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An Introduction to CLEO
Lora Hine – Education & Outreach Coordinator
at the Lab for Elementary-Particle Physics
Laura Fields – CLEO Graduate
Student at Cornell University
What Exactly Are Particles?
Particles are the most basic building block
Everything is made of particles, and nothing is smaller than a particle
They include the stuff we are
made of and see around us
–Electrons
–Quarks (the things protons and
neutrons are made of)
–Photons
• There are also exotic kinds of particles
–
–
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Heavier versions of electrons & quarks
• E.g. muons and taus
Anti-matter (particles with the same properties as matter
but opposite charge)
Other particles we haven’t yet discovered?
Creation of exotic particles requires very high energy
densities like what existed during the big bang or what
occurs during the supernova of stars or at high energy
particle accelerators like we have here at Cornell
Why should I care about particles?
• The discoveries of particle physics are unlikely to impact
our lives in the near future, so why should we bother?
•
There are lots of cool unanswered questions in
Particle physics
–
–
–
What is dark matter made out of?
If the mass of the electron were just slightly
different than what it is now, we wouldn’t be here… so
why are we here?
What happened during the big bang?
•Fundamental Science is Important
Technology is always based on discoveries that seemed to have little
application at the time
An Example: When JJ Thompson discovered the electron, he said
“Could anything at first sight seem more impractical than a body
which is so small that its mass is an insignificant fraction of the
mass of an atom of hydrogen?”
Are the things that power computers, telephones, radios,
airplanes, etc insignificant?????
Studying Particles at Cornell
•
We accelerate Electrons and
Positrons (anti-electrons) to nearly
the speed of light, and store them in
a big ring called CESR
• The electrons and positrons run
into each other at the CLEO
detector. When the collide they
produce a bunch of other particles
Courtesy of CERN
Wilson Laboratory, Cornell University
Scientists at CLEO try to figure out what happens
after the particles collide, and use this to learn
more about what the universe is made of.
Terminology: We call one
collision an “EVENT”
How Does CLEO Work?
CLEO and other particle detectors around the world are generally
used to:
1. Identify the particles
2. Determine their energy
3. Determine their direction
But CLEO doesn’t automatically spit out this information…. Getting a particle’s energy and
direction is like putting together a puzzle… sometimes it’s a very hard puzzle!
What are the puzzle pieces?
Particles are much too small to be seen directly. The only way we can observe
them is by seeing the effect they have on the material in the detector to pass by.
There are two types of particles: Charged Particles and Neutral
Particles, and we deal with the two differently
Charged particles either:
•Emit light or
•Ionize the material (that
is, create charged ions in
the material, that we can
then detect)
Neutral particles are
more difficult. Generally
we detect them when
they run into material
and create a lot of
charged particles.
CLEO III – View from East flare
Sometimes we don’t
detect them at all, and
just observe them as
missing energy
The Drift Chamber
How it works:
•The detector is filled with a mixture of heliumpropane gas.
•The chamber is also filled with OVER 40,000
hair-width wires.
•When CHARGED PARTICLES pass through the
gas, they ionize the gas inside.
•The wires in the drift chamber are held at very
high voltage, so they attract the charged ions.
•We can detect these ‘hits’ on the wires, and
knowing what wires were hit, software
reconstructs the path of the particle
A few facts about the drift chamber
•Remember, only charged particles can be detected here
•There are actually two drift chambers, one inside the
other, the whole thing is about 1 meter in diameter
•Some particles aren’t seen in the drift chamber because
they go back down the beam line. We see about 90% of the
possible directions a particle might fly.
The CLEO Magnet
All of the layers of CLEO except
one (the Muon Chamber) are
inside a powerful magnet that
creates a magnetic field.
This gives us a LOT of
information…. WHY????
 
F  qv  B
Particles CURL in a magnetic
field
The Direction they curl in tells us what charge they have. If we view a cross-section
of the detector from the front
•Particles Curling Counter Clockwise are positive
•Particles Curling Clockwise are negative
From the Radius of curvature, we can find the momentum of the particle… The more a
particle curls, the less momentum it has… This is very important for us!
The RICH Detector
RICH stands for ‘Ring Imaging Cherenkov
Radiation’ but don’t worry if you don’t
understand that!
It’s a big cylinder wrapped around the drift
chamber
It also detects only charged particles, but
instead of detecting charged ions, it detects
light that charged particles emit when the
pass through certain substances
Different Particles radiate light
differently. So this is a method of
determining the identity of charged
particles
However, it is often really hard to
tell them apart!
The Calorimeter
This detector, which is just outside the
RICH detector, is made of 7800 crystals
like this one.
This detector detects charged particles by
Detecting light that is produced when the
charged particles pass through it.
But, unlike any part of CLEO we’ve looked
at so far, this detector can detect
NEUTRAL particles as well! This is
because, unlike the previous detectors, it
contains a lot of matter for the particles
to bump into. When neutral particles bump
into the detector, they produce lots of
charged particles, which of course, it can
detect
How do we tell the difference between showers from charged particles and
showers from neutral particles???????
Showers from charged particles have tracks pointing towards them… Showers
from neutral particles don’t!
The Muon Detector
By the end of all of the detectors we’ve talked
about so far, most particles have been stopped
somewhere in the detector.
But one type of Particle hasn’t: The muon.
Remember, the muon is a heavy version of the
electron.
The muon detector is quite large compared
to CLEO’s other components, and is made up
of several detectors buried inside a lot of
Iron.
Muons don’t interact much with other kinds
of matter, so they are the only charged
particles that can go through Iron, which is
very dense.
What We See
In this view, the
beam pipe is coming
out of the page.
What We Know
These create
the other
tracks in the
event
How We Know This
• The central white section indicates the
drift chamber. Individual wire hits
clearly show the paths of the two
particles labeled Π – and Π + on the left.
• The crystal calorimeter is indicated in
the light blue where each box
represents a separate crystal. The
various colored clusters indicate energy
deposited in a group of crystals. The
particle labeled e- was identified as an
electron due to the large amount of
energy deposited as shown by the large
green cluster in the calorimeter.
• Finally, the outermost part of the
detector, the muon detector, shows a
definite pattern of hits indicated by the
red dots. This is how the particle labeled
μ – was identified as a muon.
-
Activity: “Interpreting Event Diagram Displays”
• Student Handout:
Ground Rules for Interpreting Event Diagram Displays
Three Event Diagram Displays from CLEO XD
Event Diagram Quiz
• Teacher Handout:
Includes Answers to Event Diagram Quiz
• Let’s go over this activity together…