PPT - Physics & Astronomy

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Transcript PPT - Physics & Astronomy

Discovery of the
Higgs Boson
Gavin Lawes
Department of Physics and Astronomy
Includes slides and images from:
Robert Harr, Paul Karchin, and Sean Gavin (Wayne State University)
LHC (CMS Collaboration)
Note: There is a lot more information about the LHC and the Higgs
Boson at home.web.cern.ch
What do we know
about matter?
What properties define
matter particles?
Spin. (Zero spin particles are bosons, spin ½ particles are fermions)
Electric charge.
Color charge.
Particles in the Standard model
Particle crib sheet
Photon g (massless, no charge)
Electron e (massive, charged)
Muon m (massive, charged)
Proton p (massive, charged)
Gluon g (massless, “color charge”)
Higgs H (massive, ?????)
What are gauge bosons?
Particles exert forces on one another. These forces are transmitted by
gauge bosons.
Electromagnetic force (carried by photons [1])
Strong nuclear force (carried by gluons [8])
Weak nuclear force (carried by W and Z particles [3])
Where do gauge
bosons come from?
Gauge bosons reflect underlying symmetries of the universe.
The number of gauge bosons for each force is the number of
generators for each symmetry group (e.g. the symmetry group for the
strong nuclear force has 8 generators, hence 8 gluons).
These gauge bosons should be massless.
However, the W+, W-, and Z bosons (which mediate the weak nuclear
force) are massive.
Classical mass
Mass in Relativity
(special and general)
Binding Energy
Proton mass, mP=1.6726x10-27 kg
Neutron mass, mN=1.6749x10-27 kg
mass=6.6446x10-27 kg
Less than 2mP+2mN
Proton mass
The mass of a proton is
much larger than the
mass of the quarks that
make up the proton.
Quantum fluctuations
The Heisenberg Uncertainty Principle states:
with the Planck constant h=6.636x10-34 m2 kg/s
This means that particles are popping in and out of existence
continuously (so the vacuum is actually fluctuating).
The probability distribution of particles everywhere in space is called
a field.
Higgs Mechanism
In 1964, 6 authors in 3 different papers (Brout and Englert, Higgs,
and Guralnik, Hagen, and Kibble) proposed a mechanism for making
the weak force carriers massive.
Depended on having these gauge bosons couple to something called
the Higgs field, which has a fourfold symmetry.
When the vacuum fluctuations of the Higgs field become non-zero,
the symmetry is broken, which makes the W+, W-, and Z bosons
massive and leaves a single massive particle called the Higgs boson.
Other particles (electrons, quarks, etc) can also acquire mass by
coupling to the Higgs field.
How can we find the
Higgs boson?
Build a giant particle collider. Fortunately, there was already a giant
tunnel at CERN.
27 km ring filled with superconducting magnets cooled to just above
absolute zero.
Large Hadron Collider (LHC) costs about $10B over 20 years. Note:
Hadrons are particles containing quarks.
ATLAS and CMS projects involve over 3,000 physicists
Movie of a collision
H  gg
Same event, different angle
H  ZZ  mmmm
H  ZZ  eeee
H  gg
Events / 2 GeV
Selected diphoton sample
Data 2011+2012
Sig+Bkg Fit (m =126.8 GeV)
Bkg (4th order polynomial)
ATLAS Preliminary
H®g g
s = 8 TeV, ò Ldt = 20.7 fb
s = 7 TeV, Ldt = 4.8 fb
Events - Fitted bkg
mg g [GeV]
Higgs boson summary
Two different experiments (ATLAS and CMS) find a new particle
with a mass of 125.6 GeV/c2.
This is a spin 0 boson, with properties consistent with the Standard
Model Higgs boson.
The existence of this particle confirms the point of view that mass is
an acquired property (through coupling to the Higgs field) and not an
intrinsic property of particles.
What next?
Want to investigate the properties of the Higgs boson in more detail
(e.g. decay paths, coupling to other particles, etc).
Are there other Higgs-like particles? The Standard Model assumes a
Higgs field with fourfold symmetry, but there are other models that
include more Higgs terms.
Also, other interesting physics problems to study at the LHC.
Quark-gluon plasma
Higgs search concentrated on p-p collisions (total of 6 quarks).
When colliding nuclei (each with hundreds of quarks) together, one
can produce a “quark-gluon” plasma, consisting of quarks and gluons
mixed together but not bound into hadrons.
Understanding this special state of matter can provide insight into the
evolution of the universe, which is believed to have passed through a
similar high density/high temperature state nanoseconds after the Big
The phase diagram of QCD
Early universe
critical point ?
quark-gluon plasma
hadron gas
nucleon gas
baryon density
Neutron stars
Note: 1 TeV=1,000 GeV
Nuclear Collisions
Note: Spherical nuclei look like pancakes because of
relativistic length contraction.
Hydrodynamics of quark-gluon
Quark-gluon plasma acts like a
perfect liquid
Viscosity of quark-gluon plasma
Other new physics
The LHC provides a tool for measuring the properties of fundamental
particles at high energies and high densities.
Because of (possible) phase transitions (e.g. weak symmetry
breaking, quark-gluon plasma, etc) the properties of particles under
these extreme conditions (high density and high energy) may be very
different than their properties under ambient conditions (low density
and low energy).
This can potentially change our understanding of the fundamental
symmetries that govern physical law.
The end