Note: this is probably the easiest section to skip without missing anything too crucial.

A bit about electric and magnetic fields

Let's go back to what you would have learned in school or by playing around with magnets and rubbing balloons on your head.  Our understanding of (classical) electric and magnetic fields really reached its pinnacle in the late 19th / early 20th century (I'm tempted to say at which point they'd moved beyond playing with magnets and balloons, but I actually don't think that's true).

Of course we understand the electromagnetic force far better today in the context of something called quantum electrodynamics (QED), or quantum electroweak theory.  But for now in this section we'll look at the picture from a classical perspective since (a) it's good enough for what we're talking about and (b) it's easier in terms of developing an intuitive sense of how detectors work (even, I would say, for people familiar with all the fancy math and theory!).

First, let's look at a summary of how individual, electrically charged particles produce both electric and magnetic fields.  We often talk about fields and field lines here.  Why?  Well, when we know the strengths and directions of (electric or magnetic) field lines, that lets us know how particles will react to those field lines.  That, in turn, tells us how they'll move, how they'll interact with the detector materials and so on.  A field here we treat as a mathematical construct.  Is it really filling space everywhere?  Not really.  Kind of.  I'm not going to try to tackle that question here.  The point: employing the concept of fields allows us to make predictive statements about a system.  So whether you want to think there are actually fields there (à la Obi-wan Kenobi) or not, if you follow the rules based on the theory, you get answers that can be tested by experiment (with incredible precision if you do it right!).

So here we go, producing fields from particles in a nutshell:

So that's how we produce the field lines with individual particles.

Larger macroscopic objects can also give rise to the magnetic effects you're more familiar with.  So we can think of magnetic fields in the following way too, but it's actually intrinsically connected (on a very small scale!) to the ideas above, even if it doesn't appear to be the case at first glance.

So the take-home message: we can manipulate distributions of charges or electric currents (such as those in wires) to produce electric and magnetic field lines in basically whatever orientation and strength we want.  Ok there are limitations to what we can do, but we've been doing that for a long time and we're good at it.  ...I'm not good at it, but some humans are.  The point: we can make it happen.

Next question is: why do we want to do that?

Because in the same way that the particles can produce the fields, the fields in turn can impact the trajectories of particles, such as those particles we're trying to measure.  As a result, using fields to manipulate particles allow us to make the measurements we do.

Let's pretend now we already have some electric or magnetic fields, and let's then look now at how particles behave in the presence of those fields.  In other words we, or someone else, has generated the fields, and we just want to see what their effect is on particles in their presence.

Let's look at the effects on particles (charged or neutral) due to electric fields first:

If that's harder to remember, you might be more familiar with this way of thinking about it:

You can actually convince yourself these rules make sense by drawing the relevant electric field lines around each particle in the above picture (based on the rules introduced earlier) and then looking at how the neighbouring particle will react (or not) to those field lines.

How particles interact with magnetic field lines is another story.  Here we can't just limit ourselves to directions in two dimensions – up, down, left, right, and so on.  We need three dimensions, so be prepared to think about up, down, left, right, as well as into the page, out of the page, or any admixture of those directions!

Whereas you might have been more familiar with the rules above for electric fields, be prepared for some unexpected results from magnetic fields.  You might want to read through these a few times.  Or who knows, maybe it makes intuitive sense to you!   Definitely doesn't to most people when they first come across it, so if it does, consider a change of career into physics.

Here it is, how particles react to magnetic field lines:

Again, a lot of this is convention.  Don't get bogged down by the details.  This is just for completeness (ok, recognizably saying this is a complete description is stretch of course), but again, the only thing you need to really remember here is that electric and magnetic fields can be used to manipulate (speed up, slow down, or steer) particles.

In fact, at this point, armed with this knowledge you might stop to think about how it is exactly that we're able to steer the positively charged protons around the giant LHC ring.  Any ideas?   Magnetic fields!  Well, actually we use both electric and magnetic fields, but the way we keep them on their circular trajectory is largely through the use of magnetic fields.  Very strong magnetic fields.  In general we use electromagnetic fields (to use a word that groups both kinds together) to steer, but also to focus and defocus the LHC proton beams as we like.

A case that's perhaps more familiar to you of where charged, high-energy particles can have their trajectories deflected by magnetic field lines is that of charged particles emanating from the sun and passing by the earth's north or south poles, where the concentration of magnetic field lines – and therefore the field strength – is greatest.  Yes, the northern (or southern) lights, at a basic level, are caused by such charged particles getting trapped by the earth's magnetic field lines and interacting with the gases in the upper atmosphere, which in turn give off characteristic colours of light.

Putting this concept to more practical use (in the context of ATLAS), we can say that by seeing how a particle's trajectory is modified (if at all!) by a magnetic field of known strength allows us to gain crucial information about that particular particle.

Charged Particles and the Measurement of 'Tracks'

At the very centre of the ATLAS detector it's possible to perform high-precision tracking measurements.  Charged particles interact with the detector materials in such a way that one can infer the positions of charged particles at various points.  Sometimes you just get noise, but in the cases that an actual charged, high-energy particle travels through the detector, you might think of lining up those points (a connect-the dots sort of concept) and try to identify a track!  With no magnetic fields, any particles, regardless of their mass, charge, velocity, and so on, would travel by and large in straight lines.  Flip on the magnetic fields and suddenly, zap!, we get more indirect information about the various particles just based on which way and how much their trajectories curve.

Here's an example of something we might see in part of the detector (we're not dealing with random 'noise' in this case – here we consider a real 'signal' event):

Any ideas?

Think of some of the types of particle decays you saw in earlier sections and remember that you only get tracks for charged particles (not neutral ones).  There might have been something else there that seems invisible to the detector even it really was there.  (That’s a hint).

What if I told you that there was a strong, and constant magnetic field directed into the page everywhere in the square above.  Does that help explain the root of some of those paths?

Ok, here's the answer (or one possible answer):

Think this sounds crazy?  Just trying doing a google image search for “cern bubble chamber” and see what you come up with.  If you were successful (...) what you're looking at are not just the result of some complicated mathematical algorithms – they’re real images tracing out the tracks of charged particles using, in case it wasn't obvious, something called a bubble chamber.  Not only are they real images, they used to be used by physicists to make physics measurements!   A romantic notion, no?  Nowadays of course we let the detector and computer do the work – the field of experimental particle physics has obviously changed a lot in recent decades – but the actual photographs can be more insightful (and cooler to look at).  Now any time you see one of these types of images you'll be able to explain to people more or less what it is that you're looking at.

Looking at the picture one more time but with all of the missing pieces now in place:


Of course the Z boson could have been a photon (particle of light) and we'd have seen a similar pattern.  And the electrons could have been muons, or something else.  How would we really even know?

Luckily – by design actually! – we typically have complimentary measurement techniques in ATLAS, meaning that if we run into an ambiguity somewhere, we have a way of reasoning it out with our extra information (or having a computer reason it out for us).  So we use different parts of the detector to give us the whole picture, together with our previous knowledge – from theory and from other experiments! – of some of the properties of each type of particle.

Here we highlight how deductive reasoning plays a large role in particle physics and science in general.  Consider the three separate tracks below, and assume there's a constant magnetic field everywhere which is doing the bending of the trajectories.

Can you make any deductions about the three cases of charged-particle tracks as shown?

To be fair, I haven't given you enough of the (mathematical) tools to be able to be able to deduce anything concretely, but in a way you can use your intuition.  And here's a spoiler: there are many possible options and not just one unique answer!

Below is just one possible explanation, but there are others.

One interpretation: all particles are travelling at the same speed and have the same mass.  Particle A has the greatest charge, followed by particle B, and finally particle C. If particle A is positively charged, then both particles B and C are negatively charged, or vice versa.

Again, this was just one interpretation.  One important point to note though is that regardless of the assumptions we make, there's one thing we unambiguously know for sure: the charge of particle A is definitely opposite in sign compared with both particles B and C. For the rest we'd need more info and it's a bit of detective work, but it's a start!  Again, in experimental particle physics any time there's an ambiguity, you just have to come up with a complimentary measurement that can be done to break the ambiguity and give you the real story.

To reiterate, the basic idea though is that by making using of what we know (the strength of the magnetic field being fixed for instance) together with certain assumptions, we can make clear statements about the comparative speeds, charges or maybe even masses of the three particles which left the tracks in the above cases A, B and C.  And if we're left with any ambiguities, we make some sort of a complimentary measurement until we get to the right answer (or as close as we can!).  The type, strength and orientations of the fields can be exploited for whatever type of detector or piece of electronics you're trying to produce.

So that's it in a nutshell: electric and magnetic fields and how we use them.

Particles – and the clever things that have to be done to be able to see them in our detectors – come next!