What's really going on inside of a proton
collision?
Is there a Higgs boson secretly hidden inside
a proton?
Or maybe there's some dark matter and therefore
if we hit it hard enough,
it'll knock the dark matter off.
That's not what's happening.
What happens with a proton collision is actually
the two protons get close enough
so that they can feel each other.
With a certain force.
because that question that we asked earlier,
when you're asking, "How small can I cut anything?"
you're secretly asking, "What was holding
anything together to begin with?"
And the thing that holds things together at
the particle level are forces.
But the particle level, forces are mediated
by other types of particles.
It's like a force exchange.
So it's a little bit strange for us, you know,
humans, we're big, bulky humans
that are, that evolved in this, you know,
on the surface of the Earth,
and in this certain energy range,
and we can't really intuitively understand
the quantum world.
But the quantum world is a bit odd.
So one way you can, as an analogy, you can
do it, you can get a sense of it,
is you imagine you and your friend were on
an ice skating rink.
And you're both standing right in front of
each other.
One of you has a gigantic ball, like a big
rock or something like that.
And you throw the rock to your friend.
As you throw the rock, it's really heavy,
so you're going to slide backwards, right?
And as your friend catches it, she's going
to slide backwards as well.
So in a sense, we have just exchanged a force
carrying particle.
That's the way particles interact.
But particle collisions are weird compared
to what we think of
as collisions in our everyday life,
because if were actually particles in the
quantum world,
you are your friend actually have another
option for when you throw the rock.
You can not just throw the rock to each other.
The two of you could actually smack together
and become the rock.
Perhaps let's use a different analogy.
Particle collisions are not like car collisions.
We like them to think so. We like to think
of that in a way.
But they're not like car collisions. They
have different rules.
When two particles collide at almost the speed
of light the quantum world takes over.
And in the quantum world, two particles can
actually cease to exist
and a new particle can be created in their
place
that lives for a tiny fraction of a second
before then splitting into other particles
that hit our detector.
Imagine a car collision where the two cars
vanish upon impact.
A bicycle appears in their place,
and then the bicycle explodes into two skateboards,
that hit our detector.
Hopefully not literally, but you know.
This is the sort of thing, a bit of a weirdness,
that happens at the quantum level.
The way that's possible for that to happen
is that go back to your high school physics.
I hope I'm not going to scare anybody off
by saying:
"Go back to your high school physics."
Go back to your high schools physics.
What does physics really care about?
Physics doesn't care about the names of things.
Doesn't care that this is called an electron
and this is called a positron.
We care about that for car collisions.
Is Mary OK? Is Jack OK?
Particle collisions, they don't care about
that.
Physics cares about conserved quantities.
I hope I haven't made anybody fall asleep,
just by saying those words.
Physics cares about conserved quantities.
Because think about what it is that... conservation
of energy,
conservation of momentum, conservation of
charge.
And some more arcane ones, conservation of
spin.
These are the things that particle physics
cares about.
So, as long as I conserve these things before
and after a collision,
I'm all good.
Here for example. This is a diagram, I hope
I haven't scared anybody off with the diagram.
This is a Feynman diagram, maybe you know
Richard Feynman.
On the left hand side you have two quarks
coming out of the proton
in one of our collisions.
Two quarks, at some point they are far apart
in space.
They get closer and closer together.
At some point they annihilate, they cease
to exist,
and then a Z boson appears in their place.
Then the Z boson lives for something like
1e-12 seconds. Basically nothing.
And then it splits into two muons.
That's crazy! You started with two quarks
and ended up with two muons.
How is that even possible?
And so this is what we do to look for new
particles.
Because the thing we care about is that middle
part, the Z thing.
