1. B Tagging Efficiency and Mistag Rate Measurement in ATLAS
Thomas Delemontex (LPSC)
Jin Wang (Shandong University && LPSC)

2. Objective and Motivation
B Jet Identification
Light Jet Mistag Rate
B Tagging Efficiency
B Tagging Efficiency
B Tagging Efficiency
Light Jet Mistag Rate
W+jets events
QCD events
W+jets events
QCD events

3. B-tagging Algorithm
B-tagging algorithms are used to tag the b-jets originating from b quarks. In first data, three algorithms are going to be used:
TrackCounting: use tracks with large impact parameter.
JetProb: calculate the probability of a track to be from light jet.
Secondary Vertex Reconstruction (SV0): use the signed decay length significance of the secondary vertex .

4. Definition
Each tagger provides a tag weight w. On this w distribution of all reconstructed jets, we cut at certain threshold , and define :
B-tagging efficiency :
Mistag rate :
Light jet rejection : rejection =

5. B tag efficiency measurement : pTRel method (1/2)
Definition : for a muonic jet, it’s the transverse momentum of the muon compared to the jet axis
CSC Note: CERN-OPEN
Jet axis
Soft muon PT
Form CSC note
θμ-jet
Primary vertex
pTRel is sensitive to jet flavors

6. B tag efficiency measurement : pTRel method (2/2)
Simply a fit of an unknown pTRel distribution with previous templates
Form CSC note
Jet sample without any btag requirement
Adjustment of template normalizations
We find nb jets in this sample
Same sample with a btag requirement
We find ntaggedb jets in this sample
Form CSC note
B tagging efficiency is a simple ratio

7. pTRel method : Results Form CSC note
- pTrel method is OK up to 80 GeV and then becomes less efficient due to similarities between light, c and b templates
- Soon dominated by systematic uncertainties (fragmentation function used, muon pT measurement , misalignment …)
- Can be used with ~ 20pb-1

8. B tag efficiency measurement : System8 method
Use of 3 uncorrelated taggers :
J = b tagging algorithm we want to calibrate  B tagging efficiency in the system = εb(J)
M = muon tagger (pTrel), B = presence of an opposite b tagged jet (enhance bbar events)
Q(X,Y) fractions of jet that survive the X and Y tagger requirement in data
εX efficiency for the X tagger (unknowns)
Κ(X,Y) correlation between X and Y taggers (found in MC)
8 equations and 8 unknowns (εX , fb and fcl) = system fully constraint

9. System8 method results Advantages compared to the pTrel method :
Reduce dependences in Monte Carlo
Systematics clearly defined : error on correlation factors Κ(X,Y)
Advantages compared to the pTrel method :
Problems :
Need more statistics than the pTrel method
Still some problems at high jet pT (use of soft muon tagger pTrel )
Form CSC note

10. Mistag Rate Measurement: Negative Tag Method
System

11. Test of Negative Tag Method in MC

12. Test Results of Negative Tag Method

13. Obtain Mistag Rate Matrix from Data

14. Test Results of Mistag Rate Matrix

15. Real Application in Background Estimation

16. Real Application in Background Estimation

17. Summary