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Particle Physics Handout 8 the Weak Force

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Particle Physics Handout 8 the Weak Force Subatomic Physics: Particle Physics Handout 8 The Weak Force Weak interactions W and Z interactions at low energy Fermi theory Electroweak theory W and Z bosons at high energy 6 6 SELF-INTERASELF-INTERACTIONSCTIONS hhhljkhsdlkh 1 At this point, QCDhhhljkhsdlkhlooks like a stronger version of QED. At this point, QCD looks like a stronger version of QED. This is true up to a point. However, in practice QCD This is true up to a point. However, in practice QCD behaves very differently to QED. The similarities arise from behaves very differently to QED. The similarities arise from QCD Summarythe fact that both involve the exchange of MASSLESS the fact that both involve the exchange of MASSLESS spin-1 bosons. The big difference is that GLUONS carry spin-1 bosons. The big difference is that GLUONS carry colour “cQuarksharge”. and gluons carry QCD: Quantum Gluonscolour are“c theharg e”. colour charge. GLUONS CAN INTERACT WITH OTHER GLUONS: Chromodymanics is the propagatorGLUONS of theCAN INTERACT WITH OTHER GLUONS: quantum description of strong force Gluons self-interact:g g the strong force. g gg g g Only quarks feel the g g g strong force. g gg g 3 GLUON VERTEX 4 GLUON VERTEX 3 GLUON VERTEX 4 GLUON VERTEX Hadrons can be Electromagnetic coupling constant ! decreasesEXAMPLE: Gluon-Gluon Scattering • EXAMPLE: Gluon-GluondescribedScattering as consisting as a charged particles get further apart. of partons: quarks and •Strong coupling constant !S increases as gluons, which interact + + further apart quarks become. + independently+ Quarks and gluons produced in Colour Confinement collisions hadronise: hadrons are energy required to separate produced. quarks ! " The decay products of the hadrons quarks are confined to hadrons appear e.g.in ther g detector+e.g.gb→r gr as+r+g jetsrbb→.r r+r b 2 Dr M.A. Thomson Lent 2004 Dr M.A. Thomson Lent 2004 3 ! Two types of WEAK interaction Introduction to the CHARGEDWeak CURRENTForce(CC) - Bosons NEUTRAL CURRENT (NC) - Boson The weak force is responsible for some of the most important phenomena: • Decays of the muon and tau leptons ! WEAK force mediated by MASSIVE VECTOR • Neutrino interactions BOSONS:Boson W± Z0 Decays of the lightest mesons and baryons • Mass Radioactivity, nuclear fission and fusion 80.4 91.2 • GeV/c2 Characteristics of Weak Processes: ! e.g. the WEAK interactions of electrons and • Long lifetimes 10#13 - 103 s charge, e ±1 0 electron neutrinos: #13 • Small cross sections 10 mb - spin 1 1 - e νeℏ ℏ e νe W- Z0 Z0 W+ Weak Force is propagated by massive W+, W# and Z0 bosons + + ± 0 ν ν e e • The interactions of W and Z are different e e (related by symmetry of the weak interaction)BOSON SELF-INTERACTIONS W- W- W± and Z0 can interact with each other 0 • Z γ ± ± • W and " can interact (as W bosons are charged) + + W W 6 W- WSummar- Z0 yWo-f Standarγ Wd- Modelγ VWer- tices 3 ! At this point have discussed all fundamental fermions W+ andW+theirZ0interactionsW+ withZ0 the fWor+ce carrγ ying bosons.W+ Weak Vertices! Interactions characterized by SM vertices ! also interactions with PHOTONS (W-bosons ELECTROMAGNETIC (QED) QED are charged)W-boson- e q Couples to CHARGE mediated by the exchange of mediated -by thee exchangeQ of e W e Does NOT change virtual photons boson q Dr M.A. Thomson FLAVOUR Lent 2004 e2 α = γ γ acts on all charged particles acts on all 4quarkπ and leptons 6 STRONG (QCD) coupling strength e !! coupling strength gW $!W Summary!of !Standard Model Vertices ! ! q Couples to COLOUR g ! At this point 2have discussed2 2 all fundamental fermions 2 s2 Does NOT change propagator term: 1/(q #m" )= 1/q propagator term: 1/(qq#mW ) and their interactions with the force carrying bosons. FLAVOUR g 2 α = s g For! manyInteractions processes:characteriz ed by SM verticesFor smany4π processes: 2 2 2 2 2 ELECTRM ∝ OMAe /q GNETIC (QED) WEAKM ∝ gCharW /(qg#emd WCurrent) - e q Couples to CHARGE νe d’ Changes FLAVOUR g V - e Q e - gw w ckm e q Does NOTe change u For QUARKS: coupling 2 FLAVOUR - BETWEEN generations e g 2 W + α = γ γ α = w W 4π w 4π STRONG (QCD) Recall: matrix element, M, is the amplitudeWEAK ofNeutral a process.Current q - Scattering cross section, $ !M2.Couples Decay to width,COLOUR !e !, ν M2 q - e gs e , Does NOT changeνe 4 q Does NOT change FLAVOUR q g 2 FLAVOUR α = s g 0 s 4π Z Z0 WEAK Charged Current Dr M.A. Thomson Lent 2004 νe d’ Changes FLAVOUR g V - gw w ckm e u For QUARKS: coupling - BETWEEN generations g 2 W + α = w W w 4π WEAK Neutral Current - e , ν q - e e , νe q Does NOT change 0 FLAVOUR Z Z0 Dr M.A. Thomson Lent 2004 6 Summary of Standard Model Vertices ! At this point have discussed all fundamental fermions and their interactions with the force carrying bosons. ! Interactions characterized by SM vertices 6 Summary oELECTRf StandarOMAdGNETICModel(QED)Vertices - e q Couples to CHARGE ! At this point have discussed all fundamental fermions - e Q e and their interactionsewith the force carrq ying bosons. Does NOT change FLAVOUR ! Interactions characterize2ed by SM vertices α = γ γ ELECTROMAGNETIC4π(QED) - STRONG (QCD) e q Couples to CHARGEq Couples to COLOUR - e Q e e ± Doesg NOTs change Interactions of the W bosonq q Does NOT change 2 FLAVOUR FLAVOUR e γ g 2 α = α = s γ g • Known as “charged current interactions” 4π s 4π STRONG (QCD)WEAK Charged Current W± boson interacts with all fermions (all quarks and leptons) q 6 • νe Couples to COLOURd’ Changes FLAVOUR gg g V Summar- sw y of StandarDoesw NOTckm changed Model Vertices • Charged current changes the flavour of the fermion: qe u For QUARKS: coupling FLAVOUR BETWEEN generations e.g. electron emitting an W-boson can’t remain2 an electron! At this2point ha-ve discussed all fundamental fermions • gs g W + - violates conservation of charge! α = α = w g W s 4π and wtheir4πinteractions with the force carrying bosons. an electron turns into a electron neutrino 6 • WEAK Charged!CurrentInteractions characterized by SM vertices • an up quark turns into a down quark WEAK Neutral Current Summaryν oELECTRf StandarOMA-dd’GNETICModel(QED)Vertices and vice versa! e e , νe Changes FLAVOURq g g- V - ! At- thiswpoint have ediscussedw, νeckm alle fundamental fermionsq Couples to CHARGE e u For QUARKS: couplinDoesg NOT change • Coupling strength at every vertex ! gW (more about quarks later) - q and their interactions withethe force carrBETWEENyingQ ebosons. generations 1 2 - e 0 q FLAVOURDoes NOT change gw W Z + 0 • Propagator term describing the W-boson !! α(Interactionsqw2 = m2 ) characteriz2ed by WSM vertices Z FLAVOUR −4π W e γ q is the four-momentum transferred by the W-boson α = γ • ELECTROMADrGNETICM.A. Thomson4π(QED) Lent 2004 WEAK Neutral- Current -e STRONG (QCD)q Couples to CHARGE e , νe q - 5 q e-, ν e Q e Couples to COLOUR e e q Doesg NOT change q Does NOTs change Does NOT change 2 qFLAVOUR e γ0 FLAVOUR FLAVOUR Allowed Flavourα = ChangesZ g 2 γ0 4π α = s Z g s 4π At a W-boson vertex: STRONG (QCD) Dr M.A. Thomson WEAK Charged Current Lent 2004 • Lepton numbers: Le, Lµ and L%, is conserved: q Couples to COLOUR " # # νe d’ Changes FLAVOUR Allowed lepton flavour changes: e #&e µ #&µ % #&% gs g V q- ggWw Doesw NOTckm change e For QUARKS: coupling uFLAVOUR g 2 s 2 - BETWEEN generations αs = gg 2 Wg + • Total Quark Number, Nq, is conserved 4π α = wW W αW w= 4π • Individual quark flavour numbers: NuWEAK, Nd, Ns, NCharc, Nb,g Net d Current4π are not conserved WEAK Neutral Current νe -d’ Changes FLAVOUR Allowed quark flavour changes: e , ν q - g g-wWVVckmud e (Q=+2/3 e quark) # (Q="1/3 e quark) w e ue , νe For QUARKS: coupling Does NOT change ( d s b ) # ( u c t ) - BETWEENq generations g 2 W + FLAVOUR Each of the nine possible quark flavour αchanges = w has a W0 • w 4π Z 0 different coupling strength: e.g. gWVud for u to d quarks Z (Vs are terms in CKM matrix - more later) WEAK NeutralDr M.A.CurrentThomson Lent 2004 • Main quark flavour changes are within a generation: - e , νe q d # u s # c b # t - e , νe q 6 Does NOT change 0 FLAVOUR Z Z0 Dr M.A. Thomson Lent 2004 6 Summary of Standard Model Vertices ! At this point have discussed all fundamental fermions and their interactions with the force carrying bosons. 6 ! Interactions characterizSummared byySMoverf Standartices d Model Vertices ELECTROMAGNETIC (QED) - ! At this point have discussed all fundamental fermions e q Couples to CHARGE and their interactions with the force carrying bosons. - e Q e 6 e ! Interactionsq characterizDoes NOTed bchangey SM vertices FLAVOUR e2 Summary of Standard Model Vertices α = γ ELECTROMAγGNETIC (QED) 4π - ! At this pointehave discussed allq fundamental fermions STRONG (QCD) Couples to CHARGE and-theireinteractions withQ thee force carrying bosons. e q Couples to COLOURDoes NOT change ! Interactions characterizq ed by SM vertices g FLAVOUR q e2 s Does NOT change ELECTRα = OMAγGNETIC (QED)γ 4π - FLAVOUR g 2 α = s eg q Couples to CHARGE s 4π STRONG (QCD) - e Q e q WEAK Charged Currente q DoesCouples NOT to changeCOLOUR ν 2 d’ gs FLAVOUR e e Changesq FLAVOURDoes NOT change αg = V γ γ - gw w4πckm FLAVOUR e u g 2 For QUARKS: coupling STRα =ONGs (QCD) g s 4 BETWEEN generations 2 - π gw W + q Couples to COLOUR αw = WEAK ChargWed Current 4π g 0 ν s d’ Does NOT change Interactions of theWEAK ZNeutral bosonCurrent e q Changes FLAVOUR - g g V FLAVOUR - 2w w ckm e , νe e gs q For QUARKS: coupling - αs = u g Known as “neutral current interactions” e , νe 4π BETWEEN generations • 2 - WEAKq gwChargWed CurrentDoes NOT change+ αw = W • Acts on all fermions - (all quarks and leptons) 0 4π FLAVOUR Z νe0 d’ Changes FLAVOUR • Neutral current conserves flavour of the fermion Z g V WEAK- gNeutralw Currentw ckm e - For QUARKS: coupling No allowed fermion flavour changes Dr M.A.
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