Transcript DVCS with CLAS - Jefferson Lab

JLab: Probing Hadronic Physics with
Electrons and Photons
Elton S. Smith
Jefferson Lab
V Latinamerican Symposium on
Nuclear Physics
Santos, Brazil, September 2003
Elton S. Smith
Introduction to JLab
The shape of the proton
Pentaquarks
1
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
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V Latinamerican Symposium, Sep 1-5, 2003
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CEBAF @ JLab Today
 Main 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
─ max current
─ e polarization
5.7 GeV
200 mA
80%
 Simultaneous operation in 3 halls
L[cm-2s-1]
─ A: Two High Resolution Spectrometers (pmax=4 GeV/c
1039
─ B: Large Acceptance Spectrometer for e and g induced reactions
1034
─ C: Two spectrometers (pmax= 7 and 1.8 GeV/c) + special equipment 1039
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V Latinamerican Symposium, Sep 1-5, 2003
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CEBAF accelerator site
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Three Experimental End-Stations
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GEp: Electric form factor of the proton
Goal: Determine the charge and current
distributions inside the proton
NY Times “Is a proton round?”
u
u
from G.A. Miller
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d
Naïve expectation is that the
charge and currents are
determined from the spatial
distribution of quark charges
and spins.
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Charge distribution and Form Factors
 (r )   d q F (q ) e
3
(r)
1
 (r ) 



1

F (Q 2 )  
2
2 
1

Q
/



e
0.1

 iq  r
F(Q2)
1
r
3
8p
2
2
0.1
=0.94 GeV
=0.84 GeV
=0.74 GeV
0.01
0
0.2
0.4
0.6
0.8
Radius (fm)
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0.01
0
2
4
6
Q2 (GeV2/c2)
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Decomposition of the elastic cross section
2
  GMp
G
d
 ns
dW
1 
(
2
Ep
 2 cos2 (e / 2) E
 ns  2 4
4 E sin (e / 2) E
2
 2 GMp
tan2 ( e / 2)
)
Q2
 
2
4M p
d  (1   )   2
2
2
R 
 GEp (Q2 )  GMp
(Q )

d W ns 
  {1  2(1  ) tan2 (e / 2) }1
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Proton Form Factors pre-1998
GEp ~
GMp
m
~ GD
1
GD 
Q2 2
1
0.71
(
)
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Spin transfer reaction e p → e p
e’
q
Lˆ
Tˆ
p’
Nˆ
p
e
GEp
GMp
Pt Ee  Ee '

tan(e / 2)
Pl 2 M
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Transport through magnet
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Azimuthal asymmetry in the polarimeter
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GEp from polarization transfer
E93-027, E99-007
Perdrisat, Punjabi, Jones, Brash
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World data for GEp
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Interpretation of new data
F2(Q2) is a spin-flip transition
m
i

q
m
2
P' J P ~ F2 (Q )u ( P' )
u ( P)
2M
In the absence of quark angular momentum
Q2
F2
~ mq M m
 0
q 0
F1
Quark orbital angular momentum
essential to describe data
2


F2 1
Q
2
Q ~ log  2  ~ const
F1 Q
 
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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)
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Hadron multiplets
Mesons qq
3  3  8 1
Baryons qqq
N
3  3  3  10  8  8  1
S
X
W─
Baryons built from meson-baryon basis
+
Q
8  8  27 10 10  8  8 1
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Production and decay of W─ → Xo p─
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What are pentaquarks?
 Minimum quark content is 5-quarks.
 Anti-quark has different flavor than any of 4-quarks
( qqqqQ ).
 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 pentaquark
searches after 1988.
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The Anti-decuplet predicted by Diakonov et al.
udd (uu  ss)
uud (dd  ss)
uus(dd  ss)
dds(uu  ss)
uds(uu  dd  ss)
dss(uu  dd ) uss(uu  dd )
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Reactions on deuterium
g
K─


n ( p)  Q K ( p)
g



K n
Q

 (1520)  K p
*
Q+
n
p
g p (n)  * (1520)K  (n)
K+
g
p
n
n
p
K+
K─
*
p
n
gN  f(1020) N  K+K- N
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V Latinamerican Symposium, Sep 1-5, 2003
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CEBAF Large Acceptance Spectrometer
Torus magnet
6 superconducting coils
Electromagnetic calorimeters
Lead/scintillator, 1296 photomultipliers
Liquid D2 (H2)target +
g start counter; e minitorus
Drift chambers
argon/CO2 gas, 35,000 cells
Gas Cherenkov counters
e/p separation, 256 PMTs
Time-of-flight counters
plastic scintillators, 684 photomultipliers
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Exclusive measurement using gd reactions
CLAS Collaboration (S.
Stepanyan, K. Hicks, et al.),
hep-ex/0307018
 Requires 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
*(1520) events both
nucleons detected with
p > 0.15 GeV/c
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V Latinamerican Symposium, Sep 1-5, 2003
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gd → p K+K─ (n) in CLAS
K-
K+
p
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Kaon times relative to proton
tK  t 
R
; c 
c  c
p2  mK2
pp+p-
ppp-
t (p-K─)
(ns)
p
pK+K─
t (p-K+) (ns)
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Reaction gd→pK+K-(n)

Clear peak at neutron
mass.

15% non pKK events
within ±3 of the peak.

Almost no background
under the neutron peak
after event selection with
tight timing cut.
Reconstructed Neutrons
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Identification of known resonances




Remove events with IM(KK) f(1020) by IM > 1.07 GeV
Remove events with IM(pK) (1520)
Limit K+ momentum due to g dp K Q  phase space pK < 1.0GeV/c
C. Meyer (CLAS note 03-009): checked narrow structure impossible in
gd aK+Y*N aK+(K-N)N,+ KN rescattering
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nK+ invariant mass distribution
Q+
F ( M )  GQ  GBg  P0
Distribution of (1520) events
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Q+: experimental status
Experimental evidence for Q+ 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 4.6.
DIANA collaboration at ITEP (Moscow), April 2003 – peak in the
invariant mass of pK0 at 1.538 GeV, statistical significance 4.4.
CLAS collaboration at JLAB, July 2003 – peak in the invariant
mass of the nK+ at 1.542 GeV, statistical significance 5.3.
SAPHIR collaboration at ELSA (Bonn), August 2003 – peak in the
invariant mass of the nK+ at 1.54 GeV, statistical significance 4.8.
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.
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Summary
 We 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.
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