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 qq qq 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 precisely probably ~ GeV/c 2 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, 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 !

E878 E886 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 M  *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 J p =1/2 + Isospin = 0

Spring-8 Result  1.54  0.01 GeV  < 25 MeV Gaussian significance 4.6  ++   + d  K - K + n p Cuts: no   K + K - no recoil p (  n only) Missing mass for n PRL 91, (2003)

 MeV  MeV 7.8  CLAS/JLAB Results Phys.Rev.Lett.91, (2003)  MeV  MeV 5.2   d  →  p      n  p  →  n      + Phys. Rev. Lett. 92, (2004)

Many Experiments Confirm  + ExperimentsResults Mass Width (MeV) (Mev) SPring8 DIANA CLAS SAPHIR ITEP ( ) HERMES KN elastic 1540  10  5 G    2  “ few ” G   2  5 FWhM =   4  2 G <  5 G <  2  2 G <  0.5 G < few MeV ! One theory ( c QSM) 1530 MeV G<15 MeV I=0 S=+1 J P = ½ +

Many Experiments with Null Results Noticeably (biased list) CDFp + p Very High Energy HERA-Bp + A sqrt(s) 42 GeV BES BaBare + + e - collisions ALEPH DELPHI

Possible  + Photo-Production Reactions   CLAS/JLAB LEPS/SPring-8 Exclusive Channels ! Not Solid Physical Picture!

CLAS Recent Null Results on Pentaquark Searches  d   nK +  p  pK + K - Pentaquark production in these EXCLUSIVE channels 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  NA49  -- has never been confirmed by any experiment γ + d (n ) reactions γ + p → p K s 0 γ + p → n K + K - p + K + (N) → p K s 0 lepton + D, A → p K s 0 p + A → pK s 0 + X p + p → pK s 0 +  + Other  + Upper Limits BaBar CLAS-d2 BELLE ALEPH, Z SVD2 LEPS-d2 LEPS-C CLAS-d1 DIANA SAPHIR SVD2 COSY-TOF Hermes JINR CLAS-p LEPS-d BC ZEUS BES J,  CLAS g11 SPHINX HyperCP HERA-B FOCUS WA89 CDF

K － p missing mass spectrum – New LEPS Results T. SQM2006 MMd(γ,K － p) GeV/c 2 sideband  * sum Counts/5 MeVC Excesses are seen at 1.53 GeV and at 1.6 GeV above the background level GeV peak: preliminary MMd(γ,K － p) GeV/c 2 Counts/5 MeV Normalization of  * is obtained by fit in the region of MMd < 1.52 GeV. (in the 5 bin = 25 MeV) preliminary No visible signal in sidebands. ++ ~

Au + Au Collisions at RHICSTAR Central Event (real-time Level 3)

STAR Pentaquark Searches  +  p + K S  ++  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 GeV Background – Combinatorial and Correlated Pairs dAu results M (GeV/c 2 )

dAu results Understand the Background! PeaK?!

dAu results The invariant mass distribution is fitted to a Gaussian plus a linear function. A sigma signal is seen Measured mass is about 1.53 GeV/c 2. Full width is about 15 MeV ?  ++

 ++ 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/c 2 ) K [ ] GeV/c P [ ] GeV/c K [ ] GeV/c P [ ] 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 level of pion contamination (p cut) !

Delta++ Mass K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

Can the Peak Be Real ??

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)

 Year 4 AuAu 200 GeV data  20-80% centrality bin  10.7 M events  No 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 data  0-70% centrality bin  16.5 M events  No 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 K L  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/ sqrt(s)42 GeV pA (C,Ti,W)200 M inelastic events  + /  <0.92%; 95%CL  + /  (1520)<2.7%; 95% CL Our Estimate in STAR d+Ausqrt(s) 200 GeV   /  ~ 0.35% Does this imply  (1520)/  ~ 34%? STAR  (1520)/  ~ 10% (corrected for branching ratio) !

The Puzzle Continues 1)If pK + peak at 1530 MeV/c 2 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 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  K + Interaction – Attractive !!  K Coalescence    pK + (N  K = 1575 MeV;  K>N  K; but  is very wide !)  lifetime ~ 1 fm  not easy for the coalescence process  in 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 and Rapid Hadronization -- Hadron Formation Through Parton Clustering (coalescence/recomb.)  Unique Collision Environment for Possible Exotic Particles Formation  Exotic Mesons, Pentaquarks, Di-baryons [  and Strangelets

Clustering and Surface Emission Volcanic mediate p T – Spatter (clumps) Enhancement of Clusters at intermediate p T ! (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 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

Exotic Particles Hadrons with internal structure beyond existing QCD qqq and q-qbar framework !! pp pA AA pp pA 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-baryons H – ( , uuddss) [  ] (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 ?  ++  (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 !!