The fundamental structure of matter

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The fundamental structure of matter ?
HW9 will be posted later ortomorrow
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Recap from last time (I)

• Electrons in atoms have well defined Energy
  Levels (E1, E2, E3, E4, )
• When all the atomic electrons are in their
  lowest possible energy state, this is called the
  ground state of the atom.
• An electron can be promoted to a higher energy
  state by doing workon the atom (i.e., having an
  electric current pass through a gas of
  theseatoms).
• The electron will spontaneously fall back to
  the ground state, and inthe process, emit EM
  radiation (ie., a photon).
• The energy of the photon is given by the
  difference in energy betweenthe initial final
  energy levels (ie, E3-E2).
• The wavelength of the photon can be found using
  Ehc/l. (If the energies are in eV, you must
  first convert eV ? J)

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Hydrogen atom energy levels
Quantum physics provides the tools to compute the
values ofE1, E2, E3, etcThe results are
En -13.6 / n2
These results DO DEPEND ON THE TYPE OF ATOM OR
MOLECULE
So, the difference in energy between the 3rd and
2nd quantum state is
Ediff E3 E2 -1.51 (-3.4) 1.89 eV
When this 3? 2 atomic transition occurs, this
energy is released in the form of
electromagnetic energy.
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Example
In the preceding example, what is the frequency,
wavelength of theemitted photon, and in what
part of the EM spectrum is it in?
E 1.89 eV. First convert this to J.
Since E hn ? n E/h, so
n E/h 3.0x10-19 J / 6.6x10-34 J s
4.5x1014 1/s 4.5x1014 hz
l c/n (3x108 m/s) / (4.5x1014 1/s)
6.6x10-7 m 660 nm
This corresponds to Visible - RED !
You should be able to do this kind of problem !!!
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Some Other Quantum Transitions
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Discoveries in Cosmic Rays

• 1937 Discovery of the muon. Its very
  much like a heavy electron.

• 1947 Discovery of the pion.

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The Plethora of Particles
Because one has no control over cosmic rays
(energy, types of particles, location, etc),
scientists focused their efforts on accelerating
particles in the lab and smashing them together.
Generically people refer to them as particle
accelerators. (Well come back to the particle
accelerators later) Circa 1950, these particle
accelerators began to uncover many
newparticles. Most of these particles
areunstable and decay very quickly, and hence
had not been seen in cosmic rays.Notice the
discovery of theprotons antiparticle,
theantiproton, in 1955 !Yes, more antimatter !
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From Simplicity? Complexity? Simplicity

• Around 1930, life seemed pretty good for our
  understandingof elementary (fundamental)
  particles.
• There was protons, neutrons electrons.
  Together, they made upatoms ? molecules ? DNA ?
  People !
• AAHHHHH, nature is simple, elegant, aaahhhh

But the discoveries of dozens of more particles
in accelerator experiments lead many to question
whether the proton and neutron were really
fundamental. Is nature really this cruel ?
Needless to say, the zoo of new particles that
were being discovered at accelerators appeared
to reveal that nature was not simple, but
complicated? Until.
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Quarks ?

• First things first Where did the name quarks
  come from?

Murray Gell-Mann had just been reading Finnegan's
Wake by James Joyce which contains the phrase
"three quarks for Muster Mark". He decided it
would be funny to name his particles after this
phrase. Murray Gell-Mann had a strange sense of
humor!
The quarks come in 3 types (flavors) up(u),
down(d), and strange(s) and they are fractionally
charged with respect to the electrons charge
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How sure was Gell-Mann of quarks ?
When the quark model was proposed, it was just
considered to be a convenient description of all
these particles..A mathematical convenience to
account for all these new particlesAfter all,
fractionally charged particles come on !
Well.
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Scattering Experiments
Rutherfored, deBroglie, and others taught us that
we can learn about the structure of matter by
colliding high energy particles into matter, and
seeing what happens. Recall, Rutherford
determined that the atom must contain a
densecore of positive charge to account for the
large angular deflectionsof incoming alpha
particles. Also, as we discussed earlier, in
order to probe matter of size, say A,the
wavelength which you use to probe it must be at
least this size,or smaller
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Rutherford example
What was the wavelength of the alpha particles
used inRutherfords scattering experiments on
Gold foils ? Note that ma 6.7x10-27
kg, va 1.6x107 m/s)
deBroglie taught us that particles have
wavelength given by l h/p
So, first get momentum p mv (6.7x10-27
kg)(1.6x107 m/s) 1.0x10-19 kg m/s
l h/p 6.6x10-34 / 1x10-19 6.2x10-15 m
Since the gold nucleus is about 10x10-15 , this
wavelength is small enough to resolve the fact
that there is a nucleus there
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Probing deeper into matter

• If we really want to understand if there is
  anything inside a proton or neutron (aka
  nucleon), we have to examine it with particles
  whose wavelengths are smaller than the size of a
  proton.? Since l h/p, we must produce higher
  momentum particles. That is, the higher the
  momentum of the particle, the smaller its
  deBroglie wavelength ? can see, or probe
  smaller things
• Since the protons size is very small, about
  1x10-15 m, We need very energetic beams
  of particles (high momentum) to probe its
  structure.
• By the 1960s, physicists had learned how to
  produce high energy, well-focused, beams of
  particles, such as electrons or protons
  (particle accelerators !)
• This has been the driving force behind
  understanding What is matter at its
  most fundamental level ?

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Are protons/neutrons fundamental ?
In 1969, a Stanford-MIT Collaboration was
performing scattering experiments
(X anything)
What they found was remarkable the results were
as surprising as what Rutherford had found more
than a half-century earlier!
The number of high angle scatters was far in
excess of what one would expect based on
assuming a uniformly distributed charge
distribution inside the proton.
Its as if the proton itself contained smaller
constituents
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Quarks
Since 1969, many other experiments have been
conducted to determinethe underlying structure
of protons/neutrons.All the experiments come to
the same conclusion. ? Protons and neutrons are
composed of smaller constituents.These quarks
are the same ones predicted by Gell-Mann Zweig
in 1964.
Are there any other quarks other than UP and DOWN
?
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Three Families of Quarks
Increasing mass
Woohhh,fractionallychargedparticles?
Also, each quark has a corresponding
antiquark.The antiquarks have opposite charge to
the quarks
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The 6 Quarks, when where
SLAC Stanford Linear AcceleratorBNL
Brookhaven National Lab
Notice the units of mass !!!
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Major High Energy Physics Labs
Fermilab
DESY
SLAC
KEK
CERN
CESR
BNL