Elementary particles Spring 2005, Physics 123 11/24/2018 Lecture XXV

Concepts antimatter leptons quarks fundamental interactions 11/24/2018 Lecture XXV

Mass and energy Mass and energy are interchangeable Energy can be used to create mass (matter) Mass can be destroyed and energy released 11/24/2018 Lecture XXV

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 11/24/2018 Lecture XXV

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) 11/24/2018 Lecture XXV

Particle acceleration RF cavities 11/24/2018 Lecture XXV

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 11/24/2018 Lecture XXV

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 11/24/2018 Lecture XXV

Magnetic fields are used to separate positive from negative And measure particle velocity “Mustache” = matter – antimatter pairs Bubble chamber picture 11/24/2018 Lecture XXV

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 11/24/2018 Lecture XXV

Tracking: connecting the dots 40 cm 11/24/2018 Lecture XXV

Nature’s scales Antimatter 11/24/2018 Lecture XXV

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 11/24/2018 Lecture XXV

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. 11/24/2018 Lecture XXV

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- 11/24/2018 Lecture XXV

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 11/24/2018 Lecture XXV

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 11/24/2018 Lecture XXV

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 11/24/2018 Lecture XXV

Top production Statistics up to now : 600 pb-1  3x1013collisions 4200 top pairs produced 11/24/2018 Lecture XXV

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 11/24/2018 Lecture XXV

Top event 11/24/2018 Lecture XXV

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 11/24/2018 Lecture XXV

Conservation laws Electric charge Energy and momentum Number of leptons and baryons (antilepton = -1, antibaryon = -1) Energymass Mass energy 11/24/2018 Lecture XXV

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 11/24/2018 Lecture XXV