QUARKS FOR BEGINNERS

High School Teachers

at CERN 2001

CERN HST 2001

... We are made of particles and so is everything else in the universe! We know now that just four kinds of building blocks are needed to account for all of ordinary matter. These particles are called:

• Up-quarks and down-quarks • Electrons • Electron-neutrinos

CERN HST 2001

A million atoms fit across the width of a hair, but atoms are huge compared to the particles CERN accelerates and studies.

99.99% of an atom is empty, the particles CERN is interested in fill the remaining 0.01%. To see things this small we use powerful “microscopes”: accelerators and detectors: how does that work?

The smallest particles….

CERN HST 2001

By accelerating particles… to very high energies

and smashing them into targets, or into each other, new particles are produced. This happens when matter turns into energy and back again following Einstein’s famous equation:

But… only certain masses are found!

CERN HST 2001

And quarks beginning…..

These particle collisions are organized in the middle of huge detectors. Such great concentration of energy produces new particles. With detectors we study the new born particles that fly away from the collision point.

CERN HST 2001

Up quarks and down quarks

Up-quarks and down-quarks are embedded deep inside protons and neutrons in the atomic nucleus. They are bound so tightly that it is impossible to pull an individual one out.

CERN HST 2001

The strong interactionNature uses four different

glues to make particles stick together. They are called interactions:– gravity– strong– electromagnetic – and weak

The strong interaction, carried by gluons, sticks quarks firmly together in protons and neutrons inside the atomic nucleus.

CERN HST 2001

The strong interaction has the curious property that if the quarks get farther apart the interactions between them become stronger.

That means that they are free to roam inside their nuclear cages but they can not escape, rather like animals in a zoo.

CERN HST 2001

Kinds of quarks

Quarks are fundamental building blocks of matter. They build neutrons and protons and many other particles called hadrons. There are six different types of quarks. Each quark type is called a flavor

CERN HST 2001

Flavor Mass(GeV/c2)

Electric Charge (e)

u up 0.004 +2/3

d down 0.008 -1/3

c charm 1.5 +2/3

s strange 0.15 -1/3

t top 176 +2/3

b bottom 4.7 -1/3

CERN HST 2001

How do we know quarks are real?

The answer is simple. First physicists found hundreds of different particles. These particles had only certain distinct masses, so not all different masses are found in nature. This was an important clue that something special is going on. If you assume smaller building blocks of which all these particles are built of then the picture is complete. Later on they predicted properties of other, undiscovered particles, using the building block idea. In experiments they found these unknown particles later. So, this building block idea was correct, because it could predict the truth!

CERN HST 2001

How can we detect these particles?….Detectors record the traces of particles too

small to be "seen“.

CERN HST 2001

How particle detectors work

For example,a particle passes through the detector, collides with atoms and kicks out electrons.

The electrons are attracted to a positive wire, hanging nearby in the detector

The electric pulse on the wire is amplified and sent to a computer.

From the position of the wire and the arrival time of the signal, the computer reconstructs the path of the particles coming of the collision point.

THE ENDIf you have any questions, please

ask!

More information:

http://user.web.cern.chhttp://teachers.web.cern.ch

http://www2.slac.stanford.edu/vvc/contents.html