1. DANTE: DAnae Nucleon Time-like form factor Experiment
Patrizia Rossi - 32nd LNF SC meeting May 31, 2006
Importance of the nucleon Form Factors and our actual knowledge
Measurement of the nucleon Time-Like FFs with KLOE
Competitors

2. Hadron Electromagnetic Form Factors
FFs are fundamental quantities describing the internal structure of the hadron. In a P- and T-invariant theory, the EM structure of a particle of spin S is defined by 2S+1 form factors.
Since the Frisch & Stern experiment in 1933 indicating that the proton was not a point-like particle, FFs have been extensively measured, neverthless we are far from their full understanding

3. Hadron Electromagnetic Form Factors
In the framework of one photon exchange, FFs are functions of the momentum
transfer squared of the virtual photon, t=q2=-Q2
Scattering
t<0
t>0
Annihilation
and are defined by the matrix elements of the e.m. current J(x) of the nucleon:
Dirac
Pauli
Each FF is described by an analytical function in the complex q2 plane
Spacelike form factors are real, timelike are complex and they are connected by dispersion relations

4. Nucleon Form Factors:  are of fondamental importance in themselves
 are necessary ingredients as input for the descriptions of ALL processes where nucleons are involved

5. Why hadron e.m. Form Factors are important: 1) QCD tests
Electromagnetic FFs of nucleons parametrize the nucleon internal structure from low momentum transfer Q2, where they describe the nucleon charge distribution and magnetization current, to high Q2, where they probe the valence quark distributions.
p-QCD predictions from non-perturbative to perturbative regime can be tested according to their capability to reproduce the form factors measurements for any value of the momentum transfer

6. Why nucleon e.m. Form Factors are important 2) connection with GPDs
GPDs: New powerfull theoretical framework which unifie the description of ALL reactions with hadrons
Carry simulaneously information on both the longitudinal and transverse distribution of partons in a fast moving hadron
3-D description of the nucleon
GPD: E,H,E,H of 3 kin var.: x,ξ,t ~
At present GPDs calcultions possible ONLY through models
Simple parametrizations contrained by
 general symmetry properties
limit t=0  usual DF ;  over x  nucleon FF
Adjusting a few free parameters to the exp. data on the Dirac and Pauli FFs one aims at a suitable parametrization of the GPDs
Pauli
Dirac
Axial
Pseudoscalar
Their calculation: non-pert. method

7. Why nucleon e.m. Form Factors are important 3) determination of the strange FF
Contributions of strange sea quarks to charge and magnetization distributions of the proton
PV e-scattering
experiments
Final precision hampered mainly by uncertainties in the neutron electric form factor

8. Why nucleon e.m. Form Factors are important 4)  QE scattering
Quasielastic cross section for neutrino scattering on nucleons
Same param.
GEn≠
≠ param.
Gen=0
H. Budd et al. hep-ex/

9. Nucleon e.m. Form Factors are still not fully understood

10. SPACE-LIKE nucleon e.m. Form Factors
Rosenbluth separation
Based on cross section measurement
Q2 = |q2 |
e = * polarization t = Q2/ 4M2
Measures dσ/dΩ vs  at constant Q2
intercept gives GM, slope gives GE
PROTON
NEUTRON
Proton: electric and magnetic SL FFs scaling: GMp  mpGEp
Neutron: GEn  
GMp GEp GMn: Dipole Formula
Thought well known…
Different charge and magnetization distributions
Quark angular momentum contribution?
Rosenbluth
polarization
Linear
deviation
from dipole
GE≠GM
BUT…

11. TIME-LIKE nucleon e.m. Form Factors
Time like FFs are complex functions  moduli & phases
s=4M2  |GM|=|GE| s>>4M2  |GM|>>|GE|
Moduli: extraction from , d/d meas. in e+e- NN and pp  e+e-
Phases: extraction from polarization measurements

12. Nucleon Time-like FFS are basically unknown
no independent extraction of both TL FFs has been performed
FF measurements are based on total cross section, under some theoretical assumption on their ratio
|GEp|/|GMp|
Ebeam (GeV)
DANAE
Phases of time-like FFs never measured
Only one measurement for neutron magnetic FF
Inconsistencies between data and pQCD expectations

13. NucleonTime-like FFs |GM| PROTON DATA |GM| NEUTRON DATA
Early pQCD scaling |GM| ~ Q-4
NucleonTime-like FFs
|GM|
Assumption: |GE|=|GM |
Ebeam (GeV)
PROTON DATA
Steep behaviour close threshold
Time-like FF larger than space-like
|GM|
Assumption: GE=0
NEUTRON DATA
neutron ~4 times the proton
extrapolation
pQCD predicts the limit
|GMn|≈(qd/qu)2 |GMp|= | GMp|/4

14. Anomalous behavior in total cross section
BaBar found steps in total cross section at s =2.2 and 2.9 GeV

15. Time-Like FFs put more stringent constraint on the nucleon models
Q2(GeV2)
|GEp|/|GMp|
unphysical
region
Form Factors are connected to the e.m. currents both in
Time-Like and Space-Like region: they contain the SAME INFORMATIONS but in a DIFFERENT KINEMATIC REGION
Time-Like FFs put more stringent constraint on the nucleon models
“GOOD” NUCLEON MODELS have to be able to describe ALL the 4 Form Factors in ALL the complex plane

16. Detector requirements
The 31st LNF SC Summary of Recomendation:
it invites the proponents for future experiments to consider that the designe of future experiments should be developed around a single common detector and a set of repleaceable modules specific to a given physics
Following this recomendation, we are studing the possibility to modify the KLOE detector for the fulfillment of the DANTE program.
KLOE detector has to guarantee two key points:
- detection of p-pbar n-nbar exclusive events
- polarization measurement
Moreover: given the requirements of the different experiments, the optimal choice seems to be that for each experiment a specific interaction region is implemented.

17. Measurement of the moduli
2E(GeV)
T(GeV)
P(GeV/c)
s<1.91 GeV:
1 annihilation star on beam pipe on DC wall; no track in the DC
s>1.91 GeV:
p track in the DC
p-bar annihilation or track
e+e-  pp _
key point: understand the KLOE neutron detection efficiency not known up to now
Studies in progress (KLOE data/simulation):
Preliminary results (AMADEUS collaboration)
10-20 MeV <Tn < 250 MeV  ~20%< n < 50-60%
Test beam planned
e+e-  nn _

18. Moduli: projected results (with FINUDA)
Integrated luminosity  pb-1 (~6 months measurements)
proton
Projected data assuming
|GE| = |GM| or |GE|/|GM| from DR
Integrated luminosity =100 pb-1
Constant detection efficiency =80%
KLOE wider angular coverage
neutron
Projected data assuming
|GE| = |GM| or |GE|=0
Integrated luminosity =100 pb-1
Constant detection efficiency =15%
Statistical error of the order of few percent for all the 4 nucleon FFs in the whole explored region
KLOE wider angular coverage
and maybe a better efficiency

19. Induced polarization NO beam polarization dependence! -
The complex phases of the FFs in the TL region make it possible for a single outgoing baryon to be polarized in
e+e-BB even without polarization in the initial state -
NO beam polarization
dependence!
non negligible polarization
high discriminating power between theories
extraction of FF relative phase
Py maximum at 45° and 135°

20. Polarization measurements
2nd tracking system f
1st tracking system qp qs e+ e- p P PC z’
analyzer
The polarization is measured through secondary scattering in a strong interaction process
The spin-orbit coupling causes an azimuthal asymmetry in the scattering
Analysing power
Polarization is extracted by measuring asymmetries y f + - x
Ex: Py pol( cosf)

21. Polarization measurements_
e+e-  pp
From pC elastic 
 = 2.5 % -20
= 1.8 % -20
= 0.05 % 20-35
173 cm
KLOE DC
analyzer
25 cm p e+ e-
1st tracking
system
@ 1.2 GeV For ΔP/P  30 %: ~ 1 year meas.(Tot L2.5 fb-1)
e+e-  nn _
Solution to be investigated

22. Competitors VEPP2000 BEPC PANDA-PAX
s (GeV) MN
2.0
2.4
4.2
proton
neutron
VEPP2000
max energy ~ 1 GeV per beam, luminosity ~ 1032 cm-2 s-1
pp and nn measurement. No polarization measurements
start run ~ 2007
BEPC
energy range ~ GeV, luminosity ~ 1033 cm-2 s-1
only pp measurement. No polarization measurements
start run ~ 2007
PANDA-PAX
inverse reaction pp → e+e- (no neutron measurement)
single and double polarization measurements
start run >2013
DANAE

23. “Prospects for e+e- physics at Frascati between the  and the ”
High energy program
The DANTE Collaboration confirms its interest in the high energy program discussed in the
“Prospects for e+e- physics at Frascati between the  and the ”
presented by C. Bini in this meeting

24. Conclusion
Form factors are fundamental quantities describing the internal structure of the hadron
Dispite their long investigation we are far from their full understanding
In particular in the time-like region they are pratically unknown
DANAE will provide:
The FIRST accurate measurement of the proton time-like form factors |GpE | | GpM|
The FIRST measurement of the relative phase between |GpE| and | GpM|
The FIRST measurement of the two  contribution from the p ang. dist. asymmetry
The FIRST accurate measurement of the e+e n-nbar cross section
The FIRST measurement of the neutron time-like form factors |GnE | and | GnM|
The FIRST measurement of the strange baryon form factors