This section is all about exploring the ATLAS detector: what is it? what is it used for? and what are the quantities that we can actually measure with it?  Let's start by addressing the first of those questions: what it is.  That part's easy:

That's the gist of it.  We could have just put an enormous chunk of lead surrounding the beam-crossing point, and that would have soaked up most of the energy and radiation streaming from the proton-proton collisions produced at its centre (which we learned about in Part II).  But that would have been a colossal waste of (the good tax-payers') money!   We don't just want to produce collisions, we want to produce them in order to better understand their underlying processes on the smallest possible scale, and for this we need active material in the detector – material which allows us to probe the particles and interactions we really care about.

The ATLAS detector is kind of like a giant digital camera.  Well, think of it rather as hundreds of thousands of little digital cameras, each independently giving us some little piece of information, but working in concert to paint a more cohesive, global picture.

That's one analogy.

It's also often said that the ATLAS detector is structured like an onion.  The term onion is used since there are differing types of detector components for each of the various layers.  Each layer will tell you a different story, so if you listen to all of the layers separately and put that information together you'll get a pretty comprehensive picture (or as comprehensive as you can hope for).

That's a second analogy.

So that's it: ATLAS is an onion-like collection of complimentary digital cameras.  That's a nice picture to keep in your head I guess, strange as it might seem.

I reiterate that here in Part IV we're focusing on the ATLAS detector, but you should note that while its particular design might make it unique, the basic concepts for many of the detectors used in a collider experiment – not just ATLAS or the other experiments the LHC – are more or less the same.

What quantities are we actually trying to measure?

The two main quantities we actually measure with the detector are energy and momentum.  We'll talk about momentum in a second since it's likely the less familiar of the two.

Often what we're actually looking to measure are things like production rates of certain particles (e.g. "how many of particle X do we produce from collisions over a given data-taking period"), as well as particle masses, but those quantities (and more) we really get indirectly from the energy and momentum measurements.  They're the two basic ingredients we need to make a measurement of any kind.  Ok so even those are indirect in a way too, but they're more direct than other quantities and they're the important ones to remember.

So what is momentum anyway?  It's just the product of mass and speed (which just means you get it by multiplying the two together).

There's a super useful relationship between energy, momentum, and mass.  It's a tiny bit of math again, but part of it will seem familiar to you I'm sure.

So all that time you remembered Einstein's equation as "E equals m-c-squared", but it was actually an approximation at a speed of zero! Now you can impress people by telling them the full thing, right?

Just to reiterate what was mentioned earlier in Part II, don't be tricked by the speed of light term (c) into thinking that the given particle (with energy E, momentum p, and mass m) is itself travelling at the speed of light!  It may, or it may not be (actually if it is travelling at the speed of light, then its mass is necessarily exactly zero).  Even if it's just some particle sitting there on a table, or if it's moving at some other subluminal speed, the speed of light term stays where it is in the equation above – it's what we call a 'constant' – but the particle will have some value for its energy, momentum, and a (fixed) mass value.

Interestingly, whereas the mass of a given particle stays fixed by definition, the amount of energy and momentum it has depends on the relative motion of the person who's observing it.  That's relativity!  Or pat of relativity anyway.  Ok, we won't dig too deep into the (interesting) details.  Still, you can think about it, right?  Here, consider this: assume the protons in the LHC beampipe are travelling at near (but crucially below!) the speed of light with an energy of 7 TeV.  This was the design beam energy for the LHC.  We could translate that energy into Joules (or even calories!) if we wanted to – forget about the units and what they mean.  It's seven somethings

So now, think about this: what would each proton's energy be if you were travelling along with it at the same speed?  Well, in that case it wouldn't be moving, at least relative to you, right?  So you'd measure its energy as just "m-c-squared" which is ~ 0.001 TeV, or only about 0.001% of the energy you'd measure in the case that you weren't moving.  That's much, much smaller!  So who's right?  The you who's standing still, or the you who's moving along with the proton?  How does the bookkeeping make sense in the context of all of that stuff we discussed in Part II?

At the LHC we deal with colliding protons which are travelling in opposite directions, and in a way this makes things a bit simpler.  Here's why: even if you're moving along with the protons in one beam such that their energy seems to decrease – they're moving slower in your 'reference frame' after all – the protons coming at you in the other direction will consequently be at a higher (not lower) energy.  So we can talk about the energy that the whole system has when in the context of the the collision.  Amazingly (you might think) that value stays fixed no matter who's looking at the system.  Doesn't matter who's measuring it and how fast they're travelling: when it comes to the energy of the system (sometimes called the centre-of-mass energy) everyone will agree on the same number.  Great!

So to recap, sure, two separate observers can disagree on the relative motions of the particles involved, but if there's an interesting collision between the particles either there's enough energy to produce some heavier type of particle in the collision or there's not!  If it weren't that way we'd run into a whole mess of paradoxes.

Note that this sort of relativity is somewhat different – no, not different, just altered – from the common-sense type of everyday relativity you might otherwise think of.  Here's what I mean: two cars are travelling towards each other at 50 km/h.  If someone from one car were to throw a ball forward at 10 km/h (so towards the oncoming car) you'd be tempted to say that the person in the other car would see that ball coming towards them not at 50 km/h, not 100 km/h, but at 110 km/h!  You with me?  Draw a quick picture and you'll see what I mean (from either of the drivers' perspectives they're not moving; it's the other car that is).  So not only would you be tempted to say that, but you'd be right!  But careful: nature doesn't work that way at really high speeds.  In that regime relativity works in such a way that the universe squeezes and contorts space and time to ensure that the speed of light remains constant – lengths can change, time can slow down or speed up, but crucially the speed of light (in any reference frame!) simply has to be 'c'.  The bizarre effects are increasingly significant the closer one gets to the speed of light. 

The light emanating from the flashlight of someone who's standing still will travel towards you at the speed of light.  The light emanating from the flashlight of someone who's running towards you at 10 km/h will still be the speed of light – not the speed of light plus some tiny little bit, just the same speed of light.  Rather than allowing the speed of light (which is fixed by nature!) to change, that 'tiny little bit' difference has to manifest itself in some other form – in this case the times and distances involved (as measured by both you and the person running towards you) are what change!

So, a key point to take away: if you want to talk about the total energy of a single particle, you first have to state how you're moving relative to it!  "Q: What energy does that proton have? / A: Depends on who's making the measurement!" But the total collision energy between two particles is itself a fixed quantity that we can all agree on, regardless of our relative motions.

Let's get back to momentum: it's a bit different from the other two quantities (energy and mass) in a few ways.

Firstly, you've maybe never heard of it (or maybe you'd heard the word itself, but didn't know exactly what it meant).

Secondly, it typically has a direction associated with it.  So for example you'd never say "that table has a mass of 43 kg to the left" or "he's about 100 kg too far north".  Nor could you ever talk about an implicit direction associated with energy (though that one's perhaps less intuitive).  With momentum however, there's normally a direction associated with it.

A more practical example: if I had a mass of 70 kg and were walking forward at 5 km/h, you could say that I have a total momentum in the forward direction of 350 kg km/h (even though those units sound strange – it's correct!).  But I have zero momentum in the upwards direction.  Actually to be picky, we use the word velocity for speeds with a direction, so really I should have said that momentum is the product of mass and velocity earlier, but for the purposes of this introduction, that's just jargon, so it doesn't matter a great deal.

Intuitively you might have a pretty good feel for momentum in the everyday sort of sense.  Picture a tiny car and a huge truck both flying down the highway at 130 km/h; they both have the same speed, but one (the truck) has a larger mass.  Which of the two would pack a meaner punch?  The truck, right? Both it and the tiny car have the same speed, but since it has a larger mass, it consequently has a larger momentum.  Of course if you have two identical trucks, the one with the larger speed also will have the greater momentum.  And in the case of the car and truck you could tune their speeds just right by speeding the tiny car up (or slowing the truck down) such that their momenta match perfectly.  Not that anyone would do that, but one could is all I'm saying.

This idea about momentum having a direction will come up again later on though, so hold onto that thought.  

Great, so we've got a sense of energy and momentum.  Now we're almost ready to talk about how they're measured in the ATLAS detector.

Before we do that though, we first should talk a little bit about electric and magnetic fields, since they're important and are at the root of most of the detector components' design concepts.  Feel free to skip the section of course – it's not crucial.  That said, it won't hurt...