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Richard Lindgren - UVa Cole Smith - UVa Khem Chirapatimol - Chiang Mai University E04- Status Report Precision Measurements of π 0 Electroproduction near Threshold: A Test of Chiral QCD Dynamics Hall A Collaboration Meeting June 13,

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Reaction H(e,e’p)π 0 E= MeV, 6 cm H target Measure pi0 absolute cross section from W= threshold up to 28 MeV in 1 MeV bins and Q 2 from 0.05 to 0.15 (GeV/c) 2 in bins of (GeV/c) 2 Detect electron in HRS at 12.5,14.5,16.5, and 20.5 degree Detect proton in BigBite at 43.5, 48, and 54 degrees. Cut on Missing mass Identifies pi0 Extract four structure functions from pion CM theta and phi dependence. Use to test predictions of low energy Chiral based effective theories

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Since Last Collaboration Meeting Khem Chirapatimol at Chiang Mai University, Thailand. Continuing data analysis on JLab computers Weekly Skype Meetings New and improved optimized optics data base for HRS using our hydrogen sieve data. Preliminary Results appearing in JLab 2012 Chiral Dynamics Workshop Proceedings Preliminary paper available without systematic errors U. Meissner reaffirms intention to extend HBChPT electroproduction calculation after seeing our data 3

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4 CD2012 Workshop ManuscriptPreliminary Letter

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Chiral Perturbation Theory (ChPT): Low energy effective theory based on spontaneous breaking of chiral symmetry in QCD. Long range degrees of freedom: Mesons and baryons Short distance physics encapsulated into Low Energy Constants (LECs) fitted to experiment. Precision measurement of (Q 2,W) evolution of p(e,e’p)π o reaction near threshold can test validity of low energy expansion once LECs are fixed. Total cross section, polarization observables, EM multipoles. Chiral Dynamics Important Low Energy Theory 5

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Electroproduction Data: MAMI Q 2 = 0.10: Distler et al. PRL 80, 2294 (1998) Q 2 = 0.05: Merkel et al. PRL 88, 1230 (2002) 2002 Q 2 = : Merkel et al. arXiv: Theory: RChPT, Marius Hilt, Bosen 2011 HBChPT (1996) MAID Conflicting measurements require more extensive data set

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Analysis Goals Extract total cross section for comparison with Mainz data and models listed below. Extract 4 structure functions for each W and Q 2 point using measured φ* distributions and compare with previous MAMI experiments and HBChPT, MAID07, DMT, Chiral-MAID, and SAID calculations. Extract model independent partial wave content of structure functions using Legendre polynomial fits (assume s,p-wave dominance.) Perform model dependent fits to estimate electromagnetic multipoles, needed for precise comparison to models. Extract beam asymmetry for comparison with Imaginary part of s wave Lets focus on our total cross section results 7

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Q 2 Dependence of TOT from ΔW=0.5 to 7.5 MeV Q 2 < 0.10 (GeV/c) 2 : Our σ tot data agrees with HBChPT (BKM96) in the range 0.5 < ΔW < 4.0 MeV Q 2 > 0.10 (GeV/c) 2 : Our σ tot falls off faster in Q 2 than predicted by HBChPT, shows better agreement with new 4 th order relativistic theory of M. Hilt (ChMAID). HBChPT may require non-higher order calculation. Meissner has agreed to extend HBChPT in electroproduction to 4 th order 8

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How does systematic errors on the electron arm affect our results We used a non-optimized HRS optics data set for our experiment. Since then, we have optimized the HRS optics and I’ll show the difference. We are also worried about mis-calibration of the energy/momentum and how it would affect total cross section near threshold. We did find unexplained changes in beam energy Effects on total cross section due to angle shifts in HRS vertical and horizontal seem to be negligible compared to other systematic errors. Only 1-3% on threshold cross section. 9

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E Multi-foil C (Ge Jin) E LH2 sieve run 6 cm target beginnning of exp and also at end. Non-optimized Optimized 10

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Carbon Elastic Cross Sections Cross section is sensitive to pointing errors, but not so much to energy calibration. Fourier-Bessel fit to NIKHEF-K data taken from Offermann et al. and recalculated for E=1.192 GeV using DWBA phase-shift code from J. Heisenberg (R. Lindgren). 11

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Before beam correctionAfter beam correction Projected elastic W from for each sieve slit hole using Hydrogen target for Run Vertical line is expected position for hydrogen elastic. 12

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Tantalum Red Carbon Black Hydrogen Blue Fitted centroids of elastic peaks of all Ta, C and LH2 calibration runs. Note W errors seem to not depend on target mass. Corresponding correction to beam energy required to center elastic peaks. Study of Systematics of Invariant Mass (W) ΔW (GeV) vs Run No. ΔE beam (GeV) vs Run No. 13

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Effect on Total Cross Section Due to Shift in Momentum Calibration HBChPT DMT MAID07 JLAB keV shift in W JLAB 2012 Nominal W calibration 14

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Summary First Replay data using new HRS data base optimized using our hydrogen target data. Create DSTs for fast replay at UVA. Finalize total cross section data for paper Estimates of systematic errors on cross section and Q 2 due to W mis-calibration and possible incorrectly reconstructed angles at edges of acceptance are basically known. Complete and circulate PRL paper Second Must go back and look at BigBite arm systematic errors for angular distribution measurements. Prepare long paper. Large data set with unprecedented W, Q 2, C.M. coverage and many available chiral theories to test. Prepare long paper. 16

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Comparison with MAMI 2011 Measurement JLAB 2012 E=1.192 GeV ε=0.943 MAMI 2011 E=0.880 GeV ε=0.882 MAMI 2011 H. Merkel et al., arXiv: v1 [nucl-ex]. 17

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