More on the Elementary Particles and Forces in the Universe

Dr. Mike StraussThe University of Oklahoma

1) What are the fundamental objects from which everything else in the universe is made?

2) What are the forces or interactions that hold these objects together and how do these forces work?

Two Questions Asked for Centuries

What are the fundamental objects in the universe from which everything else is made?

• This question has been pondered for over 2500 years– Ancient Greece (followers of Thales)

–Ancient Greece (Democritus)•Indivisible particles called - atomos

How are the fundamental objects held together?

or in more precise scientific language

What are the fundamental forces of nature?

• At the turn of the century, (that is in 1900) two fundamental forces were known:– Gravity– Electromagnetism

The Fundamental Particles in the Universe(Current Model)

Particles

• Leptons– Latin for “Light”– Usually found alone

• Quarks– A nonsense word in Finnegan’s Wake by James Joyce

– Always found in groups

The Atom

These electronsare fundamentalparticles (leptons).

Other fundamentalparticles (quarks) are buried deepinside the nucleus.

The Fundamental Forces in the Universe(Current Model)

Forces

• Gravity

• Electromagnetic Force

• Weak Nuclear Force

• Strong Nuclear Force– Only quarks and particles made from quarks

(hadrons) interact via this force

Electroweak

The Standard Model:A Theory of Everything (except gravity)

u c td s b

e- - -

e

six quarks(and antiquarks)

six leptons(and antileptons)

Strong force: 8 gluonsWeak force: W+, W-, Z0

Electromagnetic force:

(plus a lot of Nobel Prize winning math)

The Fundamental Particles: (Fermions)

The Fundamental Forces: (Bosons)

Charge = +2/3eCharge = 1/3e

And: Higgs Boson: H Not yet discovered

Quarks are very bizarre objects

• They have no size, but they do have mass. – (All “elementary” particles have no apparent size)

• They have charges that are fractions of the proton and electron charge.

• They cannot be isolated– No quark has ever been discovered by itself.

– They are always found in groups of three quarks or antiquarks (baryons) or one quark and one antiquark (mesons).

Terminology Review

Antiparticle: Every particle, including quarks, has an antiparticle. The charge and “quantum numbers” of the antiparticle are opposite that of the particle, and the mass is the same.

Hadron: Any particle made of quarks and/or antiquarks.

Baryon: Any particle made of three quarks. (Antibaryons are made up of three antiquarks.)

Meson: Any particle made of a quark and an antiquark.

Baryons Mesonsp: uud : udn: udd : uu uds : ud: sss K: usc:udc D: cup: uud

(electric charge) (electric charge)2/3+2/3-1/3=+1 2/3-(-1/3)=+12/3-1/3-1/3=0 2/3-2/3=02/3-1/3-1/3=0 -2/3-1/3=-1-1/3-1/3-1/3=-1 2/3-(-1/3)=+12/3-1/3+2/3=+1 2/3-2/3=0-2/3-2/3+1/3=-1

• Properties of hadrons can be explained from the properties of their constituents.

Selected Hadrons (Hundreds of hadrons have been discovered)

• Most of the visible matter in the universe is made of up and down quarks and electrons.

• Most of the known objects in the universe are made of matter and not antimatter.

The Forces of Nature

• Gravity: All objects in the universe are attracted to each other by this force.

• Electromagnetic*: Holds atoms and molecules together. Most of the phenomena we experience everyday is a result of this force.

• Weak Nuclear Force*: Responsible for radioactive decay.

• Strong Nuclear Force: Holds quarks together in hadrons and holds the nucleus together.

*A theory combining these two into an “electroweak” force was developed in the 1960’s and verified in 1983.

The Forces of Nature (continued)

Particles RelativeForce Carrier(s) Affected Strength Range

Gravity Graviton* All 10-38

EM Photon Charged 10-2

Weak W+, W-, Z0 All 10-1

<10-18 m

Strong Gluons (8) Quarks/Gluons 1 10-15 m Hadrons

*Not yet discovered. Not part of the “Standard Model”

How Do We Know the Fundamental Structure of Anything?

(How Do You Know How Your Car Works?)

• Be taught by someone who already knows

• Take it apart (or look inside)

• Put it together

Earnest Rutherford’s 1911 Experiment

“Pudding” “Plum Pudding”

“The Results”

Rutherford proposed the “Nucleus” to explain the results.

Looking Inside Very Small Objects

Early Evidence for Quarks (late 1960’s)(Looking Inside the Proton)

Proton (p)

Incomingelectron (e-)

Deep Inelastic Scattering

= h/p

h = 6.63 10-34 Jsp = mv (momentum)

The Wave Nature of MatterThe de Broglie Wavelength

In order to “see” an object, the wavelength of the probe must be smaller than the object being observed.

But How Do You Put Protons (or other particles) Together?

E = m0c2

E2 = m02c4

E2 = m02c4 + c2p2

Answer: Mass is a form of energy. If I can concentrate enough energy at any point (even energy of motion—kinetic energy), I can create any particle(s) with mass.

Particle accelerators can create matter (from other forms of energy)

Step 1: Accelerate two particles towards each other. They have a lot of energy from their motion, kinetic energy.

e- e+

Step 3: That energy can create any particle and its antiparticle with mass less than or equal to the total energy (E=mc2).

Step 2: Let them collide and annihilate each other to create energy or other particles.

“Feynman” Diagram of e+eAnnihilation

Time

Spa

ce

e+

e-

Photon or Z0

any fundamentalparticlee.g. -

the correspondingantiparticlee.g. +

Creating Hadrons

1. Quarks created from initial annihilation

2. Strong nuclear force acts like a rubber band

3. Eventually the “rubber band” breaks creating new quarks

Production of Hadrons

Time

Spa

ce

e+

e-

Photon or Z0

qq

qq

qq

qq

meson

meson

meson

meson

So Let’s Review

• What are the two classes of fundamental particles?

• Which class of fundamental particles are always bound together to make other subatomic particles?

• What are the four fundamental forces?

• Which force is so weak that it plays little role in the interactions of fundamental particles?

• Which principle of physics allows scientist to probe the structure of matter with high energy particles?

• Which principle of physics allows fundamental particles to be created in the laboratory?

Let’s Look at a Few Topics in More Detail

• Forces as Particles

• Quarks and Protons

• Benefits

The interaction between two particles can be thought of as the two particles exchanging another particle. In this case, the two people throw a basketball back and forth to change their momentum. The basketball is the “carrier” of the force or interaction.

What about the forces?Why are they described by particles?

Now consider an electron (with a negative charge)and a positron (with a positive charge) approaching each other at a rapid rate.

e-

e+

This can be thought of as the two particles exchanging a “photon” which, in turn, changes their

direction as indicted in this Feynman Diagram

Time

Spa

ce

e+

e-

Photon

e+

e-

Different quarks have different masses

The equation E=mc2 is used to define the mass of an object. In these units, a proton has a mass of about 1 billion electron volts (1 GeV/c2).

The mass of just one top quark is more than the entire mass of a gold nucleus which has 79 protons and 118 neutrons, or more than 591 up and down quarks!

(The following masses are in GeV/c2)

Up quark (u): 0.0004 Down quark (d): 0.0007Charm quark (c): 1.5 Strange quark (s): 0.15Top quark (t): 175 Bottom quark (b): 4.7

In a very basic model:

A neutron is made of 3 quarks: up, down, down (udd)

Charge: +(2/3) - (1/3) - (1/3) = 0

A proton is also made of 3 quarks: up, up, down (uud)

Charge: +(2/3) + (2/3) - (1/3) = 1

All the properties of the neutron and proton can be derived from the properties of its constituent particles.

Quarks have fractional charge

• The force that holds quarks together is called the strong nuclear force.

• There are 3 types of strong nuclear charge which can attract quarks to each other and cause them to bind together.

Why are quarks always bound together?

Strong charge

• Objects with strong charge interact via the strong force

• Three types of strong charge

Larry, Curly, MoeLarry, Curly, Moe

anti-larry, anti-curly, anti-anti-larry, anti-curly, anti-moemoe

knife, fork, spoonknife, fork, spoon

Three strong charges

colorcolor

Quantum Chromodynamics (QCD)Quantum Chromodynamics (QCD)

Every color is attracted to its anticolor

Hadrons in nature are colorlessBaryons:Baryons:• 3 quarks

– 1 green, one red, one blue

– Constantly changing color

• Antibaryons have 3 anti-quarks– With 3 different anti-colors

constantly changing

• Some Baryons– Proton

– Neutron

– Lambda

– Sigma– Anti-proton

MesonsMesons• 1 quark and 1 anti-quark

– Color and anticolor constantly changing

• Some Mesons– Pion

– Kaon

– Eta

• At any “moment” in a baryon, the three quarks are three different colors.

• At any moment in a meson, the quark is a particular color and the antiquark is the corresponding anticolor.

• Gluons can also carry color so they can interact with each other. – When gluons are exchanged between quarks, they can

change the color of the quarks. The type of quark, or flavor, cannot be changed by a gluon.

Quark and Gluon Color

A model of the Structure of a ProtonS

pace

Time

u

u

d

valence quarks

gluons

u

u

d

Virtual Particles Exist!It’s as if a tennis ball changed into a bowling ball and an “anti”-bowling ball for a brief moment, before turning back into a tennis ball.

E1 E2 E3

E1=E3

E = E2 E1

Et h/2

A more complete model of the Structure of a ProtonS

pace

Time

valence quarks virtual

“sea” quarks

q

qu

u

d

gluons

u

u

d

Neutron Decay and the Weak Force Described Using Particles

d

d

u

Time

Spa

ce

Pro

ton

u

d

u

Neu

tron

W-

e-

e

Question: The neutron has a mass of about 1 GeV/c2 and the W has a mass of about 84 GeV/c2. How is energy conserved in neutron decay?

Heisenberg’s Uncertainty Principle:

Et ≥ h/2mc2(d/c) ≥ h/2mc2 ≥ hc/2dd ≥ h/2mc

So if d < h/2mc a “virtual” particle can be produced.(h = 6.63 10-34 Js)

Answer: During the very brief period of time that the W exists, energy is not conserved? ...How can this be?

Benefits of High Energy Physics

• Answers questions about the structure and origin of the universe that have been pondered for millennia.

• Leads to future technology. Technological advances can only be made when the underlying physical principles are understood.– e.g. Electricity, Semi-conductors, Superconductors

• “Spin-off” applications result from technologies developed to accelerate, collide and detect particles.– CT scans, Proton Therapy, World Wide Web

• Builds a foundation for other areas of science.• Develops an educated work force.• Economic benefits (30% return on investment).