsarealsoimprovedsignificantly. interference. The red and green points represent the previous Theimprovementinthepolarizedgluondistributionfunctions CLAS and Hall A data, respectively. The blue curve is the comes largely from g1d(x,Q2) measured on deuterium in the VGG GPD parameterization [48] in twist-2 (solid) and twist- same kinematics range. The projected impact of an extension 3 (dashed-dotted). The dashed black line is a Regge model of the measurements at 12 GeV with the planned CLAS12 prediction [49]. spectrometer are shown with the solid lines. form factors were of little interest as the only known pro- 6 Spin structure of the nucleon and parton cess how they could be directly measured is elastic scat- distributions tering of gravitons off the nucleon. Today we know that theseformfactorsalsoappearasmomentsoftheunpolar- izedGPDs [43]. The quarkangularmomentum in the nu- The CLAS collaboration has collected very precise data oninclusivedoublepolarizationinclusivescatteringresult- cleonisgivenbyJq(t)=R−+11dx[xHq(x,ξ,t)+Eq(x,ξ,t)], ing in high quality spin structure function g1p(x,Q2) and which at t=0 results in the well known Ji sum rule, and g1d(x,Q2),aswellasfirstmomentsΓ1 =Rx1ming1(x,Q2)dx M2q(t)+4/5dq1(t)ξ2 = R−+11dxxHq(x,ξ,t). The mass and for proton, deuterons and neutrons. The world data on pressuredistributionofthequarksaregivenbythesecond CstLruAcStudreatfaunccotvieorntgh1ep(lxow,Qer2)Qa2reansdhohwignhinxBFirga.n1g0e.. TThhee emteormξe.nAtosfeGpaPrDatiHon,owfhMer2eq(tth)ealnadttdeq1r(its)prreoqbueirdesbympeaasruarme-- bulk of the data coversthe resonanceregion,however the ment of the moments in a large range of ξ. How do we precise data in the DIS region provide strong constraints accessthis information?The beamspinasymmetryofthe onQCDfits toextractpartondistributionfunctionsafter deeplyvirtualComptonscattering(DVCS)amplitudein- higher twist contributions have been properly taken into terfering with the Bethe-Heitler (BH) amplitude is sensi- account [42]. The extracted uncertainties for the polar- tiveto the GPDH(x=ξ,ξ,t), andhasbeen measuredat izedgluondistributionfunction are shownin Fig.11,and Jefferson Lab [44,45,46,47] in a wide kinematics range in indicate very significant reductions compared to results Q2,ξ,andt.Therecentazimuthalasymmetriesmeasured obtained before the CLAS data became available. by CLAS were fitted with ALU = αsinφ/(1 + βcosφ). The t-dependence of the leading term α for different val- uesofQ2 andx =2ξ/(1+ξ)isshowninFig.12.Wesee B that α has a maximum at small t and smoothly drops to 7 Generalized Parton Distributions and DVCS zero. The comparison of α with the standard VGG GPD parameterization[48]showsqualitative,evenquantitative Thenucleonmatrixelementoftheenergy-momentumten- agreementinsomekinematics,especiallyatlarge−t,how- sorcontains3formfactorsthatencodeinformationonthe ever the theoretical asymmetry exceeds the data at small angular momentum distribution of quark q in transverse −t. This could mean that at low momentum transfer the space,Jq(t),themass-energydistribution,Mq(t),andthe denominator in the asymmetry does not fully account for 2 pressure and force distribution, dq(t). For decades these all contributions to the DVCS cross section. 1 VolkerD.Burkert, for the CLAS collaboration.: Probing the nucleon structurewith CLAS 7 While the elastic GPDs are currently at the center of 7. C. Alexandrou et al., Phys.Rev.Lett.94, 021601, 2005. thedevelopmentofamorecomplexpictureofthenucleon, 8. I.Aznauryan,Phys.Rev.C76, 025212, 2007. GPDsmayalsobe definedfortransitionswherethe recoil 9. Z.p. Li, V.Burkert, Zh.Li, Phys.Rev.D46,70, 1992. baryon is not a ground state nucleon but an excited nu- 10. H. Egiyan et al.,Phys.Rev.C73, 025204, 2006. cleon, such as the ∆(1232) or any other excited nucleon 11. K. Joo et al., Phys.Rev.C72 058202, 2005. state. Measuring the DVCS with a recoiling excited state 12. M. 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Sandorfi,plenary talk at this conference. the Korea Research Foundation, and a research grant of 51. T.-S. H. Lee, plenary talk at this conference. the RussianFederation.The JeffersonScience Associates, LLC, operates Jefferson Lab under contract DE-AC05- 060R23177. References 1. K. Joo, et al., Phys.Rev.Lett.88, 122001, 2002. 2. M. Ungaro et al.,Phys.Rev.Lett.97, 112003, 2006. 3. J. J. Kelly et al.,Phys.Rev.Lett.95, 102001, 2005. 4. V. Frolov et al.,Phys.Rev.Lett.82, 45,1999. 5. N. Sparveriset al., Phys.Rev.C67, 058201, 2003. 6. N. Sparveriset al., Phys.Lett.B651, 102, 2007.