1. Elementary particles
Spring 2005, Physics 123
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Lecture XXV

2. Concepts antimatter leptons quarks fundamental interactions 11/24/2018
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3. Mass and energy Mass and energy are interchangeable
Energy can be used to create mass (matter)
Mass can be destroyed and energy released
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4. Energy, mass and momentum
Mass is energy:
vc:
Energy –momentum - mass
v=c if and only if m0=0
Mg=0
Units for mass
Units for momentum
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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)
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Lecture XXV

6. Particle acceleration
RF cavities
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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
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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
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Lecture XXV

9. Magnetic fields are used to separate positive from negative
And measure particle velocity
“Mustache” = matter – antimatter pairs
Bubble chamber picture
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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
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11. Tracking: connecting the dots
40 cm
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12. Nature’s scales
Antimatter
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13. Matter = fermions (s=1/2)
Leptons
Charge e
-1e ne m nm t nt
All fermions interact gravitationally and weakly.
All charged particles interact electromagnetically.
Only quarks interact strongly
Quarks
Charge u
+2/3e d
-1/3e c s t b
For each fermion there exists an antiparticle with opposite electric charge
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14. Periodic table of forces
Interaction
Field particle
Comment
Electromagnetic
Photon g
Holds electron in orbit
Strong
Gluon g
Holds nucleus
Weak
W+, W-, Z0-bosons
Reactions in the Sun
Gravity
Graviton G (??)
Holds planets in orbit
S=1
S=2
4 fundamental forces – others combinations of these.
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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- g e- e-
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Lecture XXV

16. Periodic table of matter and forces
Gravity
1st generation – enough to build the Universe
Why 3 generation?
Mass hierarchy?
Why top is so heavy? g
EM and weak unified
Why M(g)=0 M(W)=80GeV, M(Z)=90 GeV? – Electro Weak Symmetry Breaking
W,Z
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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
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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
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19. Top production Statistics up to now : 600 pb-1  3x1013collisions
4200 top pairs produced
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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
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21. Top event
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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
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23. Conservation laws Electric charge Energy and momentum
Number of leptons and baryons (antilepton = -1, antibaryon = -1)
Energymass
Mass energy
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24. Decays Z Z-boson e+e- 3.36% m+m- 3.36% t+t- 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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