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Thesis Defense of Luminda Kulasiri Dept. of Physics, University of Cincinnati 05.09.2005PowerPoint Presentation

Thesis Defense of Luminda Kulasiri Dept. of Physics, University of Cincinnati 05.09.2005

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Search for exclusive two body decays of B→D h at Belle

*

S

Thesis Defense of

Luminda Kulasiri

Dept. of Physics, University of Cincinnati

05.09.2005

*

Vcs

c

c

s

s

W-

W-

b

u

b

*-

*-

u

Vub

DS

DS

*

Vub

Vcs

p0

B0

B-

p+

d

d

u

u

- Decay via b → u spectator Diagram
- Clean measurement for Vub -No penguin terms
- Model independent
- Not yet seen
- Importantinput for measuring Sin(2β+γ)

CKM Matrix

*+

DS

*

Vcb

Vud

s

W

s

K-

d

u

Motivation-- Evidence for W-exchange
- First seen in B0→ Ds-K+decay
- PRL 89, 231804(2002)
- Not seen
- Role of the final state interactions
- Br can be as large as 10-4
- D+-, D00 can turn out to beDs*-K+
- (B. Block et al. PRL 78, 3999, 1997)

c

B0

Past Measurements & Theoretical Predictions

-PDG 2004

Theoretical predictions

A. Deandrea et al. Phy. Lett. B 318, 549(1993)

a1~1.0, Vub~.003, Vcs~0.97

→

→

D. Choudhry et al. Phy. Rev. D, 45, 217(1992)

KEKB Accelerator

- Asymmetric Collider with,
- 8.0 GeV e- x 3.5 GeV e+
- 22 mrad crossing angle
- Lpeak = 15.33nb-1s-1
- ∫Ldt ~400 fb-1
- Ecm = 10.58 GeV

operates at (4S) resonance

e+e-→ (4S) →BB

(4S) center of mass frame

Belle Detector

Aerogel Cherenkov cnt.

KL/µ Detector

CsI Calorimeter

3.5 GeV e+

TOF counter

SC solenoid

8GeV e-

Silicon Vertex

Detector (SVD)

Central Drift Chamber (CDC)

Belle Detector

- Silicon Vertex Detector (SVD)
- Tracks low momentum particles with CDC
- Vertex reconstruction, 18 µm
- Central Drift Chamber (CDC)Mom. of charged particles is measured from the curvature of the track traversing in the magnetic field
- PID using dE/dx - energy loss by ionization of the matter
- Aerogel Cerenkov Counter (ACC)
- Index of refraction ranges from 1.01 to 1.03
- K/ ID between 1.2 – 4.0 GeV/c

TOF counter

K/ separation using timing of plastic scintillation counters

CsI Calorimeter

Measure energy of e’ns and via detection of scintillation light from e.m. showers.

KL/µ Detector

Detect high mom.(>600 MeV) K/µ

SC Solenoid

Generates 1.5 T mag. field

Particle Identification (PID) at Belle

- Uses information from CDC, TOF, and ACC
- Combine the information using Likelihood method

Pi – likelihood for signal species

Pj – likelihood for background species

Where i, j {e, , K, , p}

- Used 250 fb-1 data at (4S) Center of Mass resonance
- 274.8 million BB events

Decay Chain

B0→ Ds*+-,

B0→ Ds*-K+,

B+→ Ds*+0

Ds*+→ Ds+

Ds+ → {+, KSK+, K*K+}

→K+K-, KS→+-, K*→K+-

Conjugate modes are also assumed

Data

Reconstruction of and, K*0

→K+K-

Kaon ID > 0.6

1.0116 < M(KK) < 1.0272 GeV

(±3 of the nominal mass)

K*0→K+-

K/ ID > 0.6

|M(KK) – 0.8961| < 0.060 GeV

(±3 of the nominal mass)

Helicity angle(θh) – Angle between momentum of DS and momentum of K in (K*) frame.

- Flat dist. for background events.
- Cos2(θ) dist. for signal events.
- Selection requirement,
- |cos(θh)| > 0.3

Bg. evts.

Signal evts.

Reconstruction of K0s

K0s→+-

Pion ID > 0.6

0.4902 < M(+-) < 0.5051 GeV

(±3 of the nominal mass)

2< 30 (vertex reconstruction fit)

Other cuts: dr > 0.009 cm; d < 0.2 rad

dr – smaller of dr1 and dr2, where dr1 and dr2 are the smallest approach from the IP to the two tracks in x-y plane

d- angle betn. the momentum vector and decay vertex displacement vector in r- plane

Signal Events

Background Events

dr(cm)

dr(cm)

d(rad)

d(rad)

Bg.

Signal

Reconstruction of DS+and DS*+

±3 of the nominal mass

(1968.5±0.6 MeV)

1.9539 < M() < 1.9833 GeV

1.9471 < M(KSK) < 1.9901 GeV

1.9495 < M(K*K) < 1.9877 GeV

momentum selection

1.7< P(cms) < 2.5 GeV

M = M(DS*)-M(DS)

M has better resolution than mass.

0.124 < M < 0.164 GeV

A large portion of the background is accounted by photons that are not really coming from DS*.

E(cms) > 110 MeV

0 veto – reject if 0.12 < M() <0.15 GeV

GeV

GeV/c2

B→Ds*+- (Ds+→+)

Selection of the B Candidate

Two quantities M(B) and E

are defined as,

E vs. M(B)

Signal MC

where Ebeam = 5.29 GeV,

Pi- mom. of B, Ei-energy of B

Signal Region,

–0.05 < E < 0.05 GeV for h {, K}

–0.10 < E < 0.05 GeV for h {0}

5.27 < M(B) < 5.29 GeV/c2

Largest background source is e+e-→ qq (q{u,d,s,c}) events

Fisher Discriminant is a powerful tool to discriminate signal and background

Background Suppression – Fisher Discriminant (FD)

- A linear combination of 9 variables
- Optimized to discriminate signal from background

- Cos(θth) – Angle between thrust axis of the B candidate
- and the thrust axis of the remaining particles.
- Cos(B) – Angle between the B momentum & beam axis
- qr x (QD) – qr contains flavor information of the other B;
- q = ±1; 0<r<1; QD – charge of Ds

Where is a unit vector s. t. it maximizes T

is the mom. of the ith particle in CM frame

s

s

Combine all 9 variables into F

Background Suppression – Fisher Discriminant (FD)

(cont.)

vs FD plots to decide the best

selection

Background Suppression – Fisher Discriminant (FD) (cont.)

All the parameters are optimized to get the maximum discrimination between signal and background

continuum

Signal

Arbitrary units

s - Signal

b - background

FD

N - inclusive Ds* yield ; - efficiency ;

Reconstruction Efficiencies

- First used Sig. MC – fitting M(B)
- Observed inconsistency among the yields of DS sub modes
- Minimized MC dependence by using inclusive M
- Efficiency for mode obtained using sig. MC
- Total eff. obtained by multiplying by eff. of the other cuts

Following relationships can be obtained

- Sidebands of Ds and M used
- 3 from lower and upper side of the signal region
- used to compare data and MC
- Background shapes were obtained

Observations:

- Bkg. shapes of data and MC agree each other
- Observed a disagreement in bkg. levels ~12% – 22%
- Bkg. of Ds not random – real Ds but not from Ds*→Ds

- Simultaneous fitting of 3 DS sub decay modes
- Common branching fraction for all 3 DS sub-decay modes
M-1D

- Signal - Gaussian shape with mean & fixed to sig. MC shape
- Bkg. - linear shape, by fitting data excluding the signal region
E-1D

- Signal - Gaussian shape with mean & fixed to sig. MC shape
- Bkg. - sideband shapes of M

Lmax - max. likelihood

L(0) – max. likelihood if

signal is zero

Ds*+-

Ds*-K+

E fit

Solid line (red) – total fit

dotted line (blue) - background

Ds*+0

DS*+-

DS*+K-

- M fit
- Solid line (red) – total fit
- dotted line (blue) - background

DS*+0

Since 3 Ds modes,

Total Syst. error = common syst. errors + indept. syst. errors

Common Errors (%)

Systematic Uncertainties-cont.

Independent Errors

RD* for Sin(21+3)

D* - strong phase,c – Cabibbo angle

D. Becirevic, Nucl. Phys. Proc. Suppl. 94, 337 (2001)

- good agreement with the expected result which is ~0.02

- Used 278.4 million events
- M fits give more consistent results
- Both M and E results agree within errors
- Obtained estimates for Vub and RD*

Summary

Combined yields from M fit (274.8 m evts.)

DS*+-

19.0±5.7 evts

DS*-K+

11.0±4.8 evts

DS*+0

9.4±5.5 evts

Combined yields from M(B) fit (274.8 m evts.)

DS*+-

21.4±5.9 evts.

DS*-K+

10.6±4.9 evts.

DS*+0

8.1±5.7 evts.

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