Introduction%20to%20High%20Energy%20Physics%20for%20Anyone%20Interested

1
Introduction to High Energy Physics for Anyone
Interested
Natalia Kuznetsova Fermi
National Accelerator Laboratory
Outline

• The main questions particle physics attempts to
  answer
• What are things made out of?
• What holds things together?
• How do we know what we know about particles?
• What are the remaining unanswered questions?
• What are the prospects for high energy physics in
  the U.S.?

2
What is particle physics?

• Particle physics addresses some of the most
  fundamental questions that people have been
  pondering for centuries
• What are the building blocks of matter?
• Why are these blocks what they are? Can we
  explain their properties, such as mass?
• What holds them together?

• In a way, particle physics is complementary to
  cosmology
• cosmology studies the largest possible objects
  (such as galaxies, with hundreds of billions of
  stars!), and particle physics studies the
  smallest possible objects imaginable.

3
What is elementary?

• What is the most elementary building block of
  matter? First, we need to define elementary
• Let us define an elementary particle as something
  that
• has no discernable internal structure
• appears pointlike.
• First, people thought that the atom was
  elementary

The atom, as it was envisioned around 1900
-- a ball with electrical charges inside,
bouncing around!
4
The atom has a rich structure!

• Eventually, it was realized that the atom is not
  elementary
• it consists of a positively charged nucleus and
  negatively charged electrons.
• The properties of outermost electrons in atoms
  give rise to chemistry and biochemistry, with all
  of its complexity!
• The electron, as far as we know, is elementary!

electron
nucleus
If the nucleus were as big as a baseball, then
the entire atom's diameter would be greater than
the length of thirty football fields!
5
Is the nucleus elementary, too?

• Unlike the electron, the nucleus is not
  structureless! It consists of protons and
  neutrons.
• But protons and neutrons are not elementary,
  either!
• They consist of quarks, which to the best of our
  knowledge are elementary.

nucleus
neutron
proton
6
What exactly are quarks?

• Quarks are elementary building blocks of matter
  that are only found inside other particles, such
  as protons and neutrons, which most of the matter
  around us is made out of (including you!).

There are 6 quarks, and they come in pairs
up
charm
top
UCSB HEP group logo
down
strange
beauty
7
Hadrons

• Quarks are never found by themselves, but only
  with other quarks inside hadrons.

Baryons three quarks
Mesons two quarks
protons and neutrons are in fact baryons, made
out of u and d quarks
examples of mesons are pions (p) and kaons (K)
8
Hadrons are everywhere you look!
Everything you can look at contains the simplest
hadrons protons and neutrons!
9
What about the electron?

• We said earlier that apart from the six quarks,
  the electron was also elementary.
• It turns out that the electron is not alone --
  it belongs to a group of six particles called
  leptons! Just like quarks, leptons come in pairs

Electron neutrino
Muon neutrino
Tau neutrino
nm (massless (?))
nt (massless (?))
ne (massless (?))
m (mass 205 x mass of e)
t (mass 3503 x mass of e)
e
electron
muon
tau
10
What are neutrinos?

• W. Pauli postulated their existence in order to
  save the energy conservation principle in certain
  types of radioactive decays, known as
  beta-decays
• E. Fermi called them "neutrinos" -- "little
  neutrons" in Italian.
• Neutrinos hardly interact with anything at all.
  In fact, the earth receives more than 40 billion
  neutrinos per second per cm2. Most of them just
  pass through the earth, as if it's not even
  there!

neutron decays into proton plus electron plus
neutrino
11
Antimatter

• Strictly speaking, the particle produced in a
  beta-decay is called an anti-neutrino.
• There is an anti-particle for every particle.
  The only difference between them is that they
  have opposite charges.
• The predominance of matter over antimatter in
  the Universe is one of the biggest mysteries of
  modern high energy physics and cosmology!

A photon (which leaves no trace) produced an
electron and an anti-electron (positron), which
curl in opposite directions in a magnetic field.
anti-electron (positron)
electron
12
The Standard Model

• The most compehensive theory developed so far
  that explains what the matter is made out of and
  what holds it together is called the Standard
  Model.
• In the Standard Model, the elementary particles
  are
• Why do quarks and leptons come in sets (which are
  called generations)? Why are there three of
  them? We don't know.
• Note that the Standard Model is still a model
  because it's really only a theory with
  predictions that need to be tested by experiment!

• 6 quarks (which come in three sets)
• 6 leptons (which also come in three sets)

13
What holds it together?

• Things are not falling apart because fundamental
  particles interact with each other.
• An interaction is an exchange of something.
• But what is it that particles exchange? There is
  no choice -- it has to be some other special type
  of particles! They are called mediating
  particles.

A rough analogy of an interaction the two tennis
players exchange a ball
14
Four fundamental interations

• There are four fundamental interactions between
  particles

Interaction
Mediating particle
Who feels this force
Strong
Gluon (g)
Quarks and qluons
Photon (g)
Electromagnetic
Everything electrically charged
Weak
W and Z
Quarks, leptons, photons, W, Z
Gravity
Graviton (?)
Everything!
15
The strong interaction

• The strong force holds together quarks in
  neutrons and protons.
• It's so strong, it's as if the quarks are
  super-glued to each other! So the mediating
  particles are called gluons.
• This force is unusual in that it becomes stronger
  as you try to pull quarks apart.
• Eventually, new quark pairs are produced, but no
  single quarks. That's called quark confinement.

QUARK
16
The electromagnetic interaction

• The residual electromagnetic interaction is
  what's holding atoms together in molecules.
• The mediating particle of the electromagnetic
  interaction is the photon.
• Visible light, x-rays, radio waves are all
  examples of photon fields of different energies.

opposite charges attract
17
The weak interaction

• Weak interactions are indeed weak
• Neutrinos can only interact with matter via weak
  interactions -- and so they can go through a
  light year of lead without experiencing one
  interaction!
• Weak interactions are also responsible for the
  decay of the heavier quarks and leptons.
• So the Universe appears to be made out of the
  lightest quarks (u and d), the least-massive
  charged lepton (electron), and neutrinos.

1 light year
nm
n
18
Gravity

• The Standard Model does not include gravity
  because no one knows how to do it.
• That's ok because the effects of gravity are tiny
  comparing to those from strong, electomagnetic,
  and weak interactions.
• People have speculated that the mediating
  particle of gravitational interactions is the
  graviton -- but it has not yet been observed.

19
How do we know what we know?

• Some of the major High Energy Physics
  laboratories
• European Organization for Nuclear Research (CERN)
• Stanford Linear Accelerator Center (SLAC)
• Fermi National Accelerator Laboratory (FNAL)
• What's actually hapenning there?
• How can we "look inside" tiny particles?
• What are accelerators?
• What are detectors?

20
CERN European Organization for Nuclear Research

• The laboratory is located on the
• Swiss-French border, near
• Geneva (an awesome location!).
• It was founded in 1954, one of
• the first examples of a major
• international endeavor.
• Currently, it includes 20
• European countries as member
• states.
• CERN is the birth place of WWW!

5.6 miles
21
SLAC Stanford Linear Accelerator Center

• SLAC is located near the beautiful Stanford
  University campus, at Menlo Park in California
  (20 min. to the ocean).
• The research performed at SLAC has been
  recognized with three Nobel Prizes in physics!
• http//www.slac.stanford.edu is the first U.S.
  Web site!

22
FNAL Fermi National Accelerator Laboratory

• Fermilab is located in Batavia, Illinois (about
  an hour west of Chicago).
• Fermilab is home to the Tevatron, the worlds
  highest-energy particle accelerator.
• Fermilab is also a park, with 1,100 acres of
  prairie-restoration land!

23
How do we study tiny particles?

• Recall how we perceive the world we detect light
  (photons) bouncing off objects.
• But we cant use light to see atoms (not to
  mention, whats in them!).
• Thats because visible light waves have too low
  energy -- or too large a wavelength.

wavelength
24
But we can use something other than light!

• Its not just light that has wave properties -
  all particles do!
• The higher the particles momentum, the smaller
  its wavelength.
• Therefore, the more sensitive it is to small
  objects.

Slow electron large wavelength wave
Fast electron small wavelength wave
25
Accelerators

• Accelerators are machines used to speed up
  particles to very high energies. This way, we
  achieve two things
• We decrease the particles wavelength, so we can
  use it to poke inside atoms.
• We increase its energy, and since E mc2, we can
  use that energy to create new, massive particles
  that we can study.

PEP-II accelerator at SLAC
26
Collisions are important events!

• After particles have been accelerated, they
  collide either with a target (fixed target
  experiments) or with each other (colliding beam
  experiments).
• These collisions are called events.
• New particles are created in such a collision.
  Most of them quickly decay, but we can look at
  their decay products using detectors.

27
Events

• Depending on the energy of the colliding
  particles, the events can be very messy, with
  lots of stuff flying out, or they can be
  relatively clean.
• The products of collisions are looked at using
  detectors.

An event from the OPAL experiment at CERN
An event from the BaBar experiment at SLAC
28
Our detectors are HUGE!
ALEPH detector at CERN
CDF detector at FNAL
A lot of HEP detectors are as big as a house --
several stories high!
29
Collaborations

• Because the experiments are so big, it takes a
  very large group of physicists and engineers to
  get things working.
• Such groups of scientists are called
  collaborations. The major collaborations around
  the world include hundreds or thousands of people
  from tens of countries!

ATLAS Collaboration at CERN nearly 2,000 people!
30
Why are they so big?

• The history of high energy physics is one of a
  relentless climb to higher and higher energies.
• Comparing to one of the first discovered
  elementary particles, the electron, some of the
  particles we are studying now are about 400,000
  times heavier!

31
Anatomy of a detector silicon vertex detector

• Many particles decay very close to where they
  were produced.
• Thats why at the heart of many detectors is a
  device needed for finding just where this
  happened.

The vertex point
The silicon vertex detector used in the BaBar
experiment at SLAC
32
Anatomy of a detector tracking chambers
3. electron (from muon)

• Charged particles leave tracks by ionizing gas in
  tracking chambers.
• We can learn a lot by studying these tracks --
  for example, a particles momentum!

2. muon (from pion)
1. pion
Glowing gas along particle tracks in a streamer
chamber!
33
Another tracking chamber example

• This tracking chamber is filled with helium gas.
• Charged particles ionize this gas and leave
  tracks in the chamber.
• Lots of wires are strung the length of the
  chamber to pick up electrical signals due to the
  ionization.

The tracking chamber used in the BaBar
experiment at SLAC
34
Anatomy of a detector particle identification

• When a charged particle travels in some medium
  (e.g., water) faster than light does, it emits
  Cherenkov light.
• By analyzing this light, physicists can in some
  cases tell what kind of a charged particle it was.

Rings of Cherenkov light from the
Super-Kamiokande experiment in Japan.
35
Anatomy of a detector calorimeters

• Calorimeters allow physicists to measure the
  total energy deposition of some particles.
• This, in turn, allows us to tell what kind of
  particles they are.

showers!
photon
positron
electron
Study of electromagnetic calorimeter performance
for the CMS detector at CERN (not yet built)
36
Anatomy of a detector bringing it all together

• Different sub-detectors in a single particle
  detector are used for detecting and analyzing
  different types of particles

37
Other detectors neutrino detectors
Sudbury Neutrino Observatory in Canada
Super-Kamiokande experiment in Japan
38
What are the unanswered questions?

1. Why is there so much matter in the Universe and
   almost no anti-matter?
2. What's "dark matter"?
3. Why are there three generations of quarks and
   leptons?
4. Are quarks and leptons really fundamental?
5. Why are the particle masses what they are?
6. How can we unify gravity with the other three
   forces?
7. .

39
Matter-antimatter asymmetry

• In the Big Bang, we think that matter and
  antimatter were created in equal amounts. So
  where did the antimatter go?
• There must be some asymmetry in the behavior of
  particles and antiparticles.
• This effect is called "CP asymmetry", and an
  example has just been observed by the BaBar
  experiment at SLAC in the decays of particles
  called B mesons!

40
Grand Unification

• One day, there will exist a theory that unifies
  all three forces electromagnetic, weak, and
  strong.
• Physicists have speculated that this merging of
  all the forces may occur at a very high energy.

All three forces may merge at an energy of 1019
GeV, which is about 1,000,000,000,000,000,000,000,
000,000 times larger than the energies we are
used to dealing with in our everyday life!
41
Where does gravity fit in?

• Theories attempting to unify gravity with the
  other three forces are still in their infancy,
  but one of them, called supersymmetry, looks
  quite promising.
• Supersymmetry, in turn, follows naturally from a
  really mind-boggling theory called string theory,
  where all particles are treated as strings, and
  which requires extra space dimensions!

extra spacial dimensions!
one string
merge into yet another string!
plus another string
42
Practical applications of high energy physics

• Basic research always pays off in the long run.
• Apart from invaluable scientific advancement, the
  tools and methods used in fundamental science
  often find important practical applications, such
  as
• Medical physics (e.g., cancer treatment, drug
  improvement).
• Environmental applications (e.g., characterizing
  environmental wastes using synchrotron
  radiation).
• Computing applications.
• Remember that WWW is one of the high energy
  physics spin-offs!
• And much much more!

43
Conclusion

• High energy physics addresses some of the most
  fundamental questions about the Universe.
• What's more, it's really fun!
• The high energy physics community in the U.S. is
  strong and thriving -- and will welcome you
  should you decide to become part of this
  excitement!

44
Prospects for particle physics in the U.S.
Start-up date or Decision Point
Experiment
U.S. Participartion,
Lab
in progress in progress 2006 2006
51/55
CDF/D0 MiniBOONe NuMI/MINOS BTeV
100
FNAL
69
74
CERN
ATLAS/CMS
2006
20
SLAC
BaBar
in progress
2003 2010 2020
Next Linear Collider Very Large Hadron
Collider Muon Collider
?
?
45
More Tracking Chamber Images
One of the first bubble chambers at CERN
Tracks in Brookhaven National Laboratory 7-foot
bubble chamber
46
Wiring a Drift Chamber
Wiring some 25,000 wires in a drift chamber for
the ZEUS detector at DESY (Germany)