Recent results from SVD-2 experiment Current status Pentaquark search Charmed mesons

A.Kubarovsky, V.Popov. Recent results from SVD-2 experiment QFTHEP17 June 2004

Recent results from SVD-2 experiment

1. Current status2. Pentaquark search3. Charmed mesons

A.Kubarovsky, V.Popov SINP MSU Moscow

(for SVD collaboration)Talk at QFTHEP 2004, Peterhof, St.Petersburg, Russia

17-23 May 2004


SVD-2 experiment

• The main goals (when started in 2000):– Cross section of charm creation at near-threshold

energies– A-dependencies– Searches for intrinsic charm of nucleons (Brodsky,...)

• Most of statistics was collected in 2002• Pentaquark analysis since ‘2003 fall


SVD-2 detector layout

1 23 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.


SVD-2 tracking detector

Si active target1 mm pitch

Pb C

Tracking detectors

25 μm 50 μm

X Y

X Y

X Y U W X Y

10 mm


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


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


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


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


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


uud +2/3

+2/3−1/3

p

K−s−1/

3

u−2/3

Meson: quark-antiquark

+

uu

d

ds+1/3 −1/

3+2/3

−1/3

+2/3

Pentaquark: 4 quarks + 1 antiquark

Exotic if antiquark is of the different flavour

Baryon: three quarks

Hadron nicknames


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)


PDG – in “Particle listings under revision”


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


Anti-decuplet of q4q-bar states

DPP model modified(DP hep-ph/0310212)

(1530) < 15 MeV

M=1760-1810

M=1650-1690

M=1862 = 5 +


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


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


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


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


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


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


Theta mass with another set of cuts applied

• Peak width:

24

18 MeV/c2


Experiments zooSpring8 ELSA

JLab-p

HERMES

ITEP

pp ++.

COSY-TOF

DIANA

SVD-2

JLab-d

ZEUSCERN/NA49H1


Pentaquark observations

A table from E.Klempt, hep-ph/0404270


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


H1: charmed pentaquark?

• M = 3099 ± 3 ± 5 MeV• Minimal quark content:

uuddc• To be confirmed!


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 15/06/04


Current status

• New analysis software is able to process “distant” V0s : – K0 - 10 times more statistics (for Θ+ analysis

and charm searches)– 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

Documents

Recent results from SVD-2 experiment Current status Pentaquark search Charmed mesons