iop march 2005

IOP March 2005

Opposite Side Flavour Tagging At DØ

Philip Lewis, Imperial College London

- B mixing at DØ- Flavour tagging- Studying the opposite side flavour tagger- Towards BS mixing measurement

IOP March 2005 Phil Lewis

B mixing at the Tevatron

Measurement of Δms / Δmd allows a stringent test of the CKM formalism of the standard model and a precise determination of Vtd

Δmd = 0.502±0.007 ps-1 (world average)Δms > 14.9 ps-1 (combined limit at 95% CL)Δms = 18.3 ps-1 S.M. (15.6 - 22.2 at 95% CL)

At present the Tevatron is the only place where Bs mixing can be measured


The DØ Detector

Run II upgrade:

- Silicon Vertex Detector (SMT)- Central Fibre Tracker (CFT)- 2T Solenoid- Forward Muon system

Coverage

- Tracking | η | < 3

- Muon | η | < 2

Mixing measurement: - B lifetime - mixed/unmixed i.e. initial state & final state

Final State: from reconstructionInitial state: same side tag - charge correlation of fragmentation pions opposite side tag - from other b in bb pair production


Flavour Tagging

B B π* π

Kμ±

μ±

Same side tag

Opposite side tag

Reconstructed B


Opposite Side Flavour Tags

O.S. Tags- Lepton tag: b→Xl-, b→Xl+ branching ratio 10% into e, μ

- Partial reconstruction of secondary vertex (SV tag)- Jet charge tagging

O.S. Tags independent of reconstructed side→ studies on B+, B0 (Bu, Bd) applicable to Bs

O.S. B meson can mix (B0, Bs) 17.4% B0, 50% Bs opposite side mesons will be mixed

B B π* π

Kμ±

μ±

Opposite side tag


Tagging Study: Samples

B+→J/ψ K+ B+→D0μ+

B+→D0→KπB0 →D*-μ+

B0 →D*-→D0π-

NB = 8130 ± 130 NB = 77460 ± 390 NB = 25820 ± 60

M(J/ψ K+) GeV/c2M(Kπ) GeV/c2 M(D0π)-M(D0) GeV/c2

460 pb-1 data

Efficiency ε = Ntagged events / Ntotal Dilution D = (Ncorrect tags – Nwrong tags) / (Ncorrect tags + Nwrong tags)Tagging power ~ εD2

Simple O.S. muon tag: charge of muon gives b flavour


Tagging Study: O. S. Muon Tag

B+→J/ψ K+

ε 5.47 ± 0.36 %

D 0.37 ± 0.04

εD2 0.76 ± 0.17 %

N tagged 444 ± 25

N correct 305 ± 20

- hits in SMT and CFT, >1 muon detector segment- PT muon > 2.5 GeV/c2

- not in reconstructed B jet

M(J/ψ K+) GeV/c2

B+→J/ψ K+

To improve ε, D we can make more sophisticated taggers, and combine different types of tagger:

Calculate dilution dependant on properties of O.S. muon e.g. - number of segments of muon detector - PT muon relative to jet - charge of muon jet

Currently use muon + SV combined tag:


Combined Flavour Tag

B+→J/ψ K+ Simple tag Combined tag

ε 5.47 ± 0.37% 7.12 ± 0.41%

D 0.37 ± 0.04 0.38 ± 0.04

εD2 0.76 ± 0.17% 1.01 ± 0.19%


Fitting Procedure for B0 Sample

For tagging studies of B+→D0μ+, B0→D*-μ+ we need to account for sample composition:

To fit B0d component need to measure oscillations.

- split into VPDL bins and measure asymmetry in each bin- compare with calculated asymmetry.- correct for K factor and detector resolution

Binned fit determines Δmd and dilution for each sample

B0d B+ Bs

B0→D* sample 84% 14% 2%

B+→D0 sample 16% 82% 2%


Dilution of Combined Tagger

Dilution (B+) = 0.47 ± 0.03 Dilution (B0) = 0.45 ± 0.04Δm = 0.558 ± 0.048

Asymmetry = (Nunmixed -Nmixed ) / (Nunmixed +Nmixed)


Towards Bs Measurement at DØ

Preliminary mixing result to be presented soon.

Bs→DsμνXBs→Ds→φπBs→Ds→φ→KK

Improving flavour tag is vital to making the best possible measurement.- increase efficiency of muon tagger- addition of electron tag

Move to unbinned fit method - can use event by event dilution- need to make sure we can calculate this reliably

World Average Δms


Summary

- Observation of Bs oscillations is a major goal of the Tevatron

- To do this need good performing, well understood flavour tagging

- Combined muon and SV opposite side flavor tagger studied at DØ

- Dilution of tagger consistent for Bd and Bu mesons; suitable for application to Bs decays

- Several improvements to the tagger in development

- Preliminary Bs mixing measurement to be presented soon

iop march 2005

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