Photoproduction of Mesons off the Deuteron

Photoproduction of mesons off light nuclei————————————————————–

B. Krusche, U. Basel

Introduction

Experimental setupsCrystal Ball & TAPS @ MAMI

Crystal Barrel & TAPS @ ELSA

Experimental resultsphotoproduction off quasi-free nucleons: η, πo, πoπo, η′, πoη, π±η, πoπ±,...

coherent photoproduction of mesons off light nuclei: η, πoη, πoπo, πoπoπo

Conclusions

Collaboration2

a

B. Krusche, Erice School, Sicily, 2011

electromagnetic excitations of the neutron————————————————————importance of measurements off the neutron:

• different resonance contributions

• needed for extraction of iso-spin composition of elm. coupl ings

complications due to use of nuclear targets (deuteron):

• coincident detection of recoil nucleons

• Fermi motion, nuclear effects like FSI, coherent contribut ions


Experiments: Crystal Ball & Crystal Barrel with TAPS—————————————————————————

10 cm

Bonn ELSA accelerator:Crystal Barrel (CsI),TAPS (BaF2) forward wall,inner detectors

Eγ ≤ 3.5 GeV,lin. pol.: available,circ. pol.: available

Mainz MAMI accelerator:Crystal Ball (NaJ),TAPS (BaF2) forward wall,inner detectors

Eγ ≤ 1.5 GeV,lin. pol.: available,circ. pol.: available


TAPS Crystal Ball - at MAMI—————————————


η-photoproduction off the proton: resonance contributions ?————————————————————————————–branching ratios and elm. couplings (PDG):

state bη [%] Ap1/2 Ap

3/2 An1/2 An

3/2

• D13(1520): 0.23±0.04 -24 166 59 139• S11(1535): 30 - 55 90 -46• S11(1650): 3 - 10 53 -15• D15(1675): 0±1 19 15 -43 -58• F15(1680): 0±1 -15 133 29 -33• D13(1700): 0±1• P11(1710): 6.2±1.0• P13(1720): 4±1

D15(1675) has stronger electromagneticcoupling to neutron than to protonbut parameters quite uncertain:

An1/2=-(21-57), An

3/2=-(30-77)

bη=0-1% (PDG), bη=17% (ETA-MAID, Chiang et al.)

interference structure in S 11-sector?

γp → ηp

1.6 1.8 2 2.2 2.4

5

10

1520 1 1.5 2 2.5

W / [GeV]

/ [GeV]γ Eb]µ / [ totσ

Data:

• TAPS: B. Krusche et al., PRL74 (1995) 3736• GRAAL: F. Renard et al., PLB528 (2002) 215• CLAS: M. Dugger et al., PRL89 (2002) 222002• Crystal Barrel: V. Crede et al., PRL94 (2005) 012004

⇑⇓

⇑

⇓

S11

S11

D15?

P11, P13


what is expected for n(γ, η)n - why is it interesting?————————————————————————total cross sectionsfor proton and neutronfrom MAID model withand without D 15(1675))

0

10

1500 1600 1700 1800

σ[µu

b]

W[MeV]

full model MAID, protonno D15(1675), proton

full model MAID, neutronno D15(1675), neutron

previous data fromMAMI only at lowerincident photon energies

0.5

1

1.5

700 800 900 1000

σ n/σ p Kaiser et al.

MAID full model

MAID only S11(1535)

d(γ,η)X, σn/σp(Eγ)d(γ,η)X, σn/σp(Eγ)

d(γ,η)X, σn/σp(E*d(γ,η)X, σn/σp(Eγ)

4He(γ,η)X, σn/σp(Eγ)4He(γ,η)X, σn/σp(Eγ)4He(γ,η)X, σn/σp(E

*4He(γ,η)X, σn/σp(Eγ)

Eγ[MeV]

GRAAL experimentreported narrowstructure in neutronexcitation function

0

0.2

0.4

0.6

0.8

1.5 1.6 1.7 1.8 1.9W,GeV

dσ/d

Ω, a

.u.

0

0.5

1

1.5 1.6 1.7 1.8 1.9W,GeV

dσ/d

Ω, a

.u.

0

0.2

0.4

0.6

0.8

1.5 1.6 1.7 1.8 1.9W,GeV

dσ/d

Ω, a

.u.

0

0.5

1

1.5 1.6 1.7 1.8 1.9W,GeV

dσ/d

Ω, a

.u.

0

0.2

0.4

0.6

0.8

1.5 1.6 1.7 1.8 1.9W,GeV

dσ/d

Ω, a

.u.

0

0.5

1

1.5 1.6 1.7 1.8 1.9W,GeV

dσ/d

Ω, a

.u.

predictions from chiral soliton models: P11-like state of the anti-decuplet hasstrong photon-coupling to the neutron and large ηN decay branching ratio


control over Fermi motion and FSI effects————————————————————–test with help of proton data:

most simple test: comparison of quasi-free proton cross sec tion with Fermifolded free proton cross section - perfect agreement

more detailed: final state kinematically completely determ ined, four-momentaof meson, participant-, and spectator-nucleon known=⇒ W follows from invariant mass of meson and participant nucleo n

momenta of spectator nucleons proton excitation functions σ(W )

0

0.2

0.4

0.6

0.8

1

0 100 200 300

neutron spectator

coun

ts[a

.u.]

ps[MeV]

datasimulationdeuteron wf.

0 100 200 300 400

proton spectator

0.60.70.80.9

1

2

3

4

56789

10

20

1600 1800 2000 2200

σ[µb

]

W[MeV]

σ(ηp), free proton

σ(ηp), quasi-free proton

B. Krusche, Erice School, Sicily, 2011 ELSA

control over detection efficiency of recoil nucleons—————————————————————————η-photoproduction

internal consistency: σincl = σp + σn + (σd)

coherent part σd negligible: σn ≈ σincl − σp

excitation functions σ distribution of differences of dσdΩ

(n), dσdΩ

(incl−p)

1

10

500 1000 1500 2000 2500

σ[µb

]

Eγ[MeV]

σpσn

σnpσp+σn

0.5

1

1.5

500 1000 1500 2000 2500

σ n/(σ

np-σ

p)

0

10

20

30

40

50

60

-5 -4 -3 -2 -1 0 1 2 3 4 5δσi/∆σi

coun

ts

fitted Gauss

σ = 1.25

µ = 0.034

standard Gauss

σ = 1.0

µ = 0.0

no indication of systematic effects from neutron detection efficiency!


quasifree η-photoproduction off the deuteron—————————————————————–total cross section for γn → ηn from as function of incident photon energy:pronounced structure around Eγ ≈1 GeV

0

5

10

1 1.5 2 2.5

σ[µb

]

Eγ[GeV]

pη

nη*3/2

pη, nη, MAID

1.5 1.7 1.9 2.1 W[GeV]

0

2.5

5

7.5

10

0.6 0.8 1 1.2 1.4

σn

σnp-σp

σ[µb

]

0.5

1

1.5

1 1.5 2 2.5

1.5 1.7 1.9 2.1 W[GeV]

σn/σp this work

Weiss et al.

Hejny et al.

MAID

Shklyar et al.

Eγ[GeV]

σ n/σ p


‘de-folding’ of Fermi motion—————————————–

for events with neutron in TAPS(cos (Θ⋆

η) < -0.1)neutron energy from time-of-flight

comparsion: W from photonenergy (Fermi smeared) -W from nucleon - meson4-vectors (resolution smeared)

de-folded proton cross sectionsimilar to free proton,de-folded neutron cross sectionshows structure around 1.7 GeV:

position: W=1683 MeVwidth: Γ=60±10 MeV(resolution dominated)

WB[MeV] WR[MeV]

0

2

4

1500 1600 1700 1800

dσ/d

Ωac

c[µb/

sr]

ηpηn

0

2

4

1500 1600 1700 1800

I. Jaegle et al.,Phys. Rev. Lett. 100 (2008) 252002


‘de-folding’ of Fermi motion - kinematical reconstruction———————————————————————————–reaction kinematics completely determined even without ne utron ToF:

• initial state:incident photon and deuteron at restknown/measured: Eγ, md, ~pd = 0

• final state:η, participant, and spectator nucleonknown/measured: ms, mp, Θp, Φp, mη, ~pηnot measured: Tp, ~ps (four variables)

• four constraintsfrom energy/momentum conservation→four-vectors ofparticipant and spectator determined

structure in n excitation functionwidth ≈(25±10) MeV(resolution dominated)

0

5

10

15

1600 1800 2000 2200σ[

µb]

W[MeV]

σ(ηp), free proton

σ(ηp), quasi-free proton

3/2σ(ηn), quasi-free neutron

0.5

1

1.5

2

2.5

1600 1800 2000 2200

σ n/σ p

W[MeV]

I. Jaegle et al., arXiv:1107.2046 (published in Eur. Phys. J . A)


Fermi de-folded proton and neutron data———————————————————–excitation functions for different bins of polar anglefree proton data ( ⋆): McNicoll et al. PRC 82 (2010); curves: MAID model

cosΘ -1. → -0.5 cosΘ -0.5 → 0. cosΘ 0. → +0.5 cosΘ +0.5 → +1.

1

σ[µb

] σ(ηp)σ(ηn)

1

2

1600 1800

σ n/σ p

1600 1800 1600 1800

W[MeV]1600 1800

‘peak’ in neutron cross section related to‘dip’ in proton cross sections (?)only ‘dip’ reproduced by MAID!

fit with:

dσdΩ = q⋆

k⋆×Σ AiPi(cos(Θ⋆))

-0.5

0

0.5 A1/A0

A3/A0

A2/A0

free proton

-0.5

0

0.5q.f. proton

Ai/A

0

-0.5

0

0.5

1.5 1.6 1.7 1.8 1.9

q.f. neutron

W[GeV]


preliminary excitation functions from MAMI————————————————————–

W = f(Eγ) W = f(n, η) ratio σn/σp

[MeV]beamW1400 1500 1600 1700 1800 1900

b]µ [σ

0

2

4

6

8

10

12

14pη→pγ

nη→nγ

[MeV]true (kin)W1400 1500 1600 1700 1800 1900

b]µ [σ

0

2

4

6

8

10

12

14

16pη→pγ

nη→nγ

[MeV]true (kin)W1400 1500 1600 1700 1800 1900

pσ / nσ

0.4

0.6

0.8

1

1.2

1.4

1.6

new MAMI data confirm statistical significance of structure b eyond any doubts

correspond to ≈30% of data taken

will allow detailed analysis of angular distributions

width of structures still resolution dominated

D. Werthmuller et al.

B. Krusche, Erice School, Sicily, 2011 MAMI

similar results for 3He target——————————————

[MeV]BW1400 1500 1600 1700 1800 1900

b]µ [σ

0

5

10

15

20

25

30Nη→ +NγN Pfeiffer et al.η→ +Nγpη→ +pγnη→ +nγN (from sum)η→ +Nγ

[MeV]RW1500 1600 1700 1800 1900

b]µ [σ

0

2

4

6

8 11 MeV± = 45 Γ

inclusive andexclusivecross sections

quasi-freenucleonsW fromtime-of-flight(only nucleonsin TAPS)

L. Witthauer et al.

[MeV]BW1400 1500 1600 1700 1800 1900

b]µ [σ

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

pη→ +pγ

nη→ +nγ

[MeV]RW1500 1600 1700 1800 1900

b]µ [σ

0

1

2

3

4

5

6

pη→ +pγ

nη→ +nγ

quasi-free nucleon,W from incidentphoton energy

quasi-free nucleon,W from N − ηfinal state

B. Krusche, Erice School, Sicily, 2011 MAMI

Summary————–

narrow structure in excitation function of γn → nη:

GRAAL: W ≈ 1680 MeV, Γ <30 MeV

Tohoku-LNS: W ≈ 1670 MeV, Γ <40 MeV

ELSA: W ≈ 1665 MeV, Γ <40 MeV

MAMI-C: W ≈ 1675 MeV, Γ <50 MeV

MAMI-C, 3He: W ≈ 1650 MeV, Γ <50 MeV

so far no information about quantum numbersof possible resonanceor whatever nature of the structure


measrement of polarization observables under way————————————————————————–

at MAMI data taken for target asymetry T (transversely polarized target anddouble polarization observable F (tranversely polarized target, circularlypolarized beam)

at ELSA double polarization observable E (’GDH’) (longitudinally polarizedtarget, circularly polarized beam)

online results free proton quasi-free proton

0 500 1000 1500 2000 2500

0

2000

4000

6000

8000

3/2σ-1/2σweighted

< 150 MeVfermi

p

Conclusions——————-Systematic investigation of meson production——————————————————————

off 2H and 3He———————

η-photoproduction off deuteron:large difference for resonance contributionsto p(γ, η)p and n(γ, η)n

narrow structure in excitation function off neutron

other channels:iso-spin dependence of the elm. nucleon excitation

outlook:quasi-free (double) polarization observables

I. JaegleI. KeshelashviliT. RostomyanM. DieterleA. KaserY. MaghrbiM. OberleF. PheronD. WerthmullerL. WitthauerTh. ChallandR. Trojer


Photoproduction of Mesons off the Deuteron

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