ATLAS muon reconstruction efficiency with a tag & probe technique on Z boson dimuon decay

Transcription

1 ATLAS muon reconstruction efficiency with a tag & probe technique on Z boson dimuon decay Alessandro Manfredini-----Muon Combined Performance group (Roma Tre University)

2 Talk structure: Introduction: LHC & ATLAS Definitions Data analysis: Selection of the Z boson Tag & Probe method on Z muon decay

3 Introduction: LHC is a PP collider running at CERN Nowadays collision energy = 7 TeV ATLAS is one of the 6 experiments now taking data at LHC ATLAS is a General purpose detector LHC & ATLAS

4 Introduction: ATLAS aims It will investigate a wide range of physics: Known processes (as heavy flavour physics, electroweak processes, ecc.. ) Existence of the Higgs Boson New physics (as SuperSymmetry ecc..) In this first phase a big effort is made in studying detector performance

5 Introduction: muon spectrometer hadronic calorimeter Electromagnetic calorimeter Inner Detector tracker ATLAS structure

6 Introduction: muons track can be reconstructed in 3 ways: muon reconstruction Inner detector (ID) track pointing to muon chamber hits Muon spectrometer (MS) Match MS + ID tracks: combined muon how much are we efficient in muon reconstruction? Efficiency depends mainly on MS performance Reconstruction efficiency ~96% (Monte Carlo) # MS chamber crossed by a muon in function of η and φ

7 Introduction: aim of the game Measure from data the efficiency to have a combined muon given a inner detector track Achieved with a Tag and Probe technique on Z dimuon decay 7

8 Introduction: aim of the game (2) Study the process PP Z + X μμ + X: study the signature of the Z boson muon decay study the backgrounds tune a set of selection cuts Develop a tag & probe method on Z μμ: good estimate of the reconstruction efficiency possibly unbiased robust and independent from cuts variation 8

9 Z selections: production/signatures Mainly produced by Drell-Yann processes leading order next to leading order Signatures of Z μμ: 2 high Pt muons invariant mass ~ MZ isolated muons small D0 w.r.t. primary vertex 9

10 Z selections: Backgrounds I neglet: cosmics and muon from light flavours decay. I consider as Backgrounds: Process bb σ (nb) notes filter: 4 GeV Pt muon cc 8400 filter: 4 GeV Pt muon tt 0.14 w μν 9 Z ττ 0.85 plus adronic muon 10

11 Z selections: invariant mass plot Dimuon invariant mass plot (cumulative backgrounds) 11

12 Z selections: cuts Preselection: Vertex_ntrack > 2 # muon >1 Trigger: L1MU6 Standard quality track requirements (SCT > 5 ecc...) Isolated: Etcone_40 < 3 GeV Impact parameter: D0_PV < 0.15 mm 2 μ Pt > 15 and Pt > 20 GeV (asymmetric cut) 2 μ with opposite charge Invariant mass: MZ Mμμ < 25 GeV No back to back in the η-plane: θ1 + θ2 -π >

13 Z selections: cut tuning Variate the cut's threshold in step for each step calculate the acceptance for signal and background Plot: signal VS background acceptances signal VS background acceptances 13

14 Z selections: acceptaces Z μμ acceptance (Monte Carlo) Selezione # event Acceptance tot geometrical acceptance Trigger μ good quality Isolation D asymmetric Pt cut M MZ < 25 GeV

15 Expected event in 330 nb 1 Process Acceptance # expected event in 330 nb-1 Z μ+ μ ± ± 13 W μν Z ττ tt bb + cc tot (1 ± 1) 10-5 (2 ± 2) 10-2 (5.0 ± 1.5) 10-5 (1.4 ± 0.4) 10-2 (3.6 ± 0.8) 10-3 (9 ± 2) 10-2 (4 ± 4) ± 0.05 Acceptances are corrected for the difference in trigger efficiency between data and MC

16 Z selections: invariant mass after cuts Dimuon invariant mass after cuts 16

17 Cross section measurement Integrated luminosity in data: 330 ± 11 % nb-1 # of Z events in data after cuts: N = 113 Cuts acceptance A = ± Estimated background event B = 0.16 ± 0.05 σdata = 0.87 ± 0.15 nb σmc = 0.85 ± 0.02 nb 17

18 Tag & Probe Data driven technique for measure the efficiency of a selection criteria Independence from the cuts In general: select two correlated object by cinematic cuts: Tag and Probe check whether or not the probe has the property you are asking for the criteria I used Z μμ for the muon reconstruction efficiency 18

19 Tag & Probe: the method Select Z events Tag: high Pt and isolated muon Probe: high Pt ID track, calorimeter energy loss compatible with a muon (Calomuon) if Mtag+probe MZ it's a Z μμ event The probe has to be reconstructed as muon 19

20 Tag & Probe: the method (2) Assume we have n Z0 μμ events 2μ reconstructed 2 μ in acceptance A TT 1μ reconstructed TP n A ε2 = TT n A 2ε(1 ε) = TP ε = muon reconstruction efficiecy A = cuts acceptace 20

21 Tag & Probe: MC test 1 MC Test: variation of the cuts thresholds Efficiency variation within the statistical error

22 Tag & Probe: MC test 2 T&P gives a good estimate of the efficiency? MC true reconstruction efficiency: true # in acceptance P t 15 GeV reconstructed = # in acceptance P t 15 GeV The result is: εtrue = ± εt&p = ±

23 Tag & Probe: Applying the method on data 330 nb-1 (preliminary measure): results TT = 88 ; TP = 11 BTT, BTP = backgrounds (neglected w.r.t statistical error) the error is statistical To be compared with εmc = ±

24 Conclusion I've studied a set of selection for Z μμ Preliminaryly measured the cross section Preliminaryly measured the muon reconstruction efficiency with a T&P Nowadays a big effort is made in studying muon performance Most recent results (8 Pb-1) confirm the mean difference of 2% between data and MC in muon reconstruction efficiency Thank you! 24

25 Backup 25

26 Z selections: Pt distributions Inclusive single muon Pt distributions 26

27 Tag & Probe: the method (3) ε is independent from A, but in practice things are more complicated n A1 ε2 = TT n A2 2ε(1 ε) = TP Choose the right cuts make A1 = A2 cut only on ID track variables (both for Tag and Probe) symmetric Pt cut each Tag muon has to be reconstructed also as a calomuon 27

28 Z selections: Et in cone distribution Isolation cut Iso cut tuning Etcone40 = Transverse Energy in a cone of ΔR = (ΔΦ2+Δϑ2)1/2 < 0.4 I chose Etcone40 < 3 GeV

29 Z selections: D0 distributions D0 cut D0 cut tuning D0 = Impact parameter w.r.t. Primary Vertex I chose D0 < 0.15 mm

30 Z selections: Pt cut Asymmetric Pt cut: μ1 Pt > 15 GeV μ2 Pt > 20 GeV Acceptance 92% Symmetric Pt cut: μ1,2 Pt > 20 GeV Acceptance 86% Gain few percent! Pt cut efficiency VS Pt threshold on μ2, μ1 fixed 30

31 why neglet cosmics Primary vertex with 3 track: ID Timing different topology Impact parameter: they must came from the primary VX No back to back in the η-plane 31

32 why neglet cosmics (2) 32

33 cosmic Pt distro 33

34 light adrons 34

35 Z selections: acceptaces Background acceptances (Monte Carlo) Selection bb cc tt w μν Z ττ tot Pt M MZ < 25 GeV geometrical acceptace Trigger 2 μ good quality Isolation D0

36 Tag & Probe: results Applying the method on backgrounds (MC) we obtain: # TT # TP w μν 0 0 Z ττ 6 0 (0.7 ± 0.1) 10 tt 47 5 (5 ± 0.7) 10 bb 0 0 cc 0 0 Tot # TT in 330 nb-1 (5 ± 0.7) 10-2 # TP in 330 nb (0.6 ± 0.2) 10 (0.6 ± 0.2) I neglect the background w.r.t. statistical error 36

37 D0 resolution 37

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