Particles and fields. Today: Review Particle Physics. Question. Quantum Electrodynamics: QED. Electrons and photons

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1 Exam 4: Fri. May 10, in-class 20 qestions, covers fission, fsion, particle physics No final exam. Essays retrne Friay Toay: Review Particle Physics Particles an fiels: a new pictre Qarks an leptons The weak interaction Unification an mass The approach of string theory Particle physics an the niverse Particles an fiels Think abot electrons an photons Moern view is that both are excitations of qantm fiels that permeate all state. Photon is an excitation of the photon fiel. Electron is an excitation of the electron fiel. Bt there is a ifference: Electron is a matter particle Photon is a force particle Phy107 Lectre 41 1 Phy107 Lectre 41 2 Qantm Electroynamics: QED A theory of electrons an photons Electromagnetic force arises from exchange of photons. Electromagnetic replsion via emission of a photon electron photon Qestion An electron an anti-electron (positron) can annihilate, so that the particles vanish. What is left after the annihilation? A. nothing B. qarks C. photons D. netrinos electron Phy107 Lectre 41 3 Phy107 Lectre 41 4 Pair proction, annihilation Electron an positron can annihilate to form two photons. Photon can isappear to form electron-positron pair. Relativity: Mass an energy are the same Go from electron mass to electromagnetic/photon energy Electrons an photons Electromagnetic force an electrons are both fiels. The fiels have qanta: photon an electrons. The Qantm fiel theory QED explains how they interact. Very sccessfl theory: explains perfectly all the interactions between electrons an photons Preicte a few things we in t expect: Antiparticles - the positron. Electrons an positions can be annihilate to photons an vice versa. Phy107 Lectre 41 5 Phy107 Lectre

2 Creating more particles All that is neee to create particles is energy. Energy can be provie by high-energy collision of particles. An example: Electron an positron annihilate to form a photon. This can then create particles with mc 2 <photon energy. All the leptons Six ifferent leptons: cats an fleas The cats have an electrical charge of -e The fleas (netrinos) have zero charge (netral). µ, Mon mass: 100MeV/c 2, electron mass 0.5 MeV/c 2 e- e-,µe+ e+,µ+ First new particle fon this way Phy107 Lectre 41 7 Phy107 Lectre 41 8 Three lepton generations Each electron-like particle an netrino paire in a generation. Qestion The ifference between the ifferent generations of leptons is Generation I Generation II Generation III e µ τ ν e ν µ ν τ A. their charge B. their mass C. their color D. their spin Phy107 Lectre 41 9 Phy107 Lectre Qarks Six ifferent kin of qarks, analogos to the six leptons All qarks have an electric charge, they cople to photon fiel. Bt they also have a color charge, an they cople to the glon fiel. Three qark generations Six ifferent qarks, bt two per generation Jst like the leptons Generation I Generation II Generation III Charge c t +2/3 s b -1/3 Copling of qarks to the glon fiel is the strong interaction Phy107 Lectre Phy107 Lectre

3 Qarks & leptons: matter particles Three generations ifferentiate primarily by mass (energy). First generation One pair of leptons, one pair of qarks Leptons: Electron, electron-netrino. Qarks: Up, own. All 3 generations seen Phy107 Lectre Biling combinations of qarks Harons are composite particles mae from qarks Baryons are harons with three qarks Mesons are harons with two qarks Qestion: a proton is a A. Meson B. Baryon C. Qark D. Fnamental particle Phy107 Lectre Protons/Netrons are composite Qestion A netron is a composite particle consisting of three qarks (p=charge +2/3e, an own = charge -1/3e). Which is the strctre of the netron. A. p, p, p B. p, p, ow C. p, own, own Total charge: +2e +e 0 Netron has zero total charge Phy107 Lectre Phy107 Lectre Many baryons Qark p own strange Q +2/3-1/3-1/3 Mass ~5 [MeV/c 2 ] ~10 [MeV/c 2 ] ~200 [MeV/c 2 ] s s s Mesons They are forme when a qark an an anti-qark bin together. So far we ve only seen 3 qark combinations. There are also 2 qark combinations. The harons: 2 qarks, meson an 3 qarks, baryon. s Lamba (Λ) Q = 0 M=1116 MeV/c 2 Excite state - Higher energy/mass s Sigma (Σ + ) Q = +1 M=1189 MeV/c 2 s Sigma (Σ 0 ) Q = 0 M=1192 MeV/c 2 s Sigma (Σ ) Q = -1 M=1197 MeV/c 2 Phy107 Lectre What s the charge of this particle? Q=+1, an it s calle a π + c What s the charge of this particle? Q= -1, an this charm Q= 0, this strange meson is calle a D - meson is calle a K 0 Phy107 Lectre s What s the charge of this particle? 3

4 How o particles interact? The for fnamental interactions all have associate fiels an meiating particles. Talke abot EM interaction between electrically charge particles by exchange of photons Strong interaction between color charge particles by glon exchange EM interaction Charge particles interact via the electromagnetic (EM) interaction A charge particle coples to the photon fiel It excites a photon (excite state of photon fiel) an loses energy. Another charge particle can absorb the energy from the photon fiel (photon isappears). Only particles with an electric charge cople to the photon fiel. Phy107 Lectre Phy107 Lectre Exchange particles Each interaction has one or more associate particles that meiate the interaction. The exchange particles are associate with the known interactions Interaction Meiating particle(s) Nmber Electro- photon (1) magnetic Weak W +, W - an Z o (3) Strong glons (8) Gravity graviton (1) These all have integer spins ( bosons ) Phy107 Lectre The Stanar Moel 6 leptons 6 qarks 4 interactions Electromagnetic, Gravitational, Strong, Weak meiate by 13 exchange bosons, which are excitations of the corresponing fiels. Phy107 Lectre Qestion Which of the following particles is not a matter particle A. Electron B. Qark C. Glon D. Netrino Many Charges A particle with electrical charge coples to the photon fiel Particle with mass charge coples to grav. fiel Particle with weak charge coples to weak int. fiel Particle with color charge coples to glon fiel Phy107 Lectre Phy107 Lectre

5 Strong Force In the Stanar Moel particles are often classifie by which forces they can participate in. Which of these particle has a color charge an interacts via the strong force? A. electron B. mon C. photon D. qark E. none of these Strong interaction Qarks interact by exchanging glons Qark has a color charge - it coples to glon fiel One qark excites glon in glon fiel Other qark absorbs it. This is the strong force Phy107 Lectre Phy107 Lectre Glon also has color charge Glon has a color charge-> coples to the glon fiel, excites more glons - stronger force As qarks get farther apart, glons have more room to interact. Force gets stronger as qarks get frther apart! Eventally have enogh energy to make more qarks. new Qark-antiQark pair The weak interaction Weak interaction is not strong, bt is important. It can change one particle into another! Mon, ta, can trn into netrinos! Qarks can trn into other qarks Particles with a weak charge (qarks an leptons) cople to the weak fiel Excitations of the weak fiel are the Z an W bosons. Phy107 Lectre Phy107 Lectre Qestion The Weak Force Qarks interact only via the A. weak force B. strong force C. gravitational force D. all of the above EM STRONG WEAK Phy107 Lectre Phy107 Lectre

6 Carriers of the weak force Like the Electromagnetic & Strong forces, the Weak force is also meiate by force carriers. For the weak force, there are three force carriers: W + W - Z 0 These weak force carriers carry electric charge also! This weak force carrier is electrically netral The charge of the weak interaction is calle weak charge Phy107 Lectre Dring raioactive beta ecay, a netron changes to a proton insie a ncles by one of the internal qarks ecaying into a ifferent qark. What particle meiates the qark ecay? A. photon B. glon C. Z o D. W E. W + Electric charge conservation. Ncles gains a positive charge, negative charge mst be proce. Phy107 Lectre Nclear beta ecay Interaction via the W explains nclear beta ecay. n W - _ ν e e - p time own qark emits a W-, changing flavor into a p qark. W ecays to an electron an anti-electron netrino. The ncleon then has two ps an one own qark, which is a proton. Lepton ecay Flavor change can occr spontaneosly if the particle is heavy enogh. Generation I e ν e Electron is stable Generation II µ ν µ Emit W - 2x10-6 secons Generation III τ ν τ Emit W - 3x10-13 secons Charge -1 0 Phy107 Lectre Phy107 Lectre Qarks an the weak force Qarks have color charge, electric charge, an weak charge other interactions swamp the weak interaction Bt similar to leptons, qarks can change their flavor (ecay) via the weak force, by emitting a W particle. Flavor change between generations Bt for qarks, not limite to within a generation! Generation I Generation II Generation III Charge Generation I Generation II Generation III Charge c t +2/3 c t +2/3 s b -1/3 s b -1/3 Emit W + 2x10-12 secons Emit W secons Emit W secons Emit W secons Phy107 Lectre Phy107 Lectre

7 Qarks an leptons have mltiple charges. Some of the bosons have charges. Electric, flavor, color, mass Flavor Electric, flavor, mass Particle charges Color None Electric, mass Key Points Differences between particles connecte to how they interact, what charges they have. Qarks have all the charges. Color charge: Qarks form composite states harons via the strong force. Flavor charge: Heavy qarks ecay to lighter qarks via the weak force. Leptons have no color change. Don t bin together to form composite states. Netrinos only interact via the weak force which means they rarely interact at all. Phy107 Lectre Phy107 Lectre The Higgs boson If the Higgs fiel were not present, particles A. wol not have ifferent charges. B. wol not interact with each other. C. wol not be confine. D. wol not have antiparticles E. wol not have mass. Mass in the SM In the stanar moel (SM), particles have mass becase they interact with something that pervaes the niverse. This something is the Higgs fiel Particles hit the Higgs fiel when yo try to accelerate them Mass = (chance of hit) x (Higgs ensity) Copling constant Phy107 Lectre Phy107 Lectre Unification Details of weak interaction sggest that Different qarks are iff. orientations of the same particle. Different leptons are iff. orientations of the same particle. Weak an EM interactions are ifferent parts of a single electroweak force. Electroweak interaction le to the introction of the Higgs Boson Gran Unifie Theories (GUTs) Will combine leptons an qarks Unify strong an electroweak interactions More Unifications? Phy107 Lectre Phy107 Lectre

8 String theory In string theory, ifferent fnamental particles appear as a. ifferent qantm vibrations of strings. b. ifferent qantm interactions between strings. c. qantm strings with ifferent spin.. qantm strings of ifferent length. e. qantm strings of ifferent velocity. String theory A string is a fnamental qantm mechanical object that is almost a point, bt not qite. A string has a tension that characterizes its behavior. Qantm mechanical vibrations of the string correspon to the particles we observe One fnamental object responsible for all particles Phy107 Lectre Phy107 Lectre String Interactions Strings interact by joining an splitting No extra meiating particles reqire 2 strings joine split into 2 String theory works only in more imensions than we are aware of. The exact way to roll these p etermines the ways in which the strings can vibrate How many hanles Size of each hanle # & location of branes # of flx lines wrappe Compactification In particlar compactifications, most of the matter an force particles we know abot can be fon. Phy107 Lectre Phy107 Lectre Particle physics - niverse connection Jst after big bang, the niverse was Very ense Very hot Very high energy Forces nifie? String theory? Phy107 Lectre Phy107 Lectre