talk at hsqcd 2004, repino, st.petersburg, russia 18-22 may 2004
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c
HSQCD21 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
A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c
HSQCD21 May 2004
SVD-2 detector layout
1 2
3 54
1.a
P-beam
K-
K-
p+
1 High precision microstrip vertex detector. 5 Gamma detector.1a Active target with Si, C and Pb planes2 Multiwire proportional chambers. 3 Magnet ( 1.18 T over 3m long region). 4 Multicell threshold Cherenkov counter.
A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c
HSQCD21 May 2004
SVD-2 tracking detector
Si active target1 mm pitch
Pb C
Tracking detectors
25 μm50 μm
X Y
X Y
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
HSQCD21 May 2004
SVD-2 trigger
Level I• Based on energy depositions in 58 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
HSQCD21 May 2004
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
A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c
HSQCD21 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
HSQCD21 May 2004
Primary vertex Z-coordinate in SVD-2 active target
SVD-2 data analysis
Si Si
Pb
Si
C Si
Si
Si – 300m; 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
HSQCD21 May 2004
K0s and Λ0 effective masses
• K0s M = 497.61 ± 0.09 (stat.), σ=4.4 MeV/c2
• Λ0 M = 1115.32 ± 0.05 (stat.), σ=1.6 MeV/c2
A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c
HSQCD21 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/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 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
HSQCD21 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 K0
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
HSQCD21 May 2004
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
A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c
HSQCD21 May 2004
pK0s spectrum analysis
• Cut applied: pK0s
system 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
A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c
HSQCD21 May 2004
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
A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c
HSQCD21 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/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
A.Kubarovsky, V.Popov Observation of narrow exotic baryon resonance by the SVD-2 experiment in pA interactions at 70 GeV/c
HSQCD21 May 2004
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)
0
0,01
0,02
0,03
0,04
C Si Pb
The ratio of Θ+ peak events to the total of K0 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
HSQCD21 May 2004
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, allowing
Ξ - decays reconstruction to search for Ξ 3/2
pentaquark claimed by NA49• Better kinematical fitting for secondaries to
improve effective mass resolution