c Swinburne University of Technology, 2010 35 OF 1 AGE P Lepton : Super-Kamiokande Collaboration Credit HET614-M03A01: The Particle Zoo: Lend Me a Lepton The Particle Zoo: Lend Me a c Swinburne University of Technology, 2010 35 OF 2 AGE P -decay led to the prediction of neutrinos; β antimatter. HET614-M03A01: The Particle Zoo: Lend Me a Lepton experimental confirmation of the neutrino’s existence; and the discovery of two additional families of leptons. how observations of • • • Before we canmentioned do until that, now – however, we need to briefly discuss a form of matter that we haven’t In this Activity, we will begin toWe examine will the investigate: particle zoo with the family of particles known as leptons. Summary c Swinburne University of Technology, 2010 35 OF 3 AGE P gravity , there also exists a corresponding 1 , so, contrary to popular belief, . However, all the exotic particles that you mass Universe electrons and protons . , or antiparticle. Quantum Numbers . A particle and its antiparticle have the same 2 HET614-M03A01: The Particle Zoo: Lend Me a Lepton Thus far, we have only discussed Particles and antiparticles also have opposite quantum numbers which will be discussed more fully Antiparticles look and behave just like theircharges corresponding matter particles, except they have opposite antimatter particle affects them in the same way. 1 are about to meet in the2 next few Modules also have corresponding antiparticles. in the Activity For all the types of charged particles that exist in the Antimatter c Swinburne University of Technology, 2010 35 OF 4 AGE P . When positron is electrically negative. An antiproton protons is electrically positive whereas an proton and anti- , on the other hand, is positively charged. It is usually referred to as a HET614-M03A01: The Particle Zoo: Lend Me a Lepton electron anti- Positrons For instance, a a matter particle and antimatter particle meet, they annihilate into pure energy. c Swinburne University of Technology, 2010 35 OF 5 AGE P : CERN Electron-positron pairs in bubble chamber. Credit Module near the end of the Unit. For now, we’ll go Cosmology to visit CERN’s website. Particle HET614-M03A01: The Particle Zoo: Lend Me a Lepton Click here The idea of antimatter isto strange, be especially composed because entirely the ofmatter, Universe matter. then appears If where there has is it as all much gone? antimatterYou as can there see is evidencewhich for shows antimatter the in productionin of this this electron-positron early chamber pairs. bubble makesparticles chamber The negative (positrons) magnetic photo, particles curl field right. (electrons) curl leftYou and will positive learnModules, more and about we’ll get differentin back the types to discussing of the antimatter matter-antimatter imbalance in the next few on to investigate the “standard model” of particle physics. Anti-matter really exists c Swinburne University of Technology, 2010 35 OF 6 AGE P HET614-M03A01: The Particle Zoo: Lend Me a Lepton In the next few Activities,the we last will 100 investigate years. the “zoo” of particles which have been discovered over The Standard Model ofinteractions particle between them. physics attemptswe The know to about: Standard describe electromagnetic, Model weak the and is fundamental strong able nuclear particles forces. to . . and explain the but nearly not gravity. all of the interactions The Standard Model c Swinburne University of Technology, 2010 35 OF 7 AGE P . The neutron has no neutron . 1 HET614-M03A01: The Particle Zoo: Lend Me a Lepton Electrons do not “feel” the strong nuclear force Nucleons “feel” the strong nuclear force. • • Neither do positrons. To see what wecharge, mean and by a so particle therewould say “feeling” is that a no the force, neutron electromagnetic does consider1 force not the feel between the electromagnetic neutrons force. and charged particles. We There is one(collectively fundamental called nucleons). difference between electrons as compared to protons and neutrons Fundamental differences c Swinburne University of Technology, 2010 35 OF 8 AGE P which carries the electromagnetic photon . An important gauge boson is the are particles that “feel” the strong force. are particles that do not “feel” the strong force. gauge bosons HET614-M03A01: The Particle Zoo: Lend Me a Lepton Hadrons Leptons • • There is an additional categorycalled for some special particles which “carry” forces. These particles are In turns out that all material particles can be classified into one of two groups: Hadrons and Leptons force between charged particles. c Swinburne University of Technology, 2010 35 OF 9 AGE P , which we discussed in the 2 − + p n e e γ +1 938.27 Hadron ParticleProton TypeNeutron Charge MassElectron Hadron 0 Symbol LeptonPositron -1 LeptonPhoton +1 939.56 Boson 0.511 0 0.511 0 HET614-M03A01: The Particle Zoo: Lend Me a Lepton (The particle masses in this table have been quoted in units of MeV/c previous Module.) In the following Activities, weis are the going first: to be showing you many tables of particle properties. Here Classifying particles c Swinburne University of Technology, 2010 35 OF 10 AGE P -decay process. β -decay: β HET614-M03A01: The Particle Zoo: Lend Me a Lepton Z is the Atomic Number = ofA protons. is the mass number = of protons + of neutrons. -Decay and a new particle • • β We have already met two verydiscovered important from leptons: observations the of electron the and the positron.Consider The the next following lepton was c Swinburne University of Technology, 2010 35 OF 11 AGE P ). e + m p (m > n − p + e → n is much heavier than the emitted electron, we can neglect the atom’s atom Mass(Atom: Z, A) = Mass(Atom: Z+1, A) + total energy of the electron -decay process is the conversion of a neutron into a proton, where the mass of the neutron is HET614-M03A01: The Particle Zoo: Lend Me a Lepton β Because the decaying recoil. Then by conservation of energy arguments, we have: greater than the combined mass of the proton and electron: m The A question of mass c Swinburne University of Technology, 2010 35 OF 12 AGE P kinetic K + E ) and its 2 2 c c e e m = m e -decay processes was found to take on a range β . . K K Mass(Atom: Z+1, A) – Mass(Atom: Z, A) = total energy(electron) = (or energy of motion), E HET614-M03A01: The Particle Zoo: Lend Me a Lepton energy But the observed kinetic energy of the electron in In this scenario, the electron kinetic energy has a unique value: of values up to the predicted E The total energy of the electron includes both the electron mass (by using E Energetic electrons c Swinburne University of Technology, 2010 35 OF 13 AGE P . Note that very few electrons K Pb. 214 -decay looks like this: -decay of β β HET614-M03A01: The Particle Zoo: Lend Me a Lepton It tells us how many electrons will occur with particular value of E have the predicted energy!Chadwick from The observations of continuous the energy spectrum was discovered in 1914 by James The electron energy spectrum from The electron energy spectrum c Swinburne University of Technology, 2010 35 OF 14 , AGE P a two body not electron antineutrino -decay is β − . More precisely, it is an something else p + e + → − n neutrino p + e → n . The “bar” indicates an antiparticle. e ¯ ν is a particle called a -decay is not β something else HET614-M03A01: The Particle Zoo: Lend Me a Lepton process. In other words, as we first wrote it, but is actually: This The total electron energy spectrum suggests something important: What does it all mean? which is given the symbol: c Swinburne University of Technology, 2010 35 OF 15 AGE P -decay spectrum. Ernest Rutherford β ` e -decay electrons were emitted with the same energy, but lost some of this energy β Visual Archives Ernest Rutherford. Credit: AIP Emilio Segr HET614-M03A01: The Particle Zoo: Lend Me a Lepton The neutrino was not the only explanation for the continuous (Re)Discovering the neutrino thought that all depending on the thickness of “atomic material” they had to pass through. c Swinburne University of Technology, 2010 35 OF 16 AGE P -decay β ´ e Deutsch, London as an alternative in 1933. , Andr neutrino : CERN Wolfgang Pauli Credit 1 Telephone Poles and Other Poems ” in : AIP Emilio Lise Meitner (ca. 1928) Credit Segre Visual Archives ”. Cosmic Gall little neutral one HET614-M03A01: The Particle Zoo: Lend Me a Lepton J.Updike, from “ In December 1930, Wolfgang Pauli proposedGeiger the and existence of Lise the Meitner neutrino (who in an had open made letter important to Hans contributions to the understanding of “Neutrinos, they are very small” herself). Pauli originallydiscovered called what we the now particle call a a neutron! “neutron”, Enrico Fermi as proposed this was two years before Chadwick (1964). 1 It means “ c Swinburne University of Technology, 2010 35 OF 17 AGE P . 1 HET614-M03A01: The Particle Zoo: Lend Me a Lepton Electron antineutrino takes away nearly all of theElectron energy, and leaving electron electron antineutrino “at both rest”. have non-zero kineticElectron energy. takes away nearly all of the energy, leaving the electron antineutrino “at rest” We have to say “at rest” because it is unlikely that either particle won’t have some small amount of 1: 2: 3: 1 kinetic energy. We can now interpret the electron energy spectrum in terms of the neutrino: Understanding the electron energy spectrum c Swinburne University of Technology, 2010 35 OF 18 AGE P ? Of course there is! e ν e e ν ν + + ¯ + − -decay -decay n + e p + e + − β β → → p n -decay process involves the decay of a neutron into a proton, an β -decay process which turns a proton into a neutron, a positron and an electron β second HET614-M03A01: The Particle Zoo: Lend Me a Lepton neutrino. We can denote these two decays as: We have now seen that the Antineutrinos and neutrinos electron and an electron antineutrino. Is there also an electron neutrino, There is a c Swinburne University of Technology, 2010 35 OF 19 AGE P ) ) e e ν ¯ ν m m + + + e e m m + + p n m m ( ( > < n p m m in a moment...) e ¯ ν m -decay process can occur in free space. That is, a lone neutron away from any other particles − HET614-M03A01: The Particle Zoo: Lend Me a Lepton β But (We will justify the addition of so the proton decay canbinding energy only of occur the when atom can the provide proton the is additional mass-energy part required. of an atom. In this case, the nuclear The can spontaneously decay into a proton, electron andAs electron we antineutrino. stated earlier, this is because No free proton decay c Swinburne University of Technology, 2010 35 OF 20 AGE P : AIP Emilio Segre Fred Reines Credit Visual Archives is a significant Sun the , Astronomy Neutrino -decay spectrum, is that neutrinos only interact very β . 1 HET614-M03A01: The Particle Zoo: Lend Me a Lepton As we will see in the Activity Experimental confirmation source of neutrinos, but mostthere of was them no pass matter through in the the Earth way. (and you!) as if Although Pauli’s proposed neutrinospectrum, seemed direct to experimental evidence explain1959, for the nearly the 30 electron neutrino years energy later. did not appearThe until reason itits took existence so from long the to directly detect a neutrino, rather than infer weakly with matter Fred Reinesat and Savannah Clyde River,of Cowan’s North uranium. experiment Carolinaantineutrinos. used with These neutron-rich a1 fission fragments nuclear fragments produce reactor copious numbers of electron c Swinburne University of Technology, 2010 35 OF 21 AGE P . 1 Particle Physics: Detectors. sec) between the two photon production events: called a “coincidence” µ HET614-M03A01: The Particle Zoo: Lend Me a Lepton We will describe scintillators in the Module: There is a time delay (a few detection. In a 200 hour experiment,1 567 coincidence signals were detected! Occasionally the antineutrinos would interactcoincidence with events protons that within were measured the with detector layers apparatus, of producing liquid scintillators c Swinburne University of Technology, 2010 35 OF 22 AGE P -decay process now β 2 c e ¯ ν m = e ¯ ν E determined from the simple two-body K , and therefore rest energy: e ¯ ν m has a mass, e ¯ ν HET614-M03A01: The Particle Zoo: Lend Me a Lepton includes the contribution from the electron mass which we neglected before. To be rigorous here, the “predicted” E Suppose that What is the mass ofspectrum the again: electron neutrino (or antineutrino)? Let’s look at the total electron energy The Mass of the neutrino c Swinburne University of Technology, 2010 35 OF 23 AGE P K muon’s discovery µ ( muon , which is about 200 times heavier than the 2 HET614-M03A01: The Particle Zoo: Lend Me a Lepton occurred between 1934, when themassive first particle, cosmic and ray 1937 tracks when revealed conclusive a results were published. Anderson and Nedermeyer wererays, and examining discovered the a tracksa particle mass of of which 105.66 cosmic MeV/c had a negative charge, but electron! This new particle was called the Muons c Swinburne University of Technology, 2010 35 OF 31 AGE P seconds. 6 − 10 × µ µ ν ν + + ¯ e e ν ν + ¯ + − + and e e → → + − µ µ , we will see why the decays take this form. : 1 Quantum Numbers is the antiparticle of the muon: the antimuon. + µ HET614-M03A01: The Particle Zoo: Lend Me a Lepton In the Activity where 1 Muons decay in the following way Provided its higher massidentical to is those taken of the into electron! account, WeThe can the muon think is electromagnetic of highly the properties unstable, muon and as of decays a with the “heavy a electron”. muon very short are lifetime of about 2.2 A heavy electron c Swinburne University of Technology, 2010 35 OF 32 AGE P , was discovered in 1975 from 2 + τ + − τ → + e + ) with mass 1777 MeV/c − τ ( e seconds, and typical decays are : 13 − tauon 10 × has a lifetime of only 3 HET614-M03A01: The Particle Zoo: Lend Me a Lepton τ The Tauon decay is much moredue complex to than the muon extra decay, mass as of there the are tauon. several possible decay modes experiments involving collisions of beams ofin electrons the and interaction was positrons. greater Provided than the 3.6 energy GeV, available a tauon and an antitauon were produced: An even “heavier electron”, the Heavier electrons c Swinburne University of Technology, 2010 35 OF 33 AGE P (and 1 ) but also antileptons - τ and − µ , − e ¯ ν τ µ , ¯ ν ¯ ν e , , τ µ , ν + , ν , ν + + , e − , τ , µ e − − τ µ ). And each of the “heavy electrons” is partnered by corresponding neutrinos Muon family: Tauon family: Electron family: + τ and + µ HET614-M03A01: The Particle Zoo: Lend Me a Lepton , + The existence of the tau neutrino was confirmed by a direct observation at Fermilab in 2000. Unfortunately we docorresponding not neutrinos. We know will see why another1 example there of this are “tripling” in two the next heavier Module. versions of the electron and their antineutrinos). We now have quite a collection of leptons, with three different “families”: (e As these last few reactions show, there are not only leptons (e Antileptons c Swinburne University of Technology, 2010 35 OF 34 AGE P − µ e τ − - ν µ ν τ ν 2 2 2 3eV/c 190keV/c 18.2MeV/c < < < ParticleElectron Type Lepton Charge Mass -1 0.511 Symbol e Neutrino Lepton 0 Muon neutrinoTauon Lepton 0 Lepton -1 1777.03 MuonTauon neutrino Lepton Lepton -1 0 105.66 except where noted. 2 HET614-M03A01: The Particle Zoo: Lend Me a Lepton Although the muon andcharged tauon elementary particle, can and decay so into it is lighterOur stable. particles, final There summary is the table nothing for electron lighter thisand it is Activity tauon can includes the neutrinos: decay the into! lightest experimental limits known on the masses of the muon The stable electron Mass in MeV/c c Swinburne University of Technology, 2010 35 OF 35 AGE P HET614-M03A01: The Particle Zoo: Lend Me a Lepton We have now looked at the threenuclear families force. of leptons. These are particles whichIn do the not feel next the Activity, strong we turnfundamental particles. our attention to the hadrons, and introduce some more ways to classify Coming up after the break