Photoproduction experiments with the Crystal Barrel/TAPS ... fileSpectroscopy - Electromagnetic interaction - Galaxie 1021 m Materie 10-1 m DNA 10-8 m Kristall 10-9 m Atom 10-10 m

Photoproduction experiments with the Crystal Barrel/TAPSdetector at ELSA

U.Thoma, Bonn University

• Introduction

• The Crystal Barrel / TAPS detector at ELSA

• η- photoproduction at the proton and neutron

• 2π0- photoproduction

• π0η- photoproduction

• Polarisation

• Current experiments - double polarisation -

• Summary

funded by the DFG within the

SFB/TR16

Introduction

Electromagnetic Interaction Strong Interaction

γ

e

e

pp

↔ QED u

d

u

d

π+g

↔ QCD

“color“

⇒ 8 Gluons

- Gluons carry “color charge“

⇒ Running coupling

constantαs

QCD: Effects of the running coupling constantαs:

Abstand [m]

Confinement

?

asymptotische Freiheit

αs small⇒ QCD-processes calculable (perturbation theory)

αs big ⇒ perturbation theory notapplicable

→ Baryons (qqq)

→ Mesons (qq)

Better understanding of strong QCD and the structure of hadrons:

• What are the relevant degrees of freedom ?

• And the effective forces ?

⇒ Baryon spectroscopy

baryons, e.g.: or ?

Spectroscopy - Electromagnetic interaction -

Galaxie

1021 m

Materie

10-1 m

DNA

10-8 m

Kristall

10-9 m

Atom

10-10 m

Atomkern

10-14 m

Nukleon

10-15 m

⇔ Remember the atom:

Emissionlines

Absorptionlines

n=1

2

3

4

S1/2

S1/2 1/2, P

P3/2

P3/2 3/2, DD5 /2

S1/2 1/2, P

S1/2

S1/2

P1/2

P3/2

D5 /2

P1/2

S1/2

P3/2

D3/2

F =0

F =1

F =0F =1

B o h r Dira c Q ED H F S

0

E

1 420 M H z

17 8 M H z

8 17 3 M H z

2 46 6 T H z

43,5 G H z

Lamb−Shift

QEDDiracBohr

⇒ Discrete spectrum of absorption and emission lines

- Bohr model of the atom- Quantum mechanics- QED ...

⇔ Study strong QCD through the excitation spectrum of the nucleon

but:N∗- spectral-lines do not really look like this:

Ebut more like this:

E

Heisenberg’s uncertainty principle:

∆E · ∆t ≥ ~

⇔ Baryon-resonances are broad and overlapping

N∗- resonances in the quark model

Galaxie

1021 m

Materie

10-1 m

DNA

10-8 m

Kristall

10-9 m

Atom

10-10 m

Atomkern

10-14 m

Nukleon

10-15 m

U. Loering, B. Metsch, H. Petry et al. (Bonn)

π2T 2JL P11 D13 1 11 1 13 1 11 1 13S DF F G I IH H191715 K 11 13 15 17 19GP

1720

1535

1675

1440

939

1900

19902000

2700

2090

1650

1520

1700

2600

1710

2100

1680

2190

22502200

2080

2220

1986

1897 1895

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−J

Mas

s [M

eV]

1000

1500

2000

2500

3000

****

*

**

****

**

* **

****

**

***

S **

***

****

****

**

****

****

****

****

***

****

****

S S

?

?

proton

??

↔

Constituentquarks

Confinement-potential

Residualinteraction

Baryon spectroscopy

Symmetric quark models:

→ many more resonances expected than observed yet- certain configurations completely missing !

• Certain configurations not realised by strong QCD ? Why ?

• Experimentally not found yet (resonances might decouple from πN)

↔ Big discovery potential of photoproduction experimentsinvestigating e.g. γp → Nη, Nη′, Nω, ∆π, Nρ, ∆η, ∆ω

Experimentally:Broad and strongly overlappingresonances

Important:→ Measurement of polarisation observables

(unambigiuos PWA)

→ Investigation of different final states

γp → pπ0 γp → pη

Photoproduction cross sections

Eγ/GeV

σ/barn

1

1.2 1.6 1.8 2.0 2.2 2.4

0 2 3

µ

10−1

210

10

1

1.4

π 0π 0ppγ π 0ppγ η

γ p ηpγ pp + −K K

+γ πpp −π π0

πpp −π+γ

p−>pXγ

γ p π 0p

K Σ+0γ p

γ p K+Λγ p K+Σ

– CB-ELSA data ,– SAPHIR data (ELSA) ,– other experiments

At high excitation energies:Multi-meson final states play a role of increasing importance

The Crystal Barrel / TAPS Experiment at ELSA

e

6.6m

inner detector

Eγ = E0 − E tagger

ELSA provides electrons

up to E=3.5 GeVTAPS

γCrystal Barrel goniometer

tagging system

• Unpolarised photons:Scattering of unpolarised electronsfrom an amorphous radiator→ production of Bremsstrahlung∼ 1/Eγ distribution

Tagging system:

- 14 Scintillators- 480 scintillating fibres- wire chamber (208 wires)

tagged range: 22 - 92% of E0

• Linearly polarized photons:

Production of linearly polarized photonsby coherent Bremsstrahlung off a diamondcrystal- coherent scattering if momentum transfercorresponds to a reciprocal lattice vectorplane fixed → linear polarisation

• Circularly polarized photons:

Scattering of longitudinally polarizedelectrons from an amorphous radiator

0

0.2

0.4

0.6

0.8

1

0.2 0.4 0.6 0.8 1

Pci

rc / P

e

Eγ / Eo

0

( Polarisation transfer, calculation: H.Olsen, L.C. Maximon)

Erzeugung von

polarisierten

Photonen

Christian

Hammann

Polarisierte

Photonen

Erzeugung

unpolarisierter

Photonen

Bremsstrahlung

Kinematik

linear polarisierte

Photonen

Impulsubertrag aufKristall

reziprokes Gitter

Bestimmung desPolarisationsgrads

Messen derPhotonpolarisation

zirkular

polarisierte

Photonen

Polarisationsubertrag

Bestimmung derElektronpolarisation

Messen derPhotonpolarisation

Spektrum der Erzeugten Photonen


γγ

p

γ

γp → N∗ →pη→pγγ

TAPS:

- 528 BaF2-crystals (6-30)

- 12 rad. length (τ=0.9/630ns)

- fast trigger, timing (σ=210ps)

- Energy resolution:σE/E = 0.79%/

p

E[GeV] + 1.8%

- charged identification via scintillators

Crystal Barrel:

- 1260 CsI(Tl) crystals

- 16 rad. length (τ= 0.9/7µs)

- angular resolution 1.2

- Energy resolution:σE/E = 2.5% /E1/4[GeV]


γ

p

γ

Crystal Barrel

- 1260 CsI(Tl) crystals

- 16 rad. length (τ= 0.9/7µs)

- angular resolution 1.2

- Energy resolution:σE/E = 2.5% /E1/4[GeV]

Inner detector:→ charged identification

- 3 layers of scintillating fibres(513 fibres, 2mm diameter)

- provides an intersection point ofcharged particle trajectories

- trigger, timing

η - Photoproduction

CB-ELSA:

] 2 [MeV/cγ γ m100 200 300 400 500 600

102

103

104

105

106

400 500 600

1000

2000

3000

4000

5000

6000

~ 78000 (a)γ 2 → η

400 500 600

500

1000

1500

2000

2500~ 21000 (b)

0π 3 → η

γ 2 → 0π~ 2.6 million

→ Photons aredetected

→ Proton directionmeasured

1000 15000

1000

2000

mass [MeV]

CBELSA/TAPS

γp → p η :

η → γγ

η → 3π0

CLAS (Jefferson Lab.):

→ Proton detected

→ η from missing mass

η′ → π0π0η

η - Photoproduction ( γp → N∗ →pη→pγγ )

DS11 13

1535

2090

1650

1520

1700

2080

1897 1895

1/2− 3/2−

* **

****

****

****

***

S S

3000

π2T 2JL 11P

1440

939

1710

2100

1986

1/2+J

Mas

s [M

eV]

1000

1500

2000

2500

****

*

S

***

****

γ

η

γ

ηProton

] 2 [MeV/cγ γ m100 200 300 400 500 600

102

103

104

105

106

400 500 600

1000

2000

3000

4000

5000

6000

~ 78000 (a)γ 2 → η

400 500 600

500

1000

1500

2000

2500~ 21000 (b)

0π 3 → η

γ 2 → 0π~ 2.6 million

CB-ELSA

→ photonsdetected (E,Θ, φ)

→ proton directionmeasured (Θ, φ)

⇒

1.6 1.8 2 2.2 2.4

5

10

1520 1 1.5 2 2.5

W [GeV]

[GeV]γEb]µ [ totσ

TAPS

GRAAL

CLASCB−ELSA

3rd S11 ?

FIT

⇔

E

broad and overlapping resonances

η - photoproduction

⇒ additional information necessary:

π2T 2JL P11 13 F15P

1720

1440

939

1900

2000

1710

2100

1680

1986

1/2+ 3/2+ 5/2+J

Mas

s [M

eV]

1000

1500

2000

2500

3000

****

*

**

****

S

***

****

****

**

DS D11 13 15

1535

1675

2090

1650

1520

1700

2200

2080

1897 1895

1/2− 3/2− 5/2−

* **

****

**

****

****

****

***

S S

⇔ Baryons have different properties

⇒ 1.) Measurement of angular distributions

γ

p

p

η

Θ

decay angular distribution

η

p

γ

LRp

helicity coupling

s−channel

(Breit−Wigner)resonance

Differential cross sections γp → pη

V.Crede, O.Bartholomy et al.,PRL 94, 012004 (2005)

CB-ELSA 4 GRAAL CLAS ? TAPS —– CB-ELSA fit

750 − 800 800 − 850 850 − 900 900 − 950 950 − 1000 1000 − 1050

1050 − 1100 1100 − 1150 1150 − 1200 1200 − 1250 1250 − 1300 1300 − 1350

1350 − 1400 1400 − 1450 1450 − 1500 1500 − 1550 1550 − 1600 1600 − 1650

0.5

1

1.5

2

0.2

0.4

0.6

0.2

0.4

0.6

−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 1−1 −0.5 0 0.5 1

cmθcos

b/sr]µ [Ω/dσd

W=1800W=1700

γ

p

p

η

Θ

η

pp

γ

R

s−channel

γ

p p

η

ρ,ω

t−channel

Total cross section γp → pηV.Crede, O.Bartholomy et al.,PRL 94, 012004 (2005)

1.6 1.8 2 2.2 2.4

5

10

1520 1 1.5 2 2.5

W [GeV]

[GeV]γEb]µ [ totσ

TAPS

GRAAL

CLASCB−ELSA

3rd S11 ?

FIT

CB-ELSA Isobar model fit:

Data included:

• γp → pη, γp → pπ0 (CB-ELSA)

• γp → pη (TAPS)

• Σ(~γp → pη), Σ(~γp → pπ0) (GRAAL)

• Σ(~γp → pπ0), σ(γp → nπ+) (SAID)

⇒ S11(1535), D13(1520), S11(1650), F15(1680), P13(1720), D13(2080)+ ... + ρ -,ω -t-channel exchange

+ new D15: m = 2068 ± 22 MeV,Γ = 295 ± 40 MeV

↔ No need for a 3rd S11!

New D15 -state

- D15(2060 ± 30, 340 ± 50):

! !" #$ % & ' %( ! ') *"+,- ./0 0 10 234 5- 6 67 - 4 - 8 . 1 2- 79 :; - 5- / 8 6 < 8- 0 = 64 0 , /> - ?- - 23 . 14 4 - 79

@ BA A C C ) %" !ED F GH " % ' @ BA A C C ) %" !ED F GH " % ' @ BA A C C ) %" !ED F GH " % ' @ BA A C C ) %" !ED F GH " % '

IJKL M NPOQR S TUV L W MXY Z T Y MV X V U W W MXY[ \ \] ^_ àb cde b `[ \ \] ^_ àb cde b `[ \ \ ] ^_ à b c de b `[ \ \ ] ^_ à b c de b `f \ \ g `h h ij klm e npo qsr tu \

i\v i\ w ^b x] a xy i\ clm e nz r nzf \ ae ] | i\ klm e npo qsr tu \

iv j \ ] ` h `_ ~ f clm e nz r nz m

\v ge b c | c w f klm e nz r z n z

@ A A C C ) %" !ED F GH " % F # !@ A A C C ) %" !ED F GH " % F # !@ A A C C ) %" ! D F GH " % F # !@ A A C C ) %" ! D F GH " % F # !

IJKL M NPOQR S TUV L W MXY Z T Y MV X V U W W MXY

j \ g `h h ij klm e npo qsr tu \

\ \v \ \ w ^b x] a xy i\ clm e nz r nz \ ae ] | i\ klm e npo qsr tu \

j \v \ ] ` h `_ ~ f clm e nz r nz m

~ v j f ge b c | c w f klm e nz r z n z

varies strongly !

⇔ Results vary strongly!

Multiparticle final states: γp → pπ0π0 , γp → pπ0η

Search for N∗/∆∗ → ∆π , ∆∗ → ∆η : CB-ELSA

γp → p4γ:

⇒ γp → pπ0π0

and γp → pπ0ηclearly observed

γp → N∗/∆∗ → ∆π0 → pπ0π0

γp → ∆∗ → ∆η → pπ0η

isospin selective ⇒ investigation ofthe ∆∗-spectrum

N*

π

π0

0

γ

Proton

∆(1232)

*∆,

, ηγp → pπ0η

) 0πm(p1000 1500 20000

500

1000

1500

2000

∆(1232)

• γp → N∗/∆∗ → Xπ0 → pπ0π0

0

20

40

60

80

100

120

140

) 0π(p2m1000 2000 3000

3x10

) 0 π

(p2

m

1000

2000

3000

3x10 ): 1800−2000 MeV0π0πm(p

13D (1520)

∆ (1232)

13D (1520)

) 0πm(p1000 1500 20000

1000

2000

3000

): 1800-1200 MeV0π0πm(p

X = ∆(1232)

X = D13(1520)

0

10

20

30

40

50

60

70

80

90

) 0π(p2m1000 2000 3000 4000

3x10

) 0 π

(p2

m

1000

2000

3000

4000

3x10 ): 2000−2200 MeV0π0πm(p

(1232)∆

X(1660)

D13(1520)

) 0πm(p1000 1500 20000

500

1000

1500): 2000-2200 MeV0π0πm(p

X = ∆(1232)

X = D13(1520)

X = X(1660)

γp → pπ0π0

U. Thoma et al., PLB 659 (2008) 87

0

2

4

6

8

10

1.2 1.3 1.4 1.5 1.6 1.7 1.8

M(γp), GeV/c2

σ tot ,

µb

CB−ELSA

GRAAL

TAPS

(ππ)sp

∆π

Results contradicting naive expectation:e.g.: D13(1520) → ∆π decay with L=0 ≈ L=2

D13(1700) → ∆π decay with L=0 < L=2

D33(1700) → ∆π decay with L=0 or L=2→ Measurement of double polarisation

observables necesarry

CB-ELSA Fit including additional data from:- single meson photoproduction,- π−p→n2π0 (CBall),- P11, S11, P33, D33- πN-partial waves

↔ Event based maximum likelihood fit

⇒ Determination of resonance properties:m , Γi (∆π0,Nσ,P11π,D13π,+...)

0

5

10

15

20

1.1 1.2 1.3 1.4 1.5 1.6

M(πp), GeV/c2

Nev

/ 20

MeV

x1033*1

0Phase space

FIT

DATA

0

1

2

3

4

5

6

7

0.2 0.4 0.6 0.8

M(ππ), GeV/c2

Nev

/ 20

MeV

x103

3*1

0

γp → pπ0ηI. Horn, Bonn

πP P33 S D31 F F D GH G II3 1135 37 39 H3 11 3 13 3 15 3 1333 35 37 3931KKL2T 2J

1232

20001950

1700

2150

1940 1930

2200

2750

1620

23502400

1900

2300

2420

2950

2390

19051920

1600

1750

1910

J 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-

Mas

s [M

eV]

1000

1500

2000

2500

3000

*

********

*****

****

**

**

****

***

**

**

****

**

**

*

*

*

*

*******

***

*∆− resonances

?

U. Loering et al.

• γp → ∆∗ → ∆(1232)η → pπ0η

) 0πm(p1000 1500 20000

500

1000

1500

2000

∆(1232) ⇒ ∆(1232) clearly observed !

but there are also additionalinteresting structures :

0

2

4

6

8

10

12

14

16

) 0π(p2m1500 2000 2500 3000 3500

3x10

) η(p

2m

3000

4000

5000

3x10 ): 2200−2400 MeVη0πm(p

(1232)∆S (1535)11

) η0πm(600 800 1000 12000

500

1000 a0(980)

γp → pπ0η CB-ELSAI.Horn, Bonn

πP P33 S D31 F F D GH G II3 1135 37 39 H3 11 3 13 3 15 3 1333 35 37 3931KKL2T 2J

1232

20001950

1700

2150

1940 1930

2200

2750

1620

23502400

1900

2300

2420

2950

2390

19051920

1600

1750

1910

J 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-

Mas

s [M

eV]

1000

1500

2000

2500

3000

*

********

*****

****

**

**

****

***

**

**

****

**

**

*

*

*

*

*******

***

*∆− resonances

?

U. Loering et al.

Total cross section

2γ

σµb

M( p) GeV/c

CB−ELSANabakayashiet al, PRC 2006

∆η

S11π

2.42.22.01.81.6 2.6

5

4

3

2

1

0

tot

−1 −0.5 0 0.5 10

200

400

600

800

1000

1200

1400

1.2 1.4 1.6 1.8 2 2.20

500

1000

1500

2000

2500

3000

−1 −0.5 0 0.5 10

200

400

600

800

1000

1200

1400

0.8 1 1.2 1.4 1.6 1.80

200

400

600

800

1000

1200

1400

1600

cos Θη cos Θp

π0π0π0m(p ) π0m( )η

Fit, +: data, phasespace

Event based maximum likelihood fit:

⇒ D33(1940) clearly contributes

~γp → pπ0η - CBELSA/TAPS -

0πφ

-150 -100 -50 0 50 100 1500

500

1000

1500

2000

2500

dσdΩ

=(

dσdΩ

)

0(1 − δl(Σ cos 2φ+ Is sin 2φ))

— PWAprediction

E. Gutz et al,

EPJ A35, (2008) 291

~γp → pη - CBELSA/TAPS -

· CB/TAPS beam-asymmetries ⇔ provide additional information for the PWA

P (1720)13

13D (1520)

BoGa−PWA

13D (1520)

P (1720)13

η −MAID

D.Elsner et al.,EPJ. A33 (2), 147 (2007)

Single pseudoscalar meson photoproduction

Complete experiment→ ≥ 8 observables needed

(W.-T. Chiang, F.Tabakin, PRC55, 2054 (2007))

Double pseudoscalar meson photoproduction

→ ≥ 15 observables needed(Roberts, Oed)

⇒ double polarisation experiments needed !

Crystal Barrel/TAPS at ELSA:

Experiments with longitudinally polarised targetand polarised beam started

Polarisation observables:

Photon Target Recoil Target–Recoil

x′ y′ z′ x′ x′ z′ z′

x y z x z x z

unpolarized σ 0 T 0 0 P 0 Tx′ -Lx′ Tz′ Lz′

linear -Σ H (-P ) -G Ox′ (-T ) Oz′ (-Lz′) (Tz′) (-Lx′) (-Tx′)

circularly 0 F 0 -E Cx′ 0 Cz′ 0 0 0 0

1 unpolarised measurement, 3 single polarisation observables,12 double polarisation observables

Example: Beam polarisation Σ

- along y-axis: dσ⊥

- along x-axis: dσ||

Σ =σ⊥ − σ||

σ⊥ + σ||

,

0πφ

-150 -100 -50 0 50 100 1500

500

1000

1500

2000

2500

dσ

dΩ=

dσ

dΩ0· (1 − PlΣcos(2φ))

Photoproduction of two pseudoscaler mesons

Double pol. experiments with: - linear or circular polarised beam- longitudinal polarised target


dσdΩ

= σ0 1 − δ lΣ cos 2ϕ − Λ z ( − δ lG sin 2ϕ + δ E)


dσdxi

= σ0 ( 1 − Λz · Pz ) + δ (I − Λz · E )

− δ l [ sin 2ϕ ( I s − Λz ·G )

+ cos 2ϕ ( Σ − Λz · Pzc ) ]

bcm

a

k2

b1

zx

pθ

φ θ

⇒ Multi-meson final states or final states with non-spin-0-mesons:

much more complicated

~γp → pη - CBELSA/TAPS -

· CB/TAPS beam-asymmetries ⇔ provide additional information for the PWA

P (1720)13

13D (1520)

BoGa−PWA

13D (1520)

P (1720)13

η −MAID

D.Elsner et al.,EPJ. A33 (2), 147 (2007)


Complete experiment→ ≥ 8 observables needed

(W.-T. Chiang, F.Tabakin, PRC55, 2054 (2007))


→ ≥ 15 observables needed(Roberts, Oed)

⇒ double polarisation experiments needed !

Crystal Barrel/TAPS at ELSA:

Experiments with longitudinally polarised targetand polarised beam started

Circularly polarized beam + longitudinally polarized targetη-photoproduction

Predictions:

η-MAID / BoGa-PWA

dσ(3/2−1/2)

dΩ=dσ3/2

dΩ−dσ1/2

dΩ

1500

0

1

2

dσ/dΩ [µb/sr] (helicity 1/2 - 3/2)

1600 1700

18001

0.5

0

1900 2000

21000.5

0

2200 2300

24000.5

-0.5

0

-1

0-1 1

2500

0-1 1

2600

0-1 1

cos θcm

Double Polarisation Experiments at ELSA

Experiments with: - linear or circular polarised beam- longitudinal polarised target (frozen spin butanol)

TAPS

Forward Detector

Crystal Barrel

+ Inner detector

Goniometer

Tagging system

Beam DumpPhoton intensity

monitor

Gas-Cherenkov

Polarized Target

The central detector:

γγ

p

Forward plug :- 90 CsI(Tl) crystals (∼12-30)- Photomultiplier readout- 1st-level trigger capabilities- timing

Crystal Barrel :- 1230 CsI(Tl) crystals (∼30-156)- Readout via photodiodes on WLS- 2nd-level trigger capabilities only

inner detector:charged part. identification

- charged identification (FP):180 scintillators in 2 layers


Experiments with: - linear or circular polarised beam- longitudinal polarised target (frozen spin butanol)

TAPS

Forward Detector

Crystal Barrel

+ Inner detector

Goniometer

Tagging system

Beam DumpPhoton intensity

monitor

Gas-Cherenkov

Polarized Target

Mean Pol ~ 70%


Online spectra: circularly polarised beam, longitudinally polarised target

γp → pη:

g p

g p

proton

500 1000 1500 2000

mass [MeV]

2000

4000

6000

8000

10000

0

co

un

ts

difference unpolarised difference

=⇒

1800

1400

1000

600

200

0

-200500 1000 1500 2000

mass [MeV]

Dc

ou

nts 1800

1400

1000

600

200

0

-200500 1000 1500 2000

mass [MeV]

Dc

ou

nts

h gg

h gg300

co

un

ts

200

100

0100 200 300 400 500 600

mass [MeV]

x103

co

un

ts 14000

12000

10000

8000

6000

4000

2000

0400 450 500 550 600 650 700

mass [MeV]

p0 gg γp → pγγ

⇒ First asymmetries observed

Double Polarisation Experiments

First online spectra: linearly polarised beam, longitudinally polarised targetdσdΩ

=(

dσdΩ

)

0(1 − δl(Σ cos 2φ− ΛzG sin 2φ))

)Θcos(-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

φ

-150

-100

-50

0

50

100

150

0

2

4

6

8

10

12

14

16

18

20

)Θcos(-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

φ

-150

-100

-50

0

50

100

150

0

2

4

6

8

10

12

14

16

18φφ −45° +45°

cos( )θ cos( )θ

200

400

600

800

1000

1200

1400

1600

1800

2000

400

600

800

1000

1200

1400

1600

1800

2000

−150 −100 −50 0 50 100 1500 −150 −100 −50 0 50 100 1500

200

projection on φ projection on φ

φ φ−45° +45°

~γ~p → pπ0

(Eγ=750-1200 MeV)

−150 −100 −50 0 50 100 1500

500

1000

1500

2000

2500

3000

3500

sum

φ

Double Polarisation Experiments

First online spectra: linearly polarised beam, longitudinally polarised targetdσdΩ

=(

dσdΩ

)

0(1 − δl(Σ cos 2φ− ΛzG sin 2φ))

−150 −100 −50 0 50 100 1500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

−150 −100 −50 0 50 100 1500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

φ φ

−45° +45°Target − Target −

−150 −100 −50 0 50 100 1500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

−150 −100 −50 0 50 100 1500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8Target + −45° Target + +45°

φ φ

~γ~p → pπ0

(Eγ=750-1200 MeV)

⇒ Clear effect

fromG observed

= further information for thepartial wave analysis !

The ” new ” states - a comparison with the quark model -

π2T 2JL P11 13 F15P

1720

1440

939

1900

2000

1710

2100

1680

1986

1/2+ 3/2+ 5/2+J

Mas

s [M

eV]

1000

1500

2000

2500

3000

****

*

**

****

S

***

****

****

**

DS D11 13 15

1535

1675

2090

1650

1520

1700

2200

2080

1897 1895

1/2− 3/2− 5/2−

* **

****

**

****

****

****

***

S S D13(1875)

11(1840)P

+ πp π electroproduction (CLAS)

P (1900)13

D15(2070)

2 states ?

Quark model calculations by:

U. Loring, B. Metsch, H. Petry et al.

... important : further polarisationexperiments!

→ confirmation ofnew resonances

↔ higher sensitivityon resonance contri-butions !

The ∆∗- states

πP P3331 F F H35 37 39 H3 11L2T 2J

1232

20001950

2300

24202390

19051920

1600

1750

1910

J 1/2+ 3/2+ 5/2+ 7/2+ 9/2+

Mas

s [M

eV]

1000

1500

2000

2500

3000

*

********

*

****

**

**

*******

***

S D D G G I3 1133 35 37 3931

1700

2150

1940 1930

2200

1620

23502400

1900

1/2− 3/2− 5/2− 7/2− 9/2− 11/2−

****

***

**

**

****

*

*

*

*

11/2+

****

Quark model

U. Loring, B. Metsch,

H. Petry et al.

model∼ 2n+ `

The ∆∗- states

πP P3331 F F H35 37 39 H3 11L2T 2J

1232

20001950

2300

24202390

19051920

1600

1750

1910

J 1/2+ 3/2+ 5/2+ 7/2+ 9/2+

Mas

s [M

eV]

1000

1500

2000

2500

3000

*

********

*

****

**

**

*******

***

S D D G G I3 1133 35 37 3931

1700

2150

1940 1930

2200

1620

23502400

1900

1/2− 3/2− 5/2− 7/2− 9/2− 11/2−

****

***

**

**

****

*

*

*

*

11/2+

****

?

in CB−ELSAdata

⇔ Additional experimental information needed !!

Quark model

U. Loring, B. Metsch,

H. Petry et al.

model∼ 2n+ `

data∼ n+ ` ?

↔ Paritydoublets ?

Summary

• High quality baryon spectroscopy data has been taken

→ Determination of resonances properties

→ First evidence for new resonances D15(2070) → pη

→ Evidence for D33(1940) → ∆η

→ Decays via higher mass resonances observed(baryon cascades e.g. D13(1520)π0, S11(1535)π0)

⇔ Already very interesting !

But there is a lot more to be discovered+ cross check new states

⇔ Polarisation experiments

⇒ Better understanding of the hadron spectrum

⇒ Detailed testing ground for quark models,lattice QCD calculations ...

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