RHIC As an Exotic Particle Factory

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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
2
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
q
q
Color Configuration
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QCD Allowed State
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Artists View of Proton and Glueball
proton
glueball
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Not Easy to Identify Glueball

• 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 !

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H0(uuddss) Detection
H0 ? p S- ? p n p-
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Strange Quark Matter Stability
Stable SQM -- Ground State of QCD gt 1116
MeV unstable Stability M/Baryon 940,1116
MeV Weak lt940 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 !
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Stable (ctgt50 ns) strange quark matter has not
been detected at the level 10-9 per central
AuPb collisions !
E886
E878
E864 Final
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Q Exotic Baryon
D. Diakonov, V. Petrov, and M. Polyakov, Z. Phys.
A 359 (1997) 305.

• Exotic S1
• Low mass 1530 MeV
• Narrow width 15 MeV
• Jp1/2

Isospin 0
M 1890-180Y MeV
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Spring-8 Result
M 1.54?0.01 GeV G lt 25 MeV Gaussian
significance 4.6s
Q

• d ? K- K n p
• Cuts no f?KK-
• no recoil p (gn only)
• Missing mass for n

PRL 91, 012002 (2003)
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CLAS/JLAB Results
g d ? p K K- n
g p ? n K K- p

• M 1542?5 MeV
• lt 21 MeV
• 5.2 s

• M 1555?10 MeV
• lt 26 MeV
• 7.8 s

Phys. Rev. Lett. 92, (2004) 032001
Phys.Rev.Lett.91, (2003) 252001
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Many Experiments Confirm Q
Experiments Results Mass Width (MeV) (Mev)
SPring8 DIANA CLAS SAPHIR ITEP (n) HERMES KN elastic 1540 ?10 ?5 G ? 25 4.6?1 1539 ?2 ?few G ? 8 4.4 1542 ?2 ?5 FWhM 21 5.3?0.5 1540 ?4 ?2 G lt 25 4.8 1533 ?5 G lt 20 6.7 1526 ?2 ?2 G lt 13 5.6?0.5 G lt few MeV !
One theory (cQSM) 1530 MeV Glt15 MeVI0 S1 JP½
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Many Experiments with Null Results
Noticeably (biased list)
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Possible Q Photo-Production Reactions
CLAS/JLAB
LEPS/SPring-8
Q
Exclusive Channels !
Not Solid Physical Picture!
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CLAS Recent Null Results on Pentaquark Searches
gd?LnK
gp?pKK-
Pentaquark production in these EXCLUSIVE channels
at JLab Kinematics is NOT favored !!
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Evidence for ? Is Disappearing
? d (n ) reactions
? p ? p Ks0
? p ? n K K- p
K (N) ? p Ks0
lepton D, A ? p Ks0
p A ? pKs0 X
p p ? pKs0 S
Other Q Upper Limits
LEPS-d2
CLAS-d2
CLAS-d1
LEPS-d
LEPS-C
CLAS g11
SAPHIR
CLAS-p
BELLE
DIANA
BaBar
nBC
ZEUS
Hermes
SPHINX
JINR
SVD2
HyperCP
SVD2
ALEPH, Z
COSY-TOF
HERA-B
BES J,Y
FOCUS
CDF
WA89
2002 2003
2004 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_at_JLab)
• NA49 X-- has never been confirmed by any
  experiment

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K-p missing mass spectrum New LEPS
Results T. Nakano _at_ SQM2006
Excesses are seen at 1.53 GeV and at 1.6 GeV
above the background level.
Q
Counts/5 MeVC
1.53-GeV peak

(in the 5 bin 25 MeV)
preliminary
No visible signal in sidebands.
preliminary
Counts/5 MeV
MMd(?,K-p) GeV/c2
sideband
L
MMd(?,K-p) GeV/c2
sum
Normalization of L is obtained by fit in the
region of MMd lt 1.52 GeV.
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Au Au Collisions at RHIC
Central Event
(real-time Level 3)
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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)
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Particle identification
Particle Identification dE/dx from TPC
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dAu results
M (GeV/c2)
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dAu results
PeaK?!
Understand the Background!
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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
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Q and L(1520) Using the Same Analysis Procedure
L(1520)
Same charge Sign (SS) and Opposite Sign (OS)
background different
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Background Shape Depends on Cuts
M (GeV/c2)
M (GeV/c2)
K 0.2-0.6 GeV/c P 0.3-1.5 GeV/c
K 0.2-0.6 GeV/c P 0.3-1.0 GeV/c
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Possible Sources of Background
Double Conversion of p0 photons p0 ? gg ?
ee- ee- Same-sign es 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 !
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D?pp and using p as K
Q
p ? K
Q
D
Slope depends on the level of pion contamination
(p cut) !
D
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D/N(1535) decay and p?K does not make the peak
K?p
Delta Mass
Delta Mass
N(1535)?pp wrong charge for q!
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Other PID Cuts
Kaon 0.2ltpptlt0.7, Proton 0.3ltpptlt1.0, no
opening angle cut
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Other PID Cuts
Kaon 0.2ltpptlt1.0, Proton 0.3ltpptlt1.5, no
opening angle cut
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Can the Peak Be Real ??
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AuAu 62.4 GeV Results

• AuAu 62 GeV data
• 20-80 centrality bin
• 5.6 M events
• Weak Signal (3sigma) if any

Kaon ppt (0.2, 0.6) Proton ppt (0.3, 1.5)
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AuAu 200 GeV Run 4 Results

• Year 4 AuAu 200 GeV data
• 20-80 centrality bin
• 10.7 M events
• No Significant Signal (2s)

Kaon ppt (0.2, 0.6) Proton ppt (0.3, 1.5)
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CuCu 62.4 GeV Run 5 Data

• Year 5 CuCu 62 GeV data
• 0-70 centrality bin
• 16.5 M events
• No signal at all !!

Kaon ppt (0.2, 0.6) Proton ppt (0.3, 1.5)
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Is There an Obvious Contradiction ?
The signal is not significant in AuAu systems
! dAu is indeed a favored system signal
strength and combinatorial background !! RHIC
should have another long dAu run !!
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??KsP

• dAu data, Ks was identified by topological
  method
• The ? 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 years AuAu data may give us an answer?

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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 lt0.92
95CL q/L(1520) lt2.7 95 CL Our Estimate in
STAR dAu sqrt(s) 200 GeV q/L 0.35
Does this imply L(1520)/L 34?
STAR L(1520)/L 10 (corrected for branching
ratio) !
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The Puzzle Continues

• 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.
• The STAR observed yield is so small such that
  many
• experiments would not have the sensitivity to
  see it.
• Within the STAR data we have not seen any
  significant
• peak signal in pp data and AuAu at
• 200 GeV and CuCu 62 GeV.
• What do these null observations mean?
• Production dynamics or data set bias unknown to
  us?
• What is so special about dAu 200 GeV (18.6 M
  events)?
• 4) Can the formation is such that
  photo-production is not favored?

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An Intriguing Production Mechanism

• pK Interaction Repulsive
• DK Interaction Attractive !!
• DK Coalescence ? Q ? pK
• (NpK 1575 MeV DKgtNpK but D is very wide !)
• lifetime 1 fm ? not easy for the coalescence
  process
• in ee, photo-production and pp collisions.
• spin 3/2, parity -1 pK d-wave decay ? narrow
  width
• pA collisions favored !

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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

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Clustering and Surface Emission
Enhancement of Clusters at intermediate pT
! (baryons and hyperons)
Search for Multi-quark (gt3) Cluster State at RHIC
!
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RHIC Exotic Particle Factory
STAR Exciting Physics Program A full TOF
and Heavy Flavor Tracker upgrade will greatly
enhance STARs capability !!
Heavy Flavor Tracker Using Active Pixel Sensor
two layers of thin silicon detector 1.5 cm
and 4 cm radius Charmed Exotics?!
Full Barrel TOF Using MRPC
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Exotic Particles
Hadrons with internal structure beyond
existing QCD qqq and q-qbar framework !!
RHIC Dedicated QCD Machine Beyond
Exotic
(Deconfinement Phase Transition)
AA
AA
pp
pA
pp
pA
(CGC,EMC)
(spin)
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Discoveries from Unexpected Areas?!
RHIC -- Frontier for bulk partonic matter
formation (quark clustering and
rapid hadronization) -- Factory for
exotic particles/phenomena
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The End
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Other PID Cuts
Kaon 0.2ltpptlt0.8, Proton 0.3ltpptlt1.5, no
opening angle cut
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Other PID Cuts
Kaon 0.2ltpptlt0.8, Proton 0.3ltpptlt1.0, no
opening angle cut
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Is There an Obvious Contradiction ?
The signal is not significant in AuAu systems
! L(1520)?pK- branching ratio 22, corrected
for the ratio. dAu is indeed a favored system
signal strength and combinatorial background
!! RHIC should have another long dAu run !!