Data analysis of April'2002 SVD-2 run. Pentaquark searches Detection capabilities Charmed mesons

A.Kubarovsky, V.Popov,V.Volkov Data analysis of April'2002 SVD-2 run. Pentaquark searches

VHMP16 April 2005

Data analysis of April'2002 SVD-2 run. Pentaquark searches

1. Detection capabilities2. Charmed mesons3. Pentaquark searches

A.Kubarovsky, V.Popov, V.Volkov SINP MSU Moscow

Talk at VHMP 2004, Dubna, Russia 16-17 April 2005


VHMP16 April 2005

• 53,000,000 events collected • ~7% are “beam only”/non-useable• Basic way of pre-analysis:

– Finding a primary vertex– Search for a secondary one(s) within different regions

and with different detectors/methods to use/CPU time to be consumed

• Average CPU time per event: up to 7 PIII-1000 seconds

• Facilities: SINP and SRCC MSU “old” clusters.


VHMP16 April 2005

P-beam

K-

K-

p+

Region I(1-35 mm)

Region II (5-60 cm)


VHMP16 April 2005

Primary vertex Z-coordinate in SVD-2 active target

SVD-2 data analysis

Si Si

Pb

SiC Si

Si

Si – 300m; Pb – 220 m; С – 500 m

z = 70÷130µm, x,y = 7÷10µm


VHMP16 April 2005

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


VHMP16 April 2005

• Reconstruction of V0s with the spectrometer (“distant” ones):– K0 : σ=5.8 MeV/c2

– Λ0 : σ=3.0 MeV/c2


VHMP16 April 2005

• The well-known resonances of the same topology as +

• Arrows correspond exactly to the PDG data


VHMP16 April 2005


VHMP16 April 2005

Near-threshold D0 production

• Search for the D0 K+π- decays

• Different reconstruction methods used (not cross-checked yet…)

• 24 events over background of 12

V.Volkov


VHMP16 April 2005

A start of pentaquark history

An idea of five-quark states was discussed since late 60’s (Jaffe, Lipkin, Strotteman, …)

First estimates in the Skyrme model (Chemtob, Praszalowicz, Walliser) – 1985...

There were no conclusive results from experimental searches

“The general prejudice against baryons not made of three quarks and the lack of any experimental activity in this area make it likely that it will be another 15 years before the issue is decided”. (PDG, 1986)


VHMP16 April 2005

PDG – in “Particle listings under revision”


VHMP16 April 2005

In the ‘2003 fall...

• uudds 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


VHMP16 April 2005

Experiments zooSpring8 ELSA

JLab-p

HERMES

ITEP

pp ++.

COSY-TOF

DIANA

SVD-2

JLab-d

ZEUSCERN/NA49H1


VHMP16 April 2005

PENTA-I : 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


VHMP16 April 2005

Penta-I:Events selection

• ≤ 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


VHMP16 April 2005

Penta I : 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


VHMP16 April 2005

Penta I : 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


VHMP16 April 2005

Penta I : 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


VHMP16 April 2005

Penta I : 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


VHMP16 April 2005

Theta masses from different experiments

• Certain mass shift can be observed between the K+n and K0p modes

Spring-8

CLAS-d

CLAS-p

SAPHIR

ITEP

HERMES

COSY

ZEUS

SVD-2

1520 1540 1560

K0p

K+n


VHMP16 April 2005

Penta I : second analysis

•To be checked…•Possibly could result in a much more narrow peak


VHMP16 April 2005

Penta II : event selection

• “Distant” K0 used• Cuts applied:

– Number of primary charged tracks: 4 to 6– Proton candidate momentum: 9 ≤ Pp ≤ 15 GeV/c– No Cerenkov detector hit for a proton

• A total of 9,000 events selected


VHMP16 April 2005

Penta II : results

• Peak is well pronounced (6 σ)

• No significant change in the effective mass

• Cross-section and systematic errors are yet to be evaluated


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Pentaquark II (“distant” K0)

• M=1524 ± 2.3 MeV/c2

• σ = 12.0 ± 2.3 MeV/c2

• Nevents = 187• Significance 5.9 σ

No significant mass difference from the previous analysis

Documents

Data analysis of April'2002 SVD-2 run. Pentaquark searches Detection capabilities Charmed mesons