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: near threshold PS170 (CERN) large q2 E835 (FNAL)

Neutron time-like form factors

FENICE (FRASCATI)

Narrow structure in e+e- hadrons nearNN threshold

Other baryon time-like form factors

Possibilities for PEP-N

THE PHYSICS OF E.M. FORM FACTORS

Small Q2 charge distribution

magnetization current

High Q2 valence quark distribution

Test of QCD from

non perturbative regime (near threshold)

to perturbative regime (large Q2 )

SPACE-LIKE REGION

pepe

Study of the reaction

space-like

Reduced Rosenbluth cross section:

Dipolar behavior and scaling for

0)2/(sin4 2'2 EEQ

222

2

)(

),(ME

rid

GGq

q

d

d

10

22 )/(10|| cGeVQ

2

2

2 ||1/

QGG

pME

22 )/(71.0 cGeV

02 Q

Study of the reactions

Differential cross section

at threshold GE=GM

uniform angular distribution

At Q2 >> 4mp2

GE contribution negligible

NNee

*222

2*222

2

sin)(4

)cos1()(4

QG

s

mQG

sC

dd

Ep

M

TIME-LIKE REGION

ppee 02 sQ

02 Q

QCD asymptotic behaviour

C and free parameters

at large Q2 (QCD, analyticity)

ratio of neutron to proton form factor (QCD)

Any prediction where the Nucleon ismostly represented in terms of valencequarks should hardly foresee

TIME-LIKE REGION

2

2

2

4 ln

QQ

CG

p

M

)()( 22 QGQG

25.0

22

u

d

p

M

n

M

qq

GG

pM

nM GG

PROTON FORM FACTOR (LOW Q2) PS170 exp. (CERN)

Nucl.Phys.B 411 (1994), 3

from threshold

to ECM 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

eepp

PS170 DETECTOR

PROTON FORM FACTOR (LOW Q2)

Steep threshold behavior

ANGULAR DISTRIBUTIONS

pM

pE

G

GIndication of decrease with energy

PROTON FORM FACTOR (HIGH Q2)E835 exp. (FNAL)

Phys. Rev. D60 (1999), 032002

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)

pp

XeeJpp )(/

pp

GeVs 4.49.2

E835 experiment at Fermilab

Ideal for study of form factors as well, through the reaction

High Q2, but cross section still detectable

High luminosity

Efficient reconstruction of e+e- pairs with high invariant mass

Low background level

eepp

E835 DETECTOR

PROTON FORM FACTOR (HIGH Q2)

The dashed line is the QCD fit.

The dot-dashed line represents the dipolebehavior of the form factor in the space-likeregion for the same values of |Q|2.

The expected Q2 behaviour is reached quite early, however there is a factor of 2 betweentimelike and spacelike data measured at thesame |Q|2.

NEUTRON FORM FACTOR FENICE exp. (Frascati)

Nucl.Phys.B 517 (1998), 3

from threshold

to ECM 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

nnee

identification isolated annihilation star +

measurement

detection efficiency 10% at 2 GeV

no signal from neutron required

FENICE DETECTOR

nnee

n n

n s

NEUTRON FORM FACTOR

74 events

The neutron form factor is bigger than that of

the proton!

2Q

nMG

14.0 pbLdt

NEUTRON ANGULAR DISTRIBUTION

nM

nE GG

From the fit of the angular distribution

PROTON FORM FACTOR ANDTOTAL HADRONIC CROSS SECTION

33ee

DM2 data

)(MeVs

MeVnb 20/)(

33 from high energy diffractive photoproduction

FNAL E687

OPEN ISSUES

?

Steep threshold behavior

(related to narrow structure in

e+e- hadrons cross section ?)

All these issues can be solved at PEP-N

pM

nM GG

nM

nE GG

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

the neutron gets enough energy

(through the Lorentz boost) to

produce an hadronic shower

2 hadronic showers easy

to detect in coincidence

1231

max10 scmL

GeVs 1.3

NN 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

s

TIME OF FLIGHT

Angular coverage between 20 and 450 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 Tracking:

measure p and p directions and momenta 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

NN DETECTOR REQUIREMENTS

N N ANGULAR DISTRIBUTIONS

cm on an event by event basis.

cm from lab

TOF (+ momentum measurement for pp)

over-constrained fit with 2 particles

fit to GE/GM

GE/GM SEPARATION

GE=GM and GE=0 cases for 6 weeks of datataking with 30% detection efficiency

top: Ee- = 490 MeV bottom: Ee- = 330 MeV

DATA TAKING TIME

Time in weeks needed for an error of 15% on

|GE/GM|2

CONCLUSIONS

PEP-N has excellent capability in the

time-like form factor field:

orders-of-magnitude better than previous experiments (for the neutron case, all the

existent statistics in less than one day!)

for the first time electric and magnetic form factors will be measured separately

for the first time measurements of time-like form factors for , , …