Elementary particles Spring 2005 Physics 123 6122021 Lecture

Elementary particles Spring 2005, Physics 123 6/12/2021 Lecture XXV 1

Concepts • • antimatter leptons quarks fundamental interactions 6/12/2021 Lecture XXV 2

Mass and energy • Mass and energy are interchangeable • Energy can be used to create mass (matter) • Mass can be destroyed and energy released 6/12/2021 Lecture XXV 3

Energy, mass and momentum • v c: • Mass is energy: • Energy –momentum - mass • Units for mass • v=c if and only if m 0=0 • Mg=0 • Units for momentum 6/12/2021 Lecture XXV 4

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) 6/12/2021 Lecture XXV 5

Particle acceleration RF cavities 6/12/2021 Lecture XXV 6

Particle accelerators • Fermilab • 40 miles west of Chicago • Tevatron – at the moment world’s highest energy collider – 1 Te. V proton beam collides with 1 Te. V antiproton beam – 6. 28 km circumference • Top quark discovery - 1996 6/12/2021 Lecture XXV 7

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 6/12/2021 Lecture XXV 8

Magnetic fields are used to separate positive from negative And measure particle velocity “Mustache” = matter – antimatter pairs Bubble chamber picture 6/12/2021 Lecture XXV 9

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 6/12/2021 Lecture XXV 10

Tracking: connecting the dots 40 cm 6/12/2021 Lecture XXV 11

Nature’s scales 6/12/2021 Lecture XXV 12

Matter = fermions (s=1/2) Leptons e ne m nm t nt Charge -1 e 0 ØAll fermions interact gravitationally and weakly. ØAll charged particles interact electromagnetically. ØOnly quarks interact strongly Quarks u d c s t b Charge +2/3 e -1/3 e For each fermion there exists an antiparticle with opposite electric charge 6/12/2021 Lecture XXV 13

Periodic table of forces S=1 Interaction Field particle Electromagnetic Photon g Comment Holds electron in orbit Strong Gluon g Holds nucleus Weak Gravity W+, W-, Z 0 -bosons Graviton G (? ? ) Reactions in the Sun Holds planets in orbit S=2 4 fundamental forces – others combinations of these. 6/12/2021 Lecture XXV 14

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: eeg 6/12/2021 e- e. Lecture XXV 15

Periodic table of matter and forces Matter: Forces Gravity • 1 st generation – enough to build the Universe • Why 3 generation? • Mass hierarchy? • Why top is so heavy? 6/12/2021 ®EM and weak unified – Why M(g)=0 M(W)=80 Ge. V, M(Z)=90 Ge. V? – Electro Weak Symmetry Breaking Lecture XXV g W, Z 16

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 6/12/2021 Lecture XXV 17

Higher generations – heavier replicas of the first generation I. I. Rabi 6/12/2021 • Muon discovered in 1930’s • Mass =105 Me. V/c 2 • Was a big surprise – first hint of extra generations • Particles of higher generations decay into particles of lower generation Lecture XXV 18

Top production • Statistics up to now : • 600 pb-1 3 x 1013 collisions • 4200 top pairs produced 6/12/2021 Lecture XXV 19

Top ID in “lepton+jets” channel • • • 6/12/2021 Fingerprint of top pair production: 2 b-jets Lepton: electron or muon Neutrino (from energy imbalance) 2 q’s – transform to jets of particles Lecture XXV 20

Top event 6/12/2021 Lecture XXV 21

Hadrons = composite quark states • Meson = combination of quark and antiquark: • Spin s=0 • Spin s=1 6/12/2021 • Baryons = combination of 3 quarks • Spin s=1/2 • Spin s=3/2 Lecture XXV 22

Conservation laws • Electric charge • Energy and momentum • Number of leptons and baryons (antilepton = -1, antibaryon = -1) Energy mass 6/12/2021 Mass energy Lecture XXV 23

Decays • • • 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% 6/12/2021 Z Lecture XXV 24