The Standard Model Particle Physics ☞ The Nucleus The nucleus consists of protons and neutrons, which are collectively known as nucleons. The following notation represents the nucleus of the atom: Mass Number or Nucleon Number Element Symbol Proton Number or Atomic Number ☜ ☞ Isotopes An isotope is any of two or more forms of a chemical element. They have the same number of protons in the nucleus, but have different numbers of neutrons. ☜ ☞ Constituents of the Atom Proton Neutron Relative SI Unit Mass 1 1.673 x 10-27 kg Charge 1 1.6 x 10-19 C Relative SI Unit Mass 1 1.675 x 10-27 kg Charge 0 0 Electron ☜ Relative SI Unit Mass 0.0005 9.11 x 10-31 kg Charge -1 -1.6 x 10-19 C ☞ Fundamental Particles Fundamental (elementary) particles are those which are not composed of other particles. The atom consists of fundamental and non-fundamental particles ☜ ☞ Strong Force The nucleus is held together by the strong force. • • Highly attractive between two protons within 1-2 femtometres Repulsive between two protons at smaller separations (cannot overlap) It is extremely short range and has no effect outside of the nucleus ☜ • ☞ Exchange Particles It is thought that exchange particles, called gluons, are responsible for the Strong Force. They act between the quarks in a neutron or proton. A gluon is a particle within the category of force-carrier particles called gauge bosons. ☜ ☞ Four Fundamental Forces There are four fundamental forces that act within a nucleus. Each is thought to have their own set of exchange particles, which “carry” the force: Fundamental Force: Acts On: Exchange particles (gauge boson): Strong Quarks Gluons Gravitational All particles with mass Gravitons (not yet observed) Electromagnetic All particles with charge Photons Weak Leptons W and Z Bosons ☜ ☞ Photon = Exchange Particle A photon is a discrete packet of electromagnetic radiation. The energy of each photon is determined by: h = Planck’s Constant = 6.63x10-34Js f = frequency of light λ = wavelength of light ☜ The photon is the exchange particle for the electromagnetic force. ☞ Unstable Nuclei Nuclei (or isotopes) with more than 83 protons are unstable. The strong force is not great enough to hold the nucleus together permanently. In this case, the nuclei are radioactive. They emit alpha, beta or gamma radiation to become more stable. ☜ ☞ Beta Decay One of the ways an unstable nuclei becomes more stable is to emit β¯ and β+ particles. β¯ Decay β+ Decay Neutron decays into a proton Proton decays into a Neutron “down” quark → “up” quark “up” quark → “down” quark Caused by Weak Interaction Caused by Weak Interaction ☜ The conservation of energy in these decays, led to the neutrino being discovered. ☞ Particles and Antiparticles β¯ and β+ decay highlights the existence of particles and antiparticles (e.g. ) Every particle has a corresponding anti-particle, with: 1. Same Mass 2. Opposite Charge ☜ Particle Symbol Rest Mass / Charge MeV electron 0.510999 - positron 0.510999 + proton 938.257 + antiproton 938.257 - neutrino 0 0 antineutrino 0 0 ☞ Annihilation When a particle and its antiparticle meet each other they annihilate each other. Two identical photons are created. The mass of the particles is converted into energy in the form of photons. ☜ ☞ Annihilation Question Calculate the wavelength of the released photons, when an electron and positron annihilate. ☜ Click Here for Answer ☞ Pair Production The opposite can also happen… High energy photons can produce a particle and its antiparticle, this is called pair production. The energy of the photons is converted into mass in the form of particles. ☜ ☞ Quarks and Antiquarks Quarks (and antiquarks) are thought to be fundamental particles that make up other particles. There are three common quark ”flavours”. Flavour Symbol charge (Q) Baryon Number (B) up u +2/3 +1/3 0 down d -1/3 +1/3 0 strange s -1/3 +1/3 -1 Strangeness (S) Each quark has a corresponding antiquark, with opposite charge and equal mass. ☜ ☞ Hadrons Hadrons are a group of particles, with the following qualities: • They are affected by the strong force • They are made up of quarks • Hadrons have two subsets: Baryons and Mesons • Protons and neutrons are hadrons. • Most hadrons are very unstable and decay instantly into different particles • Protons and neutrons are the only hadrons that are relatively stable (although neutrons are only stable when within a nucleus). ☜ ☞ Particle Colliders Particle accelerators have been designed to collide hadrons (Large Hadron Collider at CERN) and observe decay. Typical decay reactions are: ☜ ☞ Strange Particles In certain hadron reactions, scientists noticed that some particles ( ) decayed very slowly and were always discovered in pairs. They thought it very strange and called them Strange Particles! • A strange particle is given a strangeness quantum number of S=1 • Strangeness is conserved in all hadron reactions • Strange particles are produced by the strong force, but decay through the weak force. ☜ Strange ☞ Conservation Laws It was noted when studying these hadron reactions that : • • • • • Electrical charge is always conserved The baryon number is always conserved The total number of mesons may change Strangeness is always conserved Energy and momentum are always conserved in all interactions ☜ ☞ Baryons Baryons are a subset group of Hadrons • They have a Baryon Number = 1 • They are made up of three quarks Proton • Anti-baryons are made up of the corresponding anti-quarks • Protons and neutrons are baryons. • The proton is the only stable baryon, into which other baryons eventually decay. ☜ Anti-Proton ☞ Baryons Baryon Symbol Baryon No. (B) Charge (Q) Strangeness (S) Quark Structure neutron n0 1 0 0 udd proton p+ 1 +1 0 uud delta-plus ∆+ 1 +1 0 uud delta-zero ∆0 1 0 0 udd delta-minus ∆- 1 -1 0 ddd lambda 𝚲 1 0 -1 uds sigma-plus ∑+ 1 +1 -1 uus sigma-zero ∑0 1 0 -1 uds sigma-minus ∑- 1 -1 -1 dds ☜ ☞ Each baryon has a corresponding anti-baryon, with opposite charge and quark structure. Mesons Mesons are a subset group of Hadrons • They have a Baryon Number = 0 K+ • They are made up of one quark and one anti-quark • Anti-mesons are made up of the corresponding opposite quarks • All mesons are unstable. K- • They are affected by the strong force. ☜ ☞ Mesons Meson Symbol Baryon No. (B) Charge (Q) Strangeness (S) Quark Structure kay-plus K+ 0 +1 1 us kay-zero K0 0 0 1 ds kay-minus K- 0 -1 1 su phi ɸ0 0 0 0 ss pi-plus π+ 0 +1 0 ud pi-zero π0 0 0 0 uu or dd pi-minus π- 0 -1 0 du Each meson has a corresponding anti-meson, with opposite charge and quark structure. ☜ ☞ Leptons Leptons are a different family of sub-atomic particles that are affected by the weak force. • They are not affected by the strong force and do not exist within the nucleus. • They are fundamental particles • There are only 6 leptons, each with their own anti-particles. • All mesons are unstable. ☜ Lepton Symbol electron e electron neutrino νe μ νμ 𝜏 ν𝜏 muon muon neutrino tau tau neutrino Annihilation Answer The energy from each particle converts to the energy of each photon. Click Here to Return to Slides
