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B tagging in the tt all jets channel

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  1. B tagging in the tt all jets channel • B tagging, performance vertexing • Neural Net studies • tt event selection • mass reconstruction in tt events • conclusions By: Graziano Massaro Michiel Vogelvang (university student) Marcel Vreeswijk Marcel Vreeswijk (NIKHEF)

  2. Start with (non PV) selected tracks Significance>3 Make all possible 2-track vertices (vertex fits) • Keep/Kill vertices with shared tracks • Add tracks • (based on probability: • opening angle, Pt) B tagging & vertexing • Performance secprob algorithm in tt events (p05 & p08) • Reminder: Vertex fit based on impact parameters Marcel Vreeswijk (NIKHEF)

  3. Performance vertexing • Signal Events: • Efficiency SECPROB and KALMAN compatible. • No large effect from min. bias. MC samples, thanks to Suyong!!! Background events • KALMAN selects significantly more QCD jets (used without any additional cuts: what are they?) Marcel Vreeswijk (NIKHEF)

  4. Performance vertexing Performance SECPROB as func of Et S(ttbar)/B(QCD) Bjet eff. Performance KALMAN as func of Et KALMAN: higher QCD background Marcel Vreeswijk (NIKHEF)

  5. Performance vertexing • Signal Events (cuts): Bjet eff. S(ttbar)/B(QCD) S/B ratio not dependent on Decay Length Marcel Vreeswijk (NIKHEF)

  6. Event CAL Jet-Tracks Vertex in Jet vertexing and beyond • Reminder: vertex constructed based on probability (Opening angle, Et) • Now: try to find variables to discriminate between B vertices and QCD fakes, using a Probalistic Neural Network Preliminary!!!!!!! Marcel Vreeswijk (NIKHEF)

  7. c2-jet-track impact parameters Jets-QCD Bjets-ttbar Probability from NN Ratio vertexing and beyond • Strategy NN: take Et_vtx and Opening_Angle_vtx as base variables and see the effect of a third variable. • the Et_jet and c2 based on jet-track impact parameters appear promising Preliminary!!!!!!! 2bcontinued Marcel Vreeswijk (NIKHEF)

  8. Conclusions • The performance of the SECPROB and KALMAN algorithm are studied using ttbar and QCD events. • KALMAN has a slightly higher efficiency for B-vtxs, but finds significantly more QCD fake vtxs • To find discriminating variables between good/fake vtxs a NN is used as tool. • Many variables are tried: Et_jet and c2 based on jet-track impact parameters appear promising Marcel Vreeswijk (NIKHEF)

  9. tt event (pre)selection • For the ‘All jet’ channel At least 5 jets with |h|<2 tt Et of jets qcd Simple, effective, but: QCD has to be multiplied by 107 Marcel Vreeswijk (NIKHEF)

  10. tt event (pre)selection • From D0-RunI pubs: ET3= Et of jets, skipping 2 highest Et jets. Note: multiply QCD by 107 • Cut appears less effective than in RunI. Why? • In RunI: Initial jets in QCD events have large Et. The additional jets originate from QCD splittings and have low Et. Skipping 2 highest Et jets has large effect. For ttbar event effect is average: • ET3(QCD) < ET3 (ttbar) • In RunII: QCD background has significant contribution from min. bias, which dillutes this effect. Marcel Vreeswijk (NIKHEF)

  11. tt event (pre)selection • Alternative: tt Et(5-jets)/Et(jets) vs <Et(jets)> qcd QCD: low Et per jet, many jets ttbar: high Et per jet contained in not so many jets. Need many more QCD events!!!! Marcel Vreeswijk (NIKHEF)

  12. j W j t b b t j W j Mass reconstructionin tt--> all jets • A very preliminary study • Difficult final state: 4+2 jets • But, many constraints: • W mass (2x) • Both branches should yield similar top mass • Selection (no preselection): • At least 6 jets. Keep 6 highest Et jets • 2 jets have vertex--> B candidates. • Reconstruction: • 2x2 W jets lead to 3 mass combinations • These mass combinations are then assigned to B candidates: 6 mass combinations. • Take combination with best c2 based on Mw (2x) and Mt1-Mt2 Marcel Vreeswijk (NIKHEF)

  13. True mass tt QCD Mass reconstructionin tt--> all jets • Background: 5*5000000 QCD events • <--> need more MC!!!!!! ALL Marcel Vreeswijk (NIKHEF)

  14. Bad mass combs. Good mass combs. Mass reconstructionin tt--> all jets • Mass peak looks fine, but…. The mass peak seems independend on bad/good combinations of the jets?!?! Side remark: particle info in IN_PRT is corrupted as reported. In this study we attempted to take this into account properly. Marcel Vreeswijk (NIKHEF)

  15. Mass reconstructionin tt--> all jets tt W-mass (recoed) qcd tt Note: multiply QCD by 107 qcd Marcel Vreeswijk (NIKHEF)

  16. Conclusions • The performance of the SECPROB and KALMAN algorithm are studied using ttbar and QCD events. KALMAN has a slightly higher efficiency for B-vtxs, but finds significantly more QCD fake vtxs • To find discriminating variables between good/fake vtxs a NN is used as tool. Many variables are tried: Et_jet and c2 based on jet-track impact parameters appear promising • The (pre)selection of ttbar events was studied. Cuts used in RunI apeared to have less effects due to min. bias overlay. New cuts are suggested. • Can we measure the top mass in ttbar->All jet channel? A preliminary study, using all mass constraints yield a mass peak. However, this peak also show up for wrong jet-combinations(?). Marcel Vreeswijk (NIKHEF)

  17. Performance vertexing • Background Events (cuts): Marcel Vreeswijk (NIKHEF)

  18. Performance vertexing • Signal Events (cuts): Marcel Vreeswijk (NIKHEF)

  19. QCD jets B jets ttbar Probability from NN vertexing and beyond • Strategy NN: take Et_vtx and Opening_Angle_vtx as base variables and see the effect of a third variable. • the Et_jet and c2 based on jet-track impact parameters appear promising For Et-jet Ratio Marcel Vreeswijk (NIKHEF)

  20. Check p8 vs p9 • Validate ‘P9’ WH and QCD events versus ‘p8’ events (All samples from Suyong) • First check distributions. Plots added of the tracking in jets related quantities: Sum of significances of tracks in jet wrt PV Sum of significances of tracks in SV wrt SV • See Plots, distributions look ok. Differences probably due to different cuts (Et in QCD generation), #min bias events and code changes Marcel Vreeswijk (NIKHEF)

  21. QCD p8 Marcel Vreeswijk (NIKHEF)

  22. QCD p9 Marcel Vreeswijk (NIKHEF)

  23. ttbar p8 Marcel Vreeswijk (NIKHEF)

  24. WH p9 Marcel Vreeswijk (NIKHEF)

  25. Efficiencies • Signal events Number of ‘taggable’ Bjets + efficiency look fine • Background Number of tagged PV jets in QCD is significantly higher in P9. Probably explained by Et generator cut and/or #min. bias events. Marcel Vreeswijk (NIKHEF)