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Two Photon Exchange in elastic electron-proton scattering: QCD factorization approach

I. P. N. Two Photon Exchange in elastic electron-proton scattering: QCD factorization approach. P. Nikolai Kivel. in collaboration with. M. Vanderhaeghen. DSPIN- 09. Missing radiative corrections. Radiative correction at electron side :. well understood and taken care of.

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Two Photon Exchange in elastic electron-proton scattering: QCD factorization approach

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  1. I P N Two Photon Exchange in elastic electron-proton scattering:QCD factorization approach P Nikolai Kivel in collaboration with M. Vanderhaeghen DSPIN-09

  2. Missing radiative corrections Radiative correction at electron side: well understood and taken care of Box diagramsinvolve photons of all wavelength long wavelength (soft photon) is included in radiative correction (IR divergence is cancelled with electron proton brems- strahlung interference) Soft bremsstrahlung involves long-wavelength photons compositeness of the nucleon only enters through on-shell ff Maximon, Tjon PRC62, 2000

  3. TPE is responsible for the discrepancy in FF extraction Large Q2 asymptotic of the TPE contribution: motivation Chen, Afanasev et al PRL93(2004) partonic or GPD-model Blunden Melnitchouk Tjon PRL91(2003) hadronic-model ? pQCD in TPE Future JLab Hall A/C PR-07-109/09-001 pol GE/GMQ2=6-14.8GeV2 JLab Hall A PR-07-108 unpol Q2=7-17.5GeV2 ,stat. err. < 1%

  4. TPE contribution: theory Guichon Vanderhaeghen PRL91(2003) ep ep k k’ ( ) p p’

  5. TPE contribution: theory ep ep k’ k ( ) p p’ 1γ exch Form factors 2γ exch

  6. TPE contribution: theory ep ep k’ k ( ) p p’ 1γ exch Form factors Large Q2 asymptotic (pQCD) 2γ exch

  7. TPE at large-Q2: QCD factorization approach both photons have large virtualities + crossed - Breit frame n=(1,0,0,1) n=(1,0,0,-1)

  8. TPE at large-Q2: QCD factorization approach + others 23 graphs + crossed = Lowest order graphs to turn 3 collinear quarks into 3 collinear quarks moving in opposite direction

  9. TPE at large-Q2: QCD factorization approach + others 23 graphs + crossed = Lowest order graphs to turn 3 collinear quarks into 3 collinear quarks moving in opposite direction

  10. TPE at large-Q2: QCD factorization approach + others 23 graphs + crossed = Breit frame:

  11. TPE at large-Q2: QCD factorization approach + others 23 graphs + crossed = Nucleon DA

  12. TPE at large-Q2: QCD factorization approach + others 23 graphs + crossed = Hard subprocess * *

  13. TPE contribution: theory ep ep k’ k ( ) p p’ 1γ exch Form factors Large Q2 asymptotic (pQCD) 2γ exch helicity flip suppressed

  14. TPE at large-Q: QCD factorization approach Form factors: all integrals are IR-finite pole ⇒ Im part ! NK, Vanderhaeghen, 2009 Borisyuk, Kobushkin, 2008 normalization is not computed, only the ratio to nucleon pQCD FF’s

  15. TPE at large-Q: QCD factorization approach Nucleon DA assumption: drop the highest conformal moments theor. uncertainties: renorm. scale (NLO) helicity flip FF: 3 non-perturbative parameters Models for DA : QCD SR (1988),COZ QCD LCSR (2006),BLW Lattice (2006), QCDSF

  16. Reduced cross section with TPE correction Guichon Vanderhaeghen PRL91(2003) Born app Re part of TPE Empirical fit for the ratio and form factor: JLab Hall A, 2002 Brash, Kozlov, Li, Huber 2002

  17. Reduced cross section with TPE correction Cross section NK, Vanderhaeghen, 2009 1-γ pol QCDSF BLW COZ Data set SLAC NE11, 1994

  18. Golden mode: e+p vs. e-p elastic scattering JLab ClassPR-07-005 Q2=.5-2.5 GeV2ε=.1-.9 Future experiments: VEPP-3 Novosibirsk Q2=1.6 GeV2ε=.44, .92 Olympus@Desy Q2=.8-4.5 GeV2ε=.4-.9 pQCD estimate COZ Expected accuracy <2% BLW QCDSF

  19. Polarized observables with TPE: test of ε-dependence Pl Pt μpGE/GM Born Pl/Pl COZ BLW BLW COZ

  20. Polarized observables with TPE: test of ε-dependence Pl Pt JLab Hall C E-04-119 completed, new preliminaryQ2=2.5 GeV2ε=.14,.63,.785 Pre li mi na ry !!! BLW COZ GPD

  21. Asymmetries related to absorptive part Target Normal Spin Asymmetry E-02-013 Hall-C prelimin μnGE/GM ≃0.35 Elab= 4.8 GeV CLASS, 2009 Q2= 1.-4.5GeV2 GM ≃μnGD COZ PR-05-015 Hall-A Elab= 3.3, 5.3 GeV Q2= 1.0, 2.5GeV2 BLW

  22. Conclusions ☺ TPE is responsible for the discrepancy in FF extraction. This can be determined experimentally (GE/GM, non-linearity of Rosenbluth plot, ratio e+p/e-p, asymmetries sensitive to Im part of TPE) Hard QCD contribution in TPE at large Q2 canbe large and must be considered accurately ☺ ☺ TPE at large Q2depends on the structure of target and can provide us information about nucleon DA ☺ Experiments at higher Q2where the theory works better is a good opportunity to check pQCD predictions ☹ Large power corrections and/or NLO can change optimistic picture.

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