Non-pole Backgrounds in the Extraction of Fπ

H. Avakian, P. Bosted, H. Fenker, R. Feuerbach, D. Gaskell,

D. Higinbotham, T. Horn*, M. Jones, D. Mack, C. Butuceanu,

G. Huber, A. Sarty, W. Boeglin, P. Markowitz, J. Reinhold,

D. Dutta, V. Koubarovski, P. Stoler, A. Asaturyan,

A. Mkrtchyan, H. Mkrtchyan, V. Tadevosyan, E. Brash,

K. Aniol, J. Calarco, P. King, J. Roche JLab, Regina, Saint Mary’s, Florida International, Mississippi State,

RPI, Yerevan, CNU, California State, New Hampshire,

Ohio University

• Motivation• Experimental Details• Summary

Hall A Collaboration Meeting

January 2006

Extracting FExtracting Fππ from from σσLL data in data in ππ++ productionproduction

• In t-pole approximation:

• Want smallest possible -t to ensure t-channel dominance

)(QFmt

gtσ 22

2

NNπ2π

2π

L

πcos2φdφdt

dσεπcosφdφdt

dσ1)(εε2dφdt

dσεdφdtσ2d TTLTT

dφdtdσL

Results from Fπ-2

• The VGL Regge model describes σL for π+ well

• Note that at tmin (maximal pole contribution) still only have σL/σT ~ 1 at Q2=2.45 GeV2

• Constraint on non-pole backgrounds requires experimental data

Horn et al., Phys. Rev. Lett. 97, 192001 (2006)

Vanderhaeghen, Guidal and Laget, Phys. Rev. C57, 1454 (1998).

Context

• Understanding of hadronic structure via measurement of Fπ is one of the high priorities at 12 GeV

• Extraction of Fπ relies on pion pole dominance – what about other processes?

• Limited knowledge of non-pole contributions limits kinematic range of Fπ measurement

– Interpretation on experimental data widely considered reliable only below –t~0.2 GeV2

– This kinematic contraint is the primary reason why we are limited to Q2~2.5 GeV2 at JLab at 6 GeV

Size of non-pole contributions

• Carlson&Milana indicated a significant contribution of non-leading processes complicating the extraction of Fπ– Background ratio rises

dramatically once tmin>0.2

• Other theoretical predictions can be obtained from:– VGL/Regge model– GPD formalism

Carlson & Milana, Phys. Rev. Lett. 65, 1717 (1990)

Interpretation of Fπ data considered reliable for -t<0.2 GeV2

• But constructing an upper bound on -t difficult due to poor quality of existing data.

Motivation -

• Non-pole contributions can be constrained using the πo longitudinal cross section– Can be related to the one from π+ using e.g. GPD formalism

• Many studies of πo unseparated cross sections in the resonance region, but contribution of σL effectively unknown above the resonance region– JLab preliminary data from Hall A (DVCS, Q2=1.5-2.5 GeV2,

W=1.9-2.3 GeV) and Hall B (e16, Q2=1-5 GeV2) available – both unseparated

Theoretical Predictions for π+ and πo cross sections

• Theoretical models based on Regge and GPD formalism describe σL for π+ quite well

• But πo prediction for σL differs by order of magnitude– Theoretical uncertainty

quite large

• Preliminary unseparated πo data from Hall A/B in this kinematic region– No information on relative

σL contribution

Vanderhaeghen, Guidal and Laget, Phys. Rev. C57, 1454 (1998). Vanderhaeghen, Guichon and Guidal, Phys. Rev. D60 (1999).

Separated σL

Non-pole contributions in the GPD Framework

• Amplitudes for π+ and πo composed of the same GPDs, but different linear combinations

• Obtain non-pole contributions by comparing πo and π+ production amplitudes, ML~ApN+BpN

– In the limit t →(mπ)2 the π+ amplitude contains a strong singularity

(pion pole)

)~~

( dd

uup

HeHeA o

)~~

( dd

uup

EeEeB o

))(~~

( dudu

peeHHA

))(~~

( dudu

peeEEB

πo

π+

VGG/GPD prediction

Motivation Summary• Constraining the non-pole contributions in the

extraction of Fπ requires experimental data– Systematic measurement of πo cross section could

constrain the size

• If the non-pole contributions are smaller than anticipated this would significantly increase the kinematic range accessible for the Fπ measurement at 12 GeV

• Constraining the contribution of σL in πo production will allow for easier planning of Rosenbluth separations

Cross Section Separation via Rosenbluth Technique

• Cross Section Extraction– For uniformuniform φ-acceptance, σTT,

σLT –›0 when integrated over φ– Determine σT+ ε σL for high and

low ε in each t-bin for each Q2

– Isolate σL, by varying photon polarization, ε

• Small σL makes traditional Rosenbluth separation difficult due to unfavorable error propagation with two different acceptances

πcos2φdφdt

dσεπcosφdφdt

dσ1)(εε2dφdt

dσεdφdtσ2d TTLTT

dφdtdσL

22

-121

-1εε

dσL )ε(R)ε(Rdσ

21

VGL/Regge

VGG/GPD

Cross Sections via Recoil Polarization

1

ε1h

Pε1

σσ

R2

z

T

L

• Using the properties of the independent helicity amplitudes in parallel kinematics σL/σT is related to Pz

• In parallel kinematics can relate σL/σT to recoil polarization observables– Avoids some of the

adverse systematic effects due to small R in Rosenbluth technique.

εσ

σR1

0L

• From the combination of R and σ0 one can obtain σL

dRσdσ 0L

Experiment Overview• 100uA, 5.75 GeV beam, 80% polarized, 10-cm LH2 target

– Standard Hall A setup• Coincidence measurement with recoil proton into HRS with FPP

and electrons in the electron arm, H(e,e’p)πo– FPP analyzing power relatively large in this region

• Kinematics chosen to overlap with π+ data from Fπ-2 and πCT allowing for direct comparison

W

(GeV)

Q2

(GeV2)

Pp

(GeV)

tmin

(GeV/c)2

1.90 2.45 0.585 0.314

2.00 3.10 0.641 0.372

2.00 3.80 0.751 0.494

Fπ-2

πCT

Parallel Kinematics

• For recoil polarization analysis all data taken in parallel kinematics– Cuts in θ/φ select events

• Taking data to left and right of virtual photon could allow for t-dependent studies of unseparated cross section

• Radial coordinate θ• Azimuthal coordinate φ

FPP – analyzing power

• Low momentum protons <760 MeV: Los Alamos fit applicable, McNaughton et al., Nucl. Instrum. Meth. A241, 435 (1985)

• But in 2006 LEDEX took data for similar proton momenta in Hall A, so use this– FoM relatively large for

proposed kinematics

Preliminary LEDEX data courtesy of R. Gilman et al.

Hard Photon Backgrounds• Reconstructed photon

smeared out under πo peak – Mx cuts not useful– Use simulation for fitting

both peaks – Subtract bin-by-bin from

azimuthal dependence of the asymmetry in FPP

• Relative contribution of πo and hard photon background in good agreement with Hall B preliminary data

• Relative scaling based on VGL πo cross sections and VGG DVCS+BH cross sections

πo

γ

Q2=3.8, W=2.0, x=0.55

Vanderhaeghen, Guichon and Guidal, Phys. Rev. D60 (1999).

DVCS and Bethe-Heitler Contributions

• Contribution of hard photons requires full background subtraction. – Bethe-Heitler process

dominates photon cross section

– Contribution of DVCS to total cross section 10-20%

• Bethe-Heitler propagators are

no problem for these kinematics

Vanderhaeghen, Guichon and Guidal, Phys. Rev. D60 (1999).

Other Backgrounds

• End caps subtracted using dummy target data

• Online singles rates are low, removed with offline cuts

• Electron momentum is too low for elastics to be in acceptance

• Missing mass cuts separate πo/η

What is needed?

• Standard Hall A HRS configuration with FPP

• Installation of standard 10-cm LH2 cryotarget

• 5.75 GeV beam, 80% polarization

• Some FPP checkout and calibration

Projected Uncertainties

• Measure πo cross section at Q2=2.45, 4.0 GeV2, where π+ data are available

• Statistical uncertainty in recoil polarization measurement dominates uncertainty

– Measure σL/σ0 contribution to ~10%

2y0εAN

2ε1hε

1dR

πo π+

Projected Uncertainties

• Fixed W scan at three values of Q2 – up to Q2=4.0 GeV2

• Measure relative σL/σ0 contribution of to ~10%

Beam Time Estimate

W

(GeV)

Q2

(GeV2)

ε N0 dPz dR Hours

1.90 2.45 0.77 220k 0.036 0.092 106

2.00 3.10 0.65 200k 0.028 0.070 154

2.00 3.80 0.56 250k 0.028 0.076 145

Total 406

• Elastic data for proton absorption studies and calibration

• Spectrometer angle and momentum changes• Some FPP checkout• Polarization measurement

Summary• Non-pole contributions in the extraction of Fπ are an

unresolved issue; constraint on non-pole backgrounds requires experimental data.– Open door for larger kinematic reach in Fπ measurement

at 12 GeV

• Increase knowledge of largely unknown πo σL at large Q2 above resonance region– Easier planning for future Rosenbluth separation exp.

• Requires 17 days of data at 5.75 GeV

• An relatively easy experiment making use of existing Hall A standard equipment.