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

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


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.


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


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


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


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


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


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


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


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


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


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


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


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


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


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

talk at hsqcd 2004, repino, st.petersburg, russia 18-22 may 2004

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