MAMI - Nuclear Physics - University of Glasgow

nuclear.gla.ac.uk

MAMI - Nuclear Physics - University of Glasgow

Recent Results from the CB@MAMI Programme

at the Glasgow Tagged Photon Spectrometer in

Mainz

E. J. Downie

University of Glasgow


Introduction

Introduction to the CB@MAMI experimental series

Physics highlights of first round

➔ Experiment to measure the MDM of ∆ + (1232)

Glasgow Photon Tagger upgrade / MAMI-C

Anticipated physics of round II


The Need for CB@MAMI Experiments

Detailed information on nucleon & resonances → test QCD

N resonances: short lifetime ↔ difficult to measure properties

Large Breit Wigner widths → can't accurately determine spect.

Need more experimental information


Unique capabilities of CB@MAMI

Well understood probe: photon

Ability to accurately separate final states: good det. resolution

Sensitivity to small σ processes: 4π detection, large γ flux

Access to polarisation observables: pol. beam, target, recoil


The CB@MAMI Set-up – Electron Beam

MAMI B

Up to 883 MeV e - beam

MAMI-A

180 MeV

100 % duty factor

Current up to 100µA

Polarised e -

Very stable beam

MAMI-B

883 MeV


The CB@MAMI Set-up – Photon Beam

Glasgow Photon Tagging Spectrometer

Pass e - beam through radiator

Magnetic spectrometer for e -

“Tags” bremsstrahlung Ε γ

Photon beam enters expt.

Timing coincidence

Polarised e - → 80% circ. pol. γ

Crystalline rad. → lin. pol. γ


The CB@MAMI Detector Set-up

Glasgow

Photon

Tagging

Spectrometer

High rate 2MeV resolution tagged photon beam

Liquid H 2

/D 208 2,

Pb, 40 Ca, 16 O, 12 C and other nuclear targets

Two EM calorimeters → 4π spectrometer set-up

MWPC & PID for tracking and ID of charged particles

Ideal for high precision / low cross section measurements


The CB@MAMI Detector Set-up

High rate 2MeV resolution tagged photon beam

Liquid H 2

/D 208 2,

Pb, 40 Ca, 16 O, 12 C and other nuclear targets

Two EM calorimeters → 4π spectrometer set-up

MWPC & PID for tracking and ID of charged particles

Ideal for high precision / low cross section measurements


The CB@MAMI Detector Set-up – The PID

Built by Glasgow & Edinburgh

∆E (PID) / E (CB)

2mm scint. barrel

Identified p / π +/- / e - / d

PID II by Edinburgh & Glasgow

Protons

Pions


The Magnetic Dipole Moment of the ∆ + (1232)

Nucleon structure very difficult to probe

Predictions of µ very sensitive to the baryon model used

EM properties of nucleon excited states largely unmeasured

Spin precession in a magnetic field wholly unsuitable

➔ Experimental test of theoretical models


The Magnetic Dipole Moment of the ∆ + (1232)

p(uud)

∆ + ∆ +

Tagged photon beam on liquid H2

γ

∆ + (uud)

P

π 0

γ

γ

∆ + lifetime 10 -24 s ⇔ large Breit-Wigner width

Created ∆ + at upper end of B-W width

∆ + radiatively decays to another ∆ +


The Magnetic Dipole Moment of the ∆ + (1232)

p(γ,γ'π 0 p) Experimentally difficult channel:

~50nb cross section

Backgrounds: p(γ,π 0 p), 318µb; p(γ,π 0 π 0 p), 1.5µb

Comprehensive measurement required:

Measure two channels: p(γ,γ'π 0 p), p(γ,γ'π + n)

Measure several observables:

➔ Five-fold differential cross section

➔ Linearly polarised photon asymmetry

➔ Circularly polarised photon asymmetry


The Magnetic Dipole Moment of the ∆ + (1232)

M(initial ∆ + )

M(final ∆ + )

M(sum)

E miss.

data

E miss.

sim.

M 2 miss. (π0 p) data

M 2 miss. (π0 p) sim.


The Magnetic Dipole Moment of the ∆ + (1232)

Total cross section of p(γ,γ'π 0 p)

Expect improvement in statistical error:

→ kinematic fitting

→ background subtraction improvement

This Work - preliminary

Kotulla et al. P.R.L. 89 (2002)


The Magnetic Dipole Moment of the ∆ + (1232)

Differential σ of p(γ,γ'π 0 p)

Improvement in acceptance calculation expected

Fully diff. σ being calculated by Mainz / Basel students

S. Schuman, Mainz

Kotulla et al. P.R.L. 89 (2002)


The Magnetic Dipole Moment of the ∆ + (1232)

Linearly polarised photon asymmetry of p(γ,γ'π 0 p)

Variation with E(γ') important model constraint

First ever measurement!


The Magnetic Dipole Moment of the ∆ + (1232)

Chiang et al. P.R.C. 71 (2005)

Pascalutsa & Vanderhaeghen P.R.L. 94 (2005)


The Magnetic Dipole Moment of the ∆ + (1232)

Σ (linearly Polarised)

Σ (circ. Polarised)

Ε(γ') / MeV

Ε(γ') / GeV

p(γ,γ'π + n) results

PID & shower shape → Good π + energy determination

Can measure nπ + decay of ∆ + with CB !!

Proof of principle established more detailed meas. planned

D. Glazier & D. Watts, Edinburgh


Other CB@MAMI Round I Physics Highlights

η mass determination

➔ Find η prod. threshold

m

η

= −m

p

+

m

2

p

+

2⋅

m

p

E

c

thr

γ

2

A. Nikolaev, Bonn

Without

Acceptance

Correction


Other CB@MAMI Round I Physics Highlights

η mass determination

p(γ,π 0 π 0 p) production

➔ Test of χPT

➔ Circ. asymmetry

➔ Reaction mechanism?

Double π 0 Excitation Function

Bernard et al. Phys. Lett. B382 (1996)

Kotulla et al. Phys. Lett. B578 (2004)

F. Zehr, Basel

A. Nikolaev, Bonn

Without

Acceptance

Correction

➔ N resonance properties?

D. Krambrich, Mainz


Other CB@MAMI Round I Physics Highlights

η mass determination

p(γ,π 0 π 0 p) production

p(γ,π 0 p) photon asymmetry

➔ Massive statistics

➔ High resolution

➔ EM properties of

➔ E2/M1 ratio

E. Downie

R. Leukel, Mainz, 2001

E. Downie, Glasgow

VERY PRELIMINARY

>0.5 of data set


CB@MAMI Highlights I

Coherent π 0 production

➔ Do heavy stable nuclei have a neutron skin?

➔ Fundamental property of nuclear physics

➔ Size of skin gives direct information on neutron rich equation of state

➔ Skin size also gives important new insights into neutron star physics

(cooling mechanisms, mass radii relationships)

➔ Accuracy < 0.05 fm

➔ C. Tarbert, D. Watts, Edinburgh

➔ See D. Watts talk


See low E nuclear decay γ's

In coincidence !

➔ 550 MeV η invariant mass

➔ 135 MeV π 0 invariant mass

➔ 4.4 MeV Carbon decay photon

New possibilities for range of

nuclear physics studies e.g.

transition matter form factors

CB@MAMI Highlights I

4.4 MeV

12

C

C. Tarbert, Edinburgh

16

O

6.1 MeV

π 0

η

C. Tarbert, Edinburgh


MAMI-C / Photon Tagger Upgrade

MAMI-A

MAMI-B

MAMI-A


180 MeV 883 MAMI-B

MeV


MAMI-C / Photon Tagger Upgrade

MAMI – C

MAMI-A

Now

Operational!

MAMI-B

MAMI-A


180 MeV 883 MAMI-B → MAMI–C

MeV 1.5 GeV


MAMI-C / Photon Tagger Upgrade

Tagger

Tagger has to bend 1.5 GeV

Need to increase B to 1.8T

Complete Tagger dismount

➔ Reduce pole gap

➔ Increase return yoke

Tagger reassembled

2T field achieved

Sucessfully running @ 1.5GeV!


MAMI-C / Photon Tagger Upgrade


MAMI-C / Photon Tagger Upgrade

1.5 GeV Beam Spot

See Eillidh McNicoll's talk


CB@MAMI Future Facilities: Edinburgh Recoil Polarimeter

High γ flux allows possibility of recoiling nucleon polarimetry

Graphite scatterer + PID/Tracking detectors

Scatter > 10°: Hadronic interaction

n(θ,φ) =n o

(θ){1+A(θ)[P y

cos(φ)–P x

sin(φ)]}

First “complete measurement” of meson (p,n..) production

Enable first fully constrained PWAs


CB@MAMI 1.5GeV Expected Future Highlights

Recoil polarisation expts.

Frozen spin target – double pol. experiments

S 11

(1535) Magnetic Moment

Threshold strangeness production

Short range correlations

Tests of fundamental symmetries η, η' decays

Medium modification of ω mesons (χral sym. restoration)

Sum rules – GDH ... VERY PRELIMINARY


>0.5 of data set

Vector meson production

η-mesic nuclei


Conclusions

Initial round of CB@MAMI experiments complete

Results are approaching publication

MAMI C running at 1.5GeV

Tagger upgrade sucessfully complete

Broad range of nuclear and hadronic physics planned

Watch this space for interesting physics!


The CB@MAMI Detector Setup – The PID

Designed and build by Edinburgh / Glasgow Universities

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