The Discovery of the Higgs Boson with
the ATLAS Experiment at CERN
William Trischuk
University of Toronto
Tsukuba Global Science Week
29 September 2014

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Outline• Why the Higgs Boson?
– The last fifty years in five minutes– What did we know about the Higgs Boson before the LHC?

• The Large Hadron Collider’s assault on the Higgs– The technical and scientific achievements– Evidence for the Higgs Boson• What we’ve learned about the Higgs– Properties – is it the Standard Model Higgs?
– Is there anything left to learn?
Animations: ATLAS Experiment © 2012 CERN
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The Invention of Higgs (and others)• Massive W boson
introduced to explain
weakness of beta-decay

Theoretical View of the Higgs– Including a gauge
invariant mass term tricky• Higgs et al.* showed how
via spontaneous symmetry
breaking of a scalar field:
The Higgs field (1964++ )5* Brout, Englert, Guralnik, Hagen, Higgs, Kibble in 3 ~contemporaneous papers

Constraints on the Higgs Mass• W boson scattering
diverges in Standard
Model– Inclusion of Higgs boson
renders it finite• No lose theorem:
Higgs Boson, or something
like it, had to appear
below 1000 GeV
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Higgs “Known Unknown” before theLarge Hadron Collider• Top quark provides quadraticcorrections to W mass

• Higgs Boson only provides
logarithmic corrections

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The Large Hadron Collider (LHC)• A 27 km long circularsuperconducting proton
collider at CERN
• Collides bunches ofprotons on protons atevery 25-50ns• Produces up to ~800
million proton collisions
per second• Aim for ~design energy
and luminosity in 2015
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The ATLAS Experiment

9A Large Collaboration•The ATLAS collaboration has over 3000
scientists from 178 institutions in 38 countries 10

CDF Detector
ATLAS Detector
during Installation

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The ATLAS Tracking Detectors

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Detector Challenge at the LHC• Beam energy limited byexisting tunnel radius• The LHC designed for an
aggressive collision rate
– 20-50 pp interactions per
bunch crossing– Only one (at most) contains
interesting physics• Process a lot of data worldwide computing grid
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Grid Computing for the LHC
Tier-0 (CERN):
recording•DataFirst pass•reconstruction•Data distribution
Tier-1 (11 centres):storage•Permanent•Re-processing•Analysis
Tier-2 (~130 centres):• Simulation• End-user analysis

Ian Bird, CERN

Huge Datasets to Store/Analyse• TRIUMF (Vancouver) “Tier 1”• The data are reconstructed andanalyzed in a worldwide
computing “grid” with over
100,000 processors, 100
Petabytes of storage
SciNet (Toronto), “Tier 2”

CERN (Geneva) “Tier 0”

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Where to Start Looking for Higgs?

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Higgs Boson Search
HZZHgg

Higgs Boson
Boson Search
HiggsSearch
HZZ

H g g4μ candidate with mass = 124.6 GeV

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2e2μ candidate with mass= 124.3 GeV

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Combination of Channels•

Probability that the background fluctuated to produce thedistributions that were observed

ATLASCMS

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So now we’refinished?

Higgs Boson Search Measurements
HZZ

H g gHiggs Boson Mass Measurements

Higgs Decays•

Standard Model is very predictive with respect to the Higgs boson.
Once its mass is known, all couplings to SM particles predicted:

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Higgs Coupling Measurements• Initial measurements of theHiggs couplings– Good agreement with StandardModel prediction• Include twice the data sampleused for discovery

m = 1.30 ± 0.12 (stat) ± 0.13 (sys)– Flucutations have come down
since discovery– Compatible with being “the”
Standard Model Higgs at 14%28

Higgs Boson Spin•

A unique feature of the Higgs: the only fundamental Standard
Model particle with spin 029

So now we’rereally finished?

Future Challenges• The LHC will go to
design energy/lumi in2015 (10x data)• Improve precision on
all aspects of Higgs• Beyond 2022  afurther 10x sample• Higgs as a window on
physics beyond SM31

LHC Long Range Plan

Upgraded ATLAS Tracking• Installed new pixel layer in
2013-14– Resolve multiple interactions– Better b quark tagging• Preparing full replacement
tracker for 2023
– Detectors should not limit
our ability to exploit LHC
collision capabilities
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Areas of Active Higgs ResearchMarumi Kado
ICHEP14and refs. therein34

Summary•••

Our current theory that describes fundamental
particles and forces (Standard Model) predicted
the existence of a new particle: the Higgs boson
More than 40 years after it was postulated, aspin 0 force carrier was definitively observed inJuly 2012
This discovery has important implications oncosmology and our understanding of the very
early Universe

Only begun to explore the fullpotential of the Higgs discovery
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The ATLAS Higgs Channels

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The Standard Model
Standard Model describes:•12 fermions , spin 1/2 particles
in 3 generations:– 6 quarks– 6 leptons•

3 forces mediated by bosons,spin 1 particles:– electromagnetic (photons)– strong (8 gluons, massless)– weak (W+,W-,Z) (massive!)•A spin 0 particle (Higgs boson)
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LEP’s Chance to Observe the Higgs• Produced at well defined rate
in e+e- collisions– Ecms = mZ + mHiggs + 10 GeV38

Large Hadron Collider

Some LHC Facts• Need to plan these large
projects well in advance:planning started in the 80s:
two machines would behoused in the tunnel: LEP(electron-positron collider inthe 90s) and LHC in thefollowing decade• CERN needs about 200 MW
at peak consumption, abouta third of the city of Geneva• Largest vaccum system in
the world: 104 km of piping
under vaccum, 250000
welded joints, 18000
vacuum seals• “Ultra-high” vacuum in beampipe
with pressure ~10-10 to 10-11 mbar
(10-13 atm), lower pressure thanon the moon…
• Special coatings used to trap
molecules in warm sections

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Some LHC Facts• ~10000 magnets to keep beam on
track and focus it• >1200 15m-long dipole bending
magnets operated at 1.9K
• Dipoles run at 12000 amps toproduce 8 Tesla field• Largest cryogenic plant in theworld:• Dipoles operated at 1.9K (colderthan outer space)• 120 tonnes of helium• 40MW required to powercryogenics• Design energy is 14 TeV in the centre ofmass.

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Accelerating protons: Radio-Frequency
Cavities

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Images: courtesy of CERN and Fermilab

The Large Hadron Collider and ATLAS

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Lots of Data…
• If all the data from ATLAS would berecorded, it would fill 10,000 DVDs per
second: a stack of DVDs the size of theCN tower every minute• The data rate is equivalent to 50 billion
telephone calls at the same time
• ATLAS actually only records a fractionof the data (what we decide could be“interesting”) and that rate is equivalent
to 2 DVDs per minute46

Higgs Production••

Higgs production at LHC dominated by “gluon fusion” process“Weak boson fusion” is subdominant but has less background47

H ZZ(*) llll
• 4 lepton mass spectrum for the Higgs decay to two Z bosons: Left
CMS experiment, right ATLAS experiment48

H WW*lnln•

Reconstruct Higgs candidate “transverse mass”•Have to carefully take into account 9 different backgroundprocesses

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H
•gg

Production depends on coupling to top quark (in SM!)–••

Small contribution from WBF: production depends on coupling to W/Z bosons

Decay depends on coupling to top and W boson
Large backgrounds: need good photon identification–••

ATLAS EM calorimeter designed with this signal in mind

Small branching ratio, need integrated luminosity
A good discovery final state:–––

Excellent Higgs mass resolution
Looking for a resonance on top of smooth background
Robust channel with respect to pileup (advantage in 2012)

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H•gg

Diphoton mass spectrum: CMS
below, ATLAS to the right51

The Holy Grail of Higgs Physics• Higgs self-coupling not constrained by Standard Model• ILC/LHC designs now driven by this channel• This may or may not be achievable53