Egle Tomasi-Gustafsson CEA, IRFU,SPhN, Saclay,...
Transcript of Egle Tomasi-Gustafsson CEA, IRFU,SPhN, Saclay,...
Egle Tomasi-Gustafsson 1
Electromagnetic hadron form factors: status and perspectives
Erice, 21-IX-2015
Egle Tomasi-Gustafsson CEA, IRFU,SPhN,
Saclay, France
Characterize the internal structure of a particle (≠ point-like)
Elastic form factors contain information on the hadron ground state.
In a P- and T-invariant theory, the EM structure of a particle of spin S is defined by 2S+1 form factors.
Neutron and proton form factors are different.
Deuteron: 2 structure functions, but 3 form factors.
Playground for theory and experiment at low q2 probe the size of the
nucleus, at high q2 test QCD scaling
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Hadron Electromagnetic Form factors
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Recent experimental achievements: polarization
VEPP-Novosibirsk
LNS-Saclay
JINR-Dubna
Hadron polarimetry in the GeV range
… and also: polarized sources and targets, high intensity e- beams, high luminosity colliders, large acceptance spectrometers, high resolution 4π detectors…
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_ _
q2<0
e- e-
p p
q2>0
e-
e+ p
p
q2 q2=4mp2
Time-Like FFs are complex
Space-like FFs are real
GE=GM
GE(0)=1
GM(0)=µp
Asymptotics - QCD - analyticity
p+p ↔ e++e- e+p → e+p
Unp
hysi
cal r
egio
n
p+p ↔
e+
e- +π0
0
Proton Charge and Magnetic Distributions
Erice, 21-IX-2015
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GE=GM p+p ↔ e++e- e+p → e+p
_ _
q2<0
e- e-
p p
q2>0
e-
e+ p
p
q2 q2=4mp2
Time-Like FFs are complex
Space-like FFs are real U
nphy
sica
l reg
ion
GE(0)=1
GM(0)=µp
Asymptotics - QCD - analyticity
p+p ↔
e++
e- +π0
Hadron Electromagnetic Form Factors
GE=GM GE unpolarized
GM
GE Polarized
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GE=GM e+p → e+p
Hadron Electromagnetic Form Factors
GE=GM GE unpolarized
GM
GE Polarized
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The Space-Like region: low Q2
e+p → e+p
Fourier Transform
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Root mean square radius
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Rp=0.84184(67) fm (muonic atom)
Rp=0.897(18) fm
Rp=0.879(8) fm (e-p Mainz)
Rp=0.82-0.85 fm (DR PRC75 035202(2007))
Rp=0.78-0.86 fm (lattice QCD PRD 79 094001(2009))
Rp=0.8768(69) fm
The Proton Radius
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Rp=0.84184(67) fm (muonic atom)
Rp=0.897(18) fm
Rp=0.879(8) fm (e-p Mainz)
Rp=0.82-0.85 fm (DR PRC75 035202(2007))
Rp=0.78-0.86 fm (lattice QCD PRD 79 094001(2009))
Rp=0.8768(69) fm
The Proton Radius
What about extrapolation to Q2→ 0?
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• Radiative corrections • Two photon exchange • Coulomb corrections …..comments
Mainz, A1 collaboration (1400 points) Q2>0.004 GeV2
• MUSE Experiment • Jlab CLAS
G.I. Gakh, A. Dbeyssi, E.T-G, D. Marchand,V.V. Bytev, Phys.Part.Nucl.Lett. 10 (2013) 393, Phys.Rev. C84 (2011) 015212
Why I do not trust the fits
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Slide from Savely Karshenboim
GE (q2)
Why I do not trust the fits
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Slide from Savely Karshenboim
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The Space-Like region
e+p → e+p Wolfgang Pauli Niels Bohr ( ’30ies)
2M4
2Q,1
2e2)tan1(21 =
−++=
⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛
⎟⎠
⎞⎜⎝
⎛ τθ
τε
2MG
2EGR τεσ +=
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→ Holds for 1γ exchange only
Linearity of the reduced cross section
PRL 94, 142301 (2005)
→ tan2θe dependence ε
Q2 f
ixed
ep-elastic scattering : Rosenbluth separation 1950 ⎟⎟
⎠
⎞⎜⎜⎝
⎛⎟⎟
⎠
⎞
⎜⎜
⎝
⎛+
+Ω=
Ω)2(2)2(2)1(
1 QMGQEG
Mottdd
dd
ετ
τσσ
The polarization induces a term in the cross section proportional to GE GM Polarized beam and target or polarized beam and recoil proton polarization
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ep-elastic scattering : Akhiezer-Rekalo method
1967
The simultaneous measurement of Pt and Pl reduces the systematic errors
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C. Perdrisat, V. Punjabi, et al., JLab-GEp collaboration
The polarization method (exp: 2000)
A.I. Akhiezer and M.P. Rekalo, 1967 Jlab-GEp collaboration 1) "standard" dipole function
for the nucleon magnetic FFs GMp and GMn
2) linear deviation from the dipole function for the electric proton FF Gep
3) QCD scaling not reached 3) Zero crossing of Gep? 4) contradiction between
polarized and unpolarized measurements
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A.J.R. Puckett et al, PRL (2010), PRC (2012)
Polarization Experiments
• Some models (IJL 73, Di-quark, soliton..) predicted such behavior before the data appeared
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• Simultaneous description of the four nucleon form factors...
• ...in the space-like and in the time-like regions
• Consequences for the light ions description
• When pQCD starts to apply? • Source of the discrepancy
BUT
Issues
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The Time-Like region
p + p → e+ + e- (γ)
GE=GM
4M2
Time-like observables: | GE| 2 and | GM| 2 .
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As in SL region: - Dependence on q2 contained in FFs - Even dependence on cos2θ (1γ exchange) - No dependence on sign of FFs - Enhancement of magnetic term but TL form factors are complex!
A. Zichichi, S. M. Berman, N. Cabibbo, R. Gatto, Il Nuovo Cimento XXIV, 170 (1962) B. Bilenkii, C. Giunti, V. Wataghin, Z. Phys. C 59, 475 (1993). G. Gakh, E.T-G., Nucl. Phys. A761,120 (2005).
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VMD: Iachello, Jakson and Landé (1973)
Isoscalar and isovector FFs γ* ω,ρ,ϕ
Erice, 21-IX-2015
• The Experimental Status
• No individual determination of GE and GM
• TL proton FFs twice larger than in SL at the same Q2
• Steep behaviour at threshold
• Babar:Structures? Resonances?
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The Time-like Region GE=GM
Panda contribution: M.P. Rekalo, E.T-G , DAPNIA-04-01, ArXiv:0810.4245. S. Pacetti, R. Baldini-Ferroli, E.T-G , Physics Reports, 514 (2014) 1
]2 [GeV2q10 20 30 40
pF3−10
2−10
1−10
4 6 8 10
1−10
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The Time-like Region
Expected QCD scaling (q2)2
]2 [GeV2q10 20 30 40
pF
3−10
2−10
1−10
4 6 8 10
1−10
Erice, 21-IX-2015
GE=GM
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Oscillations : regular pattern in PLab pF
0.1
0.2
0.3(a)
p [GeV]0 1 2 3
D
-0.04
-0.02
0
0.02
0.04 (b)
A: Small perturbation B: damping C: r < 1fm D=0: maximum at p=0
Erice, 21-IX-2015
The relevant variable is pLab associated to the relative motion of the final hadrons.
Simple oscillatory behaviour Small number of coherent sources
A. Bianconi, E. T-G. Phys. Rev. Lett. 114,232301 (2015)
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Oscillations : regular pattern in PLab pF
0.1
0.2
0.3(a)
p [GeV]0 1 2 3
D
-0.04
-0.02
0
0.02
0.04 (b)
A: Small perturbation B: damping C: r < 1fm D=0: maximum at p=0
Erice, 21-IX-2015
The relevant variable is pLab associated to the relative motion of the final hadrons.
Simple oscillatory behaviour Small number of coherent sources
A. Bianconi, E. T-G. Phys. Rev. Lett. 114,232301 (2015)
pF
0.1
0.2
0.3(a)
p [GeV]0 1 2 3
D
0.04−
0.02−
0
0.02
0.04 (b)
pF
0.1
0.2
0.3
p [GeV]0 1 2 3
D
0.04−
0.02−
0
0.02
0.04
pF
0.1
0.2
0.3
p [GeV]0 1 2 3
D
0.04−
0.02−
0
0.02
0.04
pF
0.1
0.2
0.3
p [GeV]0 1 2 3
D
0.05−
0
0.05
(a) (b)
(c) (d)
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Fourier Transform
r (fm)0 0.5 1 1.5 2
-410
-310
-210
-110
1
10
210
)3(r) (1/fm0M
r (fm)0.6 0.8 1 1.2 1.4 1.6
-0.05
0
0.05
0.1
0.15)3M(r) (1/fm
• Rescattering processes • Large imaginary part • Related to the time evolution of the charge density?
• Consequences for the SL region? • Data expected at BESIII, PANDA
(E.A. Kuraev, E. T.-G., A. Dbeyssi, PLB712 (2012) 240)
Varenna, 15-VI-2015
F0 ==
The nucleon
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antisymmetric state of colored quarks
3 valence quarks and a neutral sea of qq pairs
Does not hold in the spatial center of the nucleon: the center of the nucleon is electrically neutral, due
to strong gluonic field
Main assumption
E.A. Kuraev, E. T-G, A. Dbeyssi, Phys.Lett. B712 (2012) 240
The annihilation channel:
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The neutral plasma acts on the distribution of the electric charge (not magnetic).
Prediction: additional suppression due to the neutral plasma similar behavior in SL and TL regions
• Implicit normalization at q2=4Mp2: |GE|=|GM| =1
• No poles in unphysical region
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The asymptotic region
Large q2 : : where the extremes meat
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E. T-G. and M. P. Rekalo, Phys. Lett. B 504, 291 (2001)
Applies to NN and NN Interaction (Pomeranchuk theorem) t=0 : not a QCD regime!
Connection with QCD asymptotics?
Phragmèn-Lindelöf theorem
Analyticity
PANDA
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• Large activity at all world facilities both in Space and Time-like regions
• Experiment: to measure
- zero crossing of GE/GM in SL? 2γ? Proton radius? - GE and GM separately in TL - complex FFs in TL region: polarization! - new structures in TL: access to hadron formation?
• Theory: unified models in SL and TL regions:
- describe proton and neutron, electric and magnetic - pointlike behavior at threshold? - understand GE, GM(SL) < GE,GM(TL);
Conclusions
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Point-like form factors?
S. Pacetti
.sm,x
sin
xxx
),x,s(W
,sm
s
Ex),m)(ppee(),x,s(W
sm
cosddm)ppee(d
e>>θ⎟⎟
⎠
⎞
⎜⎜
⎝
⎛−
θ
+−
π
α=θ
−==→σθ=θ
γ→σ γ−+−+
222
122
2
2
2
2
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e+ +e- → p + p + γ
B. Aubert ( BABAR Collaboration) Phys Rev. D73, 012005 (2006)
Radiative return (ISR)