Download - Elementary particles

04/22/23 Lecture XXV 1

Spring 2005, Physics 123

Elementary particles


Concepts

• antimatter• leptons• quarks • fundamental interactions


Mass and energy

• Mass and energy are interchangeable• Energy can be used to create mass (matter)• Mass can be destroyed and energy released

2mcE


Energy, mass and momentum • Mass is energy:

2mcE

420

222 cmcpE

• Energy –momentum - mass

• Units for mass2/ ceV

• Units for momentum

ceV /

• vc:

22

0

/1 cv

mm

• v=c if and only if m0=0• M=0

hcE


Particle acceleration

• Electric field is used to accelerate the elementary particles and thus increase their energy

• Energy is conserved, because particles receive their energy from the electric field

• Energy of accelerated particles can be used to produce new particles (matters)

eVEnergy 5000


Particle acceleration

RF cavities


Particle accelerators• Fermilab • 40 miles west of

Chicago • Tevatron – at the

moment world’s highest energy collider – 1 TeV proton beam

collides with 1 TeV antiproton beam

– 6.28 km circumference • Top quark discovery

- 1996


Large Hadron Collider (LHC)

• Next collider – LHC - is built in Europe, operational 2008

• 27 km;• 14 Tev - LHC will discover Higgs if

it exists.• Two high PT experiments _CMS and

Atlas


Magnetic fields are used to separate positive from negativeAnd measure particle velocity“Mustache” = matter – antimatter pairs

Bubble chamber picture


Detecting particles

• Tracking charged particle in magnetic field - p

• Calorimeter – collect all energy, energy loss light

• The only particle that can survive calorimeter material – muon

• Calorimeter is followed by another set of tracking devices – muon chambers


Tracking: connecting the dots

40 cm


Nature’s scales


Matter = fermions (s=1/2)

For each fermion there exists an antiparticle with opposite electric charge

ee

ee

Leptons Chargee -1ee 0

-1e 0

-1e 0

Quarks Chargeu +2/3ed -1/3ec +2/3es -1/3et +2/3eb -1/3e

etcdduu ,....;

All fermions interact gravitationally and weakly.

All charged particles interact electromagnetically.

Only quarks interact strongly


Periodic table of forces

4 fundamental forces – others combinations of these.

Interaction Field particle CommentElectromagnetic Photon Holds electron in

orbitStrong Gluon g Holds nucleus Weak W+, W-, Z0-bosons Reactions in the Sun

Gravity Graviton G (??) Holds planets in orbit

S=1

S=2


Fundamental interactions and Feynman diagrams

• Gauge bosons (photon, W, Z, gluon, graviton) mediate fundamental interactions

• Example: photon – quantum of EM field– Electron knows about the presence of another electron through

EM field. In quantum language – through exchange of photons:

e-

e-e-

e-


Periodic table of matter and forces

• 1st generation – enough to build the Universe

• Why 3 generation?• Mass hierarchy? • Why top is so heavy?

Matter:Matter: ForcesForces

Gravity

W,Z

EM and weak unified– Why M()=0

M(W)=80GeV, M(Z)=90 GeV? – Electro Weak Symmetry Breaking


Higgs boson – generator of mass• Theoretical hypothesis:

– Space is saturated with bosonic field (Higgs, s=0) with nonzero vev;

– W, Z bosons absorb a component of this field and gain mass, while photon does not and remains massless

– fermions acquire mass through interaction with Higgs boson.

• Analogy – popular person in a party (massive particle) attracts a lot of people (Higgs boson) thus effectively gaining mass.

• To test the hypothesis – find Higgs


Higher generations – heavier replicas of the first generation

• Muon discovered in 1930’s• Mass =105 MeV/c2

• Was a big surprise – first hint of extra generations

• Particles of higher generations decay into particles of lower generation

I.I. Rabi

ee


Top production • Statistics up to now :• 600 pb-1 3x1013collisions• 4200 top pairs produced


Top ID in “lepton+jets” channel

• Fingerprint of top pair production:• 2 b-jets • Lepton: electron or muon• Neutrino (from energy imbalance)• 2 q’s – transform to jets of particles

lWorqqWbWt

ttpp

'


Top event


Hadrons = composite quark states

• Meson = combination of quark and antiquark:

• Spin s=0 • Spin s=1

• Baryons = combination of 3 quarks

• Spin s=1/2• Spin s=3/2

0)31(

31;

1)32(

31;

1)32(

31;

0

QdbB

QubB

QubB

131

32

32;

031

31

32;

131

32

32;

Qduup

Quddn

Quudp


Conservation laws• Electric charge• Energy and momentum• Number of leptons and baryons (antilepton = -1,

antibaryon = -1)

23

2

0

/1091)(

/5.0)()(

cMeVZm

cMeVemem

Zee

2

20

0

/0)(

/135)(

)(

cMeVm

cMeVm

uu

Energymass Mass energy


Decays

• Z-boson• e+e- 3.36% 3.36% 3.36%• uubar 10.1%• ccbar 10.1%• ddbar 16.6%• ssbar 16.6%• bbbar 16.6%• All neutrinos 20%

Z

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