1. Alpha S. A measure of the strongest fundamental force of nature- The Strong Force

2. 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! Proton Neutron Diameter ~ 10 -10 m

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

4. LOGO 4 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.  1/130 Quarks e  leptons (not neutrinos)‏ Photon  Electro-magnetic  s 1/8 Quarks Gluon g Strong Relative coupling constant Particles that feel the force Force carrier Fundamental Force  G ! 1/ (10 39 ) ! 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.)‏

5. 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?

6. 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

7. LOGO 7 Calculating Alpha S In 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

8. LOGO 8 Calculating Alpha S 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. 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 * N 3Jets / N 2Jets