Transcript A Brief Tour of High Energy Physics
An Introduction to the Elementary Particles and Forces in the Universe
© Dr. Michael G. Strauss The University of Oklahoma 2006
Two Questions Asked for Centuries 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?
If you know the most fundamental particles of nature and how these objects are put together, you can construct a complete and accurate theory of the natural universe.
(Well, at least in theory) The modern science that studies the answer to these question is called “Elementary Particle Physics” or also “High Energy Physics”
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
Glass of Water An example: Water - H 2 O Oxygen Atom (~10 -10 m) Neutron or Proton (~10 -15 m) H 2 O Molecule (~10 -10 m) Oxygen Nucleus (~10 -14 m) Quark (<10 -18 m) ?
The Fundamental Particles in the Universe (at the turn of the new century) Particles • Leptons – Latin for “Light” – Usually found alone • Quarks – A nonsense word in
Finnegan’s Wake
by James Joyce – Always found in groups
The Fundamental Forces in the Universe (our knowledge at present) Forces • Gravity • Electromagnetic Force • Weak Nuclear Force Electroweak • Strong Nuclear Force – Only quarks and particles made from quarks (hadrons) interact via this force
The Standard Model: A Theory of Everything (except gravity) The Fundamental Particles: (Fermions) six quarks (and antiquarks) u c t d s b Charge = +2/3e Charge = 1/3e six leptons (and antileptons) e e The Fundamental Forces: (Bosons) Strong force: Weak force: Electromagnetic force: 8 gluons W + , W , Z 0 And: Higgs Boson: H Not yet discovered (plus a lot of Nobel Prize winning math)
These electrons are fundamental particles (leptons).
Other fundamental particles (quarks) are buried deep inside the nucleus.
The atom revisited
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 and electron charge.
fractions of the proton • 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).
Some Terminology 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.
(Yes Star Trek fans, there is such thing as antimatter.) 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.
Selected Hadrons (Hundreds of hadrons have been discovered) Baryons p: uud n: udd uds : sss c :udc p: uud (electric charge) 2/3+2/3-1/3=+1 2/3-1/3-1/3=0 2/3-1/3-1/3=0 -1/3-1/3-1/3=-1 2/3-1/3+2/3=+1 -2/3-2/3+1/3=-1 Mesons : ud : uu : ud K : us D : cu (electric charge) 2/3-(-1/3)=+1 2/3-2/3=0 -2/3-1/3=-1 2/3-(-1/3)=+1 2/3-2/3=0 • Properties of hadrons can be explained from the properties of their constituents. • 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.
Force Gravity EM
The Forces of Nature (continued)
Carrier(s) Graviton* Photon Particles Relative Affected Strength All 10 -38 Charged 10 -2 Range Weak W Strong + , W , Z 0 All 10 -1 Gluons (8) Quarks/Gluons Hadrons <10 -18 m 1 10 -15 m *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
Looking Inside Very Small Objects Earnest Rutherford’s 1911 Experiment “Pudding” “Plum Pudding” “The Results” Rutherford proposed the “Nucleus” to explain the results.
Early Evidence for Quarks (late 1960’s) (Looking Inside the Proton) Incoming electron (e ) Proton (p) Deep Inelastic Scattering
The Wave Nature of Matter The de Broglie Wavelength =
h
/
p p h
= 6.63 =
mv
10
-34
J
s (momentum)
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 = m 0 c 2 E 2 = m 0 2 c 4 E 2 = m 0 2 c 4 + c 2 p 2 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 2: Let them collide and annihilate each other to create energy or other particles.
Step 3: That energy can create
any
particle and its antiparticle with mass less than or equal to the total energy (
E
=
mc
2 ).
“Feynman” Diagram of e + e Annihilation e + any fundamental particle e.g. Photon or Z 0 e the corresponding antiparticle e.g. + Time
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
e + e Production of Hadrons Photon or Z 0 q q meson q q meson q q meson q q meson Time
Accelerating and Colliding Particles To accelerate a particle, the particle must be charged and stable: p, p, e , e + • Protons/antiprotons are more massive than electrons, so it is easier to produce higher energy collisions.
• Electrons/positrons are fundamental particles, so their collisions do not produce as many superfluous particles.
• Fermilab, USA: pp, 2000 GeV • SLAC, USA: e e + , 4 GeV • HERA, Germany: p,e + • KEK, Japan: e e + , 4 GeV • LHC, France: pp, 14,000 GeV (to be completed 2007)
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 • Particle accelerators and collisions • High Energy Physics and Cosmology • Questions and Benefits
What about the forces?
Why are they described by particles?
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.
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 e + e + Photon e Time e -
Different quarks have different masses
Physicists use Einstein’s equation E=mc 2 mass of an object. In these units, a proton has a mass of about 1 billion electron volts (1 GeV/c 2 to define the ). (The following masses are in GeV/c 2 ) Up quark (u): 0.0004
Charm quark (c): 1.5
Top quark (t): 175 Down quark (d): 0.0007
Strange quark (s): 0.15
Bottom quark (b): 4.7
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!
Quarks have fractional charge
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.
Why are quarks always bound together? - (part 1) • 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. • These strong charges are called “color” although they have nothing to do with real colors as we know them.
– red , anti-red – green – blue , , anti-green anti-blue Antiquarks have anticolor (The anticolor is similar to electric charges being positive or negative.)
Why are quarks always bound together? - (part 2) • Quarks are always found in nature bound together as “colorless” objects. This is called “confinement”. (We have some idea of why this is true, but not a complete understanding of it.) – Three primary colors make a colorless object (baryons) – A color and anticolor make a colorless object (mesons) – Each individual quark continually changes its color while maintaining a colorless hadron.
• The branch of physics dealing with how quarks and gluons interact with each other is called “ Q u a n t u m C h r o m o d y n a m i c s .”
Why are quarks always bound together? - (part 3) • 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.
A model of the Structure of a Proton valence quarks u u u u gluons d d Time
Neutron Decay and the Weak Force Described Using Particles e e W d d u u d u Time
Question: The neutron has a mass of about 1 GeV/c 2 and the W has a mass of about 84 GeV/c 2 . How is energy conserved in neutron decay?
Answer: During the very brief period of time that the W exists, energy is
not
conserved? ...How can this be?
Heisenberg’s Uncertainty Principle: So if
d
<
E
t
≥
h
/2
mc
2 (
d
/
c
) ≥
mc
2
d
≥
h
/2 ≥
hc
/2
d h
/2
mc h
/2
mc
a “virtual” particle can be produced.
(
h
= 6.63 10 -34 J s)
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.
E
1
E
2
E
3
E
1 =
E
3
E
=
E
2
E
t
h
/2
E
1
A more complete model of the Structure of a Proton valence quarks u virtual “sea” quarks q u q u u gluons d d Time
Colliding Objects
CDF DØ
DØ Detector
It’s a big camera that takes pictures of particle collisions
30’
DØ Detector
• Multiple components with different functions • Weighs 5000 tons • Inspect 3,000,000 collisions/second • Records 50 collisions/second 50’
Electromagnetic Calorimeter (
e
and ) Muon ( ) chambers Neutrinos? p T 0 Charged particle tracks
How quarks usually interact in the detector
q Remember that quarks are always confined!
Hadronization As quarks leave a collision, they change into a ‘shotgun blast’ of particles called a
‘jet’
q
Top Quark Production and Decay
15% 5%
Top Decay Modes
44%
Top quark events are characterized by their decay mode
15%
What Do Top Quarks Look Like?
An event in the DØ Detector that looks like a top quark
Count the number of top quark candidates and compare with a computer simulation
m t
175 GeV
High energy particle accelerators recreate the conditions shortly after the universe began when all the energy in the universe was concentrated into a very small volume.
High Energy Particle Physics is related to Cosmology and the Origin of the Universe About 15 billion years ago, or about 0.0000000001 (10 -10 ) seconds after the universe began, the energy density of the universe was very large.
In the first seconds after the big bang, the universe was very hot and very dense. All the energy of the universe was concentrated in a very small region. These are the same conditions created in modern particle accelerators.
Our study of the fundamental particles and forces has led us to the very origin of the universe.
Particle accelerators allow us to study the fundamental particles and forces in nature and to study the origin of the universe because the conditions in an accelerator reproduce conditions almost immediately after the Big Bang when: • Energy was very densely compacted • Only fundamental particles existed • The forces were combined into a “super” or “unified” force.
A History of the Universe… • The Big Bang occurred creating fundamental particles • “fundamental” quarks combined to make neutrons and protons... • which combined with electrons to make atoms … • which were attracted to each other because of gravity and collapsed together … • which created stars … • which combined together to create galaxies … • and in the galaxies stars exploded creating carbon and heavy elements … • which gathered together to make planets and us.
So studying the fundamental particles and forces helps us understand the early universe giving insight into our existence.
Now (14 billion years) Stars form (1 billion years) Atoms form (400,000 years) Nuclei form (3 minutes) Protons and neutrons form (10 -10 seconds) (10 -34 seconds?) 4x10 -12 seconds ??? (Before that)
High Energy Physics and Cosmology Big bang GUTs Quarks Hadrons Nuclei Galaxies Now Leptons Stars Time 10 -43 s 10 -35 s Temperature 10 32 K 10 27 K 10 -6 s 10 13 K Energy 10 16 TeV 10 11 TeV 1 GeV 3 min 300 kyr 15 Gyr 10 9 K 3000 K 3 K 0.1 MeV 0.3 eV 3 10 -4 eV Current accelerators probe to about .2 TeV 10 -11 s
The Unification of the Forces Strong EM Weak Electroweak Gravity 100 GeV Energy GUT TOE 10 15 GeV 10 19 GeV
The Fate of the Universe Depends on its Composition No clue what this is. Its currently causing the expansion of the universe to accelerate!
Discovered Matter Undiscovered Matter Too much matter and the universe will one day collapse!
The universe expands forever The universe will collapse A precise measurement of the cosmic background radiation.
What are some questions still unanswered?
1) Why do different particles have their different masses? Where is the Higgs Boson?
2)What about compositeness? Are quarks and leptons made of something more fundamental? (Maybe Strings?) 3) How are all three fundamental forces combined into a single force? (GUTs) 4) Why is the universe composed of so much matter and almost no antimatter?
5) Is there a more fundamental theory than the Standard Model? Something must exist at an energy about 1 TeV.
6) Are there a whole set of undiscovered particles? (supersymmetry) 7) What unexpected surprises await us?
Comments about the Higgs • “[the Higgs is] the toilet of the Standard Model; every house must have one; but no one likes to talk about it.” • “[the Higgs is] the rug of ignorance under which the problems of the Standard Model have been swept.” -David Kestenbaum • “[the Higgs] will smash open the Standard Model.” - Chris Hill • “a single Higgs is just dumb. It doesn’t explain anything.” -Chris Hill
The Standard Model and Its Limits “Physicists look on the Standard Model with a mixture of reverence and frustration. Since they have put it together, they have always known that it is incomplete... There must be a larger, more elegant theory.” - David Kestenbaum
The Large Hadron Collider (LHC)
The ATLAS Detector
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 understood.
. Technological advances can only be made when the underlying physical principles are – 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).