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Non-pole Backgrounds in the Extraction of F π

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  1. 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

  2. Extracting Fπ from σL data in π+ production • In t-pole approximation: • Want smallest possible -t to ensure t-channel dominance

  3. 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).

  4. 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

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

  6. 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

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

  8. Non-pole contributions in the GPD Framework VGG/GPD prediction • Amplitudes for π+ and πo composed of the same GPDs, but different linear combinations πo π+ • 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)

  9. 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

  10. Cross Section Separation via Rosenbluth Technique • Cross Section Extraction • For uniformφ-acceptance, σTT, σLT –›0 when integrated over φ • Determine σT+ εσL 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 VGG/GPD VGL/Regge

  11. Cross Sections via Recoil Polarization • 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. • Using the properties of the independent helicity amplitudes in parallel kinematics σL/σT is related to Pz • From the combination of R and σ0 one can obtain σL

  12. 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 Fπ-2 πCT

  13. 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

  14. 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.

  15. Hard Photon Backgrounds Q2=3.8, W=2.0, x=0.55 • 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 Vanderhaeghen, Guichon and Guidal, Phys. Rev. D60 (1999).

  16. 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).

  17. 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

  18. 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

  19. 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% π+ πo

  20. Projected Uncertainties • Fixed W scan at three values of Q2 – up to Q2=4.0 GeV2 • Measure relative σL/σ0 contribution of to ~10%

  21. Beam Time Estimate • Elastic data for proton absorption studies and calibration • Spectrometer angle and momentum changes • Some FPP checkout • Polarization measurement

  22. 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 πoσL 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.