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bTagging Issues in t t H(H-> b b)

bTagging Issues in t t H(H-> b b). Lily Asquith 23 Nov 07 ATLAS UCL. The Importance of bTagging. t t H(H->b b ): final state has 4b jets and two light jets Jet-heavy backgrounds swamp the signal, but 4 bjet final states not common. Pre-selection: ttH ttjj

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bTagging Issues in t t H(H-> b b)

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  1. bTagging Issues in ttH(H->bb) Lily Asquith 23 Nov 07 ATLAS UCL

  2. The Importance of bTagging ttH(H->bb): final state has 4b jets and two light jets Jet-heavy backgrounds swamp the signal, but 4 bjet final states not common Pre-selection: ttH ttjj 1 hard lepton +At least 6 jets ~1200 ~850000 At least 4 bTagged jets ~100 ~800 ~10%~0.1% B quarks’ large mass and low CKM matrix elements make it’s lifetime long, giving us the opportunity to identify it using tracking in the Inner Detector ( |eta|<2.5) and ‘bTag’ it. Before pre-selection: signal :background ~ 1:1000

  3. The Importance of bTagging ttH(H->bb): final state has 4b jets and two light jets Jet-heavy backgrounds swamp the signal, but 4 bjet final states not common Pre-selection: ttH ttjj 1 hard lepton +At least 6 jets ~1200 ~850000 At least 4 bTagged jets ~100 ~800 ~10%~0.1% B quarks’ large mass and low CKM matrix elements make it’s lifetime long, giving us the opportunity to identify it using tracking in the Inner Detector ( |eta|<2.5) and ‘bTag’ it. Before pre-selection: signal :background ~ 1:1000

  4. Intro: Impact Parameter bTagging used: ‘IP3D + SV1’ IP3D uses transverse IP a0 and longitudinal IP z0 3d Impact parameter gives probability of being a b versus probability of being a u Impact parameter a0 (z0) a0 / sigma(a0) Weight of track =Pb/Pu Weight(jet) = sum of weights of tracks. No tracks-> weight = - 20.72 Excellent description: http://cdsweb.cern.ch/record/686346

  5. Intro: Secondary Vertex bTagging used: ‘IP3D + SV1’ SV1 uses secondary vertex information Uses distance from beam pipe to cut out SVs due to material interaction and K0 and Lambda decays Energy in vertex/ Jet Energy Mass of pcles in SV Number of 2-track vertices + + No SV-> weight = - 1.2 Excellent description: http://cdsweb.cern.ch/record/686346

  6. bTagging Efficiency (Numbers for efficiency and rejection are dependant on the choice of bweight cut. These numbers are for a bweight cut of 4) E( b )~ 60% If it’s a b, we will call it a b ~60% of the time R( u,d,s ) ~ 100 If it’s a u,d,s we’ll call it a b ~1% of the time R( c ) ~ 8 If it’s a c, we’ll call it a b ~12% of the time Naively expect both light jets from W to be tagged as bjets roughly 8 % of time, and all 4 bjets to be tagged as a bs (0.6)4: 13% of the time These numbers do not take into account the bias introduced by our pre-selection cuts.

  7. Our bTagging efficiency (These numbers are for events where I have been able to match all 6 quarks (of status 3) to jets within dR <0.2.) 28% of events have at least one jet from the W tagged as a bjet 34% of events have either a light jet or a bjet mistagged Current analysis separates jets into light jets / bjets based on tagging. Light jets only are used for W reconstruction: we scupper our chances of correctly reconstructing the higgs 28% of the time at this point. Want to keep more jets for making W. Would like a more efficient way of deciding which jets should be used to reconstruct W,tops, higgs…

  8. Segregation: to what extent… None: use all jets to try and make a decent hadronic W mass. Use leftovers for tops (lose none) Sort Jets in terms of bweight, exclude highest weighted jets from W reconstruction • exclude top four weighted jets (lose ~27%) • exclude top three weighted jets (lose ~10%) • exclude top two weighted jets (lose ~4%) Aims: To improve significance of channel (more signal events contributing to higgs reconstruction wrt background) To improve higgs mass peak (want to make the Higgs out of the right 2 jets)

  9. To look at: Questions for me: ? Effect of larger collection for W on top reconstruction (play with W mass window) - Marseille showed bjets successful in making a W in mass window 10% of time ? Efficiencies for W, tops, higgs • standard cut-based analysis:(of events passing pre-selection) • 67% hadronic W in mass window • 44% both tops reconstructed • 17% higgs in mass window ? Effect on significance - higgs in mass window correct, incorrect, background -e.g.: currently 17% of pre-selected events have higgs in mass window, but less than half of these use the right pair of bjets

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