2. Tony Liss December 6, 1997

3. What is the World Made of? Air Fire Water Earth Since ancient times humankind has looked for fundamental constituents

4. Atoms The first modern fundamental particle was the atom, from the Greek work atmos, meaning indivisible. The organization of the known types of atoms into the Periodic Table was a hint that there was a deeper underlying structure.

5. Electrons Are the Reason n Mendeleev laid out this beautiful picture that classified the elements and predicted their properties. But why? n In 1897 Thompson discovered the electron. The negatively charged electron came from the atom proving –The atom is not elementary and, –There must be a positive charge in there too.

6. Rutherford: The First High Energy Physicist n The atomic nucleus is discovered! Gold foil  particles n Four fundamental particles describe it all: The proton, neutron, electron and photon. neutron, electron and photon.

7. What Are the Forces That Hold It Together? With each force goes a quantum field theory to describe it (but we don’t yet have a working theory for the oldest known force, gravity). Each force has a particle associated with it, a gauge boson, that allows the force to act at a distance. For the electromagnetic force the theory is quantum electrodynamics and the gauge boson is the photon. Keeps you in your seat Nuclear beta decay: n  p e - Binds electrons and protons Holds the nucleus together

8. Too Tiny To See? A typical nucleus has a radius of about 10 -15 meters, yet Rutherford was able to “see” the nucleus of a gold atom. How did he do it? We usually see by detecting light that has bounced off objects into our eyes. If the object is very small compared to the wavelength of the light, then the light diffracts around the object and the image is lost. To see very tiny objects we need very short wavelengths. <d >d

9. Enter Quantum Mechanics With the development of quantum mechanics in the 1920s, it was shown that a particle with momentum p behaves like a wave with wavelength  h/p Rutherford’s  particles had a wavelength similar to the size of a gold nucleus. Today, particle physicists still use this principle: To resolve structure on a smaller and smaller scale, we need higher and higher energies

10. And Relativity! There’s another reason for high energies too: E=Mc 2 If we want to create a particle that isn’t found in normal matter in the laboratory, we need an energy at least equal to its mass times the speed of light squared.

11. A Natural Source of High Energy Particles n Early particle physicists discovered that nature provides an excellent source of high energy particles: cosmic rays Cosmic rays are high energy charged particles, mostly protons, that impinge on the earth’s atmosphere from outer space. Wanna bang a projectile on a target? Nature does it for you all the time! Mostly Muons collision w/ air molecule protons from outer space

12. The Particle Explosion Begins n The study of cosmic ray interactions brought the discovery of a host of new particles: –1931 - The positron (e + ) –1936 - The muon (m) –1947 - Pions, kaons, hyperons n Meanwhile, Ernest Lawrence was learning how to build powerful accelerators Intensities MUCH higher than cosmic rays!

13. Where’s the Order? With high energy accelerators and a new particle detector called the bubble chamber, particle physicists went to town in the 1950s   K-K- K0K0   K+K+ e  p n   ee   

14. Quarks n In 1961 Gell-Mann & Ne’eman did for “fundamental” particles what Mendeleev had done 100 years earlier for “fundamental” atoms. np   -- 00                     K0K0 K+K+ K-K- K0K0 Q=-1 Q=0 Q=+1 Q=-1 Q=0 Q=+1 Q=+2 S=0 S=-1 S=-2 S=-3 S=+1 S=0 S=-1 A missing piece! The   : S=-3, Q=-1

15. Order  Constituents n Just as the order of the periodic table was due to the three constituents, so Gell-Mann and Zweig proposed that all the hadrons were made up from just 3 “quarks” Which, oddly enough, had electric charges (in units of e) of 2/3, -1/3. -1/3 puud nudd  + ud  0 uu  - du   uuu   uud   udd   ddd   sss K + us K 0 ds K - su K 0 sd

16. Where are the Quarks? n This is a nice picture, but no one had (or has) ever seen a free quark. n Repeat Rutherford’s experiment at MUCH higher energies! Protons electrons                                                                Evidence that the proton has something hard inside!

17. Quantum Chromodynamics n In the 1970s a quantum theory of strong interactions was developed called quantum chromodynamics (QCD). It includes a new gauge boson called the gluon. n The “charge” of the strong force is called color, and each quark comes in one of three colors: red, green or blue. The color force is transmitted by the gluon. The observed hadrons are white, i.e. they have all 3 colors, or a color and and anti-color.

18. No Free Quarks (ever) n The color force between quarks decreases with decreasing distance (asymptotic freedom) and grows large at large distances (infrared slavery) n Here’s what happens if you try to pull a (colorless) baryon apart: Energy in the field increases until... Enough E=Mc 2 for a quark-antiquark pair to be produced

19. The Standard Model Electric Magnetic Weak Strong Electromagnetic Electroweak Quarks Leptons udud cscs tbtb e e -   Gauge Bosons  W+W+ W-W- Z0Z0 g W W W W W W Simplicity regained!

20. On To Higher Energies! To test these new theories, physicists needed higher energies  COLLIDING BEAMS. “Fixed Target” Amount of energy for new particle production ~  E beam Colliding Beams Amount of energy for new particle production ~E beam

21. Detecting the Debris A typical colliding beam detector Tracking chamber - Inside magnetic field, measures charged particle momenta Electromagnetic calorimeter - Measures energy of electrons and photons Hadronic calorimeter - Measures energy of hadrons Muon tracking - If it has an electric charge and it makes it out here, it’s a muon

22. Evidence for QCD QCD tells me I can’t see a free quark. So, what happens if I whack two protons together so hard that one of the constituent quarks goes flying away?

23. A Real Two “Jet” Event

24. Tests of Electroweak Theory n With the advent of electroweak theory, three new particles were needed: the gauge bosons W +, W - and Z 0. The masses were predicted by the theory to be –M W c 2  80 GeV –M Z c 2  90 GeV That’s about the mass of bromine (z=35) and zirconium (z=40). Not badd for a pair of elementary particles! The W and Z are extremely short-lived, but can be identified by their decay modes, also predicted by electroweak theory WW e  ud cs tb(?) ZZ e + e -         qq

25. How Many Generations? n The Z decays to the leptons and antileptons of each generation, as well as the quarks and antiquarks. n The more ways there are for the Z to decay, the easier it is for it to do so and the shorter its lifetime n Heisenberg’s uncertainty principle can be written  E  t   /2 Coupled with E=Mc 2, this says that the mass of the Z when it decays is determined only up to a constant proportional to its lifetime   M t z  1

26. The Width of the Z Z width (  E) Z mass (=E/c 2 ) Number of Z decays

27. So….Six quarks And the sixth one is

28. The Hunting of the Top Quark n After the bottom quark was discovered (Fermilab, 1977) it was known that a top quark had to exist because the Standard Model requires the quarks to come in pairs. n The Standard Model tells us exactly how the top decays q, e,  q e,,,  But there is no direct prediction of its mass.

29. Making Top Quarks in Batavia, IL n The Fermilab Tevatron is the world’s highest energy accelerator. Protons and antiprotons collide at an energy of 1.8 TeV (1800000000000 electron volts).

30. Success !! n In 1994, after 17 years of trying at various accelerators, and nearly 10 years at the Tevatron, we finally found a grand total of 12 collisions that looked like they produced a top-antitop pair. Now there are more than 100. n Why was it so hard? –Because M TOP c 2 = And only 1 out of 10 10 collisions produced a top-antitop pair. Now that’s a needle in a haystack!

31. We’re Done! (NOT!) The  neutrino (we know it’s there, it’s just really hard to see directly) The  neutrino (we know it’s there, it’s just really hard to see directly) n The Higgs Boson (Not another one?!?) n Why are there three generations? n How can we explain the weird pattern of masses? n Are the fundamental particles really so? Might they too have constituents? We’re doing the Rutherford experiment yet again, this time with quark-antiquark collisions! Do quarks have constituent parts? Maybe! OK, 3 generations, all six quarks, Ws, Zs, gluons, photons. What’s missing?

32. Are there fundamental particles? Or is nature just an onion?