Download What is the Higgs Boson?

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
J. Nielsen
2
Fundamental Particles & Forces
J. Nielsen
3
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?
J. Nielsen
4
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!
J. Nielsen
5
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
J. Nielsen
6
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!
J. Nielsen
7
Why is the Top Quark So Massive?
180
mass (GeV/c2)
160
140
mt=175 GeV/c2
120
Interaction with Higgs quantum
defines mass of fermions
100
80
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!
J. Nielsen
8
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…
J. Nielsen
9
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
J. Nielsen
10
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)
J. Nielsen
11
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
J. Nielsen
12
Rare Higgs Decays (?)
(Claus Grupen)
J. Nielsen
13
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
J. Nielsen
14
One Provocative Candidate Event
HZ  bbbb selection
ECM=206.7 GeV
3 NN b-tagged jets
Reconstructed mH = 110 ± 3 GeV/c2
J. Nielsen
15
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
J. Nielsen
16
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!”
J. Nielsen
0.05
17
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!
J. Nielsen
18
bb Dijet Invariant Mass Distribution
J. Nielsen
19
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
J. Nielsen
20
Higgs Decay to Photons
Rare decay in SM
H
t
g
t
g
LHC detectors have
been optimized to
find this peak!
J. Nielsen
21
Higgs Decay to ZZ
Requires precise measurement of muon curvature
J. Nielsen
22
ATLAS Experiment at LHC
J. Nielsen
23
ATLAS Experiment at LHC
ATLAS
collaborator
J. Nielsen
24
ATLAS DETECTOR
Nov. 2005
J. Nielsen
25
Installation of inner
detector end-cap
J. Nielsen
26
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
J. Nielsen
27
Identifying Particle Signatures
ATLAS trigger system can identify specific signatures online
J. Nielsen
28
“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”
J. Nielsen
29
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
J. Nielsen
30
J. Nielsen
31