B-tagging in ATLAS

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B-tagging in ATLAS

• Monika Panušková/FZU ASCR

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Outline

• Motivation
• B-tagging basics
• Misalignment studies on ttbar events

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Motivace

• b-tagging je nezbytný na extrakci signálu od
  pozadí v rade zajímavých kanálu na LHC
• top fyzika (pár nebo osamocený t kvark)
• Higgsova fyzika (H-gtbb) objev a merení
  vlastností
• SUSY (h-gtbb, bbH/A)
• Exotické modely (malý Higgs, .)
• a samozrejme necekané
• Požadavky se liší pro každý kanál
• ttbar duraz na potlacení pozadí pro cisté
  merení,
• Efektivita není problém
• ttH(bb) taktéž velké pozadí, potrebujeme 4
  btagy a vysokou efektivitu (malý úcinný prurez
  signálu)

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Introduction to b-tagging

• Velká doba života B hadronu
• B/B0/Bs ct 470 mm
• Lb ct 390 mm
• pro Energie 50 GeV, délka letu 5 mm,
• Metody znackování
• Geometrické (impact parameter, secondary vertex)
• Soft-lepton tagging (low pT electron/muon from
  B/D )

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Impact parameter (IP)

• Signed impact parameter a0

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Tagger IP2D

• Dve statistické hypotézy b u hypotézy b(S),
  u(S)
• Pro každou dráhu pomer b(S)/u(S)
• Váha jetu standardním statistickým zpusobem

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Tagger IP2D (2)
Liší se dostatecne na to, aby se b jet odlišil
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Seznam hlavních taggeru
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IP3DSV1 combined tagger

• Momentálne nejvýkonnejší

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B-tagging performance

• B-jet efficiency eb
• Jmenovatel
• Jety definované jako b z Monte Carlo generátoru
• pT gt 15 GeV/c n lt 2.5
• Citatel
• To samé cut na váhu b-jetu
• Light-jet rejection Ru 1/eu
• R100 znamená 1 špatne oznacených jetu
• Light jet u,d,s,g

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B-tagging performance (2)

• Release 12, IP3DSV1 tagger

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B-tagging performance (3)

• Release 12, IP3DSV1 tagger

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B-tagging performance (4)

• Release 12

• B-tagging performance (3)
• Release 12, IP3DSV1 tagger

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Effects of misalignment

• Nedostatek znalosti o poloze detektoru
• Z misaligned k perfektnímu alignmentu za zhruba
  rok

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Residual misalignment

• Specializované algoritmy na alignment celého
  ATLASu existují
• Nejsou dokonalé
• Misalignment zbývající po aplikaci alignmentu
  residual misalignment

Poucený odhad misalignment v pixelovém detektoru
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Error Scaling - introduction

• A way to fight the residual misalignment is error
  scaling
• There are different measurements errors coming
  from clustering and tracking (due to
  initial/residual misalignments and/or improper
  knowledge of calibration mechanism)
• It guarantees proper tracking (track scoring,
  outlier flagging, tracking efficiency and
  parameter resolution)
• Improves calibration and alignment -gt essential
  for b-tagging

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Error Scaling - principle

• There are 2 parameters which modify the error
  matrix
• Parameter a corrects the effects correlated with
  measurement errors
• Parameter c corrects uncorrelated effects
• The parameters are not universal for the whole
  tracker
• Different parameters for Pixel X direction, Pixel
  Y direction, SCT, TRT
• 16 constants altogether
• Muon spectrometer has its own set of error
  scaling parameters

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Error Scaling - calculation

• Using reconstruction of single 200 GeV muons
• Independent of material effects gt good standard
  candles
• Reconstruction produces an ntuple with the pull
  widths

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Error Scaling calculation (2)

• Parameter a
• Je to multiplikativní rozdíl šírky pullu od 1.0,
  tj. prímo standardní odchylka sigma distribuce
  pullu získaných v posledním kroku
• Potrebujeme 2-3 iterace

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Error Scaling calculation (2)

• Parameter c
• It appears in the misaligned environment
• It is calculated from the formula

s0
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Error Scaling results
cmm
a
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B-tagging residual misalignment

• study of residual misalignment effect on
  b-tagging
• using ttbar events main background for ttH-gtbb
  channel

1.)Residual misalignment introduced by
hand 2.)Error scaling applied
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B-tagging residual misalignment

• per cents with respect to the original value

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B-tagging residual misalignment - observation

• ip3DSV1 improvement to 50 of original value
  (without residual misalignment)
• other two taggers small or no improvement
• not a real problem as only IP3DSV1 is used for
  physics

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B-tagging - conclusions

• b-tagging is crucial in extracting signal from
  background on LHC
• can play important role in several Higgs
  discovery channels (ttH-gtbb) and Higgs
  supplementary channels (WH)
• several taggers developed, for physics we use
  combination of impact parameter significance and
  secondary vertex finder (IP3DSV1)
• efficiency/rejections gets lower with higher pT
• misalignment can be treated by dedicated
  alignment algorithms
• residual misalignment can be cured to the large
  extent by introduction of Error Scaling