1

Particle Physics Particle physics – what is it? – why do it? Standard model

Quantum field theory Constituents, forces Milestones of particle physics

Particle physics experiments shortcomings of standard model Summary Webpages of interest

http://www.fnal.gov (Fermilab homepage) http://www-d0.fnal.gov (DØ homepage) http://www.cern.ch (CERN -- European Laboratory

for Particle Physics) http://cms.web.cern.ch/cms/ (CMS) http://www.hep.fsu.edu/~wahl/Quarknet/links.html

(has links to many particle physics sites and other sites of

interest) http://www.fnal.gov/pub/tour.html (Fermilab particle physics

tour) http://www.cpepweb.org/ Contemporary Physics Education

Project http://ParticleAdventure.org/ (Lawrence Berkeley Lab.)

2

Topics

what is particle physics, goals and issues historical flashback over development of the

fieldo cosmic rayso particle discoverieso forceso new theories

the standard model of particle physics

3

About Units Energy - electron-volt

1 electron-volt = kinetic energy of an electron when moving through potential difference of 1 Volt;

o 1 eV = 1.6 × 10-19 Joules = 2.1 × 10-6 W•s

o 1 kW•hr = 3.6 × 106 Joules = 2.25 × 1025 eVo 1 MeV = 106 eV, 1 GeV = 109 eV, 1 TeV = 1012

eV

mass - eV/c2

o 1 eV/c2 = 1.78 × 10-36 kgo electron mass = 0.511 MeV/c2

o proton mass = 938.27 MeV/c2 = 0.93827 GeV/ c2

o neutron mass = 939.57 MeV/c2

momentum - eV/c: o 1 eV/c = 5.3 × 10-28 kg m/so momentum of baseball at 80 mi/hr

5.29 kgm/s 9.9 × 1027 eV/c Distance

o 1 femtometer (“Fermi”) = 10- 15 m

4

Outline what is particle physics? Origins of particle physics

Atom (p, e-), radioactivity, discovery of neutron (n) (1895-1932)

Cosmic rays: positron (e+), muon (μ-), pion (π), Kaon (K±,

K0) (1932 – 1959) the advent of accelerators:

more and more particles discovered, patterns emerge (1960’s and on):

o leptons and hadronso Electromagnetic, weak, strong interactions

present scenario: Standard Model of electroweak and strong interactions Formulation and discovery (1960’s to 1980’s) Precision experimental tests (from 1990’s)

quest for new physics (beyond the standard model) Open questions, possible strategies Present and future experiments, facilities Search for Higgs particle

outlook

5

Sizes and distance scales

virus 10-7

Molecule 10-9m Atom 10-10m nucleus 10-14

m nucleon 10-15m Quark <10-

19m

6

The Building Blocks of a Dew Drop A dew drop is made up of

1021 molecules of water. Each molecule = one

oxygen atom and two hydrogen atoms (H2O).

Each atom consists of a nucleus surrounded by electrons.

Electrons are leptons that are bound to the nucleus by photons, which are bosons.

The nucleus of a hydrogen atom is just a single proton.

Protons consist of three quarks. In the proton, gluons hold the quarks together just as photons hold the electron to the nucleus in the atom

7

Contemporary Physics

Education Project

8

Goals of particle physics particle physics or high energy physics

is looking for the smallest constituents of matter (the “ultimate building blocks”) and for the fundamental forces between them (“interactions”);

aim is to find description in terms of the smallest number of particles and forces

Try to describe matter in terms of specific set of constituents which can be treated as fundamental;

With deeper probing (at shorter length scale), these fundamental constituents may turn out to consist of smaller parts (be “composite”).

“Smallest constituents” vs time:o in 19th century, atoms were considered smallest building

blocks,o early 20th century research: electrons, protons, neutrons;o now evidence that nucleons have substructure - quarks; o going down the size ladder: atoms -- nuclei -- nucleons --

quarks – ???... ??? (preons, toohoos, voohoos,….????)

9

Issues of High Energy Physics Basic questions:

Are there irreducible building blocks?o How many?o What are their properties?

mass? charge? flavor? What is mass? What is charge ? How do the building blocks interact?

o forces? o Differences? similarities?

Why more matter than antimatter? why is our universe the way it is?

o Coincidence? o Theoretical necessityo Design?

Why do we want to know? Curiosity Understanding constituents may help in understanding composites Implications for origin and destiny of Universe

10

Cosmic rays Discovered by Victor Hess (1912) Observations on mountains and in balloon: intensity of cosmic

radiation increases with height above surface of Earth – must come from “outer space”

Much of cosmic radiation from sun (rather low energy protons) Very high energy radiation from outside solar system,

but probably from within galaxy

11

Victor Hess’ Balloon ride 1912

12

13

Positron Positron (anti-electron)

Predicted by Dirac (1928) -- needed for relativistic quantum mechanics

existence of antiparticles doubled the number of known particles!!!

Positron track going upward through lead plate

o Photographed by Carl Anderson (Aug. 2, 1932)

o Particle moving upward, as determined by increase in curvature

of the top half of the track after it passed through lead plate,

o and curving to the left, meaning its charge is positive

14

Anderson and his cloud chamber

15

Neutron Bothe + Becker (1930):

Some light elements (e.g. Be), when bombarded with alpha particles, emit neutral radiation, “penetrating”– gamma?

Curie-Joliot and Joliot (1932): This radiation from Be and B able to eject protons from

material containing hydrogen Chadwick (1932)

Doubts interpretation of this radiation as gamma Performs new experiments; protons ejected not only

from hydrogen, but also from other light elements; measures energy of ejected protons

(by measuring their range), results not compatible with assumption that unknown

radiation consists of gamma radiation (contradiction with energy-momentum conservation), but are compatible with assumption of neutral particles with mass approximately equal to that of proton – calls it “neutron”

Neutron assumed to be “proton and electron in close

association”

16

Chadwick’s experiment

17

Nuclear force – field quantum photon carries the electromagnetic force. Analogy: postulate particle as carrier of nuclear

force (Hideki Yukawa, 1935) with mass intermediate between the electron and the proton

This particle also to be responsible for beta-decay.

potential energy between the nucleon field quanta has the form

m = mass of the exchanged quantum; from observed range of nuclear force:

mass of the exchanged particle 200MeV

2

2

/exp

4 mcc

R

R

gU

18

More particles: Muon 1937: “mesotron” is

observed in cosmic rays (Carl Anderson, Seth Neddermeyer) – first mistaken for Yukawa’s particle

However it was shown in 1941 that mesotrons didn’tinteract strongly with matter.

19

Discovery of pion Lattes, Occhialini and Powell (Bristol, 1947)

(+ graduate student Hugh Muirhead): observed decay of a new particle into two particles

decay products: muon (discovered by Neddermeyer), the other is invisible (Pauli's neutrino).

muon in turn also decays into electron and neutrino

20

Kaons First observation of

Kaons: Experiment

by Clifford Butler and George Rochester at Manchester

Cloud chamber exposed to cosmic rays Left picture: neutral Kaon decay (1946) Right picture: charged Kaon decay

into muon and neutrino Kaons first called “V” particles Called “strange” because they behaved

differently from others

21

“Strange particles”

Kaon: discovered 1946; first called “V” particles

K0 production and decayin a bubble chamber

22

Bubble chamber

p p p n K0 K- + - 0

n + p 3 pions0 , e+ e-K0 + -

-

---

23

Particle Zoo

1940’s to 1960’s : Plethora of new particles discovered

(mainly in cosmic rays): e-, p, n, ν, μ-, π±, π0, Λ0, Σ+ , Σ0 , Ξ,….

question: Can nature be so messy? are all these particles really

intrinsically different? or can we recognize patterns or

symmetries in their nature (charge, mass, flavor) or the way they behave (decays)?

24

Particle spectroscopy era 1950’s – 1960’s: accelerators, better detectors even more new particles are found, many of them

extremely short-lived (decay after 10-21 sec) “particle spectroscopy era”

Bubble chamber allows detailed study of reactions, reconstruction of all particles created in the reactions

find that often observed particles actually originate from decay of very short-lived particles (“resonances”)

1962: “eightfold way”, “flavor SU(3)” symmetry (Gell-Mann, Ne’eman)

allows classification of particles into “multiplets” Mass formula relating masses of particles in same

multiplet Allows prediction of new particle Ω- , with all of its

properties (mass, spin, expected decay modes,..) subsequent observation of Ω- with expected properties

at BNL (1964)

25

Ω-

The bubble chamber picture of the first omega-minus. An incoming K- meson ( of momentum 5.0 GeV/c ) interacts with a proton of the liquid hydrogen in the bubble chamber and produces an omega-minus, a K° and a K+ meson:

K- + p ―› Ω- + K+ + K0

The omega minus then decays: Ω- ―> Ξ0 + π-, with subsequent decay Ξ0 ―> Λ + π0; Λ ―> p + π- π0 ―> γ γ γ ―> e+ e-

26

Ω-

27

Particle nomenclature by mass:

baryons – heavy particles .. p, n, Λ, Σ, Ω-, ++ ….

o nucleons and their excited states: p, n, N*, ++ , ……

o hyperons: Λ, Σ, Ξ, Ω- , and their excited states mesons – medium-heavy particles … π, K, K*, ρ, ω … leptons – light particles e, μ, νe, ν …

by spin:

fermions: spin = odd-integer multiple of ½: ½, 3/2, 5/2,……

o leptons and baryons bosons: integer spin 0, 1, 2, …. – mesons are bosons

by interaction: hadrons: partake in strong interactions leptons: no strong interaction

by lifetime: stable particles: lifetime > 10-17 sec

(decay by weak or electromagnetic interaction)

unstable particles (“resonances”) lifetime <10-20 sec (decay by strong interaction)

28

Spin and statistics Fermions obey “Fermi-Dirac” statistics:

multi-fermion states are antisymmetric with respect to exchange of any two identical fermions (wave function changes sign)

|f1, f2, f3 > = - |f2, f1 , f3 >

Pauli exclusion principle is special case of this Bosons obey “Bose-Einstein” statistics:

multi-boson states are symmetric with respect to exchange of any two identical bosons (wave function stays the same) |b1, b2 , b3 > = |b2, b1 , b3 >

consequence: bosons like to “stick together” (e.g. Bose-Einstein

condensate)

29

Towards the standard model

Quark Model (Gell-Mann, Zweig, 1964) observed SU(3) symmetry can be explained by

assuming that all hadrons are made of “quarks” There are 3 quarks: u (up), d (down), s

(strange); quarks have non-integer charge:

o u 2/3, d -1/3, s -1/3 baryons are made of 3 quarks:

o p = uud, n = udd, Λ = uds, Σ = uus Ξ = uss, Ω- = sss

mesons are made of quark-antiquark pairs:o π+ = ud, π- = u d, π0 = u u + d d, ……..

_ _ __

30

The 8-fold way

K0

-

K+

+0

K- K0sd

ud

su

du

ds us

uu,dd,ss

0

-

+

+0

- 0uss

uus

dss

dds

udd uud

uds

-

ddd++

uuu

-

sss

n p

mesonsqq

baryons qqq

31

color charge problem with quark model

Quarks have spin ½, i.e. are fermions must obey Pauli principle

Ω- = sss, has spin 3/2; spins of 3 s quarks must be aligned, i.e. Ω- has 3 quarks in identical state --- forbidden

similarly for ++ = uuu, spin 3/2

way out: quarks have additional hidden property – “color charge”

3 colors: green, red, blue each quark can carry one of three colors

o red blue green antiquarks carry anticolor

o anti-red anti-blue anti-green observed particles are “color-neutral”; only colorless (“white”) combinations of

quarks and antiquarks can form particles:

qqq qq

32

“Elementary” particles?

“leptons” (electron, muon and their neutrinos) are fundamental, interact electromagnetically and “weakly”

“hadrons” (p, n, Λ, Σ, Ω-, ++ , π, K, K*, ρ, ω,…) are not fundamental particles – are made of quarks, interact electromagnetically, “weakly”, and “strongly”

33

A theoretical model of interactions of elementary particles, based on quantum field theory

Symmetry: SU(3) x SU(2) x U(1)

“Matter particles” Quarks: up, down,

charm,strange, top, bottom Leptons: electron, muon, tau,

neutrinos “Force particles”

Gauge Bosonso (electromagnetic force)o W, Z (weak, electromagnetic)o g gluons (strong force)

Higgs boson spontaneous symmetry

breaking of SU(2) mass

Standard Model

34

Matter and forces Fundamental forces (mediated by “force particles”)

strong interaction between quarks, mediated by gluons (which themselves feel the force) (QCD)

o leads to all sorts of interesting behavior, like the existence of hadrons (proton, neutron) and the failure to find free quarks

Electroweak interaction between quarks and leptons, mediated by photons (electromagnetism) and W and Z bosons (weak force)

Fundamental constituent particles Leptons q = quarks q =

-1 e 2/3 u c t 0 e –1/3 d s b

Role of symmetry: Symmetry (invariance under certain

transformations) governs behavior of physical systems:

o Invariance “conservation laws” (Noether)o Invariance under “local gauge transformations”

interactions (forces)

35

36From Contemporary Physics Education Project

http://www.cpepweb.org/particles.html

37From Contemporary Physics Education Project

http://www.cpepweb.org/particles.html

38

From Contemporary Physics Education Project http://www.cpepweb.org/particles.html

39From Contemporary Physics Education Project

http://www.cpepweb.org/particles.html

40

Strong quark interactions quarks carry “color charge” (red,

blue, green) and interact exchanging gluons, the carriers of the strong force

theory of strong interaction is “gauge theory”; form of interaction governed by invariance under local SU(3)c (“color SU(3)”)

8 gluons carry color charge interact with each other

41

Electroweak interactions

leptons (and also quarks) carry a “weak charge” (in addition to usual electric charge)

they interact exchanging neutral EW force

carriers: photon , Z0

charged EW force carriers: W±

theory describing EW interaction is gauge theory; gauge symmetry group SU(2)xU(1)

42

Some milestones Quantum electrodynamics (QED) (1950’s)

(Feynman, Schwinger, Tomonaga) electroweak unification – the standard model

(1960s) (Glashow, Weinberg, Salam) deep inelastic scattering experiments – partons

(SLAC/MIT) (1956 – 1973) Quark Model (1964) (Gell-Mann, Zweig) Quantum Chromodynamics (1970s)

(Gross, Wilczek, Politzer) neutral weak current (1973) (Gargamelle, CERN) Charm discovery (1974) (S. Ting, B. Richter) Bottom quark discovery (1977) (L. Lederman) gluon observation (1979) (DESY) W,Z observation (1983) (UA1, UA2, C. Rubbia,

CERN) top quark (1995) (DØ, CDF, Fermilab) Higgs (2012) LHC experiments

43

Brief History of the Standard Model Late 1920’s - early 1930’s: Dirac, Heisenberg, Pauli, & others

extend Maxwell’s theory of EM to include Special Relativity & QM (QED) - but it only works to lowest order!

1933: Enrico Fermi introduces 1st theory of weak interactions, analogous to QED, to explain b decay.

1935: Hideki Yukawa predicts the pion as carrier of a new, strong force to explain recently observed hadronic resonances.

1937: muon is observed in cosmic rays – first mistaken for Yukawa’s particle

1938: heavy W as mediator of weak interactions? (Klein) 1947: pion is observed in cosmic rays

1949: Dyson, Feynman, Schwinger, and Tomonaga introduce renormalization into QED - most accurate theory to date!

1954: Yang and Mills develop Gauge Theories

1950’s - early 1960’s: more than 100 hadronic “resonances” have been observed !

1962 two neutrinos! 1964: Gell-Mann & Zweig propose a scheme whereby

resonances are interpreted as composites of 3 “quarks”. (up, down, strange)

44

Brief History of the Standard Model - 2

1970: Glashow, Iliopoulos, Maiani: 4th quark (charm) explains suppression of K decay into

1964-1967: spontaneous symmetry breaking (Higgs, Kibble)

1967: Weinberg & Salam propose a unified Gauge Theory of electroweak interactions, introducing the W±,Z as force carriers and the Higgs field to provide the symmetry breaking mechanism.

1967: deep inelastic scattering shows “Bjorken scaling” 1969: “parton” picture (Feynman, Bjorken)

1971-1972: Gauge theories are renormalizable (even when symmetry is spontaneoulsy broken) (t’Hooft, Veltman, Lee, Zinn-Justin..)

1972: high pt pions observed at the CERN ISR

1973: Quantum Chromodynamics (Gross, Wilczek, Politzer, Gell-Mann & Fritzsch) : quarks are held together by a Gauge-Field whose quanta, gluons, mediate the strong force

1973: “neutral currents” observed (Gargamelle bubble chamber at

CERN)

45

Brief History of the Standard Model - 3

1974: J/ discovered at BNL/SLAC; 1975: J/ interpreted as cc bound state (“charmonium”) 1976: t lepton discovered at SLAC 1977: discovered at Fermilab in 1977, interpreted as bb

bound state (“bottomonium”) 3rd generation

1979: gluon – jets observed at DESY 1982: direct evidence for jets in hadron hadron interactions at

CERN (pp collider)

1983: W±, Z observed at CERN (pp collider built for that purpose)

1995: top quark found at Fermilab (DØ, CDF) 1999: indications for “neutrino oscillations” (Super-

Kamiokande experiment) 2000: direct evidence for tau neutrino () at Fermilab

(DONUT experiment) 2005: clear evidence for neutrino oscillations (Kamiokande,

SNO)

-

-

-

-

-

-

46

Feynman diagrams Feynman Diagrams

In our current understanding, all interactions arise from the exchange of quanta

The mathematics describing such interactions can be represented by a diagram, called a Feynman diagram

ee

Feynmandiagram

47

Present scenario

Most of what’s around us is made of very few particles: electrons, protons, neutrons (e, u, d)

this is because our world lives at very low energy

all other particles were created at high energies during very early stages of our universe

can recreate some of them (albeit for very short time) in our laboratories (high energy accelerators and colliders)

this allows us to study their nature, test the standard model, and discover direct or indirect signals for new physics

48

Study of high energy interactions -- going back in time

49

Homework set 5

HW 5.1 go to Particle Data Group website:

http://pdg.lbl.gov find masses (in MeV or GeV) ,

principal decay modes and lifetimes of the following particles: π±, π0, K0, J/Ψ, p, n, Λ0, Σ+ , Σ0, Ξ, Ω-

give the quark composition of π±, π0, K0, J/Ψ, p, n, Λ0, Ω-

50

Summary

Particle physics was born during last century, grew out of atomic and nuclear physics

huge progress in understanding over last 50 years, due to revolutionary ideas in both theory and experiment

intense dialog between experimenters and theorists precision tests of standard model ongoing, looking

for hints of new physics next:

symmetries tests of standard model, experiments,

accelerators problems and shortcomings of standard model new projects, outlook