A first path towards unification: from hadrons to quarks

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A first path towards unification from hadrons to
quarks
Circa 1950, the first particle accelerators began
to uncover many new particles, at a much faster
rate than cosmic ray experiments. Most of these
particles areunstable and decay very quickly,
and hence had not been seen in cosmic
rays. Could all these particles be fundamental?
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M. Gell-Mann and the eightfold way
Symmetry operations on an octahedron illustrate
the theory of quarks. Theorist Murray Gell-Mann
(and, independently, Yuval Ne'eman) discovered a
theory that organized all the particles into
families with properties mathematically the same
as those of a "group of eight" in abstract
algebra. Gell-Mann called it "The Eightfold Way."
When physicists recognized that underlying
fundamental particles could explain the eightfold
pattern, the idea of quarks was born. In the
1970s, experiments at the Department of Energy's
SLAC showed that quarks were not just
mathematical constructs but real building blocks
of protons and neutrons.
? quarks were established as the fundamental
building blocks of hadrons
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Quarks and leptons the building blocks
Ordinary matter
Cosmic rays/ particle physics accelerators
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1974 The discovery of charm
sc
the fourth quark was discovered simultaneously on
the West Coast, at DOEs SLAC, using the MARK I
detector, (above left) and on the East Coast, at
DOEs Brookhaven Laboratory. Physicist Burton
Richter, (left), led the SLAC team, and Sam Ting
led the Brookhaven group, (right).
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Our path towards elementary constituents does it
end here?
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Fundamental forces I
Electromagnetic force
The electromagnetic force binds electrons to
atomic nuclei (clusters of protons and neutrons)
to form atoms.
gravity
This is the force that is part of our everyday
life but is the only one that does not fit in the
quantum theory that I am describing
Weak force
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Fundamental forces II the strong force

• Each quark carries one of the three types of
  "strong charge," also called "color charge."
  These charges have nothing to do with the colors
  of visible light.
• There are eight possible types of color charge
  for gluons. Just as electrically-charged
  particles interact by exchanging photons, in
  strong interactions color-charged particles
  interact by exchanging gluons.

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Force in the microworld
an exchange mechanism
interactions proceed via the exchange of a
force-carrier called bosons.
Particles transmit forces among each other by
exchanging force-carrying particles called
bosons. These force mediators carry discrete
amounts of energy, called quanta, from one
particle to another. You could think of the
energy transfer due to boson exchange as
something like the passing of a basketball
between two players
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Bosons the force carriers
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electroweak unification

• Standard Model of ElectroWeak interactions
  (Glashow, Salam, Weinberg)
• unification of em and weak forces
• W inferred from b decay etc
• no experimental evidence of the Z at that time
• W?, Z mass ? 100 GeV ? weak force is weak and
  short range
• BUT the SM construction applies only to massless
  particles
• unless the minimum energy state is non zero
  .
• this implies that a particle called
  Higgs exists.

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experimental evidence
1973 discovery of weak neutral currents at CERN
in nme scattering interactions (indirect evidence
for the Z boson)
The theory was in good shape but there was still
a lot to verify
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The mystery of mass
When you get on the scale in the morning, you may
be hoping that it registers a smaller number than
the day before -- you may be hoping that you've
lost weight. It's the quantity of mass in you,
plus the force of gravity, that determines your
weight. But what determines your mass?That's
one of the most-asked, most-hotly pursued
questions in physics today. Many of the
experiments circulating in the world's particle
accelerators are looking into the mechanism that
gives rise to mass. We are hoping to find the
"Higgs boson." Higgs, they believe, is a
particle, or set of particles, that might give
others mass.
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The hunt for the Higgs particle(s)
This is the remaining piece of the puzzle that
is yet to be discovered LHC, the most powerful
pp collider is being built at CERN to discover
this object and, maybe, some even more exotic
matter!
CERN
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Putting it all together

• The Standard Model summarizes the current
  knowledge in Particle Physics and is consistent
  with an impressive amount of very precise data.
  It is the quantum theory that explains all our
  present observations of the subatomic world on
  the basis of two fundamental components
• the theory of strong interactions (quantum
  chromodynamics or QCD)
• the unified theory of weak and electromagnetic
  interactions (electro-weak).

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Are we done?

• We do not think so
• Mass hierarchy is a big mystery
• mt/mu ?108
• the n mass ? quark mass different scale
• Why are there 3 families?

What about gravity?
A more complete theory may be looming at the
horizon!
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What is next?
A deeper level of unification?
the coupling constant are not really constant
a1 em a2 weak a3 strong
at TeV scale the coupling constants start to
deviate from the SM predictions if there is SUSY
expect to see sparticles at TeV energy
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Final remark the study of these fundamental
particles can be seen as a search for our origins
particle accelerator time machine recreate at
microscopic scale the physics soon after the Big
Bang