Measurement of Nucleon Form Factors with DAFNE2

1. 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 Measurement of Nucleon Form Factors with DAFNE2 Marco Mirazita INFN-LNF LNF SCIENTIFIC COMMITTEE November 29 2005

2. 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 L (S) 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 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

3. 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) + t F2(q2)t=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)

4. 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 mpGEp •  charge and magnetization have the same distribution • Neutron electric SL FF GEn 0 within errors • All 3 non-zero FFs are well described by the dipole formula corresponding to the r and w meson resonances in the time-like region and to exponential distribution in the coordinate space No substantial deviations from this picture were expected

5. Space-like FFs in the XX century - 2 PROTON NEUTRON

6. 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? Time-like FFs in the XX century

7. new generation of beams and detectors JLab Space-Like FFs in the XXI century - 1 Accuracy of form-factor measurements significantly improved by measuring the interference term GEGM through the beam helicity asymmetry with a polarized target or with recoil polarimetry Recoil 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

8. Rosenbluth polarization The new data imply a completely different picture of the proton Fourier transform of GM and GE : charge and magnetization distributions Space-Like FFs in the XXI century Quark angular momentum contribution? Second “spin crisis” of the proton

9. 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 Why a new measurement of time-like FFs in the XXI century? Time-like FFS are basically unknown • 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?

10. Tentative extraction of FF ratio from angular distributions DR analysis DAFNE2 Very suitable energy window Electric to magnetic FF ratio Different hypothesis on GE/GM strongly affect the GM extraction, mainly in the low energy region

11. Feasible with minimal modifications • 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 FINUDA well suitable 1 m Experimental requirements • 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 • Beam 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) • 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

12. Add Scintillator slabs • antineutron converter • polarimeter or carbon cylinder remove nuclear targets ISIM OSIM nuclear targets TOFino vertex region 1 cm Minimal changes required in FINUDA drift chambers straw tubes 10 cm TOFone

13. - 1.5 cm carbon converter A. Filippi, INFN Torino e+e-nn with FINUDA s = 1890 MeV, B = 0.2 T

14. e+e-pp with FINUDA: typical topology s = 1890 MeV, B = 0.2 T

15. Projected data assuming |GE| = |GM| (black) or |GE|/|GM| from DR (red) • Integrated luminosity L=100 pb-1 • Constant detection efficiency e=80% • fit of angular distributions in the FINUDA coverage • F(q)=A(1+cos2q)+Bsin2q |GM| |GE| FINUDA Max sensitivity to |GE| Proton angular distributions

16. |GM| |GE| FINUDA Neutron angular distributions • Projected data assuming |GE| = |GM| (black) or |GE| = 0 (red) • Integrated luminosity L=100 pb-1 • Constant detection efficiency e=15% • fit of angular distributions in the FINUDA coverage • F(q)=A(1+cos2q)+Bsin2q

17. proton neutron FF measurement: projected accuracy Integrated luminosity  700-1000 pb-1 KLOE in last 12 months: 1800 pb-1 at F Statistical error of the order of few percent for all the 4 nucleon FFs in the whole explored region

18. B y z x • non negligible polarization • Py maximal at 45° and 135° • high discriminating power between theories • extraction of FF relative phase e- e+ B Induced polarization Polarization normal to the scattering plane No beam polarization

19. The polarization is measured through secondary scattering in a strong interaction process • The spin-orbit coupling causes an azimuthal asymmetry in the scattering p z’ tracking system drift chambers straw tubes TOFone f qs Analysing power p tracking system analyzer Vertex region OSIM e- e+ P PC qp Polarization measurement

20. y Polarization is extracted by measuring asymmetries f - + For Pypol( cosf) x Polarization ~ 15% max (pQCD model) Averaged analysing power ~ 50 % Polarization measurement 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)

21. Integrated luminosity

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

23. VEPP2000 • max. energy ~ 1 GeV per beam, luminosity ~ 1032 cm-2 s-1 • measure pp and nn final state • start run ~ 2007 • BEPC • 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 DAFNE2 proton neutron s (GeV) 2.0 2.4 4.2 MN Time-like FF measurement competitors

24. 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