Option 212: UNIT 2 Elementary Particles Department of Physics and Astronomy SCHEDULE  5-Feb pm Physics LRA Dr M Burleigh Intro lecture  9-Feb am Eng 1 Dr M Burleigh Problem solving  (12-Feb am Physics F2Problem Workshop) 16-Feb am Eng 1 Dr M Burleigh Follow-up

UNIT 2: OUTLINE SYLLABUS: 1st Lecture Introduction Hadrons and Leptons Spin & Anti-Particles The conservation laws: Lepton Number Baryon number Strangeness 2nd Lecture 3rd Lecture Follow-up Fundamental forces and field particles The standard model Problem solving Check a decay for violation of conservation laws Quarks Properties of a particle given quark combination

Tipler Chap 41 Q8 State which of the following decays or reactions violates one or more of the conservation laws, and give the law(s) violated in each case: (a) p -> n + e + + e (b) n -> p +   (c) e + + e - ->  (d) p + p ->  (e) e + p -> n + e + (a) m p < m n : energy conservation is violated. Also L e =0 on lhs, but L e =-2 on rhs (b) m n < m p + m   energy conservation is violated (c) Momentum conservation is violated: in pair annihilation, two photons (  rays) must be emitted to conserve momentum(d) Allowed(e) L e =-1 on both sides, but m p < m n so energy conservation violated

Tipler Chap 41 Q29 Consider the following decay chain          p               e - + e +  (a) write the overall decay reaction for   to the final decay products (b) are the final decay products stable? (c) Check the overall decay reaction for the conservation of electric charge, baryon number, and lepton number (d) Check the overall decay reaction for conservation of strangeness. Is the reaction possible via the weak or strong interactions?

Tipler Chap 41 Q29  a    p    e - + e +  (b) Use Table The proton is stable for years. In contrast, the neutron is only stable for 930secs. Answer: yes, stable. (c) Charge conservation: 0 -> p + e - = 0: conserved. Baryon number 1 -> 1: conserved. Lepton number L e : 0 -> e - + e = 1 + (-1) = 0: conserved. L  : 0 -> = 0. (d) See Tipler p Strangeness must be conserved if reaction occurs via strong interaction. Here S=-2 on lhs and S=0 on rhs. But if  S=+/-1, then can occur via weak interaction. In first two parts of reaction,  S=1 (  0 has S=-1) so is allowed via weak interaction.

True or false? (a) Leptons consist of three quarks (b) Mesons consist of a quark and an anti- quark (c) The six flavors of quark are up, down, charmed, strange, left and right (d) Neutrons have no charm (a) False: leptons are fundamental particles e.g e - (b) True (c) False: there is no left and right quark, but there are top andbottom quarks (d) True: neutrons are made of udd quarks

Quark confinement No isolated quark has ever been observed Believed impossible to obtain an isolated quark If the PE between quarks increases with separation distance, an infinite amount of energy may be required to separate them When a large amount of energy is added to a quark system, like a nucleon, a quark-antiquark pair is created –Original quarks remain confined in the original system Because quarks always confined, their mass cannot be accurately known

Quark color Consider the    particle, which consists of three strange quarks Remember that quarks have spin ½ The  - has spin 3/2, so its three strange quarks must be arranged thus: But Pauli exclusion principle forbids these identical (same flavor, same mag of spin, same direction of spin) quarks occupying identical quantum states The only way for this to work is if each quark possesses a further property, color: Quarks in a baryon always have these three colours, such that when combined they are “color-less” ( q r, q y, q b ) In a meson, a red quark and its “anti-red” quark attract to form the particle

Field Particles (Tipler P.1325) In addition to the six fundamental leptons (e -,      e,     and six quarks, there are field particles associated with the fundamental forces (weak, strong, gravity and electro- magnetic) For example, the photon mediates the electro-magnetic interaction, in which particles are given the property “charge” –The theory governing electro-magnetic interactions at the quantum level is called Quantum Electrodynamics (QED) Similarly, gravity is mediated by the graviton –The “charge” in gravity is mass –The graviton has not been observed

Field Particles The weak force, which is experienced by quarks and leptons, is carried by the W +, W -, and Z 0 particles –These have been observed and are massive (~100 GeV/c2) –The “charge” they mediate is flavor The strong force, which is experienced by quarks and hadrons, is carried by a particle called a gluon –The gluon has not been observed –The “charge” is color –The field theory for strong interactions (analagous to QED) is called Quantum Chromodynamics (QCD)

Electroweak theory The electromagnetic and weak interactions are considered to be two manifestations of a more fundamental electroweak interaction At very high energies, >100GeV the electroweak interaction would be mediated (or carried) by four particles: W +, W -, W 0, and B 0 The W 0 and B 0 cannot be observed directly But at ordinary energies they combine to form either the Z 0 or the massless photon In order to work, electroweak theory requires the existence of a particle called the Higgs Boson –The Higgs Boson is expected have a rest mass > 1TeV/c 2 –Head-on collisions between protons at energies ~20TeV are required to produce a Higgs Boson (if they exist) –Such energies will only be achieved by the next generation of particle accelerators (eg Large Hadron Collider at CERN)

The Standard Model (Tipler P.1327) The combination of the quark model, electroweak theory and QCD is called the Standard Model In this model, the fundamental particles are the leptons, the quarks and the force carriers (photon, W +, W -, Z 0, and gluons) All matter is made up of leptons or quarks –Leptons can only exist as isolated particles –Hadrons (baryons and mesons) are composite particles made of quarks For every particle there is an anti-particle Leptons and Baryons obey conservation laws Every force in nature is due to one of four basic interactions: –Stong, electromagnetic, weak and gravitational A particle experiences one of these basic interactions if it carries a charge associated with that interaction

Properties of the basic interactions GravityWeakElectro- magnetic Strong Acts on MassFlavorElectric charge Color Particles participating AllQuarks, leptons Electrical ly charged Quarks, Hadrons Mediating particle GravitonW +, W -, Z 0 PhotonGluon

Grand Unified Theories (GUTs) In a GUT, leptons and quarks are considered to be two aspects of a single class of particle –Under certain conditions a quark could change into a lepton and vice-versa –Particle quantum numbers are not conserved These conditions are thought to have existed in the very early Universe –A fraction of a second after the Big Bang –In this period a slight excess of quarks over anti-quarks existed, which is why there is more matter than anti-matter in out Universe today One of the predictions of GUTs is that the proton will decay after years –In order to observe one decay, a large number of protons must be observed –Such experiments are being attempted

Crib sheet (or what you need to know to pass the exam) The zoo of particles and their properties –Leptons (e -,      e      –Hadrons (baryons and mesons) –Their anti-particles –The conservation laws and how to apply them (energy, momentum, baryon number, lepton numbers, strangeness) Quarks and their properties –Flavors: up, down, strange, charm, top,bottom –How to combine quarks to form baryons and mesons –Quark spin and color –The eight-fold way patterns Fundamental forces and field particles The standard model And from special relativity, its important to understand the concepts of rest mass and energy, and the equations of conservation of relativistic energy and momentum