Talk - Hadronic Nuclear Physics Group

Cornell University Cleo lunch talk Volker Credé 

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 (2000/2001) 

– Baryon spectroscopy (photoproduction of neutral mesons: E γ < 3 GeV ) 

– γ p → p π 0 (p η) ⇒ differential and total cross sections 

– γ p → p π 0 π 0 (p π 0 η) ⇒ total cross sections and PWA 

• Summary and outlook: polarization measurements with CB/TAPS (2003) 

Wilson Laboratory for Nuclear Studies, Cornell University, Ithaca, NY 14853


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?


Renaissance of Hadron Spectroscopy 

Recently discovered new states ➜ Beginning of new spectroscopy ? 

Mesons 

• D ∗+ 

sJ (2317) → D+ s 

π 0 at BaBar 

(B. Aubert et al., Phys. Rev. Lett. 90 (2003) 242001) 

• D + sJ 

(2457) → D∗+ s 

π 0 at CLEO 

(D. Besson et al., Phys. Rev. D68 (2003) 032002) 

⇒ Observation of missing 

0 + 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 = −2 

(C. Alt et al. [NA49 Coll.], hep–ex/0310014) 

⇒ 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’03 conference) 

(S.K. Choi et al., hep–ex/0303032)


The quark model of hadrons 

• Mesons (qq) q ⊗ q = 3 ⊗ 3 = 8 ⊕ 1 

• Baryons (qqq) q ⊗ q ⊗ q = 3 ⊗ 3 ⊗ 3 = 10 ⊕ 8 ⊕ 8 ⊕ 1 

} {{ } 

Ordinary matter, however, QCD also predicts so–called exotic states 

⇒ simplest possibility: q ⊗ q ⊗ q = 15 ⊕ 6 ⊕ 3 ⊕ 3 

} {{ } 

➜ 

Does not work: color singlets needed ! 

⇒ multiple of (qqq) and (qq) necessary 

• Glueballs: g ⊗ g = 8 ⊗ 8 = 27 ⊕ 10 ⊕ 10 ⊕ 8 ⊕ 8 ⊕ 1 

• Hybrids: q ⊗ q ⊗ g = 27 ⊕ 10 ⊕ 10 ⊕ 8 ⊕ 8 ⊕ 8 ⊕ 1 ⇒ (qq) l ((q) 3 ) m (g) n , 

l + m ≥ 1 for n = 1



• Mesons (qq) q ⊗ q = 3 ⊗ 3 = 8 ⊕ 1 


} {{ } 



} {{ } 

➜ 



• Glueballs: g ⊗ g = 8 ⊗ 8 = 27 ⊕ 10 ⊕ 10 ⊕ 8 ⊕ 8 ⊕ 1 

Exciting the flux tube 

➜ 

Hybrids 


l + m ≥ 1 for n = 1



• Mesons (qq) q ⊗ q = 3 ⊗ 3 = 8 ⊕ 1 


} {{ } 



} {{ } 

➜ 



• Glueballs: g ⊗ g = 8 ⊗ 8 = 27 ⊕ 10 ⊕ 10 ⊕ 8 ⊕ 8 ⊕ 1 

Exciting the flux tube 

➜ 

Hybrids 


• Multi–quark states l + m ≥ 1 for n = 1


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 = −2







Ξ −− with Q = −2 

2. States with exotic combinations of J P C 

⇒ 

forbidden for ordinary qq states: 

0 +− , 0 −− , 1 −+ , 2 +− , 3 −+ , etc. 

π 1 (1400) → ηπ − 

π 1 (1600) → η ′ π − ⎫ 

⎬ 

⎭ J P C = 1 −+







Ξ −− with Q = −2 

2. States with exotic combinations of J P C 

⇒ 

forbidden for ordinary qq states: 

0 +− , 0 −− , 1 −+ , 2 +− , 3 −+ , etc. 

π 1 (1400) → ηπ − 

π 1 (1600) → η ′ π − ⎫ 

⎬ 

⎭ J P C = 1 −+ 

3. States with hidden exotic properties 

⇒ Problem: predicted glueballs can 

mix with ordinary qq states 

f 0 (1500) 

} 

J P C = 0 ++ 

Evidence far from solid 

⇒ Details needed for a full understanding are missing


Spectroscopy with the Crystal–Barrel Detector 

2000 - present (University of Bonn) 

• Photoproduction experiments at ELSA 

⇒ Spectroscopy of light baryons 

(... and mesons) 

1989 - 1996 (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


Photoproduction with the Crystal–Barrel Detector at ELSA 

Photoproduction 

2.0 < m pπ 0 π 0 < 2.1 (in GeV/c 2 ) 

➜ 

➜ 

200 

150 

100 

50 

γ p → pπ 0 π 0 

0 

1000 1200 1400 1600 1800 2000 

Invariant pπ 0 mass [ MeV/c 2 ] 

➜ 

Spectroscopy 

Baryon spectrosopy 

• Search for missing 

resonances


General Physical Motivation 

⇒ Search for missing resonances 

Quark models predict many more baryons than have been observed 

**** *** ** * 

N spectrum 11 3 6 2 

∆ spectrum 7 3 6 6 

⇒ 

⇒ 

according to PDG 

(Phys. Rev. D66 (2002) 010001) 

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)


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) 094030) 

• One–gluon exchange (S. Capstick and N. Isgur, Phys. Rev. D34 (1986) 2809) 

(relativized quark model) 1. wrong spin–orbit couplings 

2. no explanation for parity doublets 

• Instanton–induced interaction 

(relativistic quark model) 

1. acceptable Regge trajectories 

2. natural explanation for parity doublets 

Which is the right model? 

⇒ Do we have a correct model? Is there really one interaction that dominates?


Symmetries and classification 

| qqq 〉 = | colour 〉 A · | space; spin, flavour 〉 S 

O(6) SU(6) → SU(2)| spin ⊗ SU(3)| flavour 

⇒ Total wave function antisymmetric with respect to exchange of two quarks 

SU(6) symmetry: (Notation: 2S+1 multiplet) 

6 ⊗ 6 ⊗ 6 = 56 S ⊕ 70 M ⊕ 70 M ⊕ 20 A 

⇒ 56 = 4 10 ⊕ 2 8 

70 = 2 10 ⊕ 4 8 ⊕ 2 8 ⊕ 2 1 

20 = 2 8 ⊕ 4 1 

Notation for baryon resonances: 

(example) 

S 11 (1535) ⇐⇒ N 1 

2 

−(1535) 

Classification of multiplets (D, L P N ): 

ground state (56, 0 + 0 ): 

∆ 3 

2 

+(1232) ɛ 4 10 

N 1 

2 

+(939) ɛ 2 8 

1. excited state (70, 1 − 1 ): 

∆ 1 −(1620), 

2 

∆ 3 −(1700) 

2 

ɛ 2 10 

N 1 −(1535), 

2 

N 3 −(1520) 

2 

ɛ 2 8 

N 1 −(1650), N 3 −(1700) 

2 

2 

N 5 −(1675) 

2 

ɛ 4 8

¢¡ 

¥¤ £ 

§¦ 

¥¨¤ 

¦ ¢©¦ ¨¤ 


3000 

2500 

—— U. Löhring, B.C. Metsch and H.R. Petry, Eur. Phys. J. A10, 395-446 (2001) 

missing 

resonances 

2700 

** 

2600 

*** 

Mass [MeV] 

2000 

1500 

2100 

* 

1986 

2000 1990 

S 

1900 

** ** 

** 

1710 1720 

1680 

*** 

**** 

**** 

1440 

**** 

2220 

**** 

2090 

2080 

* ** 

1897 1895 

S S 

1700 

1650 *** 

**** 

1535 1520 

**** **** 

2200 

** 

1675 

**** 

2190 

**** 

2250 

**** 

1000 

L 

J π 

2T 2J 

939 

**** 

Bonn model: residual short–range interaction 

based on instanton–induced forces 

1/2+ 3/2+ 5/2+ 7/2+ 9/2+ 11/2+ 13/2+ 1/2- 3/2- 5/2- 7/2- 9/2- 11/2- 13/2- 

P11 P F15 F17 

H19 

H K S11 D13 D15 G17 G19 

I I 

13 1 11 1 13 1 11 1 13


Nucleon resonances 

—— S. Capstick and N. Isgur, Phys. Rev. D34 (1986) 2809 

3000 

** 

2500 

*** 

**** 

** **** 

**** 

Mass [MeV] 

2000 

* 

S 

*** 

** 

**** 

** ** 

**** 

* ** 

S S 

*** 

**** 

**** 

1500 

**** 

**** 

**** 

1000 

**** 

OGE model: residual short–range interaction 

based on one–gluon exchange 

J π 1/2+ 3/2+ 5/2+ 7/2+ 9/2+ 11/2+ 13/2+ 1/2- 3/2- 5/2- 7/2- 9/2- 11/2- 13/2-


—— U. Löhring, B.C. Metsch and H.R. Petry, Eur. Phys. J. A10, 395-446 (2001) 

3000 

2950 

** 

2750 

** 

2500 

2390 

* 

2300 

** 

2420 

**** 

2150 

* 

2350 

* 

2200 

* 

2400 

** 

Mass [MeV] 

2000 

1500 

1910 

**** 

1750 

* 

1920 

*** 

1600 

*** 

2000 

** 

1905 

**** 

➜ 

1950 

**** 

too low in mass 

➜ 

1900 

** 

1620 

**** 

1940 1930 

* *** 

1700 

**** 

**** 

Bonn model: residual short–range interaction 

based on instanton–induced forces 

1232 

1000 

J π 1/2+ 3/2+ 5/2+ 7/2+ 9/2+ 11/2+ 13/2+ 15/2+ 1/2- 3/2- 5/2- 7/2- 9/2- 11/2- 13/2- 15/2- 

L 2T 2J P P 33 H K K S D 

G I 

35 37 39 H 3 11 3 13 3 15 33 35 37 39 I 3 11 3 13 

31 F F 31 D G


∆ resonances 

—— S. Capstick and N. Isgur, Phys. Rev. D34 (1986) 2809 

3000 

** 

** 

2500 

* 

** 

**** 

* 

** 

* 

* 

Mass [MeV] 

2000 

1500 

**** 

* 

*** **** 

*** 

** 

➜ 

**** 

too low in mass 

➜ 

** 

**** 

* *** 

**** 

1000 

**** 

OGE model: residual short–range interaction 

based on one–gluon exchange 

J π 1/2+ 3/2+ 5/2+ 7/2+ 9/2+ 11/2+ 13/2+ 15/2+ 1/2- 3/2- 5/2- 7/2- 9/2- 11/2- 13/2- 15/2-


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


The Crystal–Barrel Experiment at ELSA 

Photoproduction experiment using a liquid H 2 target 

(unpolarized in a first series of experiments → 2000/2001) 

Inner Detector 

Crystal Barrel 

Tagging Magnet 

Tagging range: 

25 % − 93 % of incoming E e − 

Target 

Tagger 

Radiator 

E e − = 1.4 GeV 

0.35 GeV ≤ E γ ≤ 1.3 GeV 

1.2 GeV/c 2 ≤ √ s ≤ 1.8 GeV/c 2 

γ e - 

e - - 

E e − = 3.2 GeV 

0.8 GeV ≤ E γ ≤ 3.0 GeV 

1.5 GeV/c 2 ≤ √ s ≤ 2.6 GeV/c 2 

E = E - E 

γ 0 e


The tagging system 

• 14 scintillation counters 

• 2 wire chambers (352 wires) 

• tagging range: 25 % − 92 % 

electrons 

radiator 

magnet 

photons 

primary electron beam 

beam dump 

high photon energies 

low photon energies


Target and inner detector 

The inner detector: 

• 3 layers of scintillating fibres 

(⇒ additional reconstruction point) 

• Trigger on charged tracks 

1 cm


The Crystal–Barrel Experiment at ELSA 

➜ 

Tagging magnet 

➜ 

Barrel calorimeter


Cross section of a CsI(Tl) module 

LEAR: 

operation in a magnetic field of 1.5 T 

⇓ 

silicon photodiodes to read out 

scintillation light 

0.1 mm titanium can 

1 

circuit board 

6 

5 

light fibre 

2 mm Titan 

CsI(Tl) 

4 

preamplifier 

10 cm 

3 

photodiode 

2 

wavelength shifter


Segmentation in Φ 

Segmentation in Θ 

20 × 60 crystals (type 1-10), opening angle 6 ◦ 

6 × 30 crystals (type 11-13), opening angle 12 ◦ 

26 rings of crystals, opening angle 6 ◦ 

The barrel calorimeter

$ %& 

! # 

" 

' 

!" ) 

(!) " 

('& 

" %$# ! 


Arrangement and parameters of crystal modules 

6 o 12 o Φ 

8 

9 

10 

11 

12 

13 

7 

6 

5 

4 

3 

2 

1 

1 2 3 4 5 6 7 8 

9 

10 

11 

12 

13 

Θ 

Density 4.53 g/cm 3 

Radiation length L 0 

1.86 cm 

Moliére radius 

3.8 cm 

13 

12 

11 

10 

9 

8 

7 

6 

5 

4 

3 

2 

1 

13 

12 

11 

10 

9 

1 2 3 4 5 6 7 8 

Light output 

0.85 relative to NaI 

Maximum emission 

550 nm 

Decay times 0.9 and 7 µs 

Photon yield 

4.5 × 10 4 /MeV 

Crystal length 30 cm = 16.1 L 0 

Solid angle 

97.8 % of 4π 

Typ. signal 

2000 photons / MeV 

Total weight 

5 t


The FACE trigger 

projection onto x axis 

projection onto y axis 

• cellular logic 

• decision time ≈ 4 µsec 

1 

1 

1 

1 

marking a cluster 

counting and removing


Photoproduction of π 0 / 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 !


• Selection according to the number 

of clusters in the Crystal Barrel 

• Identification of protons via inner 

scintillating fibre detector 

• Kinematic fitting 

(e.g. 3π 0 and missing proton) 

10 6 6000 

5000 ~ 78000 (a) 

4000 η → 2 γ 

3000 

2000 

10 5 

1000 

400 500 600 

10 4 

Reconstruction 

2500 

2000 

1500 

1000 

500 

~ 21000 (b) 

0 

η → 3 π 

400 500 600 

10 3 

10 5 2 

10 4 

10 3 

10 2 

10 

1 

π 0 

η 

200 400 600 800 

MeV/c 

10 2 

100 200 300 400 500 600 

m γ γ [MeV/c 

ω 

η 

′ from hep-ph/0311045 

on eta photoproduction 

➜ 

Invariant γγ mass 

➜ 

2 

]


Differential cross sections for the reaction γ p → p π 0 

d σ/d Ω [µb/sr] 

—– SAID predictions —— CB–ELSA fit 

300 - 325 325 - 350 350 - 375 375 - 400 400 - 425 425 - 450 

20 

0 

10 

5 

0 

6 

4 

450 - 475 475 - 500 500 - 525 525 - 550 550 - 575 575 - 600 

Preliminary 

600 - 625 625 - 650 650 - 675 675 - 700 700 - 725 725 - 750 

0 

-3 

0 

-1 

2 

0 

-1 -0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

cos θ cm 

0 

-0.4 

PhD thesis Harald van Pee, Bonn 2003



d σ/d Ω 

6 

4 

[µb/sr] 


750 - 775 775 - 800 800 - 825 825 - 850 850 - 875 875 - 900 

2 

0 

4 

2 

0 

3 

2 

900 - 925 925 - 950 950 - 975 975 - 1000 1000 - 1025 1025 - 1050 

Preliminary 

1050 - 1075 1075 - 1100 1100 - 1125 1125 - 1150 1150 - 1175 1175 - 1200 

0 

-0.4 

0 

-0.3 

1 

0 

-1 -0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

cos θ cm 

0 

-0.3 

PhD thesis Harald van Pee, Bonn 2003



d σ/d Ω 

3 

2 

[µb/sr] 


1200 - 1225 1225 - 1250 1250 - 1275 1275 - 1300 1300 - 1350 1350 - 1400 

1 

0 

3 

2 

1 

0 

1 

1400 - 1450 1450 - 1500 1500 - 1550 1550 - 1600 1600 - 1650 1650 - 1700 

Preliminary 

1700 - 1750 1750 - 1800 1800 - 1850 1850 - 1900 1900 - 1950 1950 - 2000 

0 

-0.2 

0 

-0.2 

0 

-1 -0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

cos θ cm 

0 

-0.1 

PhD thesis Olivia Bartholomy, Bonn 2004



d σ/d Ω [µb/sr] 


2000 - 2050 2050 - 2100 2100 - 2150 2150 - 2200 2200 - 2250 2250 - 2300 

1 

0 

2300 - 2400 2400 - 2500 2500 - 2600 2600 - 2800 2800 - 3000 -0.5 0 0.5 1 

1 

0 

0 

-0.1 

-1 -0.5 0 0.5 1 -0.5 0 0.5 1 -0.5 0 0.5 1 -0.5 0 0.5 1 -0.5 0 0.5 1 

cos θ cm 

Preliminary 

0 

-0.1 



Total cross section for the reaction γ p → p π 0 

d σ/d Ω 

1 

[µb/sr] 

σ tot 

[µb] 

E γ 

[GeV] 

2000 - 2050 2050 - 2100 2100 - 2150 2150 - 2200 2200 - 2250 2250 - 2300 

400 

300 

200 

0.5 1 1.5 2 2.5 

0 

1 

0 

100 

2300 - 2400 2400 - 2500 2500 - 2600 2600 - 2800 2800 - 3000 

10 

-1 -0.5 0 0.5 1 -0.5 0 0.5 1 -0.5 0 0.5 1 -0.5 0 0.5 1 -0.5 0 0.5 1 

Preliminary 

-0.5 0 0.5 1 

0 

-0.1 

cos 

θ cm 

0 

-0.1 

1.4 1.6 1.8 2 2.2 2.4 

W [GeV] 



Investigation of the reaction γ p → p π 0 

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 0 π 0 , γ p → p π 0 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 > 2 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 2.4 GeV (presumably exchange of mesons in the t–channel)


Investigation of the reaction γ p → p η 

7000 

6000 

5000 

η → 3π 0 

➜ 

⎧ 

⎨ 

⎩ 

η → 3π 0 

(CB − ELSA) 

4000 

3000 

2000 

1000 

N 

η = 16105 

σ = 9.71013 

0 

300 400 500 600 700 

Invariant π 0 π 0 π 0 mass [ MeV/c 2 ] 

γ p → p X (missing mass) 

(CLAS) 

⎫ 

⎬ 

⎭ 

➜


Investigation of the reaction γ p → p η 

120 

100 

80 

60 

R = 0.825 ± 0.001 ± 0.005 

BR(η → 3π 0 ) ≈ 32.5 % 

BR(η → γγ) ≈ 39.4 % 

R = BR (η → 3π0 ) 

BR (η → 2γ) 

for each bin in dσ/dΩ 

weighted by 1/σ 2 

40 

PDG: 

20 

0 

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 

• 0.826 ± 0.024 

• 0.832 ± 0.013 

• 0.841 ± 0.037 

SND 

TAPS 

CBAR 

⇒ Data for η → γγ and η → 3π 0 can be added up !


2 

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/0311045, submitted to Phys. Rev. Lett. 

750 - 800 800 - 850 850 - 900 900 - 950 950 - 1000 1000 - 1050 

1 

0 

0.5 

1050 - 1100 1100 - 1150 1150 - 1200 1200 - 1250 1250 - 1300 1300 - 1350 

0 

-0.5 

0 

0.5 

1350 - 1400 1400 - 1450 1450 - 1500 1500 - 1550 1550 - 1600 1600 - 1650 

0 

-0.05 

0 

0 

-1 -0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.05 

cos θ cm


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/0311045, submitted to Phys. Rev. Lett. 

0.5 

1650 - 1700 1700 - 1750 1750 - 1800 1800 - 1850 1850 - 1900 1900 - 1950 

0 

0.5 

1950 - 2000 2000 - 2050 2050 - 2100 2100 - 2150 2150 - 2200 2200 - 2250 

0 

-0.05 

0 

0.5 

2250 - 2300 2300 - 2400 2400 - 2500 2500 - 2600 2600 - 2800 2800 - 3000 

0 

-0.05 

0 

0 

-1 -0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.5 0 0.5 1 

-0.05 

cos θ cm


Models describing differential cross sections for γ p → p η 

• Multipole analysis (L. Tiator et al.: PRC 60 (1999) 035210) 

Bonn as well as Mainz cross section data, 

Graal beam polarization up to 1100 MeV 

⇒ S 11 (1535), D 13 (1520), S 11 (1650), D 15 (1675), P 11 (1440), F 15 (1680) 

• Isobar model (ETA-MAID by W.T. Chiang, L. Tiator) 

⇒ S 11 (1535), D 13 (1520), S 11 (1650), D 15 (1675), F 15 (1680), D 13 (1700), 

P 11 (1710), P 13 (1720) + (ρ, ω) exchange in the t–channel 

• Coupled–channel analysis (C. Bennhold et al.: PRC 58 (1998), 457 PRC 59 (1999), 460) 

⇒ all known spin–1/2 and spin–3/2 resonances included up to 2 GeV 

+ (ρ, ω) exchange in the t–channel 

⇒ only S 11 (1535), D 13 (1520), S 11 (1650), D 13 (1700) of importance 

• Chiral constituent quark model (B. Saghai and Z. Li: proceedings of N ∗ conference 2002) 

⇒ all known *** and **** resonances included, no t–exchange contributions ! 

⇒ third S 11 (m = 1780 MeV, Λ = 280 MeV): CLAS data up to E γ = 2 GeV


Total cross section for the reaction γ p → p η 

σ tot 

20 

15 

10 

5 

[µb] 

E γ 

1 1.5 2 2.5 

[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 (2080)D 15 

(hep-ph/0311045) 

⇒ Needs confirmation ! 

1.6 1.8 2 2.2 2.4 

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 1780 MeV ? 

• How are η mesons produced at high energies ?


Hidden symmetry for η decays ? 

L=3 

S=3/2 

S=1/2 

ND13 

N(2200) D15 N(2190)G17 N(2250)G19 

N(2080)D15 N(2080)G17 

L=2 

S=3/2 

S=1/2 

N(2100)P11 

N(1900)P13 N(2000)F15 N(1990)F17 

N(1720)P13 

N(1680)F15 

L=1 

S=3/2 

S=1/2 

N(1650)S11 N(1700)D13 N(1675)D15 

N(1535)S11 

N(1520)D13 

J=1/2 

J=3/2 J=5/2 J=7/2 J=9/2


Investigation of the reaction γ p → p π 0 η 

Data comprises full statistics of a 2001 production run with E e 

− 

= 3.2 GeV: 

Events 

—– CL CL >10% 

pπ pπ0 10 4 0 γγ 

> 10% γγ 

10 3 

10 2 

10 

1 

⇒ 

—– CL CL >10% and CL 10% and γγ CL pπ 0 η 

< pπ 0 η —– CL CL >10% and CL pπ0 10% and γγ CL pπ 0 π 0 pπ< 0 π 0 

100 200 300 400 500 600 700 800 900 1000 

γ p → p π 0 γγ ⇒ M γγ [MeV/c 2 ] 

Signal to background ratio for mesons: 

Selection 

• for η meson in the reaction γ p → p π 0 η: ≈ 16 : 1 

• for π 0 meson in the reaction γ p → p π 0 π 0 : ≈ 30 : 1 

• 5-particle final states 

• proton identification: 

inner detector 

• kinematic fitting 

– energy as well as 

momentum conservation 

– mass constraints


Mass plots for the reaction γ p → p π 0 η (E e − = 3.2 GeV) 

450 

400 

350 

300 

250 

200 

150 

100 

50 

a b 

0 

1000 1500 2000 2500 3000 

Invariant pπ 0 η mass [ MeV/c 2 ] 

Do we observe ∆ ∗ → ∆(1232)η ? 

γ 

⇒ 

✕ 

∆ ∗ 

✙ 

p 

❘ 

π 0 

η 

c 

∆(1232) 

1.8 < √ s < 2.0 (in GeV/c 2 ) 

forming invariant 

pπ 0 mass for (b) 

➜ 

2.2 < √ s < 2.35 (in GeV/c 2 ) 


pπ 0 mass for (c) 

➜ 

600 

500 

400 

300 

200 

100 

120 

100 

0 

1000 1200 1400 1600 


80 

60 

40 

20 

∆(1232) 

∆(1232) 

0 

1000 1200 1400 1600 1800 2000 


?


Mass plots for the reaction γ p → p π 0 η (E e − = 3.2 GeV) 

500 

400 

300 

200 

γγp p →pπ ηp π 0 η 


π 0 η mass 

➜ 

1.8 < √ s < 2.0 (in GeV/c 2 ) 

0 

600 650 700 750 800 850 900 950 1000 1050 

350 

300 

250 

200 

150 

100 

50 

0 

Invariant pπ η mass 1800-2000 [MeV/c ] 

2 

100 

0 

600 700 800 900 1000 11001200 1300 1400 1500 

0 

2 

Invariant π η mass [MeV/c ] 

Parameters for a 0 (980) production: 

• E threshold ≈ 1490 MeV 

⇒ √ s ≈ 1920 MeV/c 2 

Do we observe γ p → ∆ ∗ → a 0 (980)p? 


π 0 η mass 

➜ 

0 

300 

250 

200 

150 

100 

50 

0 

2 


2.0 < √ s < 2.2 (in GeV/c 2 ) 

0 

Invariant pπ η mass 2000-2200 [MeV/c ] 

a 0 (980) 

0 

600 700 800 900 1000 1100 1200 1300 

0 

2 


2


Total cross section of γ p → p π 0 η 

Performing a PWA: first results 

µb] 

σ [ 

Total cross sections with 20 20 MeV bin size 

14 

12 

10 

8 

6 

4 

0 

γp → pπ η 

γp → pπ 0 π 0 

GRAAL 

Preliminary 

• New resonances necessary in order 

to describe data: 

⇒ evidence for (I = 3 − 

2 , J = 

3 − 

2 ) 

at ≈ 2.2 GeV 

• Hints for contributions from the 

reaction ∆ ∗ → a 0 (980)p 

2 

1600 1800 2000 2200 2400 

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 1900 MeV: 

⇒ solutions ambiguous 

(polarization necessary) 

⇒ CB/TAPS


1200 

1000 

800 

600 

400 

200 

Investigation of the reaction γ p → p π 0 π 0 (E e − = 3.2 GeV) 

1.6 < √ s < 1.75 (in GeV) 1.87 < √ s < 1.98 (in GeV) 2.0 < √ s < 2.1 (in GeV) 

∆(1232) 

500 

400 

300 

200 

100 

∆(1232) 

D 13 (1520) 

x 

0 

1000 1200 1400 1600 1800 2000 

0 

1000 1200 1400 1600 1800 2000 

0 

1000 1200 1400 1600 1800 2000 




x 10 3 

x 10 3 

10 3 

40 4000 

20 

2800 

140 3250 

35 

18 

3500 

120 

16 

30 

2400 

2750 

100 

3000 

14 

25 

12 

2000 

80 2250 

20 2500 

10 

60 

15 

8 

1600 

1750 

2000 

6 

40 

10 

1200 

20 

1500 

4 

1250 

5 

2 

0 

0 1000 

0 

1000 1500 2000 2500 3000 

1000 1500 2000 2500 3000 3500 

1500 2500 3500 

m 2 x 10 3 x 10 3 x 10 3 pπ 0 [MeV/c 2 ] 2 m 2 pπ 0 [MeV/c 2 ] 2 m 2 pπ 0 [MeV/c 2 ] 2 

250 

200 

150 

100 

50


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 

• 180 CsI crystals removed 

in the forward direction 

Data taking: Sep. 2002 – Dec. 2003 

• (un)polarized photon beam 

• liquid H 2 , deuterium 

• solid targets 

TAPS 

• 512 BaF crystals 

• forward detector 

– high granularity 

– fast trigger


CB–TAPS: ⃗γ p → p π 0 η 

Polarization observables provide additional pieces of information 

⇒ Results of PWA for the reaction γ p → p π 0 η are ambiguous 

Calculation → (P T = 100 %) 

0.04 

0 

-0.02 

-0.06 

-0.1 

-1 -0.5 0 0.5 1 

0.05 

0 

-0.05 

-0.1 

-0.15 

toy MC 

toy MC 

P y (cos θ η ) 

-0.2 

-1 -0.5 0 0.5 1 

0.04 

0 

-0.02 

-0.06 

-0.1 

-0.12 

-1 -0.5 0 0.5 1 

0.05 

0 

-0.05 

-0.1 

-0.15 

toy MC 

toy MC 

P y (cos θ p ) 

P y (cos θ πη ) P y (cos θ pπ ) 

-0.2 

-1 -0.5 0 0.5 1 

➜ 

Example for 2-body final state (E fixed): 

N(φ) = N 0 (1 + P T · Σ · cos(2φ)) 

P T : transversal photon polarization 

Σ: photon asymmetry of the reaction 

P y : P T · Σ (one for 2-body decay) 

⇒ more for 3-body decay 

8 

>< 

>: 

J P = 1/2 + (polarized and unpolarized) 

J P = 3/2 − (unpolarized) 

J P = 3/2 − (polarized)


Polarization and observed asymmetries 

Diamond crystal used to create 

coherent bremsstrahlung 

Polarization 

Asymmetry Σ PT 

E pol = 1600 MeV 

E γ [MeV] 

E photon [MeV] 

⇒ Agreement between polarization and 

observed asymmetry very good !


CB–TAPS (2003 data): ⃗γ p → p π 0 η 

120 

100 

80 

60 

40 

20 

0 

-3 -2 -1 0 1 2 3 

φ [rad] 

120 

100 

80 

60 

40 

20 

0 

-3 -2 -1 0 1 2 3 

φ [rad] 

Σ P T 

0 

-0.04 

-0.08 

-0.12 

-0.16 

-1 -0.5 0 0.5 1 

cos ( θ) 

140 

120 

100 

80 

60 

40 

20 

0 

-3 -2 -1 0 1 2 3 

φ [rad] 

70 

60 

50 

40 

30 

20 

10 

0 

-3 -2 -1 0 1 2 3 

φ [rad] 

[ -1.0, -0.5 ] [ -0.5, 0.0 ] 

[ 0.0, 0.5 ] [ 0.5, 1.0 ] 

➜ 

Discrimination of ambiguous solutions in the 

PWA of the unpolarized data 

→ higher sensitivity 

(small contributions may have a big effect 

in certain polarization variables) 

170 000 events of the type ⃗γ p → p π 0 η 

(30 % of total statistics) 

⎧ 

⎪⎨ 

⎪⎩ 

Φ π 0 for different cos Θ π 0 bins 

(data is not yet acceptance corrected !) 

(1440 MeV ≤ E γ ≤ 1640 MeV)


Summary and conclusion 

A: Baryon spectroscopy: Touching upon the question of missing resonances 

• Observation of baryon cascades in γ p → p π 0 π 0 

as well as γ p → p π 0 η 

• Hint for new N ∗ resonance N(2080)D 15 observed in its decay to pη 

(V. Credé et al., submitted to Phys. Rev. Lett.) 

• Hints for further states: ∆ 3 

2 

− around 2.2 GeV/c 2 (in the reaction γ p → p π 0 η) 

There is yet a lot more to be discovered ! 

⇒ Polarization (beam and target) 

⎧ 

⎨ 

⎩ 

Crystal Barrel at ELSA 

CLAS at JLab, etc.


Summary and conclusion 

A: Baryon spectroscopy: Touching upon the question of missing resonances 

• Observation of baryon cascades in γ p → p π 0 π 0 

as well as γ p → p π 0 η 

• Hint for new N ∗ resonance N(2080)D 15 observed in its decay to pη 

(V. Credé et al., submitted to Phys. Rev. Lett.) 

• Hints for further states: ∆ 3 

2 

− around 2.2 GeV/c 2 (in the reaction γ p → p π 0 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

Talk - Hadronic Nuclear Physics Group

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