What is the Higgs Boson?
And how do we search for it?
Jason Nielsen
SCIPP / UC Santa Cruz
June 25, 2007
J. Nielsen
VERTEX 2004
1
Challenge of Particle Physics
•
Unification of the basic forces
and the origin of mass for the
fundamental particles
•
Unexpected new physics
or extra dimensions not
included in Standard Model
•
Unknown new physics
(forces or particles)
hinted at by cosmology
Particle collisions at the energy frontier enable us to
pursue these and other questions about nature
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Fundamental Particles & Forces
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Force Carrier Quanta
Photon (electromagnetic)
• verified 1922
• mass of photon = 0
W,Z bosons (weak force)
• verified 1983
• mW, mZ: 80 GeV/c2, 91 GeV/c2
Gauge symmetry is fundamental to electrodynamics
• when extended to electroweak theory, requires massless W,Z
• how to accomodate their large masses?
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Higgs Mechanism in Field Theory
Electroweak “Standard Model” relies on broken symmetry
Additional fields with constructed potential
• just like gravitational field, electric field
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Introduction of a pervasive Higgs field
• Rotationally symmetric potential
• But the stable minimum breaks the symmetry!
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Spontaneous Symmetry Breaking
Came to particle physics from condensed matter physics
above Tc
below Tc
Pencil on point
Heisenberg ferromagnet
Theory has rotational invariance; ground state is not invariant
 Symmetry has been broken by external factor
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Higgs Mechanism in Field Theory
Spontaneous symmetry breaking
• Lost degree of freedom -> Goldstone bosons
Goldstone bosons give mass to W±,Z
• One physical scalar boson: Higgs boson
whose mass is unknown
Discovery of the Higgs boson would help verify this approach
Otherwise, much head-scratching and new theories!
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Why is the Top Quark So Massive?
mass (GeV/c 2)
180
160
140
120
100
80
mt=175 GeV/c2
Interaction with Higgs quantum
defines mass of fermions
60
40
20
0
u
d
s
c
b
t
Schwinger (1957): a coupling produces effective mass terms
through the action of the vacuum fluctuations (Higgs boson)
Top quark most affected by this “Higgs field molasses”
Note: Higgs couplings explain fundamental
fermion mass but not everyday mass!
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So What IS the Higgs boson?
Higgs boson is a physical
condensate of the pervasive
postulated Higgs field
Similar to photon, except Higgs boson is not a force carrier
What kinds of particles do it couple to?
• Its couplings are proportional to the fermion masses
• So it couples most strongly to the most massive particles
This makes it clear how to search for it, if it exists…
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Wringing Out the Higgs Condensate
Physical Higgs bosons can be
produced, given enough energy
e+
H
Z*
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
(Here ECM > mH + mZ)
Z
eThat’s where the collider comes in
But Higgs boson is fleeting:
decays immediately to
characteristic “final state”
b
b
H
Z
q
q
That’s where the detector comes in
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Recent Physics Results
Effects of the Higgs boson are felt via loop interactions
Precision measurements
are sensitive to the Higgs mass
Updated winter 2007 with new
Tevatron mW=80.4±0.04 GeV
mH < 182 GeV/c2 at 95% CL (including previous searches)
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How does the Higgs Boson Decay?
Notice coupling to massive
particles (bb, tt, WW, ZZ)
For low mass Higgs,
expect decay to b quark
pairs;
For very high mass Higgs
expect decay to ZZ
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Rare Higgs Decays (?)
(Claus Grupen)
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Identifying b Quarks from Higgs
B hadrons have lifetimes of 1.5 ps: find the decay vertex!
proton-antiproton
Interaction point
B hadron
Fit tracks together to form secondary vertex
• measure flight distance of B hadron
• typical flight distance is 0.5 cm from interaction point
• close, precise measurement provided by silicon is crucial
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One Provocative Candidate Event
HZ  bbbb selection
ECM=206.7 GeV
3 NN b-tagged jets
Reconstructed mH = 110 ± 3 GeV/c2
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Bumps in the Mass Spectrum
Decay products of the Higgs boson form a mass resonance
- similar to resonances from past discoveries of new particles
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Strategy for identifying Higgs boson production:
1. Excess of events in Wbb signature (or other signature)
2. Higgs decay products form a invariant mass peak
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Tevatron Cross Section Hierarchy
In proton-antiproton collisions at s = 1.96 TeV:
b-jet pairs from QCD
high-energy leptons
1
Particle production
rates vary widely:
the Higgs is the
“needle in the haystack!”
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0.05
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What kind of unit is a “barn?”
Manhattan Project physicists
gave the name to the
typical nuclear cross-section
defined as 10-24 cm2
Practically “as big as a barn”
where (sub)-nuclear processes
are concerned
Photo: Reidar Hahn, Fermilab
the term “barn” wasn't officially declassified until 1948
Apparently there was also a unit called the “shed”: 10- 48 cm2
This summer CDF will have collected 3 giga-sheds of data!
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bb Dijet Invariant Mass Distribution
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Large Hadron Collider at CERN
Next generation collider: startup scheduled for 2008
Italy
p
14 TeV
Luminosity target: 1034cm-2 s-1
p
Increased production of heavy
particles like Higgs, top quark
CMS
ATLAS
More particles at higher energy
requires new detector design
and technology
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Higgs Decay to Photons
Rare decay in SM
H
t
g
t
g
LHC detectors have
been optimized to
find this peak!
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Higgs Decay to ZZ
Requires precise measurement of muon curvature
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ATLAS Experiment at LHC
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ATLAS Experiment at LHC
ATLAS
collaborator
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ATLAS DETECTOR
Nov. 2005
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Installation of inner
detector end-cap
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Prospects for SM Higgs at LHC
Should discover SM Higgs
regardless of mass value
Low-mass Higgs channels:
• Hgg ! (sm =1.5 GeV/c2)
• W,Z boson fusion to Higgs:
then HWW or Htt
• ttH: top quark again!
High-mass Higgs channels:
• golden mode 4e/ opens >2mZ
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Identifying Particle Signatures
ATLAS trigger system can identify specific signatures online
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“Hunt for Higgs” WWW Site
One of the best I’ve seen at describing what really happens
http://www.sciencemuseum.org.uk/antenna/bigbang/huntforhiggs/index.asp
Let’s have a look together at the “Hunt for Higgs”
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Future of the Higgs Search
• Tevatron experiments still searching
• LHC turns on in 2008
– Commissioning and calibrating detectors
• Understand non-Higgs backgrounds
• Find the Higgs boson peak above the bkgd!
• My guess is that it will take a few years to
collect enough events to convince ourselves
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