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?

• There are also exotic kinds of particles– 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?

Particles are the most basic building blockEverything 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

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?

• 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 ImportantTechnology 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?????

• The discoveries of particle physics are unlikely to impact our lives in the near future, so why should we bother?

Wilson Laboratory, Cornell University

 • 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

Studying Particles at Cornell

Courte

sy o

f CER

N

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”

CLEO and other particle detectors around the world are generally used to: 1. Identify the particles 2. Determine their energy 3. Determine their direction

How Does CLEO Work?

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!

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.

What are the puzzle pieces?

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.

Sometimes we don’t detect them at all, and just observe them as

missing energy

CLEO III – View from East flare

The Drift Chamber

How it works:

•The detector is filled with a mixture of helium-propane 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 particleA 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????

BvqF

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!

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 Calorimeter

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.

In this view, the beam pipe is coming out of the page.

What We See

-

These create the other tracks in the event

What We Know

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…