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# Explanation.
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# The physics
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This animation is a representation of the flight path of muons, created
in the CMS detector by collisions of protons accelerated in the 
LHC (Large Hadron Collider).

The LHC accelerates protons to 4 TeV (tera-electron volts). This is the 
same as converting 4000 protons into pure energy, and pouring all that
energy into speeding up a proton!

When 2 protons collide (p-p collision), there is a tremendous amount energy focused in a
very small volume. This energy is released as new particles are created.
Some of these particles live for such a short period of time, that they
(the parent particle) decay almost instantaneously to other particles
(the daughter particles). You can see an example of these particles moving
through the CMS detector in the .png image, Resources/cms_2_muons.png.

Some of these daughter particles are called muons. These are the red lines
in the image. Some particles, like the Higgs, might decay to two muons;
specifically a muon with +1 electrical charge and a muon with -1 electrical 
charge. 

Sometimes, different parent particles will create two muons in the same
p-p collision, but we do not know who decayed into whom. So when physicists 
analyze the data, we assume that the muons came from the same parent particle
and we calculate the mass of that parent particle. We do this calculation
using the formulas of Special Relativity, taught to us by Albert
Einstein.



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# The animation.
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A small red dot represents the coordinates of CERN, which straddles the borders of
France and Switzerland. 

A pulsating square of green, red, and black is located at the coordinates
of Nairobi, Kenya, the location for this Science Hack Day. 

A pink line, pointing from CERN to the west, is the definition of the z-axis, 
along which LHC is colliding the proton beams inside of the CMS detector.

We read in events, provided for us by the CMS experiment, that have two muons 
in them. We are given the energy, 3-momentum (px,py,pz), and the charge of each
muon.

We animate the muons, moving in the x-z plane. Since the y-direction is sticking
up and out of the Earth (or into it) we do not show that in this animation.
Muons with +1 charge are orange and muons with -1 charge are blue. 
The size of the muon is proportional to its energy.

We create a fading dot at CERN when the muons are created. This represents
the theoretical parent particle. We say ``theoretical" because there is no 
way to know, for any given pair of muons, whether or not they came from
the same parent. If the two muons are opposite charges (+1 and -1) the parent
particle is coloured gray. Otherwise, it is the same color as the daughter
muons. The size is proportional to its mass/energy.


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# Some key physics concepts to point out.
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1) The muons are not actually shown at real speed in the animation. They travel
close to the speed of light, which means that to travel from CERN to Nairobi (about
6000 kilometers) would only take about 2/100's of a second!

2) The muons are traveling at the same speed in the animation, even when they have
very different energies. Why? The theory of special relativity tells us that nothing
can go faster than the speed of light. Which means that as you put more and more energy
into a particle like a muon, it gets closer and closer to the speed of light...but
it never quite gets there. The muons produced at CERN are all at high enough energies
that they are effectively moving at the same speed. Some are 99% the speed of light.
Some are 99.99%. Some are 99.9999823% the speed of light! Those are all close
enough to the same speed, that we cannot even measure the difference in those
speeds in the physics instrumentation!

3) The mass/energy of the parent particle sometimes appears to be smaller 
than the energy of the largest muon (by comparing the size). That is because
sometimes the parent particle is moving itself and that energy is transferred
to the muons. We did not represent the movement of the parent particle in this
animation.