Open questions in hadron physics Spectroscopy with the Crystal Barrel Detector Volker Credé

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1 Open questions in hadron physics Spectroscopy with the Crystal Barrel Detector Volker Credé Introduction The Crystal Barrel Detector at LEAR (CERN) Meson spectroscopy in pp reactions at rest and the search for exotic states The Crystal Barrel Experiment at ELSA: initial configuration (/1) Baryon spectroscopy (photoproduction of neutral mesons: E γ < 3 GeV ) γ p p π (p η) differential and total cross sections γ p p π π (p π η) total cross sections and PWA Summary and outlook: polarization measurements with CB/TAPS (3) Wilson Laboratory for Nuclear Studies, Cornell University, Ithaca, NY 14853

2 Access to QCD and fundamental questions Breaking of chiral symmetry can be treated perturbatively very small energies resonance region at intermediate energies Application of perturbation theory allows access to QCD very large energies Do we understand bound systems within the framework of QCD? Development of QCD-inspired models can help to find answers to the fundamental questions What are the relevant degrees of freedom and the corresponding effective interactions responsible for hadronic phenomena? What are the mechanisms for confinement and for chiral symmetry breaking?

3 Renaissance of Hadron Spectroscopy Recently discovered new states Beginning of new spectroscopy? Mesons D + sj (317) D+ s π at BaBar (B. Aubert et al., Phys. Rev. Lett. 9 (3) 41) D + sj (457) D + s π at CLEO (D. Besson et al., Phys. Rev. D68 (3) 3) Observation of missing + and 1 + states in the D + s (cs) system?... or even DK molecules or tetraquarks? B ± K ± π + π J/ψ at BELLE Baryons Observation of Θ + with S = 1 (LEPS, CLAS, DIANA, SAPHIR, etc.) Observation of Ξ with Q = (C. Alt et al. [NA49 Coll.], hep ex/3114) Observation of pentaquark states as members of an antidecuplet? Observation of an enhancement at pp threshold in J/ψ γ η c γ pp (BES Coll. at HADRON 3 conference) (S.K. Choi et al., hep ex/333)

4 The quark model of hadrons Mesons (qq) q q = 3 3 = 8 1 Baryons (qqq) q q q = = }{{} Ordinary matter, however, QCD also predicts so called exotic states simplest possibility: q q q = }{{} Does not work: color singlets needed! multiple of (qqq) and (qq) necessary Glueballs: g g = 8 8 = Hybrids: q q g = (qq) l ((q) 3 ) m (g) n, l + m 1 for n = 1

5 The quark model of hadrons Mesons (qq) q q = 3 3 = 8 1 Baryons (qqq) q q q = = }{{} Ordinary matter, however, QCD also predicts so called exotic states simplest possibility: q q q = }{{} Does not work: color singlets needed! multiple of (qqq) and (qq) necessary Glueballs: g g = 8 8 = Exciting the flux tube Hybrids Hybrids: q q g = (qq) l ((q) 3 ) m (g) n, l + m 1 for n = 1

6 The quark model of hadrons Mesons (qq) q q = 3 3 = 8 1 Baryons (qqq) q q q = = }{{} Ordinary matter, however, QCD also predicts so called exotic states simplest possibility: q q q = }{{} Does not work: color singlets needed! multiple of (qqq) and (qq) necessary Glueballs: g g = 8 8 = Exciting the flux tube Hybrids Hybrids: q q g = (qq) l ((q) 3 ) m (g) n, Multi quark states l + m 1 for n = 1

7 The search for new forms of matter All exotic states of hadrons can be subdivided into three groups: 1. States with explicitly exotic values of principal quantum numbers Θ + with S = 1 Ξ with Q =

8 The search for new forms of matter All exotic states of hadrons can be subdivided into three groups: 1. States with explicitly exotic values of principal quantum numbers Θ + with S = 1 Ξ with Q =. States with exotic combinations of J P C forbidden for ordinary qq states: +,, 1 +, +, 3 +, etc. π 1 (14) ηπ π 1 (16) η π J P C = 1 +

9 The search for new forms of matter All exotic states of hadrons can be subdivided into three groups: 1. States with explicitly exotic values of principal quantum numbers Θ + with S = 1 Ξ with Q =. States with exotic combinations of J P C forbidden for ordinary qq states: +,, 1 +, +, 3 +, etc. π 1 (14) ηπ π 1 (16) η π J P C = States with hidden exotic properties Problem: predicted glueballs can mix with ordinary qq states f (15) } J P C = ++ Evidence far from solid Details needed for a full understanding are missing

10 Spectroscopy with the Crystal Barrel Detector - present (University of Bonn) Photoproduction experiments at ELSA Spectroscopy of light baryons (... and mesons) (at LEAR/CERN) Investigation of pp and pd annihilations Annihilation dynamics in the non-perturbative regime of QCD Search for baryonium (pp bound states) Spectroscopy of light mesons

11 Photoproduction with the Crystal Barrel Detector at ELSA Photoproduction. < m pπ π <.1 (in GeV/c ) γ p pπ π Invariant pπ mass [ MeV/c ] Spectroscopy Baryon spectrosopy Search for missing resonances

12 General Physical Motivation Search for missing resonances Quark models predict many more baryons than have been observed *** ** * N spectrum spectrum according to PDG (Phys. Rev. D66 () 11) little known (many open questions left) Possible solutions: a) Quark-diquark structure one of the internal degrees of freedom is frozen b) They have not been observed, yet Nearly all existing data result from πn scattering experiments If the missing resonances did not couple to Nπ, they would not have been discovered!! (supported by theory)

13 Quark models and experimental overview Effective theories and models necessary to make spectroscopic predictions Basic assumption: linear confinement potential + residual short range interaction Goldstone boson (pion) exchange (L.Y. Glozman, W. Plessas, K. Varga and R.F. Wagenbrunn, Phys. Rev. D58 (1998) 943) One gluon exchange (S. Capstick and N. Isgur, Phys. Rev. D34 (1986) 89) (relativized quark model) 1. wrong spin orbit couplings. no explanation for parity doublets Instanton induced interaction (relativistic quark model) 1. acceptable Regge trajectories. natural explanation for parity doublets Which is the right model? Do we have a correct model? Is there really one interaction that dominates?

14 Symmetries and classification qqq = colour A space; spin, flavour S O(6) SU(6) SU() spin SU(3) flavour Total wave function antisymmetric with respect to exchange of two quarks SU(6) symmetry: (Notation: S+1 multiplet) = 56 S 7 M 7 M A 56 = = = Notation for baryon resonances: (example) S 11 (1535) N 1 (1535) Classification of multiplets (D, L P N ): ground state (56, + ): 3 +(13) ɛ 4 1 N 1 +(939) ɛ 8 1. excited state (7, 1 1 ): 1 (16), 3 (17) ɛ 1 N 1 (1535), N 3 (15) ɛ 8 N 1 (165), N 3 (17) N 5 (1675) ɛ 4 8

15 Cornell University Cleo lunch talk Volker Credé 3 5 U. Löhring, B.C. Metsch and H.R. Petry, Eur. Phys. J. A1, (1) missing resonances 7 ** 6 *** Mass [MeV] 15 1 * S 19 ** ** ** *** * ** S S *** ** L J π T J 939 Bonn model: residual short range interaction based on instanton induced forces 1/+ 3/+ 5/+ 7/+ 9/+ 11/+ 13/+ 1/- 3/- 5/- 7/- 9/- 11/- 13/- P11 P F15 F17 H19 H K S11 D13 D15 G17 G19 I I

16 Nucleon resonances S. Capstick and N. Isgur, Phys. Rev. D34 (1986) 89 3 ** 5 *** ** Mass [MeV] * S *** ** ** ** * ** S S *** 15 1 OGE model: residual short range interaction based on one gluon exchange J π 1/+ 3/+ 5/+ 7/+ 9/+ 11/+ 13/+ 1/- 3/- 5/- 7/- 9/- 11/- 13/-

17 U. Löhring, B.C. Metsch and H.R. Petry, Eur. Phys. J. A1, (1) 3 95 ** 75 ** 5 39 * 3 ** 4 15 * 35 * * 4 ** Mass [MeV] * 19 *** 16 *** ** too low in mass 19 ** * *** Bonn model: residual short range interaction based on instanton induced forces J π 1/+ 3/+ 5/+ 7/+ 9/+ 11/+ 13/+ 15/+ 1/- 3/- 5/- 7/- 9/- 11/- 13/- 15/- L T J P31 P 33 F F H K K S 31 D 33 D 35 G 37 G 39 I 3 11 I H

18 resonances S. Capstick and N. Isgur, Phys. Rev. D34 (1986) 89 3 ** ** 5 * ** * ** * * Mass [MeV] 15 * *** *** ** too low in mass ** * *** 1 OGE model: residual short range interaction based on one gluon exchange J π 1/+ 3/+ 5/+ 7/+ 9/+ 11/+ 13/+ 15/+ 1/- 3/- 5/- 7/- 9/- 11/- 13/- 15/-

19 The Electron Stretcher Ring ELSA Investigation of the nucleon structure with a 4π high-resolution photon detector at the Electron Stretcher Accelerator ELSA in Bonn Collaboration of Basel Bochum Stretcher Ring Bonn Dresden Erlangen Gatchina Gießen Groningen Münster

20 The Crystal Barrel Experiment at ELSA Photoproduction experiment using a liquid H target (unpolarized in a first series of experiments /1) Inner Detector Crystal Barrel Tagging Magnet Tagging range: 5 % 93 % of incoming E e Target Tagger Radiator E e = 1.4 GeV.35 GeV E γ 1.3 GeV 1. GeV/c s 1.8 GeV/c γ e - e - E e = 3. GeV.8 GeV E γ 3. GeV 1.5 GeV/c s.6 GeV/c E = E - E γ e -

21 The tagging system 14 scintillation counters wire chambers (35 wires) tagging range: 5 % 9 % electrons radiator magnet photons primary electron beam beam dump high photon energies low photon energies

22 Target and inner detector The inner detector: 3 layers of scintillating fibres ( additional reconstruction point) Trigger on charged tracks 1 cm

23 The Crystal Barrel Experiment at ELSA Tagging magnet Barrel calorimeter

24 Cross section of a CsI(Tl) module LEAR: operation in a magnetic field of 1.5 T silicon photodiodes to read out scintillation light.1 mm titanium can 1 circuit board 6 5 light fibre mm Titan CsI(Tl) 4 preamplifier 1 cm 3 photodiode wavelength shifter

25 Segmentation in Φ Segmentation in Θ 6 crystals (type 1-1), opening angle crystals (type 11-13), opening angle 1 6 rings of crystals, opening angle 6 The barrel calorimeter

26 Cornell University Cleo lunch talk Volker Credé Arrangement and parameters of crystal modules 6 o o Θ Φ ('& %$# "!!" # $ & % ' )!(" "! ) Density 4.53 g/cm 3 Radiation length L 1.86 cm Moliére radius 3.8 cm Light output.85 relative to NaI Maximum emission 55 nm Decay times.9 and 7 µs Photon yield /MeV Crystal length 3 cm = 16.1 L Solid angle 97.8 % of 4π Typ. signal photons / MeV Total weight 5 t

27 The FACE trigger projection onto x axis projection onto y axis cellular logic decision time 4 µsec marking a cluster counting and removing

28 Photoproduction of π / η mesons Excitation spectra and quantum numbers alone do not provide very sensitive tests of hadron models Models also have to... predict transitions between states Unknown branching fractions for η decays link observables to fundamental questions Little known on η decays!

29 Selection according to the number of clusters in the Crystal Barrel Identification of protons via inner scintillating fibre detector Kinematic fitting (e.g. 3π and missing proton) Reconstruction ~ 78 (a) η γ ~ 1 (b) η 3 π π η MeV/c m γ γ [MeV/c ω η from hep-ph/31145 on eta photoproduction Invariant γγ mass ]

30 Differential cross sections for the reaction γ p p π d σ/d Ω [µ b/sr] SAID predictions CB ELSA fit Preliminary cos θ cm -.4 PhD thesis Harald van Pee, Bonn 3

31 Differential cross sections for the reaction γ p p π d σ/d Ω 6 4 [µ b/sr] SAID predictions CB ELSA fit Preliminary cos θ cm -.3 PhD thesis Harald van Pee, Bonn 3

32 Differential cross sections for the reaction γ p p π d σ/d Ω 3 [µ b/sr] SAID predictions CB ELSA fit Preliminary cos θ cm -.1 PhD thesis Olivia Bartholomy, Bonn 4

33 Differential cross sections for the reaction γ p p π d σ/d Ω [µ b/sr] SAID predictions CB ELSA fit cos θ cm Preliminary -.1 PhD thesis Olivia Bartholomy, Bonn 4

34 Total cross section for the reaction γ p p π d σ/d Ω 1 [µ b/sr] σ tot [µb] E γ [GeV] Preliminary cos θ cm W [GeV] PhD thesis Olivia Bartholomy, Bonn 4

35 Investigation of the reaction γ p p π Aspects of pion production for CB-ELSA Understanding of detector acceptances in preparation for partial wave analyses also for other channels (γ p p π π, γ p p π η, etc.) Normalisation (photon flux) by fitting angular distributions to known SAID predictions cross check with hardware measurements Better understanding of high-energy behaviour E γ > GeV: t channel vector meson exchanges are better described in terms of Regge trajectories allows extrapolation towards lower energies Resonances are excited up to the highest available photon energies, however, strong production of pions in the forward direction is observed above.4 GeV (presumably exchange of mesons in the t channel)

36 Investigation of the reaction γ p p η η 3π η 3π (CB ELSA) N η = 1615 σ = Invariant π π π mass [ MeV/c ] γ p p X (missing mass) (CLAS)

37 Investigation of the reaction γ p p η R =.85 ±.1 ±.5 BR(η 3π ) 3.5 % BR(η γγ) 39.4 % R = BR (η 3π ) BR (η γ) for each bin in dσ/dω weighted by 1/σ 4 PDG: ±.4.83 ± ±.37 SND TAPS CBAR Data for η γγ and η 3π can be added up!

38 Differential cross sections for the reaction γ p p η CB-ELSA CB ELSA fit GRAAL CLAS TAPS dσ/dω [ µb/sr ] Crede et al., hep-ph/31145, submitted to Phys. Rev. Lett cos θ cm

39 Differential cross sections for the reaction γ p p η CB-ELSA CB ELSA fit CLAS dσ/dω [ µb/sr ] Crede et al., hep-ph/31145, submitted to Phys. Rev. Lett cos θ cm

40 Models describing differential cross sections for γ p p η Multipole analysis (L. Tiator et al.: PRC 6 (1999) 351) Bonn as well as Mainz cross section data, Graal beam polarization up to 11 MeV S 11 (1535), D 13 (15), S 11 (165), D 15 (1675), P 11 (144), F 15 (168) Isobar model (ETA-MAID by W.T. Chiang, L. Tiator) S 11 (1535), D 13 (15), S 11 (165), D 15 (1675), F 15 (168), D 13 (17), P 11 (171), P 13 (17) + (ρ, ω) exchange in the t channel Coupled channel analysis (C. Bennhold et al.: PRC 58 (1998), 457 PRC 59 (1999), 46) all known spin 1/ and spin 3/ resonances included up to GeV + (ρ, ω) exchange in the t channel only S 11 (1535), D 13 (15), S 11 (165), D 13 (17) of importance Chiral constituent quark model (B. Saghai and Z. Li: proceedings of N conference ) all known *** and resonances included, no t exchange contributions! third S 11 (m = 178 MeV, Λ = 8 MeV): CLAS data up to E γ = GeV

41 Total cross section for the reaction γ p p η σ tot [µb] E γ [GeV] CB ELSA data CB ELSA fit GRAAL CLAS TAPS The angular coverage of new CB ELSA data allows determination of the total cross section CB-ELSA: Hint for N resonance (8)D 15 (hep-ph/31145) Needs confirmation! W [GeV] No need for third S 11 Remaining questions: How do baryon resonances couple to η mesons? Is there evidence for a third S 11 at 178 MeV? How are η mesons produced at high energies?

42 Hidden symmetry for η decays? L=3 S=3/ S=1/ ND13 N() D15 N(19)G17 N(5)G19 N(8)D15 N(8)G17 L= S=3/ S=1/ N(1)P11 N(19)P13 N()F15 N(199)F17 N(17)P13 N(168)F15 L=1 S=3/ S=1/ N(165)S11 N(17)D13 N(1675)D15 N(1535)S11 N(15)D13 J=1/ J=3/ J=5/ J=7/ J=9/

43 Investigation of the reaction γ p p π η Data comprises full statistics of a 1 production run with E e = 3. GeV: Events CL CL >1% pπ pπ 1 4 γγ > 1% γγ CL CL >1% and CL <1% pπ pπ γγ > 1% and γγ CL pπ η < pπ η CL CL >1% and CL pπ <1% pπ γγ > 1% and γγ CL pπ π pπ< π γ p p π γγ M γγ [MeV/c ] Signal to background ratio for mesons: Selection for η meson in the reaction γ p p π η: 16 : 1 for π meson in the reaction γ p p π π : 3 : 1 5-particle final states proton identification: inner detector kinematic fitting energy as well as momentum conservation mass constraints

44 Mass plots for the reaction γ p p π η (E e = 3. GeV) a b Invariant pπ η mass [ MeV/c ] Do we observe (13)η? γ p π η c (13) 1.8 < s <. (in GeV/c ) forming invariant pπ mass for (b). < s <.35 (in GeV/c ) forming invariant pπ mass for (c) Invariant pπ mass [ MeV/c ] (13) (13) Invariant pπ mass [ MeV/c ]?

45 Mass plots for the reaction γ p p π η (E e = 3. GeV) γγp p pπ ηp π η forming invariant π η mass 1.8 < s <. (in GeV/c ) Invariant pπ η mass 18- [MeV/c ] Invariant π η mass [MeV/c ] Parameters for a (98) production: E threshold 149 MeV s 19 MeV/c Do we observe γ p a (98)p? forming invariant π η mass Invariant π η mass [MeV/c ]. < s <. (in GeV/c ) Invariant pπ η mass - [MeV/c ] a (98) Invariant π η mass [MeV/c ]

46 Total cross section of γ p p π η Performing a PWA: first results µb] σ [ Total cross sections with MeV bin size γp pπ η γp pπ π GRAAL Preliminary New resonances necessary in order to describe data: evidence for (I = 3, J = 3 ) at. GeV Hints for contributions from the reaction a (98)p s [ MeV ] Unbinned maximum likelihood fit: event-based fit (important for #particles 3) takes all correlations properly into account (5 independent variables) Question of negative parity states around 19 MeV: solutions ambiguous (polarization necessary) CB/TAPS

47 Investigation of the reaction γ p p π π (E e = 3. GeV) 1.6 < s < 1.75 (in GeV) 1.87 < s < 1.98 (in GeV). < s <.1 (in GeV) (13) (13) D 13 (15) Invariant pπ mass [ MeV/c ] Invariant pπ mass [ MeV/c ] Invariant pπ mass [ MeV/c ] x 1 3 x 1 3 x m pπ [MeV/c ] x 1 3 m pπ [MeV/c ] x 1 3 m pπ [MeV/c ] x

48 New challenges: linear polarization and CB TAPS Goniometer v amorphous radiators screen z h empty position wires for determination of beam profiles diamond crystal Crystal Barrel 18 CsI crystals removed in the forward direction Data taking: Sep. Dec. 3 (un)polarized photon beam liquid H, deuterium solid targets TAPS 51 BaF crystals forward detector high granularity fast trigger

49 CB TAPS: γ p p π η Polarization observables provide additional pieces of information Results of PWA for the reaction γ p p π η are ambiguous Calculation (P T = 1 %) toy MC toy MC P y (cos θ η ) toy MC toy MC P y (cos θ p ) P y (cos θ πη ) P y (cos θ pπ ) Example for -body final state (E fixed): N(φ) = N (1 + P T Σ cos(φ)) P T : transversal photon polarization Σ: photon asymmetry of the reaction P y : P T Σ (one for -body decay) more for 3-body decay 8 >< >: J P = 1/ + (polarized and unpolarized) J P = 3/ (unpolarized) J P = 3/ (polarized)

50 Polarization and observed asymmetries Diamond crystal used to create coherent bremsstrahlung Polarization Asymmetry Σ PT E pol = 16 MeV E γ [MeV] E photon [MeV] Agreement between polarization and observed asymmetry very good!

51 CB TAPS (3 data): γ p p π η φ [rad] φ [rad] Σ P T cos ( θ) φ [rad] φ [rad] [ -1., -.5 ] [ -.5,. ] [.,.5 ] [.5, 1. ] Discrimination of ambiguous solutions in the PWA of the unpolarized data higher sensitivity (small contributions may have a big effect in certain polarization variables) 17 events of the type γ p p π η (3 % of total statistics) Φ π for different cos Θ π bins (data is not yet acceptance corrected!) (144 MeV E γ 164 MeV)

52 Summary and conclusion A: Baryon spectroscopy: Touching upon the question of missing resonances Observation of baryon cascades in γ p p π π as well as γ p p π η Hint for new N resonance N(8)D 15 observed in its decay to pη (V. Credé et al., submitted to Phys. Rev. Lett.) Hints for further states: 3 around. GeV/c (in the reaction γ p p π η) There is yet a lot more to be discovered! Polarization (beam and target) Crystal Barrel at ELSA CLAS at JLab, etc.

53 Summary and conclusion A: Baryon spectroscopy: Touching upon the question of missing resonances Observation of baryon cascades in γ p p π π as well as γ p p π η Hint for new N resonance N(8)D 15 observed in its decay to pη (V. Credé et al., submitted to Phys. Rev. Lett.) Hints for further states: 3 around. GeV/c (in the reaction γ p p π η) There is yet a lot more to be discovered! Polarization (beam and target) Crystal Barrel at ELSA CLAS at JLab, etc. B: Meson spectroscopy: The search for new forms of matter There are certainly striking observations ( J PC = 1 + )! ( However, our knowledge is far from solid!) CLAS at JLab CLEO-c at Cornell GlueX at JLab