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 PhysicsTsinghua University

QCD Structure of Hadrons

Confinement – Hadrons must be color singlet !

Existing Hadrons:meson – quark and anti-quark pairBaryon – three quark system

qq q q

q

Color Configuration

QCD Allowed State

1) ggg – Glue Ball

2) qqQQ – Tetra-quark

3) qqqqQ – Penta-quark

4) qqqqqq – Di-baryons: H(uuddss), di- (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 preciselyprobably ~ 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 - p n -

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 !

E878E886

E864 Final

Stable (c>50 ns) strange quark matter has not been detected at the level ~10-9 per central

Au+Pb collisions !

+ Exotic Baryon

M1890-180*Y] MeV

D. Diakonov, V. Petrov, and M. Polyakov, Z. Phys. A 359 (1997) 305.

• Exotic: S=+1

• Low mass: 1530 MeV

• Narrow width: ~ 15 MeV

• Jp=1/2+

Isospin = 0

Spring-8 Result

1.540.01 GeV< 25 MeVGaussian significance 4.6

+

+ d K- K+ n p

Cuts: no K+K-

no recoil p (n only)Missing mass for n

PRL 91, 012002 (2003)

MeVMeV7.8

CLAS/JLAB Results

Phys.Rev.Lett.91, (2003) 252001

MeVMeV5.2

d→p n p→n +

Phys. Rev. Lett. 92, (2004) 032001

Many Experiments Confirm +

Experiments

ResultsMass Width (MeV) (Mev)

SPring8DIANACLASSAPHIRITEP ()HERMESKN elastic

1540 10 5 25 4.61

1539 2 “few” 8 4.4

1542 2 5 FWhM= 21 5.30.5

1540 4 225 4.8

1533 5 < 20 6.7

1526 2 2 5.60.5

<few MeV !

One theory

(QSM)

1530 MeV MeVI=0 S=+1 JP=½+

( / )s bN N

Many Experiments with Null Results

Noticeably (biased list)

CDF p + p Very High Energy

HERA-B p + A sqrt(s) 42 GeV

BESBaBar e+ + e- collisionsALEPHDELPHI

Possible + Photo-Production Reactions

CLAS/JLABLEPS/SPring-8

Exclusive Channels !

Not Solid Physical Picture!

CLAS Recent Null Results on Pentaquark Searches

dnK+ ppK+K-

Pentaquark production in these EXCLUSIVEchannels at JLab Kinematics is NOT favored !!

Evidence for + Is Disappearing

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 -- has never been confirmed by any experiment

γ + d (n ) reactions

γ + p → p Ks0 　

γ + p → n K+ K- p+

K + (N) → p Ks0

lepton + D, A → p Ks0

p + A → pKs0 + X

p + p → pKs0 + +

Other + Upper Limits

BaBar

CLAS-d2

BELLE

ALEPH, ZSVD2

LEPS-d2LEPS-C CLAS-d1

DIANA

SAPHIR

SVD2

COSY-TOF

Hermes

JINR

CLAS-p

LEPS-d

BCZEUS

BES J,

CLAS g11

SPHINXHyperCP

HERA-B

FOCUS WA89CDF

2002 2003 　 2004 2005

K － p missing mass spectrum – New LEPS Results

T. Nakano @ SQM2006

MMd(γ,K － p) GeV/c2

sideband

*

sum

5S

S B

Co

un

ts/5

MeV

C Excesses are seen at 1.53 GeV and at 1.6 GeV above the background level.

1.53-GeV peak:

pre

lim

inar

y

MMd(γ,K － p) GeV/c2

Co

un

ts/5

MeV

Normalization of * is obtained by fit in the region of MMd < 1.52 GeV.

(in the 5 bin = 25 MeV)

pre

lim

inar

y

No visible signal in sidebands.

+

~

Au + Au Collisions at RHIC

STARSTAR

Central Event

(real-time Level 3)

STAR Pentaquark Searches

+ p + KS

++ 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

pK+ and pK- from 18.6 M d+Au at 200 GeVBackground – Combinatorial and Correlated Pairs

dAu results

M (GeV/c2)

dAu results

Understand the Background!

PeaK?!

dAu results

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

?++

KppK KppK

++ and (1520) Using the Same Analysis Procedure

(1520)

Same charge Sign (SS) and Opposite Sign (OS) background different

Background Shape Depends on Cuts

M (GeV/c2)M (GeV/c2)

K [0.2-0.6] GeV/cP [0.3-1.5] GeV/c

K [0.2-0.6] GeV/cP [0.3-1.0] GeV/c

Possible Sources of Background

Double Conversion of 0 photons

0 e+e- e+e-

Same-sign e’s within the K and p bands

mostly in the low mass region opening angle cut very effective

Associated production K+ p

These background sources contribute to the residuals in the event-mixing. But they do not produce a narrow peak !

+++p and using as K

K

Slope depends on the levelof pion contamination (p cut) !

Delta++ Mass Delta++ Mass

K

N*(1535)decay and K does not make the peak

N*(1535)p wrong charge for ++!

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

pK Kp

KppK

Can the Peak Be Real ??

AuAu 62.4 GeV Results

AuAu 62 GeV data20-80% centrality bin5.6 M eventsWeak Signal (3sigma) if any

Kaon p&pt (0.2, 0.6)Proton p&pt (0.3, 1.5)

Year 4 AuAu 200 GeV data20-80% centrality bin10.7 M eventsNo Significant Signal (2)Kaon p&pt (0.2, 0.6)

Proton p&pt (0.3, 1.5)

AuAu 200 GeV Run 4 Results

Cu+Cu 62.4 GeV Run 5 Data

Year 5 CuCu 62 GeV data0-70% centrality bin16.5 M eventsNo signal at all !!

Kaon p&pt (0.2, 0.6)Proton p&pt (0.3, 1.5)

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 !!

+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 year’s AuAu data may give us an answer?

A Stringent Limit from HERA-B

HERA-B hep-ex/0408048sqrt(s) 42 GeVpA (C,Ti,W) 200 M inelastic events+/ <0.92%; 95%CL+/(1520) <2.7%; 95% CL

Our Estimate in STARd+Au sqrt(s) 200 GeV/ ~ 0.35%

Does this imply (1520)/ ~ 34%?

STAR(1520)/ ~ 10% (corrected for branching ratio) !

The Puzzle Continues1) If pK+ peak at 1530 MeV/c2 is a real pentaquark, then

I = 1 likely, there must be a +. But the recent JLab null result on + casts serious doubt on the observation

of +.

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 significantpeak 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

K+ Interaction – Attractive !!

K Coalescence pK+ (NK = 1575 MeV; K>NK; but is very wide !)

lifetime ~ 1 fm not easy for the coalescence processin e+e, photo-production and p+p collisions.

spin 3/2, parity -1; pK d-wave decay narrow width

p+A collisions favored !

RHIC – Exotic Particle Factory

RHIC – Very Dense Partonic Matter andRapid Hadronization

-- Hadron Formation Through PartonClustering (coalescence/recomb.)

Unique Collision Environment for Possible Exotic Particles Formation

Exotic Mesons, Pentaquarks, Di-baryons [ and Strangelets

Clustering and Surface Emission

Volcanic mediate pT – Spatter (clumps)

Enhancement of Clustersat intermediate pT !(baryons and hyperons)

Search for Multi-quark (>3) Cluster State at RHIC !

STAR – Exciting Physics Program A full TOF and Heavy Flavor Tracker upgrade will greatly enhance STAR’s capability !!

RHIC – Exotic Particle Factory

Heavy Flavor TrackerUsing Active Pixel Sensor

two layers of thin silicon detector 1.5 cm and 4 cm radius Charmed Exotics?!Full Barrel TOF Using MRPC

Exotic Particles Hadrons with internal structure beyond existing

QCD qqq and q-qbar framework !!

pp pA

AA

pppA

AA

Exotic

RHIC – Dedicated QCD Machine & Beyond

(spin) (CGC,EMC)

(Deconfinement Phase Transition)

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-baryonsH – (, uuddss) [] (ssssss)

strange quark matter

meta-stable Parity/CP odd vacuum bubblesdisoriented 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 ?

++

(1520)

The signal is not significant in Au+Au systems !(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 !!