The Need for a

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The Need for a Strong Force
Why do protons stay together in the nucleus,
despite the fact that they have the same
electric charge?
? They should repel since they are like charge
Why do protons and neutrons in the nucleus bind
together?
? Since the neutron is electrically neutral,
there shouldbe no EM binding between protons and
neutrons.
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Announcements
This Friday and then Monday I will begin
reviewing what we havelearned up to now.This
will also serve to focus you on the important
concepts.I will also hand out a review sheet
next Monday which summarizeswhat the exam will
cover.Friday the 26th will be devoted to a pure
QA session.
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Search for a Theory of Strong Interactions

• By the 1960s, Feynman et al, had fully
  developed a quantumtheory which accounted for
  all EM phenomenon. This theory iscalled Quantum
  Electrodynamics (or QED for short).
• Because of this remarkable success, scientists
  developed ananalogous theory to describe the
  strong interaction. It is calledQuantum
  Chromodynamics (or QCD for short).
• Scientists conjectured that, like the EM force,
  there is also a quantum of the strong force, and
  called it the gluon.

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The Strong Force

• For the EM interactions, we learned that The
  photon mediates the interaction between objects
  which carry electrical charge

• For the Strong Interactions, we conjecture
  that A force carrier, called the gluon mediates
  the interaction between objects which carry
  color charge (that is, the quarks, and gluons
  !).

• The most striking difference between the gluon
  and thephoton is The gluon carries color
  charge, but the photon does not carry electric
  charge. ? Gluons can interact with other gluons
  !!!!

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Comparison Strong and EM force
Property EM Strong
Force Carrier Photon (g) Gluon (g)
Mass 0 0
Charge ? None Yes, color charge
Charge types , - red, green, blue
Couples to All objects with electrical charge All objects with color charge
Range Infinite (1/d2) ?10-14 m(inside hadrons)
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Color Charge of Quarks

• Recall, we stated, without much explanation,
  that quarks come in3 colors.

• color charge ?? strong-force
  as electrical charge ?? EM force.

• Experiments show that there are 3 colors not 2,
  not 4, but 3.

• Again, this does not mean that if you could see
  quarks, you would see them as being colored.
  This color that we refer to isan intrinsic
  property and color is just a nice way to
  visualize it.

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Color of Hadrons (II)
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Color of Gluons
Each of the 8 color combinationshave a color
and an anti-color
When quarks interact, they exchange color
charge.
Dontworryaboutwhat thismeans
Quark 1
Quark 2
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Color the Strong Force
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Flow of Color Charge
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Color Exchange
Quarks interact by the exchange of a
gluon.Since gluons carry color charge, it is
fair to say that the interaction between quarks
results in the exchange of color charge (or just
color) !
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Gluons Important Points

• Gluons are the force carrier of the strong
  force.
• They only interact with object which have color,
  or color charge.
• Therefore, gluons cannot interact with
  leptonsbecause leptons do not have color charge !

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Feynman Diagrams forthe Strong Interaction

• As before, we can draw Feynman diagrams to
  represent the strong interactions between
  quarks.
• The method is more or less analogous to the case
  of EM interactions.
• When drawing Feynman diagrams, we dont show
  theflow of color charge (oh goody). Its
  understood to be occurring.
• Lets look at a few Feynman diagrams

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Feynman Diagrams (Quark Scattering)
q
Quark-antiquarkAnnihilation
g
q
Quark-quarkScattering Could also
beQuark-antiquarkScatteringorAntiquark-antiqua
rkScattering
q
q
Position
g
q
q
time
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Where do we get quark and antiquarks from?
Quarks
PROTON
And, antiquarks?
ANTIPROTON
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Flashback to EM Interactions
Recall that photons do not interact with each
other.Why? Because photons only interact with
objects which have electric charge, and photons
do not have electric charge !
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BUT GLUONS DO !!!
Gluons carry the charge of the strong force,
which is color charge, or just color !
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Ok, so heres where it gets hairy!
Since gluons carry color charge, they can
interact with each other !(Photons cant do that)
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And quark-gluon interactions as well!
Since both quarks and gluons have color, they can
interact witheach other !!!
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Where do the gluons come from ?

• The gluons are all overinside hadrons!!
• In fact there are a lot more than shown here
  !!!
• Notice sizes here
• In fact quarks are lt 1/1000th of the size of
  the proton, so they are still too big in this
  picture !
• Even protons and neutronsare mostly empty space
  !!!

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Confinement
Since the strong force increases as quarks move
apart, they can only get so farThe quarks are
confined together inside hadrons.Hadron jail !
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Hadronization
In this way, you can see that quarksare always
confined inside hadrons (thats CONFINEMENT) !
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What holds the nucleus together?
The strong force !

• Inside the nucleus, the attractive strong force
  is stronger thanthe repulsive electromagnetic
  force.
• Protons and neutrons both experience the
  strong force.
• The actual binding that occurs between
  proton-proton andproton-neutron is the residual
  of the strong interaction.

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Food for thought
Recall Mass of Proton 938 MeV/c2 Proton
constituents 2 up quarks 2 (5
MeV/c2) 10 MeV/c2 1 down quark 1
10 MeV/c2 10 MeV/c2 Total quark mass
in proton 20 MeV/c2
Wheres all the rest of the mass ?????
Its incorporated in the binding energy
associated with the gluons !
? 98 of our mass comes from glue-ons !!!!
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Summary (I)

• The property which gives rise to the strong
  force is color charge
• There are 3 types of colors, RED, GREEN and
  BLUE.
• Quarks have color charge, and interact via the
  mediator of thestrong force, the gluon.
• The gluon is massless like the photon, but
  differs dramaticallyin that
• It has color charge
• Its force acts over a very short range (inside
  the nucleus)

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Summary (II)

• Because gluons carry color charge, they can
  interact among themselves.
• Quarks and gluons are confined inside hadrons
  because of the nature of the strong force.
• Only 50 of a protons energy is carried by the
  quarks. Theremaining 50 is carried by gluons.
• We learn about the strong force by hadron-hadron
  scatteringexperiments.