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Advanced Techniques in B-Tagging for ATLAS Experiments

Explore the significance of b-tagging in signal extraction across various channels at LHC, encompassing top, Higgs, SUSY, and exotic physics with specific requirements for each channel. Delve into impact parameters, taggers, and error scaling techniques for efficient b-tagging performance and misalignment studies.

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Advanced Techniques in B-Tagging for ATLAS Experiments

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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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