# two photon exchange in elastic electron-proton scattering: qcd factorization approach

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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. - PowerPoint PPT PresentationTRANSCRIPT

Two Photon Exchange in elastic electron-proton scattering:QCD factorization approachNikolai Kivel in collaboration withM. Vanderhaeghen DSPIN-09PNPI

Missing radiative correctionsRadiative correction at electron side:well understood and taken care ofSoft bremsstrahlung involves long-wavelength photonscompositeness of the nucleon only enters through on-shell ffBox diagrams involve photons of all wavelengthlong wavelength (soft photon) is included in radiative correction (IR divergence is cancelled with electron proton brems- strahlung interference)Maximon, Tjon PRC62, 2000

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

TPE contribution: theoryep epkkpp( )Guichon Vanderhaeghen PRL91(2003)

TPE contribution: theoryep epkkppForm factors1 exch2 exch( )

TPE contribution: theoryep epkkppForm factors1 exch2 exch( )Large Q2 asymptotic (pQCD)

TPE at large-Q2: QCD factorization approach both photons have large virtualitiesBreit framen=(1,0,0,1) n=(1,0,0,-1)-+ crossed

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

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

+ crossed =+ others 23 graphsTPE at large-Q2: QCD factorization approach Breit frame:

+ crossed =+ others 23 graphsTPE at large-Q2: QCD factorization approach Nucleon DA

+ crossed =+ others 23 graphsTPE at large-Q2: QCD factorization approach Hard subprocess**

TPE contribution: theoryep epkkppForm factors1 exch2 exch( )Large Q2 asymptotic (pQCD)helicity flip suppressed

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

TPE at large-Q: QCD factorization approach Nucleon DAassumption: drop the highest conformal momentstheor. uncertainties: renorm. scale (NLO) Models for DA :QCD SR (1988), COZQCD LCSR (2006), BLWLattice (2006), QCDSF3 non-perturbative parametershelicity flip FF:

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

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

- Golden mode: e+p vs. e-p elastic scattering JLab Class PR-07-005 Q2=.5-2.5 GeV2 =.1-.9Future experiments: VEPP-3 Novosibirsk Q2=1.6 GeV2 =.44, .92 Olympus@Desy Q2=.8-4.5 GeV2 =.4-.9BLW COZ pQCD estimate Expected accuracy
Polarized observables with TPE: test of -dependencePl Pt pGE/GMCOZBLWPl/PlBornBLWCOZ

Polarized observables with TPE: test of -dependence JLab Hall C E-04-119 completed, new preliminary Q2=2.5 GeV2 =.14,.63,.785GPDCOZBLWPl Pt Preliminary !!!

Asymmetries related to absorptive partTarget Normal Spin AsymmetryPR-05-015 Hall-A Elab= 3.3, 5.3 GeVQ2= 1.0, 2.5GeV2Elab= 4.8 GeVCOZBLWnGE/GM 0.35E-02-013 Hall-C preliminGM nGDCLASS, 2009 Q2= 1.-4.5GeV2

Conclusions Hard QCD contribution in TPE at large Q2 can be large and must be considered accuratelyTPE at large Q2 depends on the structure of target and can provide us information about nucleon DAExperiments at higher Q2 where the theory works better is a good opportunity to check pQCD predictions 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)Large power corrections and/or NLO can change optimistic picture.

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