Nucleon Form Factors in the Timelike Region Diego

Nucleon Form Factors in the Timelike Region Diego Bettoni Istituto Nazionale di Fisica Nucleare, Ferrara Workshop on Nucleon Form Factors Frascati, 12 -14 October 2005

Outline • Introduction – Definitions – Properties – Predictions • Proton Form Factors – Overview of measurements – Main Features • Neutron Form Factors • Form Factor Phases • Outlook Diego Bettoni Timelike Form Factors 2

Introduction p 0 j p J k k Dirac and Pauli Form Factors Diego Bettoni Timelike Form Factors 3

Sachs Form Factors • GE and GM are Fourier transforms of nucleon charge and magnetization density distributions (in the Breit Frame). • Spacelike form factors are real, timelike are complex. • The analytic structure of the timelike form factors is connected by dispersion relations to the timelike regime. • By definition they do not interfere in the expression of the cross section, therefore, in the timelike case, only polarization observables allow to get the relative phase. Diego Bettoni Timelike Form Factors 4

Diego Bettoni Timelike Form Factors 5

C is the Coulomb correction factor, taking into account the QED coulomb interaction. Important at threshold. finite There is no Coulomb correction in the neutron case. Diego Bettoni Timelike Form Factors 6

Form Factor Properties • At threshold GE=GM by definition, if F 1 and F 2 are analytic functions with a continuous behaviour through threshold. GE (4 mp 2) = GM (4 mp 2) • Timelike GE and GM are the analytical continuation of non spin flip and, respectively, spin flip spacelike form factors. Since timelike form factors are complex functions, this continuity requirement imposes theoretical constraints. • Two-photon contribution can be measured from asymmetry in angular distribution. Diego Bettoni Timelike Form Factors 7

Form Factor Properties • Perturbative QCD and analyticity relate timelike and spacelike form factors, predicting a continuous transition and spacelike-timelike equalitity at high Q 2. • At high Q 2 PQCD predicts: • PQCD and analyticity predict: Diego Bettoni Timelike Form Factors 8

Proton Form Factors • The moduli of the Form Factors can be derived from measurements of the cross sections for e+e- pp • Due to the low value of the cross sections and the consequent limited statistics, most experiments could not determine |GM| and |GE| separately from the analysis of the angular distributions, but extracted |GM| using the (arbitrary) assumption |GE| = |GM|. • The magnetic form factor has been derived in this way by many e+eand pp experiments. The timelike electric form factor is basically unknown. • Recently Ba. Bar has attempted to measure |GM|/ |GE| by means of ISR, but the final result is quoted using |GE| = |GM|. Diego Bettoni Timelike Form Factors 9

Proton Magnetic Form Factor |GM| The first experiment to produce a positive result for the proton timelike form factor was carried out at ADONE in Frascati e+e- pp The measurement was based on 0. 2 pb-1 of data at 4. 4 Ge. V 2 yielding 25 events. Diego Bettoni Timelike Form Factors 10

Proton Magnetic Form Factor |GM| The first measurement of the timelike form factors at threshold is due to the ELPAR experiment at CERN. They observed 34 events of pp annihilation at rest in a liquid H 2 target. The measurement assumes |GE|=|GM| Diego Bettoni Timelike Form Factors 11

Proton Magnetic Form Factor |GM| Various measurements of the proton form factors were carried out at DCI in Orsay using e+e- pp The first experiment was DM 1 which recorded 63 events in 4 data points. Diego Bettoni Timelike Form Factors 12

Proton Magnetic Form Factor |GM| At DCI in ORSAY the DM 2 collected data in three data taking runs for a total of 0. 7 pb-1. With a total of 112 events in 6 points they attempted to measure the angular distribution, from which they could fit |GM|/|GE|=0. 34, but |GE|=|GM| was still allowed. Diego Bettoni Timelike Form Factors 13

Proton Magnetic Form Factor |GM| The first high-statistics measurement of the timelike form factors was carried out at LEAR by the PS 170 collaboration. They recorded a total of 3667 pp e+eevents in 9 data points. The angular distribution is compatible with |GE|=|GM|. First indication of steep rise near threshold. Diego Bettoni Timelike Form Factors 14

Proton Magnetic Form Factor |GM| The E 760 experiment at Fermilab produced the first measurement of the form factors at high Q 2 pp e+e. Very difficult measurement due to very small cross section. They recorded 29 events. The measurement assumes |GE|=|GM|. Diego Bettoni Timelike Form Factors 15

Proton Magnetic Form Factor |GM| The FENICE experiment at ADONE, primarily devoted to the measurement of the neutron form factor, produced also a measurement of the proton magnetic form factor with 69 events in 4 points. Diego Bettoni Timelike Form Factors 16

Proton Magnetic Form Factor |GM| E 835 at FNAL, continuation of E 760, made further measurements at high Q 2 with a total of 206 events in 2 data taking runs. Diego Bettoni Timelike Form Factors 17

Proton Magnetic Form Factor |GM| A new measurement at high Q 2 was recently made by the CLEO at CESR in e+e- pp. It assumes |GE|=|GM|. The measurement is based on 14 events. Diego Bettoni Timelike Form Factors 18

Proton Magnetic Form Factor |GM| Another measurement of the proton timelike form factors has been reported by BES. The measurement covers 9 data points from (2. 0 Ge. V)2 to (3. 07 Ge. V)2 using the hypothesis |GE|=|GM|. Diego Bettoni Timelike Form Factors 19

Proton Magnetic Form Factor |GM| Ba. Bar measurement using Initial State Radiation (ISR) e+e- pp Advantages: • All energies at the same time fewer systematics • CMS boost easier measurement at threshold Disadvantages • Luminosity proportional to invariant mass bin L s • More background Diego Bettoni Timelike Form Factors 20

Asymptotic Behavior The dashed line is a fit to the PQCD prediction The expected Q 2 behaviour is reached quite early, however. . . Diego Bettoni Timelike Form Factors 21

Asymptotic Behavior The dashed line is a fit to the PQCD prediction The expected Q 2 behaviour is reached quite early, however. . . there is still a factor of 2 between timelike and spacelike. Diego Bettoni Timelike Form Factors 22

Threshold Q 2 Dependence Steep behavior near threshold observed by PS 170 at LEAR (2000 events). Diego Bettoni Timelike Form Factors 23

Ba. Bar Measurement using ISR Ba. Bar measurement very near threshold confirms steep rise of Form Factor Diego Bettoni Timelike Form Factors 24

Threshold Enhancement observed by BES fitted peak location acceptance weighted BW J/y gpp +3 +5 M=1859 -10 -25 Me. V/c 2 G < 30 Me. V/c 2 (90% CL) c 2/dof=56/56 0 0. 1 0. 2 M(pp)-2 mp (Ge. V) 3 -body phase space Diego Bettoni Timelike Form Factors 0. 3 acceptance 25

Possible Explanations • Tail of a narrow resonance below threshold (baryonium ? ). • Dominance of exchange in pp final state interaction. • Underestimation of the Coulomb correction factor. Possible test for baryonium: a vector meson with very small coupling to e+e- (and relatively small hadronic width), lying on top of a / recurrence, should show up as a dip in some hadronic cross section. Diego Bettoni Timelike Form Factors 26

Resonant Structures The dip in the total multihadronic cross section and the steep variation of the proton form factor near threshold may be fitted with a narrow vector meson resonance, with a mass M 1. 87 Ge. V and a width 10 -20 Me. V, consistent with an N N bound state. Diego Bettoni Timelike Form Factors 27

• Dip observed in 6 diffractive photoproduction by E 687 at Fermilab • New results from Babar expected soon Diego Bettoni Timelike Form Factors 28

Neutron Timelike Form Factor Only one measurement: FENICE at ADONE 80 events The neutron form factor is bigger than that of the proton !!! Diego Bettoni Timelike Form Factors 29

Neutron Angular Distribution (1+cos 2 ) ? isotropic Diego Bettoni Timelike Form Factors 30

Measuring the Phase between GE and GM The relative phase ME between GM and GE can only be measured by means of single- or double-polarization experiments. The polarization Py of the outgoing nucleon normal to the scattering plane is given by: It takes the maximum value near scattering angles of 450 and 1350 and vanishes at 900. Once this phase is known, by measuring the ratio of the two components of the nucleon polarizations in the scattering plane with longitudinally polarized beams, the ratio |GM|/|GE| can be obtained with small systematic uncertainties. Diego Bettoni Timelike Form Factors 31

Future Opportunities • Proton Timelike Form Factors – Ba. Bar, Belle, BES. (Druzhinin, Kuo, Hu) Ba. Bar will quadruple statistics by 2008 – PANDA at GSI will measure pp e+e- up to ~ 20 Ge. V 2 (Rosner) – PAX will use polarization variables to measure the phase of the timelike form factors (Nikolaev) – New measurements of the proton form factors at VEPP-2000 (Serednyakov) • Neutron Timelike Form Factors –. . . • DA NE-2 plans to measure both the proton and the neutron (Mirazita) Diego Bettoni Timelike Form Factors 32

Summary In spite of more than forty years of measurements our knowledge of the timelike nucleon form factors is far from complete. • Proton Form Factors – Only |GM| has been measured. Almost no information on |GE| and phases. – Steep behavior near threshold poses interesting challenge (baryonium, dips in hadronic cross sections. . . ). – Asymptotic Q 2 regime reached quite early, but still far from spacelike. – Ba. Bar data suggest steps rather than smooth behavior. • Neutron Form Factor, measured by a single (low statistics) experiment – |GMn| > |GMp| contrary to expectations – |GMn|>> |GEn| Diego Bettoni Timelike Form Factors 33

Outlook These considerations strongly support the importance of a new measurement of the neutron and proton timelike form factors with much higher statistics than previous work and with the capability of separately determining the electric and magnetic form factors. Near and below the threshold a measurement of the various multihadronic e+e- channels is also of great importance to understand if there are indeed N N bound states. We can look forward to many more years of exciting Form Factor Physics ! Diego Bettoni Timelike Form Factors 34