Particles, Colliders, and the Higgs Boson

1. Particles, Colliders, and the Higgs Boson Tim Wiser Splash P2506 3 Nov 2012

2. Plan • Standard Model of Particle Physics • Particles, interactions, and the Higgs field • Drawing Feynman diagrams • These simple pictures are actually calculational tools for physicists! • Particle colliders • How they tell us what stuff is made of • What’s left? • Incompleteness of the Standard Model • Q&A

3. What is matter made of? • Atoms (~100 elements) • Protons (p), neutrons (n), and electrons (e-) • p, n made of quarks (up & down type) • All the matter around us is made of u, d, e- • But there’s more! • 6 quarks and 6 leptons, plus antiparticles

4. What holds it together? • Four fundamental forces: • Gravity • Electromagnetism • Strong force • Weak force • Gravity is by far the weakest, and it’s different than all of the others. So we will ignore it today!

5. Forces in Particle Physics • In the Standard Model, all forces work the same way: by exchanging particles. • E&M: photon • Strong force: gluon • Weak force: W, Z bosons • Two electrons can “toss” a photon back and forth between them, and repel each other as a result.

6. e-/e- repulsion

7. What about attractive forces…? • You might think that exchanging particles can only result in repulsive forces. • But the exchanged particles are not “real”… • Virtual particles can move left but carry rightward momentum! • Kind of like throwing a boomerang…

8. Feynman Diagrams • There are three rules in particle physics: • Conserve energy • Conserve momentum • Conserve charges (electric, and more…) • As long as those rules are satisfied, everything that is allowed WILL happen with some probability! • Feynman diagrams automatically obey the 3rd rule. “Calculating” the diagram tells us the probability. • There are usually lots of diagrams for the same process, so we will need to add them all together.

9. QED (Quantum Electrodynamics) • QED is the simplest part of the Standard Model. There is only one possible Feynman vertex:

10. Electron Scattering • Let’s say we want to see how two electrons “scatter” off of each other. We need to draw all Feynman diagrams with two electrons in and two electrons out. • We already saw one:

11. How can we possibly deal with an infinite number of Feynman diagrams??

12. Order of importance • Fortunately for us, the more complicated the diagram, the smaller its value! • Each vertex multiplies the probability by a small number (in QED, 1/137) • Every loop divides the probability by about 25,000! • So, we only need to think about the simplest possible diagrams.

13. “Bending” diagrams • It’s not against the rules to have electron lines go “backwards in time” • Such electrons would act exactly like oppositely-charged particles moving forward in time—antimatter! • (This doesn’t make time travel possible. Sorry!)

14. Pair production • If a photon has enough energy (rule #1!) it can produce an electron and its antiparticle, the positron. • (It turns out that this can only happen if the photon hits something first, due to rule #2.)

15. Annihilation • If we read the diagram the other way, we see that an electron and positron can “annihilate” and produce a photon. • (Well, actually two photons—we need to conserve momentum!)

16. Evidence for QED • Besides the fact that we have detected electrons, positrons, and photons and they work just like QED says… • QED predicts the “g-factor” of an electron to be almost, but not quite, 2. • Prediction: 2.0023228 (1 loop) • Measured: 2.0023193 • If you add in the 2-loop correction, they agree to 10 decimal places!

17. Protons & Neutrons • For a while, scientists thought that these were elementary particles like the electron and photon. • If that were true, gp=2 and gn=0 • But… • Experimentally, gp=5.6 and gn=-3.8 • This can only happen if the proton and neutron are made of smaller particles!

18. So, what’s inside? • We only have one good way of finding out what’s inside of particles… • Smash them together!

19. A plenitude of particles • When we started smashing protons and neutrons together, we started discovering all sorts of new particles: • 8 mesons: 3 pions, 4 kaons, and the eta • 8 baryons: p, n, 3 sigmas, 2 xis, and the lambda • But as we built bigger, better colliders we found even more: there are now hundreds of mesons and baryons known.

20. Simplifying • We wanted to find what protons and neutrons were made of… • But we found a bunch of composite particles like them instead! • We can explain the structure of hadrons (mesons and baryons) if we guess that there are three “quarks”—up, down, and strange. • But we’ve never seen quarks by themselves, so the force that holds them together must be really strong!

21. Hadron Structure • Mesons: • Baryons:

22. Quantum Chromodynamics (QCD) • In fact, there is a way for this all to work… • Three quarks: up, down, and strange • In addition to electric charge, “color charge” • Call them red, green, and blue • Force carrier particle: gluon

23. QCD Feynman Diagrams

24. Confinement • Because gluons themselves have color charge, the force between two quarks doesn’t get weaker as they get further apart! • If you pull hard enough, you will just create new particles until everything is color neutral. • This explains why we see mesons (quark-antiquark pairs) and baryons (three quarks or three antiquarks) but never quarks or gluons by themselves.

25. Jets • If we never see quarks or gluons in nature, why are they useful? • It turns out that QCD gets weaker at high energies! • So we can describe collider physics with quarks and gluons… • which “hadronize” as they leave the collision point. • The resulting bunches of hadrons are called jets.

27. Weak Interactions • In nature, we observe “flavor-changing” interactions • Nuclear beta decay (n->p+e+?) • d quark -> u quark • How can we explain this? QED and QCD are “flavor-blind”

28. Neutrinos • It looks like beta decay doesn’t conserve momentum! • That’s ridiculous, there must just be an invisible particle as well. • Call it a “neutrino” (quasi-Italian for little neutral particle)

29. Changing Flavors • To change from a d quark to a u quark, we must emit a charge -1 particle • That particle must then emit an electron and an anti-neutrino. • W- boson (there is also a W+ boson, of course.) • To explain the “weakness” of the weak force, the W bosons must be heavy. (This will be important later!)

30. Constructing the Standard Model • A series of surprises, predictions, and experiments. • Prediction: pion as nuclear force mediator • Surprise: muon (a heavier electron!) • Experiment: quarks are real • Prediction: charm quark (confirmed!) • Prediction: W and Z bosons (confirmed!) • Surprise: 3rd generation of matter

32. Practice with Feynman Diagrams • Beta decay • e+e- -> μ+μ- • π+ -> μ+νμ • K0 -> K0 bar

33. Testing the Standard Model • High-energy tests • Particle colliders • Cosmic rays • Precision tests • g-2 experiments • Rare particle decays

34. Colliders • 2 things come in, n things go out • Higher energy means we can make heavier particles in the collision • Two main types: linear (like SLC) and circular (like LHC)

35. Electron Colliders • The easiest particles to accelerate • Since they’re elementary particles, easy to calculate and to measure the results • Hard to make circular colliders (LEP was one) • Lots of linear colliders, including one at SLAC! • Link

36. A few discoveries made by e+e- colliders • Countless hadrons • Charm quark (in the form of the J/ψ meson) • Tau lepton • Precision measurements of W and Z bosons

37. Hadron Colliders • Protons and/or antiprotons • Tevatron (p-pbar) and LHC (p-p) are the major HCs • Pros: high energy, can be circular (cheaper), strong interactions • Cons: hadrons are composite, strong interactions

38. Discoveries at Hadron Colliders • Bottom and top quarks (Tevatron) • W and Z bosons (SPS) • Countless MORE hadrons

39. Collider Physics I: Acceleration • Powerful electric fields speed up charged particles • In practice, “RF cavities” are used • Kind of like a tuned microwave oven… • In a linear collider, we get one shot to accelerate • In a circular collider, we can accelerate it over and over again

40. Collider Physics II: Bending and Focusing • Electric fields speed up the particles, but we use magnetic fields to focus and aim the beam • Magnets have to be kept very cold so that the wires superconduct and produce very strong magnetic fields

41. Collider Physics III: Collision • Finally, two beams of particles will collide with each other • How do we see what is produced? • Massive detectors around the collision point can track the paths of particles and measure their energies

42. The LHC • Large Hadron Collider • At CERN, near Geneva, Switzerland • 17 mile circumference, >150 ft underground • Passes under both Switzerland and France • 2 primary detectors, ATLAS and CMS • 2 special-purpose detectors, LHCb and ALICE • Several minor detectors

44. ATLAS

46. CMS

48. The Higgs Boson • The Standard Model as we have talked about so far makes a prediction: • All elementary particles are massless! • (Composite particles like hadrons can still have mass, though.) • This is obviously not true…but the Standard Model works so well, we have to try and save it.

49. Symmetry • The SM has a property called “gauge symmetry” which describes the properties of the three forces • Mass is incompatible with gauge symmetry! • But removing gauge symmetry gets rid of all of the predictive power.

50. Broken Symmetry • In quantum field theory, particles are actually ripples of fields • Most fields have the value of 0 in the lowest-energy state. • If a field’s lowest energy state is not zero, then it is said to “break” a symmetry. • The symmetry still exists, but it is ‘hidden’ at low energies.