Impact Parameter b-jet tagging at Level-3 presented by Per Jonsson Imperial College London Thanks to Chris, Ray, Tim, Michele, Amber, Makoto, Andre, Michiel, Jon, Stephen All DØ Meeting June 25 th 2004

ADM 06/25/04Per Jonsson - Imperial College London2 Outline Introduction –What’s the aim? Level-3 tools –Primary Vertex reconstruction Beam spot –Impact Parameter b-tagger: L3TBTagIP Performance –MC comparison –Real data offline/online Physics –Trigger list v13: hbb Z->bb ZH->nnbb

ADM 06/25/04Per Jonsson - Imperial College London3 Introduction Why? –Many search signals are swamped by light quark QCD background, e.g. Z->bb and Higgs –Light quark rejection needed already at trigger level to achieve acceptable trigger rate at L > 50E30: alternative is pre-scaling! Method: –2 dimensional Signed Impact Parameter based b-jet tag –Use reconstructed Level-3 Tracks, Jets and Primary Vertex from other tools Beam spot position and tilt from Vertex Examine Tool Results: –Track, jet and event probabilities –Continuous variables [0,1]: Easy to control rate/efficiency by simple cuts Aim: provide b-tagging at Level-3 for high p T jet physics events, enable various topology dependent physics filters. A set of track only based IP tools/filters for B-physics triggering also exist at L3, see talks by C. Barnes, R. Jesik

ADM 06/25/04Per Jonsson - Imperial College London4 Jet based IP b-tagging an Overview Select well reconstructed 3D SMT+CFT tracks –Need tracker: L3TGlobalTracker; p T > 0.5 GeV Recalculate track parameters wrt Primary Vertex –Need Primary Vertex reconstruction: L3TCFTVertex and L3TXYVertex Match each track to closest jet –Need jet-tool: L3TJet; 0.5 cone jets, E T > 9 GeV Calculate the signed Impact Parameter of each track –Sign from crossing of track-jet axis: positive if downstream of PV negative if upstream of PV, i.e. non-physical solution Construct track probability based on IP significance and fitted resolution function –Need well determined track errors, also true for PV rec. Combine track probabilities to jet and event tags What do we want to do and what tools do we need? Construct physics filters to reject background and achieve acceptable rate

Overview Jet/Event tag

ADM 06/25/04Per Jonsson - Imperial College London6 Primary Vertexing; the Z-coordinate  Uses P t weighted histogramming: histogram tracks according to z0 weight bin by total P t choose highest 2 adjacent bins and average values B s -> D s π min bias event From Chris Also important to sharpen jet turn-ons ….and reject events outside SMT Resolution < 2 mm min biashard scatter

Chris Barnes, Imperial CollegeFresno June 2004 DCA resolution in data P scat = P sin 3/2  Material causes multiple scattering Large P scat – low multiple scattering ~ 15  m asymptotic value! Impact parameter minimization in 2 dimensions using linear tracks Requires inversion of one 2×2 matrix - FAST Minimise Primary Vertexing; the XY-coordinates From Chris Average number of PV tracks in data =6

ADM 06/25/04Per Jonsson - Imperial College London8 Beam spot information Why do we need the beam spot information? –Starting/reference point for the Primary Vertex tool: Linearization of tracks Constraint for Primary Vertex fit –Enters equation with weight of ~3 GeV track, non removable Returned as PV if algorithm fails, Beam tilt, sizeable correction. How does the information reach Level-3? –The beam position/tilt is measured by Vertex Examine every few minutes: DCA vs phi/Z fit to samples of L3 tracks –COOR command: l3update transmits the information to Level-3 –ScriptRunner writes the names and values to NameValueTable –L3TXYVertex reads the information at RunInit. Beam spot stored both in database and in data: debugchunk

ADM 06/25/04Per Jonsson - Imperial College London9 Beam spot dependence What if the beam position moves within a run? –Beam spot rather stable, < 30  m, within stores –Impact of larger beam shifts has been tested with MC and real data –Mean Reconstructed PV will shift by about 50% of beam shift –Leads to larger IPs on average and higher trigger rates No loss of signal efficiency, but more bkg. triggers –Cure: stop and restart new run Shifters: Vertex Examine must run at ALL times in the control room! Beam spot position picked up by Vertex tool at beginning of each run

ADM 06/25/04Per Jonsson - Imperial College London10 Impact Parameter b-tag; track selection Track p T > 0.5 GeV DCA error < 0.75 mm Matched to jet in 0.7 cone –Jet E T > 9 GeV –Jet rank =< pjets; pjets=3,6 Distance to PVZ < 2 cm 0 < Impact Parameter < 0.25 cm Decay Length < 1 cm Loose track selection: Average of 3 tracks/event Track quality Min. bias rej. Long lived bkg

ADM 06/25/04Per Jonsson - Imperial College London11 Multiple Scattering correction to DCA error Track errors from L3TGlobalTracker simple function of number of SMT hits Use QCD MC to obtain MS correction: –No beam size or PV uncertainty –Expected dependence on the width of DCA significance: f(P scat )=(c0 2 + (c1/P scat ) 2 ) 0.5 New error correction in L3TGlobalTracker, used by both PV and b-tagger QCD MC Data after corr.

ADM 06/25/04Per Jonsson - Imperial College London12 From IP to b-tag Fit resolution function, Gauss + 2 exp, to negative side of real data Use to calculate track/jet/event probabilities bb data QCD Data: Track prob Data: event tag “b-signal”

ADM 06/25/04Per Jonsson - Imperial College London13 Performance MC How well does it work compared to offline JLIP? Z->bbhbb Black = 2 jets Red = 2 JLIP tags Black = 3 jets Red = 3 JLIP tags Eff vs L3 btag-cut

Inefficiency MC Where do we lose efficiency wrt JLIP? ADM 06/25/04Per Jonsson - Imperial College London14 Z->bb MC, 2 JLIP tags, L3 btag < 0.1 Eff vs E T Eff vs  Eff vs nr. of tracksEff vs PVZ Less eff at low E T, ntrack, high  PVZ

ADM 06/25/04Per Jonsson - Imperial College London15 Performance: online Special mu+jet L3 IP tag test run A track only based version of L3TBtagIP has also successfully been running online since last August collecting B-physics events From Tim L3 Single JLIP tagged events

ADM 06/25/04Per Jonsson - Imperial College London16 Trigger list v13 Triggering on b-jet events w/o leptons at lum above 50E30 is challenging: –Rates driven up by non-b backgrounds –Aim to have un-prescaled triggers Alternatives to trigger level b-tagging are largely exhausted! The Higgs group will use the L3 b-jet tag for the following triggers in v13: –hbb: JT2_3JT15L_IP_VX, with L3 btag < 0.05 –Z->bb: ZBB_TLM3_2JBID_V, with L3 btag < 0.1 MUJ1_2JT12_LMB_V and MUJ2_2JT12_LMB_V, with L3 btag < 0.05 –ZH-> bb: JT1_ACO_MHT_BDV, with L3 btag < 0.3 Many very nice studies by Tim, Amber, Makoto et al. –See Agenda server since last fall: Higgs, Trigger, Multijet and TB meetings for details The first data collected with these triggers are currently being analyzed: –A first look confirms enhanced b content with L3 IP based triggering!

Latest results from Makoto Special v13 test runs: ZH-> bb trigger: JT1_ACO_MHT_BDV, L3 btag<0.3 ADM 06/25/04Per Jonsson - Imperial College London17 Black: all events JLIP prob Red: JT1_ACO_MHT_BDV Nr of tagged jets Loose L3 tag, so still dominated by light quarks, but nice b enhancement

ADM 06/25/04Per Jonsson - Imperial College London18 Summary & Outlook L3TBTagIP is tagging b-jet events at Level-3 Will be used by the Higgs group for several triggers in the imminent trigger list v13 The first data taken by this trigger list are just being analyzed D0 notes describing the L3 tools are coming –Future Improvements (p17): Tuning of vertex errors Improved fit of resolution function 3D version –Longer term: Soft lepton tag 2ndary Vertex tool Combination