Download 2.4 Particle interactions

2.4Particleinteractions
Particlesinteractthroughthefourfundamentalforces–theelectrostaticforce,
thestrongnuclearforce,theweaknuclearforceandgravitationalforce.
• Allparticleswithchargefeeltheelectromagneticforce.
• Allparticleswithmassfeelthegravitationalforce.
• Onlyparticlescontainingquarksexperiencethestrongnuclearforce.
• Bothhadronsandleptonsfeeltheweaknuclearforce.
AfamousAmericanphysicist,RichardFeynmansuggestedthattheinteractionis
achievedbytheparticlesexchanging‘virtualparticles’.Inthecaseofcharged
particles(e.g.proton-proton),theelectrostaticforceexperiencedbytheparticles
comesfromaninteractionachievedbytheexchangeof‘virtualphotons’.
Feynmancameupwithavisualwayofdescribingtheseinteractions.Theyare
calledFeynmandiagrams.
BelowisaFeynmandiagramshowingtheelectrostaticinteractionbetweentwo
protons.
p
p
time
γ
p
p
space
Thesediagramsshouldbereadfrombottomtotop.Thisshowstheeventsas
timeprogresses.Wecanseefromtheexampleabovethattwoprotonsmove
towardseachother,exchangeavirtualgamma(γ)photonandthenmoveapart.
(theyrepeleachother).Thewavylinerepresentstheexchangeofavirtual
gammaphoton.
Amodelforexchange
Belowisamodeltopicturehowexchangeofvirtualparticlesresultsinaforceon
twointeractingparticles.
SkaterAthrowsaheavyball
toskaterB.SkaterA
experiencesaforcetotheleft.
Oncatching,skaterB
experiencesaforcetothe
right.
B
A
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(1)!UsingNewton’sthirdlaw,explainwhyAmovestotheleftonthrowingthe
ball.
(2)!UsingNewton’sthirdlaw,explainwhyBmovestotherightoncatchingthe
ball.
(3)!Explainhowthemomentumofthesystem(containingtwoskaters)is
conserved.
Theseconddiagrammodelstheeffectofattractionbetweenparticles(e.g.
electrostaticinteractionbetweentheelectronandproton).PersonAthrowsa
boomerangtotheleft,whichpushesthemtotheright.PersonBcatchesthe
boomerang,whichmovesthemtotheleft.Theoveralleffectisattractive.
B
A
Thestrongnuclearforce
Thestrongnuclearforceactsbetweenallparticlescontainingquarks,whichare
constituentsofparticles(neutronsandprotons)inthenucleus.Itactsovera
veryshortrange.Theexchangeparticleforthestrongnuclearforceisthepion.
(4)!Overwhatrangeisthestrongnuclearforceattractive?
Gravitationalforce
Theexchangeparticleforthegravitationalforceisthegraviton.Thesehaven’t
beendetectedyet.
Theweaknuclearforce
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Theweaknuclearforceisresponsibleforinteractionswhichresultinchangesto
theparticlesinvolved.Wbosonsaretheexchangeparticlesfortheweaknuclear
force.
Consideraninteractionbetweenanelectronneutrino(𝜐" )andaneutron(n).
(Suchinteractionsareextremelyrare.)
𝑛 + 𝜈" 𝑝 + 𝛽 ( Wecanseethattheelectronneutrinointeractswiththeneutrontoproducea
proton(p)andabetaminusparticle(𝛽 ( ).TheFeynmandiagramforthis
interactionisshownbelow:
Note:Itisagoodidea
todrawanarrowto
𝛽( p
showthedirectionthe
exchangeparticle
moves.Inthiscase,we
couldsubstitutea𝑊 * (
𝑊
bosonmovinginthe
𝜐" oppositedirection.
n
WecanseetheWminusexchangeparticle(𝑊 ( )isinvolvedinchanginga
neutronintoaproton.Notethatthechargeisconserved–theoverallchargeis
zerobeforeandaftertheinteraction.
(5)!Ananti-electronneutrino(𝜐" )canalso(rarely)interactwithaproton,
producinganeutronandabetaplus(𝛽 * )particle.A𝑊 * bosonistheexchange
particle.DrawaFeynmandiagramforthisinteraction.
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Theweaknuclearforceisresponsibleforchangeswhichoccurtonucleons
duringbetadecay.Forexample,inbetaminusdecayaneutrondecaystoa
protonandemitsabetaminusparticleandananti-electronneutrino:
𝑛 𝑝 + 𝛽 ( + 𝜐" Wecanshowthis‘weakinteraction’usingaFeynmandiagram:
Inthiscasetheexchange
𝛽( particleisa𝑊 ( boson.
p
𝜐̅" Youcanseethatchargeis
(
conservedasthe𝑊 ( 𝑊 bosoncarriesanegative
n
chargeawayfromthe
neutrontothe𝛽 ( (an
electron).
Inbetaplusdecay,aprotonchangestoaneutron
andemitsabetaplusparticleandanelectronneutrino.
(6)!Writeasymbolequationforbetaplusdecay.
(7)!DrawaFeynmandiagramforbetaplusdecay.
Electroncapture
Insomecases,whereanucleusisprotonrich,aprotonwillcaptureaninner
shellelectronandtransformtoaneutron.
(8)!Whatotherparticleisproducedbyelectroncapture?
(9)!DrawaFeynmandiagramforelectroncapture.
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Electron-protoncollision
Inacollisionbetweenanelectronandaproton,aneutroncanbeproducedand
anotherparticle.
(10)!Whatotherparticleisproducedbyelectron-protoncollision?
(11)!DrawaFeynmandiagramforelectron-protoncollision.
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