Folie 1

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Alpha S. A measure of the strongest fundamental
force of nature- The Strong Force
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What is Alpha S?
Our hands, bodies and all material things we see
around us are made of atoms.
Alpha S holds Protons (and all hadrons) together
with a force STRONGER than any other in the
universe!
Electron orbits
Nucleus
Proton
Neutron
Diameter 10 -10 m
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What is Alpha S?
Something called a coupling constant can
determine how strong a fundamental force is.
Alpha S is the coupling constant of the strong
force, and the strong force is felt between
quarks and gluons.
Alpha S determines how strongly quarks and gluons
interact, how strongly they are held together (or
are coupled together) within protons and neutrons.
Quark
Gluon
Proton
Nucleus
Electron orbits
Atom
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The Fundamental Forces
The Strong Force is the strongest of the four
fundamental forces. You can see how much weaker
the other forces coupling constants are from
their relative strengths.
Relative coupling constant
Particles that feel the force
Force carrier
Fundamental Force

• ?s
• 1/8

Quarks
Gluon g
Strong

• ?W
• 1/30

Quarks all Leptons
W Z boson W Z
Weak

• ?
• 1/130

Quarks e???????? leptons (not neutrinos)?

• Photon
• ?

Electro-magnetic
?

• ?G
• 1/ (1039) !

Quarks all Leptons
Graviton (not discovered)? G
Gravity
?
For each force can you think of an example where
it would be experienced? (The pictures in the
force column might help.)?
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How do we Calculate Alpha S?
So how can we make a measurement of the strength
of the strong force in a scientific experiment?
Alpha S can be measured from the rate of
production of multijet final states.
What are multijet final states?
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What are multijet final states?
Particle detectors are used to detect new
particles produced in high energy collisions.
Electrons and photons can be detected but you
cant detect single quarks on their own, only
objects called Jets. When two or more jets are
produced this is called a multijet.

• Quarks cannot exist on their own, they must be
  in twos or threes.
• When a quark is forced away from its partner(s),
  new quarks and gluons pop into existence to keep
  the isolated quark company.
• This forms a jet of particles.

Jet
Jet
Jet
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Calculating Alpha S
In ee- collisions lots of Z bosons are produced.
Zs can then decay into
If one of the quarks emits a gluon early on, this
will form a jet producing a final state of three
jets.
Z ? qq
Z ? qq ? qqg
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Calculating Alpha S
Alpha S can be measured from the rate of
production of multijet final states.
So to calculate Alpha S (?S) all we have to do is
determine how many three jet events there are
compared to how many two jet events there.
Since the number of jets detected is dependant on
how the jets are defined a calibration factor k
is needed. k depends on how jets are defined and
reconstructed.
?S k N3Jets / N2Jets
k constant 0.2 N Number of jets
Further thinking
?
Can you now calculate the strengths of all four
forces?

?
How would you calculate Alpha S with events with
four jets in them?