From Last Time Discussed the weak interaction All quarks and leptons have a weak charge They interact through the weak interaction Weak interaction often swamped by electromagnetic or strong interaction. Most clearly manifested in particle decays. Today Unification of the electromagnetic and the weak force the Standard Model Where do particles get mass? The Higgs mechanism and the Higgs boson(s) Prospects for unifying all of particle physics Grand Unified Theories Fundamental Matter Particles Three generations of leptons and quarks All feel the weak force. Charged leptons feel weak & EM forces Similar particles All six quarks can change flavor via the weak interaction. Leptons can change flavor within a generation, and neutrinos between generations View 12 fundamental matter particles as only two different types: different flavors are just diff. orientations. Quarks feel weak, EM, & strong And several different interactions Exchange Bosons (force carriers) Remember that interactions are due to exchange of bosons. EM interaction - exchange photons Weak interaction - exchange W +, W, Z o Strong interaction - exchange gluons (8) EM Weak 1
Electro-weak unification Some similarities here The standard model says that the electromagnetic interaction (photon exchange) & the weak interaction (W +, W -, Z o exchange) are different pieces of the same electroweak interaction Neutral weak Electromagnetic W + W - Pos. weak Neg. weak W + W - Mass=91 GeV/c 2 Mass=0 GeV/c 2 Range ~ inf. Pos. charge Neg. charge Neutral weak Mass=91 GeV/c 2 Electromagnetic Mass=0 GeV/c 2 Range ~ inf. Pos. weak Pos. charge Neg. weak Neg. charge These two both exchange neutral bosons Neither boson changes the lepton flavor (remains electron) These two both exchange charged bosons. Both bosons change the lepton flavor Similar indeed In fact, the Z o interaction and the photon interaction are so similar that they are very difficult to distinguish experimentally. The Z o was predicted by the Standard Model, and then found experimentally. It s existence was forced by symmetries on which the standard model is based. Symmetries in the SM The standard model is based on symmetries, but they are a little subtle. The symmetry of the electroweak interaction is SU(2)xU(1) This is (flavor)x( charge ) This results in four exchange bosons. W +, W o, W - for flavor B o for charge Symmetry breaking The standard model says that at high energies, this symmetry is apparent We see a single electroweak interaction. At low energies it is broken We see distinct electromagnetic and weak interactions W o and B o quantum mechanically mix together Results in the two bosons we observe 1) electromagnetic photon interaction 2) Z o (neutral part of weak interaction) Particle mass in the SM The standard model has great successes, but in its first incarnation all particles were required to have zero mass. Otherwise the mathematics gives non-physical results. Clearly a major problem, since many particles do have mass. 2
Mass Here s the experimental masses of SM particles. Original SM gives zero mass for all particles. But can give particles mass by coupling to a new field, the Higgs field. Higgs boson is an (unobserved) excitation of the Higgs field. What is mass? Think of inertial mass: inertial mass is a particle s resistance to changes in velocity. When you apply the same force to particles, the smaller the mass, the larger the acceleration. What is the origin of mass? Mass in the SM In the standard model (SM), particles have mass because they interact with something that pervades the universe. This something is the Higgs field Particles hit the Higgs field when you try to accelerate them Mass = (chance of hit) x (Higgs density) Coupling constant Mass and the Higgs field Now a popular person enters the room, attracting a cluster of hangers-on that impede her motion As she moves she attracts the people she comes close to- the ones she has left return to their even spacing. Her motion is impeded - she has become more massive. Imagine a party in a room packed full of people. Nobody is moving around much, just standing and talking. In this example, the popular person plays the role of an electron, or a W boson, or any particle. The people in the room represent the Higgs field. The interaction with the Higgs field gives the particle its mass A particularly unpopular person could move through the room quite easily, with almost no mass. This is the analog of a low-mass particle. In present theories, the popularity of each particle is an input parameter. The Higgs boson The people get quite excited. They cluster to pass on the rumor, and the cluster propagates thru the room. Since the information is carried by clusters of people, and since it was clustering which gave extra mass to the popular person, the clusters also have mass. The Higgs boson is a quantum excitation of the Higgs field. In analogy, suppose an interesting rumor is shouted in thru the door. 3
How can we see the Higgs? The Higgs boson is predicted to be just such a clustering in the Higgs field. We will find it much easier to believe that the field exists, and that the mechanism for giving other particles mass is true, if we actually see the Higgs particle itself. The Higgs boson needs to be created in order to see it. Einstein showed that energy and mass are equivalent, through E = mc 2. This requires an energy of at least the rest mass energy of the Higgs particle. The Higgs mass is unknown, but present experiments suggest m Higgs > 100 Gev m Higgs < 200 Gev Symmetry breaking again Higgs boson: use at your own risk The theory says that the Higgs field has a vacuum expectation value. That is, the Higgs field is always nonzero. This is different than other fields we have talked about. At high energies, this is not true, and all particles are massless. The Higgs Boson wheel isn't really a wheel. It isn't even designed to roll on; it's designed to grind with. *WARNING- THE HIGGS BOSON IS NOT A TRADITIONAL INLINE-SKATING WHEEL. THE SHAPE AND URETHANE ARE NOT DESIGNED FOR ROLLING. USE AT YOUR OWN RISK. Beyond the standard model? Standard model has been enormously successful. Consistent picture of particles and their interactions. Predictive power with unusual accuracy. But SM w/ Higgs mechanism has 19 input parameters. No suggestion of why there are 3 generations of leptons/quarks. No explanation of left-right asymmetries. Quarks and leptons are unrelated fundamental particles More Unifications? 4
Grand Unified Theories Not all that easy What do we really need to unify particle physics? Maxwell unified the electric and magnetic interactions into electromagnetic (EM) The standard model unified the EM and weak interactions into the electroweak interaction What s left is the strong force. What kind of theory is needed to unify this? What GUTs might do Flavor changing interactions in quarks (e.g. changing a top quark to a bottom quark by emitting a W + ) suggest that quarks can be viewed as 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 of the same fundamental object. The price of unification When the SM unified EM and weak interactions, we ended with more force-carrying bosons (e.g. the Z o ) This is because our fundamental particle increased in complexity e.g. from an electron to an electron-neutrino pair If our particle now encompasses both leptons and quarks, the interaction also becomes more complex. In one particular GUT, we get 24 exchange bosons (W +,W -,Z 0, photon, 8 gluons, and 12 new ones) What is left? Unifications: now and the future There is a left-right asymmetry in nature that has no explanation. Most particle masses are inputs to the theory. In GUTs, EM, weak, and strong forces are unified. But gravity is left out. How could we reconcile a quantized gravity with Einstein s geometrical theory (warped space-time)? These are unanswered questions! 5
Summary Details of weak interaction suggest that Different quarks are diff. orientations of the same particle. Different leptons are diff. orientations of the same particle. Weak and EM interactions are different parts of a single electroweak force. Mass Particles get mass by interacting with Higgs field Higgs boson is an excitation of the Higgs field Grand Unified Theories (GUTs) Will combine letptons and quarks Unify strong and electroweak interactions 6