1. 1 Meson production with CLAS Volker D. Burkert Jefferson Lab 24 th Student’s Workshop on Electromagnetic Interactions September 9–14, 2007, Bosen, Germany

2. 2 Overview Introduction, N*’s and Δ’s, Multiplets, SU(6)xO(3) Meson production as a tool to study excited baryons Analysis Tools, CLAS Electromagnetic Excitation of resonances - the Δ(1232) and the shape of the nucleon The “Roper” resonance and other lower mass resonances, S 11 (1535), D 13 (1520), in single π and η production. Search for “Missing Resonances” in –Double pion production –Strangeness production. III IIIIV

3. 3 Additional Reading Material V.D. Burkert and T.-S. H. Lee, Electromagnetic Meson Production in the Nucleon Resonance Region, International Journal of Modern Physics E, Vol. 13, Nummer 6, (2004) F.E. Close, A. Donnachie, D. Shaw (editors) Electromagnetic Interactions of Hadrons (2007)

4. 4 Why N*’s are important?  Nucleons represent the real world, they must be at the center of any discussion on “why the world is the way it is”  Nucleons represent the simplest system where “the non- abelian character of QCD is manifest”. N*’s can teach us a lot about quark confinement in the basic building blocks of matter which make up 99.9% of the mass of the visible part of the universe.  Nucleons are complex enough to “reveal physics hidden from us in mesons”. Gell-Mann & Zweig - Quark Model: 3 x 3 x 3 = 10 + 8 + 8 + 1 O. Greenberg - The   problem and “color” Gluon flux of system of 3 heavy quarks. 3-gluon vertex (Nathan Isgur at N*2000, JLab)

5. 5  (1232) Phys. Rev. 85, 936 (1952)   p X   p X Evidence for the first excited state of the proton and a hint for more.

6. 6 p  (GeV/c) Total cross section   p X

7. 7 Δ ++ π + p → Δ ++ is the largest πN cross section, but the Δ ++ state is not allowed in CQM without “color”. O. Greenberg introduces a new quantum number to get asymmetric w.f. u u u  s  flavor  spin  ++  as  flavor  spin  color u u uu u u  ++ The Δ ++ (1232) leads to “color”

8. 8 Baryon multiplets Y=B+S I3I3 +1/21 -1/2 1 2 Symmetry of Baryons made from u, d, s quarks: N    ─  ++ --   The symmetry underlying the QM predicts the existence of a new particle made from (sss).

9. 9 Production and decay of Ω - → Ξ o π - V.E. Barnes et. al., Phys. Rev. Lett. 8, 204 (1964)       e-e- e+e+ e-e- e+e+ K0K0  – K+K+ –– p K–K–

10. 10 Baryon Resonances and SU(6)xO(3) 3 Flavors: {u,d,s} SU(3) {qqq}: 3 3 3 = 10 8 8 1 +++ ++ SU(6) multiplets decompose into flavor multiplets: 56 = 4 10 2 8 + + + 70 = 2 10 4 8 2 8 2 1 + + 20 = 2 8 4 1 |Baryon> :  |qqq> +  |qqq(qq)| +  |qqqG> +.. Quark spin s q = ½ SU(2) +++ ++ {qqq }:6 6 6 = 56 70 70 20 Baryon spin: J = L +  s i  parity: P = (-1) L O(3)

11. 11 D 13 (1520) S 11 (1535) P 11 (1440) F 15 (1680) P 33 (1232) D 33 (1700) SU(6)xO(3) Classification of Baryons Missing States?

12. 12 Configuration Mixing in [70,1 - ] States with same I, J p quantum numbers and different total quark spins S q = 1/2 or S q = 3/2, mix with mixing angle  M. The pure quark states |N 2, 1/2 - > and |N 4, 1/2 - > in [70,1-] project onto physical states S 11 (1535) and S 11 (1650). S q = 1/2S q = 3/2 | S 11 (1535)> = cos  1 |N 2, 1/2 - > - sin  1 |N 4, 1/2 - > |S 11 (1650)> = sin  1 |N 2,1 /2 - > + cos  1 |N 4, 1/2 - >  1 = 31 o (measured in hadronic decays). |D 13 (1520)> = cos  2 |N 2, 3/2 - > - sin  2 |N 4, 3/2 - > |D 13 (1700)> = sin  2 |N 2, 3/2 - > +cos  2 |N 4, 3/2 - >  2 ~10 o Similarly for |N 2,3/2 - > and |N 4,3/2 - > The |N 4,5/2 - > quark state has no N 2 partner, and cannot mix. | D 15 (1675) > = |N 4, 5/2 - > Notation: L 2I,2J p

13. 13 Analysis Tools

14. 14 Simple searches for resonances For a 2-body decay one can search for resonance structures in the invariant mass distribution. P, M p 1, m 1 p 2, m 2 proton pion M 2 = (p p + p  ) 2 4-vectors M Rarely are resonances observed just in mass distributions, e.g. if the state is narrow, or if strongly excited. It also gives no information on quantum numbers other than isospin if one sees the “peak” in two channels.

15. 15 A resonance at m 12 = 1.8 GeV will not appear like a resonance in m 23 A narrow resonance at m 23 will not appear as a narrow structure in m 12 A known narrow resonance at m 12 = 2.0 GeV may appear like an enhancement in m 23 at a different mass (kinematical reflection). P, M p 1, m 1 p 2, m 2 p 3, m 3 Dalitz Plot for 3-body decay (e.g. pπ + K 0 ) 3-body decay

16. 16    γp pK + K - Dalitz Plot: E  = 1.6-3.5 GeV  (1020)

17. 17 Argand Diagram f(k,  ) = 1/k (2l+1)a l P l (cos  )  l Elastic scattering amplitude of spinless particle with momentum k in cms: For purely elastic scattering :  l = 1, (e.g.  N →  N) d  /d  = |f(k,  )| 2 Optical theorem:  tot = 4  /k[Im f(k,0)] Cross section for l th partial wave is bounded:  l = 4  /k 2 (2l+1) 2 |a l | 2 < 4  (2l+1) 2 /k 2 a l = (  l e 2i  l – 1)/2i, 0 <  l < 1,  l : phase shift of l th partial wave

18. 18 Im a 1 Re a -1/2 +1/2  1/2 alal  /2 Argand Diagram, cont’d inelasticity sets in a l : partial wave amplitude evolving with energy. The amplitude leaves the unitary circle where inelasticity sets in. a l = (  l e 2i  l – 1)/2i resonance mass increasing energy

19. 19 Breit-Wigner Form  B-W (non-relativistic) form for an elastic amplitude a l with a resonance at cm energy E R and elastic width Γ el and total width Γ tot is al =al =  el /2 E R – E – i  tot /2  Relativistic form : al =al =  m  el s – m 2 – im  tot  Many other B-W forms exist, dependent on process dynamics.

20. 20 Argand Diagrams for some P–waves P 33 - the elastic wave  Crosses indicate every 50 MeV step in W  Dots correspond to Breit-Wigner, W R = M R

21. 21 Some resonances in  →   (1920)***  (1600)*** N(1710)*** N(2100)*

22. 22 A comment Nearly all known nucleon resonances have been observed in elastic πN → πN scattering. If ε is branching ratio of resonance R into πN, then the cross section for πN → R → Nπ is σ ~ ε 2. If ε is small (e.g. ε = 0.05) then  may be a very small number, and many resonances may not be seen in the elastic channel. We need to look for other decay channels as well!

23. 23 Hadron Structure with e.m. Probes? resolution of probe low high N π Allows to address central question: What are the relevant degrees-of-freedom at varying distance scale? LQCD/DSE/Instantons e.m. probe q Constituent quark model with fixed quark masses only justified at photon point and low q.

24. 24 Reach of Current Accelerators JLAB Spring-8 Neutrals Charged & some neutrals

25. 25 E max ~ 6 GeV I max ~ 200  A Duty Factor ~ 100%  E /E ~ 2.5 10 -5 Beam P ~ 85% E g (tagged) ~ 0.8-5.5GeV CLAS JLab Site: The 6 GeV CW Electron Accelerator

26. 26 C EBAF Large Acceptance Spectrometer Liquid D 2 (H 2 )target +  start counter; e mini-torus; shielding solenoid Drift chambers argon/CO 2 gas, 35,000 cells Electromagnetic calorimeters Lead/scintillator, 1296 PMTs Torus magnet 6 superconducting coils Gas Cherenkov counters e/p separation, 216 PMTs Time-of-flight counters plastic scintillators, 684 PMTs Large angle calorimeters Lead/scintillator, 512 PMTs In addition, polarized targets for electron and photon beams are in use.

27. 27 CLAS during maintenance

28. 28 CLAS Torus Magnet Lines of constant absolute total field. The high field is concentrated at forward angles. Note that the field in the inner part drops very rapidly to zero. large ∫Bdl small ∫Bdl Target

29. 29 CLAS Momentum and angle resolution

30. 30 Neutron Detection in the EC The EC is a scintillator-lead sandwich calorimeter with 40 layers of 1 cm thick plastic scintillators. This provides excellent neutron detection efficiency of up to 60% at high momentum.

31. 31 ` Particle Identification by Time-of-Flight An important aspect in any production experiment is particle identification. Charged particles are often identified through either time-of-flight or/and through energy loss measurements. In CLAS the main tool is precise timing information of 150psec(σ). In electron scattering the start time at the target can be measured precisely since β=1 for identified electrons. With known path length from the target, the start time and the RF beam bucket can be determined. As the RF time is given with pico-second accuracy, the uncertainty of the electron timing can be eliminated. e-e- e-e- Beam time structure i

32. 32 Particle Identification by Time-of-Flight β P(GeV/c) Mass by TOF

33. 33 The Hall B Photon Energy Tagger Energy resolution ΔE/E = 10 -3

34. 34 PID with over-determined kinematics γp → pX  P1P1 p2p2 pXpX p3p3 p X = (p 1 + p 2 –p 3 ) p X 2 = M X 2 =(p 1 + p 2 –p 3 ) 2 2-body process with one unmeasured particle pγpγ p pXpXp

35. 35 Example of Single Event γd → p K + K ─ X K-K- K+K+ p

36. 36 Electromagnetic Excitation of Baryon Resonances

37. 37 Electromagnetic Excitation of N*’s Primary Goals:  Extract electro-coupling amplitudes for known △,N* resonances in Nπ, Nη, Nππ –Partial wave and isospin decomposition of hadronic decay –Assume em and strong interaction vertices factorize –Helicity amplitudes A 3/2 A 1/2 S 1/2 and their Q 2 dependence  Study 3-quark wave function and underlying symmetries  Quark models: relativity, gluons vs. mesons.  Search for “missing” resonances predicted in SU(6) x O(3) symmetry group e e’ γvγv N N’,Λ N*,△ A 3/2, A 1/2,S 1/2  p  p   p  

38. 38 Inclusive Electron Scattering ep → e’X  Resonances cannot be uniquely separated in inclusive scattering → exclusive processes Q 2 =-(e-e’) 2 ; W 2 = M X 2 =(e-e’+p) 2 (G E, G M )  (1232) N(1440) N(1520)N(1535)  (1620) N(1680) ep →ep

39. 39 W-Dependence of selected channels at 4 GeV e’ Measurement of various final states needed to probe different resonances, and to determine isospin. From panels 2 and 3 we can find immediately the isospins of the first and second resonances. The big broad strength near 1.35 GeV in panel 3, and not seen in panel 2 hints at another I=1/2 state. From panels 3 and 4 we see that there are 5 resonances. Panel 5 indicates there might be a 6 th resonance. 1 2 3 4 5

40. 40 End of 1 st part

41. 41 γNΔ(1232) Transition

42. 42 N-Δ(1232) Quadrupole Transition SU(6): E 1+ =S 1+ =0

43. 43 NΔ - in Single Quark Transition M1 N(938)Δ(1232) Magnetic single quark transition. Δ(1232) N(938) C2 Coulomb single quark transition.

44. 44 Multipole Ratios R EM, R SM before 1999 Sign? Q 2 dependence?  Data could not determine sign or Q 2 dependence

45. 45 N ∆ electroproduction experiments in the JLAB era ReactionObservableWQ2Q2 Author, Conference, PublicationLAB p(e,e’p)π 0 σ 0 σ TT σ LT σ LTP 1.2210.060S. Stave, EPJA, 30, 471 (2006) MAMI p(e,e’p)π 0 1.2320.121H. Schmieden, EPJA, 28, 91 (2006) MAMI p(e,e’p)π 0 1.2320.121Th. Pospischil, PRL 86, 2959 (2001) MAMI p(e,e’p)π 0 σ 0 σ TT σ LT σ LTP 1.2320.127C. Mertz, PRL 86, 2963 (2001) C. Kunz, PLB 564, 21 (2003) N. Sparveris, PRL 94, 22003 (2005) BATES p(e,e’p)π 0 σ 0 σ TT σ LT σ LTP 1.232 1.221 0.127 0.200 N. Sparveris, SOH Workshop (2006) N. Sparveris, nucl-ex/611033 MAMI p(e,e’p)π 0 A LT A LTP 1.2320.200P. Bartsch, PRL 88, 142001 (2002) D. Elsner, EPJA, 27, 91 (2006) MAMI p(e,e’p)π 0 p(e,e’π+)n σ 0 σ TT σ LT σ LTP 1.10-1.400.16-0.35C. Smith, SOH Workshop (2006) JLAB / CLAS p(e,e’p)π 0 σ 0 σ TT σ LT 1.11-1.700.4-1.8K. Joo, PRL 88, 122001 (2001) JLAB / CLAS p(e,e’p)π 0 p(e,e’π+)n σ LTP 1.11-1.700.40,0.65K. Joo, PRC 68, 32201 (2003) K. Joo, PRC 70, 42201 (2004) K. Joo, PRC 72, 58202 (2005) JLAB / CLAS p(e,e’π+)nσ 0 σ TT σ LT 1.11-1.600.3-0.6H. Egiyan, PRC 73, 25204 (2006) JLAB / CLAS p(e,e’p)π 0 16 response functions1.17-1.351.0J. Kelly, PRL 95, 102001 (2005) JLAB / Hall A p(e,e’π+)nσ 0 σ TT σ LT σ LTP 1.1-1.71.7-4.5K. Park, Collaboration review JLAB / CLAS p(e,e’p)π 0 σ 0 σ TT σ LT 1.10-1.403.0-6.0M. Ungaro, PRL 97, 112003 (2006) JLAB / CLAS p(e,e’p)π 0 σ 0 σ TT σ LT 1.10-1.352.8, 4.0 V. Frolov, PRL 82, 45 (1999) JLAB / Hall C p(e,e’p)π 0 σ 0 σ TT σ LT 1.10-1.406.5, 7.5A. Villano, ongoing analysis JLAB / Hall C

46. 46 Pion Electroproduction Structure Functions Structure functions extracted from fits to  * distributions for each (Q 2,W, cosθ * ) point. LT and TT interference sensitive to weak quadrupole and longitudinal multipoles. + : J = l + ½ - : J = l - ½

47. 47 Unpolarized structure function –Amplify small resonant longitudinal multipole by interfering with a large resonance transverse multipole The Power of Interference I  LT ~ Re(L*T) = Re(L)Re(T) + Im(L)Im(T) Large Small P 33 (1232) Im(S 1+ ) Im(M 1+ )

48. 48 Typical Cross Sections vs cos  * and  * Q 2 = 0.2 GeV 2 W=1.22 GeV

49. 49 NΔ(1232) - Small Q 2 Behavior Structure Functions → Legendre expansion

50. 50 Structure Functions - Invariant Mass W

51. 51 Legendre Expansion of Structure Functions Resonant Multipoles Non-Resonant Multipoles (M 1+ dominance)