1. Barion Form Factors at FAIR
P.Gianotti
INFN
Laoratori Nazionali Frascati

2. Form Factors: definitions
Space Like
Time Like
q2<0
MN q2>0
Annihilation or reversed channel
Electron scattering

3. Form Factors: definitions
In the approximation of single g exchange
ds/dW  [ |GM(q2)|2(1+cos2q)+4mN2/q2 |GE(q2)|2 sin2q)]
Sachs and Pauli/Dirac FF
in the Space-Like region  Fourier transform of charge and magnetization distributions
At threshold (t=q2/4MN2=1)  |GE| = |GM|
F1 and F2 are analytic functions of q2 real in Space-Like, but complex in Time-Like.
Time-Like GE and GM FF are analytic prolongations of Space-Like non spin-flip (GE) and spin flip (GM) form factors.
GE = F1 + t F2
GM = F1 + F2

4. Why measuring nucleon TL FF?
FF are fundamental quantities of the nucleon (m, m, …)
Understanding the Nucleon structure
Comparison with Space-Like data that seems to show the non validity of the Born approximation
Testing the hypothesis GE/GM= 1
Test different theoretical approaches

5. Form Factors: open questions
Perturbative QCD (q2  ∞)
F1(q2)  q F2(q2)  q-6
q2F2/F1  constant
exp data seems to indicate qF2/F1  constant
limq2→∞ |GM|time like = |GM|space like
exp data gives |GM|time like ≈ 2 |GM|space like

6. SL FF present situation
Data on the ratio GE/GM for the proton including the older Rosenbluth separation data (crosses), most recent JLab Rosenbluth separation data (filled circles),
and polarization transfer data (triangles)
New Jlab data have been obtained with the recoil polarization method
stating that:
PRL (2005)
The old assumption seems no more valid

7. FF TL: data extraction
|GE| and |GM| in the Time-Like region can be determined by the reactions
pp ↔ e+e-
Presently statistics is limited  no real separation |GE|/ |GM|
|GM| extracted assuming |GE| = |GM| (true at threshold)
GE in Time-Like region is today unknown
Recent data from BaBar  extraction of the ratio R = |GE|/ |GM| through the ISR method (e+e-  gpp) (q2<7 (GeV/c)2)

8. FF Time-Like: angular distribution
In the approximation of single g exchange
ds/dW  [ |GM(q2)|2(1+cos2q)+4mN2/q2 |GE(q2)|2 sin2q)]
 is cross section isotropic?
With PANDA we will have access to almost total angular range
Direct access to |GM(q2)| and |GE(q2)|
The sensitivity to |GE(q2)| decreases while energy increase (4mN2/q2 = 0.25 at 10 GeV, and only 0.14 at 15 GeV)
2g: odd cosq contributions in the cross-section. Are these an explanation? (PL B )

9. FF Time-Like: present data
A new bulk of data up to 20 GeV came from BaBar using the technique of ISR
|GM|2
10-1
10-2
For E2 < 2.1 GeV2 the ratio |GE|/ |GM|has been found >1 in disagreement with previous LEAR data
|GEp|/|GMp|
E (GeV) 5 6 7 8 10 4 9 20
Ebeam (GeV)
PRD

10. Angular distributions
BaBar data
e+ e- → g p p
Dashed: |GM|
LEAR data
p p → e+ e-
Dot-dashed: |GE|
Both data sets have big errors!
Mpp=2.05 GeV/c2
Mpp=1.99GeV/c2
Mpp=1.96 GeV/c2
Dashed: |GE|/|GM| free
Solid: |GE|=|GM|
Mpp=1.92GeV/c2
Mpp=1.94GeV/c2
Mpp≈1.91 GeV/c2
Mpp≈1.99 GeV/c2
Mpp≈2.06 GeV/c2
Mpp≈2.15 GeV/c2
Mpp≈2.30 GeV/c2
Mpp≈2.70 GeV/c2
PLB
PRD

11. Facility for Antiproton and Ion Research
Primary Beams
2(4)x1013/s 30 GeV protons
Secondary Beams
Antiproton production target
Antiprotons GeV
100 m
HESR
High resolution mode
dp/p ~ 10-5 (electron cooling)
Luminosity = 1031 cm-2 s-1
Storage and Cooler Rings
1011 stored and cooled GeV antiprotons
High luminosity mode
Luminosity = 2 x 1032 cm-2 s-1
dp/p ~ 10-4 (stochastic cooling)

12. PANDA: the detector calorimeter MVD DIRC STT/TPC Forward Spectrometer
Forward tracking
Solenoid
target

13. PANDA: Physics program
Charmonium and open charm spectroscopy;
Charmed hybrids and glueballs:
Meson mass modification in the nuclear matter;
Double hypernuclei produced via Ξ-baryon capture;
Wide angle compton scattering;
Drell-Yan reactions;
Baryon-Antibaryon production;
CP-Violation (Λ,D).

14. Time-Like FF : world data
|GM|2
10-1
10-2
in e+e-
VEPP L=1032
Frascati L= ?
BEPC/BES L=1033
Present typical errors 5%
20%
50%
PANDA range :
5 to 30 (GeV/c)2
at L =
PANDA errors
Belle ?? S=100 GeV2 ( centered on the upsilon 4S). ISR possible but far away from the « measurable «  region  los by at least a factor 1000
0.1% % %

15. PAX program Plans for the TL FF Separate measurement of |GE| et |GM|
Precisions on the ratio R=|GE|/|GM| and sR
DR/R <1% at low Q2
DR/R = 10% at Q2=10 (GeV/c)2
Separation possible up to Q2=15 (GeV/c)2
Test of the 1 g hypothesis (symmetry of the angular distribution)
Measure |GM| up to Q2=25 (GeV/c)2
Possibility to measure the phase difference j(GE) - j(GM) in case of transverse polarization
pp↑ ↔ e+e-
Double spin asymmetry will allow the independent measurements of GE and GM
PAX program

16. Experimental challenge
≈106
2 μb/8pb at q2=9 (GeV /c)2
106 times higher background
Thanks to PANDA PID capabilities we expect to reach a pion rejection power at the level of 10-8
Cherenkov radiation: above 1 GeV
Radiators: quartz, aerogel, C4F10
Energy loss: below 1 GeV
Tracking devices: best accuracy with a TPC
Electromagnetic processes: EMC for e/g
Time of flight
Problem: no start detector, huge material

17. PANDA PID
To allow electron-pion separation at 10-8 level PANDA must have excellent PID capability
MVD PID capability
Beam
Space resolution 50μm
Low material 5% X/X0
dE/dx capability
High rate 107 ev/s

18. PANDA tracking system Two options Straw Tube Tracker
Space resolution: r < 150µm z ~ few mm
Momentum resolution: pt / pt ~ 1.2 %
Material budget: X/X0 ~ %
dE/dx capability
Two options
Straw Tube Tracker
Time Projection Chamber

19. dE/dx performances, stt vs tpc
supposing the particles are hitting 22 tubes
Gaussian distribution ( ~ 6 %):
probability of misidentified pion
3% at 1 GeV/c , 10% at 6 GeV/c
stt
tpc [Panda TPR]

20. DIRC Concept Detection of Internally Reflected Cherenkov light 2.05
Different Cherenkov angles give different
reflection angles
PANDA DIRC similar to BaBar
96 Fused silica bars, 2.6m length
Water tank & 7000 PMTs
Alternative readout: (x,y,t), mirrors
Momentum (GeV/c)
e -
(deg)
Fraction of
misidentified pions (DIRC)
(εelectron=90%)
0.5
2.05
0.8
0.01
1.5 %
1.0
0.52
12%
1.5
0.23
27%
Schwiening
Schwiening

21. Electromagnetic Calorimeters
Forward Endcap
4000 PWO crystals
High occupancy in center
Readout LA APD or
vacuum triodes
Backward Endcap
800 Crystals
Worse resolution due to
service lines of trackers
Needed for hermeticity
Barrel Calorimeter
11000 PWO Crystals
LA APD readout
σ(E)/E~1.5%/√E + const.
1.0 GeV/c  e
Energy deposit in EMC
Forward Shashlyk
(after dipole magnet)
350 channels
Readout via PMTs
σ(E)/E~4%/√E + const.
Alternative Designs:
Spiral Shashlyk
Segmented composite
Shashlyk-Sandwich for
for (e/h/µ)

22. Pion rejection is OK 1. 5.4 2.5 8.2 5. 12.9 10. 22.3 MPtot 2particles
Tp_bar
(GeV) Q2
(GeV/c)2
qCM
qlab
plab
(GeV/c)
Misident. Probability
ECAL×DIRC×dE/dx
MPtot
2particles 1.
5.4
20°
13°
2.2
0.001 × 0.5 ×0.05 =
132°
0.57
0.033 × ×0.03 =
90°
54°
1.43
0.001 × 0.3 ×0.03 =
0.001 × 0.3 × 0.03 =
2.5
8.2
10°
3.7
0.001 × 1. ×0.05 =
117°
0.7
0.014 × × 0.03 =
41°
0.001 × 1. × 0.03 = 5.
12.9
7.4°
6.1
0.001 × 1. × 0.1 = 10-4
102°
0.8
32°
3.4
0.001 × 1. × 0.05 =
10.
22.3
5.4°
10.9
0.001 × 1. × 0.3 =
85°
1.0
0.005 × 0.12 × 0.03 =
24°
5.95
0.001 × 1. × 0.1 =
Pion rejection is OK

23. ds/dW  [ |GM(q2)|2(1+cos2q)+4mN2/q2 |GE(q2)|2 sin2q)]
FF Time-Like : GE et GM
~100 days, L = cm-2s-1
ds/dW  [ |GM(q2)|2(1+cos2q)+4mN2/q2 |GE(q2)|2 sin2q)] -
en 10 7 s
Nb of counts for pp→e+e-
cos(cm)
106 counts
3000 counts
6 104 counts
Nb of counts for pp→e+e-
Nb of counts for pp→e+e- - 10
B. Ramstein
J. Van de Wiele
6 104 counts 5
106 counts
3000 counts
80 counts
Q²=22.3 (GeV/c)²
cos
R=|GE|/|GM|
s=q2=5.4 (GeV/c) q2= 8.2 (GeV/c) q2=12.9 (GeV/c) q2=22.3 (GeV/c)2
Ntot= Ntot= Ntot= Ntot=82
Elab=1 GeV Elab=2.5 GeV Elab=5 GeV Elab=10 GeV
DR=0.6% DR=3% DR=25% DR= 100%
09/03/ B. Ramstein
EM working group

24. Summary - electromagnetic form factors: fundamental
property of nucleon
- poorly known in time-like region
- PANDA offers a unique possibility to measure in time-like domain up to q2 = 20 GeV2
- crucial: high luminosity & excellent particle ID