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The Hunt for the Higgs (and other interesting stuff at the Tevatron)
Robert RoserFermi National Accelerator Laboratory
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~90 years ago
1
10,000
~60 years ago
1
10
~40 years ago
1
100,000
Present
electron
1
100,000
of human hair
thickness
up quark
down quark
atom
nucleus proton
neutron
The Past Century…
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The Big Picture!
• World is comprised
of quarks and
leptons
• Each particle has its
own anti particle
• Quarks have
fractional charge!
• Good description of
particles and their
interactions
• Extensively tested
Periodic Table of the Particles
The Standard Model of Particle Physics states:
The world is comprised of Quarks and Leptons that interact by exchanging Bosons
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An example – Water!
• H20
H
O
H
P
e
HydrogenOxygen
8N
8P
8e
28 Up Quarks
26 down quarks
10 electrons
H2O
UU
d
d
d
U
Proton Neutron
2/3+2/3-1/3 2/3-1/3-1/3
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The $64,000 Question
top quark
ZW
. . .
ne nm nt e m t u d s c b
photons
gluons
“Why are there so many particles?”“Where does mass come from?”
Why is top so heavy?
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Enter the Higgs Mechanism
Analogy by Prof. David Miller
University College of London
Popularity Mass
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7
So in honor of their work ...
2010 Sakurai Prize
Guralnik Hagen KibbleHiggsEnglert Brout
PRL 13, 508-509 (1964)PRL 13, 321-323 (1964) PRL 13, 585-587 (1964)
... for "elucidation of the properties of spontaneous symmetry breaking
in four-dimensional relativistic gauge theory and of the mechanism for
the consistent generation of vector boson masses."
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The Solution: Brout-Englert-Higgs-Hagen-
Guralnik-Kibble (BEHHGK) mechanism[pronounced “beck”, preserves author grouping, publication ordering, and much catchier than “EBHGHK”]
Add scalar field throughout the universe
Potential is symmetric
Ground state breaks symmetry
Cleverly
Masses are generated for the fermions due to their interaction
with this non-zero field
Theory preserves symmetry (gauge invariance)
Standard Model calculations no longer fail
A new particle is predicted: the BEHHGK boson
Finding the BEHHGK boson
Means BEHHGK field exists
Means we confirm our theory for the origin of mass
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9
I will use “Higgs” and
“BEHHGK” boson
interchangeably in this talk,
but will refer to
the H particle
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How does one search for the
BEHHGK (Higgs) boson?
(or anything else for that matter)
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Making Particle X
E = mc2
Thanks to Einstein we know that a high energy collision of particle
A and B can result in the creation of particle X
Energy
particle beam
energy
anti-particle beam
energyEa Eb
Mx
As long as Ea + Eb >> Mxc2
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It’s a bit more complicated…
Eu Eu Mt Mt
d
u
u
u
u
d
Proton Anti-Proton
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America’s Most Powerful Accelerator:
Fermilab’s Tevatron
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Colliding Beam Detectors
• Detector design is always a
compromise
– $$$
– available space
– technological risk
– readout time and
construction time
• Goal is to completely
surround collision with
detectors
• Arrange detectors in layers
based on functionality
– Measure particle’s position,
momentum and charge first
– Type and kinetic energy
second
CDF II Detector cross section
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Collecting the Data You Want!
• The Collider Challenge– 1.7 Million Collisions/second inside our
detectors
• Detectors– Very complicated with lots of information
available on each collision
• The problem– You can’t write out each collision to tape!
– Don’t worry – not every collision is interesting and warrants saving…
• The Solution – A Device called a “trigger”
– Examines every event in real time and identifies the most “interesting”
– Reject 99.991% of events and collect data at ~50-100 hz
Level-1/2 Triggers
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The Life of an Experimentalist…
• Our “camera” is not fast enough to take a picture of
say a top quark! We have to infer based on the
information provided!!!!
• What do we know?
– Conservation of Energy
– Conservation of Momentum
– E=mc2
• What do we want to identify?
– Electrons
– Muons
– Quarks
– Neutrinos
– b quarks
(Helps!)
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The D0 Experiment
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The CDF Experiment
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CDF
15 Countries
63 institutions
602 authors
D0
18 Countries
90 institutions
507 authors
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Principle of a Collider Detector
Basic principle is the interaction
of particles with matter
• Momentum/Charge Measurement:
- need to affect particle as little as
possible
- use dilute/thin absorber medium
(gas, thin silicon wafers): Tracker
• Energy Measurement:
- want to fully absorb particle
- use thick absorber medium
(lead, steel, uranium): Calorimeter
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A Single
Top quark
Candidate
vent
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Top Quark Discovered!
1995
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You Can Not make a Discovery with one Collision
It is a Statistical Process!
Today we
have 70
times more
data
collected
than we used
to discover
the top quark
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New Physics Shows Up Throughout
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Tevatron Physics Output
Tevatron Experiments publishing >100 papers/year
Over the last few years, ~60 PhD’s/year
Present >200 talks at conferences each year
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The Tevatron Research Program
Precision, New Research
Discoveries
• Mixing, CKM Constraints and
CP Violation
• Heavy Flavor Spectroscopy
• New Heavy Baryon States
• Tests of Quantum
Chromodynamics
• Precise measurement of Top-
quark and W-boson Masses
• Top Quark Properties
• Di-Boson production and SM
Gauge Couplings
• New Exclusive/Diffractive
Processes
Unique Window into the
unknown
• Searches for Supersymmetry,
Extra Dimensions, Exotica
• Still at the Energy Frontier
– Proobing the Terascale as
the luminosity increases
• The Standard Model BEHHGK
(Higgs) Boson is within reach!
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A Roadmap to discovery…
9
Ha
rde
r to
Pro
du
ce
Harder to Observe
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W Mass Summary
Mw = 80.399 0.023 GeV
Tevatron has
worlds best
measurement
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Summary of Top Mass
We now know the mass of
the top quark with better
precision than any other
quark!!!
15 short years from
discovery to this….
Mt = 173.1 1.2 GeV
<1% Precision
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Where is the BEHHGK (Higgs) Hiding?
MH < 157 GeV at 95% C.L. Preferred MH – 87+35
-26 GeVMw vs Mtop
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Higgs Production and Decay
Higgs are produced in several diferent ways…
Higgs decay into different “final states”
depending on the mass of the Higgs
To find it, we need to look at all these final decay states
and combine the results
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The Challenge
These are production numbers –
trigger, acceptance etc.not yet factored in…
# of Events produced/exp in 1 fb-1
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Doing the Math…
This is a Hard Way to Make a Living!!!
From the previos page…. we expect to make 40-60 Higgs
events inside each detector for every 1/fb of data
We now have 6 /fb of data – so we expect to have made
250-350 BEHHGK (Higgs) candidates
If our efficiency for finding them is a few percent, we are
trying locate a handful of events out of billions of
collisons!
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Decoding Limit Plots 101F
ac
tor
aw
ay
in
se
ns
itiv
ity
fro
m S
M
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An Example -- Higgs →WW
• Signature: Two high pT leptons and Missing Energy
– Primary backgrounds: WW and top in di-lepton decay channel
– Key issue: Maximizing lepton acceptance
• Most sensitive Higgs search channel at the Tevatron
g Hf
ν
W+
gW-f
l+
l-
ν
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One Experiment’s Channel CombinationF
ac
tor
aw
ay
in
se
ns
itiv
ity
fro
m S
M
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One Experiment’s Channel CombinationF
ac
tor
aw
ay
in
se
ns
itiv
ity
fro
m S
M
• Statistically combine channels
• Use a procedure to account for correlated
uncertainties
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Combine ExperimentsF
ac
tor
aw
ay
in
se
ns
itiv
ity
fro
m S
MNeither experiment has sufficient power to span the entire mass range
using the luminosity we expect to acquire in Run II
SM Higgs Excluded: mH = 163-166 GeV
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We are Making Steady Progress…
For a 160 GeV Higgs
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How Well Can We Do???
2 3 Projections with 5, 10 fb-1 data sets/experiment
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We are Getting Serious!
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Reaching for the Higgs Horizon
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A Very Rich Program
dsb
ddb
uub
2.7 fb-1
ssb
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Conclusions
• High Energy Colliders provide physicists with a
tool to explore the fundamental questions about
nature
• The Tevatron has been remarkably successful
from the discovery to the top quark, to the
observation of B_s mixing to the remarkable
precision measurements of the Top quark and W
boson mass just to name a few
• Evidence for the Higgs is within reach and the
Tevatron. Its going to be an exciting next couple
of years !!!
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Backup
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The Multistep Process
• Step 1 – select events using simple kinematic cuts
(high pt lepton, missing energy, b jets in the event
• Step 2 – Make use of other distinguishing features
– Fits to kinematic distributions
– Multivariate techniques (neural nets) to make optimal use of
the information in each event
• Step 3 -- Optimize
– Improve triggering
– Improve lepton acceptance
– Improve background rejection
– All of this is very hard pain staking work
• Step 4 – Combine efforts
– Combine all the different decay channels together
– Combine results from both experiments
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