1 Creating more particles What have we learned? All that is - PDF document

Particles as fields Final Exam : Sat. Dec. 19, 2:45-4:45 pm, 2103 Cham. Exam is cumulative, covering all material • Electromagnetic field spread out over space. Review Chap. 18: Particle Physics – Stronger near the the source of the electric/magnetic charge - weaker farther away. • Particles and fields: a new picture • Electromagnetic radiation, the photon, is the quanta of the field. • Quarks and leptons • The weak interaction • Describe electron particles as fields: • Unification and mass – Makes sense - the electron was spread out around the hydrogen atom. • The approach of string theory – Wasn’t in one place - had locations it was more or less probable to be. Stronger and weaker like the electromagnetic field. • Electron is the quanta of the electron field. Phy107 Lecture 40 Phy107 Lecture 40 1 2 Pair production, annihilation Quantum Electrodynamics: QED • Electron and positron can ‘annihilate’ to form two photons. • Normal electromagnetic force comes about from exchange of photons. • Photon can ‘disappear’ to form electron-positron pair. electron • Relativity: Mass and energy are the same Electromagnetic repulsion via emission – Go from electron mass to electromagnetic/photon energy of a photon photon electron Phy107 Lecture 40 Phy107 Lecture 40 3 4 Seeing antiparticles The story so far • Electromagnetic force and electrons are both fields. • Photons shot into a tank of • The fields have quanta: photon and electrons. liquid hydrogen • The Quantum field theory QED explains how they in a magnetic interact. field. • Electrons and • Very successful theory: explains perfectly all the positrons bend interactions between electrons and photons in opposite • Predicted a few things we didn’t expect: directions and, losing energy – Antiparticles - the positron. to ionization, – Electrons and positions can be annihilated to photons and spiral to rest. vice versa. Phy107 Lecture 40 5 Phy107 Lecture 40 6 1

Creating more particles What have we learned? • All that is needed to create particles is energy. Matter is made of atoms • Energy can be provided by high-energy collision of particles. An example: – Electron and positron annihilate to form a photon. – This can then create particles with mc 2 <photon energy. µ , Muon mass: 100MeV/c 2 , Atoms are made of leptons and quarks electron mass 0.5 MeV/c 2 “ Atoms are made of leptons and quarks “ u ν e e- e-, µ - Leptons Quarks First new particle e d found this way Interact via different forces carried by particles, e+, µ + photons…, simple except for the muon e+ Phy107 Lecture 40 Phy107 Lecture 40 7 8 Three ‘generatations’ of particles The ‘generations’ • Three generations differentiated primarily by mass (energy). • First generation Light – One pair of leptons, one pair of quarks • Leptons: Heavier – Electron, electron-neutrino. • Quarks: Heaviest – Up, down. • All 3 generations seen Phy107 Lecture 40 Phy107 Lecture 40 9 10 Energy uncertainty Charge • To make a very short pulse in time, • These are the exchange bosons. need to combine a range of frequencies. • What are they exchanged between? • Frequency related to quantum energy by E =h f . • Or on what are the corresponding forces exerted? • Heisenberg uncertainty relation can also be • Example: stated – When a photon is exchanged between two particles, (Energy uncertainty)x(time uncertainty) there is a electromagnetic or Coulomb force. ~ (Planck’s constant) – We know that only particles with electrical charge interact via the Coulomb force In other words, if a particle of energy E – Zero charge -> zero Coulomb interaction only exists for a time less than h / E, it doesn’t require any energy to create it! Phy107 Lecture 40 11 Phy107 Lecture 40 12 2

A little complicated Many Charges • Quarks and leptons have multiple charges. • In this language, we say that the • Some of the bosons have charges. electrical charge is a ‘source’ of an EM field. • A mass ‘charge’ is the source of a gravitational field Color Electric, flavor, • A weak ‘charge’ (sometimes called ‘flavor’) color, mass is the source of a weak interaction field None • A strong ‘charge’ (sometimes called ‘color’) Flavor is the source of a strong interaction field Electric, Electric, flavor, mass mass Phy107 Lecture 40 Phy107 Lecture 40 13 14 Interactions through Exchange Feynman Diagrams (Quark Scattering) of Color Charge rg rg rg u Quark-quark u Position Scattering Could also be Emission of Gluon Quark-antiquark Scattering d Initially After gluon emission d or RED  RED-ANTIBLUE + BLUE time Antiquark-antiquark (quark) (gluon) (quark) Scattering d u d Re-absorption of Gluon g Quark-antiquark Before gluon absorption After gluon absorption u Annihilation g RED-ANTIBLUE + BLUE  RED u d (gluon) (quark) (quark) Phy107 Lecture 40 Phy107 Lecture 40 15 16 More Baryons Gluon interactions Quark up down strange Since gluons carry “color charge”, Q +2/3 -1/3 -1/3 they can interact with each other ! Mass ~5 [MeV/ c 2 ] ~10 [MeV/ c 2 ] ~200 [MeV/ c 2 ] (Photons can’t do that) u u u d d d s s s Gluon-gluon Scattering Gluon-gluon Fusion Excited state - Higher energy/mass g g g g u u u d g d u d d s s s s g Lambda ( Λ ) Sigma ( Σ + ) Sigma ( Σ 0 ) Sigma ( Σ − ) g g g Q = 0 Q = +1 Q = 0 Q = -1 g M=1116 MeV/ c 2 M=1189 MeV/ c 2 M=1192 MeV/ c 2 M=1197 MeV/ c 2 Phy107 Lecture 40 17 Phy107 Lecture 40 18 3

Mesons Carriers of the weak force • They are formed when a quark and an anti-quark • Like the Electromagnetic & Strong forces, “bind” together. the Weak force is also mediated by “force carriers”. • So far we’ve only seen 3 quark combinations. There are also 2 quark combinations. • For the weak force, there are three force carriers: • The hadrons: 2 quarks, meson and 3 quarks, baryon. W + W - Z 0 d d d c s u This “weak force” carrier These “weak force” carriers is electrically neutral carry electric charge also ! What’s the charge What’s the charge What’s the charge of this particle? of this particle? of this particle? The “charge” of the weak interaction Q=+1, and it’s Q= -1, and this charm Q= 0, this strange is called “weak charge” called a π + meson is called a D - meson is called a K 0 Phy107 Lecture 40 Phy107 Lecture 40 19 20 Range of the interaction Scattering from quarks in a nucleus • What Ice Cube looks for is neutrinos emerging from • Electron doesn’t have enough energy to create Z o. collisions as muons. • Z o only present due to uncertainty relation µ - ν µ • The neutrino interacts with (Energy uncertainty)x(Time uncertainty)~Planck cnst quarks bound inside nucleons in the nucleus. • Neutrino emits W + , changing W + It can only exist for a time determined by flavor into muon. • Down quark bound in a neutron Time uncertainty ~ Planck cnst absorbs W + , changing into a up quark. Particle mass d u • The nucleon then has two ups n p u u and one down quark, which is a Farthest it can travel in that time is proton. d d Range ~(Light Speed)x Planck cnst ~ 10 -18 m time Particle Mass Phy107 Lecture 40 Phy107 Lecture 40 21 22 Lepton decay Similar to nuclear beta decay • Flavor change can occur spontaneously if the • Interaction via the W explains nuclear beta decay. particle is heavy enough. • s quark emits a W-, changing _ flavor into a u quark. Generation I Generation II Generation III Charge ν e • W decays to an electron and anti-electron neutrino. W - µ — τ — e — -1 • The nucleon then has two ups e - and one down quark, which is a proton. • Similar to the rotated Feynman d u ν e ν τ 0 diagram we studies with the ν µ n p u u electromagnetic force d d Electron is stable Emit W - Emit W - 2x10 -6 seconds 3x10 -13 seconds time Phy107 Lecture 40 23 Phy107 Lecture 40 24 4