DANTE: DAnae Nucleon Timelike form factor Experiment

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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

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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 2S1 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

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Hadron Electromagnetic Form Factors
In the framework of one photon exchange, FFs are
functions of the momentum transfer squared of
the virtual photon, tq2-Q2
and are defined by the matrix elements of the
e.m. current J?(x) of the nucleon
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
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Nucleon Form Factors ? are of fondamental
importance in themselves ? are necessary
ingredients as input for the descriptions of ALL
processes where nucleons are involved
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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
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Why nucleon e.m. Form Factors are important 2)
connection with GPDs
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Why nucleon e.m. Form Factors are important 3)
determination of the strange FF
Final precision hampered mainly by uncertainties
in the neutron electric form factor
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Why nucleon e.m. Form Factors are important 4) ?
QE scattering
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Nucleon e.m. Form Factors are still not fully
understood
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SPACE-LIKE nucleon e.m. Form Factors
Thought well known
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TIME-LIKE nucleon e.m. Form Factors
Time like FFs are complex functions ? moduli
phases
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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

• Phases of time-like FFs never measured
• Only one measurement for neutron magnetic FF
• Inconsistencies between data and pQCD
  expectations

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NucleonTime-like FFs
PROTON DATA

• Steep behaviour close threshold

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Anomalous behavior in total cross section
BaBar found steps in total cross section at ?s
2.2 and 2.9 GeV
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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
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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.
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Measurement of the moduli
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Moduli projected results (with FINUDA)
Integrated luminosity ? 700- 1000 pb-1 (6
months measurements)
KLOE wider angular coverage
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
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Induced polarization

• extraction of FF relative phase
• Py maximum at 45 and 135

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Polarization measurements
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Polarization measurements
_
ee- ? pp
From pC elastic ? ?? 2.5 5?-20?
1.8 7?-20? 0.05 20?-35?
173 cm
KLOE DC
analyzer
1st tracking system
_at_ 1.2 GeV For ?P/P ? 30 1 year meas.(Tot
L?2.5 fb-1)
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Competitors
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High energy program
The DANTE Collaboration confirms its interest in
the high energy program discussed in the
Prospects for ee- physics at Frascati between
the ? and the ? presented by C. Bini in this
meeting
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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
• DANTE _at_ 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 ee?
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
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Hyperon Form Factors

• Y Y final state could be identified by
  detecting the decay of one hyperon
• Angular distributions ? moduli of FF
• For weakly decaying hyperons (L(1116),
  S(1189)) the polarization measurement does not
  require a polarimeter (can be studied by looking
  at the angular distribution of the decay products)

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Summary Haiming Hu NUCLEON05, Frascati, Italy
Collider luminosity increase about 100 times from
BEPC (1031 cm-1s-1) to upgraded BEPCII
(1033cm-1s-1 ). It is expected we may take enough
data samples to measure pp form factors
BESIII will have much better space and momentum
resolution, and the particle identification
ability has been improved greatly than BESII. But
probably neutrons nn can not measure well.
It is possible that to obtain large data samples
in energy region of 2.4 4.2 GeV, and collect
enough statistics for pp . Using the scan data,
one may measure the polar angle distribution, and
then fit the electric GE(s) and magnetic GM
(s) form factors at same time as the function of
the center-of-mass energy s.