Experimental Review of Pentaquarks: Positive and Null Results Forum on Pentaquarks (DESY) February 1, 2005 Ken Hicks (Ohio University)

February 1, 2005K. Hicks, Ohio U. Outline Preliminary Comments and Opinions Evidence for the  + The Null Experiments Some common “myths” New Data (SPring-8) Conclusions

February 1, 2005K. Hicks, Ohio U. Preliminary Remark Congratulations to the ESA on a MAJOR success:

February 1, 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 1, 2005K. Hicks, Ohio U. More Opinions If the  + exists, data suggests it likely favors certain production mechanisms. –This is an exotic baryon. –It may have an exotic production mechanism. To make solid scientific statements: –Calculate the expected rate of production. –Understand the rate of the background. –Compare with acceptance-corrected data.

February 1, 2005K. Hicks, Ohio U. If it exists, what is it? The first  + search was motivated by the chiral soliton model of DPP. –Is it is possible that there is another interpretation of the  + ? We should not be biased toward a particular theory. Lattice QCD suggests that the  + has negative parity (opposite to DPP). –But these are not “gold-plated” calculations Diakonov, Petrov and Polyakov, Z. Phys. A359, 305 (1997).

February 1, 2005K. Hicks, Ohio U. Positive results

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

February 1, 2005K. Hicks, Ohio U. Background well described by D* MC and “wrong charge D” from data Apply mass difference technique M(D*p)=m(K  p)-m(K  )+M PDG (D*) no enhancement in D* Monte Carlo no enhancement in wrong charge D Signal is visible in different data taking periods But no signal seen in ZEUS data (question: different D* accep.?) narrow resonance at M=3099  3(stat.)  5 (syst.) MeV Results from H1 (From Karin Daum)

February 1, 2005K. Hicks, Ohio U. Null Results

February 1, 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 1, 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 1, 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 1, 2005K. Hicks, Ohio U. Slope for mesons Slope for baryons Slope for pentaquarks??

February 1, 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 1, 2005K. Hicks, Ohio U. Some common “myths”

February 1, 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 2 /f 2 “reflections”.

February 1, 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 1, 2005K. Hicks, Ohio U. New Data

February 1, 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 1, 2005K. Hicks, Ohio U. LEPS: Fermi motion corrections MM  (GeV) No large difference among the three Fermi motion correction methods  (1520) resonance

February 1, 2005K. Hicks, Ohio U. Fermi motion corrections:  + MM  (GeV) No large differences among the three Fermi motion corrections.

February 1, 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 1, 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 1, 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 1, 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 1, 2005K. Hicks, Ohio U. K － p missing mass for events with missing momentum > 0.35 GeV/c MMd(γ,K － p) GeV/c 2 sideband regions select VERY PRELIMINARY!

February 1, 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 data: internal review in ~1 month.

February 1, 2005K. Hicks, Ohio U. Outlook There are several new experiments that will help settle the existence question: –SPring-8: LEPS (deuterium: higher statistics) –JLAB: CLAS (g10, g11, eg3) –COSY: TOF –DESY? We still need to understand the null experiments: –background? production mechanism?

February 1, 2005K. Hicks, Ohio U. Model-independent Parity   p p At threshold S-wave dominant If S = 0, then L i = even, P = even ==> P(  ) = + If S = 1, then L i = odd, P = odd ==> P(  ) = - T = 1 K, or K* Thomas, Hosaka, KH, Prog. Theor. Phys. 111, 291 (2004). See full calculation: C. Hanhart et al., hep-ph/

February 1, 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 1, 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 1, 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 1, 2005K. Hicks, Ohio U. A di-quark model for pentaquarks JW hep-ph/ JM 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.