1Phy107 Fall 20061Final Exam• Thursday, Dec. 21: 2:45 - 4:45 pm113 Psychology Building• Note sheet: one double-sided page• Cumulative exam-covers all material, 40 questions– 11 questions from exam 1 material– 11 questions from exam 2 material– 11 questions from exam 3 material– 7 questions from post-exam 3 materialCourse evaluation Wednesday, Review FridayStudy Hint: download blank hour exams from website and take them closed-book, with note sheet only.Solution for Exams will all be posted this week.Phy107 Fall 20062• The forces– Matter particles interact via the forces according tothe types of charge they carry.– Each force carried by a particle.– Matter particles are fermions, force carriers arebosons. Described by quantum field theories.• Unification– Noticed that electromagnetic force and weak forcecarried by Z boson are nearly identical.– At high energy they are identical. Same types ofinteractions and same strength– Requires an extra particle. The Higgs boson.From the last time…Phy107 Fall 20063Symmetry and Symmetry breaking• The standard model says that at high energies,this symmetry is apparent– We see a single electroweak interaction.– Zo and interact exactly the same way with the samestrength.• At low energies thesymmetry is broken– We see distinctelectromagnetic andweak interactions• However needs one more element. Something togive the W and Z massPhy107 Fall 20064Mass• Here’s the experimentalmasses of SM particles.• Original SM gives zeromass for all particles.• But can give particlesmass by coupling to a newfield, the Higgs field.• Higgs boson is the(unobserved) quanta ofthe Higgs field.Phy107 Fall 20065What is mass?• Think of inertial mass:– inertial mass is a particle’sresistance to changes in velocity.• When you apply the same force to particles,the smaller the mass, the larger theacceleration.• What is the origin of mass?Phy107 Fall 20066Mass in the SM• In the standard model (SM),particles have mass because they interactwith something that pervades the universe.This something is theHiggs fieldParticles ‘hit’ the Higgsfield when you try toaccelerate themMass =(chance of hit) x (Higgs density)Coupling constant2Phy107 Fall 20067Mass and the Higgs fieldImagine a party in a roompacked full of people.Now a popular person entersthe room, attracting acluster of hangers-on thatimpede her motionshe has become moremassivePhy107 Fall 20068The Higgs bosonThe Higgs boson is a quantumexcitation of the Higgs field.In analogy, suppose an interestingrumor is shouted in thru the door.The people get quite excited.They cluster to pass on the rumor, and thecluster propagates thru the room.Looks very similar to the popular/massiveperson who entered the roomGood way to think of other quantumexcitations. All the other force carriersPhy107 Fall 20069The Higgs BosonHow much mass do youthing the Higgs BosonhasA. No massB. Light like an up ordown quarkC. Very massive like atop quarkPhy107 Fall 200610How can we ‘see’ the Higgs?e-Zoe+HZo• The Higgs boson needs to be created in order tosee it. E = mc2• Not found yet•mH > 114GeV•mH < 186GeVPhy107 Fall 200611Grand Unified Theories• What do we really need to unify particle physics?• Maxwell unified the electric and magneticinteractions into electromagnetic (EM)• The standard model unified the EM and weakinteractions into the electroweak interaction• Start with the strong force.• What kind of theory is needed to unify this?Phy107 Fall 200612Not all that easy3Phy107 Fall 200613Grand Unified Theories• Flavor changing interactions in quarks(e.g. changing a top quark to a bottom quark byemitting a W+) suggest that quarks can be viewedas different ‘orientations’ of the same object.• Have found the same thing for leptons.• But maybe there should be a lepto-quark field?– Quarks could turn into leptons, leptons into quarks– All matter particles would be different ‘orientations’ ofthe same fundamental object.• If we unify leptons and quarks then weak andstrong forces may be shown to be two aspects ofone force.Phy107 Fall 200614The price of unification• When the SM unified EM and weak interactions, weended with more force-carrying bosons (e.g. the Zo)• This is because our fundamental ‘particle’ increasedin complexity– e.g. from an electron to an electron-neutrino pair• If our ‘particle’ now encompasses both leptons andquarks, the interaction also becomes more complex.• In one particular GUT, we get 24 exchange bosons(W+,W-,Z0, photon, 8 gluons, and 12 new ones)Phy107 Fall 200615Beyond the standard model?• Standard model has been enormously successful.• Consistent picture of particles and theirinteractions.• Predictive power with unusual accuracy.• Questions:– Why 3 generations?– What determines all themass values and interaction strengths?– Can we relate the quarks and leptons and the forces?Phy107 Fall 200616What does the SM say?• We can calculate how interactions would work atenergies like those of the big bang.– The results don’t make sense.• Astrophysics observations indicate that there ismore mass in the galaxy and universe than we cansee: Dark Matter– No standard model particle could explain this.• All the standard model interactions create electronsand positrons or quarks and antiquarks in pairs.– However, everything around us is made of quarks andelectrons. Where did the positrons and antiquarks go?None of these things can be explained by the SM!Phy107 Fall 200617Grand Unified Theories: GUTs• Unify all the forces: strong force and gravity• Quantize the forces - QFT very successful• Unify the particles: quarks, leptons - 3 generations• Explain all the different masses and strengths• Explain dark matter• Explain why universe is mostly mater• Explain physics at very high energy - big bangPhy107 Fall 200618Unifications: now and the future4Phy107 Fall 200619Fermions and bosons• Matter (fermions) and forces (bosons) behave differently.Which drawing below best represents fermions?Fermions BosonsA. AB. BABPhy107 Fall 200620Supersymmetry (SuSy) Superpartners (compare to anti-particles)Every fermion has a boson partner and vice versaStarts to relate the fermions and bosonsPhy107 Fall 200621Supersymmetry Successes• Designed to explain behavior at very high energy• Forces merge in SUSY– Same strength at highenergy.• Lightest SUSYparticles don’t decay.• Dark
View Full Document
Unlocking...