The Search for Zbb at DO: Unveiling the Z->bb Story

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

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

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.

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.

Triggering on Zbb Hope to incorporate a L2 STT Zbb trigger term in v14… • 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 Ultimately we are limited by the BR(b)  10%

v12 and v13 Trigger Selection • 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.

The v13 Suite of Triggers

How Effective are our Triggers? Offline cuts: 1 tight muon; 2 or more good jets; jet  < 2.5; jet pT > 15 GeV; 1st & 2nd leading jets are taggable and tight-SVT b-tagged.

Data and Monte Carlo Samples 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 pT spectrum (see next slide). • Full jet energy scale corrections are applied to all samples using JetCorr v5.3.

Monte Carlo Samples p14.07.00

Kinematic Handles • 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

Using dphi as a Discriminator Passing v13 triggers njets >= 2 njets = 2 • data • Zbb MC • bb MC njets = 3

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 pT > 15 GeV; • - ensure 1st & 2nd 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

Signal Peak in Monte Carlo Z mass low!

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

Background Subtraction njet = 2, dphi > 3.0 “IN ZONE” • Key to this analysis is understanding the background • The method (a la CDF Run I): • 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. 3.0 “OUTSIDE ZONE” dphi njet 2 3

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

1. The Invariant-Mass Based TRF (cont’d) Estimated Background Excess High mass excess

2. Correction for  Dependence of TRF • Ratio of double to single only tight SVT tagged events with dphi < 3.0 (black) and 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

2. Correction for  Dependence of TRF (cont’d) • We observe a linear dependence of TRF on , in both MC and data: • 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. Treat as if no signal in this region – this is conservative

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 1st leading jet is single-SVT tagged • For these events, we then consider 2nd ldg jet. In three eta regions for the 2nd leading jet, we calculate the TRF as function of ET. • 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.

Background Subtraction Using this Refined Method S/(S+B) = 5 • Excess seen in v12 data: Estimated background 490  22 events Background shape well-modelled

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  22events. • Analysis cuts are being finalised. • The Analysis Note is being prepared for group review.

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

Sources of Error • JES • Btagging • Trigger efficiencies • Luminosity • Jet reco/ID • Statistics outside signal zone • TRF dep on dphi

The Search for Zbb at DO: Unveiling the Z->bb Story