The Search for the
SM Higgs Boson at DØ

Dr. Andy Haas

Columbia University

DØ / ATLAS

University of Chicago

Particle Physics Seminar

January 14, 2008

The Standard Model

3 families of matter

3 forces

“gauge symmetries”:
U(1)Y x SU(2)L x SU(3)C

Massive W,Z gauge bosons

Scalar Higgs field, non-zero VEV

W,Z get masses through “Higgs mechanism”

Fermions can get Yukawa masses:

Higgs boson: excitation of the Higgs field - mH

Precision EW Constraints

EW variables sensitive to mH via radiative corrections:

LEP II direct: mH>114.4 GeV

mH<182 GeV (including direct limit)

mH<144 GeV

(at 95% CL)

The Tevatron at Fermilab

Running (again) since ~2003

proton on anti-proton

sqrt(s)=1.96 TeV

DØ

Jan. '03

~90% data taking efficiency

3.4 fb-1

3.0 fb-1

The DØ Detector

New for Run II

Magnetic tracker

2 T solenoid

Silicon vertex tracker

Scintillating fiber tracker

Preshower detectors

Forward muon detector

Front-end electronics

Trigger and DAQ

Retained from Run I

LAr calorimeter

Central muon detector

Muon toroid

Higgs Production at the Tevatron

time

gluon fusion

Vector-boson
fusion

Associated
production

Higgs Decays

Coupling  fermion mass

Main channels:

bb (low mass)

WW (high mass)

Main Higgs Analyses

H→bb

H→WW

pp→H

pp→WH

pp→ZH

H→WW→
ee/eµ/µµ+MET

W/Z+H→W/Z+WW→
l+l- l+ / l+l+jj + MET

H→bb

WH→Wbb→
e/µ+bb

ZH→Zbb→
ee/µµ+bb

MET+bb

Main Higgs Analyses

H→bb

H→WW

pp→WH

pp→ZH

H→WW→
ee/eµ/µµ+MET

H→bb

WH→Wbb→
e/µ+bb

W/Z+H→W/Z+WW→
l+l- l+ / l+l+jj + MET

pp→H

ZH→Zbb→
ee/µµ+bb

MET+bb

b-Jet Tagging

B hadrons are “long”-lived

Reconstruct charged particles tracks

Reconstruct “vertices” where tracks overlap

Identify jets with:

large impact parameter significance tracks

large decay length significance vertices

~3mm

Beam

IP significance = IP / σ(IP)

b-Jet Tagging

Look at jets with
very loose b-tag

Many variables with
separation power:

Vertex: DLS, #tracks, #vertices, mass, chi2

# high IP tracks,
combined light-jet probability

...

Vertex
significance

# vertex tracks

# high IP tracks

Neural Network b-Jet Tagging

Train NN on simulated events

optimized inputs, training method, network topology

Test NN eff. and fake rate using data

NN Output

Cut-based
b-jet tagging
algorithm

NN b-jet tagging

algorithm

Equivalent to 2.5x as much data
for double-tag analysis

ZH -> µ+ µ- b b Search

Easy to observe Z decay

reduces backgrounds

provides trigger

Good b-tagging is essential

Reconstruct Higgs mass
from two b-jets

µ+

µ-

b-jet

b-jet

Transverse view

Z + 2 jets

Select events:

2 μ, isolated*,
pT>10 GeV, |eta|<2

>=2 jets, pT>15 GeV

65<mZ<115 GeV

Good agreement of data /
simulation at Z peak

Trigger eff. ~ 100%

QCD background determined
from (less-isolated) data

Main background: Z+jets

Invariant Mass of Muons (GeV)

Signal

*scalediso = (cal iso + trk iso) / pT
scalediso1 x scalediso2 < 0.1

Z + 2 b-jets

Require >=2 “NN b-tags”

NN tag cut for optimal expected limit:
~65% eff.
~2% fake

Higgs->bb forms bump in di-b-jet mass spectrum

Signal

Mass of b-tagged jets (GeV)

Di-b-jet mass resolution is key!

Single b-tag

Also use events with a single “tight” NN b-tag:
~40% eff.
~0.5% fake

(AND NOT two “loose” tags)

Compared to double-b-tag:

Similar amount of signal

Double amount of background

Like having 25% more data

Combine during limit-setting

Signal

Additional Variables

Di-b-jet mass is the best variable

But other variables also have some separation power

ttbar

Missing ET

Angle between two muons

Signal

Signal

Z rapidity

Signal

Neural Net Event Selection

Loose kinematic cuts

Multivariate discriminant

Separate NN for single/double b-tag

Train for
each mH

Sensitivity 37% greater than

using Mbb alone!

Signal

Neural Net Cross-Checks

All input variables are
well-modeled

Also check shape of NN output before b-tagging

High statistics

Negligible signal

Variables / correlations are well-modeled in Z+jets

Neural Net Cross-Checks

Train NN on ttbar as signal

A “standard candle”

Cross-section well-known

Expected ttbar is observed in both single and double b-tag channels

Single
b-tag

Double
b-tag

ttbar signal

ttbar signal

Neural Net Cross-Checks

Study variable shapes after cutting on NN output (>0.5)

NN is selecting signal-like region of phase-space

No separation power remains

NN>0.5

Di-b-jet Mass (GeV)

Di-b-jet Mass (GeV)

Systematic Uncertainties

Luminosity, 6.1%

Lepton ID, 2%

Background cross-
sections, 5-30%

QCD estimation, 20%

Jet-energy scale*

b-tagging*

*Affects shape of NN output
as well as normalization

ZH -> l+ l- b b Limits

Use whole shape of NN
outputs to set limits

Need ~20x more sensitivity
to see the Higgs in this channel alone (at 115 GeV)

Other channels

Combine with CDF

More data (2.4 fb-1 this winter)

Reduced systematics

Further improvements in analysis technique

20

Higgs Mass (GeV)

H -> W+W- -> µ+ µ- + MET

Select events:

2 µ, isolated,
pT>10 GeV, |eta|<2

>=2 jets, pT > 15 GeV

MET > 20 GeV*

MET “scaled” > 5

Min µ transverse mass > 20 *

15 < mµµ < 70 *

Signal

Missing ET (GeV)

*(depending on mH)

MET projected
onto jet direction

H -> W+W- -> µ+ µ- + MET

Main backgrounds after
pre-selection:

Z+jets : Fake MET

W+jets : Jet->Muon

WW : Nearly irreducible

tt : Two b-jets

Muon

MET

Low TM

Mis-measured muon

High TM

Real W event

Signal

Signal

Min Trans. Mass (GeV)

M(µµ) (GeV)

φ

H -> W+W- -> µ+ µ- + MET

Higgs is a scalar

Muons from W's tend to be more aligned

Δφ between two muons

Signal

H -> W+W- -> µ+ µ- + MET

Train a NN for each
simulated signal mH

NN Output

NN>0.5

W+jets: 8.5

WW: 8.1

Z+jets: 4.0

ttbar: 0.9

QCD: 0.3

Signal
(x100)!

pT (μ1)

pT (μ2)

m(μ1μ2)

φ(μ1,μ2)

ET

φ(μ1,ET)

φ(μ2,ET)

MinTM(μ1,μ2)

H -> W+W- -> µ+ µ- + MET

Combine with ee,
eµ channels

3x SM

Combine with CDF

More data (2.4 fb-1 this winter)

Further improvements in analysis technique

4th Generation

Already excluding from 145-195 GeV

Sensitive up to
260 GeV by 2010

4th generation

production

9x more gg->H production!

DØ Results in Main Higgs Channels

Combining the Channels

6x SM

3x SM

Combining the Experiments

Observed limit: 1.4x SM !

4x SM

2x SM

Improving Sensitivity

With no improvements, would
need much more luminosity
to be sensitive:

~20 fb-1 at 115 GeV

~7 fb-1 at 160 GeV

Expect 6.8 fb-1 by 2010

Improvements underway:

Better b-tagging

Di-jet mass resolution

Lepton efficiency

Matrix Element techniques

Further improvements in analysis technique...

Reducing Backgrounds to H->WW

Z+jets:

MET likely comes from mis-measured jet(s)

Look at HT, the scalar sum of the jets' pT

Useful as an additional NN variable to reduce Z+jets background

Sum pT of all jets (HT) (GeV)

Signal
(x100)!

Z+jets

Reducing Backgrounds to H->WW

W+jets, QCD:

Muon(s) comes from a jet

Usually a low-pT muon incorrectly matched to a high-pT track

Look at the worst χ2 between track, muon

Useful as an additional NN variable to reduce QCD and W+jets

Log10( Max(Chi2 match of track to muon) )

Signal

QCD

W+jets

Reducing Backgrounds to H->WW

Original NN

Using additional variables: HT , pT(μ1μ2),
UE, SET, χ2(track,μ), Min(μQ)

NN>0.5

W+jets: 8.5

WW: 8.1

Z+jets: 4.0

ttbar: 0.9

QCD: 0.3

NN>0.5

W+jets: 4.9

WW: 10.4

Z+jets: 1.4

ttbar: 0.7

QCD: 0.1

Sensitivity improved by 30%

Di-jet Mass Resolution

Undertaking a major effort to improve jet energy resolution

Add “pre-shower” energy

Correct for jet “width”

Track-based corrections

(H1-style) cell energy weighting

First a simple trick: measurements of 0.5 and 0.7 cone jets are different

0.5 less sensitive to noise, pileup, overlap

0.7 captures more jet energy

0.5 cone

0.7 cone

Pre-shower

Average Cone Jets

mH=115 GeV

backgrounds

signal (scaled!)

0.5 cone

0.7 cone

Average cone

Use 0.5 cone jets for event selection and b-tagging

Match to 0.7 cone jet and average their 4-vectors

~8% di-jet mass resolution improvement

angles are also
measured better

~5% energy resolution improvement observed in γ+jet and di-jet data

Translates into 8% more

sensitivity for low-mass

Higgs searches

Muon Efficiency

Z->µµ efficiency is only ~60%

Limited muon acceptance

20% of Z's with muon |eta|>2

Use tracker, good acceptance
to |eta|=~3.5

Require one standard muon,
look for isolated track

pT > 10 GeV

Tune track and isolation criteria

ZH→µµbb
mH = 115 GeV

Muon eta

Similar methods also underway

to improve electron ID

Muon Acceptance

|eta|=2

|eta|=1

Bottom “holes”

Muon Efficiency

Muons recovered in regions with poor muon acceptance

Track eta

Track phi

Forward-muon octants

Muon Efficiency

Combine track-only muon with:

wider Z mass window

muons with no central track

muons failing timing criteria

looser muon isolation

At pre-selection:

~60% more ZH->µµbb signal

~90% more background

Equivalent to ~30% more data after training NN

ZH→µµbb
mH = 115 GeV

Di-muon
data

Sensitivity Estimates

Better b-tagging

Semi-leptonic tagging (5%)

Silicon Layer-0 (8%)

Di-jet mass resolution (20%)

Lepton efficiency (10%)

Matrix Element (20%)

Sensitive to mH=160 GeV
with data already on tape

May be sensitive up to 200 GeV by 2010

2007

2.4 fb-1

Higgs Mass (GeV)

LHC

Tevatron

LHC

proton on proton

7x Tevatron energy

~100x the luminosity

Collisions this summer?

ATLAS

Taking cosmic data with all detector subsystems

Barrel LAr / Tile calorimeter

Higgs at the LHC

Discovery possible in whole mass range after 30/fb (~2011)

mH<~130 GeV difficult

TeV: W/Z+H, H->bb decays

LHC: H, qqH, H->ττ/γγ decays

ttH(->bb) is very hard

Important to measure many channels, to test SM

Conclusions

The Higgs Boson would complete verification of the SM

Will also lead to new puzzles – and perhaps to new physics

DØ is hunting the Higgs, using advanced techniques

NN b-tagging, single/double b-tag, NN event selection

Combined with CDF: 4.3x SM @115 GeV, 1.9 @160 GeV exp.

Finding the Higgs will take further innovations

Better b-tagging, di-b-jet mass resolution, lepton ID, ME's...

May be sensitive to SM Higgs @160 GeV this winter

Possible to be sensitive up to 200 GeV by 2010

And we're always ready for surprises!

1.4 observed!

Backup

Precision EW Constraints

EW variables sensitive to mH via radiative corrections:

LEP II: mH>114.4 GeV

mH<97 GeV

(at 95% CL)

(leptonic only)

Why is the Higgs so Light?

The Higgs mass is unstable

Large radiative corrections
(it's a scalar)

Hierarchy problem:

mH << mGUT

Why is the Higgs so Light?

We wouldn't be here if it wasn't

A small Higgs VEV seems necessary for life

Same reason for a small cosmological constant?

Why is the Higgs so Light?

New physics: Supersymmetry

Particles come in fermion-boson pairs

Corrections to Higgs mass nearly cancel,
if boson and fermion masses are similar

DØ SUSY Higgs Search

Search for bbb bump on bbb background

Interpret as limits in
mA / tanB plane

PRL 95, 151801 (2005)

b

b

Why is the Higgs so Light?

616 papers on hep-ph in 2007 on “Higgs”

Lots of great ideas

Need experimental input

LEP @ CERN in 2000

Circular e+ e- collider

Maximum E of 200-210 GeV

Look for e+ e- -> Z+H(->bb)

Slight excess around 115 GeV

Higgs mass > 114.4 GeV

A good, but not the
only variable...

Loose

Medium

Tight

Limit Setting

Sensitivity

2003 study

ZH->µµbb Pre-b-tagging

Event Kinematics

No true MET in µµbb events

Should assign observed MET to some combination of muons and jets – correct their pt's

Depends on:

angles between MET, muons, and jets

pt balance of Z and di-jets

invariant mass constraint of Z and H->bb

Too complicated for NN

µ+

µ-

b-jet

b-jet

Transverse view

MET

Matrix Elements

Calculate “cross-section” for an observed event to be from Z+bb or Z+(H->bb)

Use MC integration methods

Include as inputs to NN

ZH

Z+bb

µ

Jet

MET = µ1+µ2+j1+j2+σ(MET)

Tracking

Silicon Microstrip Tracker

Central Fiber Tracker

IP resolution

γ→e+e-

Layer 0 of Silicon Tracker

Silicon detectors mounted just outside the beampipe

Installed fall '06

Better track impact-parameter resolution
-> Better b-jet tagging

Effect of Layer 0

in recent data

IP Resolution (um)

pT (GeV)

Layer 0 being inserted into the silicon tracker

b-Jet Tagging Reality

B

b-jet

Fiber tracker

Silicon tracker

2 m

b-Tagging Measurement

System 8 method:

Correlation coefficients, measured in MC:

Lepton Efficiency

MET from non-reconstructed muon is present

And points in the direction of the isolated track

Isolated

Background

Isolated

Background

Before correction

After correction

Standard Z's

ZH→µµbb
mh = 115 GeV

4th Generation

9x more gg->H production!

Reduced H->bb,tautau

Enhanced H->gg

Relieves mH EW tension

DAQ System

The Higgs in Supersymmetry Decays

Look for Higgs bosons in the decays of supersymmetric particles

Require MET>250 GeV to
reduce SM background

BR X2->X1H = 19%

Missing ET (GeV)

SM background (top)

Susy background

Susy Higgs signal

The Higgs in Supersymmetry Decays

Look for bump from H->bb decays

30 fb-1

bb Invariant Mass (GeV)

Andy Haas

2004-07-07T15:59:13