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Talk at HSQCD 2004, Repino, St.Petersburg, Russia 18-22 May 2004

Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c A.Kubarovsky, V.Popov SINP MSU Moscow (for SVD collaboration). Talk at HSQCD 2004, Repino, St.Petersburg, Russia 18-22 May 2004. K -. .  . .  . P-beam. K -. .  .  . 1.a.

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Talk at HSQCD 2004, Repino, St.Petersburg, Russia 18-22 May 2004

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  1. Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c A.Kubarovsky, V.Popov SINP MSU Moscow (for SVD collaboration) Talk at HSQCD 2004, Repino, St.Petersburg, Russia 18-22 May 2004

  2. K-     P-beam K-    1.a p+  1 2 3 5 4 1 High precision microstrip vertex detector. 5 Gamma detector. 1a Active target with Si, C and Pb planes 2 Multiwire proportional chambers. 3 Magnet ( 1.18 T over 3m long region). 4 Multicell threshold Cherenkov counter. SVD-2 detector layout

  3. SVD-2 tracking detector Tracking detectors 50 μm 25 μm Si active target 1 mm pitch Pb C X Y 10 mm X Y U W X Y X Y

  4. SVD-2 trigger Level I • Based on energy depositions in 58 Si active target strips • Basic principle is to look for the >2...3 MIPs depositions in consecutive planes • Decision time: 220 ns • "Non-usable" events contamination: < 10% Level II • Not implemented yet... • Basic idea - a search of "secondary" activity • A simplified "fast" information from vertex tracking detector to be used

  5. SVD-2 run I (April '2002) • IHEP Protvino U-70 accelerator • Proton beam 70 GeV/c • Intensity 5-6×105 1/cycle (1.2 sec) • Total target thickness ~0.5% of hadronic length • 400…600 events/cycle registered • 53,000,000 events stored

  6. Experimental accuracies • X,Y-resolutions for tracks fitted: 8…10 m • Z-resolution: • for the primary vertex: 70… 130 m • for the secondary vertex: 200… 300 m • Impact parameter resolution: ~ 14 m • Momentum resolution for the 5 GeV tracks: 1% • Effective mass resolution: • K0: 4.4 MeV • : 1.6 MeV

  7. Primary vertex Z-coordinate in SVD-2 active target Si – 300m; Pb – 220 m; С – 500 m SVD-2 data analysis Pb C Si Si Si Si Si sz = 70÷130µm, sx,y = 7÷10µm

  8. K0s and Λ0 effective masses • K0s M = 497.61 ± 0.09 (stat.), σ=4.4 MeV/c2 • Λ0M = 1115.32 ± 0.05 (stat.), σ=1.6 MeV/c2

  9. Pentaquark searches • uudd(sbar) quark content • Predicted by Diakonov,Petrov and Polyakov Z.Phys A 359, 305(1997) • Supposed to be of 1530 MeV/c2 mass, width <15 MeV/c2 and to decay into nK+ or pK0 • Observed by several experiments (LEPS,DIANA,CLAS,…) in both decay modes • The pK0 mode can be searched for by SVD-2:good resolution for K0s andproton identification

  10. Data sample • ~35,000 events sample for D-meson searches was used • Primary selection mainly based on the presence of well-defined secondary vertices in the close-to-target region • Secondary tracks are measured by vertex detector • Allowed decay length is 1…35 mm (av. 20 mm), means that only ~10% of K0s to be reconstructed

  11. Events selection for pentaquark analysis • ≤ 5 charged tracks from primary vertex to reduce combinatorial background • K0s selection: • 2 oppositely charged tracks from secondary vertex • 490 ≤ Mππ≤ 505 MeV/c2 • Mpπ≥ 1120 MeV/c2 (→ not Λ0) • Proton selection: • Positive track from primary vertex • Momentum of 4…21 GeV/c • Absence of a Cerenkov detector hit

  12. pK0s spectrum analysis • Cut applied: pK0ssystem goes forward in center-of-mass system • Narrow peak at 1526 MeV/c2 is observed • FRITIOF simulated background fails to reproduce the real shape • Peaks in higher mass region can be attribu-ted to Σ*+ bumps

  13. Cut applied: Pp>PK0 to eliminate bumps (cf. B.Levchenko, arXiv:hep-ph/0401122) • Background can be described in different ways: • FRITIOF simulations • Plain polynom fitting • Random-star (not shown) • All three give the shape compatible to the experimental one

  14. Pentaquark parameters at SVD-2 • Number of events within a peak: 50 over background of 78, gives a significance of 5.6 σ • Mass: 1526 ± 3(stat.) ± 3(syst.) MeV/c2 • Width Γ < 24 MeV/c2 (below experimental resolution) • Strangeness could not be defined directly, but there are no reported S= -1 states in this mass region We interpret this state as Θ+ pentaquark

  15. Cross section and A-dependence • We estimate the total cross-section of pA→Θ+ + X as30÷120 μb/nucleon • We fail to find the difference in A-dependence of Θ+events from that of total for inelastic events (~A0.7) The ratio of Θ+ peakevents to the total of K0 events for different target materials

  16. Current status • New analysis software is able to process “distant” V0s : • K0 - 10 times more statistics (for Θ+analysis) • Λ – up to 100 times more statistics, allowingΞ- decays reconstruction to search for Ξ3/2pentaquark claimed by NA49 • Better kinematical fitting for secondaries to improve effective mass resolution The end

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