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B-tagging in ATLAS

B-tagging in ATLAS. Monika Pa nušková /FZU ASCR. Outline. Motivation B -tagging basics Misalignment studies on ttbar events. Motiva ce. b-tagging je nezbytn ý na extrakci signálu od pozadí v řadě zajímavých kanálů na LHC: • top fyzika (pár nebo osamocený t kvark)

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B-tagging in ATLAS

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  1. B-tagging in ATLAS Monika Panušková/FZU ASCR

  2. Outline • Motivation • B-tagging basics • Misalignment studies on ttbar events B-tagging in ATLAS Malá Skála

  3. Motivace b-tagging je nezbytný na extrakci signálu od pozadí v řadě zajímavých kanálů na LHC: • top fyzika (pár nebo osamocený t kvark) • Higgsova fyzika (H->bb): objev a měření vlastností • SUSY (h->bb, bbH/A) • Exotické modely (malý Higgs, ….) • a samozřejmě nečekané… Požadavky se liší pro každý kanál: • ttbar: důraz na potlačení pozadí pro čisté měření, Efektivita není problém • ttH(bb): taktéž velké pozadí, potřebujeme 4 btagy a vysokou efektivitu (malý účinný průřez signálu) B-tagging in ATLAS Malá Skála

  4. Introduction to b-tagging • Velká doba života B hadronů • B+/B0/Bs :ct ~ 470 mm • Lb :ct ~ 390 mm • pro Energie ~ 50 GeV, délka letu ~ 5 mm, • Metody značkování • Geometrické (impact parameter, secondary vertex) • Soft-lepton tagging (low pT electron/muon from B/D ) B-tagging in ATLAS Malá Skála

  5. Impact parameter (IP) • Signed impact parameter a0 B-tagging in ATLAS Malá Skála

  6. Tagger IP2D • Dvě statistické hypotézy b | u hypotézy: b(S), u(S) • Pro každou dráhu: poměr b(S)/u(S) • Váha jetu standardním statistickým způsobem: B-tagging in ATLAS Malá Skála

  7. Tagger IP2D (2) Liší se dostatečně na to, aby se b jet odlišil B-tagging in ATLAS Malá Skála

  8. Seznamhlavních taggerů B-tagging in ATLAS Malá Skála

  9. IP3D+SV1 combined tagger • Momentálně nejvýkonnější B-tagging in ATLAS Malá Skála

  10. B-tagging performance • B-jet efficiency eb • Jmenovatel: • Jetydefinované jako b z Monte Carlo generátoru • pT > 15 GeV/c |n| < 2.5 • Čitatel • To samé + cut na váhu b-jetu • Light-jet rejection Ru = 1/eu • R=100 znamená 1% špatně označených jetů • Light jet = u,d,s,g B-tagging in ATLAS Malá Skála

  11. B-tagging performance (2) Release 12, IP3D+SV1 tagger B-tagging in ATLAS Malá Skála

  12. B-tagging performance (3) Release 12, IP3D+SV1 tagger B-tagging in ATLAS Malá Skála

  13. B-tagging performance (4) Release 12 B-tagging performance (3) Release 12, IP3D+SV1 tagger B-tagging in ATLAS Malá Skála

  14. Effects of misalignment • Nedostatek znalosti o poloze detektorů • Z misaligned k perfektnímu alignmentu za zhruba rok B-tagging in ATLAS Malá Skála

  15. Residual misalignment • Specializované algoritmy na alignment celého ATLASu existují • Nejsou dokonalé • Misalignment zbývající po aplikaci alignmentu = residual misalignment Poučený odhad misalignment v pixelovém detektoru: B-tagging in ATLAS Malá Skála

  16. Error Scaling - introduction • A way to fight the residual misalignment is error scaling: • There are different measurements errors comingfrom clustering and tracking(due to initial/residualmisalignments and/or improper knowledge ofcalibration mechanism) • It guarantees proper tracking (track scoring,outlier flagging, tracking efficiencyand parameterresolution) • Improves calibration and alignment -> essentialfor b-tagging B-tagging in ATLAS Malá Skála

  17. 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 B-tagging in ATLAS Malá Skála

  18. Error Scaling - calculation • Using reconstruction of single 200 GeV muons • Independent of material effects => good standard candles • Reconstruction produces an ntuple with the pull widths B-tagging in ATLAS Malá Skála

  19. Error Scaling – calculation (2) • Parameter a • Je tomultiplikativnírozdílšířky pulluod 1.0, tj.přímo standardníodchylka sigma distribuce pullůzískaných v posledním kroku • Potřebujeme 2-3 iterace B-tagging in ATLAS Malá Skála

  20. Error Scaling – calculation (2) • Parameter c • It appears in the misaligned environment • It is calculated from the formula s0 = … B-tagging in ATLAS Malá Skála

  21. Error Scaling – results a c[mm] B-tagging in ATLAS Malá Skála

  22. B-tagging & residual misalignment • study of residual misalignment effect on b-tagging • using ttbar events = main background for ttH->bb channel 1.)Residual misalignment introduced by hand 2.)Error scaling applied B-tagging in ATLAS Malá Skála

  23. B-tagging & residual misalignment • per cents with respect to the original value B-tagging in ATLAS Malá Skála

  24. B-tagging & residual misalignment - observation • ip3D+SV1 –improvement to 50 % of original value (=without residual misalignment) • other two taggers – small or no improvement • not a real problem as only IP3D+SV1 is used for physics B-tagging in ATLAS Malá Skála

  25. B-tagging - conclusions • b-tagging is crucial in extracting signal from background on LHC • can play important role in several Higgs discovery channels (ttH->bb) and Higgs supplementary channels (WH) • several taggers developed, for physics we use combination of impact parameter significance and secondary vertex finder (IP3D+SV1) • 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 B-tagging in ATLAS Malá Skála

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