status of the recoil nucleon polarimeter dan watts, derek glazier, mark sikora (supa phd student)...
TRANSCRIPT
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
)