Proposta per la Misura dei Fattori di Forma del nucleone a DAFNE

Transcription

1 Proposta per la Misura dei Fattori di Forma del nucleone a DAFNE Nicola Bianchi INFN Laboratori Nazionali di Frascati Riunione Commissione Nazionale Scientifica I Napoli, 19 settembre 2005

2 Why a new measurement of the Nucleon e.m. Form Factors today? (1) FFs are fundamental quantities describing the internal structure of the nucleon Wavelength of the probe can be tuned by selecting momentum transfer Q 2 < 0.1 GeV 2 integral quantities (charge radius, ) GeV 2 internal structure of nucleon > 20 GeV 2 pqcd scaling Stern (1932) measured the proton magnetic moment indicating that the proton was not a point-like particle Hofstadter (1950 s) provided the first measurement of the proton s radius through elastic electron scattering (Nobel Prize in 1961) Subsequent Space-Like (SL) data ( 2000) were based on Rosenbluth separation with limited accuracy for G Ep for Q 2 >1 GeV 2 Early interpretation based on Vector-Meson Dominance Good description with phenomenological dipole form factor

3 Why a new measurement of the Nucleon e.m. Form Factors today? (2) Before 2000, the picture seemed to be well established and understood: - Proton electric and magnetic SL FFs scaling: G Mp μ p G E p charge and magnetization have the same distribution - Neutron electric SL FF G En small and badly measured - All 3 non-zero FFs are described 20% by the dipole formula G G G D i D = Λ = Λ + Q a n 2 Q n 2 Λ 0. 8 GeV corresponding to the ρ and ω meson resonances in the time-like region and to exponential distribution in the coordinate space No substantial deviations from this picture were expected

4 Why a new measurement of the Nucleon e.m. Form Factors today? (3) The new SL JLab results (2000) using polarization measurement in ep elastic scattering dramatically changed the picture of the Nucleon: - G Ep /G Mp decreases with Q 2 - data suggest G Ep crosses 0 at Q 2 8 GeV 2 Charge and magnetization distributions are different ( second proton spin crisis ) New strong interest triggered from these results - Reanalysis of old SLAC experimental data - New measurements are performed and planned in JLab - Theoretical effort to explain the discrepancies The Time-Like region is much less studied Intrinsic interest in itself (possible resonance at threshold) Need to reconsider some discrepancy found in the TL region and with respect to the SL Good capabilitiy to disciminate among different descriptions

5 Nucleon Form Factors general properties FFs are analytic complex functions of q 2 = (p p ) 2 Two FFs in the one-photon-exchange approximation: Pauli-Dirac (F 1 and F 2 ) or Sachs (G E and G M ) G M (q 2 ) = F 1 (q 2 ) + F 2 (q 2 ) G E (q 2 ) = F 1 (q 2 ) + τ F 2 (q 2 ) τ=q 2 /4M 2 In the Breit reference system, Sachs FFs are the Fourier transform of the charge and magnetization distributions T-invariance in the space-like region implies real FFs Dispersion relations connect the Space-Like (q 2 > 0) and Time-Like (q 2 < 0) regions FFs are connected with GPDs

6 How to measure Space-Like Form Factors Rosenbluth separation Based on cross section measurement dσ dω Q 2 = q 2 ε = photon polarization τ = Q 2 / 4M 2 Polarization observables For example, electron-to-proton polarization transfer G G M = 2 τ = σ G E + G ε 2 0 M Pt P l Ee E 2M E + e' θ tan 2 Recoil polarization measurements have been 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 for the new generation of beams and detectors JLab

7 Proton SL Form Factors JLab 2005 SLAC re-analysis Rosenbluth technique Rosenbluth polarization JLab polarization technique G E =0 at some Q 2 (~8 GeV 2 )? Asymptotic scaling: pqcd F 2 / F 1 ~ Q -2 pol. data F 2 / F 1 ~ Q -1 radiative corrections? quark angular momentum contribution? -> HERMES data of non-zero Sivers asymmetries

8 2-photon exchange A.Afanasev and C.Carlson PRL 94 (2005) complex space-like FFs - correction to the cross section are of the same order as electric contribution - corrections to polarization observables are expected to be much smaller Rosenbluth and polarization data could be reconciled? Calculations have simple parametrization of 2-γ exchange Other authors found negligible contributions to the cross section

9 Outlook in the space-like region at JLab New measurements of elastic scattering planned at JLab for higher Q 2 run in 2006 Study of 2-photon contribution comparing e - p and e + p scattering

10 Time-Like FFs measurements FF extraction from e + e - N N τ = s/ 4M 2 2 dσ α βc 2( 2 ) = G M 1+ cos θ + G E sin θ dω 4s τ G M = G E at the physical threshold s = 4M 2 isotropic distributions G M dominates the cross section for s >> 4M 2 Up to now, no independent extraction of both TL FFs has been performed (σ 1 nb) FF measurements are based on total cross section, under some theoretical assumption on their ratio G M can be more easily extracted, but it s model-dependent G E remains unmeasured

11 Time-Like FFs : proton data G M from cross section and with the assumption G E = G M Early pqcd scaling G M ~ Q -4 Time-like FF larger than space-like Steep behaviour close threshold

12 Electric to magnetic FF ratio Tentative extraction of FF ratio from angular distributions DAΦNE2 DR analysis Different hypothesis on G E /G M strongly affect also G M extraction in the low energy region

13 Time-Like FFs : neutron data Only the FENICE data Assuming G E = 0 Angular distribution dσ/dω ~ 1+cos 2 θ compatible with G E =0 neutron G M bigger than proton pqcd scaling?

14 Hyperon Form Factors Hyperons can also be produced in e + e - interactions (Λ, Σ, ) energy threshold: s ~ 2 M Λ ~ 2.23 GeV E BEAM ~1.12 GeV Y Y final state could be identified by detecting the decay of one hyperon Angular distributions moduli of FF For weakly decaying hyperons (Λ(1116), Σ ± (1189)) the polarization measurement does not require a polarimeter

15 Proposal for nucleon FFs measurement at DAΦNE Study of nucleon FFs measurement in the DAΦNE energy upgrade using the present detectors - Euridice Workshop 19 October 2002, Frascati (A. Filippi with FINUDA) - Workshop on e + e - in the 1-2 GeV range: Physiscs and Accelerartor Prospects September 2003, Alghero (A. Filippi with FINUDA; C. Ligi, R. Ricci, G. Benedetti for the machine) - LNF Spring School May 2005, Frascati (M. Mirazita with FINUDA; M. Preger for the machine) - A. Antonelli et. al., Future Research and Activities at LNF: Working Group Report, in print - M. Mirazita et al., Measurement of the Nucleon Form Factors in the time-like region at DAΦNE, Letter of Intent, first draft - D. Alesini et al., Preliminary considerations on machine requirements for a Nucleon FF experiment at Frascati DAΦNE Technical Note G-63

16 DAΦNE parameters for 1.2 GeV operation Main machine changes: - dipole magnets (normal conducting) -interaction region

17 The FINUDA detector carbon layer: antineutron converter proton polarimeter remove nuclear targets

18 e+e- nn with FINUDA: typical topology s = 1890 MeV, B = 0.2 T

19 e+e- pp with FINUDA: typical topology s = 1890 MeV, B = 0.2 T

20 Efficiency and resolution for pp tracks 2 d α βc = dω 4s 2 ( 1+ cos θ ) σ G M + G E τ G E is better measured around 90 o sin B=10 kgauss θ E=1.2 GeV E=1.1 GeV E=1.0 GeV

21 FINUDA projected accuracy proton neutron prototne Integrated luminosity pb -1 cfr FENICE cm -2 s -1 (cfr KLOE in 2004: 780 pb -1 )

22 Angular distributions neutron projected data red: G E =0 black: G E =G M Fitting function f(θ)=a(1+cos 2 θ) + B sin 2 θ

23 Induced polarization Polarization normal to the scattering plane No beam polarization M ( 2θ ) cos( δ E δ M ) 2 ρ ( θ ) + sin ( θ ) 1 ρ sin PN = σ τ cos τ GE ρ = G non negligible polarization P y maximal at 45 and 135 high discriminating power between theories extraction of FF relative phase

24 Proton polarimeter The polarization is measured through secondary scattering in a strong interaction process The spin-orbit coupling causes an azimuthal asymmetry in the scattering dσ = σ 0 dω ( θ T )[ 1+ A( θ, T )( P cosφ sinφ) ] y P, x in-plane polarization components can be measured non-normal polarimeter crossing and/or magnetic field require back-tracing of the measured polarization small efficiency to reject multiple scattering background

25 Polarization measurement Polarization is extracted by measuring asymmetries For example, for P y pol ( cosφ) ± dσ N = N d d N 0 φ = 0σ 0 ± 2 ± Ω Δφ ( ) pol π A C P y - y φ + x R = N N N N = 2 π Averaged analysing power ~ 50 % A C P y pol Polarization ~ 15 % max (pqcd model) Expected effect of the order of few % at E BEAM = 1.2 GeV For ΔR/R 30 %: total luminosity 2500 pb -1 (1 year with average cm -2 s -1 )

26 Summary The increase of DAΦNE2 energy above NN threshold would allow: The first accurate and independent measurement of the timelike proton and neutron FFs The first measurement of the outgoing proton polarization, to get the relative phase between electric and magnetic FFs The first accurate measurement of the proton angular distribution, to study possible 2-photon contributions The first measurements of strange baryon form factors, if DAΦNE energy will exceed the production threshold (1.15 GeV) As a by-product results: accurate measurement of various cross section, to study narrow unexpected structures, in part pointed out by FENICE and other experiments study of baryon transition FFs, for example N Δ

27 Conclusions No large changes in the DAΦNE machine are needed to increase the energy above the nucleon-antinucleon threshold FINUDA is well suited for the Nucleon FF measurement - good proton efficiency/resolution - neutron detection - easy implementation of a proton polarimeter Possible improvements of the detector for neutron measurement due to the high geometrical flexibility of FINUDA - two (or more) converters - new array of scintillators just before the end-cap - neutron polarimeter

28 Workshop on Nucleon Form Factors Frascati, October, 2005 Jointly organized by INFN and JLab 47 speakers (39 foreigners) Scientific Program Session I : Overview: A. Zichichi (Bologna) The electromagnetic Form Factors of the nucleon D.-O. Riska (Helsinki)The structure of the baryons through their FFs Session II : Form Factors in Space-like region: Experiment Session III: Two-Photon Physics Session IV : Theory Session V : Form Factors in Time-like region: Experiment Session VI : Generalized Parton Distributions Session VII: Parity Violation and Strangeness Concluding Session S. J. Brodsky (SLAC) New Perspectives on the Structure and Interactions of the Nucleon

29 LoI for the Measurement of the Nucleon Form Factors in the time-like region at DAΦNE 21 (27) Institutions, 6 (8) Countries, 56 (>73) people NE (draft) LoI already Signed Interest for LoI 1 INFN-LNF: M. Mirazita, V. Muccifora, A. Fantoni, E. De Sanctis, N. Bianchi, P. Rossi, F. Ronchetti, P. Gianotti, C. Petrascu, R. Baldini, S. Pacetti, L. Benussi, M. Bertani, S. Bianco, F.L. Fabbri, V. Lucherini, F. Pompili, S. Tomassini 2 INFN-Sezione di Torino: D. Calvo, A. Feliciello, A. Filippi 3 Dipartimento di Fisica Sperimentale, Università di Torino: E. Botta, T. Bressani, S. Bufalino, F. De Mori, S. Marcello 4 Dipartimento di Fisica Generale, Università di Torino: L. Busso, D. Faso 5 INAF-IFSI Sezione di Torino: O. Morra 6 Dipartimento di Fisica, Politecnico di Torino: M. Agnello 7 INFN-Sezione di Bari: G. D erasmo, E. Fiore, A. Pantaleo, V. Paticchio 8 INFN-Sezione di Trieste: M. Bregant, N. Grion, G. Margagliotti, S. Piano 9 Dipartimento di Meccanica, Università di Brescia e INFN Sezione di Pavia: G. Bonomi, A. Donzella, A. Zenoni 10 INFN-Sezione di Roma: G. Salmé 11 Università di Roma Tor Vergata: E. Pace 12 DAPNIA/SPhN, CEA-Saclay (France): E. Tomasi-Gustafsson (interest in the polarimeter) 13 JLab-Hall A (USA): K. de Jager, F. Gross, et al. 14 Triangle U. Nuclear lab. and Duke U., Durham, NC (USA) : D. Dutta, H. Gao 15 Center for Theoretical Physics, Yale (USA): F. Iachello 16 Department of Physics, University of Virginia (USA): D. Day, S. Liuti et al. 17 Yerevan Physics Institute, Armenia: N. Akopov, R. Avakian, et al. 18 Petersburg Nuclear Physics Institute of Academy of Science (Russia): S. Belostotsky et al. 19 ITEP, Moscow (Russia): A. Gasparyan, A. Kaidalov, L. Kondratyuk 20 Institute of Nuclear Research, Russian Academy of Sciences, Moscow (Russia): V. Grishina 21 Shanghai Institute for Nuclear Research (China): Wang Xu 22 Budker Institute, Novosibirsk (Russia): S. Skrinsky, E. Solodov, et al. (interest in the of machine magnets) 23 Institute of Nuclear Research, Moscow (Russia): N. Piskunov e Y. Zanevski (interest in the polarimeter) 24 Institut fuer Kernphysik, Mainz U. (Germany): T. Walcher et al. (interest in the polarimeter) 25 Experimental Physics Center, Institue of High Energy Physics, CAS Beijing (China): Y. Xie, H. Hu 26 Shanghai Institute for Nuclear Research (China): Yu-Gang Ma 27 Stanford (USA): S. Brodsky 28 Institut de Physique Nucleaire, Orsay (France): M. Guidal et al. 29 DAPNIA/SPhN, CEA-Saclay (France): M. Garcon, et al. 30 Sao Paulo U (Brazil): A. Deppman, S. Pereira Afanelos, G. De Oleiveira Echeinberg

30 Two-photon contribution Interference between 1- and 2-γ amplitudes Small effect (of the order of α em ) Some asymmetry is expected in proton (antiproton) angular distribution with respect to electron direction σ(θ) σ(180-θ) e - p θ e + A ( θ ) = σ σ ( θ ) σ ( 180 θ ) ( θ ) + σ ( 180 θ ) Could be estimated using: - γ γ p p exp. data - e + e - γ γ cross section Asymmetry ~ few %?

31 Integrated luminosity proton neutron s [GeV 2 ] E beam [MeV] L [pb -1 ] L [pb -1 ] cross (100) section (100) (100) (100) polarization