PDF of Powerpoint Presentation 2

The Standard Model of
Particles and Interactions:
What is the Universe Made of
and What Forces Drive its
Evolution
16 July 2007
Stu Loken

Standard Model Part 2
• As you learned Friday many particles were
discovered in cosmic rays and early
accelerators
• These led to the Quark Model
• Today we will go over what we have
learned about elementary particles since
the early experiments
• The theory that has evolved from these is
called the Standard Model of Particles and
Interactions

New Particles

Modern History

More lepton and quark generations
(heavy flavors)
• Discovery of charm quark Ting, Richter 1974
— m c ~1.5 GeV
• Discovery of tau lepton Perl 1975
— m τ ~1.8 GeV
• Discovery of bottom quark Lederman 1977
— m b ~4.7 GeV
• Discovery of mediators of weak interactions: W ± ,Z 0 Rubia 1983
— m Z ~ 90 GeV
— m W ~80 GeV
• Discovery of top quark D0 and CDF 1995
— m t ~176 GeV !!!

The Nucleus
• Nucleus is small and
dense; it was thought for a
while to be fundamental
• But still as many nuclei as
atoms
• Simplification – all nucleii
are made up of charged
protons and neutral
neutrons

Quarks
• We now know that even protons
and neutrons are not
fundamental
• They are made up of smaller
particles called quarks
• So far, quarks appear to be
fundamental (“point-like”)

The Modern Atom
• A cloud of electrons in
constant motion around
the nucleus
• And protons and neutrons
in motion in the nucleus
• And quarks in motion
within the protons and
neutrons

Generations
• The six quarks and the six
leptons are each organized into
three generations
• The generations are heavier
“Xerox” copies
• “Who ordered the 2 nd and 3 rd
generations?”
• The quarks have fractional
charges (+2/3 and -1/3) The
leptons have charge -1 or 0

How did we discover the fundamental
particles?
At a nuclear reactor
In cosmic rays
With accelerators

Size inside atoms
• The nucleus is 10,000
times smaller than the
atom
• Proton and neutron are 10
times smaller than nucleus
• No evidence that quarks
have any size at all !

What is the World made of?
• The real world is not made of individual quarks
(more on that later)
• Quarks exist only in groups making up what we
call hadrons: (proton and neutron are hadrons)
• E.g. a proton is 2 up quarks and 1 down quark
• We are all made from up and down quarks and
electrons

Matter and Antimatter
• For every particle ever
found, there is a
corresponding antimatter
particle or antiparticle
• They look just like matter
but have the opposite
charge
• Particles are created or
destroyed in pairs

Particles can decay
• Particles may decay, i.e.
transform from one to
another
• Most are unstable
• Proton and electron are
stable
• Neutron can decay to
electron and a proton
• Energy appears to be
missing. It is carried off by
a neutrino

What about Leptons?
• There are six leptons, three charged and three
neutral
• They appear to be point-like particles with no
internal structure
• Electrons are the most common and are the only
ones found in ordinary matter
• Muons (µ) and taus (τ) are heavier and charged like
the electron
• Neutrinos have no charge and very little mass

Matter Summary
• So all the universe is made of First Generation
quarks and leptons
• We now turn to how the quarks and leptons
interact with each other, stick together and decay

Four Forces
• There are four fundamental
interactions in nature
• All forces can be attributed to
these interactions
• Gravity is attractive; others can
be repulsive
• Interactions are also
responsible for decay

Strong Force
• In addition to their electric
charge, quarks also carry
a new kind of charge
called color charge
• The force between color
charged particles is the
“strong force”

The Gluon
• The strong force holds
quarks together to form
hadrons
• Its carrier particles are
called gluons; there are 8
of these
• The strong force only acts
on very short distances

Weak Force
• Weak interactions are responsible
for the decay of massive quarks
and leptons into lighter quarks and
leptons
• Example: neutron to decay into
proton + electron + neutrino
• This is why all matter consists of
the lightest quarks and leptons
(plus neutrinos)

Electroweak Force
• In the Standard Model, the
weak and the electromagnetic
forces have been combined
into a unified electroweak
theory
• At very short distances (~10 -18
meters), the weak and
electromagnetic interactions
have comparable strengths
• Force particles are photon, W
and Z

What about Gravity?
• Gravity is very weak
• Relevant at macroscopic
distances
• The gravity force carrier, the
graviton, is predicted but has
never been seen

Interaction Summary

Neutron Beta Decay

Electron-Positron Annihilation

Top Production

Mysteries and Failures
• The Standard Model is a theory of the universe
• It provides a good description of phenomena
observed by experiments
• It is still incomplete in many ways: why 3
generations? What is dark matter?

Is the Standard Model Wrong?
• We need to go beyond the Standard Model in the
same way that Einstein’s Theory of Relativity
extended Newton’s laws of mechanics
• We will need to extend the Standard Model with
something new to explain mass, gravity, etc.

Three Generations
• There are three sets of
quarks and three sets of
leptons
• Why are there exactly
three generations of
matter?
• Why do we see only one
in the real world?

What About Masses?
• The Standard Model cannot
explain why a particle has a
certain mass
• Physicists have theorized
the existence of a new field,
called the Higgs field,
which interacts with other
particles to give them mass
• So far, the Higgs has not
been seen by experiment

Grand Unified Theory
• We believe that GUT will
unify the strong, weak and
electromagnetic forces
• All three forces would be
different aspects of the
same, unified interaction
• The three forces would
merge into one at high
enough energy

Supersymmetry
• Some physicists
attempting to unify gravity
with the other fundamental
forces have suggested
that every fundamental
particle should have a
massive “shadow” particle

• Modern physics has theories for quantum
mechanics, relativity and gravity but they do not
quite work with each other
• If we lived in a world of more than three spatial
dimensions, these problems can be resolved
• String theory suggests that in a world with three
ordinary dimensions and some additional very
small dimensions, particles are strings and
membranes

• String Theory requires
more than three space
dimensions
Extra Dimensions
• These extra dimensions
could be very small so that
we do not see them
• Experiments are now
searching for evidence of
extra dimensions

Dark Matter
• It appears that the
universe is not made of the
same kind of matter as our
sun and the stars
• The dark matter does exert
a gravitational attraction
on ordinary matter but has
not been detected directly

The Accelerating Universe
• Recent experiments
using Type Ia
Supernovae have shown
that the universe is still
expanding and the rate of
expansion is increasing
• This acceleration must be
driven by a new
mechanism which has
been named dark energy

The Expanding Universe
• Studies of the most
distant supernova ever
detected indicates that
the universe did go
through a phase where
the expansion slowed
down
• It is now speeding up

Recent Discoveries Indicate
Particle Physics Has Reached a Singular Moment

• Today –Tevatron
• Soon – Large
Hadron Collider
• Tomorrow –
Linear Collider
Current and Future
Experimental Work

ATLAS at CERN

ATLAS Detector at CERN’s Large
Hadron Collider
Inner Tracking Detector

ATLAS – November 4, 2005 AM

Einstein’s Dream
• Is there an underlying
simplicity behind vast
phenomena in Nature?
• Einstein dreamed of
coming up with a unified
description of the forces
• But he failed to unify
electromagnetism and
gravity (GR)

planets apple
gravity
GR
String theory?
mechanics
History of Unification
Special relativity
electric magnetic
electromagnetism
Quantum ElectroDynamics
Electroweak theory
Grand Unification?
atoms
Quantum mechanics
Weak force
γ-decay
Strong force
β-decay
α-decay

A Promising Model –
Supersymmetry(SUSY)
• Additional symmetries have to be
proposed to extend the SM to
solve the Dark Matter problem
• One approach is to double the
particle spectrum by positing a
symmetry that links fermions to
bosons…giving a sparticle paired
with each SM particle, differing by
_ unit of spin
• Originally studied for other
reasons…

Problems Possibly ‘Solved’ by
Supersymmetry
• Dark Matter problem – the lightest SUSY particle is stable => candidate for
Dark Matter
• Explanation for how mass is generated in the SM => requires a heavy top
quark, as was observed after the SUSY ‘prediction’
Introduces a higher
level of symmetry
that stabilizes the
theory against higher
order corrections
(solves the hierarchy
problem)
Provides for
Unification of the
forces into a single
force at very high
energies
(alternatively, at very
short distances or in
the very early
Universe)

Standard Model and Supersymmetry

SUSY and Dark Matter
• Most SUSY models have unbroken R-parity that guarantees
that lightest sparticle (LSP) is stable
• LSP must be neutral – candidates are B W0 ~ ~ H ~ ν and ~ G ~
• ~ ν is strongly disfavored by LEP and direct searches
• Parameters m 1/2 (gaugino masses), m 0 (squark masses), tan
β, sign(µ), A, specified at GUT scale, fully describes model.
• LSP is usually B ~
and mass controlled by m1/2

• Answers to
origins of Dark
Matter and Dark
Energy likely
very soon
• Many exciting
possibilities
• One of several
thrusts in
particle physics
today
Answers coming?
Expect a 21 st Century Revolution in Particle Physics

Conclusion
• The Standard Model is a powerful synthesis that
explains a huge number of observations in a
simple framework. It is to physics what evolution
is to biology.
• There are many important questions beyond the
Standard Model