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

22
Two Photon Exchange Two Photon Exchange in elastic electron- in elastic electron- proton scattering: proton scattering: QCD factorization QCD factorization approach approach Nikolai Kivel in collaboration with M. Vanderhaeghen DSPIN-09 P N P I

Upload: fala

Post on 12-Jan-2016

45 views

Category:

Documents


0 download

DESCRIPTION

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 Presentation

TRANSCRIPT

Page 1: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

Two Photon Exchange Two Photon Exchange in elastic electron-proton in elastic electron-proton

scattering:scattering:QCD factorization approachQCD factorization approach

Nikolai Kivel

in collaboration with

M. Vanderhaeghen

DSPIN-09

PNPI

Page 2: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

Missing radiative correctionsRadiative correction at electron side:well understood and taken care of

Soft bremsstrahlung involves long-wavelength photonscompositeness of the nucleon only enters through on-shell ff

Box 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

Page 3: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

Large Q2 asymptotic of the TPE contribution: motivation

? pQCD in TPE

TPE is responsible for the discrepancy in FF extraction

Chen, Afanasev et al PRL93(2004)

Blunden Melnitchouk Tjon PRL91(2003)

partonic or GPD-model

hadronic-model

JLab 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

Page 4: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

TPE contribution: theory

ep ep

k k’

p p’ ( )

Guichon Vanderhaeghen PRL91(2003)

Page 5: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

TPE contribution: theory

ep ep

k k’

p p’

Form factors1γ exch

2γ exch

( )

Page 6: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

TPE contribution: theory

ep ep

k k’

p p’

Form factors1γ exch

2γ exch

( )

Large Q2 asymptotic (pQCD)

Page 7: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

TPE at large-Q2: QCD factorization approach

both photons have large virtualities

Breit frame n=(1,0,0,1) n=(1,0,0,-1)-

+ crossed

Page 8: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

+ crossed = + others 23 graphs

TPE at large-Q2: QCD factorization approach

Lowest order graphs to turn 3 collinear quarksinto 3 collinear quarks moving in opposite

direction

Page 9: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

+ crossed = + others 23 graphs

TPE at large-Q2: QCD factorization approach

Lowest order graphs to turn 3 collinear quarksinto 3 collinear quarks moving in opposite direction

Page 10: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

+ crossed = + others 23 graphs

TPE at large-Q2: QCD factorization approach

Breit frame:

Page 11: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

+ crossed = + others 23 graphs

TPE at large-Q2: QCD factorization approach

Nucleon DA

Page 12: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

+ crossed = + others 23 graphs

TPE at large-Q2: QCD factorization approach

Hard subprocess

* *

Page 13: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

TPE contribution: theory

ep ep

k k’

p p’

Form factors1γ exch

2γ exch

( )

Large Q2 asymptotic (pQCD)

helicity flip suppressed

Page 14: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

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 FF’s

NK, Vanderhaeghen, 2009

Page 15: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

TPE at large-Q: QCD factorization approach

Nucleon DA

assumption: drop the highest conformal moments

theor. uncertainties: renorm. scale (NLO)

Models for DA :QCD SR (1988), COZ

QCD LCSR (2006), BLW

Lattice (2006), QCDSF

3 non-perturbative parameters

helicity flip FF:

Page 16: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

Reduced cross section with TPE correction

Born app

Empirical fit for the ratio and form factor:

Brash, Kozlov, Li, Huber 2002

JLab Hall A, 2002

Re part of TPE

Guichon Vanderhaeghen PRL91(2003)

Page 17: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

Reduced cross section with TPE correction Cross section

Data set SLAC NE11, 1994

BLW

1-γ pol

COZ

NK, Vanderhaeghen, 2009

QCDSF

Page 18: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

Golden mode: e+p vs. e-p elastic scattering

JLab Class PR-07-005 Q2=.5-2.5 GeV2 ε=.1-.9Future experiments:

VEPP-3 Novosibirsk Q2=1.6 GeV2 ε=.44, .92

Olympus@Desy Q2=.8-4.5 GeV2 ε=.4-.9

BLW

COZ

pQCD estimate

Expected accuracy <2%

QCDSF

Page 19: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

Polarized observables with TPE: test of ε-dependence

Pl

Pt

μpGE/GM

COZBLW

Pl/PlBorn

BLW

COZ

Page 20: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

Polarized observables with TPE: test of ε-dependence

JLab Hall C E-04-119 completed, new preliminary Q2=2.5 GeV2 ε=.14,.63,.785

GPD

COZ BLW

Pl

Pt

Preli

minary !!!

Page 21: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

Asymmetries related to absorptive part

Target Normal Spin Asymmetry

PR-05-015 Hall-A Elab= 3.3, 5.3 GeVQ2= 1.0, 2.5GeV2

Elab= 4.8 GeV

COZ

BLW

μnGE/GM ≃0.35E-02-013 Hall-C prelimin

GM ≃μnGD CLASS, 2009 Q2= 1.-4.5GeV2

Page 22: Two Photon Exchange  in elastic electron-proton scattering: QCD factorization approach

Conclusions

Hard QCD contribution in TPE at large Q2 can be large and must be considered accurately

☺TPE at large Q2 depends on the structure of target and can provide us information about nucleon DA

☺Experiments 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.