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RHIC As an Exotic Particle Factory
-- Recent Result on Pentaquark
Searches
Huan Z. Huang
Department of Physics and Astronomy
University of California, Los Angeles
Department of Engineering Physics
Tsinghua University
QCD Structure of Hadrons
Confinement – Hadrons must be color singlet !
Existing Hadrons:
meson – quark and anti-quark pair
Baryon – three quark system
q
q
q
Color Configuration
q
q
QCD Allowed State
1) ggg – Glue Ball
2) qqQQ – Tetra-quark
3) qqqqQ – Penta-quark
4) qqqqqq – Di-baryons: H(uuddss), di-W (ssssss)
5) Strange Quark Matter
Artist’s View of Proton and Glueball
proton
glueball
Not Easy to Identify Glueball
1) We do not know the mass precisely
probably ~ 1.5-2.5 GeV/c2
2) The width is probably wide
Buried in many normal quark resonance states
in the same mass region !
H0(uuddss) Detection
H0  p S-  p n p-
Strange Quark Matter Stability
Stable SQM -- Ground State of QCD
> 1116 MeV unstable
Stability M/Baryon
[940,1116] MeV Weak
<940 MeV Stable
{
Witten, Phys. Rev. D30, 272 (1984)
E. Farhi and R.L. Jaffe, Phys. Rev. D30, 2379.
Theoretical Prediction of Stable Particles Often not Reliable
-- Particle Mass ~ a few 1000 MeV
-- Binding Energy a few MeV (deuteron)
No QCD or Phenomenological calculation
has better than 1% accuracy !
Stable (ct>50 ns) strange quark matter has not been
detected at the level ~10-9 per central Au+Pb collisions !
E886
E878
E864 Final
Q+ Exotic Baryon
D. Diakonov, V. Petrov, and M. Polyakov,
Z. Phys. A 359 (1997) 305.
•
•
•
•
M = [1890-180*Y] MeV
Exotic: S=+1
Low mass: 1530 MeV
Narrow width: ~ 15 MeV
Jp=1/2+
Isospin = 0
Spring-8 Result
M = 1.540.01 GeV
G < 25 MeV
Gaussian significance 4.6s
Q+
+ d  K- K+ n p
Cuts: no fK+Kno recoil p (n only)
Missing mass for n
PRL 91, 012002 (2003)
Q+
CLAS/JLAB Results
 d  p K+ K- n
 p  n K+ K- p+
M = 15425 MeV
G< 21 MeV
5.2 s
M = 155510 MeV
G < 26 MeV
7.8 s
Phys.Rev.Lett.91, (2003) 252001
Phys. Rev. Lett. 92, (2004) 032001
Many Experiments Confirm Q+
Experiments
Results
Mass
(MeV)
SPring8
DIANA
CLAS
SAPHIR
ITEP (n)
HERMES
KN elastic
1540 10 5
One theory
1530 MeV
(cQSM)
1539 2 “few”
1542 2 5
1540 4 2
1533 5
1526 2 2
I=0
S=+1
Width
(Mev)
G < 25
G< 8
FWhM = 21
G < 25
G < 20
G < 13
G < few MeV !
G<15 MeV
JP=½+
s (= Ns / Nb )
4.61
4.4
5.30.5
4.8
6.7
5.60.5
Many Experiments with Null Results
Noticeably (biased list)
CDF
p + p Very High Energy
HERA-B
p + A sqrt(s) 42 GeV
BES
BaBar
ALEPH
DELPHI
e+ + e- collisions
Possible Q+ Photo-Production Reactions
CLAS/JLAB
LEPS/SPring-8
Q
+
Q+
Exclusive Channels !
Not Solid Physical Picture!
CLAS Recent Null Results on Pentaquark Searches
dLnK+
ppK+K-
Pentaquark production in these EXCLUSIVE
channels at JLab Kinematics is NOT favored !!
Evidence for Q+ Is Disappearing
γ + d (n )
reactions
LEPS-C
CLAS-d1
γ + p → p Ks0
LEPS-d
LEPS-d2
CLAS g11
SAPHIR
γ + p → n K+ K- p+
CLAS-d2
CLAS-p
BELLE
DIANA
K + (N) → p Ks0
Hermes
lepton + D, A → p Ks0
SVD2
p + A → pKs0 + X
p + p → pKs0 + S+
Other Q+ Upper Limits
BES J,Y
2002
2003
2004
BaBar
ZEUS nBC
JINR SPHINX HyperCP
ALEPH, Z
COSY-TOF
HERA-B
CDF
FOCUS
SVD2
WA89
2005
 Over ten experiments reported positive evidence initially
 Since the mid of 2003, null results dominate
 Two dedicated high statistic experiments reported null
results, and contradicts with previous positive
observations from the same collaboration (CLAS@JLab)
 NA49 X-- has never been confirmed by any experiment
K－p missing mass spectrum – New LEPS Results
Excesses are seen at 1.53
GeV and at 1.6 GeV above
the background level.
MMd(γ,K－p) GeV/c2
1.53-GeV peak:
(in the 5 bin = 25 MeV)
S
S+B
~
5
preliminary
No visible signal in sidebands.
Counts/5 MeV
Q+
preliminary
Counts/5 MeVC
T. Nakano @ SQM2006
sideband
L*
sum
MMd(γ,K－p) GeV/c2
Normalization of L* is obtained by fit in the region of MMd < 1.52 GeV.
Au + Au Collisions at RHIC
Central Event
STAR
(real-time Level 3)
STAR Pentaquark Searches
Q +  p + KS
Q++  p + K+
Data Set:
Au + Au 200 GeV run 2 (~1.7 M, 30-80%)
p + p data 200 GeV run 2 (~6.5 M)
d + Au 200 GeV run 3 (18.6 M)
Au + Au 63 GeV run 4 (5.6 M)
Cu + Cu 63 GeV run 5 (16.5 M)
Au + Au 200 GeV Run 4 (10.7 M, 20-80%)
Particle identification
Particle Identification: dE/dx from TPC
dAu results
M (GeV/c2)
pK+ and pK- from 18.6 M d+Au at 200 GeV
Background – Combinatorial and Correlated Pairs
dAu results
PeaK?!
Understand the Background!
dAu results
?++
D
The invariant mass distribution is fitted to a Gaussian plus a linear
function. A 3.5-5.0 sigma signal is seen
Measured mass is about 1.53 GeV/c2. Full width is about 15 MeV
Q++ and L(1520) Using the Same Analysis Procedure
pK + + pK -
pK - + pK +
L(1520)
Same charge Sign (SS) and Opposite Sign (OS) background different
Background Shape Depends on Cuts
M (GeV/c2)
K [0.2-0.6] GeV/c
P [0.3-1.5] GeV/c
M (GeV/c2)
K [0.2-0.6] GeV/c
P [0.3-1.0] GeV/c
Possible Sources of Background
Double Conversion of
p0 photons
p0    e+e- e+eSame-sign e’s within the K and p bands
mostly in the low mass region
opening angle cut  very effective
Associated production
LK+  pp- + K+
These background sources
contribute to the residuals
in the event-mixing. But they
do not produce a narrow peak !
D++p+p and using p as K
Q
p K
Q
D
D
Slope depends on the level
of pion contamination (p cut) !
D/N*(1535) decay and pK does not make the peak
Delta++ Mass
Delta++ Mass
N*(1535)pp wrong charge for q++!
Other PID Cuts
Kaon 0.2<p&pt<0.7, Proton 0.3<p&pt<1.0, no opening angle cut
Other PID Cuts
Kaon 0.2<p&pt<1.0, Proton 0.3<p&pt<1.5, no opening angle cut
Can the Peak Be Real ??
pK + + pK -
pK +
pK -
AuAu 62.4 GeV Results
AuAu 62 GeV data
20-80% centrality bin
5.6 M events
Weak Signal (3sigma) if any
Kaon p&pt (0.2, 0.6)
Proton p&pt (0.3, 1.5)
AuAu 200 GeV Run 4 Results
Kaon p&pt (0.2, 0.6)
Proton p&pt (0.3, 1.5)
Year 4 AuAu 200 GeV data
20-80% centrality bin
10.7 M events
No Significant Signal (2s)
Cu+Cu 62.4 GeV Run 5 Data
Kaon p&pt (0.2, 0.6)
Proton p&pt (0.3, 1.5)
Year 5 CuCu 62 GeV data
0-70% centrality bin
16.5 M events
No signal at all !!
Is There an Obvious Contradiction ?
The signal is not significant in Au+Au systems !
d+Au is indeed a favored system:
signal strength and combinatorial background !!
RHIC should have another long d+Au run !!
Q+KsP
 dAu data, Ks was identified by topological method
 The Q+ is probably there, several reasons may be
responsible for the less significant signal:
 Smaller branching ratios
 Half of the Ks will become KL
 Efficiency for finding Ks at low pt is low
 This year’s AuAu data may give us an answer?
A Stringent Limit from HERA-B
HERA-B
hep-ex/0408048
sqrt(s)
42 GeV
pA (C,Ti,W) 200 M inelastic events
q+/L
<0.92%; 95%CL
q+/L(1520)
<2.7%; 95% CL
Does this imply L(1520)/L ~ 34%?
Our Estimate in STAR
d+Au
sqrt(s) 200 GeV
q++/L
~ 0.35%
STAR L(1520)/L ~ 10% (corrected for branching ratio) !
The Puzzle Continues
1) If pK+ peak at 1530 MeV/c2 is a real pentaquark, then
I = 1 likely, there must be a q+. But the recent JLab
null result on q+ casts serious doubt on the observation
of q+.
2) The STAR observed yield is so small such that many
experiments would not have the sensitivity to see it.
3) Within the STAR data we have not seen any significant
peak signal in p+p data and Au+Au at
200 GeV and Cu+Cu 62 GeV.
What do these null observations mean?
Production dynamics or data set bias unknown to us?
What is so special about d+Au 200 GeV (18.6 M events)?
4) Can the formation is such that photo-production is not favored?
An Intriguing Production Mechanism
pK+ Interaction – Repulsive
DK+ Interaction – Attractive !!
DK Coalescence  Q  pK+
(NpK = 1575 MeV; DK>NpK; but D is very wide !)
D lifetime ~ 1 fm  not easy for the coalescence process
in e+e, photo-production and p+p collisions.
Q spin 3/2, parity -1; pK d-wave decay  narrow width
p+A collisions favored !
RHIC – Exotic Particle Factory
RHIC – Very Dense Partonic Matter and
Rapid Hadronization
-- Hadron Formation Through Parton
Clustering (coalescence/recomb.)
Unique Collision Environment for
Possible Exotic Particles Formation
Exotic Mesons, Pentaquarks, Di-baryons
[WW] and Strangelets
Clustering and Surface Emission
Volcanic mediate pT – Spatter (clumps)
Enhancement of Clusters
at intermediate pT !
(baryons and hyperons)
Search for Multi-quark (>3) Cluster State at RHIC !
RHIC – Exotic Particle Factory
STAR – Exciting Physics Program
A full TOF and Heavy Flavor Tracker upgrade will
greatly enhance STAR’s capability !!
Full Barrel TOF Using MRPC
Heavy Flavor Tracker
Using Active Pixel Sensor
two layers of thin silicon detector
1.5 cm and 4 cm radius
Charmed Exotics?!
Exotic Particles
Hadrons with internal structure beyond existing
QCD qqq and q-qbar framework !!
RHIC –
Dedicated QCD Machine &
Beyond
Exotic
AA
pp
(spin)
(Deconfinement
Phase Transition)
pA
(CGC,EMC)
Discoveries from Unexpected Areas?!
RHIC -- Frontier for bulk partonic matter formation
(quark clustering and rapid hadronization)
-- Factory for exotic particles/phenomena
Potential exotic particles/phenomena:
penta-quark states (uudds, uudds!)
di-baryons
H – (L-L, uuddss)
[W-W] (ssssss)
strange quark matter
meta-stable Parity/CP odd vacuum bubbles
disoriented chiral condensate
……
The End
Other PID Cuts
Kaon 0.2<p&pt<0.8, Proton 0.3<p&pt<1.5, no opening angle cut
Other PID Cuts
Kaon 0.2<p&pt<0.8, Proton 0.3<p&pt<1.0, no opening angle cut
Is There an Obvious Contradiction ?
Q++
L(1520)
The signal is not significant in Au+Au systems !
L(1520)pK- branching ratio ~ 22%, corrected for the ratio.
d+Au is indeed a favored system:
signal strength and combinatorial background !!
RHIC should have another long d+Au run !!