Experimental Review of Pentaquarks: Positive and Null Results SLAC Seminar February 17, 2005 Ken Hicks (Ohio University)

February 17, 2005K. Hicks, Ohio U. Outline The prediction The excitement The pendulum swings back Some common “myths” The quest for higher statistics Conclusions

February 17, 2005K. Hicks, Ohio U. The Prediction

February 17, 2005K. Hicks, Ohio U. The Anti-decuplet predicted by Diakonov et al. Symmetries give an equal spacing between “tiers” Narrow width!

February 17, 2005K. Hicks, Ohio U. Postdiction: diquark model JW hep-ph/ L=1 (ud) s L=1, one unit of orbital angular momentum needed to get J=1/2 + as in  SM Uncorrelated quarks: J P = 1/2 − Decay Width:   MeV  Additional width suppression may come from w.f. overlap.

February 17, 2005K. Hicks, Ohio U. PDG Review Table

February 17, 2005K. Hicks, Ohio U. Width: Indirect Limits Nussinov ( hep-ph/ ):   < 6 MeV Arndt et al. ( nucl-th/ ):   < 1 MeV Haidenbauer ( hep-ph/ ):   < 5 MeV Cahn, Trilling ( hep-ph/ ):   ~ 0.9 MeV Sibertsev et al. ( hep-ph/ ):   < 1 MeV Gibbs ( nucl-th/ ):   ~ 0.9 MeV

February 17, 2005K. Hicks, Ohio U. Width: Possible   Signal? Input mass background (non-reson.) Gibbs, nucl-th/ Widths range: MeV 0.9 MeV = solid Conclude: width  must be ~1 MeV

February 17, 2005K. Hicks, Ohio U. Comments: Width and Parity If the KN database is correct, it is likely that the  + width is  ~1 MeV. If the width is 1 MeV, the parity is almost surely positive. –negative parity width goes up by ~50. If the lattice results are correct, the width is almost surely negative. This problem of width/parity is the most worrisome aspect to the existence of the  +.

February 17, 2005K. Hicks, Ohio U. Positive results

February 17, 2005K. Hicks, Ohio U. Opinions on Pentaquarks: There are valid criticisms for both positive and null experimental results. –A “scorecard” approach will not work. We need better, higher-statistics, data. Science versus emotion –There have been strong statements on both sides of the existence question. – Let’s make scientifically sound statements.

February 17, 2005K. Hicks, Ohio U. Comparison of  + Experiments WhereReactionMassWidth  ’s* LEPS  C  K + K - X < DIANAK + Xe  K 0 p X < 94.4 CLAS  d  K + K - p(n) < SAPHIR  p  K + K 0 (n) < ITEP A  K 0 p X < CLAS  p   + K - K + (n) < HERMESe + d  K 0 p X ~5 ZEUSe + p  e’K 0 p X ~5 COSYpp  K 0 p  < *Gaussian statistical significance: estimated background fluctuation

February 17, 2005K. Hicks, Ohio U. Evidence for Pentaquark States Spring8 SAPHIR JLab-p HERMES ITEP pp   +  +. COSY-TOF DIANA SVD/IHEP JLab-d ZEUS CERN/NA49 H1 Nomad This is a lot of evidence

February 17, 2005K. Hicks, Ohio U. Critical Comments For many experiments, the background shape is not clearly known. Some experiments have harsh angle cuts that could affect the mass spectra. In all cases, the signal is weak compared with standard resonances. –Cuts are necessary to lower background.

February 17, 2005K. Hicks, Ohio U. CLAS: deuterium result ++ Mass = GeV  < 21 MeV Significance 5.2±0.6  N   = 43 events ? Significance = ? Two different background shapes Events in the  (1520) peak.

February 17, 2005K. Hicks, Ohio U. Official CLAS statement “Further analysis of the deuterium data find that the significance of the observed peak may not be as large as indicated.” –We really need a calculation of the background before the statistical significance of the peak can be known. Eventually the new experiment, with much higher statistics, will settle the question. –The g10 experiment (x10 statistics) is now complete, and final results are expected at end of Feb –“Why is it taking so long?” --> It’s only 8 months!!

February 17, 2005K. Hicks, Ohio U. M(nK + ) [GeV] CLAS:  + from the proton no cuts  p →  + K - K + (n) K* 0 Prominent K* 0

February 17, 2005K. Hicks, Ohio U. CLAS:  + from the proton  p →  + K - K + (n) CLAS Collaboration PRL 92, (2004). E  = 3 – 5.5 GeV M(nK+) [GeV] M=1555±10 MeV  < 26 MeV Cos  *(  + ) > 0.8 Cos  *(K + ) < 0.6 n K+K+ K−K− ++  p  N* −−

February 17, 2005K. Hicks, Ohio U. Results from ZEUS NOTES: 1.  + peak is evident only for Q 2 > 20 GeV 2. --> ZEUS suggests that this condition gives the  + enough transverse momentum to get into their detector acceptance. 2. There is an assumption of background shape. --> A different background changes the stat. signifig.

February 17, 2005K. Hicks, Ohio U. HERMES  + spectra signal / background 2:1 standard cuts applied + K * and  veto signal / background: 1:3 add additional 

February 17, 2005K. Hicks, Ohio U. Null Results

February 17, 2005K. Hicks, Ohio U. Production Mechanism? If the  + exists, data suggests it likely favors certain production mechanisms. –This is an exotic baryon. –It may have an exotic production mechanism.

February 17, 2005K. Hicks, Ohio U. Published Null Experiments GroupReactionLimitSensitivity? BES e + e - J/  -->  * <1.1x10 -5 No? Belle e + e -  (2S) --> pK 0 <0.6x10 -5 ?? BaBar e + e -  (4S) -->pK s 0 <1.1x10 -4 ?? ALEPHe + e - ->Z -> pK s 0 <0.6x10 -5 ?? HERA-BpA --> pK s 0 X <0.02x  * No? CDFpp* --> pK s 0 X <0.03x  * No? HyperCPpCu --> pK s 0 X<0.3% K 0 pNo? PHENIXAuAu -->n*K - not given?? BelleK + Si -->pK s 0 X <0.02x  * Yes?

February 17, 2005K. Hicks, Ohio U.  + : Null Results HERA-B data on Carbon target: invariant mass of pK 0 shows no   peak. Could kinematics be an issue? If   is not produced by fragmentation, HERA-B may not see it. 920 GeV beam!

February 17, 2005K. Hicks, Ohio U. Critical Comments Inclusive versus Exclusive measurement –inclusive has better resolution, but more background (especially at higher energy) Backgrounds: combinatorial and from other resonances. Can we estimate? Production mechanism: projectile or target fragmentation? –Is it calculable in some model?

February 17, 2005K. Hicks, Ohio U. Titov: inclusive production (fragmentation region) fast slow Ratio: pentaquark to baryon production Regge exchange dominates (2 = diquarks as quasi-partons)

February 17, 2005K. Hicks, Ohio U.

February 17, 2005K. Hicks, Ohio U. Slope for mesons Slope for baryons Slope for pentaquarks??

February 17, 2005K. Hicks, Ohio U. Hadron production in e + e - Slope: Pseudoscalar mesons: ~ /GeV/c 2 (need to generate one qq pair) Baryons: ~ /GeV/c 2 (need two more pairs) Pentaquarks: ~ /GeV/c 2 (?) (need 4 more pairs) Slope for Pentaquark?? Slope for baryons Slope for p.s. mesons we don’t know the production mechanism!!

February 17, 2005K. Hicks, Ohio U. Some common “myths”

February 17, 2005K. Hicks, Ohio U. Myth #1 “Kinematic reflection of the a 2 and f 2 tensor mesons explain the CLAS data” Some people use a Regge trajectory ( ,  1,  2, etc.) Near theshold (E  <3 GeV) pion exchange dominates Regge exchange. --> For T=(a 2 0 and f 2 ), the  -  -T vertex violates C-parity! --> calculations using diagrams that do not violate C-parity (Y. Oh et al., hep-ph/ ) give  T far too small to explain CLAS data as a “reflection”.

February 17, 2005K. Hicks, Ohio U. Myth #2 “Ghost tracks could be responsible for the peaks seen in the pK 0 mass spectra” This only can happen if there is an error in the tacking software. --> The same track must be used twice! --> All pentaquark (pK 0 ) data analysis has been checked, and no such tracking error is found.

February 17, 2005K. Hicks, Ohio U. New Data

February 17, 2005K. Hicks, Ohio U. Detected nuclear reactions pK nKnp nK  pK  pn         )1520( )() ()( )()( * *   n  K─K─ K+K+ n ++ pp p  K+K+ K─K─ p  nn  N   (1020) N   K + K - N S=-1 S=+1 background

February 17, 2005K. Hicks, Ohio U. New data: LEPS deuterium* MM  (GeV) MM  (GeV) Preliminary   Minimal cuts: vertex, MM  KK =M N, no , E  < 2.35 GeV *in collaboration with T. Nakano

February 17, 2005K. Hicks, Ohio U. LEPS: Fermi motion corrections MM  (GeV) No large difference among the three Fermi motion correction methods  (1520) resonance

February 17, 2005K. Hicks, Ohio U. Fermi motion corrections:  + MM  (GeV) No large differences among the three Fermi motion corrections.

February 17, 2005K. Hicks, Ohio U. LEPS: K － p detection mode (New and Preliminary results) Inclusive production: Θ ＋ is identified by K － p missing mass from deuteron. ⇒ No Fermi correction is needed. γ p n Θ＋Θ＋ K－K－ p γ p n Θ＋Θ＋ K－K－ p  (1520)

February 17, 2005K. Hicks, Ohio U. Event selections in K － p mode MMp(γ,K － p) GeV/c 2 γp→K － pKπ π － mis-ID as K － K ＋ mass M(K － p) GeV/c 2 Λ(1520) Λ(1520) is tightly selected in 1.50–1.54 GeV/c 2 Non-resonant KKp

February 17, 2005K. Hicks, Ohio U. K － p missing mass for events in the  (1520) peak MMd(γ,K － p) GeV/c 2 Small enhancement at 1.53 GeV. But the statistics is not large enough. Hydrogen target data

February 17, 2005K. Hicks, Ohio U. A possible reaction mechanism  + can be produced by re-scattering of K +. K momentum spectrum is soft for forward going  (1520). γ p/n n/p  (1520) K + /K 0  P K GeV/c P K obtained by missing momentum Formation momentum

February 17, 2005K. Hicks, Ohio U. G10 run (E ): March 13 - May 16 Tagged photons in the energy range from 0.8 GeV to 3.59 GeV; Target – 24 cm long liquid deuterium at Z=-25cm; Trigger – two charged particles in CLAS. Data are taken at 2 settings of CLAS toroidal magnet. At each setting integrated luminosity (25pb -1 ) is about 10 times higher than in published deuterium data.

February 17, 2005K. Hicks, Ohio U. First analysis meeting (11/29) Discuss corrections: Eloss, E , momentum; Kinematic fitting ; PID, cuts Physics channels to be analyzed:

February 17, 2005K. Hicks, Ohio U. CLAS:  d ’ K + K - p(n) high-statistics Eloss and E  corrections. Kinematic fit Kinematic fit works. After corrections, it has small effect on masses and resolutions.

February 17, 2005K. Hicks, Ohio U. Silvia Niccolai -  d ’  K + (n) and  d ’  K 0 p (both data set)

February 17, 2005K. Hicks, Ohio U. Summary There is reason for caution about the existence of the  +. –Need better experiments (pos. and null). Experiments need to have better control over the background shape. –Can backgrounds be calculated? The new LEPS data for the  + is interesting, but not conclusive. –CLAS: high-statistics data under review.