TIME-LIKE BARYON FORM FACTORS: EXPERIMENTAL SITUATION AND POSSIBILITIES FOR PEP-N Roberto Calabrese Dipartimento di Fisica and I.N.F.N. Ferrara, Italy Working Group E2, SNOWMASS 2001

OUTLINE  Introduction  Proton time-like form factors: è near thresholdPS170 (CERN) è large q 2 E835 (FNAL)  Neutron time-like form factors FENICE (FRASCATI)  Narrow structure in e + e -  hadrons near  NN threshold  Other baryon time-like form factors  Possibilities for PEP-N

THE PHYSICS OF E.M. FORM FACTORS  Small Q 2 è charge distribution magnetization current  High Q 2 è valence quark distribution Test of QCD from non perturbative regime (near threshold) to perturbative regime (large Q 2 )

SPACE-LIKE REGION  Study of the reaction space-like  Reduced Rosenbluth cross section:  Dipolar behavior and scaling for

 Study of the reactions  Differential cross section è at thresholdG E =G M  uniform angular distribution è At Q 2 >> 4m p 2  G E contribution negligible TIME-LIKE REGION

è QCD asymptotic behaviour C and  free parameters è at large Q 2 (QCD, analyticity) è ratio of neutron to proton form factor (QCD) Any prediction where the Nucleon is mostly represented in terms of valence quarks should hardly foresee TIME-LIKE REGION

PROTON FORM FACTOR (LOW Q 2 ) PS170 exp. (CERN) Nucl.Phys.B 411 (1994), 3  from threshold to E CM  2 GeV at LEAR-CERN  Selection of e + e - pairs in high hadronic background  threshold Čerenkov counter + shower detector  Two body reconstruction  tracking system (MWPC, drift tubes)  About 2000 e + e - events above threshold

PS170 DETECTOR

PROTON FORM FACTOR (LOW Q 2 ) Steep threshold behavior

ANGULAR DISTRIBUTIONS Indication of decrease with energy

PROTON FORM FACTOR (HIGH Q 2 ) E835 exp. (FNAL) Phys. Rev. D60 (1999),  E835 study the charmonium spectroscopy in annihilations into electromagnetic final states:  Resonance are scanned by decelerating the antiproton beam.  Data collected in the energy range 1996/97 run (143 pb -1 ) 2000 run (113 pb -1 )

E835 experiment at Fermilab  Ideal for study of form factors as well, through the reaction èHigh Q 2, but cross section still detectable èHigh luminosity èEfficient reconstruction of e + e - pairs with high invariant mass èLow background level

E835 DETECTOR

PROTON FORM FACTOR (HIGH Q 2 ) The dashed line is the QCD fit. The dot-dashed line represents the dipole behavior of the form factor in the space-like region for the same values of |Q| 2. The expected Q 2 behaviour is reached quite early, however there is a factor of 2 between timelike and spacelike data measured at the same |Q| 2.

NEUTRON FORM FACTOR FENICE exp. (Frascati) Nucl.Phys.B 517 (1998), 3  from threshold to E CM  2.5 GeV at ADONE (Frascati)  Antineutron annihilation in nuclei: many prong event (“star topology”)  iron converters + limited streamer tubes (tracking)  Low antineutron velocity  hodoscopes for TOF measurement  Low luminosity  shield against cosmic ray background

 identification  isolated annihilation star + measurement  detection efficiency 10% at 2 GeV  no signal from neutron required FENICE DETECTOR

NEUTRON FORM FACTOR 74 events The neutron form factor is bigger than that of the proton!

NEUTRON ANGULAR DISTRIBUTION From the fit of the angular distribution

PROTON FORM FACTOR AND TOTAL HADRONIC CROSS SECTION

DM2 data from high energy diffractive photoproduction FNAL E687

OPEN ISSUES è è ? è Steep threshold behavior (related to narrow structure in e + e -  hadrons cross section ?) All these issues can be solved at PEP-N

OTHER BARYON FORM FACTOR MEASUREMENTS , ,   -N,  0 -  transition form factors  No data exists (only  form factor with poor statistical accuracy)  Flavor symmetry relates the hyperon form factors to those of the nucleons  accurate prediction of flavor- symmetry breaking as test of QCD

MEASUREMENT OF NUCLEON TIME-LIKE FORM FACTORS AT PEP-N  High luminosity  few hundred events/day  Asymmetric e + e - collisions up to èthe neutron gets enough energy (through the Lorentz boost) to produce an hadronic shower  2 hadronic showers easy to detect in coincidence

N  N IDENTIFICATION AND MEASUREMENT  Angular Correlation  Time-of-flight to identify events and reject prompt photons and other fast backgrounds  Momentum analysis for  pp  Calorimetric measurement

KINEMATICS FOR PEP-N  Increasing larger spread in nucleon momenta larger spread in lab angles

TIME OF FLIGHT

 Angular coverage between 2 0 and 45 0 in polar angle.  Detection of nucleons and antinucleons between 0.5 and 2.5 GeV/c with good efficiency.  Momentum analysis for p and  p. Tracking device + Calorimeter è Tracking: measure p and  p directions and momenta è Calorimeter: efficient for both neutrons and antineutrons; TOF and position of both n and  n.  Distance L between detector face and interaction point: TOF and angular resolution vs acceptance N  N DETECTOR REQUIREMENTS

N  N ANGULAR DISTRIBUTIONS   cm on an event by event basis.   cm from  lab  TOF (+ momentum measurement for p  p)  over-constrained fit with 2 particles  fit to G E /G M

G E /G M SEPARATION G E =G M and G E =0 cases for 6 weeks of data taking with 30% detection efficiency top: E e - = 490 MeV bottom: E e - = 330 MeV

DATA TAKING TIME Time in weeks needed for an error of 15% on |G E /G M | 2

CONCLUSIONS  PEP-N has excellent capability in the time-like form factor field: èorders-of-magnitude better than previous experiments (for the neutron case, all the existent statistics in less than one day!) èfor the first time electric and magnetic form factors will be measured separately èfor the first time measurements of time- like form factors for , ,  …