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Non-pole Backgrounds in the Extraction of F π 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 Marys, Florida International, Mississippi State, RPI, Yerevan, CNU, California State, New Hampshire, Ohio University Motivation Experimental Details Summary Hall A Collaboration Meeting January 2006

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Extracting F π from σ L data in π + production In t-pole approximation: Want smallest possible -t to ensure t -channel dominance

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Results from F π -2 The VGL Regge model describes σ L for π + well Note that at t min (maximal pole contribution) still only have σ L /σ T ~ 1 at Q 2 =2.45 GeV 2 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).

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Context Understanding of hadronic structure via measurement of F π is one of the high priorities at 12 GeV Extraction of F π relies on pion pole dominance – what about other processes? Limited knowledge of non-pole contributions limits kinematic range of F π measurement –Interpretation on experimental data widely considered reliable only below –t~0.2 GeV 2 –This kinematic contraint is the primary reason why we are limited to Q 2 ~2.5 GeV 2 at JLab at 6 GeV

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Size of non-pole contributions Carlson&Milana indicated a significant contribution of non-leading processes complicating the extraction of F π –Background ratio rises dramatically once t min >0.2 Other theoretical predictions can be obtained from: –VGL/Regge model –GPD formalism Carlson & Milana, Phys. Rev. Lett. 65, 1717 (1990) Interpretation of F π data considered reliable for -t<0.2 GeV 2 But constructing an upper bound on -t difficult due to poor quality of existing data.

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Motivation - Non-pole contributions can be constrained using the π o longitudinal cross section –Can be related to the one from π + using e.g. GPD formalism Many studies of π o unseparated cross sections in the resonance region, but contribution of σ L effectively unknown above the resonance region –JLab preliminary data from Hall A (DVCS, Q 2 =1.5-2.5 GeV 2, W=1.9-2.3 GeV) and Hall B (e16, Q2=1-5 GeV 2 ) available – both unseparated

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Theoretical Predictions for π + and π o cross sections Theoretical models based on Regge and GPD formalism describe σ L for π + quite well But π o prediction for σ L differs by order of magnitude –Theoretical uncertainty quite large Preliminary unseparated π o data from Hall A/B in this kinematic region –No information on relative σ L contribution Vanderhaeghen, Guidal and Laget, Phys. Rev. C57, 1454 (1998). Vanderhaeghen, Guichon and Guidal, Phys. Rev. D60 (1999). Separated σ L

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Non-pole contributions in the GPD Framework Amplitudes for π + and π o composed of the same GPDs, but different linear combinations Obtain non-pole contributions by comparing π o and π + production amplitudes, M L ~A pN +B pN –In the limit t (m π) 2 the π + amplitude contains a strong singularity (pion pole) πo πo π+ π+ VGG/GPD prediction

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Motivation Summary Constraining the non-pole contributions in the extraction of F π requires experimental data –Systematic measurement of π o 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 F π measurement at 12 GeV Constraining the contribution of σ L in π o production will allow for easier planning of Rosenbluth separations

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Cross Section Separation via Rosenbluth Technique Cross Section Extraction uniform –For uniform φ-acceptance, σ TT, σ LT –0 when integrated over φ –Determine σ T + ε σ L for high and low ε in each t-bin for each Q 2 –Isolate σ L, by varying photon polarization, ε Small σ L makes traditional Rosenbluth separation difficult due to unfavorable error propagation with two different acceptances VGL/Regge VGG/GPD

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Cross Sections via Recoil Polarization Using the properties of the independent helicity amplitudes in parallel kinematics σ L /σ T is related to Pz In parallel kinematics can relate σ L /σ T to recoil polarization observables –Avoids some of the adverse systematic effects due to small R in Rosenbluth technique. From the combination of R and σ 0 one can obtain σ L

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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,ep) π o –FPP analyzing power relatively large in this region Kinematics chosen to overlap with π + data from F π -2 and πCT allowing for direct comparison W (GeV) Q2 (GeV 2 ) P p (GeV) t min (GeV/c) 2 1.902.450.5850.314 2.003.100.6410.372 2.003.800.7510.494 F π - 2 π CT

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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

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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.

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Hard Photon Backgrounds Reconstructed photon smeared out under π o peak – M x cuts not useful –Use simulation for fitting both peaks –Subtract bin-by-bin from azimuthal dependence of the asymmetry in FPP Relative contribution of π o and hard photon background in good agreement with Hall B preliminary data Relative scaling based on VGL π o cross sections and VGG DVCS+BH cross sections πo πo γ Q 2 =3.8, W=2.0, x=0.55 Vanderhaeghen, Guichon and Guidal, Phys. Rev. D60 (1999).

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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).

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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 π o /η

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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

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Projected Uncertainties Measure π o cross section at Q 2 =2.45, 4.0 GeV 2, where π + data are available Statistical uncertainty in recoil polarization measurement dominates uncertainty –Measure σ L /σ 0 contribution to ~10% πo πo π+ π+

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Projected Uncertainties Fixed W scan at three values of Q 2 – up to Q 2 =4.0 GeV 2 Measure relative σ L /σ 0 contribution of to ~10%

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Beam Time Estimate W (GeV) Q 2 (GeV 2 ) ε N0N0 dP z dRHours 1.902.450.77220k0.0360.092106 2.003.100.65200k0.0280.070154 2.003.800.56250k0.0280.076145 Total406 Elastic data for proton absorption studies and calibration Spectrometer angle and momentum changes Some FPP checkout Polarization measurement

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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 F π measurement at 12 GeV Increase knowledge of largely unknown π o σ L at large Q 2 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.

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