Transcript Document

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.?
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.
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!
The atom has a rich structure!
electron
• Eventually, it was realized that the
atom is not elementary:
– it consists of a positively charged
nucleus and negatively charged
electrons.
nucleus
• 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!
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!
Is the nucleus elementary, too?
• Unlike the electron, the nucleus is
not structureless! It consists of
protons and neutrons.
nucleus
• But protons and neutrons are not
elementary, either!
• They consist of quarks, which to the
best of our knowledge are
elementary.
neutron
proton
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
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)
Hadrons are everywhere you look!
Everything you can look at
contains the simplest hadrons:
protons and neutrons!
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
ne
(massless (?))
e
electron
Muon neutrino
nm
Tau neutrino
nt
(massless (?))
(massless (?))
m
t
(mass = 205 x mass of e)
muon
(mass = 3503 x mass of e)
tau
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:
n  p  e n
neutron decays into proton plus electron plus neutrino
 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!
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.
anti-electron
(positron)
A photon (which leaves no trace) produced an
electron and an anti-electron (positron), which
curl in opposite directions in a magnetic field.
electron
 The predominance of matter over antimatter in the Universe is one of
the biggest mysteries of modern high energy physics and cosmology!
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:
 6 quarks (which come in three sets)
 6 leptons (which also come in three sets)
 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!
What holds it together?
Things are not falling apart because fundamental particles
interact with each other.
An interaction is an exchange of something.
A rough analogy of an interaction:
the two tennis players exchange a ball
? 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.
Four fundamental interations
 There are four fundamental interactions between particles:
Interaction
Strong
Mediating particle
Gluon (g)
Who feels this force
Quarks and qluons
Photon (g)
Everything electrically charged
Weak
W and Z
Quarks, leptons, photons, W, Z
Gravity
Graviton (?)
Electromagnetic
Everything!
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.
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.
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.
nm
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.
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?
CERN: European Organization for Nuclear Research
 The laboratory is located on the
Swiss-French border, near
Geneva (an awesome location!).
5.6 miles
 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!
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!
FNAL: Fermi National Accelerator Laboratory
 Fermilab is located in Batavia,
Illinois (about an hour west of
Chicago).
 Fermilab is home to the
Tevatron, the world’s highestenergy particle accelerator.
 Fermilab is also a park, with
1,100 acres of prairierestoration land!
How do we study tiny particles?
Recall how we perceive the
world: we detect light (photons)
bouncing off objects.
 But we can’t use light to see
atoms (not to mention, what’s in
them!).
That’s because visible light
waves have too low energy -- or
too large a wavelength.
But we can use something other than light!
It’s not just light that has wave properties - all particles do!
The higher the particle’s momentum, the smaller its
wavelength.
 Therefore, the more sensitive it is to small objects.
Slow electron = large wavelength wave
Fast electron = small wavelength wave
Accelerators
Accelerators are machines used to speed up particles to
very high energies. This way, we achieve two things:
 We decrease the particle’s 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
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.
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.
An event from the OPAL
experiment at CERN
An event from the BaBar
experiment at SLAC
 The products of collisions are looked at using detectors.
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!
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!
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!
Anatomy of a detector: silicon vertex detector
 Many particles decay very close
to where they were produced.
 That’s 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
Anatomy of a detector: tracking chambers
3. electron (from muon)
 Charged particles leave tracks
by ionizing gas in tracking
chambers.
2. muon (from pion)
1. pion
Glowing gas along particle
tracks in a streamer chamber!
 We can learn a lot by studying
these tracks -- for example, a
particle’s momentum!
Another tracking chamber example
 This tracking chamber is filled
with helium gas.
 Charged particles ionize this
gas and leave tracks in the
chamber.
The tracking chamber used in
the BaBar experiment at SLAC
 Lots of wires are strung the
length of the chamber to pick
up electrical signals due to the
ionization.
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 SuperKamiokande experiment in Japan.
Anatomy of a detector: calorimeters
showers!
 Calorimeters allow
physicists to measure the
total energy deposition of
some particles.
photon
electron
positron
 This, in turn, allows us to
tell what kind of particles
they are.
Study of electromagnetic calorimeter
performance for the CMS detector at
CERN (not yet built)
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
Other detectors: neutrino detectors
Sudbury Neutrino Observatory in Canada
Super-Kamiokande experiment in Japan
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. ….
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!
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!
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 mindboggling 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
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:
1.
2.
3.
Medical physics (e.g., cancer treatment, drug improvement).
Environmental applications (e.g., characterizing environmental
wastes using synchrotron radiation).
Computing applications.

4.
Remember that WWW is one of the high energy physics spin-offs!
And much much more!
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!
Prospects for particle physics in the U.S.
Lab
FNAL
Experiment
CDF/D0
MiniBOONe
NuMI/MINOS
BTeV
Start-up date or
Decision Point
in progress
in progress
2006
2006
CERN
ATLAS/CMS
2006
SLAC
BaBar
in progress
?
Next Linear Collider
Very Large Hadron Collider
Muon Collider
2003
2010
2020
U.S. Participartion, %
51/55
100
69
74
20
?
More Tracking Chamber Images
One of the first bubble chambers at CERN
Tracks in Brookhaven National
Laboratory 7-foot bubble chamber
Wiring a Drift Chamber
Wiring some 25,000 wires in
a drift chamber for the ZEUS
detector at DESY (Germany)