1. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 1 The Search for Z  bb at DO Amber Jenkins Imperial College London DO Winter Physics Workshop 28 February 2005 On behalf of Per Jonsson, Andy Haas, Gavin Davies and myself

2. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 2 The Z->bb Story Motivation for the search How to trigger on Z  bb events How efficient are our triggers? The Data. The Monte Carlo. Choosing the signal box Subtracting the background Looking in data… Conclusions and outlook

3. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 3 The Story Begins… Z->bb is not revolutionary new physics. But its observation at a proton-antiproton collider is very important. –For calibration of the b-JES, relevant to much of D0 physics; –As a crucial test of our jet energy and dijet mass resolution; –As an ideal testbed for the decay of a light Higgs. Back in the heyday of Run I, DO lacked the tracking or b-tagging capabilities needed for Z->bb. CDF claimed to observe 91 ± 30 ± 19 events using their Silicon Vertex Detector.

4. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 4 The Challenge In Run II, however, Z->bb is within our reach. With good muon detection, b-tagging & the prospect of the Silicon Track Trigger, all the tools are at our disposal. The challenge is to fight down the massive QCD background swamping the signal. Triggering is crucial. We need to achieve sufficient light-quark rejection such that trigger rates are acceptable at high luminosity.

5. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 5 Triggering on Z  bb The natural Z->bb trigger would be a low energy jet trigger. However, rates would be unmanageable. Ideally we would trigger on dijet events with displaced vertices at Level 2. This will soon be possible with the STT. In the meantime, we rely on semileptonic decay of b-jets to 1 or more muons  Use single-muon & dimuon triggers at Level 1  Require additional jet,  & track terms at Levels 2 and 3  Capitalise on power of our Impact Parameter b-tagging at L3 Hope to incorporate a L2 STT Zbb trigger term in v14… Ultimately we are limited by the BR(b  )  10%

6. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 6 The analysis exploits data collected with both v12 and v13. For v12, pre-existing muon triggers are used; 61 in total. For the v13 trigger list we have designed 5 dedicated triggers which optimise Z->bb signal efficiency while achieving required background rejection for luminosities up to 80E30. They went online last summer. v12 and v13 Trigger Selection

7. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 7 The v13 Suite of Triggers v13 TriggerDescription Good Luminosity Collected in PASS2 Data (pb -1 ) DMU1_JT12_TLM3 L1: dimu; L2: 1 med  ; L3: 1 jet>12 GeV, 2  (1 with trk-match > 3 GeV) 47.5 ZBB_TLM3_2LM0_2J L1: 1 trk-matched , pT>3 GeV; L2: 1 med  ; L3: 2 loose , 2 jets>12 GeV 41.2 ZBB_TLM3_2JBID_V L1: 1 trk-matched , pT>3 GeV; L2: 1 med  ; L3: IP 12 GeV, PVZ<35 41.2 MUJ1_2JT12_LMB_V L1: 1 , 2 trig towers>3 GeV; L2: 1 med , 1 jet>8 GeV; L3: 1loose , 2 jets>12 GeV, IP<0.05, PVZ<35 44.0 MUJ2_2JT12_LMB_V L1: 1 , 1 trig tower>5 GeV; L2: 1 med , 1 jet>8 GeV; L3: 1 loose , 2 jets>12 GeV, IP<0.05, PVZ<35 42.7

8. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 8 v13 TriggerL1L2L3 Trigger Efficiency w.r.t. Offline Cuts (%)  (jet1,jet2) > 2.85 radians  (jet1,jet2) > 2.90 radians  (jet1,jet2) > 3.0 radians DMU1_JT12_TLM3161516 ZBB_TLM3_2LM0_2J171821 ZBB_TLM3_2JBID_V677072 MUJ1_2JT12_LMB_V878889 MUJ2_2JT12_LMB_V88 89 Offline cuts: 1 tight muon; 2 or more good jets; jet  15 GeV; 1 st & 2 nd leading jets are taggable and tight-SVT b-tagged. How Effective are our Triggers?

9. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 9 Data and Monte Carlo Samples Data  Run selection: Remove bad CAL/MET runs, runs with bad luminosity blocks & select good CAL/MET runs.  v13 Dataset (70 pb -1 ): PASS2 Higgs skim; 45M events containing our Z->bb triggers collected from June 2004  August 2004.  v12 Dataset (~200 pb -1 ): PASS2 BID skim; 90M events containing at least 1 loose muon & 1 0.7 cone jet PYTHIA Monte Carlo: signal plus bb and light-quark inclusive QCD backgrounds covering a wide p T spectrum (see next slide). Full jet energy scale corrections are applied to all samples using JetCorr v5.3. Data and Monte Carlo Samples

10. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 10 PYTHIA-Generated SampleNumber of Events Z->bb (p14.03.02) 84,000 bb QCD, 5<p T <20 GeV202,240 bb QCD, 20<p T <40 GeV123,750 bb QCD, 40<p T <80 GeV148,750 bb QCD, 80<p T <160 GeV72,500 bb QCD, 160<p T <320 GeV23,250 Light-quark QCD, 5<p T <10 GeV200,000 Light-quark QCD, 10<p T <20 GeV200,000 Light-quark QCD, 20<p T <40 GeV300,000 Light-quark QCD, 40<p T <80 GeV250,000 Light-quark QCD, 80<p T <160 GeV300,000 Light-quark QCD, 160<p T <320 GeV50,000 Monte Carlo Samples p14.07.00

11. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 11 We investigate which kinematic tools provide best discrimination between signal & background. To eliminate essentially all of the light-quark QCD, it is sufficient to require 2 b-tagged jets in each event. There are few handles which really cut back the bb background… After b-tagging, the main difference between Z->bb & bb QCD background is colour flow in the events: –Expect more colour radiation in QCD processes –Expect pattern of radiation to be different  Study number of jets per event, njets  Study azimuthal angle between 2 b-quark jets,  12 Kinematic Handles

12. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 12 Using dphi as a Discriminator njets = 3 njets >= 2 njets = 2 - data - Zbb MC - bb MC Passing v13 triggers

13. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 13 Offline Event Selection Initial dataset is cleaned up by – removal of noisy jets – requiring a tight offline muon After triggering we apply: - jet  < 2.5; - jet p T > 15 GeV; - ensure 1 st & 2 nd leading jets are taggable & tight-SVT b-tagged; - require njets >= 2 - require  > 3.0 We are completing the tuning of these cuts, after v13 trigger selection. - Zbb MC - bb MC Mass window cut

14. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 14 Signal Peak in Monte Carlo Z mass low!

15. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 15 Signal Event Predictions Calculate no. of signal events expected per trigger, accounting for luminosity, cross-section, trigger and offline efficiency Predictions are for  > 3.0 and njets >= 2 TriggerPredicted Number of Signal Events ZBB_TLM3_2JBID_V134 ZBB_TLM3_2LM0_2J39 MUJ1_2JT12_LMB_V166 MUJ2_2JT12_LMB_V166 DMU1_JT12_TLM330

16. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 16 Background Subtraction Key to this analysis is understanding the background The method (a la CDF Run I): 1.Define 2 regions – IN SIGNAL ZONE: evts which pass njets = 2 & dphi>3.0 - OUTSIDE ZONE: all other evts (which fail above IN ZONE conditions) 2. Calc. Tag Rate Function (TRF) of double/single tagged evts OUTSIDE ZONE 3. Expected Bkg IN ZONE = TRF * (single-tagged evts IN ZONE) i.e. N ++ exp,IN = N + obs,IN * (N ++ obs,OUT /N + obs,OUT ) 4. IN ZONE, Subtract Expected Bkg from Observed Events. An excess around 90 GeV is bias-free evidence for a signal. “IN ZONE” “OUTSIDE ZONE” njet dphi njet = 2, dphi > 3.0 3.0 23

17. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 17 1. The Invariant-Mass Based TRF Construct an invariant-mass based TRF The single-tag mass histogram IN ZONE is multiplied bin-by-bin by this TRF to give a background estimate Background then subtracted

18. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 18 1. The Invariant-Mass Based TRF (cont’d) Excess Estimated Background High mass excess

19. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 19 2. Correction for  Dependence of TRF Ratio of double to single only tight SVT tagged events with dphi 3.0 (red) –Peaked increase in the Z region for dphi> 3.0 –The probabilities do not, however, converge at higher masses –This implies we are underestimating the bkgd in this region

20. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 20 2. Correction for  Dependence of TRF (cont’d) We observe a linear dependence of TRF on , in both MC and data: Treat as if no signal in this region – this is conservative We derive the TRF correction outside the signal zone. The correction is ~ 15% for 2.8<  <3.1. Note that the signal estimate is conservative. We assume Zbb is only found in signal zone, which is not accurate.

21. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 21 3. The Jet-Based TRF We improve the background estimation by moving to using a jet-based TRF a la the Hbb analysis (see Andy’s talk) We consider events where the 1 st leading jet is single-SVT tagged For these events, we then consider 2 nd ldg jet. In three eta regions for the 2 nd leading jet, we calculate the TRF as function of E T. This generates a TRF per jet. It is still based on events outside the signal zone. Each event is then weighed accordingly. This is likely to be a more accurate method as it provides a finer resolution to the correction. We still include the ~15% correction for TRF dependence on dphi.

22. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 22 Background Subtraction Using this Refined Method Excess seen in v12 data: Estimated background 490  22 events S/  (S+B) = 5 Background shape well-modelled

23. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 23 Conclusions We are searching for Z  bb in v12 and v13 data Different methods to estimate the background are being tested. Out of 9294 selected double-tagged events, we observe an excess after background subtraction of 490  22 events. Analysis cuts are being finalised. The Analysis Note is being prepared for group review.

24. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 24 To Do List Complete tuning of cuts in v13 triggers Include extra ~100 pb -1 of data from v11 runs and before Systematic errors Update note Take to Wine & Cheese seminar

25. DO Winter Physics Workshop 28 February 2005 Amber Jenkins Imperial College London 25 Sources of Error JES Btagging Trigger efficiencies Luminosity Jet reco/ID Statistics outside signal zone TRF dep on dphi