A Method to measure +/ detection efficiency asymmetry at LHCb

Liming Zhang

08/15/07

2

Idea

• Partially reconstruct D+ sD0, D0KS

0+without => partial yield

• Combine to fully reconstruct D0 => full yield • full/partial yield => absolute efficiency

• Br(bD*+X) = (17.3 2.0)%

• Br(D*+ D0 KS0(++ ) = (1.36 0.09)%

3

MC Samples

• 491 k signal MC B0D*+, D*to this channel• 680 k inclusive b MC• 766 k inclusive b biased MC

4

Signal MC

M(KS +)

m = M(KS ++) M(KS +)

M(KS +) in [1.5, 1.7] GeV

M(KS +) in [1.0, 1.3] GeV

Reconstructed Distributions

Only consider this region here

5

Optimization of Selection Criteria

• Maximize Figure of Merit FOM = S/sqrt(S+B) on inclusive b MC

• S: signal yield in this inclusive b MC, predicted by using efficiency from signal MC

• B: background yield scaled from m sideband of this inclusive b MC

• The signal region: m<15 MeV

• Sideband region: [50, 150] MeV

m background distribution

Sideband region

Signal

region

6

KS0 reconstruction

• KSDD (2/3) (only used here) • KSLL (1/3)

• Daughters IP significance > 2• Daughters track fit • Vertex • Pt > 1 GeV/c• Proper time/life time > 0.17• Vr>21 mm• Vz [150, 2330] mm

• Mass window 17 MeV

7

Distributions of KS0

purity 80% ~ 10 MeV

0

000

)(S

SS

Kz

Dz

Kz

K

P

VVmt

> 21 mm

>0.17

[150,2330] mm

17 MeV

Signal

Background

8

Selection Criteria

• Prompt + from D0:– PID

– IP significance > 5

– Pt > 700 MeV

Signal

Background

9

Selection Criteria

• slows+:

– IP significance > 1

– Pt > 220 MeV

Signal

Background

10

Selection Criteria

• D0 vertex • D* vertex • Multiply candidates selection

– Same D0 combines with several slow pion

– Choose the one which has the best D* vertex

– B/S reduced from 2.0 to 1.7

• Not helpful from cuts on IP significance, flight distance (significance) etc.

11

Results of partial reconstruction

• Efficiency (before Trigger) = NREC/NGEN GEN = 1656/391k 20% = (0.85 0.02) 103

• Annual yield (before Trigger)– S = 2fb1500b2(17.32.0)%(1.360.09)%(0.85 0.02)

103

= (4.0 0.3) 106

• Signal from bb inclusive MC data– Expect 31

– 29 found

– Fit gives 339

– B/N ~ 1.7

12

TO DO

• To check background sources and further suppress background level if possible

• Trigger efficiency

• To fully reconstruct D0 by combining with partially reconstructed candidate

• To find efficiency of and give the prediction of stat. error on detection asymmetry from this method