two photon exchange in elastic electron-proton scattering: qcd factorization approach
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 PresentationTRANSCRIPT
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
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
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
TPE contribution: theory
ep ep
k k’
p p’ ( )
Guichon Vanderhaeghen PRL91(2003)
TPE contribution: theory
ep ep
k k’
p p’
Form factors1γ exch
2γ exch
( )
TPE contribution: theory
ep ep
k k’
p p’
Form factors1γ exch
2γ exch
( )
Large Q2 asymptotic (pQCD)
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
+ 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
+ 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
+ crossed = + others 23 graphs
TPE at large-Q2: QCD factorization approach
Breit frame:
+ crossed = + others 23 graphs
TPE at large-Q2: QCD factorization approach
Nucleon DA
+ crossed = + others 23 graphs
TPE at large-Q2: QCD factorization approach
Hard subprocess
* *
TPE contribution: theory
ep ep
k k’
p p’
Form factors1γ exch
2γ 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 FF’s
NK, Vanderhaeghen, 2009
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:
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)
Reduced cross section with TPE correction Cross section
Data set SLAC NE11, 1994
BLW
1-γ pol
COZ
NK, Vanderhaeghen, 2009
QCDSF
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
Polarized observables with TPE: test of ε-dependence
Pl
Pt
μpGE/GM
COZBLW
Pl/PlBorn
BLW
COZ
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 !!!
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
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.