ΛΛ reconstruction in the magnetic field reconstruction in the magnetic field

Evgeny Kryshen

(PNPI, SPbSPU)

25.02.2005 Simulation meeting 2

Simulation frameworkSimulation framework

• 10 000 central Au-Au collisions at 25 AGeV

• UrQMD generator

• CBMROOT framework OCT04

• Magnetic field v03b

• Strip hits with 10 μm resolution

• No particle ID

• Ideal track finding (more than 3 MC points)

• Momentum and vertex reconstruction with Kalman filter (4-order Runge-Kutta extrapolation in magnetic field)

25.02.2005 Simulation meeting 3

Acceptance for Acceptance for Λ

Total acceptance STS: 15.8%

Initial distribution Acceptance

25.02.2005 Simulation meeting 4

Momentum fit performanceMomentum fit performance

25.02.2005 Simulation meeting 5

Secondary vertex fit performanceSecondary vertex fit performance

Z-vertex resolution: 340 μm Momentum resolution: 0.64%

Invariant mass resolution: 0.78 MeV0.86 MeV for 1% momentum resolution assumption1.3 MeV for parabolic approximation (TSR)

25.02.2005 Simulation meeting 6

Applying cutsApplying cuts

The following variables were considered to cut the background:

•bpp – impact parameter of accepted particles•pca – z position of secondary vertex•dca – distance of closest approach•bla – impact parameter of pair momentum

25.02.2005 Simulation meeting 7

Impact parameter of Impact parameter of ΛΛ daughter candidatesdaughter candidates

Positively charged particles Negatively charged particles

25.02.2005 Simulation meeting 8

Vertex parametersVertex parameters

Z position of the fitted vertex Distance of closest approach

25.02.2005 Simulation meeting 9

Pair impact parameter (bla)Pair impact parameter (bla)

25.02.2005 Simulation meeting 10

Selection of the first cutSelection of the first cut

Cut Description Cut value S/B ratio

Signi-ficance

Effi-ciency

-- No cuts -- 0.007 4.5 1.000

bpp Impact parameter for Λ daughters >0.10 cm 0.285 21.0 0.709

bla Pair impact parameter <0.02 cm 0.009 4.92 0.936

pca Z-vertex >2.0 cm 0.060 11.9 0.884

dca Distance of closest approach <0.03 cm 0.010 5.11 0.939

Conclusion: impact parameter cut is the strongest one

25.02.2005 Simulation meeting 11

Cut Description Cut value S/B ratio

Signi-ficance

Effi-ciency

bpp Impact parameter for Λ daughters >0.1 cm 0.285 109 0.709

bpp+bla bpp + pair impact parameter cut < 0.01 cm 75.1 184 0.599

bpp+pca bpp +z-vertex cut >4.0 cm 0.69 123 0.670

bpp+dca bpp +distance of closest approach cut <0.02 cm 8.97 181 0.650

Selection of the second cutSelection of the second cut

25.02.2005 Simulation meeting 12

Selection of the optimal cut valueSelection of the optimal cut value

25.02.2005 Simulation meeting 13

Invariant mass spectraInvariant mass spectra

25.02.2005 Simulation meeting 14

Reconstruction of initial distributionsReconstruction of initial distributions

Corrected signalCorrected signal Initial distributionInitial distribution

25.02.2005 Simulation meeting 15

Reconstructed pReconstructed pTT distributions distributions

y = 1.0 – 1.2 y = 1.8 – 2.0

25.02.2005 Simulation meeting 16

Reconstructed rapidity spectrumReconstructed rapidity spectrum

Initial yield: 40.46

Reconstructed: 40.30