Status of Beam Asymmetry Measurements for Non-Strange Meson Photoproduction from g8b

Status of Beam Asymmetry Measurements for Meson PhotoproductionASU Meson Physics Group

Hadron Spectroscopy WG Meeting – June 2009

Status of Beam Asymmetry Status of Beam Asymmetry Measurements for Non-Strange Measurements for Non-Strange

Meson Photoproduction from g8bMeson Photoproduction from g8b

ASU Meson Physics Group* (and friends)

•Work at ASU is supported by the National Science Foundation



Spin observablesSpin observables• Differential cross sections are (relatively) simple• Spin observables access interferences

I. S. Barker, A. Donnachie, J. K. Storrow, Nucl. Phys. B 95, 347 (1975)

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g8b running periodg8b running period(June 20- Sept 1, 2005)(June 20- Sept 1, 2005)

• Incident electron Ee = 4.55 GeV

• 50- diamond bremsstrahlung radiator

• Linearly polarized photon beam (┴ , ║)

• Coherent edge set at 1.3, 1.5, … 2.1 GeV → E = 1.1 to 2.1 GeV

• 40-cm-long LH2 target

• Trigger: single charged particle + tagger

Ori

ente

d /

Un

-ori

ente

d



Photon beam polarization Photon beam polarization PP• Analytical QED coherent

bremsstrahlung calculation fit to actual spectrum (Livingston/Glasgow)

• Unstable coherent edge location

• Determined regions of reliable photon beam polarization P

• Estimated systematic uncertainty in P: ~3%

Ori

ente

d/U

n-o

rien

ted

Po

lari

zati

on



““Para/Perp” approach for extracting Para/Perp” approach for extracting

• Use missing mass for reaction + p → p + X• Construct kinematical bins for parallel, perpendicular,

and unpolarized data subsets• Slice each kinematical bin into azimuthal angle φ slices

(10, 20, 30 or so): “φ slice method”• Determine photoproduced meson yield in each slice• Fit φ dependence to A(1 + B cos(2φ)), where B = P .• Preliminary results shown are from Patrick Collins and

Mike Dugger



Example: Example: p → p → n n



Extracting Extracting for for p → p → n n

High yield and/or high asymmetry is easy High yield and/or high asymmetry is easy



A much harder job: A much harder job: p →η´ pp →η´ p

Low yield and/or low asymmetry is tough Low yield and/or low asymmetry is tough



Fourier moment methodFourier moment method(CLAS Note 2008-35)(CLAS Note 2008-35)

• Simultaneously uses full azimuthal φ acceptance of data set.

• Only a few (in principle, only 2) histograms per kinematic bin need to be fit.

• Re-binning data is comparatively easier than with the φ slice method.



Flux-normalized yieldsFlux-normalized yields• Define a function f i,j for each kinematic bin i,j

i

i

j

j

E

E

ji dEddEdd

dELf

1 1

cos

cos

3, )(cos

)(cos),,()(

2

0

,

,

,

)( dfN

Y jia

a

ji

ji

2

0

,

,

,

)2cos(1)( dPfN

Y jiaji

ji

2

0

||,

||

,

,

)2cos(1)( dPfN

Y jiaji

ji

These are related to the yields normalized to integrated flux:

Subscript denotes polarization: a ↔ unpolarized

┴ ↔ perpendicular || ↔ parallel



Moments of flux-normalized yieldsMoments of flux-normalized yields• Expand the f i, j in Fourier series. For example,

1

0, sincos)(

mmm

jia mbmaaf

2

0

, )cos()( dnfH jiaan

2

0

,2

0

, )cos()2cos(1)()cos()( dnPfdnfH jia

jin

2

0

||,

2

0

,|||| )cos()2cos(1)()cos()( dnPfdnfH ji

aji

n

Find the nth moment for the f i,j (called Hn ):

orthogonality



Solving these for Solving these for

• Putting the pieces together, for the i,j kinematic bin, we find:

• The Hn histograms are created by weighting each event by cos(nφ) for each i,j kinematic bin.

4||0||40||

2||22

HHPHHP

HH

||

||



Example: Example: γγ p → p → ππ00 p p

numeratornumerator

denominatordenominator



Preliminary results for Preliminary results for from g8b from g8b• Preliminary results: 0, +, and for 1.3 GeV

coherent edge setting – Fourier moment method– Results from Mike Dugger’s work– “First and worst” coherent edge setting– Uncertainties in less than 5% (more often ~1%)

• Two ways to view preliminary results– Fixed energy (~9-MeV-wide photon energy bins)– Fixed angle (0.1-wide bins in cos )– Both ways: compare with SAID predictions



Preliminary Preliminary results for results for 00

Fixed energy – 4 slides

Fixed angle – 3 slide
























Preliminary Preliminary results for results for ++

Fixed energy – 4 slides

Fixed angle – 3 slides
























Preliminary Preliminary results for results for



Acceptance problem in g8bAcceptance problem in g8b

• Clump of dead/bad TOF counters give “divot” in γ p → p missing mass

• Simple background fit is problematic

• Monte Carlo backgrounds should work (next step)



Choose a branch: Choose a branch: → p → p ++00 ((~22.7%)~22.7%)

• Look at p → p X • Look at MY for

p X → p +Y

• Cut on MY= 0 mass• Much less

background under peak

• Lower acceptance (~ [0.5]2 0.05)



Using branch for Using branch for in g8b in g8b

• Apply Fourier moment method for quick preliminary look.– Even with branch, lots of events in g8b– Ultimately will go back to proton yield in γ p → p

with good background MC




4 E-counters / bin




2 E-counters / bin




2 E-counters / bin



SummarySummary• Good progress for the first/worst 200 MeV of photon

energy range– Finish line is in sight for for + , 0 , and – Getting for ′ will be tough but doable

• The preliminary results for + , 0 , and …– Look very good, even with fine binning– Incredibly stringent test for nucleon models– No indication of any narrow-width structures in any channel

• Goal: All four mesons ready for review by end of year



Meson Physics Group

Patrick Collins

Mike Dugger

Brian Morrison

Eugene Pasyuk

Barry Ritchie

Work at ASU is supported by the National Science Foundation

Documents

Status of Beam Asymmetry Measurements for Non-Strange Meson Photoproduction from g8b