Measurement of the Z production cross section with ATLAS

Transcription

1 Measurement of the Z production cross section with ATLAS Frank Seifert supervised by Arno Straessner, Wolfgang Mader on behalf of the ATLAS Z ττ analysis working group 1

2 Outline 1. Motivation 2. Tau identification 3. Cut flow and W suppression cuts 4. Results and combination 5. Summary 2

3 1. Motivation Measurement of the Z cross section in ATLAS is challenging, because: h One hadronically decaying tau needs to be identified. Huge Multijet cross section at LHC Several background processes with similar signature Multijets and W+jets can produce leptons and fake τ Z (e,µ) 3 Low trigger thresholds for the leptons are needed Unprescaled trigger thresholds become higher with more pile up 3

4 2. Tau identification τ leptons have a a mean lifetime of s. decays ~2/5 into e±/μ± and ~3/5 into hadrons. Multijets and hadronically decaying τ have similar signature and can be distinguished on a statistical basis. 4

5 2. Tau identification τ leptons have a a mean lifetime of s. decays ~2/5 into e±/μ± and ~3/5 into hadrons. Multijets and hadronically decaying τ have similar signature and can be distinguished on a statistical basis. Variables with discriminating power are e.g.: Number of tracks Width pf the decay producs Calorimeter and track radius Invariant cluster and track mass Transverse flight path significance 5

6 2. Tau identification Variables with significant discriminating power are put into a BDT. A cut on the score with an optimum between signal efficiency and background rejection is chosen. 6

7 2. Tau identification Variables with significant discriminating power are put into a BDT. A cut on the score with an optimum between signal efficiency and background rejection is chosen. Boosted decision tree (BDT): multivariate method The cut on the BDT score depends on the pt of the tau candidate (reach flat signal efficiency). Working point with ~60% signal efficiency [1] is chosen (BDT medium) as optimal one for the analysis. [1] The ATLAS Collaboration, Performance of the Reconstruction and Identification of Hadronic Tau Decays with ATLAS, ATLAS CONF , Nov,

8 3. Cut flow and W suppression cuts Dilepton veto Against Z ll background Opposite charge of lepton and h Cuts against W l + jets background h : Nprong= 1 or 3, charge = 1 8

9 3. Cut flow and W suppression cuts Dilepton veto Against Z ll background Opposite charge of lepton and h Cuts against W l + jets background (mu τh channel): mt = sqrt( 2pTlep MET (1 cos[δϕ(lep, MET)]) ) Cut values: ΣcosΔϕ = cos[δϕ(lep, MET)] + cos[δϕ(tau, MET)] Σ cosδϕ > 0.15 mt < 50 GeV 9

10 Cuts against W l + jets background (el τ h channel): mt = sqrt( 2pTlep MET (1 cos[δϕ(lep, MET)]) ) ΣcosΔϕ = cos[δϕ(lep, MET)] + cos[δϕ(tau, MET)] 10

11 Visible mass distributions e τ had channel μ τ had channel The electron τhad fake rate is much higher than the muon τhad fake rate! For the final measurement, the events within the mass window of 35GeV < mvis < 75GeV are taken to further reduce Z ee or Z μμ background. 11

12 4. Results and combination Summary of systematic uncertainties: Most dominant systematics contributions. 12

13 4. Results and combination Combination with the BLUE (best linear unbiased estimate) method [1] Combining correlated estimates of a single physical quantity In correlated case: error matrix E BLUE method looks for an estimate y, which is a linear, unbiased combination with the minimal variance σ2. Constant weighting factors αi of the individual measurements. y = Σαiyi Σαi = 1 σ2 = αte α = ΣiΣj E αiαj ij Finding the αi which minimize σ2. Method is very sensitive to large uncertainties with correlations between the measurements. Therefore the biggest correlated systematics have been excluded from E for the combination and added on the combined result by normal error propagation. Gives a safe combination value with a slightly higher final uncertainty. [1] L. Lyons, D. Gibaut, and P. Clifford, How to combine correlated estimates of a single physical 390 quantity, Nucl. Instrum. Meth. A270 (1988)

14 4. Results and combination The Z ττ cross section is calculated as: Ndata Nbackground σ(z ττ) = AZ x CZ x Luminosity x BR with BR = branching ratio ( ττ μ-τh,el τh, el μ ) μ τh channel el τh channel el μ channel Ndata Nbackground CZ AZ fiducial σ(z ττ)xbr [pb] total σ(z ττ) [pb] Uncertainties are stat. syst. lumi. (Summary of systematics in backup.) Combination of these three channels leads to: σ = pb Ztautau analysis 14

15 5. Summary Systematics dominated result in agreement with the theory prediction and to the measured Z ee/ cross section Ztautau analysis 15

16 Backup 16

17 Correlations between the systematic uncertainties 17