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Elementary particles. Spring 2005, Physics 123. 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. Mass is energy:. - PowerPoint PPT Presentation
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04/22/23 Lecture XXV 1
Spring 2005, Physics 123
Elementary particles
04/22/23 Lecture XXV 2
Concepts
• antimatter• leptons• quarks • fundamental interactions
04/22/23 Lecture XXV 3
Mass and energy
• Mass and energy are interchangeable• Energy can be used to create mass (matter)• Mass can be destroyed and energy released
2mcE
04/22/23 Lecture XXV 4
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
04/22/23 Lecture XXV 5
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
04/22/23 Lecture XXV 6
Particle acceleration
RF cavities
04/22/23 Lecture XXV 7
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
04/22/23 Lecture XXV 8
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
04/22/23 Lecture XXV 9
Magnetic fields are used to separate positive from negativeAnd measure particle velocity“Mustache” = matter – antimatter pairs
Bubble chamber picture
04/22/23 Lecture XXV 10
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
04/22/23 Lecture XXV 11
Tracking: connecting the dots
40 cm
04/22/23 Lecture XXV 12
Nature’s scales
04/22/23 Lecture XXV 13
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
04/22/23 Lecture XXV 14
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
04/22/23 Lecture XXV 15
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-
04/22/23 Lecture XXV 16
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
04/22/23 Lecture XXV 17
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
04/22/23 Lecture XXV 18
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
04/22/23 Lecture XXV 19
Top production • Statistics up to now :• 600 pb-1 3x1013collisions• 4200 top pairs produced
04/22/23 Lecture XXV 20
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
'
04/22/23 Lecture XXV 21
Top event
04/22/23 Lecture XXV 22
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
04/22/23 Lecture XXV 23
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
04/22/23 Lecture XXV 24
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