Determination of Z τ + τ e + e + 4ν Background Shapes From Data in Context of the b Quark Associated Higgs Boson Production with ATLAS at the LHC

Transcription

1 Determination of Z τ + τ e + e + 4ν Background Shapes From Data in Context of the b Quark Associated Higgs Boson Production with ATLAS at the LHC K.Leonhardt@physik.tu-dresden.de Institut für Kern- und Teilchenphysik January 15, 2009

2 1 Introduction

3 1 Introduction 2 3 The b quark associated Higgs decay Background processes 4 5 on the m ττ mass on the shower characteristics 6

4 Problem The electroweak theory forbids boson mass terms as well as fermion mass terms of the form m Ψ ΨΨ = mψ ( ΨL Ψ R + Ψ R Ψ L ) Solution One solution is the Spontaneous Symmetry Breaking and the introduction of Higgs Mechanism. Consequence Existance of Higgs bosons as excitations of the Higgs field. Standard Model H SUSY (MSSM) h,h,a,h +,H

5 1 Introduction 2 3 The b quark associated Higgs decay Background processes 4 5 on the m ττ mass on the shower characteristics 6

6 The Large Hadron Collider at CERN was built to discover new physics at the TeV scale. circumference of 27km two 7 TeV proton beams design luminosity cm 2 s bunches per beam and a revolution frequency of 11kHz lead to 23 proton-proton interactions per bunch crossing Figure: The LHC with its four main experiments.

7 The ATLAS detector y z x

8 The inner detector The ATLAS detector y z x

9 The inner detector The electromagnetic calorimeter The ATLAS detector y z x

10 The inner detector The electromagnetic calorimeter The hadronic calorimeter The ATLAS detector y z x

11 The inner detector The electromagnetic calorimeter The hadronic calorimeter The muon system The ATLAS detector y z x

12 The inner detector The electromagnetic calorimeter The hadronic calorimeter The muon system The ATLAS detector y pseudorapidity η = lntan (θ/2) transverse plane p T = px 2 + py 2 z x

13 1 Introduction 2 Introduction 3 The b quark associated Higgs decay Background processes 4 5 on the m ττ mass on the shower characteristics 6 The b quark associated Higgs decay Background processes

14 The b quark associated Higgs decay Background processes Study focused on the Minimal Supersymmetric Standard Model existance of two Higgs doublets Φ u, Φ d if tanβ = νu ν d is large enhanced cross sections of h/a/h for b quark associated production. b b h/a/h gb h/a/h b h/a/h ττ

15 1 Introduction 2 Introduction 3 The b quark associated Higgs decay Background processes 4 5 on the m ττ mass on the shower characteristics 6 The b quark associated Higgs decay Background processes

16 The b quark associated Higgs decay Background processes Background processes are: (a) Z + Jets σ = pb (b) t t σ = 449.8pb (c) W + Jets σ = pb (d) WW + Jets σ 130pb

17 The b quark associated Higgs decay Background processes Problem Selection of the Higgs signal through cuts. After applied cuts, Z ττ is the remaining dominant and irreducible background. Question How to estimate the background without the existance of a Higgs signal-free control sample? G. Aad et al., Expected Performance of the ATLAS Experiment, CERN-OPEN

18 The b quark associated Higgs decay Background processes Idea Estimate the Z τ τ ee + 4ν background with the help of the Z ee background Z ττ ee + 4ν irreducible from Higgs signal 3-body decay with missing energy Z ee pure selection in sideband 2-body decay no missing energy Especially with early data Z ee channel has a better resolution.

19 1 Introduction 2 Introduction 3 The b quark associated Higgs decay Background processes 4 5 on the m ττ mass on the shower characteristics 6

20 How does the method work? Data

21 How does the method work? Data Z ee Selection

22 How does the method work? Data Selection Z ee Reweighting Z ττ ee + 4ν

23 How does the method work? Data Selection Z ee Reweighting from MC Z ττ ee + 4ν Z ττ ee + 4ν

24 1 Introduction 2 Introduction 3 The b quark associated Higgs decay Background processes 4 5 on the m ττ mass on the shower characteristics 6

25 Z ee can be selected in a Higgs signal-free region, the sideband. Selection-cuts are motivated from distributions of background samples. Background now contains Higgs signal, W + Jets, Z + Jets and t t Selection cuts Trigger: one electron with E > 25GeV or two with E > 15GeV existance of an electron-positron pair, both with p T > 10GeV invariant mass of lepton pair constrained to 75GeV < m ee < 100GeV number of jets less than three, with at least one b jet

26 Entries (Nb of Jets < 3) Z->ee Z->µµ 8 Z->ττ h/a/h->ττ ttbar 6 W->eν W->µν 4 W->τν Wbbar m ττ/gev Entries (Nb of Jets < 3) 7 10 Z->ee Z->µµ Z->ττ 6 h/a/h->ττ 10 ttbar W->eν W->µν 5 10 W->τν Wbbar m ττ/gev Entries (b-tagging weight > 3) <100GeV) Entries (75GeV<M ττ Z->ee Z->µµ 0.25 Z->ττ h/a/h->ττ 0.2 ttbar W->eν 0.15 W->µν W->τν 0.1 Wbbar m ττ/gev Z->ee Z->µµ 40 Z->ττ 35 h/a/h->ττ 30 ttbar 25 W->eν W->µν 20 W->τν 15 Wbbar m ττ/gev Entries (b-tagging weight > 3) <100GeV) ττ Entries (75GeV<m Z->ee Z->µµ 5 10 Z->ττ h/a/h->ττ ttbar W->eν 4 W->µν 10 W->τν Wbbar m ττ/gev Z->ee Z->µµ Z->ττ h/a/h->ττ ttbar 4 10 W->eν W->µν W->τν Wbbar m ττ/gev Selection of Z ee leads to a purity of P = 0.973

27 1 Introduction 2 Introduction 3 The b quark associated Higgs decay Background processes 4 5 on the m ττ mass on the shower characteristics 6

28 Electrons produce a shower in the em. calorimeter. Clusters are reconstructed from the shower. Usage of TopoClusters (based on cells of the em. calo.).

29 TopoClusters Introduction Used to reconstruct clusters in the em. calorimeter. TopoClusters are able to reflect the showershape. 1 Starts with cells with E/σ noise > 4. 2 Adjoins all neighbouring cells with E/σ noise > 2. 3 Adds alls bordering cells E/σ noise > 0.

30 Realisation of the reweighting method 1 Identify reconstructed e + e pair of selected Z ee sample. 2 The four-vectors of e + e are combined to the Z boson, both are boosted to its rest-frame. Calculation of the Gottfried Jackson Angle (GJA). 3 Random energies from the Z ττ ee + 4ν reference histogram are taken dependent on the GJA.

31 4 With new energies new four-vectors are built and boosted back to the lab-frame. 5 All cells of electron clusters are reweighted with a factor from the original and the new electron energies in the lab-frame. E cell, rw = E cell, orig Eref,ττ E ee 6 Sample with new cell list is reconstructed once more.

32 1 Introduction 2 Introduction 3 The b quark associated Higgs decay Background processes 4 5 on the m ττ mass on the shower characteristics 6

33 Five 2D reference histograms from Monte Carlo calculations, sorted by cosine of the absolut value of GJA. Histograms contain energies of e + and e from τ decays in the Z rest-frame. According to GJA in Z ee sample a random pair of energies is taken. GJA sensitive to polarisation of τ leptons. E/GeV of e < cos(θ 90 GJA ) < E/GeV of e Entries

34 1 Introduction 2 Introduction 3 The b quark associated Higgs decay Background processes 4 5 on the m ττ mass on the shower characteristics 6 on the m ττ mass on the shower characteristics

35 on the m ττ mass on the shower characteristics Reminder Invariant τ τ mass is used for extraction of Higgs signal. m ττ mass for Z ττ background has to be known very precisely.

36 on the m ττ mass on the shower characteristics Invariant mass of Z τ τ ee + 4ν can be calculated in collinaer approximation. Assumption Large mass of the Z boson decay products of τ leptons have appr. same direction p T of Z boson has to be non-zero otherwise vectorial sum of missing transverse momentum would be zero Missing transverse momentum results from neutrinos of τ decay, exclusively Lepton masses are negligible with respect to the Z boson mass.

37 on the m ττ mass on the shower characteristics Invariant mass of Z τ τ ee + 4ν can be calculated in collinaer approximation. with p T conservation p T,τ1 + p T,τ2 = p T,e1 + p T,e2 + p T,miss and x i = p T,e i p T,τi the invariant mass becomes m ττ = mee x1 x 2

38 1 Introduction 2 Introduction 3 The b quark associated Higgs decay Background processes 4 5 on the m ττ mass on the shower characteristics 6 on the m ττ mass on the shower characteristics

39 on the m ττ mass on the shower characteristics Comparison of rescaled Z ee sample with Z τ τ sample

40 on the m ττ mass on the shower characteristics Fit of the mass distributions with a Crystal Ball shape A e 1 x x0 2 2 σ x < x 0 + σ n f CB (x) = 1 A ) n x > x 0 + σ n ( x x0 σ e n

41 on the m ττ mass on the shower characteristics Fit of the mass distributions with a Crystal Ball shape 8 >< A e 2 1 f CB (x) = A >: «x x0 2 σ x x0 σ 1 q en n x < x 0 + σ n x > x 0 + σ n

42 1 Introduction 2 Introduction 3 The b quark associated Higgs decay Background processes 4 5 on the m ττ mass on the shower characteristics 6 on the m ττ mass on the shower characteristics

43 on the m ττ mass on the shower characteristics Comparison of energy per reconstructed candidate and the highest cell energy. Entries Z->ττ Z->ee, scaled Z->ee E per candidate/gev Entries 0.14 Z->ττ Z->ee, scaled 0.12 Z->ee highest cell energy/gev

44 on the m ττ mass on the shower characteristics Comparison of clusters per reconstructed candidate. Entries Z->ττ Z->ee, scaled Z->ee # clusters per candidate # clusters, Z->ee # clusters, Z->ee, scaled

45 on the m ττ mass on the shower characteristics Comparison of fraction of total energy per different em. calorimeter layers. Entries 0.3 PreSampler, Barrel Z->ττ 0.25 Z->ee, scaled Z->ee Entries Layer 1, Barrel Z->ττ Z->ee, scaled Z->ee fraction of total energy fraction of total energy Entries Layer 2, Barrel Z->ττ Z->ee, scaled Z->ee Distributions remain the same, as the fraction of energy per layer was not changed fraction of total energy

46 on the m ττ mass on the shower characteristics Comparison of number cells per cluster in each em. calorimeter layer. Entries Entries 0.2 PreSampler, Barrel Z->ττ Z->ee, scaled Z->ee #cells per cluster 0.2 Layer 2, Barrel 0.18 Z->ττ Z->ee, scaled 0.16 Z->ee #cells per cluster Entries Entries 0.2 Layer 1, Barrel Z->ττ Z->ee, scaled Z->ee #cells per cluster 0.2 Layer 3, Barrel 0.18 Z->ττ Z->ee, scaled 0.16 Z->ee #cells per cluster Distributions do not follow the Z ττ distribution. This is still a challenge.

47 1 Introduction 2 3 The b quark associated Higgs decay Background processes 4 5 on the m ττ mass on the shower characteristics 6

48 Reweighting method is applicable. Invariant mass distribution is acceptable. Shower variables have to be improved. Next steps Reweighting with respect to shower shapes.