Proposta per la Misura deiFattori di Forma del nucleone a

DAFNE

Riunione Commissione Nazionale Scientifica INapoli, 19 settembre 2005

Nicola BianchiINFN – Laboratori Nazionali di Frascati

FFs are fundamental quantities describing the internal structure of the nucleonWavelength of the probe can be tuned by selecting momentum transfer Q2

< 0.1 GeV2 integral quantities (charge radius,…)0.1-10 GeV2 internal structure of nucleon> 20 GeV2 pQCD scaling

• Stern (1932) measured the proton magnetic moment indicating thatthe 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 Rosenbluthseparation with limited accuracy for GE

p for Q2>1 GeV2

• Early interpretation based on Vector-Meson Dominance• Good description with phenomenological dipole form factor

Why a new measurement of the Nucleon e.m. Form Factors today? (1)

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:

GMp ≈ μp GE

p

⇒ charge and magnetization have the same distribution- Neutron electric SL FF GE

n small and badly measured - All 3 non-zero FFs are described ≈ 20% by the dipole formula

∑+=

≈Λ⎟⎟⎠

⎞⎜⎜⎝

⎛+ΛΛ

=

nn

D

i

D

QaGG

GeV.Q

G

1

802

22

2

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

The new SL JLab results (2000) using polarization measurement in epelastic scattering dramatically changed the picture of the Nucleon:

- GEp/GM

p decreases with Q2

- data suggest GEp crosses 0 at Q2 ≈8 GeV2

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

Charge and magnetization distributions are different (“second proton spin crisis”)

Why a new measurement of the Nucleon e.m. Form Factors today? (3)

Nucleon Form Factors – general properties• FFs are analytic complex functions of q2 = (p – p’)2

• 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) + τ F2(q2) τ=q2/4M2

• 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 (q2 > 0) and Time-Like (q2 < 0) regions

• FFs are connected with GPDs

How to measure Space-Like Form Factors• Rosenbluth separationBased on cross section measurement

Q2 = |q2 |ε = photon polarizationτ = Q2/ 4M2

• Polarization observablesFor example, electron-to-proton polarization transfer

2tan

2' θ

MEE

PP

GG ee

l

t

M

E +−=

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)

⎥⎦⎤

⎢⎣⎡ +=⎟

⎠⎞

⎜⎝⎛

Ω 22

0 ME GGdd

ετσσ

JLabHad to wait for the new generation of beams and detectors

polarization

Rosenbluth

Proton SL Form Factors

• GE =0 at some Q2 (~8 GeV2)? • Asymptotic scaling: pQCD F2 / F1 ~ Q-2

pol. data F2 / F1 ~ Q-1

quark angular momentum contribution? -> HERMESdata of non-zero Sivers asymmetries

• radiative corrections?

JLab 2000-2

JLab 2005

SLAC re-analysis

Rosenbluthtechnique

polarizationtechnique

A.Afanasev and C.CarlsonPRL 94 (2005) 2123012-photon exchange

- 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

Outlook in the Outlook in the spacespace--likelike regionregion at at JLabJLab• New measurements of elastic scattering planned at JLab for higher Q2

• Study of 2-photon contribution comparing e-p and e+p scattering

run in 2006

TimeTime--Like Like FFsFFs measurementsmeasurements

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

• |GM| can be more easily extracted, but it’s model-dependent • |GE| remains unmeasured

• |GM| = |GE| at the physical threshold s = 4M2 ⇒ isotropic distributions

• GM dominates the cross section for s >> 4M2

( )⎭⎬⎫

⎩⎨⎧ ++=

Ωθ

τθβασ 222

2 114

sin cos 2EM GG

sC

dd

FF extraction from e+e-→N N τ = s/ 4M2

TimeTime--Like Like FFsFFs : proton data: proton data

GM from cross section and with the assumption |GE| = |GM|

• Early pQCD scaling |GM| ~ Q-4

• Time-like FF larger than space-like• Steep behaviour close threshold

ElectricElectric toto magneticmagnetic FF ratioFF ratio

Different hypothesis on GE/GMstrongly affect also GM extraction in the low energy region

DR analysis

Tentative extraction of FF ratio from angular distributions

DAΦNE2

TimeTime--Like Like FFsFFs : neutron data: neutron data

Only the FENICE data

• Angular distribution dσ/dΩ ~ 1+cos2θcompatible with |GE|=0

• neutron GM bigger than proton

• pQCD scaling?

Assuming |GE| = 0

Hyperon Form FactorsHyperons can also be produced in e+ e- interactions (Λ, Σ, …)

energy threshold: √s ~ 2 MΛ ~ 2.23 GeV ⇒ EBEAM~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

Proposal for nucleon Proposal for nucleon FFsFFs measurement at DAmeasurement at DAΦΦNENE

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 AccelerartorProspects” – 10-13 September 2003, Alghero (A. Filippi with FINUDA; C. Ligi, R. Ricci, G. Benedetti for the machine)

- LNF Spring School – 16-20 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 fora Nucleon FF experiment at Frascati” – DAΦNE Technical Note G-63

DAΦNE parameters for 1.2 GeV operation

Main machine changes: - dipole magnets (normal conducting)- interaction region

The FINUDA detector

remove nuclear targetscarbon layer:

• antineutron converter• proton polarimeter

√s = 1890 MeV, B = 0.2 T

e+e-→nn with FINUDA: typical topology

√s = 1890 MeV, B = 0.2 T

e+e-→pp with FINUDA: typical topology

Efficiency and resolution for pp tracks

E=1.0 GeV

E=1.1 GeV

E=1.2 GeV

B=10 kgauss( )⎭⎬⎫

⎩⎨⎧ ++=

Ωθ

τθβασ 222

2 114

sin cos 2EM GG

sC

dd

GE is better measured around 90o

FINUDA projected accuracy

Integrated luminosity ≈ 700-1000 pb-1

(cfr KLOE in 2004: 780 pb-1)

prototne

proton neutron

cfr FENICE 1029cm-2 s-1

Angular distributions

neutron projected data

red: GE=0

black: GE=GM

Fitting functionf(θ)=A(1+cos2θ) + B sin2θ

Induced polarization

• non negligible polarization • Py maximal at 45° and 135° • high discriminating power between theories

• extraction of FF relative phase

( ) ( )( ) ( )

M

E

MEN

GG

P

=

++

−=

ρ

θτρθ

δδθρτσ 220 1

21

sincos

cossinPolarization normal to the scattering planeNo beam polarization

Proton polarimeter

( ) ( ) ( )[ ]φφθθσσ sincos ,, xy PPTATdd

−+=Ω

10

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

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

• in-plane polarization components can be measured

• non-normal polarimetercrossing and/or magnetic field require back-tracing of the measured polarization

• small efficiency to reject multiple scattering background

Polarization measurement

( )polyC PANd

ddNN 2000 ±=Ω

= ∫±Δ

± πσφσ

φ

Polarization is extracted by measuring asymmetriesy

φ+-x

For example, for Pypol(∝ cosφ)

polyC PA

NNNNR

π2

=+−

=−+

−+

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)

SummarySummary

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

• The first accurate and independent measurement of the time-like 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)

The increase of DAΦNE2 energy above NN threshold would allow:

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

Workshop on Nucleon Workshop on Nucleon Form FactorsForm Factors

Frascati, 12-14 October, 2005

Jointly organized by INFN and JLab47 speakers (39 foreigners)

Scientific ProgramScientific ProgramSession I : Overview:

A. Zichichi (Bologna) The electromagnetic Form Factors of the nucleonD.-O. Riska (Helsinki)The structure of the baryons through their FFs

Session II : Form Factors in Space-like region: ExperimentSession III: Two-Photon Physics Session IV : Theory Session V : Form Factors in Time-like region: ExperimentSession VI : Generalized Parton DistributionsSession VII: Parity Violation and StrangenessConcluding Session

S. J. Brodsky (SLAC) New Perspectives on the Structure and Interactions of the Nucleon

http://www.lnf.infn.it/conference/nucleon05/

LoILoI for the Measurement of the Nucleon Form for the Measurement of the Nucleon Form Factors in the timeFactors in the time--like region at like region at DADAΦΦNENE (draft)(draft)

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. Filippi3 Dipartimento di Fisica Sperimentale, Università di Torino: E. Botta, T. Bressani, S. Bufalino, F. De Mori, S.

Marcello4 Dipartimento di Fisica Generale, Università di Torino: L. Busso, D. Faso5 INAF-IFSI Sezione di Torino: O. Morra6 Dipartimento di Fisica, Politecnico di Torino: M. Agnello7 INFN-Sezione di Bari: G. D’erasmo, E. Fiore, A. Pantaleo, V. Paticchio8 INFN-Sezione di Trieste: M. Bregant, N. Grion, G. Margagliotti, S. Piano9 Dipartimento di Meccanica, Università di Brescia e INFN Sezione di Pavia: G. Bonomi, A. Donzella, A.

Zenoni10 INFN-Sezione di Roma: G. Salmé11 Università di Roma Tor Vergata: E. Pace12 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. Gao15 Center for Theoretical Physics, Yale (USA): F. Iachello16 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. Kondratyuk20 Institute of Nuclear Research, Russian Academy of Sciences, Moscow (Russia): V. Grishina21 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. Hu26 Shanghai Institute for Nuclear Research (China): Yu-Gang Ma27 Stanford (USA): S. Brodsky28 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

21 (27) Institutions, 6 (8) Countries, 56 (>73) people

Interestfor LoI

LoIalready Signed

TwoTwo--photon contributionphoton contributionInterference between 1- and 2-γ amplitudesSmall effect (of the order of αem)

Could be estimated using:- γ γ → p p exp. data- e+ e- → γ γ cross section

Asymmetry ~ few % ?

Some asymmetry is expected in proton (antiproton) angular distribution with respect to electron direction

σ(θ) ≠ σ(180-θ)

e+e-

p

θ

( ) ( ) ( )( ) ( )θσθσ

θσθσθ−+−−

=180180A

Integrated luminosity

proton neutron

5.55 1180 2500 - polarization

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

3.55 942 300 300 cross4.05 1010 100 (100) section4.55 1060 100 (100)5.05 1120 100 (100)5.55 1180 100 (100)