Search for pentaquarks: the experimental program at CLAS S. Niccolai, IPN Orsay for the CLAS collaboration Epiphany Conference Krakov, Introduction Published CLAS results on Θ + Pentaquark program at CLAS Outlook

Krakow, S. Niccolai, IPN Orsay What are pentaquarks? New form of quark matter: baryons whose minimum quark content is 5 (qqqqq) “Non-exotic” pentaquarks  The antiquark has the same flavor as one of the other quarks  Difficult to distinguish from 3-quark baryons s + 1/3 d − 1/3 u +2/3 d − 1/3 u +2/3 Baryons with S = +1 cannot be made by qqq !!!!! “Exotic” pentaquarks (qqqqQ)  The antiquark has a different flavor from the other 4 quarks  Quantum numbers different from any 3-quark baryon Example: uudds (exotic):  Baryon number = 1/3 + 1/3 + 1/3 + 1/3 - 1/3 = 1  Strangeness = = +1

“The general prejudice against baryons not made of three quarks and the lack of any experimental activity in this area make it likely that it will be another 15 years before the issue is decided” 1986, Particle Data Group: Almost 15 years later experiments reported evidences for pentaquark states since 2003 Over 200 theory papers published Started series of dedicated international workshops: JLab (2003), SPring-8 (2004), Genova (2005)  + listed in PDG 2004 (***)! Research on pentaquarks is one of the central programs in many labs Existence of pentaquark is not forbidden by QCD General idea of a five-quark states has been around since late 60’s (Jaffe, Lipkin, Strotteman…) However…

The pentaquark anti-decuplet Experimental searches were motivated by predictions of chiral soliton model Diakonov, Petrov, Polyakov, Z.Physics A359 (1997) S = +1 S = 0 S = -1 S = -2 I=-3/2 I=-1/2 I=1/2 I=3/2 I=1 : exotic states I=-1 I=0 uudds ddssu uussd  + (1539) N( )  ( )   (1862) Baryon octets, decuplets and antidecuplets are seen as rotational excitations of the same soliton field

The pentaquark anti-decuplet Experimental searches were motivated by predictions of chiral soliton model Diakonov, Petrov, Polyakov, Z.Physics A359 (1997) S = +1 S = 0 S = -1 S = -2 : exotic states I=-3/2 I=-1/2 I=1/2 I=3/2 I=1 I=-1 I=0  + (1539) N( )  ( )   (1862) uudds ddssu uussd narrow width (few MeV) decay modes nK +,pK 0 J  =1/2 + can be detected experimentally!!

The pentaquark anti-decuplet Experimental searches were motivated by predictions of chiral soliton model Diakonov, Petrov, Polyakov, Z.Physics A359 (1997) S = +1 S = 0 S = -1 S = -2 : exotic states  + (1539) N( )  ( )   (1862) 5-quark states predicted also by lattice QCD and other models: Skyrme model, MIT bag model, QCD sum rules, …

First observation: LEPS/SPring-8 T. Nakano et al., PRL91, (2003) ++  n→K + K - (n) (the neutron is bound inside 12 C)

The Jefferson Lab electron accelerator Newport News, USA E max ~ 6 GeV I max ~ 200  A Duty Factor ~ 100%  E /E ~ Beam Pol ~ 80%

Krakow, S. Niccolai, IPN Orsay The CLAS detector at JLAB Toroidal magnetic field (6 supercond. coils) Drift chambers (argon/CO 2 gas, 35,000 cells) Time-of-flight scintillators (684 PMTs) Electromagnetic calorimeters (lead/scintillator, 1296 PMTs) Cherenkov Counters (e/  separation, 216 PMTs) Bremsstrahlung photon tagger

Krakow, S. Niccolai, IPN Orsay The CLAS detector at JLAB Bremsstrahlung photon tagger Performances: large acceptance for charged particles 8°<  <142°, 0.2< p p <4 GeV/c, 0.1<p  <4 GeV/c good momentum and angular resolution  p/p ≤ 1.5%,  ≤ 1 mrad

Krakow, S. Niccolai, IPN Orsay The CLAS detector at JLAB Bremsstrahlung photon tagger CLAS is designed to measure exclusive reactions with multi-particle final states

The bremsstrahlung photon tagger Tagging range: (20% - 95%)E 0 E   = E 0 – E e (E 0 known, E e measured)

CLAS-d: first  + exclusive measurement  d → K - pK + (n) Exclusivity: No Fermi motion corrections Final state identified with less background Experimental data from 1999 run Tagged photons with E γmax = 3 GeV Target: 10 cm long liquid deuterium Possible reaction mechanism non-spectator proton: pK - rescattering required to detect p in CLAS (p min (p) = 300 MeV/c)

Krakow, S. Niccolai, IPN Orsay CLAS-d: pK + K - event selection Main background: p  +  - pp  - p+-p+- pK + K -

 d → p K + K ─ (n) in CLAS p K+K+ K-K- K-K- K+K+ p

Krakow, S. Niccolai, IPN Orsay CLAS-d: neutron ID 15% of non pK + K - events within ±3  of the peak Almost no background under the neutron peak with tight timing cut,  t K  d → K - pK + (n) The neutron is detected by missing mass

Krakow, S. Niccolai, IPN Orsay CLAS-d : cut on known resonances Several other known processes can contribute to the pK + K - (n) final state:  d→  p(n),  →K + K -  d→  (1520)K + (n),  (1520)→pK - Both reactions proceed predominantly on the proton (neutron is a spectator) Kinematics of both reactions are not a good match for  + production.

Krakow, S. Niccolai, IPN Orsay CLAS-d: background reduction Cut on the missing neutron momentum, p n >0.08 GeV/c Cut on the K + momentum, pK + <1 GeV/c, based on the 3-body phase space Monte Carlo (  d→  + K - p,  + →nK + ) Before p K+ cut

Krakow, S. Niccolai, IPN Orsay CLAS-d: result  d → K - pK + (n) S. Stepanyan et al., PRL91, (2003) Distribution of  (1520) events Gaussian background Simulated background M = (1.542 ± 0.005) GeV/c 2  = 21 MeV/c 2 Only ~40 events in the  + peak Statistical significance: 4.6  - , depending on the background shape. No significant  + signal was found in the spectator analysis (like SPring-8) due to not optimal torus field settings (limited forward acceptance for K - ). No  ++ peak observed in M(pK + )

Is “Θ + ” a kinematical reflection? High mass mesons (a 2 (1320), f 2 (1270)) decaying to K + K - can produce structures in M(NK) at low mass (A. Dzierba et al., hep-ph/ )

Is “Θ + ” a kinematical reflection? K. Hicks et al., hep-ph/ , found inconsistencies in Djerba’s approach

Is “Θ + ” a kinematical reflection? Open issue: needed measurements of Θ + in different final states (no K + K - )

Krakow, S. Niccolai, IPN Orsay First observation on the proton: CLAS-p  p→K -  + K + (n) E  = 3.0 – 5.47 GeV After PID: neutron identified by missing mass No clear Θ + signal!! Cut on  : M(K + K - )<1.06 GeV/c 2

First observation on the proton: CLAS-p  p→K -  + K + (n) V. Kubarovski et al., PRL92, (2004) M = (1.555 ± 0.010) GeV/c 2 Statistical significance: (7.8 ± 1.0) σ  MeV/c  t-channel process a) selected and background processes eliminated with the cuts (c.m.): cosθ *  + > 0.8 and cosθ* K+ < 0.6

First observation on the proton: CLAS-p  p→K -  + K + (n) V. Kubarovski et al., PRL92, (2004) cut to understand production mechanism look at M(nK + K - )   + production through N * resonance decays?

Krakow, S. Niccolai, IPN Orsay First observation on the proton: CLAS-p  p→K -  + K + (n) Cut on M(nK + ) in  + region outside  + region  - p cross section data in PDG have a gap in the mass range 2.3–2.43 GeV intermediate baryon state?

S. Niccolai, IPN Orsay Krakow, Summary on Θ + : positive results Spring-8 DIANA CLAS-d CLAS-p SAPHIR ITEP HERMES COSY ZEUS IHEP ~ 10 MeV gap between measured masses statistical significance between 4 and 7.8 σ often heavy cuts backgrounds not well understood 10 observations different reaction mechanisms BUT:

Summary on Θ + : negative results high statistics clearly see some of the known resonances NEEDED DEDICATED, HIGH-STATISTICS, EXCLUSIVE MEASUREMENTS!!!!! HERA B: pA→pK 0 X CDF: pp→pK 0 X Phenix: Au+Au→nK - X BaBar: e + e - →pK 0 X Belle: e + e - →B 0 B 0 →ppK 0 BES:e + e - →J/Ψ→ΘΘ _ _ _ _ _ BUT: in e + e - :  there are no quarks in initial state  no theoretical predictions on  production in the other cases:  fragmentation processes,  + could be suppressed (Titov et al., Phys. Rev. C 70, (2004))  high multiplicity in the final state, combinatorial backgrounds _

Decuplet partners of Θ + : search for  3/2 −−,    ddssu uudds uussd Exotic, S = -2, Q = -2 Non exotic, S = -2, Q = 0

Decuplet partners of Θ + : search for  3/2 −−,    p+A 920 GeV/c HERA-B hep-ex/ Combined spectra C. Alt, et al., Phys.Rev.Lett.92, (2004) E cm = 17.2 GeV M=1.862± GeV Statistical significance = 5.6σ  MeV/c  NA49 pp No peaks observed!         Ξ 0 also observedNo Ξ 0

Krakow, S. Niccolai, IPN Orsay Pentaquark today: open issues confirmation of existence of Θ + with high statistics precise determination of the mass of Θ + properties of Θ + : spin, isospin, parity production mechanisms possible excited states of Θ + coupling N* to Θ + confirmation existence of other exotic members of decuplet An experimental program is currently underway at CLAS to address these issues

Krakow, S. Niccolai, IPN Orsay Pentaquark Searches at CLAS  Search for      * ++ on the proton - G11 run  p → various final states (data taking completed)  Search for  pentaquarks - EG3 run  v d→   --,    X (data taking just finished)  Exotic hadron spectroscopy - G12 run  p at 6 GeV, high luminosity (to run in 2005/6) Experimental program approved and underway at CLAS  Search for  + on deuterium - G10 run  d→K - K + p(n) and other final states (data taking completed)

Krakow, S. Niccolai, IPN Orsay CLAS G10: search for Θ + on deuterium Reactions channels to study:  d→pK - K + n Θ + →nK +  d→ΛK +(0) n(p); Θ + →nK +,pK 0  d→pK -   (p) Θ + →pK 0  ”n”→K - K + n Θ + →nK + (with Fermi momentum corrections, to compare with SPring-8) data taking finished end of May 10 billion of events collected data processing almost done results in the spring  tagged photons in the energy range 0.8 GeV to 3.59 GeV  target: 24 cm long liquid deuterium  data were taken at 2 settings of CLAS toroidal magnet (2250 A and 3375 A), lower field to increase acceptance for K - (Spring-8)  at each setting integrated luminosity (2.5pb -1 ) is about 10 times higher than in published deuterium data

Krakow, S. Niccolai, IPN Orsay CLAS G10: how to avoid a fake signal Consider only signals with statistical uncertainty <10%, and statistical significance greater than 7  Study of the NK system: –for pK 0 and nK + final states –in both the missing mass and invariant mass distributions Production of NK system together with a hyperon ( ,  ) Divide data set into two independent sets. Analyze one set, then apply the same analysis (cuts, etc) to the second set Inspect  + candidates in CLAS event display Full simulation of possible background final states

n non-spectator neutron events  CLAS G10: data quality  d → K - pK + (n) 50% of the high-field data  (1520)

Krakow, S. Niccolai, IPN Orsay CLAS G10: γd→ΛΘ +  p n d K+K+  No possibility of kinematical reflections (only one K, from Θ + decay, in the final state) S=+1 both for nK + and pK 0, thanks to Λ No background channels to remove Reaction already studied on 3 He CLAS data, but statistics were too low

Krakow, S. Niccolai, IPN Orsay CLAS G10: γd→ΛΘ + p n d  K+K+  n K+K+ p  Decay modes under study: Λ→pπ - Θ + →K + n No possibility of kinematical reflections (only one K, from Θ + decay, in the final state) S=+1 both for nK + and pK 0, thanks to Λ No background channels to remove

Krakow, S. Niccolai, IPN Orsay p n  K+K+  p  p K0K0   Λ→pπ - Θ + →K 0 p K 0 → π+π- d CLAS G10: γd→ΛΘ + Decay modes under study: Λ→pπ - Θ + →K + n No possibility of kinematical reflections (only one K, from Θ + decay, in the final state) S=+1 both for nK + and pK 0, thanks to Λ No background channels to remove

p n  K+K+  p  n K+K+ d CLAS G10: γd→ΛΘ + Decay mode: Λ→pπ - Θ+→K+nΘ+→K+n n 

p n  K+K+  p  p K0K0   Λ→pπ - Θ + →K 0 p K 0 → π+π- d CLAS G10: γd→ΛΘ + Decay mode:  K0K0

Krakow, S. Niccolai, IPN Orsay CLAS G11: search for Θ + on proton Reaction channels to study:  p→K 0 K + (n) Θ + →nK +  p→K 0 K 0 p Θ + →pK 0  p→pK -   Θ ++ →pK +  p→K - K     n) Θ + →nK + data taking finished end of July 10 times more statistics than old  p E γ = 0.8 – 3.59 GeV search for ground and first excited states of  + search for  ++ data are being processed

Krakow, S. Niccolai, IPN Orsay CLAS G11: data quality 6% of statistics p --  

CLAS G11: data quality      K0K0 n 6% of statistics

Summary and outlook  possible existence of pentaquarks gave new boost to hadronic physics and QCD spectroscopy  10 Θ + signals published so far, but:  several reports of non-observations  need of high-statistics dedicated experiments to: low statistics discrepancies in measured masses establish existence of Θ + study its properties find possible other pentaquarks (  5 ’s ?) STAY TUNED…  the new CLAS experimental program should solve these issues, with: two experiments completed (G10, G11), results coming soon one experiment finishing in these days (EG3) one experiment to run later on in 2005

Krakow, S. Niccolai, IPN Orsay CLAS-d: variation of cuts No cuts on the K + momentum.. Tighter timing cuts.  T ( P-K + ) < 0.75 ns.