Supported in parts by the U.S. National Science Foundation: NSF PHY-1205782

Steffen Strauch for the CLAS Collaboration University of South Carolina

Nucleon Resonances: From Photoproduction to High Photon Virtualities, ECT*, Trento, Italy, October 12 - 16, 2015

Baryon spectroscopy with polarization observables from CLAS

1. Unpolarized target 2. Polarized proton target (FROST) 3. Polarized neutron target (HDice) 4. CLAS 12 Forward Tagger

Relevant degrees of freedom and missing resonance problem

2

Quark Models

• Constituent Quark Models predict many more of excited states than have been observed; some of the states may only couple weakly to πN.

• Quark-Diquark Models predict fewer states. • Quark and Flux-Tube Models predict increased number of states.CQM: U. Löring, B.C. Metsch, and H.R. Petry, Eur. Phys. J. A10 395 (2001)

Degr

ees

of f

reed

om

π2T 2J

L P11

D13 1 11 1 13 1 11 1 13

S DF F G I IH H191715

K11 13 15 17 19

GP

1720

1535

1675

1440

939

1900

19902000

2700

2090

1650

1520

1700

2600

1710

2100

1680

2190

2250

2200

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

Mass [

MeV

]

1000

1500

2000

2500

3000

****

*

**

****

**

* **

****

**

***

S **

***

****

****

**

****

****

****

****

***

****

****

S S

N Resonance Spectrum — the low-energy signature of QCD

MB

3* or 4*

Resonance spectrum in Lattice QCD

3R.G. Edwards, J.J. Dudek, D.G. Richards, and S.J. Wallace, Phys. Rev. D 84, 074508 (2011)

LQCD predicts states with the same quantum numbers as CQMs with underlying SU(6) x O(3) symmetry; more states than have been identified experimentally.

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

N(938)

Δ(1232)1 1

2 3 2 1

2 2 1

4 5 3 1

mπ = 396 MeV

Hadron spectrum collaboration

Extracting nucleon-resonance information from experimental data

4

DataReaction Theory

Amplitude Analysis

N*, Δ*

LQCD

Models

QCD

Extraction of physics

Nucleon resonance information: mass, spin, parity, coupling constants, decay parameters, …

Cross sections and polarization observables

Coupled-channel analysis especially important

What we measure with CLAS

5

γ p→ π 0p,π +nγ p→ηp, ′η pγ p→ KY (K +Λ,K +Σ0,K 0Σ+ )γ p→ π +π − p,ω p, ρ p,φp…

γ n→ π − pγ n→ π +π −nγ n→ Σ−K + ,ΛK 0

…

Prot

on tar

get

Neut

ron

targ

et

Unpolarized, circularly polarized, linearly polarized beam

Unpolarized, longitudinally polarized, transversally polarized target

Recoil polarization (asymmetry in the weak decay of the hyperon)

Cross section and polarization observables

e.g. CLAS frozen spin target (FROST)

e.g. unpolarized deuterium target (g13), polarized HD-Ice target (g14)

Observables in pseudoscalar meson photoproduction

4 complex amplitudes ⇒ 16 possible (not independent) observables

6A.M. Sandorfi, S. Hoblit, H. Kamano and T.-S.H. Lee, J. Phys. G, Nucl. Part. Phys. 38, 053001 (2011)

e.g, FROST and HDice

e.g, Hyperon weak decay

Beam

Target Recoil Target + Recoil

x’ y’ z’ x’ x’ x’ y’ y’ y’ z’ z’ z’

x y z x y z x y z x y z

unpolarized dσ0 T P Tx’ Lx’ Σ Tz’ Lz’

PLγ sin(2φγ) H G Ox’ Oz’ Cz’ E F -Cx’

PLγ cos(2φγ) Σ -P -T -Lx’ Tz’ -dσ0 Lx’ -Tx’

circular Pcγ dσ0 F -E Cx’ Cz’ -Oz’ G -H Ox’

-

double pion photoproduction fills empty cells in the table

coherent and incoherent Bremsstrahlung

CEBAF Large Acceptance Spectrometer in Hall B (1997 - 2012)

7

Polarized electron beam Energies up to Ee = 6 GeV (now up to 11 GeV)

CEBAF Large Acceptance

Spectrometer (CLAS)

Photon TaggerTarget unpolarized p or d, polarized FROST, HDiceEγ = Ee − E ′e

B.A. Mecking et al., Nucl. Instr. and Meth. A 503, 513 (2003).

Beam asymmetry Σ for π+ and π0 photo production on the proton

Largest changes from previous fits are for “well known” Δ(1700)3/2- and Δ(1905)5/2+ states.

8

dσdΩ

⎛⎝⎜

⎞⎠⎟ =

dσdΩ

⎛⎝⎜

⎞⎠⎟ 01− PL

γ Σcos(2ϕγ )( )

M. Dugger et. al (CLAS Collaboration), Phys. Rev. C88, 065203 (2013)

γ

p

π0

p

θcmπ

Models: SAID DU13 (CM12), MAID07, BG2011-02

high-energy π0 data: 1904 MeV < W < 2091 MeV

!γ p→ π 0p

θ (deg)

Polarization Transfer Observables Cx, Cz

9

Cx (•), Cz () for K+Λ channel

!γ p→ K +!Λ

Bonn-Gatchina coupled-channel isobar model: N(1900)P13 needed in PWA of Nikonov et al. Strongest contributions to γp → KΛ: S11-wave, P13(1720), P13(1900), P11(1840)

with N(1900)P13

without N(1900)P13

CLAS Data: R. Bradford, et al., Phys. Rev. C 75, 035205 (2007). Analysis: V.A. Nikonov et al., Phys. Lett. B 662, 245 (2008)

State confirmed in more recent analyses.

N(1900)P13 found in qqq models, not expected in quark-diquark models.

Recoil Polarization P

10

γ p→ K +!Λ

Kaon-MAID model (green) Single-channel BW resonance fits No longer up-to-date

Data: M. McCracken et al, (CLAS) Phys. Rev. C 81, 025201 (2010).

F.X. Lee et al., Nucl. Phys. A695, 237 (2001)

Bonn-Gatchina model (blue) Multi-channel, unitary, BW resonance fit Large suite of N* contributions Was not predictive for recoil polarization

R. Schumacher, CIPANP Vail 2015

BnGa: A.V. Sarantsev et al., Eur. Phys. J., A 25, 441 (2005).

Simultaneous fit to five polarization observables: Σ, P, T, Ox, and Oz.

11

g8 analysis:Dave Ireland (U. of Glasgow)

prelim. CLAS g8 data

dσdΩ

⎛⎝⎜

⎞⎠⎟ =

dσdΩ

⎛⎝⎜

⎞⎠⎟ unpol

1− PLγ Σcos(2ϕγ )(

+Px 'R PL

γOx ' sin(2ϕγ )⎡⎣ ⎤⎦+Py '

R P − PLγT cos(2ϕγ )⎡⎣ ⎤⎦

−Pz 'R PL

γOz ' sin(2ϕγ )⎡⎣ ⎤⎦ )

Hyperon photoproduction with linearly polarized photons

!γ p→ K +Σ

!γ p→ K +!Λ1700 MeV < W < 2200 MeV

Self-analyzing weak hyperon decay gives access to recoil polarization.

W (GeV)

W (GeV)

The FROST Target

12

Through dynamic nuclear polarization the high electron polarization is transferred to the proton spin system via microwave induced transitions.

Thermal equilibrium polarization of a spin-1/2 particle

superconducting holding coils (0.5 T)

transverse polarization (g9b)

longitudinal polarization (g9a)

Target: Frozen beads of butanol (C4H9OH) at 50 mK

Temperature (K)

-210 -110 1 10210

Pola

rizat

ion

-310

-210

-110

1

B = 5 T

proton

electronDNP

P = tanh µB2kT

⎛⎝⎜

⎞⎠⎟

FROST Target: C.D. Keith et al., Nucl. Instrum. and Methods A 684, 27 (2012)

The simple idea of the experiment

• Measure polarized free-proton and unpolarized bound-nucleon yield off butanol.

• Measure simultaneously unpolarized bound-nucleon yield off 12C target.

• Determine the polarized proton yield in the difference.

13

N p ≈ NB −αNC

12C CH2Butanol

γ p→π +XW = 1240 − 2260 MeV−0.9 ≤ cos(θπ

cm ) ≤ +0.9

ΔW = 20 MeV, ncosθ = 30

• Data analysis in 900 bins

!γ !p→ π +n

S.S. et al. (CLAS Collaboration), PLB 750, 53 (2015)

Double Polarization Observable E in π+n

14

Partial Wave Analyses Good overall description after fit, however, not with identical results. S.S. et al. (CLAS Collaboration), PLB 750, 53 (2015)

!γ !p→ π +n

)cm

πθcos(

-0.5 0 0.5

E

-0.5

0

0.5W = 1.640 - 1.660 GeVSAID ST14Juelich14BnGa11E

)cm

πθcos(

-0.5 0 0.5

E

-0.5

0

0.5W = 1.640 - 1.660 GeVSAID ST14EJuelich14EBnGa14E

)cm

πθcos(

-0.5 0 0.5

E

-0.5

0

0.5W = 1.900 - 1.940 GeV

)cm

πθcos(

-0.5 0 0.5

E

-0.5

0

0.5W = 1.900 - 1.940 GeV )

cm

πθcos(

-0.5 0 0.5

E

-0.5

0

0.5W = 2.140 - 2.200 GeV

)cm

πθcos(

-0.5 0 0.5

E-0.5

0

0.5W = 2.140 - 2.200 GeV

beforeafter

W = 1.650 GeV W = 1.920 GeV W = 2.170 GeV

dσdΩ

⎛⎝⎜

⎞⎠⎟ =

dσdΩ

⎛⎝⎜

⎞⎠⎟ 01− PzP⊙E( ) W = 1240 − 2260 MeV

−0.9 ≤ cos(θπcm ) ≤ +0.9

Is chiral symmetry effectively restored in highly excited mesons and baryons?

15

An important consequence of the spontaneous breaking of the chiral symmetry is the large mass gap between chiral partners:

ρ(770)

a1(1260)Jp = 1+

Jp = 1-500 MeV

N(938)

N*(1535)Jp = 1/2-

Jp = 1/2+

600 MeV

Mesons and baryons at higher masses are often observed in parity doublets.Example: four positive-parity and four negative-parity ∆∗ resonances at about 1900 MeV

Δ(1910)1 / 2+ Δ(1920)3 / 2+ Δ(1905)5 / 2+ Δ(1950)7 / 2+ (****)Δ(1900)1 / 2− Δ(1940)3 / 2− Δ(1930)5 / 2− Δ(2200)7 / 2− (*)

A.V. Anisovich et al., arXiv:1503.05774 [nucl-ex]

New evidence forΔ(2200)7/2- resonance

16A.V. Anisovich et al., arXiv:1503.05774 [nucl-ex]

Δ(1950)7/2+ **** Δ(2200)7/2- *

Parity partner of Δ(1950)7/2+ is poorly known.

Evidence found for ∆(2200)7/2− in a preliminary analysis of the Bonn/Gatchina group.

M(Δ7/2-) ≈ 2180 MeV

… and not ≈ 1950 MeV.Chiral symmetry is not restored in high-mass hadrons.

BnGa analysis of CLAS and CBELSA/TAPS data

0

0.5

1 2142dσ/dΩ, µb/sr

γp → π0p

2142dσ/dΩ, µb/sr

γp → π+n

-0.5

0

0.52085

T2131

T

-0.5

0

0.52092

Σ

2093

Σ

-0.5

0

0.52157E 2191E

-0.5

0

0.5

-1 -0.5 0 0.5 1

2208E

cos θ-1 -0.5 0 0.5 1

2227E

cos θ

FROST π+ data

with Δ(2200)7/2-

without

χ2 in

crea

se

0

200006r2 total7/2+ 7/2<

0

100006r2 /0p

0

2000

40006r2 /+n

0

1000

6r2 KY

0

10006r2 /0/0p

0

200

1800 1900 2000 2100 2200 2300

6r2 /0dp

M, MeV

0

200

400

0

200

400

0

100

200

0

25

50

0

50

100

0

5

7/2-7/2+

All isospin channels are important to constrain coupled-channel analyses: two examples for π0p

17

)0

π (θcos-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

F

-1.0

-0.5

0.0

0.5

1.0

CLAS preliminarySN11MAID2007BG2011-02

W = 1.87 GeV

)0

π (θcos-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

T

-1.0

-0.5

0.0

0.5

1.0

CLAS preliminarySN11MAID2007BG2011-02

W = 1.87 GeV

Polarized beam and target (up/down)Polarized target (left/right)

dσdΩ

⎛⎝⎜

⎞⎠⎟ =

dσdΩ

⎛⎝⎜

⎞⎠⎟ 01+ PyT + PxP⊙F

g9 analysis: Hao Jiang (USC)

transverse polarized target

γ

p

π0

p

θcmz

xy

γ

p

π0

p

θcmz

xy

T F

!γ !p→ π 0p

Helicity asymmetry E in eta photoproduction on the proton

η photoproduction isolates N*(I=1/2) states in the resonance spectrum.

Narrow structure seen in MAMI γp → ηp cross section data. [predicted in πN PWA: Phys. Rev. C 69, 035208 (2004)]

Present CLAS E data do not demand the presence of a narrow resonance with a width of 40 MeV or less at about 1.7 GeV.

18

!γ !p→ηp

-1

-0.5

0

0.5

1

1.5 <-0.8cmθ-1.0<cos

Juelich 2014 (fit)

1500 1600 1700 1800 1900 2000 2100-1

-0.5

0

0.5

1

1.5 <0.0cmθ-0.4<cos

<-0.4cmθ-0.8<cos

1500 1600 1700 1800 1900 2000 2100

<0.4cmθ 0.0<cos

W (MeV)

E(W)

E

E

W (MeV)I. Senderovich et al. (CLAS Collaboration), arXiv:1507.00325 [nucl-ex]

19

CLAS g9b preliminary —— RPR-Ghent —— KAON-MAID …… Bonn-Gatchina

g9 analysis:Natalie Walford (CUA, now Basel)

First measurement of the polarization observable F up to 2.30 GeV.

no sufficient previous constraints exist; all models fail to describe the data.

1.85 < W < 1.90 GeV1.80 < W < 1.85 GeV1.70 < W < 1.80 GeV

2.00 < W < 2.05 GeV1.85 < W < 2.00 GeV1.90 < W < 1.85 GeV

2.15 < W < 2.20 GeV2.10 < W < 2.15 GeV2.05 < W < 2.10 GeV

2.20 < W < 2.30 GeV

γ

p

K+

Σ0

θcmz

xy

!γ !p→ K +Σ0Hyperon photoproduction

FFROST data for Λ, Σ photoprouction

Double-pion photoproduction as a tool in the study of excited nucleons

Nππ is a dominant decay channel of highly excited nucleons.

Essential part in coupled-channel calculations.

Allows for the study of sequential decays.

20

γ p→ N * →πΔγ p→ N * → ρp

cm

kp'

π+

π−

z’x’

θθ∗φ∗

p α

transversely polarized target

circularly polarized beam

d 5σdm(π +π − ) dΩπ+

* d cosθ

Example:

Parity conservation yields to symmetry properties of observables

21

* [degrees]φ0 90 180 270 360

I

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

* [degrees]φ0 90 180 270 360

xP

-0.1

-0.05

0

0.05

0.1

* [degrees]φ0 90 180 270 360

yP

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

* [degrees]φ0 90 180 270 360

xP

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

* [degrees]φ0 90 180 270 360

yP

-0.4

-0.2

0

0.2

0.4

M −λN − ′λN−λγ (θ ,θ1,φ1) = (−1)

λγ −λN + ′λN M λN ′λNλγ (θ ,θ1,2π −φ1)

I⊙ Px

Py Py⊙

Px⊙

g9 analysis: Aneta Net (USC)

odd observables: do not exist in single

meson final states. even observables:

Py and Px⊙ correspond

to T and F, respectively.

circ

ular

ly p

olar

ized

pho

tons

-

tran

sver

sely

polar

ized

tar

get

γ p→ π +π − p

Preliminary results (g9a) for Pcz

22

A. Fix and H. Arenhövel, Eur. Phys. J. A 25, 115 (2005); Preliminary data: Yuqing Mao (USC)

Yuqing Mao (USC)

cm

kp'

π+

π−

z’x’

θθ∗φ∗β

p

I = I0 1+ ΛzPz ++δ ℓ sin2β I s + ΛzPz

s( )+δ ℓ cos2β I c + ΛzPz

c( )

* * *N N N∆

π πρ

ππ σ

ππ

Effective Lagrangian Model (A. Fix)

Exchange mesons, π,ρ,σ, and resonances, Δ(1232), N*(1440), N*(1520), N*(1535), Δ(1620), N*(1675), N*(1680), Δ(1700), N*(1720), Nucleon and Delta Born terms; Resonance terms:

𝝓* (deg)

Pola

rizat

ion

Obs

erva

ble

Pc zPo

lariza

tion

Obs

erva

ble

Pcz

W = 1.46 MeV to W = 2.30 GeV

γ p→ π +π − p

Preliminary results (g9b) for Px and Py

23

g9 analysis: Priyashree Roy (FSU)

) <-0.8+πθ-1.0< cos(

) <0.2+πθ0.0< cos(

) <-0.6+πθ-0.8< cos(

) <0.4+πθ0.2< cos(

) <-0.4+πθ-0.6< cos(

) <0.6+πθ0.4< cos(

) <-0.2+πθ-0.4< cos(

) <0.8+πθ0.6< cos(

) <-0.0+πθ-0.2< cos(

) <1.0+πθ0.8< cos(

) <-0.8+πθ-1.0< cos(

) <0.2+πθ0.0< cos(

) <-0.6+πθ-0.8< cos(

) <0.4+πθ0.2< cos(

) <-0.4+πθ-0.6< cos(

) <0.6+πθ0.4< cos(

) <-0.2+πθ-0.4< cos(

) <0.8+πθ0.6< cos(

) <-0.0+πθ-0.2< cos(

) <1.0+πθ0.8< cos(

1100 MeV < Eγ < 1200 MeV

cm

kp'

π+

π−

z’x’

θθ∗φ∗

p α

Data binned in Eγ, Φ*, and cos θ*,

fit with Fourier series.

I = I0 1+ Λcos(α )Px +Λsin(α )Py( )

γ p→ π +π − p

Intermediate Δ(1232) Resonance

24

π

N(1520)

(1232)∆

p = N(938)π−

+

CLAS Data: S.S. et al (CLAS Collaboration), Phys. Rev. Lett 95, 162003 (2005); Model: A. Fix and H. Arenhövel, Eur. Phys. J. A 25, 115 (2005)

I⊙ = ak sin(kφ)∑Fourier coefficients of the angular distribution

Example of sequential decays

-0.5

-0.25

0

0.25

0.5a 1

W = 1.520 GeV

-0.2

-0.1

0

0.1

0.2

1.1 1.2 1.3 1.4M(pπ+) (GeV)

a 2

-0.5

-0.25

0

0.25

0.5

a 1

W = 1.520 GeV

-0.2

-0.1

0

0.1

0.2

1.1 1.2 1.3 1.4M(pπ-) (GeV)

a 2

N(1520)→ π −Δ++ → pππ N(1520)→ π +Δ0 → pππγ p→ N * → πΔ

γ p→ π +π − p

Beam and target asymmetries inomega photoproduction (g9b)

25

Prelim

inary

Σ

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

E: 1150 MeV

FROST (g9b-linear)

GRAAL (2013)

GRAAL (2006)

c.m.ωΘ

0 20 40 60 80 100 120 140 160 180-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100 120 140 160 180

E: 1250 MeV

E: 1450 MeVE: 1350 MeVPre

limina

ry: 1250 MeVγE

: 1650 MeVγE

: 2050 MeVγE

: 1350 MeVγE

: 1750 MeVγE

: 2150 MeVγE

: 1450 MeVγE

: 1850 MeVγE

: 2250 MeVγE

: 1550 MeVγE

: 1950 MeVγE

: 2350 MeVγE

-0.8

0

0.8

-0.8

0

0.8

-0.8

0

0.8

-1 -0.5 0 0.5 1-1 -0.5 0 0.5 1-1 -0.5 0 0.5 1-1 -0.5 0 0.5 1c.m.

)ωΘcos(

T

γ p→ω p

g9 analysis: Priyashree Roy (FSU)

Beam asymmetry Target asymmetry

All isospin channels are important to constrain coupled-channel analyses

26Figs. A.M. Sandorfi, Spin’2014 Symposium

g14 analysis: Tsuneo Kageya (Jlab)PWA solutions (already outdated) show need to measure all isospin channels.

Beam-Target helicity asymmetry (observable E) for proton and neutron targets

!γ !p→ π +n

W = 1900 MeVW = 1880 MeV

!γ !n(p)→ π − p(p)

Double pion beam-helicity asymmetry for proton and neutron targets

27

g14 analysis: Peng Peng (UVa)

γ p→ π +π − pγ p(n)→ π +π − p(n) γ n(p)→ π +π −n(p)

I ⊙ I ⊙

φ * (rad) φ * (rad)

Integrated Io asymmetries off proton and neutron targets are comparable.

preliminarypreliminary

Moller Shield

Calorimeter

Tracker

Scintillation

Hodoscope

HTCC Moller

cup

CAD implementation

CLAS 12 Forward Tagger

28

• Small angle e- scattering (2.5o to 5o) • Eγ = 6.5 - 10.5 GeV • 0.01 (GeV/c)2 < Q2 < 0.3 (GeV/c)2 virtual photon, (almost) real photon • Quasi real photons are linearly polarized wrt to scattering plane • High luminosity

e- beam

Fig. from M.Battaglieri, ECT* Workshop Lattice QCD and hadronic physics (2014)

e

ForwardTagger

CLAS12

Nγ∗

e’

Quasi-Real Photoproduction with CLAS12

Baryon Spectroscopy

• E12-11-005A - Cascade, γp → K+K+Ξ-

(millions of reconstructed Ξ) - Omega, γp → K+K+K0Ω-

(4,000 reconstructed Ω)

• LOI12-15-004 - Search for Hybrid Baryons

(focus on lowest mass hybrid baryons)

29

L. Guo et al., Phys. Rev. C 76, 025208 (2007)

)2) (GeV/c+K+MM(K1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

)2C

ount

s/(5

MeV

/c0

500

1000

1500

2000

2500 Ξ−

Ξ− (1530)

γ p→ K +K +X

presentCLAS data

L. Guo et al., Jefferson Lab Experiment E12-11-005A, “Photoproduction of the Very Strangest Baryons on a Proton Target in CLAS12” V. Burkert et al., Jefferson Lab LOI12-15-004, “Search for Hybrid Baryons with CLAS12 in Hall B"

Summary and outlook

New CLAS polarized photoproduction data off

polarized and unpolarized, proton and neutron targets

contribute to complete or nearly complete experiments.

Evidence of new states found in coupled-channel analyses.

Large impact expected as data analyses are being finalized.

30

PDG

bary

on s

ummar

y ta

ble

for

N* r

eson

ance

s

Table adapted from: V. Crede and W. Roberts, Rep. Prog. Phys. 76 (2013) 076301

future

… f

utur

e up

date

s …