1. What is the Higgs Boson?
And how do we search for it?
Jason Nielsen
SCIPP / UC Santa Cruz
June 25, 2007
VERTEX 2004
J. Nielsen 1

2. 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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3. Fundamental Particles & Forces
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4. Force Carrier Quanta
Photon (electromagnetic) W,Z bosons (weak force)
• verified 1922 • verified 1983
• mass of photon = 0 • 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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5. Higgs Mechanism in Field Theory
Electroweak “Standard Model” relies on broken symmetry
Additional fields with constructed potential
• just like gravitational field, electric field
QuickTime™ an d a
TIFF (Uncompressed) decompressor
are need ed to see this p icture .
Introduction of a pervasive Higgs field
• Rotationally symmetric potential
• But the stable minimum breaks the symmetry!
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6. 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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7. 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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8. Why is the Top Quark So Massive?
180
160
140
mt=175 GeV/c2 Interaction with Higgs quantum
mass (GeV/c2)
120
100
80 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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9. 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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10. Wringing Out the Higgs Condensate
e+ H
Physical Higgs bosons can be
produced, given enough energy Z*
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this pic ture.
(Here ECM > mH + mZ)
e- Z
That’s where the collider comes in
But Higgs boson is fleeting: b
b
decays immediately to H
characteristic “final state”
Z
q
q
That’s where the detector comes in
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11. 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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12. How does the Higgs Boson Decay?
Notice coupling to massive
particles (bb, , 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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13. Rare Higgs Decays (?)
(Claus Grupen)
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14. 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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15. 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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16. 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) decomp resso r
are neede d 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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17. 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: 0.05
the Higgs is the
“needle in the haystack!”
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18. 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
Photo: Reidar Hahn, Fermilab
are concerned
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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19. bb Dijet Invariant Mass Distribution
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20. Large Hadron Collider at CERN
Next generation collider: startup scheduled for 2008
Italy
Luminosity target: 1034cm-2 s-1
p 14 TeV p
Increased production of heavy
particles like Higgs, top quark
ATLAS
CMS
More particles at higher energy
requires new detector design
and technology
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21. Higgs Decay to Photons
Rare decay in SM
t
H
t
LHC detectors have
been optimized to
find this peak!
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22. Higgs Decay to ZZ
Requires precise measurement of muon curvature
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23. ATLAS Experiment at LHC
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24. ATLAS Experiment at LHC
ATLAS
collaborator
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25. ATLAS DETECTOR
Nov. 2005
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26. Installation of inner
detector end-cap
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27. Prospects for SM Higgs at LHC
Should discover SM Higgs
regardless of mass value
Low-mass Higgs channels:
•H !( m =1.5 GeV/c2)
• W,Z boson fusion to Higgs:
then H WW or H
• ttH: top quark again!
High-mass Higgs channels:
• golden mode 4e/ opens >2mZ
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28. Identifying Particle Signatures
ATLAS trigger system can identify specific signatures online
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29. “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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30. 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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