JLab: Probing Hadronic Physics with Electrons and Photons › ... › Elton_Latin_Sym_2003.pdfElton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 6 G Ep: Electric form factor of

JLab: Probing Hadronic Physics with Electrons and Photons

Elton S. SmithJefferson Lab

V Latinamerican Symposium on Nuclear Physics

Santos, Brazil, September 2003

Introduction to JLabThe shape of the protonPentaquarks

1Elton S. Smith

2Elton S. Smith V Latinamerican Symposium, Sep 1-5, 2003

Why use electron and photon probes?

Electromagnetic interaction is well-known

e e’

p p

F(Q2)

Elastic Form Factors Inelastic transitions

Size probed ~ 1/√Q2


CEBAF @ JLab TodayMain physics programs─ nucleon electromagnetic form factors (including strange form factors)─ N → N* electromagnetic transition form factors─ spin structure functions of the nucleon─ form factors and structure of light nuclei

Superconducting recirculating electron accelerator─ max. energy 5.7 GeV─ max current 200 µA─ e polarization 80%

Simultaneous operation in 3 halls L[cm-2s-1]─ A: Two High Resolution Spectrometers (pmax=4 GeV/c 10

39

─ B: Large Acceptance Spectrometer for e and γ induced reactions 1034

─ C: Two spectrometers (pmax= 7 and 1.8 GeV/c) + special equipment 1039


CEBAF accelerator site


Three Experimental End-Stations

High Resolution SpectrometersElectron or Hadron Arm

Detector

Q2Q1Dipole

Q3

53 m

30 m

HALL C

46 m

HALL B

HALL A

High MomentumSpectrometer

Short OrbitSpectrometer

Pair of identical High ResolutionSpectrometers (HRS2)

CEBAF's Large Acceptance Spectrometer(CLAS) and Bremsstrahlung Photon Tagger

High Momentum Spectrometer (HMS)and Short Orbit Spectrometer (SOS)


GEp: Electric form factor of the proton

Goal: Determine the charge and current distributions inside the proton

NY Times “Is a proton round?”

from G.A. Miller

u

u d

Naïve expectation is that the charge and currents are determined from the spatial distribution of quark charges and spins.


Charge distribution and Form Factors

0 0.2 0.4 0.6 0.8 10.01

0.1

1

0 2 4 60.01

0.1

1

∫ ⋅−= rqieqFqdrrr

)()( 23ρρ(r) F(Q2)

2

222

/11)( ⎟⎟

⎠

⎞⎜⎜⎝

⎛Λ+

=Q

QF

Λ=0.94 GeVΛ=0.84 GeVΛ=0.74 GeV

rρ =)(re Λ−Λ

π8

3

Radius (fm) Q2 (GeV2/c2)


Charge Distributions and Form Factors

1)( 2 =QF

rrf e Λ−Λ=π8

)(3 2

222

/11)( ⎟⎟

⎠

⎞⎜⎜⎝

⎛Λ+

=Q

QF

)()( orrrf −= δ

rr

rf e Λ−Λ=π4

)(2

222

/11)(

Λ+=

QQF

⎟⎠⎞

⎜⎝⎛ Λ−

⎟⎟⎠

⎞⎜⎜⎝

⎛ Λ=

2221

2)(

2 rrf e

π

unitypoint

exponential dipole

Yukawa pole

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛Λ−

=

2/221

)( 2Q

QF eGaussian Gaussian


Decomposition of the elastic cross section

)( )2/(tan21

2222

eMpMpEp

ns GGG

dd ϑτ

ττ

σσ +++

=Ω

2

2

4 pMQ

=τEE

E ee

ns′

=)2/(sin4)2/(cos

42

22

ϑϑασ

)()()1( 2222 QGQGdd

MpEpns

R +=+

Ω=

τε

τσετσσ

12 })2/(tan)1(21{ −++= eϑτε


Proton Form Factors pre-1998

DMp

Ep GG

G ~~µ

271.0

1

1

)( 2QGD

+=


Spin transfer reaction e p → e p

e

e’

p

p’

N̂

T̂ L̂θ

)2/tan(2

' ϑM

EEPP

GG ee

l

t

Mp

Ep +−= e


Transport through magnet


Azimuthal asymmetry in the polarimeter


GEp from polarization transfer

E93-027, E99-007 Perdrisat, Punjabi, Jones, Brash


World data for GEp


Interpretation of new data

F2(Q2) is a spin-flip transition

)(2

)'()(~' 22 PuMqiPuQFPJP ↓↑↓↑

αµν

µ σ

In the absence of quark angular momentum

0~ 01

22 ⎯⎯ →⎯ →qmq MmFFQ

Quark orbital angular momentum essential to describe data

constQQF

FQ ~log1~ 22

2

1

2⎟⎟⎠

⎞⎜⎜⎝

⎛Λ


Pentaquark: Baryon with five quarks

Goal: Determine quark content of colorless hadrons

Expectation from the quark model is that the properties of baryons are determined by three valence quarks (qqq)


Hadron multiplets

Mesons qq1833 ⊕=⊗

Baryons qqq

18810333 ⊕⊕⊕=⊗⊗

18810102788 ⊕⊕⊕⊕⊕=⊗

Baryons built from meson-baryon basis Ω─

Θ+

N

Σ

Ξ


Production and decay of Ω─→ Ξο π─


What are pentaquarks?Minimum quark content is 5-quarks.Anti-quark has different flavor than any of 4-quarks ( ).Quantum numbers can not be defined by 3-quarks.

General idea of a five-quark states has been around since late 60’s.However, searches did not give any conclusive results.PDG dropped the discussion on pentaquarksearches after 1988.

Qqqqq


The Anti-decuplet predicted by Diakonov et al.

( )uud dd ss+

( )uus dd ss+

( )udd uu ss+

( )dds uu ss+

( )dss uu dd+ ( )uss uu dd+

( )uds uu dd ss+ +


Reactions on deuterium

n

γK─

K+

nΘ+

p p

p

γK+

K─

pΛ∗

n npK

nKnp

nKΘ+pΘ+pn

−

+

+

K−

→Λ

Λ→

→

→

)1520()()1520()(

)()(

*

*γ

γ

γN → φ(1020) N → K+K- N


CEBAF Large Acceptance Spectrometer

Liquid D2 (H2)target +γ start counter; e minitorus

Torus magnet6 superconducting coils

Drift chambersargon/CO2 gas, 35,000 cells

Time-of-flight countersplastic scintillators, 684 photomultipliers

Gas Cherenkov counterse/π separation, 256 PMTs

Electromagnetic calorimetersLead/scintillator, 1296 photomultipliers


Exclusive measurement using γd reactions

CLAS Collaboration (S. Stepanyan, K. Hicks, et al.), hep-ex/0307018Requires FSI – both nucleons involved─ No Fermi motion

correction necessary─ FSI puts K- at larger lab

angles: better CLAS acceptance

─ FSI not rare: in ~50% of Λ∗(1520) events both nucleons detected with p > 0.15 GeV/c


γd → p K+K─ (n) in CLASK- K+

p


Kaon times relative to proton

ppπ- pπ+π-

22;

Kc

cK

mpp

cRtt

+=

⋅−=∆ β

β

∆t (p-K─)(ns)

∆t (p-K+) (ns)

pK+K─


Reaction γd→pK+K-(n)

Clear peak at neutron mass.

15% non pKK events within ±3σ of the peak.

Almost no background under the neutron peak after event selection with tight timing cut.

MM(pK+K-) [ GeV/c2 ]

Eve

nts

MM(pK+K-) [ GeV/c2 ]

Eve

nts

|∆tpK| ≤ 0.75 ns

0

100

200

300

400

500

0.8 0.85 0.9 0.95 1 1.05 1.1 1.15 1.2 1.25

0

50

100

150

200

250

300

350

0.8 0.9 1 1.1

Reconstructed Neutrons


Identification of known resonances

Remove events with IM(K+K−) φ(1020) by IM > 1.07 GeVRemove events with IM(pK−) Λ(1520)Limit K+ momentum due to γ d p K− Θ + phase space pK+ < 1.0GeV/cC. Meyer (CLAS note 03-009): checked narrow structure impossible inγd K+Y*N K+(K-N)N,+ KN rescattering


nK+ invariant mass distribution

MM(nK+) [ GeV/c2 ]

Eve

nts

NΘ = 43

MΘ = 1.542 GeV

σΘ = 0.009 GeVNΘ/√NBg = 5.8σ

0

5

10

15

20

25

30

35

1.5 1.6 1.7 1.8 1.9

0)( PGGMF Bg ++= +Θ

Distribution of Λ(1520) events

Θ+


Θ+: experimental statusExperimental evidence for Θ+ have been reported at four laboratories.

─ LEPS collaboration at Spring-8 (Japan), January 2003 - peak in the invariant mass of the nK+ at 1.54 GeV with statistical significance of 4.6σ.

─ DIANA collaboration at ITEP (Moscow), April 2003 – peak in the invariant mass of pK0 at 1.538 GeV, statistical significance 4.4σ.

─ CLAS collaboration at JLAB, July 2003 – peak in the invariant mass of the nK+ at 1.542 GeV, statistical significance 5.3σ.

─ SAPHIR collaboration at ELSA (Bonn), August 2003 – peak in the invariant mass of the nK+ at 1.54 GeV, statistical significance 4.8σ.

All experiments observe a narrow width.Spin, isospin and parity not yet established.Subject of intense interest and research.

─ Penta-Quark 2003 Workshop at JLab in November.


SummaryWe have presented two examples which highlight the physics program at Jefferson Lab.The electromagnetic interaction can be used to probe deep into the structure of nucleons.─ From measurements of GEp up to a Q2 = 5.6 GeV2 we

have gained new insights into the shape of the proton.─ Orbital angular momentum of quarks is a key ingredient in

our understanding of proton structure.A key question in non-perturbative QCD is the structure of hadrons─ We have presented evidence for an exotic baryon with

S = +1, which would have a minimal quark content of five quarks (uudds).

─ This baryon represents a new class of colorless hadrons.


Scaled F2/F1 ratio

JLab: Probing Hadronic Physics with Electrons and PhotonsWhy use electron and photon probes?CEBAF @ JLab TodayCEBAF accelerator siteThree Experimental End-StationsGEp: Electric form factor of the protonCharge distribution and Form FactorsCharge Distributions and Form FactorsDecomposition of the elastic cross sectionProton Form Factors pre-1998Spin transfer reaction e p → e pTransport through magnetAzimuthal asymmetry in the polarimeterGEp from polarization transferWorld data for GEpInterpretation of new dataPentaquark: Baryon with five quarksHadron multipletsProduction and decay of W─ → Xo p─What are pentaquarks?The Anti-decuplet predicted by Diakonov et al.Reactions on deuteriumCEBAF Large Acceptance SpectrometerExclusive measurement using gd reactionsgd → p K+K─ (n) in CLASKaon times relative to protonReaction gd→pK+K-(n)Identification of known resonancesnK+ invariant mass distributionQ+: experimental statusSummaryScaled F2/F1 ratio

Documents

JLab: Probing Hadronic Physics with Electrons and Photons › ... › Elton_Latin_Sym_2003.pdfElton S. Smith V Latinamerican Symposium, Sep 1-5, 2003 6 G Ep: Electric form factor of