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Two is too many: A personal review of Two-Photon Physics. Chung-Wen Kao Chung-Yuan Christian University, Taiwan. In Collaboration with Hai-Qing Zhou, Yu-Chun Chen, Shin-Nan Yang. 23.5.2008 National Taiwan University, Lattice QCD Journal Club. 2. PD. hadronizatio n.
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Two is too many: A personal review of Two-Photon Physics Chung-Wen Kao Chung-Yuan Christian University, Taiwan In Collaboration with Hai-Qing Zhou, Yu-Chun Chen, Shin-Nan Yang 23.5.2008 National Taiwan University, Lattice QCD Journal Club
2 PD hadronization One-Photon Physics: From Low to High High Q2, deeply inelastic, inclusive Low Q2, elastic, exclusive Parton distribution Form factors
Nucleon Form Factors • Hofstadterdetermined the precise size of the proton and neutron by measuring their form factor.
Why bother to go beyond One-photon-exchange framework? • Since αEM=1/137, the two-photon-exchange effect is just few percent. • However, few percent may be crucial for high precision electroweak experiments. • Surprisingly, two-photon-exchange effect is more important than we thought !
Rosenbluth Separation Method Within one-photon-exchange framework:
Polarization Transfer Method Polarization transfer cannot determine the values of GE and GM but can determine their ratio R.
Two methods, Two Results! SLAC, JLab Rosenbluth data JLab/HallA Polarization data Jones et al. (2000) Gayou et al (2002)
How to explain it? Go beyond One-Photon Exchange…. New Structure
Two-Photon-Exchange Effects on two techniques small large
Possible explanation • 2-photon-exchange effect can be large on Rosenbluth method when Q2 is large. • 2-Photon-exchange effect is much smaller on polarization transfer method. • Therefore 2-photon-exchange may explain the difference between two results. Guichon, Vanderhaeghen, PRL 91 (2003)
One way or another……. • There are two ways to estimate the TPE effect: Use models to calculate Two-Photon-Exchange diagrams: Like parton model, hadronic model and so on….. Direct analyze the cross section data by including the TPE effects: One-Photon-exchange Two-photon-exchange
Hadronic Model Result + Cross diagram Insert on-shell form factors Blunden, Tjon, Melnitchouk (2003, 2005)
Results of hadronic model Blunden, Tjon, Melnitchouk (2003, 2005)
Partonic ModelCalculation GPDs Y.C.Chen, Afanasev,Brodsky, Carlson, Vanderhaeghen (2004)
Model-independent analysis Determined from polarization transfer data TPE effects Inputs From crossing symmetry and charge conjugation:
YC Chen, CWK ,SN Yang, PLB B652 (2007) Our Choice of F(Q2, ε) ε→ 1, y→0, F→0 ε→0, y→1, F≠0 Fit (A) Fit (B)
Result of fits Dashed Line: Rosenbluth Solid line: Fit (A) Dotted line: Fit (B)
Result of fits Dashed Line: Rosenbluth Solid line: Fit (A) Dotted line: Fit (B)
Puzzle about nonlinearity V.Tvaskis et al, PRC 73, 2005 Purely due to TPE
TPE vs OPE Fit (A) : Fit (B):
TPE contribution to slope Dashed Line : Fit (B) Solid line: Fit (A) SLOPE(TPE)/SLOPE(OPE) =C1/(GE/τ)
Fit (A) Fit (B)
Common features of Two fits • GM increase few percents compared with Rosenbluth results • GE are much smaller than Rosenbluth • Result at high Q2 • OPE-TPE interference effects are always destructive • TPE play important role in the slope • TPE give very small curvature
Any other places for TPE? Normal spin asymmetries in elastic eN scattering directly proportional to the imaginary part of 2-photon exchange amplitudes spin of beam OR target NORMAL to scattering plane Comparison of e-p/e+p : Amp(e-p)=Amp(1γ)+Amp(2γ) Due to Charge conjugation Amp(e+p)=Amp(1γ)-Amp(2γ)
R=σ(e+p) / σ(e-p) Fit (B) Fit (A) Q^2=5 GeV^2 Q^2=5 GeV^2 Q^2=3.25 GeV^2 Q^2=3.25 GeV^2 Q^2=1.75 GeV^2 Q^2=1.75 GeV^2
Strangeness in the nucleon « sea » • s quark: cleanest candidate to study the sea Goal:Determine the contributions of the strange quark sea ( ) to the charge and current/spin distributions in the nucleon : “strange form factors” GsE and GsM
Parity Violating Electron Scattering Interference: ~ |MEM |2 + |MNC |2 + 2Re(MEM*)MNC Interference with EM amplitude makes Neutral Current (NC) amplitude accessible Tiny (~10-6) cross section asymmetry isolates weak interaction
Flavour decomposition NC probes same hadronic flavor structure, with different couplings: • GZE/M provide an important new benchmark for testing • non-perturbative QCD structure of the nucleon
Gg,pE,M GuE,M GpE,M Well Measured Charge symmetry Gg,nE,M GdE,M Shuffle GnE,M GsE,M GZ,pE,M GsE,M <N| sgm s |N> Charge Symmetry
Backward angle Forward angle Isolating the form factors: vary the kinematics or target For a proton: ~ few parts per million
Extraction of strange form factors Strange form factors ρand κare from electroweak radiative corrections
Zero Transfer Momentum Approximations Q2=(p-q)^2 Approximation made in previous analysis: p=q=k p q Marciano, Sirlin (1984)
Impact of our results Avoid double counting Old New
Change of the results of Strange form factors -14.6 -45.05
GMs = 0.28 +/- 0.20 GEs = -0.006 +/- 0.016 ~3% +/- 2.3% of proton magnetic moment ~20% +/- 15% of isoscalar magnetic moment ~0.2 +/- 0.5% of Electric distribution Preliminary This above plot should be modified !!!
Summary and Outlook • Two-Photon physics is important to obtain the information of nucleon structure even it is small! • Two-photon-exchange effect is crucial to extract electric and magnetic form factors. • Two-boson exchange effect is also crucial to extract the strange form factors! • More TPE-related research is going: N→ Δ transition form factor, normal beam asymmetry and so on… • In particular, low energy precision measurement of electroweak experiments which is sensitive to NEW PHYSICS rely on the good theoretical understand of Two-photon physics!
Thank you for listening Two photons may be too many, but two cups of Italian ice cream are not………