The Search for the
SM Higgs Boson at DØ

Dr. Andy Haas

Columbia University

DØ / ATLAS

SLAC

Particle Physics Seminar

January 24, 2008

µ+

µ-

b-jet

b-jet

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

Higgs Mass Constraints

EW variables sensitive to mH via radiative corrections:

LEP II direct: mH>114.4 GeV

mH<144 GeV

(at 95% CL)

mH<182 GeV (including direct limit)

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

b-jet
tagging

1000 Hz

Level 3

Trigger Farm

100 Hz

The DØ Collaboration

600 physicists from 18 nations

100 postdocs and 140 students

b-Physics Top Higgs

New Phenomena W/Z QCD

Track/Vertex b-Tagging

Muon Calorimetry

2005-2007
~20 people

~10 students

2007-
~200 people

~70 students

Finding the Higgs is now the main goal for DØ

Higgs Production at the Tevatron

Higgs Mass (GeV)

gluon fusion

time

Higgs Production at the Tevatron

gluon fusion

Higgs Mass (GeV)

Vector-boson
fusion

time

Higgs Production at the Tevatron

Associated
production

Higgs Mass (GeV)

time

Higgs Decays

Coupling  fermion mass

Main channels:

WW (high mass)

bb (low mass)

Need good b-jet tagging!

Main Higgs Analyses

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

H→bb

(low mass)

H→WW
(high mass)

Main Higgs Analyses

H→bb

(low mass)

H→WW
(high mass)

pp→WH

pp→ZH

H→WW→
ee/eµ/µµ+MET

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

H→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

IP significance = IP / σ(IP)

µ+

µ-

b-jet

b-jet

MET

Outer
Tracker

Silicon Tracker

(high-resolution)

Simulated

ZH→μμbb

event

Silicon
Tracker

Beampipe

r=1.6 cm

Secondary

Vertex

(r=0.3 cm)

Beampipe
r=1.6 cm

b-Jet Tagging

Many variables with
separation power:

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

#high IP sig. tracks,
combined light-jet prob.

...

Vertex DL
significance

# vertex tracks

# high IP tracks

~3mm

IP significance = IP / σ(IP)

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 a double-b-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)

Orthogonal event sample!

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

Other variables also have some separation power

ttbar

Missing ET

Angle between two muons

Signal

Signal

Z rapidity

Signal

Neural Net Event Selection

Combine variables with NN

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

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 transverse mass:

Mis-measured muon

High transverse mass:

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

Only 3x away from SM cross-section (at mH=160 GeV)

3x SM

Combine with CDF

More data (2.4 fb-1 this winter)

Further improvements in analysis technique

Higgs Mass (GeV)

4th Generation

Already excluding from 145-195 GeV

Sensitive up to
260 GeV by 2010

4th generation

production

9x more gg->H production!

Higgs Mass (GeV)

DØ Results in Main Higgs Channels

ZH→llbb

WH→lnubb

ZH→nunubb

H->WW*

WWW*

Combining the Channels

6x SM

3x SM

Low mass (115 GeV)

ZH→llbb 20x SM

WH→lnubb 10x SM
ZH→nunubb 12x SM

High mass (160 GeV)

H->WW*→ll 3.1x SM

WWW* 18x SM

Limit / SM cross-section

Combining the Experiments

Observed limit: 1.4x SM !

4x SM

2x SM

Limit / SM cross-section

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:

Di-jet mass resolution

Lepton efficiency

Further improvements in analysis technique...

Matrix Element techniques

Better b-tagging

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

HT: Sum pT of all jets (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

Multiple jet-cone sizes

0.5 less sensitive to noise, pileup, overlap

0.7 captures more jet energy

Jet-by-jet showering /
FSR correction

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 Acceptance

|eta|=2

|eta|=1

Bottom “holes”

ZH→µµbb
mH = 115 GeV

Muon eta

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

Use tracker:
good acceptance to |eta|~3.5

Muon Efficiency

Muons recovered in regions with poor muon acceptance

Track eta

Track phi

Forward
muon
octants

Z data

Z data

Muon

Z-mass +

MET constraints

Track

only

Tune the track quality selections

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

ZH→µµbb
mH = 115 GeV

Di-muon
data

Equivalent to 30% more data

after training NN

Sensitivity Estimates

Di-jet mass resolution (20%)

Lepton efficiency (10%)

Improved analyses (?%)

Matrix Element (20%)

Better b-tagging

Semi-leptonic tagging (5%)

Silicon Layer-0 (8%)

Should be sensitive to
mH=160 GeV ~now

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

Complimentary:

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

LHC: H, qqH : H->ττ/γγ decays

ttH(->bb) is very hard

Experience and methods from the Tevatron
are vital to LHC analyses!

Conclusions

A Higgs Boson, consistent with EW constraints,
would be fantastic verification of the SM

DØ has a large and dedicated Higgs group,
covering many analyses, 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 takes further innovations, underway

Better di-b-jet mass res., lepton ID, analysis methods, ...

May be sensitive to SM Higgs @160 GeV this winter

Possible to be sensitive up to 200 GeV by 2010

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

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

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