1DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

The Search for Zbb at DO

Amber JenkinsImperial College London

DO Winter Physics Workshop28 February 2005

On behalf of Per Jonsson, Andy Haas, Gavin Davies and myself

2DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

The Z->bb StoryMotivation for the searchHow to trigger on Zbb eventsHow efficient are our triggers?The Data. The Monte Carlo.Choosing the signal boxSubtracting the backgroundLooking in data…Conclusions and outlook

3DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

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.

4DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

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.

5DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

Triggering on Zbb• 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%

6DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

• 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

7DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

The v13 Suite of Triggersv13 Trigger Description

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<0.1, 2 jets>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

8DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

v13 Trigger L1L2L3 Trigger Efficiency w.r.t. Offline Cuts (%)

(jet1,jet2) > 2.85 radians

(jet1,jet2) > 2.90 radians

(jet1,jet2) > 3.0 radians

DMU1_JT12_TLM3 16 15 16ZBB_TLM3_2LM0_2J

17 18 21

ZBB_TLM3_2JBID_V

67 70 72

MUJ1_2JT12_LMB_V

87 88 89

MUJ2_2JT12_LMB_V

88 88 89

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.

How Effective are our Triggers?

9DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

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.

Data and Monte Carlo Samples

10DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

PYTHIA-Generated Sample Number of EventsZ->bb (p14.03.02) 84,000bb QCD, 5<pT<20 GeV 202,240bb QCD, 20<pT<40 GeV 123,750bb QCD, 40<pT<80 GeV 148,750bb QCD, 80<pT<160 GeV 72,500bb QCD, 160<pT<320 GeV 23,250Light-quark QCD, 5<pT<10 GeV 200,000Light-quark QCD, 10<pT<20 GeV

200,000

Light-quark QCD, 20<pT<40 GeV

300,000

Light-quark QCD, 40<pT<80 GeV

250,000

Light-quark QCD, 80<pT<160 GeV

300,000

Light-quark QCD, 160<pT<320 GeV

50,000

Monte Carlo Samplesp1

4.07

.00

11DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

• 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

12DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

Using dphi as a Discriminator

njets = 3

njets >= 2

njets = 2

- data- Zbb MC- bb MC

Passing v13 triggers

13DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

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

14DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

Signal Peak in Monte Carlo

Z mass low!

15DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

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

Trigger Predicted Number

of Signal Events

ZBB_TLM3_2JBID_V 134ZBB_TLM3_2LM0_2J 39MUJ1_2JT12_LMB_V 166MUJ2_2JT12_LMB_V 166DMU1_JT12_TLM3 30

16DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

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 ZONE3. 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

2 3

17DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

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

18DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

1. The Invariant-Mass Based TRF (cont’d)

Excess

Estimated Background

High mass excess

19DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

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

20DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

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.

21DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

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.

22DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

Background Subtraction Using this Refined Method

• Excess seen in v12 data:

Estimated background 490 22

events

S/(S+B) = 5

Background shape well-modelled

23DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

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.

24DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

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

25DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

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