Transcript Experimental Status of Pentaquark States Phys.Rev.Lett. 91 (2003) 012002      Introduction Experimental evidence Production mechanisms What do we know about the Q+? Exotic cascades states Special Thanks CLAS Collaborators SPring-8 uudds Mass =

Experimental Status of
Pentaquark States
Phys.Rev.Lett. 91 (2003) 012002
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Introduction
Experimental evidence
Production mechanisms
What do we know about the Q+?
Exotic cascades states
Special Thanks
CLAS Collaborators
SPring-8
uudds
Mass = 1.54 GeV
Elton S. Smith / APS Meeting / Denver May 1-4, 2004
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Quarks are confined inside colorless hadrons
Quarks combine to “neutralize” color force
q
q
q
q
q
Mystery remains:
Of the many possibilities for combining quarks
with color into colorless hadrons, only two
configurations were found, till now…
Elton S. Smith
APS, Denver
May 1-4, 2004
2
What are pentaquarks?
 Minimum quark content is 4 quarks and 1 antiquark
 “Exotic” pentaquarks are those where the antiquark has a
different flavor than the other 4 quarks qqqqQ
 Quantum numbers cannot be defined by 3 quarks alone.


Example: uudss, non-exotic
Baryon number = 1/3 + 1/3 + 1/3 + 1/3 – 1/3 = 1
Strangeness = 0 + 0 + 0 − 1 + 1 = 0
Example: uudds, exotic
Baryon number = 1/3 + 1/3 + 1/3 + 1/3 – 1/3 = 1
Strangeness = 0 + 0 + 0 + 0 + 1 = +1
Elton S. Smith
APS, Denver
May 1-4, 2004
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Pentaquarks – two approaches
Chiral soliton model: (Diakonov,
Petrov, Polyakov)
Pentaquark comes out
naturally from these models
as they represent rotational
excitations of the soliton
[rigid core (q3) surrounded
by meson fields (qq)]
Quark cluster models, e.g.
di-quark description (Jaffe, Wilczek)
(ud)
L=1
s
(ud)
Soliton:
(simplified)
L=1, one unit of orbital angular
momentum needed to get
J=1/2+ as in cSM
Meson
fields
Lattice QCD => JP = 1/2Elton S. Smith
APS, Denver
May 1-4, 2004
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The Anti-decuplet of SU(3)f
Ten observations
Null Results?
X5−−
X50
One experiment
D. Diakonov, V. Petrov, hep-ph/0310212
Elton S. Smith
APS, Denver
May 1-4, 2004
(revised version)
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Experimental Evidence
 Many experiments
 No dedicated experiments to date
─ but,… dedicated experiments are starting to take data
For new data from SPring-8 see Hicks Session J2 Sun 10:45
 Walk through the analysis from CLAS
 Selected examples from other experiments
Elton S. Smith
APS, Denver
May 1-4, 2004
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Quark lines for the reaction
g
us 
us K+
Q+
n
K−
 ddu
ddu  n
Q+ is composed of (uudds) quarks
Elton S. Smith
APS, Denver
May 1-4, 2004
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Production mechanisms
g
n
p
K+
n (p)
K+(K0)
Q+
pspec
Control Reactions
K+
g
g
n
p
K−
K−
S−
n
p−
pspec
Elton S. Smith
p
n
APS, Denver
K+
K−
L*1520
p
K−
nspec
May 1-4, 2004
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JLab accelerator CEBAF
Continuous Electron Beam
• Energy 0.8 ─ 5.7 GeV
• 200 mA, polarization 75%
• 1499 MHz operation
• Simultaneous delivery 3 halls
Elton S. Smith
APS, Denver
May 1-4, 2004
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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
Elton S. Smith
APS, Denver
May 1-4, 2004
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gd → p K+K─ (n) in CLAS
K+
K-
p
Elton S. Smith
APS, Denver
May 1-4, 2004
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Particle identification by time-of-flight
m
Elton S. Smith
APS, Denver
May 1-4, 2004
p
g c
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Reaction gd→pK+K-(n)

Clear peak at neutron
mass.

15% non-pKK events
within ±3s of the peak.

Almost no background
under the neutron peak
after event selection with
tight timing cut.
Reconstructed Neutrons
Elton S. Smith
APS, Denver
May 1-4, 2004
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Deuterium: nK+ invariant mass distribution
Q+
NQ = 43 events
Mass = 1.542 GeV
< 21 MeV
Significance 5.2±0.6 s
Two different
Background shapes
Distribution of
L*(1520) events
Elton S. Smith
APS, Denver
May 1-4, 2004
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Searching for Q+ on a proton target
gp→p+K- K+ (n)
Prominent K*0
no cuts
K*0
M(nK+) [GeV]
Elton S. Smith
APS, Denver
May 1-4, 2004
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Searching for the Q+ on a proton target
gp→p+K-K+(n)
Eg = 3 – 5.5 GeV
g
Cosq*(p+) > 0.8
p−
Q+
p
7.8s
p+
n
Q+
N*
K−
K+
M=1555±10 MeV
< 26 MeV
Cosq*(p+) > 0.8
Cosq*(K+) < 0.6
cut
CLAS Collaboration
PRL 92, 032001-1 (2004).
M(nK+) [GeV]
Elton S. Smith
APS, Denver
May 1-4, 2004
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Q+ ─ N* production mechanism?
g
cuts outside
outside
cuts Q+
p−
p
N* ?
N*
p+
n
Q+
K−
K+
 What do p-p scattering
data say?
M(nK+K−) [GeV]
Elton S. Smith
 p-p cross section data in PDG
have a gap in the mass range
2.3–2.43 GeV.
APS, Denver
May 1-4, 2004
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Diffractive mechanism?
K-
g
K*0
p+
Q+
p
Require forward K0*
Cos q*(K−p+) > 0.5
Elton S. Smith
APS, Denver
May 1-4, 2004
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Q+ NOT produced in association with K*0
K*0 events
Non K*0 events
Elton S. Smith
APS, Denver
May 1-4, 2004
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HERMES
e+d→(pKs0) X
Airapetian et al.
Hep-ph/0312044
27.6 GeV positrons
M=1528±3.3 MeV
 ~ 17±9 MeV
Mass (K0p) [GeV]
Elton S. Smith
APS, Denver
May 1-4, 2004
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New results from COSY-TOF
COSY-TOF hep-ex/0403011
pp → S+ K0 p
M=1530±5 MeV
< 18 MeV
s ~ 0.4±0.1 mb
2.95 GeV
M(K0p)
GeV/c2
The TOF spectrometer at the COSY facility in Juelich, Germany found
evidence for the Q+ in the reaction: p + p → S+ + Q+.
Elton S. Smith
APS, Denver
May 1-4, 2004
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From ZEUS at DESY…
ZEUS hep-ex/0403051
ep →eKs0p X
M=1521.5±1.5 MeV
~ 8±4 MeV
√s ~ 310 GeV
Q2 > 20 GeV2
Mass (K0p) [GeV]
Elton S. Smith
APS, Denver
May 1-4, 2004
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What do we know about this S=+1 state?
1.57
Mass (GeV)
1.56
1.55
LEPS
: gC→(nK+) K−X
DIANA
: K+Xe→(pK0) X
CLAS-d
: gd→(nK+) K−p
CLAS-p
: gp→(nK+) p+K−
SAPHIR : gp→(nK+) K0
1.54
ITEP
: n d,Ne→(pK0) K0
1.53
HERMES : e+d→(pK0) X
1.52
COSY-TOF: pp→(pK0) S+
1.51
0
10
20
Upper limit or estimate
of  (GeV)
Elton S. Smith
ZEUS
: ep→e (pK0) X
SVD-2
: pA→(pK0) X
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( ) Strangeness undetermined
Decay products in parenthesis
APS, Denver
May 1-4, 2004
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Search for pentaquarks in HERA-B
HERA-B hep-ex/0403020
L*(1520)
Q+ ?
M(pK0s) (GeV)
M(pK−) (GeV)
pA →K0p X
Null result at HERA-B
Q+(1540)
√s ~ 41.6 GeV
L*(1520)
Elton S. Smith
APS, Denver
< 0.02
May 1-4, 2004
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There is much more to learn
 Spin, parity
─ Chiral soliton model predicts Jp=½+ (p-wave)
─ Quark model naïve expectation is Jp=½− (s-wave)
─ Lattice calculations predict Jp=½−
 Isospin
─ Likely I=0, since searches for pK+ partners unsuccessful
 Width (lifetime)
─ Measurements mostly limited by experimental resolution.
─ Theoretical problem remains why the state is so narrow.
─ Analysis of existing K+d scattering data indicate that
 < 1-2 MeV.
 Complete determination of the pentaquark multiplet
Elton S. Smith
APS, Denver
May 1-4, 2004
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A new cousin: observation of exotic X5−−
X5−−
ssd du
M=1.862± 0.002 GeV
ssddu
X50
ssd ud
Q+
X(1530)
ssd ud
X5−−
X5 0
ssd du
NA49 CERN SPS
Phys. Rev. Lett. 92 (2004) 042003
Elton S. Smith
APS, Denver
May 1-4, 2004
26
But, can it be reproduced??
HERA-B hep-ex/0403020
X(1530)
X−p+ + X+p−
X−p− + X+p+
Mass (GeV/c2)
 HERA-B collaboration at DESY (Germany)
 Null result with much higher statistics!
 They also have a null result for the Q+
Elton S. Smith
APS, Denver
May 1-4, 2004
27
Predictions depend on dynamics
Decay modes are sensitive
to dynamical picture
Number of states and
mass spectra
Ss
X X5
Ns
X5
BR(X5 → K− S−)
BR(X5 → p− X−)
S
N
LS
Q
BR(X5 → p0 X−)
BR(X5 → p− X0)
Q
N
Di-quark
Soliton
Note: Models adjusted to experiment
Elton S. Smith
APS, Denver
May 1-4, 2004
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Search for exotic cascades X5−− and X5−
Experiment 04-10, scheduled to run this fall
X −− → p− X−
L → p− p
X − → p− L
Electron
beam
gn→K+K+X5−−
X− ct = 4.9 cm
L ct = 7.9 cm
Two decay vertices,
Negatives bend outwards
Elton S. Smith
APS, Denver
May 1-4, 2004
<g> ~ 1.5
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Current activities at Jlab
 Pentaquark experiments in Hall B
─ g10 (currently taking data) gd → Q+ Eg~1 – 3.5 GeV
─ g11 (starts in mid-May)
gp → Q+ Eg~1 – 3.5 GeV
─ eg3 (November)
gvd→ X5−−, X5− Eg> 3.9 GeV
─ High energy data
gp → Q+, X5 Eg~1.5 – 5.4 GeV
 Pentaquark experiment in Hall A
─ E04-012 (May), search for excited Q++ and Q0 states.
Elton S. Smith
APS, Denver
May 1-4, 2004
30
Summary
 A key question in non-perturbative QCD is the structure of
hadrons.
 We have reviewed the evidence for the existence of a new
class of colorless hadrons with quantum numbers which
cannot be generated from solely three quarks:
─ There is substantial corroborating evidence for an exotic baryon with
S = +1, which would have a minimal quark content of (uudds).
─ The observation of a doubly negative S=−2 baryon (ddssu) is
consistent with a second corner of the anti-decuplet the family of
pentaquarks, but needs additional confirmation.
 Dedicated experiments are being mounted which should
easily establish (or refute) the observations to date.
Elton S. Smith
APS, Denver
May 1-4, 2004
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