Status of the recoil nucleon polarimeter

Dan Watts, Derek Glazier, Mark Sikora (SUPA PhD student)(University of Edinburgh, UK)

Outline

• Physics motivation

• Polarimeter operation

• Beam test - proof of polarimeter concept

• First results - beam helicity transfer observabes (Cx)

• Outlook

Physics motivation: Nucleon excitation spectrum

Excitation spectrum is fundamental to nucleon structure - but not firmly established

Particularly disappointing given the potential advances from theory

Lattice QCD

Holographic dual of QCD

Constituent quark models

QCD models

+ N → m

Polarisation observables

Linear Polarisation

Circular polarisation

Recoil polarimeter - enable the first complete measurement of observables

Fully constrain the reaction amplitudes

Longitudinally polarised proton target Transversely polarised

• just one of 16 observables in pseudo scalar meson photoproduction• Complete measurement requires 8 well chosen observables• Only possible with double polarisation measurements

Double-polarisation in pseudo-scalar meson photoproduction

Polarisation of

target recoil

Observable

Analysing power of scatterer

Polar angle distributionfor unpolarised nucleons

x and y (transverse) components of nucleon polarisation

Number of nucleons scattered In the direction

n() =no(){1+A()[Pycos()–Pxsin()]

Nucleon scattering and polarisation

The polarimeter setup

Test data results - p() yield

Ee=1.5 GeV

Test data analysis – p(0)p Cx’

• 2 x 3 day beam times (Ee=0.85 and 1.5 GeV) - First data for Cx!!

Photon energy (MeV) Photon energy (MeV)

Cx’

Cx’

20 – test of helicity bit

Single spin beam helicity asymmetry

Test data analysis - p()

First measurement of beam helicity transfer in photoproduction!!

Azimuthal scatter angle in polarimeter

E <0.9 geVAll

E =0.9 -1.1 GeVAll

Summary and outlook

• Succesful nucleon polarimeter test - now ready for production beamtime

• Formalism for extraction of Ox, T we developed for CB proposal used succesfully to extract observables in JLAB kaon photoproduction measurement

• New Edinburgh PhD student to work on project

• CB@MAMI poised to provide unique measurements of double-polarisation observables for for and meson photoproduction

MAID predictions and expected data accuracy - p()N

300 hrs MAMI B

500 hrs MAMI C

cm=120o±10

For events with nuclear scatter in polarimeter

Tagged nucleon events

• Present PWA solutions indicate sensitivity of observables to specific resonances

Sensitivity to Roper P11(1440)

MAID PWANo Roper

Cross section Linear PolarisationAsymmetry

Linear Polarisation+ RECOIL

E = 500 MeV

• Recoil observables give large sensitivities to poorly established resonances e.g. Roper P11(1440)

• High quality meson photoproduction data with polarisation observables can be expected from MAMI

• Determination of beam, target and recoil polarisation will give a “complete” measurement of observables

• Commissioning data for nucleon polarimeter expected in 2007

Summary

Double polarisation in meson photoproduction

• Many overlapping resonancesare a problem

• Double polarisation observables give new constraints on resonanceproperties and reactionmechanisms

Polarised beams

+ p → N + meson

Polarised targets

target recoil

For events Scattered in polarimeter

Present knowledge of the spectrum

“Roper” ResonanceMass ~ ±20 MeVWidth ~ ±100 MeV!!

Large discrepancies between analyses of same experimental data with different amplitude analysis methods

(1

23

2)

P11

(14

40

)D

13(1

52

0)

F15

(16

80

)

Intense tagged photon beam, circularly or linearlypolarised

Longitudinally polarised proton and neutron targets

Approved programme of measurements

• 4 complex amplitudes - 16 observables in meson photoproduction

• To fix the 4 amplitudes unambiguously need to measure 8 real quantities

• d + 3 single polarisation + 5 double polarisation

• Cannot choose from same set Need recoil polarisation

measurements

target recoil

Why measure double polarisation observables?

Photon Tagger upgrade

• Predicted sensitivity to poorly established resonances

• Resonance parameters from quark model (Capstick and Roberts)

Solid – SAIDDashed – background + **** Dotdash- background + **** +N-

3/2(1960)

Dutta, Gao and Lee, PRC 65, 044619 (2002)

Cx’ ( + recoil) – theoretical predictions

P

T

Previous experimental data – SAID database

Data for all CM breakup angles

Ox’ Cx’

Recent JLAB datanot in database

• First determination p(,p)0 in 2002Hall A JLab

• MAID & SAID poor description of new data

Recent Cx’ measurement at JLab

Po

lari

sati

on

tra

nsf

er C

x’

Photon energy (MeV)

The proposed experimental setup

Graphite sheet

TAPS

Crystal Ball

beam

Hydrogen target cell

Initial path of proton Polarimeter acceptance : ±20o polar angle (target at centre)Most events suffer only coulomb scattering

Useful scattered eventSelect events with scattering angleslarger than ~10 degrees : arising from nuclear interaction

n() =no(){1+A()[Pycos()–Pxsin()]

GEANT simulation of polarimeter

No GraphiteWith Graphite scatterer

• Simulation includes realisticsmearing of energy deposits due to experimental energy resolutionand proper cluster finding algorithms

• Finite target size and E resolution included

Angle between N(E,) and TAPS hit

CM) >~130o

E=150 MeVE=200Eg=300E=500E=750E=1000E=1500

Polarimeteracceptance

Nucleon angle in lab (deg)

Pio

n a

ngle

in C

M (

deg)

Kinematic acceptance of polarimeter

p()N

• More forward recoils than for pion production.

• Almost all recoils are incident on polarimeter up to ~0.8 GeV

Eg=720Eg=820Eg=920Eg=1520

Lab nucleon angle (degrees)

CM

a

ng

le (

deg

rees)

Polarimeter acceptance

Kinematic acceptance of polarimeter

p()N

Expected data accuracy

Common parameters:

Photon beam: 2.5x105 sec-1 MeV-1 Bin ±12.5 MeVTarget: 2.11023 nuclei / cm2

Meson: Bin ±10o

Polarimeter: 3% probability for a (detected) nuclear scatter Average analysing power ~0.4

MAID predictions and expected data accuracy - p()N

300 hrs MAMI B

500 hrs MAMI C

MAID predictions and expected data accuracy - p()N

300 hrs MAMI B

Full MAID

No P11(1440)

Summary

• The different sensitivities offered by recoil polarisation observables will give new constraints on the excitation spectrum of the nucleon.

• Data will be complimentary to the beam-target measurement programmes in place at MAMI and other facilities

• UK EPSRC grant already awarded to help setup the facility (including 2 year postdoc and graphite)

Cx’ – Extraction and expected accuracy

Plot difference in distributions for two helicity states (cut on region of with reasonable A())

Left with simple sin() Dependence. Extract Px

0 180 360

Photon energy (MeV)

Cx’

P=0.7, E=±25MeV, =130±10

~ 1 b/sr → Cx ~ 0.015

~ 0.1 b/sr → Cx ~0.05

Greatly improved data quality

Ox’ – linearly polarised and recoil

• One measurement : p(+)n Yerevan 80’s

P~2/√(A2N)

P=0.4, E=±25 MeV, m=130±10

~ 1 b/sr → Ox ~ 0.04

~ 0.1 b/sr → Ox ~0.12

• Polarimeter - full acceptance - determine T as the y component.

Periodically change polarisation direction by ±45o - eliminate detector effects.

Lx (Longitudinally polarised Target + recoil)

• No previous measurements

• Mainz target: ~80% polarisation

PT=0.7, E=±25MeV, m=130±10

~ 1 b/sr → Lx ~ 0.015

~ 0.1 b/sr → Lx ~0.05

BUT:• Limitations in beam intensity and dilution

from polarised target

• Must measure background contribution from non-proton events. Prompt to background 1:1 worsens error by √2

Transversely polarised (Tx)?

Cross sections

• Eg bin +-25 MeV

Pion bin +-10 degrees,

500 hrbeamtime

• p(,N) Cross sections

as low as 1b/sr

(>2*106 n bin1-)

• p(,N)p(,N) Cross sections as low as 0.1b/sr (0.2*106 nucleons per bin)

• Assume 1% of nucleons undergo nuclear interaction in proposed graphite sheet (select high analysing power with theta cut)

Estimate of polarimetry accuracy

Take dd~1b/sr, =130±10 DA ~CB-TAPS~0.7, N=2.5x10

5 sec

-1 MeV

-1

NNucleons = NT x N x DA x CB-TAPS x

2222 day-1 MeV-1

500 hour beamtime have 2.3x106 nucleons in E=±25MeV bin

Polarimeter efficiency 2% gives 4.6x104 useful nucleons

Absolute error in polarisation P~√(2/A2N) ~ 0.02 (A~0.4 for 12C)

For 0.1 b/sr absolute uncertainty in polarisation P~0.06

For double polarisation must divide error by beam(target) polarisation

0

0.2

0.4

0.6

0.8

1

1.2

1.4

600 700 800 900 1000 1100 1200 1300 1400 1500

Series1

0

5

10

15

20

25

0 200 400 600 800 1000 1200 1400 1600

Series1

p(0)p

p()p

=130

dd~1b/sr, =130±10, DA ~0.7; CB-TAPS~0.5, N=2.5x105 s-1MeV-1

NNucleons = NT x N x DA x CB-TAPS x

day-1 MeV-1

20 days beam, E=±25MeV → 5.5x106 nucleons

polarimeter)~2% → 11.1x104 useful nucleons

Analysing power A~0.4 for 12C

~1 b/sr → P~√(2/A2N) ~ 0.010 (abs. error)

~0.1 b/sr → P~0.026

For double polarisation must include further effects of degree of beam(target) polarisation

Estimate of polarimetry accuracy

0

0.2

0.4

0.6

0.8

1

1.2

1.4

600 700 800 900 1000 1100 1200 1300 1400 1500

Series1

0

5

10

15

20

25

0 200 400 600 800 1000 1200 1400 1600

Series1

p(0)p

p()p

=130

=130

Principles of nucleon polarimetry

Well established technique – relies on spin-orbit interaction in Nucleon-Nucleon interaction

Polarimeters - exploited nucleon or nuclear targets (2H, 4He, 12C, 28Si) – tended to use materials with well known analysing powers

pomme

A1 FPP

GEn Polarimeter

Kent state

Measure direction of nucleon before and after the scatterer with sufficient accuracy to determine an analysing reaction has taken place.

Polarimetry basics

For incident protons also have multiple (coulomb) scattering

scat=5-20o

scat

Scattered nucleon detection in TAPS

1 TAPS block ~ position resolution for hit TAPS~0.9m from scatterer

N

Straight through10o scatter20o scatter

Detrimental side-effects of scatterer material

To hit polarimeter TN>100 MeV in (p,)N

above the

Proton energy loss

<10 MeV for Tp>100 MeV.

Multiple scattering

<1o FWHM for Tp>100 MeV

0.37 radiation lengths conversion ~ 30%

Tp incident proton (MeV)

Tp e

xit

pro

ton

(M

eV

)

Tp after graphiteEnergy loss

0

0.5

1

1.5

2

2.5

3

0 200 400 600 800 1000 1200

Series1

Coulomb scattering

Proton energy (MeV)FW

HM

scatt

eri

ng

an

gle

(d

eg

)