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

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

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 .

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 =

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-2008-020 Jet axis Soft muon PT Form CSC note θμ-jet Primary vertex pTRel is sensitive to jet flavors

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

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

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

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

Mistag Rate Measurement: Negative Tag Method System

Test of Negative Tag Method in MC

Test Results of Negative Tag Method

Obtain Mistag Rate Matrix from Data

Test Results of Mistag Rate Matrix

Real Application in Background Estimation

Real Application in Background Estimation

Summary