Measurement of Nucleon Form Factors with DAFNE2

Marco Mirazita INFN-LNF

LNF SCIENTIFIC COMMITTEE November 29 2005

•Introduction•Form Factors in the space-like region•Form Factors in the time-like region

•Measurement of Nucleon FFs with DAFNE2–Angular distribution measurements–Polarization measurements

•Conclusion

The DAFNE2 opportunity

Letter of Intent by 80 physicist from 24 institution in 7 countries: http://www.lnf.infn.it/conference/nucleon05/loi_06.pdf

Very positive feedback from the international community at the Workshop on Nucleon Form Factors, 12-14 October 2005, Frascati

The DAFNE energy upgrade offer the opportunity to make a detailed study of the nucleon Form Factors, providing:•accurate measurement of pp and nn cross section

model-independent extraction of proton and neutron FFs•first measurement of outgoing nucleon polarization

relative phase between GE and GM

•First measurement of baryon production (including polarization)

strange baryon FF•Study of angular asymmetry in pp (nn) distributions

look for 2-photon contribution•Measurement of e+ e-→hadrons and other exclusive multipion processes

sub-threshold NN resonance … and many others

Nucleon Form Factors – general properties

•FFs are analytic complex functions of q2 = (p – p’)2

•T-invariance in the space-like region implies real FFs

•Dispersion relations connect the Space-Like (q2 > 0) and Time-Like (q2 < 0) regions

•Two FFs in the one-photon-exchange approximation: Pauli-Dirac (F1 and F2) or Sachs (GE and GM)

GM(q2) = F1(q2) + F2(q2)GE(q2) = F1(q2) + F2(q2) =q2/4M2

• In the Breit reference system, Sachs FFs are the Fourier transform of the charge and magnetization distributions

•FFs are connected with GPDs ( quark angular momentum contributions)

Space-like FFs in the XX century - 1

Before 2000, the picture was well established and understood:- Proton electric and magnetic SL FFs scaling:

GMp p GE

p charge and magnetization have the same

distribution- Neutron electric SL FF GE

n 0 within errors

- All 3 non-zero FFs are well described by the dipole formula

nn

D

i

D

QaGG

GeV. Q

G

1

80 22

22

2

corresponding to the and meson resonances in the time-like region and to exponential distribution in the coordinate spaceNo substantial deviations from this picture were

expected

Space-like FFs in the XX century - 2

PROTON NEUTRON

Time-like FFs in the XX century

Proton data• Assuming |GE| = |GM| no |GE|• Early pQCD scaling |GM| ~ Q-4 • Time-like FF larger than space-like• Steep behaviour close threshold

Neutron data• Assuming |GE| = 0• neutron ~4 times the proton extrapolation• pQCD scaling?

Space-Like FFs in the XXI century - 1Accuracy of form-factor measurements significantly improved by measuring the interference term GEGM through the beam helicity asymmetry with a polarized target or with recoil polarimetryRecoil polarization measurements proposed more than 40 years ago as the best way to reach high accuracy in the FF measurement

Akhiezer et al., Sov. Phys. Jept. 6, 588 (1958)Arnold, Carlson, Gross, PR C23, 363 (1981)

had to wait over 30 years for development of- polarized beam with high intensity (~100 µA) and high polarization (>70 %)- beam polarimeters with 1-3 % absolute accuracy- polarized targets with a high polarization or- ejectile polarimeters with large analyzing powers

JLabnew generation of beams and detectors

polarization

Rosenbluth

Space-Like FFs in the XXI century

The new data imply a completely different picture of the proton

Fourier transform of GM and GE :charge and magnetization distributions

Quark angular momentum contribution?

Second “spin crisis” of the proton

Why a new measurement of time-like FFs in the XXI century?

• Time-like data can discriminate between models that fit equally well the space-like region• Space-like data could perhaps be reconciled with 2-photon exchange contributions. What in the time-like region?

• Jlab measurements showed that |GE| = |GM| in the space-like region is no more a valid assumption for the proton. Why should be valid in the time-like?• The inconsistencies between data and pQCD expectations could be just a consequence of the basic wrong assumption |GE| = |GM| • Neutron need a much more careful investigation

• Phases of time-like FFs never measured

Time-like FFS are basically unknown

Electric to magnetic FF Electric to magnetic FF ratioratio

Different hypothesis on GE/GM strongly affect the GM extraction, mainly in the low energy region

DR analysis

Tentative extraction of FF ratio from angular distributions

Very suitable energy window

DAFNE2

1 m

FINUDA well suitable FINUDA well suitable

Feasible with minimal Feasible with minimal modificationsmodifications

• interaction region (only one)• vacuum chambers• dipoles (normal conducting)• control system• diagnostics

Injection at 510 MeV keeping the present injection system• ramp up time ~ minutes• beam life time ~ hours

Experimental Experimental requirementsrequirementsBeam requirements: • beam energy 1.2 GeV• high luminosity ~1032 cm-

2s-1

(cross section ~ 0.1-1 nb) • beam polarization not required (but could help)

Detector requirements: • high detection acceptance for

charged and neutral particles• identification of exclusive final

state- protons momentum+TOF- high neutron efficiency- detection of antinucleons converter

• installation of a polarimeter - carbon analyzer + 2 tracking systems

•Good p-resolution•Adequate n-detection•Easy implementation of a polarimeter•Possibility to improve n-detection - more converters - new array of scintillators just before the end-cap - n-polarimeter

Minimal changes required in Minimal changes required in FINUDAFINUDA

1 cmvertex region

OSIM

nucleartargets

TOFino

ISIM

10 cm

driftchambers

TOFone

strawtubes

Add Scintillator slabs

• antineutron converter• polarimeter

or carbon cylinder

remove nuclear targets

e+e-nn with FINUDA-

s = 1890 MeV, B = 0.2 T1.5 cm carbon converter

A. Filippi, INFN Torino

s = 1890 MeV, B = 0.2 T

e+e-pp with FINUDA: typical topology

Proton angular distributions

• Projected data assuming |GE| = |GM| (black) or |GE|/|GM| from DR (red)

• Integrated luminosity L=100 pb-1

• Constant detection efficiency =80%

• fit of angular distributions in the FINUDA coverage

F()=A(1+cos2)+Bsin2

|GE||GM|FINUDA

Max sensitivity to |GE|

222

2 114

sin cos 2EM GG

sC

dd

Neutron angular distributions

• Projected data assuming |GE| = |GM| (black) or |GE| = 0 (red)

• Integrated luminosity L=100 pb-1

• Constant detection efficiency =15%

• fit of angular distributions in the FINUDA coverage

F()=A(1+cos2)+Bsin2

FINUDA|GE||GM|

FF measurement: projected accuracy

Integrated luminosity 700-1000 pb-1 KLOE in last 12 months: 1800 pb-1 at

proton

neutron

Statistical error of the order of few percent for all the 4 nucleon FFs in the whole explored region

Induced polarization

•non negligible polarization

•Py maximal at 45° and 135°

•high discriminating power between theories

•extraction of FF relative phase

M

E

MEy

GG

P

220 1

21

sincos

sinsinPolarization normal to the scattering planeNo beam polarization

z

x

y

B

B

e+e-

Polarization measurement

sincos ,, xy PPTATdd

10

• The polarization is measured through secondary scattering in a strong interaction process

• The spin-orbit coupling causes an azimuthal asymmetry in the scattering

tracking system

tracking systemanalyzer

p

s

e+ e-

p

p

P

PC

z’

drift chambers straw tubes TOFone

Vertex region OSIM

Analysing power

Polarization measurement

polyC PANd

ddNN 2000

Polarization is extracted by measuring asymmetries

For Py pol( cos)

polyC PA

NNNNR

2

Averaged analysing power~ 50 %

Polarization~ 15% max (pQCD model)

Expected effect of the order of few %at EBEAM = 1.2 GeV

For ΔR/R 30 %: total luminosity 2500 pb-1 (1 year with average 1032 cm-2 s-1)

Integrated luminosity

proton neutron

s [GeV2] Ebeam [MeV] L [pb-1] L [pb-1]

3.55 941 300 300 cross4.06 1010 100 (100) section4.58 1070 100 (100)5.20 1140 100 (100)5.76 1200 100 (100)5.76 1200 2500 polarization

Possible improvements of the detector

• Neutron polarization measurement- Use scintillator slabs as analyzer carbon for protons and hydrogen for neutrons- The scintillators can be used to increase neutron detection efficiency

• Improve nn detection capability- Double converter increase antineutron efficiency- A second layer of scintillators double neutron efficiency- Extend angular coverage of TOFone barrel

Time-like FF measurement competitors

s (GeV)

MN 2.0 2.4 4.2

proton

neutron

DAFNE2

VEPP2000• max. energy ~ 1 GeV per beam, luminosity ~ 1032 cm-2 s-1 • measure pp and nn final state• start run ~ 2007BEPC• energy ~ 2.4-4.2 GeV, luminosity ~ 1033 cm-2 s-1 • measure pp final state only• start run ~ 2007

PAX• inverse reaction pp → e+e- (no neutron measurement)• single and double polarization measurements• start run >2013

Conclusions•DAFNE2 at 1.2 GeV provides a very interesting energy region for an accurate determination of nucleon (and hyperon) form factors in the time-like sector.

•The FINUDA detector with minor modifications is well suitable for the measurements.

•An integrated luminosity between 100 and 300 pb-1 per beam energy allows measurements of |GM| and |GE| at the few percent level for the proton and below 10% for the neutron.

•Measurement of the nucleon polarization is feasible, providing the first determination of the relative phase between the electric and magnetic FFs.

•Other interesting measurements are also possible