A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c HSQCD 21 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 May 2004

A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c HSQCD 21 May 2004 SVD-2 detector layout

A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c HSQCD 21 May 2004 SVD-2 tracking detector Si active target 1 mm pitch PbC Tracking detectors 25 μm 50 μm X Y U W X Y 10 mm

A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c HSQCD 21 May 2004 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

A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c HSQCD 21 May 2004 SVD-2 run I (April '2002) IHEP Protvino U-70 accelerator Proton beam 70 GeV/c Intensity 5-6×10 5 1/cycle (1.2 sec) Total target thickness ~0.5% of hadronic length 400…600 events/cycle registered 53,000,000 events stored

A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c HSQCD 21 May 2004 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

A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c HSQCD 21 May 2004 Primary vertex Z-coordinate in SVD-2 active target SVD-2 data analysis Si Pb Si C Si – 300  m; Pb – 220  m; С – 500  m  z = 70÷130µm,  x,y = 7÷10µm

A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c HSQCD 21 May 2004 K 0 s and Λ 0 effective masses K 0 s M = ± 0.09 (stat.), σ=4.4 MeV/c 2 Λ 0 M = ± 0.05 (stat.), σ=1.6 MeV/c 2

A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c HSQCD 21 May 2004 Pentaquark searches uudd(sbar) quark content Predicted by Diakonov,Petrov and Polyakov Z.Phys A 359, 305(1997) Supposed to be of 1530 MeV/c 2 mass, width <15 MeV/c 2 and to decay into nK + or pK 0 Observed by several experiments (LEPS,DIANA,CLAS,…) in both decay modes The pK 0 mode can be searched for by SVD-2: good resolution for K 0 s and proton identification

A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c HSQCD 21 May 2004 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 K 0 s to be reconstructed

A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c HSQCD 21 May 2004 Events selection for pentaquark analysis ≤ 5 charged tracks from primary vertex to reduce combinatorial background K 0 s selection: –2 oppositely charged tracks from secondary vertex –490 ≤ M ππ ≤ 505 MeV/c 2 –M pπ ≥ 1120 MeV/c 2 (→ not Λ 0 ) Proton selection: –Positive track from primary vertex –Momentum of 4…21 GeV/c –Absence of a Cerenkov detector hit

A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c HSQCD 21 May 2004 pK 0 s spectrum analysis Cut applied: pK 0 s system goes forward in center-of-mass system Narrow peak at 1526 MeV/c 2 is observed FRITIOF simulated background fails to reproduce the real shape Peaks in higher mass region can be attribu- ted to Σ* + bumps

A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c HSQCD 21 May 2004 Cut applied: P p >P K0 to eliminate bumps (cf. B.Levchenko, arXiv:hep-ph/ ) 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

A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c HSQCD 21 May 2004 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/c 2 Width Γ < 24 MeV/c 2 (below experimental resolution) Strangeness could not be defined directly, but there are no reported S= -1 states in this mass region

A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c HSQCD 21 May 2004 Cross section and A-dependence We estimate the total cross-section of pA→Θ + + X as 30÷120 μb/nucleon We fail to find the difference in A-dependence of Θ + events from that of total for inelastic events (~A 0.7 ) The ratio of Θ+ peak events to the total of K 0 events for different target materials

A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c HSQCD 21 May 2004 Current status New analysis software is able to process “distant” V 0 s : –K times more statistics (for Θ + analysis) –Λ – up to 100 times more statistics, allowing Ξ - decays reconstruction to search for Ξ 3/2 pentaquark claimed by NA49 Better kinematical fitting for secondaries to improve effective mass resolution