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 HSQCD 21 May 2004

SVD-2 detector layout K    

P-beam

K -

1.a

   

1 2 3 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.

  p +

5

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

Si active target 1 mm pitch

SVD-2 tracking detector Tracking detectors

50 μm 25 μm Pb C 10 mm

X Y X Y X Y

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

U W X Y

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

Si – 300

SVD-2 data analysis

Pb Si C Si Si Si Si s

z = 70÷130µm ,

s

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

• Λ

0

M = 497.61 ± 0.09 (stat.), σ=4.4 MeV/c 2 M = 1115.32 ± 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 MeV/c 2 and to decay into nK + mass, width <15 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 – M p π ≤ 505 MeV/c ≥ 1120 MeV/c 2 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/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 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

) 0,04 0,03 0,02 0,01 0 C Si Pb 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 0 - 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 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