Download Par cles and Interac ons

Par$clesandInterac$ons
Prof.MarinaCobal
FisicaSperimentaleNucleareeSubnucleare
2016/2017
Whatistheworldmadeof?
•  Intheancient*me:4elements
•  19°century–atoms
•  Beginning20thcentury–
electrons,protons,neutrons
•  Today–quarksandleptons
Allthingsaroundusaremadeofatoms
Human Hair
~ 50 µm = 50 10-6 m
= 0.000050 m
Atom ~ 10-10 m
= 0.0000000001 m
Magritte
Theatominthe20thcentury...
•  Atoms reacts through chemical reactions
•  More than 100 atoms known (H, He, Fe …)
•  The internal structure is not well known
Tableofelements
•  Atomsaregroupedinfamilieswhichpresentsimilarproper*es
Theelementstableisdone
•  Thissymmetrysuggestsastructurewithsimplercons*tuents.
AtomicModel
•  Withtheexperimentsnowweareableto
“breakup”atoms
•  Lightpar*cles(electrons)withnega*ve
chargearoundaposi*veandheavy
nucleus
•  Prac*cally,theatomisempty!
Rutherford:atomsarenotelementary
par*cles!
1911
Rutherford found a nucleus in the atom
by firing alpha particles at gold and
observing them bounce back
Precursor of modern scattering experiments at accelerator
Atoms
Atoms are all similarly made of:
- protons and neutrons in the nucleus
- electrons orbiting around
proton
The electron was the
first elementary
particle to be
discovered
(JJ Thomson 1897)
Protons,
neutrons are
made up of
quarks
electron
neutron
Nucleus
•  Thenucleusissmallanddense.For
awhileitwasthoughttobepointlike.
•  However,thereweresomany
differentnucleaasmanyatoms
•  Simplifica*on:allnucleaaremade
ofneutronsandprotons!
NewPar$cles
•  CollisionsofeleYronsandnucleainthecosmicraysandin
thepar*cleacceleratorsatthebeginningofthe30ies,
broughttothediscoveryofmanyotherpar*cles.
•  Someofthemwerepredicted,otherwerediscoveredas
surprises,beingcompletelyunexpected.
•  Atthebeginningitwasthoughtthatallthesepar*cles
werefundamentals.
Atthebeginningjustafew…
And many others…
Introducingthequarks..
"Youngman,ifIcouldremember
thenamesofthesepar8cles,
Iwouldhavebeenabotanist!“
E.Fermitohisstudent
L.Lederman(bothNobellaureates)
ThePar*clePhysicist’sBible:
Par*cleDataBook
hYps://pdg.lbl.gov
Most particles are not stable and can decay to lighter particles..
Quarks
•  Todayweknowthatalsoprotonsand
neutronsarenotfundamentalunits.
•  Theyaremadeofsmallerpar*clescalled
quarks
•  Forthemomentlookslikequarksare
pointlike
Sub-atomicdimensions
TheModernAtom
•  Acloudofelectronsmovingconstantly
aroundthenucleus
•  Protonsandneutronsmovinginthe
nucleus
•  Quarksmovinginprotonsand
neutrons
Whatisfundamental?
•  Physicistshavefoundhundredsofnewpar*cles.
•  Todayweknowthatmostofthemarenotfundamental
•  Atheoryhasbeendevelopedthatseemstoexplainquitewell
whatwedoobserveinnature:thetheoryiscalledStandard
Model
•  Thismodelincludes6quarks,6leptonsand13par*cleswhich
carrytheforceinbetweenquarksandleptons.
IsthewholeUniversemadeonlyof
quarksandelectrons?
No!Therearealsoneutrinos!
ν
Electron, proton and neutrons are rarities!
For each of them in the Universe there is 1 billion neutrinos Neutrinos are the most abundant matter-particles in the
Universe!
Within each cm3 of
space: ~300 neutrinos
from Big Bang
ν ν ν ν ν νννννννν
νν ν ν νν ν ννννννν
ν νν νν νν ν ν ν ν ν νν ν ν νν ν νννννν
ν νν νν νν ννννννν
νν νν ν ννν ννννννν
νν νν ν ννν ννννννν
1 cm
1 cm
Neutrinos are everywhere!
in the outer space, on Earth, in our bodies..
Neutrinosgetunderyourskin!
Every cm2 of Earth surface is
crossed every second by more
than 10 billion (1010) neutrinos
produced in the Sun
1014 neutrinos
per second from Sun
are zipping through you
Within your body at any
instant: roughly 30 million
neutrinos from the Big Bang
ν
No worries!
Neutrinos do not harm us.
Our bodies are transparent to neutrinos
QuarksandLeptons
Structurelessbuildingblocksdowntoaspa*alextensionof10-18m
Welldefinedspinandcharge
Leptonshavewelldefinedmassaswell
LowMass
MaYer
Cons*tuentsunder
ordinarycondi*ons
(lowenergy/T)
Cons*tuentsofunstable
par*cle(producedathigh
energy,inastrophysical
systems).Theydecayto
lowermasspar*cles
HighMass20
Families
•  The6quarksandleptonsare
organizedinfamilies
•  The3familieshaveanalogies
•  Quarkshavecharge+2/3e-1/3.
Leptonshavecharge-1and0.
Fermions
Elementarypar*cles:fermions
1st generation
2nd generation
Massa (MeV)
Why3families?Are
theremore?
3rd generation
Charge (e)
2/3
2/3
-1/3
-1/3
0
0
-1
-1
Quarks
Leptons
Whatistheworldmadeof?
•  Realworldisnotdonebysinglequarks
•  Quarksexistonlyingroups,toformtheso-calledhadrons
(protonsandneutronsarehadrons)
•  Example:aprotonismadeoftwoquarksofuptypeand
onequarkoftypedown.
•  ThemaYeraround,andeveneachofus,ismadeof
quarksupanddownandofelectrons.
Areduc*onistexample:
theDeuteriumAtom
p = (u, u, d )
n = (u, d , d )
10-15 m
10-10 m
Andtheleptons?
•  Thereare6leptons:3chargedand3neutral.
•  Theylooklikepointlikepar*clewithoutaninternal
structure.
•  Electronsarethemostcommonandarefoundinthe
ordinarymaYer.
•  Muons(µ)andtaus(τ)areheavierandchargedasthe
electrons.
•  Neutrinos(ν)havenochargeandtheyhaveanextremely
smallmass.
Wave-par$cledualityofNature
Central concept of quantum mechanics:
all particles present wave-like properties
λ
Not only light has a dual nature
De Broglie showed that moving particles have an
equivalent wavelength λ
1
λ∝
p
So high momentum gives us
short wavelengths so we can
make out small details
Example: electron microscope
Electron Microscope Image
Gold atoms
(0.2 nm apart)
Copyright © FEI
Energyanddistance
Probingtheproton
Quarksdetectedwithinprotons
Freeway 280
2 miles long accelerator
End Station A
experimental area
Stanford (SLAC), California, late 1960s
Fire electrons at proton: big deflections seen!
Quarkmasses
180
160
140
120
Mass 100
(GeV) 80
60
40
20
0
175 GeV
Top
(discovered 1995)
E= mc2
1 proton mass ~ 1GeV (10-27 Kg)
0.003
0.006
0.095
1.2
4.5
Quarks
Up
Down
Strange
Charm
Bottom
Top
The mass grows larger in each successive family
QuantumMechanics
•  Atomsandpar*clesbehaviorisdescribedbytheQuantum
Mechanics
•  Someproper*es,likeenergyforexample,canonlyassume
somediscretevalues,theydon’tbelongtoacon*nuum
•  Par*cleproper*esaredescribedbythesevalues(quantum
numbers).Someexamples:
–  Electriccharge
–  Colourcharge
–  Flavour
–  Spin
Pauliprinciple
•  Wecanusethepar*clequan*s*c
proper*estoclassifythem
•  Somepar*cles,calledfermions,
obeytothePauliprinciple.While
someothers–thebosons–do
not.
TheSpin/Sta$s$csTheorem
Half-integer Spin Particles
1 3
! , ! , .... Fermions
2 2
Fermi-Dirac Statistics
(W. Pauli, 1940)
Bose-Einstein Statistics
0, ! , 2 ! , ....
Bosons
IntegerSpinPar*cles
ConsequencesoftheSpin/Sta*s*csTheorem:
• formal:wavefunc*ons,fieldoperatorscommuta*onrules
• experimental:nuclearandatomicstructure,Bose-Einstein
condensates
33
FermionsandBosons
TheWaveFunc*onmusthavethecorrectsymmetryunder
interchangeofiden*calpar*cles.If1,2areiden*calpar*cles:
2
ψ ( x1 , x2 ) = ψ ( x2 , x1 )
ψ → +ψ
(1↔ 2)
ψ → −ψ
(1↔ 2)
2
(probabilitymustbe
conserveduponexcange
ofiden*calpar*cle)
Iden*calBosons
(symmetric)
Iden*calFermions
(an*symm.)
AconsequenceoftheSpin/Sta*s*csTheorem:fortwoiden*cal
Fermions1,2inthesamequantumstatex:
ψ (x1, x2 )= ψ (x2 , x1 )=− ψ (x1, x2 ) ⇒ ψ (x1, x2 ) = 0
Because
identical
Spin/Statistics
Theorem
Pauli Exclusion Principle!
Par$cles/An$par$cles:the“birth”ofPar$clePhysics
1928:DiracEqua*on,mergingSpecialRela*vityandQuantumMech.
Arela*vis*cinvariantEqua*onforspin½par*cles.E.g.theelectron
(i γ µ ∂ µ − m) ψ = 0
•  E>0, s=+1/2
•  E>0, s=-1/2
•  E<0, s= +1/2
•  E<0, s= -1/2
Electron, s=+1/2
Electron, s=-1/2
Positron, s=1/2
Positron, s=-1/2
Rest frame
solutions: 4 independent states:
Uponreinterpreta*on
ofnega*ve-energy
statesasan*par*cles
oftheelectron:
Thepositron,apar*cleiden*caltotheelectron
e-butwithaposi*vecharge:e+.Thefirst
predic*onoftherela*vis*cquantumtheory.
An$-maXer
•  Foreveryfundamentalpar*cleofmaYerthereisanan*-par*clewith
samemassandproper*esbutoppositecharge
Matter
Anti-Matter
+2/3
0
u
νe
d
+1
e+
-2/3
u
νe
-1/3
-1
e-
0
Bar on top
to indicate
anti-particle
+1/3
d
positron
•  Correspondent anti-particles exist for all three families
•  Anti-matter can be produced using accelerators
An$par$cles
MaXer-an$maXerpaircrea$on
• Electron-positron pair created out of
photons hitting the bubble-chamber
liquid
• Example of conversion of photon
energy into matter and anti-matter
• Matter and anti-matter spiral in
opposite directions in the magnetic
field due to the opposite charge
• Energy and momentum is conserved
4Forces
•  Thereare4fundamental
interac*onsinNature
•  Allforcescanbebroughtbackto
theseinterac*ons
•  ThegravityisaYrac*ve,allother
forcescanalsoberepulsive
•  Theinterac*onsarealso
responsibleofthenucleusdecays
Howdopar$clesinteract?
•  Objectscaninteractwithoutbeingin
contact
•  HowdomagnetsaYractorrepulse?
•  HowdoestheSunaYracttheEarth?
•  Aforceissomethingwhichis
propaga*nginbetweenobjects
TheconceptofForce
In Classical Physics :
In Quantum Physics :
•  Instantaneous action at a
distance
•  Field (Faraday, Maxwell)
•  Exchange of Quanta
Inverse square law
k
F= 2
r
Constituents
properties:
Quarks:
•  Electric charge
•  Color
•  Effective mass
•  Spin (1/2)
Leptons:
•  Electric charge
•  Mass
•  Spin (1/2)
3 families
All Contituents (Quarks,
Leptons) are Fermions.
Costituents of Matter
Force
Carriers
43
ForcesinNature
Force
Intensity
Strong Nuclear ~ 1
Carriers
Happens in
Gluons
(massless)
Atomic nuclea
Elettromagnetic
~ 10-3
Photons
(massless)
Atomic levels
Weak Nuclear
~10-5
W+,W-,Z0
(heavy)
Beta radioactive
decay
Gravitation
~10-38
Gravitons (?)
Heavy bodies
Classical/QuantumForce
Letusconsidertwopar*clesatadistancer
.
r
.
Ifasourcepar*cleemitsaquantumthatreachestheotherpar*cle,
thechangeinmomentumwillbe:
r Δp ≈ !
Andsince:
Wehave:
c Δt ≅ r
Δp ! c
≈
Δt r r
Δp
k
F =
≈ 2
Δt r
45
Rangeofinterac$ons
Forcesanddistances
R>106m(gravita*onalforce)
R~10-10m(forzaelettromagne*ca)
R~10-15m(forzaforte)
Electromagne$sm
•  Theelectromagne*cforcesaresuch
thatoppositechargesaYractand
equalchargesrepulse
•  Theforcecarrieristhephoton(γ)
•  •  Thephotonismasslessandmoveat
thespeedoflight
Electromagne$cforce
The repulsive force that two approaching electrons “feel”
e-
e-
Photon is the particle
associated to the electromagnetic force
“smallest bundle” of force
Photon
Photonexchange
FeynmanDiagram
e-
e-
γ
e-
e-
Electromagne$cresidualforce
•  Normallytheatomsare
neutralhavingthesame
numberofprotonsand
neutrons
•  Thechargedpartofanatom
canaYractthechargedpart
ofanotheratom
•  Theatomscanlink
themselvestoform
molecules
Whynucleadonotexplode?
•  Aheavynucleuscontains
manyprotons,allwith
posi*vecharges
•  Theseprotonsrepulse
eachothers
•  Whythenucleusdoesnot
explode?
Strongforce
•  InaggiuntaallacaricaeleYrica,i
quarksportanoancheunnuovo*po
dicarica,deYa“caricadicolore”
•  Laforzatralepar*cellechehanno
caricadicoloree’deYaforzaforte
Thegluon
•  Thestrongforcekeepsthequark
together,toformthehadrons.
•  Theforcecarriersarethegluons:
thereare8differentgluons
•  Thestrongforceactsonlyonshort
distances
Colourandan$-colour
•  Thereare3colourchargesand3an*-colourcharges
•  Notethatthesecoloursarenotatallrelatedwiththe
standardcolourandwiththevisiblelight.Itisjustaway
todescribethephysics(quantumnumbers)
Quarksandcolour
All quark flavours come in 3 versions, called “colours”
uu
u
up
+2/3
dd
d
down
-1/3
Quarks combine together to form colourless particles
- Baryons (three quarks: red+ green + blue = white)
Strong forces
“glue” quarks
together in
bound states
proton
p
- Mesons (quark-antiquark pair) pion
such as red+anti-red u-ubar state π
u
u
Buildingmorepar$cles
B mesons (bq)
c c
b u
u b
J/ψ
B-
B+
b d
d b
B0
B0
b b
Y
Many more mesons and baryons…
Colouredquarksandgluons
•  Everyquarkhasoneofthethree
colourchargesandeveryan*quark
hasoneofthethreean*colour
charges
•  Barionsandmesonsareneutralin
colour
Quarksconfinement
•  Thecolourforceincreasewiththe
distance
•  Partcileswithcolourchargecannot
existasisolatedpar*cles
•  Quarksareconfinedwithother
quarkstoformhadrons
•  Hadronsareneutralincolour
Quarksemitgluons
•  Whenaquarkemitsorabsorbsagluon,
thecolourofthequarkchanges,in
ordertosavethecolourcharge
•  Aredquarkemitsared/an*-bluegluon,
andbecomesblu
Strongforce:gluons
Gluons interact with quarks
Gluons interact with other gluons
Quarkconfinement
•  There are no free quarks, quarks and antiquarks are “confined”
in colourless doublet (mesons) or triplets (baryons) by the
exchange of gluons
Deca
y
Z
0
Gluon hold quarks
together as they move
further apart
until the gluon connection
snaps, and other quark-antiquark
pairs are created out of the
energy released
The new quarks bound
to the old quarks
and form new mesons
Residualstrongforce
•  Thestrongforcebetweenthequarksofaprotonandthe
quarksofanotherprotonisstrongenoughtowinonthe
repulsiveelectromagne*cforce
Weakforce
•  Weakinterac*onsareresponsibleforthe
decaysofheavyquarksandleptons
•  Example:theneutrondecaysinaproton+
electrons+neutrino
•  ThisexplainswhyallmaYeriscomposedby
thelighterleptonsandquarks
Weakforce:betadecay
n
p
Antineutrin
o
Electron
Neutronβ-decay
At quark level: d→ u e- νe
u d
d
n
15 min
p
u u
d
e- νe
A (free) neutron decays after 15 min
Long life time (15min is an eternity in particle physics!) ⇒
“weak”
Electroweakforce
•  IntheSMtheelectromagne*candweak
forceshavebeenunifiedinasingle
electro-weakforce
•  Atveryshortdistances(~10-18meters),the
weakandelectromagne*cinterac*ons
havethesameintensi*es
•  Thecarriersarephotons,WandZs.
Weakforce:W-,W+,Z0
β-decay
n→peνe
WElectric charge
conserved at each
vertex
Muondecay
•  Exampleofapar*cledecay
•  Heretheproducedpar*clesare
notpiecesoftheini*alpar*cle,
butarecompletelynewones.
Howdoesapar$cledecay?
•  Whenapar*cledecay,itistransformedinalighter
par*cleandinapar*clewhichisoneofthecarriersofthe
weakforce(theWboson)
•  Apar*cledecaysifitstotalmassishigherthanthesumof
themassesofthedecayproductsandifthereisaforce
whichmediatesthedecay
Missingmass
•  Inmostofthedecays,thepar*cles
andthenucleuswhichareleohavea
totalmasssmallerthanthatofthe
ini*alpar*cleoroftheorigina*ng
nucleus
•  Themissingmassitistransformedin
kine*cenergyofthedecayproducts
Theunstablenucleus
•  Wesawthatstrongforceskeepthe
nucleustogether,againsttherepulsive
forcebetweenprotons.
•  However,notallnucleaarestable
•  Someofthemdecay
•  Thenucleuscansplitinsmallernuclea
•  Thishappensforexampleinanuclear
reactor
VirtualPar$cles
•  Par*clesdecayviapar*cleswhichareforce-carriers
•  Insomecases,apar*clemaydecayviaaforce-carrierthat
ismoremassivethantheini*alpar*cle
•  Theforce-carrierpar*cleisimmediatelytransformedinto
lower-masspar*cles
•  Theshort-livedmassivepar*cleappearstoviolatethelaw
ofenergyconserva*on
Annihila$on
•  Annihila*onsarenotdecaysbut,inthesameway,they
takeplacethankstovirtualpar*cles
•  Theannihila*onoflightquarksathighenergiescanbring
totheproduc*onofheavyquarksinthelaboratory
•  Thisbubblechambershows
anan*-protonwhichhits
againstaproton,
annihilatesandproduces8
pions.Oneofthepionthen
decaysinamuonanda
neutrino(neutrinodoesnot
leaveanytrace)
Electron-Positronannihila$on
Andthegravity?
•  Gravityisveryweak
•  Isimportantatmacroscopicdistances
•  Theforcecarriers(gravitons)ispredicted
bytheorybutnotyetobserved
MysteriesandfailuresofSM
•  TheSMisatheoryoftheUniverse
•  Itgivesagooddescrip*onofthephenomenawhichwe
observeexperimentally
•  Undermanyrespectsisnotcomplete:whythereare3
genera*ons?WhatisthedarkmaYer?
•  AsEnsteinhasextendedthelawsofmechanicsofNewton
withtheRela*vitytheory,wenowhavetogobeyondthe
SM
•  Wehavetodoittoexplainmasses,gravityetc..
Threefamilies
•  Thereare3familiesof
fundamentalpar*cles
•  Whyonly3?
•  Andwhywejustseeone
ofthemintherealworld?
Whatcanwesayaboutmass?
•  TheSMcannotexplainwhya
givenpar*cleischaracterizedby
itsmass.
•  Physicists“invented”anewfield,
calledHiggsfield,whichinteracts
withalltheotherpar*clestogive
theirmasses
•  TheHiggsbosonhasbeen
discoveredwiththeLHC
experimentin2013aoer~70
yearsofsearch!.
BeyondtheStandard
Model:Unifica*onofforces
ELECTROMAGNETIC
GRAVITY
UNIFIED
FORCE?
STRONG
WEAK
Looking for a simple elegant unified theory
TheoryofGrandUnifica$on
•  ItisbelievedthataGUTwill
unifythestrong,weak,and
electromagne*cforces.
•  These3forceswillthenbe
visibleasdifferent
manifesta*ons–atlowenergy
–ofauniqueforce
•  The3forceswillunifyatavery
highenergy
Supersymmetry
•  Somephysicists,intheaYempt
tounifygravitywiththeother
fundamentalforces,have
suggestedthatevery
fundamentalpar*cleshould
haveashadowpar*cle.Itis
morethan20yearsthatweare
lookingforthese
supersymmetricpar*cles!
•  Today,physicshasatheoryforquantummechanic,for
rela*vityandforgravity,butthese3theoriesarenot
unified..
•  Ifwewouldliveinaworldwithmorethan3spa*al
dimensions,maybethisproblemcouldbeoverpassed.
•  Thestringtheorysuggeststhatinaworldwherethereare
the3standarddimensionsandsomeaddi*onal(but
smaller)dimensions,thepar*cleswouldbelikestrings.
ExtraDimensions
•  TheStringtheoryrequiresmorethanjust3dimensions
•  Theseextra-dimensionscanbesosmallthatwecannotseethem
•  Experimentsnowlookforevidenceoftheseextra-dimensions
DarkMaXer
•  ItseemsthattheUniverseis
madebyadifferentmaYer
thanourSunandstars
•  ThedarkmaYeraYracts
normalmaYer,butithasnot
beeniden*fiedyet