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The search for Pentaquarks at

The search for Pentaquarks at. E.C. Aschenauer. DESY. on behalf of the HERMES Collaboration. HERMES @ HERA. HERA @ DESY. Polarized Deep Inelastic Scattering. self-polarised electrons: e. <P b >~ 53 ± 2.5 %. 1 H → <|P t |> ~ 85 ±3.8 % 2 H → <|P t |> ~ 84 ±3.5 %

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The search for Pentaquarks at

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  1. The search for Pentaquarks at E.C. Aschenauer DESY on behalf of the HERMES Collaboration Exotic States 2005

  2. HERMES @ HERA HERA @ DESY Polarized Deep Inelastic Scattering self-polarised electrons: e <Pb>~ 53±2.5 % 1H→<|Pt|> ~ 85 ±3.8 % 2H→<|Pt|> ~ 84 ±3.5 % 1H <|Pt|> ~ 74 ±4.2 % pure nuclear-polarized atomic gas, flipped at 90s time intervals 27.5 GeV (e+/e-) <Pb>~ 53±2.5 % HERA: e+/e- (27GeV) - proton collider Exotic States 2005

  3. The HERMES Spectrometer Internal Gas Target: unpol: H2,D2,He,N,Ne,Kr, Xe He , H , D , H Kinematic Range: 0.02 < x < 0.8 at Q2 > 1 GeV2 and W > 2 GeV Reconstruction:Dp/p < 2%, Dq < 1 mrad Particle ID:TRD, Preshower, Calorimeter a1997: Cherenkov, 1998a: RICH + Muon-ID Exotic States 2005

  4. Particle Identification hadron/positron separation combining signals from: TRD, calorimeter, preshower, RICH Aerogel; n=1.03 p C4F10; n=1.0014 K hadron separation Dual radiator RICH forp,K, p Exotic States 2005

  5. Particle Reconstruction • Define appropriate event topology • Invariant Mass calibration ± 2MeV direct reconstruction: detection of each decay particle, invariant mass reconstruction • Hadron identification: RICH • p: 1 - 15 GeV p: 4 – 9 GeV • Suppress contamination from • L(1116) a pp- Exotic States 2005

  6. Results after all cuts add p to invariant mass • Peak at: • 1528 ±2.6 MeV • = 8 ± 2 MeV Significance: 3.7 Un-binned fit (root): Results are more stable Exotic States 2005

  7. The signal and its background Mixedevent background excited S* hyperons (not included in Pythia6) PYTHIA • Peak at: • 1527 ±2.3 MeV • = 9.2 ± 2 MeV Significance: 4.3 Exotic States 2005

  8. Is there a Q++ • Clear Signal for • L(1520) a pK- • with acceptance: 1.5% • No peak structure for • Q++a pK+ • Zero counts at 91% CL • Q+ not isotensor • probably isosinglet Exotic States 2005

  9. The signal width • Q+ Monte Carlo with complete • detector simulation generated: M=1540 MeV , s=2 MeV reconstructed: M=1539.5 MeV , s=6.2 MeV Detector resolution: FWHM: 10-14.6 MeV measured: FWHM: 19-24MeV intrinsic width: G = 17 +/- 9 +/- 3 MeV Exotic States 2005

  10. How real is the Q+ • check for • “kinematical reflections” • detector acceptance and cuts (pythia6 MC / Toy MC) • Q+vsS*+ • is the Q+a pentaquark or a previously unobservedS*+ ? • add a fourth hadron • is the peak still there? • can we suppress background? • can we guess the production process for theQ+ ? • What about Q-( )? • It would purely be produced in fragmentation. • ~ 4 / 1 no clear Q- signal • strong influence from target remnant to Q+ Exotic States 2005

  11. Q vs S*+ No peak inLp+spectrum near 1530 MeV • Is peak a newS*+or a pentaquark state ? • If peak isS*+⇒also see a peak inM(Lp+) B.R.: (Lp+)/(pKs)  3/2 X- S*+ S*- HERMES preliminary HERMES preliminary Exotic States 2005

  12. More on the Q+ spectra add additional p • signal / background • 2:1 • standard cuts applied • + K* and L veto • signal / background:1:3 Exotic States 2005

  13. Comparison with World Data Kn Mean: Exotic States 2005

  14. What about the X-- • Define appropriate event topology • Hadron identification: RICH • p:0.25 - 15 GeVp: 2 – 15 GeV • Select events from L(1116) a pp- within ±3s of ML • Select events from X(1321) aLp- within ±3s of MX Exotic States 2005

  15. Final spectra for X–-and X0 X0 Mixedevent background UL for X--(1860) cross-section: 1.0 - 2.1 nb UL for X0 (1860) cross-section: 1.2 - 2.5 nb Cross-section for X0(1530): 8.8 – 24 nb Cross section and UL very sensitive to pt and pz distributions Exotic States 2005

  16. Production cross sections and UL Pythia-6 pt and pz spectra • Integrated luminosity: 290 pb-1 • all measurements done in quasi-real • photo-production (Q2<<1GeV2) • Acceptance from MC: - L(1520): 1.5% • - Q+: 0.05% - X0(1530): 0.036-0.1% - X0(1860): - 0.065% - X--(1860): - 0.031% • HERMES estimate: • s(L(1520)) = 62 ± 11 nbs(Q+)= 100-220 nb ± 25% (stat) • (additional factor 2 from production kinematics) • s(X0(1530)) = 8.8 – 24 nb or pt and pz spectra from L measurements of HERMES Exotic States 2005

  17. HERMES contribution to exotic searches X0 Phys. Lett. B 585 (2004) 213 hep-ex/0412027 Exotic States 2005

  18. Conclusion and Outlook • Direct reconstruction ofQ+ invariant mass • Mass: • FWHM-Resolution: • Significance:~ 4 s • Q+ is probably an iso-singlet • Additional p improves signal / background • no signal observed for X--and X0 for • all cross sections and c.s. U.L determined in quasi-real • photo-production (Q2<<1 GeV2) Continue data taking for more statistics Exotic States 2005

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