Crystal Ball Experiment at MAMIRecent Results

W.J. Briscoe for the A2 Collaboration

(thanks for the sabbatical support)MESONS 2010

SFB443

Overview of MAMI and the Crystal Ball experimental setup Technical capabilities: pion production, strangeness

productionfrom the proton and neutron Selected physics topics:

Coherent pion photoproduction Eta photoproduction

Complete measurements: Transverse spin observables in pion and eta photoproduction Conclusions & Outlook

Overview

MAMI

MAMI

Maximum Energy 1604 MeV, ΔE = 100 KeV

100 % duty cycle

Current ≤ 100 μA

Electron Polarization ~ 85%

~7000 hours beam / year

Photon Tagging Facility

Detection of radiating electrons: Eγ = Ee – Ee'

Energy resolution 2-4 MeV

Tagger Microscope ~6x better E res.

Circularly pol. γ from e- pol, upto 85%

Linearly pol. γ from crystalline rad., upto 70%

Collimation upgrade will give +5% pol.

End Point Tagger awaiting funding

Pe-→Pγcirc.

CB@MAMI Detector System

Crystal Ball

TAPS

PID Detector

MWPCs Target

First report of σ(γ,π0) for a specific excited state

Simultaneous detection of π0 and 4.4 MeV decay γ in CB

Important first step in isolation of coherent process

PRL 100, 132301 (2008)

Technical Capabilities: Incoherent π0 photoproduction on 12C

Decay γ spectrum in coinc. with π0 4.4 MeV 2+ state

γ12C→12 C* π0

↓ 12C γ(4.4 MeV)

Technical Capabilities: Kaon Photoproduction

Decay sub-cluster energy

Incident and decay sub-cluster time difference

K+ missing mass

Incident subcluster from K+ ~3ns

Decay sub-cluster from K+→μ+ν

μ decay ~

20ns

Tom JudeEdinburgh University

Coherent π0 photoproduction on 208Pb

Eγ = 160 - 170 MeV Eγ = 190 - 200 MeVEγ = 170 - 180 MeV Eγ = 180 - 190 MeV

Do heavy stable nuclei have a neutron skin?

Size of skin gives direct information on equation of state of n-rich matter

Skin size gives important new insights into neutron star physics!

Measurements planned on Sn, Ca isotope chains

Accuracy ~0.05 fm D. P. Watts and C. Tarbert, Edinburgh

Prelim

inar

y

Prelim

inar

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Prelim

inar

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Prelim

inar

y

Prelim

inar

y

Prelim

inar

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Prelim

inar

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Prelim

inar

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Many resonances: broad and overlapping

Accurate separation of final states → good detector resolution

Sensitivity to small σ processes → 4π detector acceptance, large γ flux

Access to polarization observables → polarized beam, target, recoil

Excitations of the Nucleon

Δ(1232) Δ(1232)

η photoproduction: γp→ηp

S11(1535) dominant resonance in η production

“Dip” in cross section due to interference with less dominant resonances

Need polarization observables to extract full resonance composition

TAPS/MAMI

GRAAL/ESRF

CLAS/JLAB

CB/ELSA

S11(1535) dominant resonance in η production

“Dip” in cross section due to interference with less dominant resonances

Need polarization observables to extract full resonance composition

JLAB ELSA

CB@MAMI preliminary(S. Prakhov)

S11

(1535)JP = 1/2 -

JP = 3/2 -

D13

(1520)

η photoproduction: γp→ηp

Complete Experiment

16 possible unpolarised, single & double polarization observables in pseudoscalar meson photoproduction

Need 8 carefully selected observables to fully constrain partial wave analyses

These have to include single & double polarization observables

All polarization degrees of freedom now uniquely accessible in Mainz!

Recoil Polarimetry

π0 / η decays & is detected as normal

Reconstruct π0 / η

Recoiling proton then tagged

Preconstructed = γbeam + ptarget – π0

Large scattering angle → nucl. interaction

Asymmetry gives pol. transfer

DataG4 totalG4 no nuclear int

Proton scattering angle in graphite

D. P. Watts , Edinburgh D. Glazier, Edinburgh M. Sikora, Edinburgh.D, Howdle, Glasgow

Recoil Polarimetry – π0 Photoproduction

Photon Energy

Deg

ree o

f P

ola

risati

on

Tra

nsfe

r C

x'

p(γ,π0)p polarisation transfer: circ. polarised beam to recoil proton

D. P. Watts, EdinburghD. Glazier, EdinburghM. Sikora, EdinburghD, Howdle, Glasgow

Polarized Frozen Spin Target

H. Ortega Spina

Uses DNP to achieve ~ 90 % proton, 80 % deuteron

Needs: Horiz. Dilution cryostat, polarizing magnet, microwave, NMR

Two holding coils: solenoid → longitudinal, saddle coil → transverse

Uses DNP to achieve ~ 90 % proton, 80 % deuteron

Needs: Horiz. Dilution cryostat, polarizing magnet, microwave, NMR

Two holding coils: solenoid → longitudinal, saddle coil → transverse

Polarised Frozen Spin Target

Polarised Frozen Spin Target

Frozen spin target fully functioning – Polarization > 90%

~1000 hours relaxation time & low He usage – long measurement time!

Running with transverse polarized target!

N. Froemmgen

P=P0exp(t/τ)

First measurement of transverse spin observable F in γp→π0p

F asymmetry: circ. polarised photons, transverse pol. Target

Need to seperate out contribution from 12C and 16O and 3/4He

Requiring proton removes coherent contributions

Other kinematic cuts and remaining underground fitted & subtracted

Data shown from 39 hours minus, 39 hours plus pol. test data, no TAPS

Test beamtime ended 07:00 08.03.10, results first shown 10:00 10.03.10

VERY PRELIMINARY!

N. Froemmgen

EPGPP

PPTP

FPHPP

Pd

d

d

d

circlinz

liny

circlinx

lin

unpol

2sin

2cos

2sin

2cos1

Asymmetry calculated for each bin (above)

Normalized to sin(φ) (target polarization angle corr.)

Weighted average for points with |sin(φ)|>0.3

Background Subtraction on MM(π0)

Eγ = 500 –

600 MeV

Eγ = 400 –

500 MeV

Eγ = 400 – 500 MeV

Eγ = 500 – 600 MeV

Eγ = 300 – 400 MeV

cos(φ

π-9

0)

= s

in (

φπ)

0

First measurement of transverse spin observable F in γp→π0p

World first measurement of F – VERY PRELIMINARY!

Need more work on Pγ (currently standard conditions assumed)

Need to extend to full solid angle coverage (measure with TAPS)

Ptarg from average over time – need event-by-event normalisation

However – everything works!

PRELIMINARY PRELIMINARY PRELIMINARY

F

First measurement of transverse spin observable F in γp→π0p

SAIDMAID

V. Kashevarov

Conclusions & Outlook

F

PRELIMINARY PRELIMINARY PRELIMINARY

SAIDMAID

F

The CB@MAMI experimental setup is a highly flexible 4π detector system

Complete measurements of π and η production within next five years

Allow full investigation of: P33(1232), P11(1440), S11(1535)

Double meson production (ππ, πη) → other resonance studies e.g. D33(1700)

Compton scattering: access to nucleon vector polarisabilities

Strangeness photoproduction, coherent π0 studies of isotope chains

η/η' decays & more...

V. Kashevarov

Polarized Target

2 cm

Uses DNP to achieve ~ 90 % proton, 80 % deuteron

Needs: Horiz. Dilution cryostat, polarising magnet, microwave, NMR

Two holding coils: solenoid → longitudinal, saddle coil → transverse

Detectors have to move

Polarized Target

Uses DNP to achieve ~ 90 % proton, 80 % deuteron

Needs: Horiz. Dilution cryostat, polarising magnet, microwave, NMR

Two holding coils: solenoid → longitudinal, saddle coil → transverse

Detectors have to move

Polarized Target

Polarised Target

Uses DNP to achieve ~ 90 % proton, 80 % deuteron

Needs: Horiz. Dilution cryostat, polarising magnet, microwave, NMR

Two holding coils: solenoid → longitudinal, saddle coil → transverse

Detectors have to move

Conclusions

• We are running a full program of Transverse proton and neutron (deuteron) polarized target measurements. (Longitudinal will follow.)

• Circularly and linearly polarized tagged photons.

• Have preliminary results for F.

• MAMI B and MAMI C experiments are being analyzed and prepared for publication by a large group of students.

• Expect at least a 5 year program with CB and TAPS at MAMI!