You've probably seen a few of my videos by now, so this makes us old friends.
You've also probably deduced that I'm not always an urbane and charming video host.
In fact, during the day, I'm a physicist.
Well, still urbane and charming to be sure, but a physicist.
And, to be more specific, I'm an experimental particle physicist, which is the most awesome
kind of scientist to be.
So how does a particle physicist make measurements?
Well, in my case, I take two particle beams and smash them together like two cars in a
head on collision.
In these collisions, we can heat matter to incredible temperatures- as much as 100,000
times hotter than the center of the sun.
This allows me to study the conditions of the universe about a trillionth of a second
after the Big Bang.
The fact that we can do this continues to amaze me.
I mean, if the fact that mankind has developed this kind of power doesn't make you at least
a little proud of your humanity, I don't know what will.
So how is it that we are able to understand matter under these extreme conditions?
You've no doubt seen animations like these.
Two beam particles come together and collide.
After the collision, hundreds of particles can come out of it.
The actual collision takes the unimaginably short time of under 10 to the -23 seconds.
Even the relatively long time that the collision persists in our detector is still incredibly
short, say in the ballpark of a couple billionths of a second or so.
So how is that we are able to make measurements of the particles coming out of the collision?
I mean it's not like we can watch the particles themselves.
Unless they are interacting somehow with our detector, they are just simply invisible.
Yet we scientists study billions of collisions.
What's our trick?
I can explain this pretty easily by means of an analogy.
This might surprise you, but particle physicists employ the same basic techniques as those
used by bomb squad investigators.
So how do bomb squad investigators do it?
Well, the first thing is they don't stand by the bomb when it goes off.
Let's imagine how that would work.
Ow!
Man, I hate it when that happens.
Don't let anyone tell you that you get used to getting blown up.
It hurts every time.
You can see why this isn't a good way to understand an explosion.
Maestro�can you- can you bring me back?
That's better.
So how do the experts do it?
To give you an idea, we asked help from a local bomb squad.
We asked them if they could show us how it is done.
And luckily they allowed us to come along on a training exercise.
Now, I hope I don't have to tell you guys, �Kids, don't try this at home.�
5, 4, 3, 2, 1.
So that was totally, incredibly cool.
But now let's pursue our metaphor.
So, we know what happened, because we set the charge.
But let's see what message the room tells us about the charge.
For instance, we see right here on the ground, we see a darkened area.
We see another darkened area over here.
And those two darkened areas are the result of heat damaging the floor.
So just from that simple bit of information we know the explosion was in this area rather
than another part of the room.
We also can take a look at the corners of the room here.
For instance, we see the corner over here is significantly damaged, and in addition,
this corner here is also completely destroyed.
In contrast, the other side of the room, the corners are far less damaged.
That information in addition confirms the location of the explosion, because corners
focus the effect of the explosion.
And so both of these message confirm the message of the blast damage on the floor.
The explosion was on this side of the room.
In addition, if we look at the ceiling up here, we see several holes in the room, and
that is shrapnel damage, where the bomb punched holes in the ceiling.
These could be ball bearings, or, um, or nuts, or something like that, some piece of metal.
Now, we happen to know exactly what we did.
We set a bomb right here in the cabinet, which we see is completely destroyed, and we put
some ball bearings that fired in that direction.
So what we know is that the room is confirming what actually occurred.
And that is how you do a bomb investigation.
You're never there when the explosion occurs, instead, you ask yourself �what do the surroundings
tell us?� and from that, we reconstruct the bomb, the location, the yield, and all
that information.
And that's exactly what we do, or analogous to what we do, in particle physics collisions.
So how does this relate to detecting particle physics collisions?
Well, scientists can't directly study the particles coming out of a collision, so what
they do is to arrange the collisions to occur inside a detector.
And this is the crucial point- it's not just any detector, it's a detector about which
they know a great deal.
They have carefully designed it keeping in mind the myriad of particles generated in
a collision.
These particles have many names, like hadrons, electrons, photons, muons and neutrinos.
Each of these interacts differently with matter.
For instance, electrons, muons and some hadrons have electrical charge.
A particle with electrical charge generates an electrical field around it that knocks
electrons off of atoms as it goes through matter.
The technical name for this phenomenon is �ionization.� Thus scientists look for
the effects of free electrons in their equipment.
If they see a string of free electrons, there is a good chance that an electrically-charged
particle has passed through their detector.
However, photons, neutrinos and some hadrons don't interact in this way and such a detector
wouldn't see them.
In order to see these unseen particles, we need to add other detectors which react to
different ways in which particles interact with matter.
For instance, hadrons, electrons and photons smash into atoms and do a most curious thing.
They make more particles.
For instance, one of these particles hits an atom and two or five or even more particles
come out.
These new particles can then interact with more atoms.
Using the simple scenario in which a single particle interacting with an atom makes two
particles, we can see how a single particle makes many.
Starting with a single electron, one particle goes to two, which goes to four, then 8, then
16 and so on.
Very quickly a single particle can turn into thousands or tens of thousands.
Since energy is conserved, every time new particles are made, they have lower energy.
For example, in the case of one particle in and two out, the two out each have on average
50% of the energy of the first particle.
We scientists call this phenomenon a �particle shower.� Hadron initiated showers are similar
and occur over longer distances.
Electron and photon induced showers are faster and develop more quickly.
Thus a hadron shower penetrates more deeply into the detector than one from an electron
or photon.
There are many other ways in which particles can interact with matter than scientists can
exploit, but just using the two kinds of showering and the atomic shaking of ionization, we can
see the basic idea of how most particle detectors work using just two components.
This table shows us how the different particles interact in matter.
Photons don't ionize, but make short showers.
Electrons both ionize and make short showers.
Neutral hadrons don't ionize and make long showers, while the charged ones both ionize
and make long showers.
Muons ionize but don't make showers.
And neutrinos neither ionize, nor shower.
So we see that even though we can't directly observe particles leaving the collision, we
can carefully design a detector that surrounds the collision and see how the particles interact
in the detector.
From that, we can infer which particles left the collision and then, finally, we can work
our way backwards to understand what physical process caused what we saw in our equipment.
In essence, the same thinking whereby the bomb squad investigator can figure out the
type of explosion by looking for scorch marks, the depth of penetration of shrapnel in walls,
overturned and damaged furniture, and so forth, is what we particle physicists do to reconstruct
our collisions.
You know, �Subatomic bomb squad investigators� has a really nice ring to it, don't you think?