Alpha S.Alpha S.A measure of the strongest A measure of the strongest

fundamental force of nature-fundamental force of nature-The Strong ForceThe Strong Force

LOGO 2

What is Alpha S?

Our hands, bodies and all material things we see around us are made of atoms.

Electron orbits

Nucleus

Alpha S holds Protons (and all hadrons) together with a force STRONGER than any other in the universe!

ProtonNeutron

Diameter ~ 10 -10 m

LOGO 3

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.

Electron orbits

Nucleus

AtomAtom

QuarkGluon

Alpha S determines how strongly quarks and gluons interact, how strongly they are held together (or are coupled together) within protons and neutrons.

ProtonProton

LOGO 4

The Fundamental ForcesThe 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.

1/130

Quarks

e

leptons(not neutrinos)

Photon

Electro-magnetic

s

1/8

QuarksGluon

gStrong

Relative coupling constant

Particles that feel the force

Force carrier

Fundamental Force

G

1/ (1039) !!

Quarks & all

Leptons

Graviton(not discovered)

G

Gravity

W

1/30

Quarks & all

Leptons

W & Z boson

W Z

Weak

?For each force can you think of an example where it would be experienced? (The pictures in the force column might help.)

LOGO 5

How do we Calculate Alpha S?

What are multijet final states?

Alpha S can be measured from

the rate of production of multijet final

states.

So how can we make a measurement of the strength of the strong force in a scientific experiment?

LOGO 6

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 can’t 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

LOGO 7

Calculating Alpha SIn e+e- collisions lots of Z bosons are produced. Z’s can then decay into

Z → qq

If one of the quarks emits a gluon early on, this will form a jet producing a final state of three jets.

Z → qq → qqg

LOGO 8

Calculating Alpha SSo 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.

Alpha S can be measured from

the rate of production of multijet final

states.

?

?

Can you now calculate the strengths of all four forces?

÷

Further thinking …

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

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.k = constant = 0.2

N = Number of jets

S= k * N3Jets / N2Jets