1. 29/03/04A. Geiser, Recent Spectroscopy Results from ZEUS 1 Recent Spectroscopy Results from the (H1 and) ZEUS Experiments nSelected main topic: evidence/search for pentaquarks  Introduction  Evidence for a resonance consistent with     p and its antiparticle (ZEUS)  ( Evidence/search for a charmed pentaquark (H1/ZEUS) -> see talk by K. Lipka tomorrow) Achim Geiser, DESY Hamburg for the ZEUS collaboration XXXIXth Rencontres de Moriond QCD and Hadronic Interactions at High Energy La Thuile, Italy, 29. March 04 hep-ex/04040xx (soon)

2. 29/03/04A. Geiser, Recent Spectroscopy Results from ZEUS 2 nQCD: only colourless states can exist as free particles (confinement) nconventional wisdom: bound multiquark states (hadrons): qq (mesons) qqq (baryons) nbut: other colourless combinations not forbidden: qqqq (tetraquarks) qqqqq (pentaquarks)... ncan they be bound tightly enough to have narrow width? What are pentaquarks?

3. 29/03/04A. Geiser, Recent Spectroscopy Results from ZEUS 3 nsome QCD models: yes! e.g. Diakonov et al. 1997: uudds =   m~1530 MeV,  < 15 MeV nrecently observed by several fixed target experiments:    n K +, LEPS, CLAS, SAPHIR (establishes exotic quantum numbers) m~1540+-5 MeV, width ~ resolution    p K 0 s, DIANA, ITEP, HERMES, SVD, COSY-TOF (more precise measurements) m~1527+-3 MeV, width ~ resolution nevidence for uussd, ddssu by NA49 (not seen by HERA-B) Evidence for pentaquarks fixed target => pentaquark uses valence quarks

4. 29/03/04A. Geiser, Recent Spectroscopy Results from ZEUS 4  s = 300 (318) GeV ep CM energy before (after) 1998 W = m(  p)  p CM energy Q 2 = -q 2 photon virtuality, squared momentum transfer Q 2 > 1 GeV 2 : DIS Q 2 < 1 GeV 2 : photoproduction uudds =   „established“, I=0 no  ,   (?) what about uudds =  - ? (this talk) (can only be produced in fragmentation) what about uuddc/uuddc =  c ? -> talk K. Lipka tomorrow (need high CMS energy) nlook in high energy collider experiments, e.g. deep inelastic ep scattering (DIS) at HERA: Pentaquarks in high energy collisions? p q2q2

5. 29/03/04A. Geiser, Recent Spectroscopy Results from ZEUS 5 ninclusive DIS event sample: Q 2 > 1 GeV 2 data 1996-2000, 121 pb -1 nK 0 selection: p T (K 0 ) > 0.3 GeV eliminate  and photon conversion candidates 866800 K 0 ‘s Search for  + p K 0 s /  - p K 0 s in ZEUS

6. 29/03/04A. Geiser, Recent Spectroscopy Results from ZEUS 6 nseveral resonances observed nproduced in gluon-rich environment nf 0 (1710) = glueball candidate?? Intermezzo: look for K 0 s K 0 s resonances

7. 29/03/04A. Geiser, Recent Spectroscopy Results from ZEUS 7 nselect protons by defining ionization band in dE/dx dE/dx > 1.15 mips, p(p) ~60% proton purity Search for  + p K 0 s /  - p K 0 s in ZEUS pp K+K+ K-K- ++ 

8. 29/03/04A. Geiser, Recent Spectroscopy Results from ZEUS 8 nwell established narrow d-wave uds state, isospin 0, charge 0 isospin 1 partner is expected to have higher mass => do NOT expect  + or X ++ in same place nselect p,K using dE/dx resonance seen in p K - + c.c., not seen in p K + + c.c. nmass and natural width consistent with PDG: M = 1519.5 +- 1.0 MeV  = 15.6 +- 1.0 MeV nno evidence for hypothetical  ++ (1530) (I=1 pentaquark) Check: look for   (1520) p K + + c.c.

9. 29/03/04A. Geiser, Recent Spectroscopy Results from ZEUS 9 nselect p/p and K 0 s in inclusive DIS (Q 2 >1 GeV 2 ) nplot m(pK 0 s ) + m(pK 0 s ) nstructure emerges for increasing Q 2 or low W around m~1525 MeV Search for  + p K 0 s /  - p K 0 s in ZEUS

10. 29/03/04A. Geiser, Recent Spectroscopy Results from ZEUS 10 nfit bg + single Gaussian peak at 1522+-2 MeV width consistent with resolution statistical significance ~ 3.9  still structure left of peak? also try different background shape Search for  + p K 0 s /  - p K 0 s in ZEUS

11. 29/03/04A. Geiser, Recent Spectroscopy Results from ZEUS 11 nfit bg + two Gaussians (2nd Gaussian parametrizes potential resonance or empirical shoulder in background shape) m = 1521.5 +- 1.5 (stat) +2.8 -1.7 (sys) MeV Gaussian width 6.1 +- 1.5 MeV still compatible with experimental resolution of ~2 MeV (Breit-Wigner fit:  = 8+-4 MeV) significance ~ 4.6  signal seen in both charges (inset) if interpreted as  + +  - antipentaquark! Search for  + p K 0 s /  - p K 0 s in ZEUS

12. 29/03/04A. Geiser, Recent Spectroscopy Results from ZEUS 12 nsummary of fit results: p(fluctuation) m = 1500-1560 MeV  = 1.5-12 MeV: 6 x 10 -5 nother systematic checks (not part of the default selection):  remove proton candidates consistent with misidentified  from K* -> K 0   momentum imbalance: require E(p) > E(K 0 ) => peak significance increases further (including Q 2 >1 GeV 2 ), but complicated background shape -> do not use  check that reflections from D s +- -> K 0 K +- or other charm resonances can not produce such a peak  assign proton mass to pion enriched sample using dE/dx; mix K 0 and p from different events -> no peaks Search for  + p K 0 s /  - p K 0 s in ZEUS

13. 29/03/04A. Geiser, Recent Spectroscopy Results from ZEUS 13 npentaquarks and other exotic multiquark states are allowed in principle in QCD, but are they narrow enough to be seen? recent evidence from fixed target experiments, suggested by some QCD models: yes! (  + = uudds, m ~ 1530 MeV) nZEUS finds evidence for a narrow resonance in p K 0 s and p K 0 s in DIS events at HERA, consistent with  + +  - m = 1521.5 +- 1.5(stat) +2.8 -1.7 (sys) MeV width still consistent with exp. resolution of 2 MeV (  = 8+-4 MeV) if pentaquark interpretation confirmed: first observation of the  - state (antipentaquark) nnew era in spectroscopy: this is just the beginning! Summary and Conclusions