The Search for the Higgs Boson
- An Opening to New Physics

Dr. Andy Haas

Columbia University

DØ / ATLAS

University of Washington

Physics Colloquium

January 22, 2007

Outline

What is the Higgs?

Current searches for Higgs

Standard Higgs

Extra Higgses from supersymmetry

Is something else going on?

Looking for some new things

The big deal in Switzerland

A new way to see Higgses over there

The Standard Model

3 families of matter

quarks, leptons

3 forces

EM, Weak, Strong forces

photon, W/Z, gluons

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

(oh, and there's gravity)

The Weak Force is Weird!

Experiments in the 1950's showed that the Weak force is left-handed

particles go in a preferred direction based on the spin direction

You can tell the real world from the world in a mirror!

muon

electron

decay

muon

electron

decay

Mirror

That “L” Makes a Big Difference

Relativity: physics looks the same no matter how you are moving

In a spaceship going faster than the electrons, you'd see them going down

That violates our rule!

It must not be possible to go faster than an electron!
-> The electron must be massless!

SU(2)L gauge invariance
-> All particles must be massless!

How do we give particles mass?

muon

electron

decay

muon

electron

decay

Go fast in
spaceship

The Higgs Boson

Gives masses to the quarks and leptons, and W/Z bosons

but leaves the photon alone (thankfully)

The Higgs field costs less than nothing to have around – like a helpful friend!

Empty space prefers a non-zero Higgs density

Particles get effective mass by interacting with the Higgs field of empty space

muon

electron

decay

muon

Higgs

Searching for the Higgs

Sounds good, but how do we know it's true?

Excite the Higgs field

make Higgs particles !!!

Need to collide particles with enough energy to create the mass of the Higgs

E=mc2

Indirect evidence, Higgs mass is about 10-1000 GeV

Build a big accelerator!

LEP @ CERN in 2000

Circular e+ e- collider

Energy was 200-210 GeV

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

Slight excess around 115 GeV?

Higgs mass > 114.4 GeV

A good, but not the
only variable...

Loose

Medium

Tight

The Tevatron at Fermilab

Running (again) since ~2003

proton on anti-proton

1 km radius

Each (anti)proton has E = 1 TeV

1000x the rest mass!

2.5 million collisions / sec.

Can look for very rare processes
(like Higgs)

DØ

time

What the Higgs “Looks Like”

The Higgs has a short life

What it decays to depends on its mass

Main focus at the Tevatron is:

bb (low mass)

WW (high mass)

Higgs Analyses at DØ

Wh -> e nu b b

Wh -> mu nu b b

Wh -> tau nu b b

Zh -> nu nu b b

Zh -> e+ e- b b

Zh -> mu+ mu- b b

h -> WW -> e+ nu e- nu

h -> WW -> mu+ nu mu- nu

h -> WW -> e+- nu mu-+ nu

Wh -> WWW -> e+- e+- +nu's + X

Wh -> WWW -> mu+- mu+- +nu's + X

The Zh -> mu+ mu- b b Search

Look for h+Z final state

Z decays to muons

Higgs decays to bb

Easy to observe Z decay

reduces backgrounds

b's form b-jets

can identify and separate from other jets (gluon, u,d,s)

reduces backgrounds

mu+

mu-

b-jet

b-jet

Transverse view

The DØ Detector

b-jet

b-jet

mu+

mu-

Me

b-Jet Tagging

How do we know when a jet is a b-jet?

B hadrons are “long”-lived
(a picosecond is forever...)

Reconstruct charged particles tracks

Identify jets with

large impact parameter significance tracks

large decay length significance vertices

~3mm

Beam

b-Jet Tagging Reality

B

b-jet?

Fiber tracker

Silicon tracker

NN b-Jet Tagging

Combine properties of the reconstructed vertex and tracks using an artificial Neural Network

Increases S/sqrt(B) by ~30% per b-jet

Like having 2.5x as much data!

Track-only
b-jet tagging
algorithm

NN b-jet tagging

algorithm

The Zh -> mu+ mu- b b Search

Let's look at the data!

And apply the b-tagging technique

Z + 2 jets Data

First, select events with a Z

Main background is Z+(non-b-)jets

b-jet tagging will help reduce this

Invariant Mass of Muons (GeV)

Z + 2 b-jets Data

Require at least 2 of the jets to have NN b-tags

Z+(non-b-)jets background greatly reduced

Look for bump from Higgs->bb

Zh -> l+ l- b b Limits

Combine with
Zh-> e+ e- b b
channel

Set an upper limit on the rate of Zh production

We need a factor of ~50x more sensitivity to see the Higgs in this channel alone
(at 115 GeV)

50

~70 fb

Combining the Channels

Fortunately, there are many channels

A factor of ~15 away from the Higgs (115 GeV)

15

Combining the Experiments

And there's another experiment at the Tevatron (CDF)

Only a factor of ~7 away from seeing the Higgs (115 GeV)

7

Improving Sensitivity

Now: 7.0

More data (2/fb per exp. by this year) : 5.0

NN b-tagging for all analyses: 3.2

Neural Net Event Weighting

Train an artificial Neural Net to discriminate between Higgs-like and background-like event kinematics

Variables:

bb invariant mass

mu+ mu- invariant mass

pT's of b-jets

Angle between muons

Angle between jets

Boost between jets

Missing ET

+

+
....

b-jet pT

Muon
angle

Final NN
output

Improving Sensitivity

Now: 7.0

More data (2/fb per exp. by this year) : 5.0

NN b-tagging for all analyses: 3.2

NN event weighting: 2.0

Better Higgs mass resolution: 1.6

Use forward electrons in WH: 1.45

Add tau channels: 1.32

Better calibrations, reduced systematics: 1.2

Small, difficult
improvements

Layer 0 of Silicon Tracker

Silicon detectors mounted just outside the beampipe

Installed last Fall

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

Improving Sensitivity

Now: 7.0

More data (2/fb per exp. by this year) : 5.0

NN b-tagging for all analyses: 3.2

NN event weighting: 2.0

Better Higgs mass resolution: 1.6

Use forward electrons in WH: 1.45

Add tau channels: 1.32

Better calibrations, reduced systematics: 1.2

Added Layer 0 silicon: 1.0

We'll get there! But it's a lot of hard work.

Small, difficult
improvements

Why is the Higgs so Light?

Where does the Higgs get its mass from?

The Higgs mass is unstable

Large radiative corrections
(it's a scalar – spin 0)

A delicate balance between many large forces

Hierarchy problem

Why is the Higgs so Light?

We wouldn't be here if it wasn't?

A small Higgs mass 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

fermion =
spin 1/2, 3/2, ...

boson =

spin 0, 1, ...

Higgses in Supersymmetry

Requires two Higgs (doublet) fields, 5 Higgs bosons:

h -> like the standard Higgs boson

H, A -> heavier, neutral also

H+, H- -> heavier, charged

tanB -> ratio of Higgs fields' VEVs

H/A +bb production proportional to tan2B

Should be able to see H/A + bb at high tanB...

H/A decays to bb 90% of the time (the rest to tau's)

Look for: H/A + bb -> bb bb

DØ Search for H/A + bb -> bb bb

Look for bump in bb invariant mass spectrum in events with at least 3 b-jets

Background determined from 2 b-jet data

No excess observed for any H/A mass

Interpret as limits in the Higgs mass vs. tanB plane

PRL 95, 151801 (2005)

H/A Sensitivity at the Tevatron

Combine with the H/A -> tau tau decay channel

Combine with CDF

Sensitive down to tanB=40 soon for H/A masses up to ~200 GeV

A nice explanation for the t/b mass ratio?

~0.3 fb-1

Trouble with Minimal Supersymmetry?

We should have found the Higgs already?!

Supersymmetry predicts a light Higgs

Comparisons of the W and top masses point to a light Higgs

But searches at LEP exclude a light Higgs!

Trouble with Minimal Supersymmetry?

CDF just measured the W mass

Include their measurement into the world average and...

A Hint of Something New?

LEP didn't really exclude a light Higgs

They only excluded a standard light Higgs!

What if it decays to some new particles?

Higgs Decays to Intermediate Particles

New light scalar particle: “a”

favored in many models

a decays to 2 taus

h -> a a -> tau tau tau tau

a is light, so tau's are
close to each other

Hard to identify!

Not excluded by LEP

Analysis in progress...

tau

tau

tau
e

tau
mu

Higgs Decays to Long-lived Particles

New long-lived particle: “P”

P decays to 2 b-jets

h -> P P -> b b b b

No one's ever looked for long-lived particles decaying to b-jets

Not excluded by LEP

Analysis in progress...

P

P

b

b

b

b

Silicon tracker

The LHC at CERN

proton on proton

4.3 km radius

Each proton has 7x the Tevatron energy

100x as many collisions/sec.
as the Tevatron

Starts in 2008!
(low-energy test run this year)

Tevatron

LHC

The ATLAS Detector

DZero

ATLAS is Real

Full central calorimeter tested with cosmic rays, in final position

All subsystems should be ready for beam in September!

The ATLAS Control Room

Higgs Searches at ATLAS

The light Higgs will be found
(if it exists)

H/A and H+- may also be seen

But these discoveries won't be easy or quick!

3 years for 30/fb

5 years for 300/fb

h->γγ

The Higgs in Supersymmetry Decays

Look for Higgs bosons in the decays of supersymmetric particles

A possible shortcut to Higgs discovery

Require lots of missing transverse energy

Backgrounds are greatly reduced...

BR X2->X1h = 19%

Missing ET (GeV)

SM background (top)

Susy background

Susy Higgs signal

The Higgs in Supersymmetry Decays

Remaining background is mostly supersymmetric!

Look for bump from H->bb decays

30/fb

bb Invariant Mass (GeV)

Conclusions

We'll soon find some kind of Higgs(es), if they exist

Possibly at the Tevatron, certainly at the LHC

Maybe just a plain-old Higgs

Nothing to stabilize its mass?

Maybe more interesting Higgs(es), with new physics

Supersymmetry?

Maybe a Higgs connected to stranger new physics

Extended supersymmetry?, Long-lived particles?, ...

Maybe we rule out the Higgs completely

Certainly new physics!

Conclusions

A very exciting time ahead!

We have little idea what's right around the corner...

Keep our minds open and look in all places!

Andy Haas

2004-07-07T15:59:13