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

1DO Winter Physics Workshop

28 February 2005

Amber JenkinsImperial College London

The Search for Zbb at DO


DO Winter Physics Workshop28 February 2005

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


28 February 2005


The Z->bb Story

Motivation 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


28 February 2005


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.


28 February 2005


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.


28 February 2005


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%


28 February 2005


• 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


28 February 2005


The v13 Suite of Triggers

v13 Trigger DescriptionGood

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


28 February 2005


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

(jet1,jet2) > 2.85 radians



DMU1_JT12_TLM3 16 15 16

ZBB_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?


28 February 2005


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


28 February 2005


PYTHIA-Generated Sample Number of Events

Z->bb (p14.03.02) 84,000

bb QCD, 5<pT<20 GeV 202,240

bb QCD, 20<pT<40 GeV 123,750

bb QCD, 40<pT<80 GeV 148,750

bb QCD, 80<pT<160 GeV 72,500

bb QCD, 160<pT<320 GeV 23,250

Light-quark QCD, 5<pT<10 GeV 200,000

Light-quark QCD, 10<pT<20 GeV

200,000


300,000


250,000


300,000


50,000

Monte Carlo Samplesp14

.07.0

0


28 February 2005


• 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


28 February 2005


Using dphi as a Discriminator

njets = 3

njets >= 2

njets = 2

- data- Zbb MC- bb MC

Passing v13 triggers


28 February 2005


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


28 February 2005


Signal Peak in Monte Carlo

Z mass low!


28 February 2005


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 134

ZBB_TLM3_2LM0_2J 39

MUJ1_2JT12_LMB_V 166

MUJ2_2JT12_LMB_V 166

DMU1_JT12_TLM3 30


28 February 2005


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

2 3


28 February 2005


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


28 February 2005


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

Excess

Estimated Background

High mass excess


28 February 2005


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


28 February 2005


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.


28 February 2005


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.


28 February 2005


Background Subtraction Using this Refined Method

• Excess seen in v12 data:

Estimated background

490 22 events

S/(S+B) = 5

Background shape well-modelled


28 February 2005


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.


28 February 2005


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


28 February 2005


Sources of Error

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

Documents

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