Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 1

Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 2

Physics 301: Physics Today Prof. Wesley Smith, wsmith@hep.wisc.edu Undergraduate Physics Colloquium

•! Discussions of current research topics in physics by the scientists involved in those studies

•! You are encouraged to contact these researchers to find out more about, and possibly participate in their research programs.

Coursework •! Paper describing a particular piece of physics

research being actively pursued this year. •! Outline due April 7 •! Paper (7-10 pages) due on May 5 •! More information on course web page

Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 3

Physics 301 Schedule Wesley Smith Introduction/Particle Physics Jan. 20, 2015

Matt Herndon The Higgs Boson Jan. 27, 2015Mark Rzchowski Building new physics with atomic-level design Feb. 3, 2015

Franz Himpsel Photovoltaics Feb. 10, 2015Albrecht Karle High Energy Neutrino Astrophysics with Ice Cube Feb. 17, 2015Duncan Carlsmith Direct Search for Dark Matter Feb. 24, 2015

Peter Timbie Physics of the Early Universe March 3, 2015John Kelley South Pole Ultra-high-energy neutrinos w/ARA March 10, 2015

Maxim Vavilov Quantum Information March 17, 2015Mark Saffman Atoms and Computers March 24, 2015Clint Sprott Chaos April 7, 2015Yang Bai Puzzles for Particle Physics April 14, 2015Mark Eriksson Semiconductor Quantum Dot-Based Qubits April 21, 2015Dan McCammon X-ray Astronomy April 28, 2015Bob Joynt Superconductivity May 5, 2015

Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 4

Particle Physics

Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 5

Interactions between matter particles

Matter particles interact via exchange of force particles

Nuclei- need a strong interaction to overcome coulomb repulsion of the protons. “Gluons” are the force carriers

Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 6

Normal electromagnetic force comes about from exchange of photons.

Electromagnetic repulsion via emission of a photon

electron

electron

photons

•! For a very quick interaction we can see individual photon exchanges

Exchange of many photons allows for a smooth force (EM field)

Normal electromagnetic force comes about Electromagnetic Force

Probability proportional to coupling strength divided by momentum

Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 7

The new EM Theory has one very interesting additional feature Can rotate diagrams in any direction

Time goes from left to right. What is an electron going backward in time?

electron

electron

photon electron

electron

photon positron

positron

Antiparticles! Anti-electron or positron. This is going to be a useful way to make new particles.

Also learned from studying EM force that the proton and neutron were made of smaller particles. up and down quarks. p=uud, n=udd

Annihilation

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Forces and Field Particles

Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 9

Many particles discovered Leptons Electron, neutrino, and 4 others

Paired with 6 antiparticles Fundamental – don’t subdivide Relatively light mass

Hadrons Proton, neutron, and many others Paired with antiparticles Interact through the strong force Can decay through weak force Not fundamental – subdivide into quarks Two groups: Mesons – made of 2 quarks – intermediate mass Baryons – made of 3 quarks – heavier mass

Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 10

Many particles discovered

Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 10

Many particles discovered

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Quarks Since 1969, many other experiments have been conducted to determine the underlying structure of protons/neutrons. (~GeV) All the experiments come to the same conclusion. ! Protons and neutrons are composed of smaller constituents.

(1.6 x 10-15 m)

1x 10-18 m (at most)

"!Protons 2 “up” quarks 1 “down” quark

"!Neutrons 1 “up” charge 2/3 2 “down” -1/3

Are there any other quarks other than UP and DOWN ?

Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 12

Three Families of Quarks Generations

I II III

Charge = -1/3

d (down) s

(strange) b

(bottom)

Charge = +2/3

u (up)

c (charm)

t (top)

Also, each quark has a corresponding antiquark The antiquarks have opposite charge to the quarks Many hadrons possible: 3 quarks baryons, 1 quark +1 antiquark mesons

Note: fractionally charged particles!

Increasing mass

u,d, s,...

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Leptons: Muon

µ-e-

m=0.51 MeV/c2 m=106 MeV/c2

The muon was discovered in cosmic ray experiments (1937). It was also used in the experimental test of time dilation. We find that a muon behaves almost identical to an electron, except its mass is about 200 times more than the electron’s mass. Also neutral leptons: neutrinos Neither bind to form hadrons. Don’t feel strong force

Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 14

Three happy families… ! In 1975, researchers at the Stanford Linear Accelerator discovered

a third charged lepton, with a mass about 3500 times that of the electron. It was named the !-lepton. ! In 2000, first evidence of the !’s partner, the tau-neutrino ("!)

was announced at Fermi National Accelerator Lab.

Family Leptons Q = -1 Q = 0

1 e- "e

2 µ "µ

3 ! "!

3 families, just like the quarks… interesting !!!

Q = +1

e+

µ+

!+

Q = 0

"e

"µ"!

Anti-Lepton

Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 15

Conservation Law: Baryon Number Whenever a baryon is created in a reaction or a

decay, an antibaryon is also created B is the Baryon Number

•! B = +1 for baryons •! B = -1 for antibaryons •! B = 0 for all other particles

The sum of the baryon numbers before a reaction or a decay must equal the sum of baryon numbers after the process

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Conservation of Lepton Number There are three conservation laws, one for each variety

of lepton Law of Conservation of Electron-Lepton Number states

that the sum of electron-lepton numbers before a reaction or a decay must equal the sum of the electron-lepton number after the process

Assigning electron-lepton numbers •! Le = 1 for the electron and the electron neutrino •! Le = -1 for the positron and the electron antineutrino •! Le = 0 for all other particles

Similarly, when a process involves muons, muon-lepton number must be conserved and when a process involves tau particles, tau-lepton numbers must be conserved •! Muon- and tau-lepton numbers are assigned similarly to

electron-lepton numbers

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Strange Particles Some particles discovered in the 1950’s were found to exhibit

unusual properties in their production and decay and were given the name strange particles •! These include K, !, ".

Peculiar features include •! Always produced in pairs •! Although produced by the strong interaction, they do not decay into

particles that interact via the strong interaction, but instead into particles that interact via weak interactions •! They decay much more slowly than particles decaying via strong interactions

To explain these unusual properties, a new law, conservation of strangeness, was introduced •! Also needed a new quantum number, S

The Law of Conservation of Strangeness states that the sum of strangeness numbers before a reaction or a decay must equal the sum of the strangeness numbers after the process

Strong and electromagnetic interactions obey the law of conservation of strangeness, but the weak interactions do not

Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 18

‘Charge’

What does it really mean for a particle to have electric charge ?

It means the particle has an attribute which allows it to talk to (or ‘couple to’) the photon, the mediator of the electromagnetic interaction.

u Electric charge = +2/3

e Electric charge = -1

The ‘strength’ of the interaction depends on the amount of charge.

u u e e

Which of these might you expect experiences a larger electrical repulsion?

Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 19

Proposed a massive force-carrying particle for the weak force

P ! F2 ! e4/(p2+MB2)2

Coupling strength: Same as EM force

p momentum of the W or Z bosons

•! If the mass of the W boson was large compared to the momentum the probability of a weak interaction could be very low!

•! Same formula for electric and weak forces: •! Put in MB large for the W boson •! Put in MB=0 for the photon •! Unified the forces! •! MW = 80GeV •! Later seen directly

u u d

d u d

n p

e -

"e W -

Proposed a massive force-carrying particle for the Weak Force

Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 20

EM Weak Strong Gravity

Couples to: Particles with

electric charge

Weak charge: quarks and

leptons

Color charge: quarks

All particles with mass

Example

Attraction between

protons and electrons

Nuclear beta decay and

nuclear fission

Holds protons and neutrons together the

nucleus

Only attractive

Quanta: Force Carrier

Photon W and Z Boson Gluon Graviton

Mass 0 80 and 91 GeV 0 0

Strength in an Atom F = 2.3x10-8N

Decays can take thousands

of years F = 2.3x102N F = 2.3x10-47N

EM Weak Strong Gravity The Forces Revisited

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Fermions and Bosons A fermion is any particle that has an odd half-integer (like 1/2,

3/2, and so forth) spin. Quarks and leptons, as well as most composite particles, like protons and neutrons, are fermions.

A consequence of the odd half-integer spin is that fermions obey the Pauli Exclusion Principle and therefore cannot co-exist in the same state at same location at the same time.

Bosons are those particles which have an integer spin (0, 1, 2...). All the force carrier particles are bosons, as are those composite

particles with an even number of fermion particles (like mesons).

*Graviton has spin 2

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What is the Standard Model? •! Explains the hundreds of

common particles: atoms - protons, neutrons and electrons

•! Explains the interactions between them

Basic building blocks •! 6 quarks: up, down… •! 6 leptons: electrons… •! Bosons: force carrier particles

All common matter particles are composites of the quarks and leptons and interact by exchange of the bosons

What is the Standard Model? What is the Standard Model? The Standard Model

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Origin of mass - the Higgs mechanism

Simplest theory – all particles are massless !! A field pervades the universe Particles interacting with this field acquire mass – the stronger the interaction the larger the mass The field is a quantum field – the quantum is the Higgs boson Finding the Higgs particle establishes the presence of the field

Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 24

Higgs Mechanism

Which of these falls more slowly? •! An unopened parachute •! Fully opened parachute

The greater the interaction with the medium (air) the lower the falling speed.

Higgs field permeates all space, particles interact with differing strengths with the Higgs field. The higher the interaction the larger the mass of the particle.

The simplest theory with Higgs fields results in a new self-interacting particle: the Higgs boson, which itself has a mass – but, theory can’t predict its mass.

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Further investigations: The Problems of Standard Model

Higgs self energy corrections •! Higgs couples to itself •! However, this coupling becomes infinite! •! Contributions ot this from fermions (leptons and quarks)

and vector bosons (W, Z) come with opposite sign •! SM particles can mitigate to about 1 TeV energy scale •! However, new physics should show up at ! few TeV

Super-symmetry •! What if there are equal number of fermions and bosons in

the real theory at high masses (~few TeV)? •! Many new fundamental scalar and fermionic fields # must be massive to fit observations

•! But, this could solve the problem of Higgs divergences #

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Supersymmetry

A new physics theory which doubles known particles again – but the new particles have very large mass

LHC may be able to produce them Dark matter candidates LHC may be able to produce them

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The Bullet Cluster (1E 0657-56). Two galaxies colliding. Red shows concentration of visible matter. Blue shows dark matter inferred by gravitational lensing.

Gravitational lensing

What is dark matter composed of? •!Supersymmetric particles perhaps? The lightest supersymmetric particle predicted by theory has all the right properties!

Outstanding Mysteries – Dark Matter

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Dark Matter & Energy?

The calculated makeup of the Universe We only understand 4.6% of it after 100s of years of trying !!

Don’t know what Dark Matter is Don’t know what Dark Energy is but SOMETHING is accelerating the expansion of our Universe

Supersymmetry ? The ‘Neutralino’ particle? A new force field particle, like the Higgs,