par$cle(physics((senior(honours)playfer/mystery.pdfmystery.pptx author: steve playfer created date:...

Par$cle Physics (Senior Honours)

•  Lectures : Tuesday & Friday 12:10-‐13:00 (JCMB 6206)

There will be 18 lectures There will be no lectures in weeks 9 & 11 •  Tutorials : Thursday 9:00-‐9:50 (JCMB 5215)

There will be 5 tutorials in weeks 3,5,7,8,10 Tutorial slots may be exchanged with a lecture •  Office Hours : Thursday 15:00-‐17:00 (JCMB 5420)

Prof. Steve Playfer Spring 2010

Par$cle Physics Lecture 1 Steve Playfer 12/1/10 1

Synopsis

•  Introduc$on •  Feynman Diagrams

•  Dirac Spinors •  Electromagne$c interac$ons

•  Weak interac$ons •  Electron-‐proton sca[ering •  Evidence for partons •  Strong interac$ons •  Quark model of hadrons

•  Hadron produc$on & Jets •  Weak decays of hadrons

•  Symmetries •  Mixing & CP Viola$on

•  Neutrino oscilla$ons •  Electroweak theory •  Higgs bosons •  Beyond the Standard Model •  Physics in the LHC era

An overview of the Standard Model of par$cle physics and its extensions. Experimental data will be shown, and theore$cal ideas will be introduced. The Quantum Physics & Subatomic Physics courses are prerequisites.


References & Websites

•  D.Griffiths – Introduc$on to Elementary Par$cles (Wiley 2008)

•  B.R.Mar$n & G.Shaw – Par$cle Physics (Wiley 1997) •  D.H.Perkins – Introduc$on to High Energy Physics (CUP 2000)

•  F.Halzen & A.D.Mar$n – Quarks & Leptons (Wiley 1984)

•  A.Seiden – Par$cle Physics: A Comprehensive Introduc$on

(Addison-‐Wesley 2005) •  I.J.R.Aitchison & A.J.G.Hey – Gauge Theories in Par$cle Physics (Hilger 1989)

  Par$cle Data Group (PDG) h[p://pdg.lbl.gov   CERN/LHC h[p://public.web.cern.ch/public


Introductory textbooks

More advanced textbooks

Useful websites

Lecture 1 -‐ Introduc$on to Par$cle Physics

H0 Higgs ?

Large Hadron Collider


On studying particle physics

“Elementary” said he

“ It is one of those instances where the reasoner can produce an effect which seems remarkable to his neighbours, because the la[er has missed the one li[le point which is the basis of the deduc$on.”

“I have been guilty of several monographs. Here is the one ‘Upon the dis$nc$on between the Ashes of the Various Tobaccos.’”


Q Charge

+2/3 e

-‐1/3 e

-‐1 e

0

I3 B L Isospin

Baryon Number

Lepton Number

+1/2

-‐1/2

+1/2

-‐1/2 0

0

+1/3

+1/3

+1

+1

0

0


Fermion Masses These are well measured (apart from νi ) but the hierarchy is hard to understand:

•  Logarithmic scale covers 15 orders of magnitude

•  Charged lepton (i=e,µ,τ), up (ui = u,c,t) and down (di=d,s,b) quark masses are similar but pa[erns are not iden$cal

•  Neutrinos (νi) are much lighter Absolute scale is unknown (<1eV)

•  Only the two νi mass differences are known: Δm12

2 = (7.6±0.2) x 10-‐5 eV2 , Δm232 = (2.4±0.1) x 10-‐3 eV2


An$ma[er

•  Masses (and life$mes) are equal

•  Charges are equal and opposite •  Baryon/lepton numbers equal and opposite

•  Massless fermions/an$fermions are leq/right-‐handed •  Fermion/an$fermion annihilate into energy (bosons)

… or can be created as a pair at high energy colliders

•  The universe is made of ma[er (fermions)

According to Quantum Mechanics (Dirac equa$on) _ Every S=1/2 fermion ( f ) has an an$fermion partner ( f )

_ _ Positron e+ (1933), An$neutrino νe (1956), An$proton p (1955)


Mysteries of the Fermions

•  Are the fermions really point-‐like objects (re < 10-‐20m)?

•  Why are there exactly twelve elementary fermions?

•  Why are there three “genera$ons” with different “flavours”?

•  Why do they have different masses and charges?

•  Why do charged fermions have anomalous magne$c moments (g-‐2), but no electric dipole moments?


Mysteries of the Fermions (cont.)

•  Why do quarks have strong interac$ons with three “colours”?

•  Why do weak interac$ons change quark flavour, but not lepton flavour?

•  Why do neutrinos have flavour oscilla$ons?

•  What are the differences between fermions and an$fermions?

•  How do we explain the Baryon asymmetry of the Universe?


Interac$ons of Leptons •  Electromagne$c

•  Weak (Charged)

All charged lepton flavours (i=e,µ,τ) have the same couplings (lepton universality) No flavour-‐changing γ,W,Z interac$ons (but there is νi mixing!)

•  Weak (Neutral)

•  There are no strong interac$ons

i

i i

i

γ (photon)

√α

W boson

Z boson

i i

i

i

νi

νi

νi

νi

gL,gR!

gL !

gL !

gL !

√α

gL(gR) are leq (right)-‐handed couplings


Interac$ons of Quarks •  Electromagne$c

•  Weak (Charged)

All quark flavours (ui=u,c,t, di=d,s,b) have the same strong couplings No flavour-‐changing g, γ, Z interac$ons … but W has flavour-‐changing couplings to all possible ui and di

•  Strong Interac$ons

•  Weak (Neutral)

ui

di di

ui

γ (photon)

√α

W boson

g (gluon)

ui ui

ui

ui

di

di

gL !

gL !

√α di

√αs

√αs

di

ui ui

di di

gL,gR!

gL,gR!

Z boson


The CKM Matrix (see Lecture 11)

Flavour-‐changing couplings of W bosons are described by:

λ = 0.226±0.001, Α = 0.81±0.02, ρ = 0.14±0.03, η =0.35±0.02 measured values (free parameters in the Standard Model)

Diagonal elements are close to 1. Vub & Vtd are very small.

+ O(λ4)


Neutrino Mixing (see Lecture 14) ν3

ν1

ν2

“Normal” mass hierarchy

ντ νµ νe

θ12 = (34.5±1.2)o, θ23 = 45o, θ13 < 13o measured values (angles and phase δ are free parameters in Standard Model)

Leads to neutrino flavour oscilla$ons


Mysteries of the Bosons

•  Electromagne$c and weak interac$ons are unified at the Electroweak scale v=246GeV.

•  Is there a “grand unified” scale where the strong interac$on is also included?

•  What is the mechanism that breaks electroweak symmetry, and how does it explain the large masses of the W and Z bosons?

•  Are there 0, 1 or many Higgs bosons?

•  What are the masses and couplings of Higgs bosons?

•  How do we include gravity?

Par$cle Physics Lecture 1 Steve Playfer

H0 ? Higgs Mass=?

12/1/10 15

Electroweak Theory (see Lectures 15 & 16)

•  Electroweak unifica$on scale: v = 2MW = 246 GeV gL

•  Electromagne$c and weak couplings are related by: e = gL sin θW (Weinberg angle sin2 θW = 0.2221)

•  W and Z boson masses are related by: MW = MZ cos θW (MW = 80.42 GeV, MZ = 91.19 GeV)

•  Standard Model explana$on is that v is the “vacuum expecta$on value” of a Higgs field which has one Higgs boson associated with it.

•  Experimental bounds on Standard Model Higgs boson mass: 115 < MH < 163 GeV (from LEP & Tevatron)


Beyond the Standard Model (Lecture 17) •  Composite models of fermions and bosons

•  Mul$plets of Higgs bosons

•  Supersymmetric partners (SUSY): S=0 squarks and sleptons S=1/2 neutralinos, charginos, higgsinos

•  Grand unified theories (GUTs) of strong/electroweak GUT scale 1011-‐ 1016 GeV New leptoquark couplings

•  Heavy neutrino(s) at GUT scale can explain neutrino oscilla$ons and light neutrino masses (“seesaw” mechanism)

•  What about gravity? (strings, branes, supergravity…) Planck scale 1019 GeV is where quantum gravity effects become strong


The Dark Side

•  WMAP measurements of cosmic microwave background fluctua$ons tell us only 4.6% of the universe is atoms (baryons).

•  To account for rota$on curves of galaxies, gravita$onal lensing and large scale structure need:

23.3% “Dark Ma[er”

Must be weakly interac$ve massive par$cles (WIMPs)

Best candidate would be the lightest SUSY neutralino

•  To account for accelera$on of expansion of the universe need:

72.1% “Dark Energy”

Either introduce a cosmological constant Λ Or a new field theory (“quintessence”)


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