DANTE : DA nae N ucleon T ime-like form factor E xperiment Status and Plans

1. DANTE: DAnae Nucleon Time-like form factor ExperimentStatus and Plans Marco Mirazita LNF Scientific Committee Frascati, 27 november 2006

2. Nucleon Form Factors and QCD • QCD has been extensively tested in the high energy domain • asymptotic freedom  perturbative calculations (at percent level) • QCD at low energy is much more complicated • non-perturbative methods (lattice, chiral theory, …) • no completely satisfactory agreement with data Nucleon FFare fundamental quantities describing the internal structure of the nucleon from low (charge and magnetization distributions) to high Q2 regimes (valence quark distributions) - simplest non pQCD observables in the hadronic physics - their calculation includes all the complications of the non-pQCD regime A succesfull non perturbative QCD model must be able to reproduce nucleon FF in all the q2 plane Nucleon FF are also important for all process where nucleons are involved - GPDs - strange content of the nucleon - neutrino experiments

3. SL  TL → FF • Sachs Form Factors • Fourier transforms of nucleon charge and magnetization density distributions (in the Breit Frame). • Space-like form factors are real, time-like are complex. • The FF are analitic functions, thus space-like and time-like regions are connected by dispersion relations. • By definition GM and GE do not interfere in the expression of the cross section, therefore, in the timelike case, only polarization observables allow to get the relative phase. Dirac Pauli Nucleon EM Form Factors

4. old cross section data new polarization data The new data imply a completely different picture of the proton Fourier transform of GM and GE : charge and magnetization distributions Proton FF in the SL region JLab measurement with polarization transfer technique in ep → ep scattering • 2-photon exchange contributions? • Quark angular momentum?

5. Science & Technology Review DOE S&T review was held June 2006. Excerpts from the Executive Summary Electromagnetic Form Factors V. Burkert, CLAS Collaboration Meeting, 11/02-04 2006

6. |GM| proton • Time-like FF are complex  4 quantities to be measured • In general mesured from total cross section and making some (arbitrary) assumption on the |GE|/|GM| ratio • Besides several questions posed by the data, basically GE remains unmeasured |GM| neutron |GE|=|GM| Independent extraction of |GE| and |GM| has been tried (LEAR and BaBar) - inconsistent results - large statistical errors |GE|=0 FF in the Time-like region

7. One example: S. Pacetti et al. – Eur. Phys. J. C46,421 use of DR to fit the experimental data of the ratio |GE|/|GM| yellow band: fit of SL + BaBar TL data green band: fit of SL + LEAR TL data unphysical region ASYMPTOTICS space-like time-like Scaling “restoration” • Results not clear • Available data are not sufficient to get FF in the whole q2 plane QCD limit An important tool: DR Time-like Form Factors HAVE TO BE MEASURED (polarization)

8. Moduli: extraction from d/d measurement q B y z x e- e+ relative phase B FF measurement in TL region Phases: extraction from polarization measurements No beam polarization needed

9. |GM| |GE| proton neutron |GM| FF measurement: projected accuracy • Projected results at 5 beam energies, 100pb-1 per each value • finer energy scan with lower statistics can be done • e(n)~40-50% of the KLOE calorimeter •  neutron and proton with comparable errors

10. p z’ PC is the polarization projected on the analyzer plane tracking system f qs p tracking system analyzer e- e+ P PC qp • Analyzer: • few cm of carbon for protons • scintillator array for neutrons Polarization measurement Based on secondary scattering in strong interaction process Analizing power Unpolarized cross section

11. Protons, T=0.2 GeV Proton Polarimeter http://www.lnf.infn.it/conference/nucleon05/FF/polarimeter_study_2.pdf Counting rate is determined by the convolution of - multiple scattering (small angle, Molière) - strong nuclear scattering (large angle, exp. unpol. cross section and analyzing power) Higher analyzer thickness  higher rate but - larger Molière angle qm - lower tracking resolution

12. Main parameters of the calculation - total luminosity 2500 pb-1 - electron beam energy 1.14 GeV (Tp=0.2 GeV) - total pp cross section: s=0.34 nb - angular pp distribution from DR  sin(dE-dM)=0.26 - carbon thickness T=1.5 cm, inner radius R = 25 cm - magnetic field B=0.5 T PyC = Py cosc Proton Polarimeter

13. p z’ fs qs p qp e- e+ P PC Scattering angle distributions 12 qp bins of 15° Good events for polarization measurement qc < qs < 40°

14. max=1.6% Polarimeter acceptance

15. L-R asymmetry with respect to the scat = 90º axis N+ N- N+ Azimuthal angle distributions • Fit of angular distributions • N(scat) = p1 • (1+Accosscat+ Assinscat) • PyPx

16. Py Fit with theoretical curve • sin(E-M) = 0.2382 ± 0.0255 (FIT) • 0.2376 ± 0.0289 (ASYM) • consistent with input value • e~ 10% Polarization extraction

17. Energy loss To keep Dp/p below ~10%  T~ 3-4 cm  pol. acceptance e ~3%  error on the phase below 10% Acceptance Polarimeter thickness

18. Conclusions and outlook • PROTON: measurement feasible • - modulus: very high precision from threshold up to the highest beam energy; can be easily integrated with other measurements of the high energy program • - phase: measurement with good precision; dedicated run required (polarimeter) • NEUTRON: measurement feasible • - modulus: statistical error comparable to the proton • - phase: high neutron detection efficiency makes easier the measurement NEXT STEPS - detailed study of the inner detector region with the polarimeter - neutron polarimeter