Turning Ab Into My Personal Blog, #1: So What Is Particle Physics Anyway?

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jim360 | 20:11 Sat 29th May 2021 | Science
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I was hoping to post semi-regular updates about what I'm doing, partly because it's good to keep AB science active, partly because it's helpful for me to think about what I'm doing, and partly because hopefully it's even interesting. At some point I'm bound to deliver talks on it, albeit to a more specialist audience, but communicating science is always important.

So, anyway. The project I'm working on to stay with is related to "the lifetimes of singly/doubly charmed baryons". I simply couldn't see a point in going any further than that without trying to define all the words in that sentence, to set the scene a little. In other words, what even is Particle Physics?

The best way to begin is with atoms. As everybody knows, hopefully, atoms are the building blocks of molecules, they represent all the known elements, and discovering and understanding them is one of the triumphs of modern science. Everybody hopefully also knows that atoms have structure: a tiny, tiny nucleus, made of protons and neutrons, orbited at a distance by electrons. What's mind-blowing about all this already is the scale: even the largest atoms are only a few ten-billionths of a metre across, and the nucleus is another hundred thousand times smaller still!

It's worth stepping back in time to try and appreciate this. It's now a staple of A-level physics to present the ground-breaking experiments that revealed the size of a nucleus as something matter-of-fact, but of course at the time (around 1910) people like Ernest Rutherford had to be geniuses to interpret what was going on. But anyway, physicists soon realised that atoms had structure, and set about trying to understand it. Particle physics is the result of this quest.

It has led to some crazy places, and what's most staggering to me about those places is how unnecessary they all seem to be, at least at first glance. For example, if all life on Earth is made from protons, neutrons, and electrons, then why in hell do you need anything like a muon, which is a particle exactly the same as an electron, only 200 times heavier, and so unstable that it always breaks up after a couple of hundred thousandths of a second?! Likewise, why is antimatter an actual thing, when it is similarly short-lived? I'll try to answer these questions, at least partly, later. Moreover, it isn't just those two new things, but hundreds more, apparently without end. It wasn't until the 1960s that people finally made sense of all of this and realised that, in reality, there are really a few fundamental building blocks.

The key breakthrough was the prediction of quarks. These are the (six) particles that make up protons, neutrons, everything like them. The "baryons" in my project title are anything made from three quarks. The names of the quarks are: up, down, strange, charm, bottom, and top. There is no reason for these names other than that physicists like them and in the 1960s everybody was also clearly on LSD. That answers the second question about my project title: I'm going to be looking at baryons containing one charm quark (or possibly two).

The "lifetime" of the title speaks to the fact that these baryons do not last very long at all (roughly 0.0000000000002 seconds) on average. It's worth stressing that the lifetime is similar to a radioactive half-life, and is more useful when you are seeing hundreds or thousands of these particles as opposed to a mere handful. This is why particle detectors like CERN have to run their experiments for so long -- you need to create and observe a *lot* of these particles to get anything meaningful, especially when there's so many other things that could be created.

continued shortly...


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I understood all of that, apart from the last paragraph. There are some things, in that, that I can't yet get my head round.
The rest is very clear. Thank you.
jim thanks, just want you to know I'm reading and digesting, I'm not in a position tonight to respond intelligently but I will tomorrow.
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One thing I wanted to say about baryons in general that is worth mentioning: we don't understand them very well. Earlier I said that baryons were made from three quarks, and while this is true it's also the same as saying that houses are made from bricks: It doesn't begin to capture the complexity of either houses or baryons to speak in that way. This is even true of atoms, to a lesser extent, and what is really comes down to is that, for baryons, and for the small world in general, the whole is very much not the sum of the parts.

But what can we do? Particle physics arrived in the 1960s-1970s at a picture, called the Standard Model, that contained six quarks, the electron and its relatives, and three forces, and is able to describe extremely well how they all fit together, but is also stuck talking about quarks when we never see them separately. But if you are trying to work out how long a house is going to last, then you need to understand not just the materials but also how they fit together, and there is no concrete answer so far to this puzzle. I'll probably explore this a lot more later when I talk about my second project, but for now it's basically worth noting that a baryon is a lot more complicated than its three quarks.

Having laid the groundwork, I might try to delve more into the technical aspects later, eg the history of my own project, what I actually do in research, etc.

Finally, in answer to Tilly, hopefully this helps:

A Radioactive atom is any atom that isn't stable enough to last forever. At some point it will decay (into a smaller, more stable atom). However, if you take just one atom, then it is completely impossible to say when that decay will occur. It could be today, tomorrow, next century, a million years from now, no idea.

However, the remarkable truth is that if you gather millions and millions of the same radioactive atom, you *can* say something concrete, namely that exactly half of those atoms will have decayed in a fixed amount of time. And then half of those left will decay in the same fixed amount of time. And so on. This is the famous "half-life", but it only has meaning if you have a huge number of atoms to look at. It's the same (more or less) with particle physics, although since most particles have half-lives measured in tiny fractions of a second, and only a few of any given type are produced, it can be tricky to measure these things accurately.
All nice and simple then, Jim ;-)
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I was going to get to that paper in a few blogs' time, Buen :P
I welcome this - you are fantastic for starting this project.

If I may make a small suggestion, most people here would not know a baryon from a lepton.
It might be worth just distinguishing between the different classes of particle and why and how physicists differentiate between the different classes, based on the properties of their constituent quarks.

I guess it will be a bit like developing a year-0 particle physics course. You'll need to prepare the course to build up understanding of the particles; the forces and how they interact and then, of course, the whole quantum realm thing.

Also, the issue with saying 'we don't understand baryons very well' is that the statement is true at your level, but it is not true at the level of the average AB-er.

At the level of the average AB-er we know exactly what protons and neutrons are. On the other hand, very few here would have any idea about a charmed Ξ′ .

Did you ever watch the series 5 Levels Video Series by Wired?

I find them quite brilliant.

They approach difficult subjects in different ways, according to the capabilities of the audience.

I guess you have to assess where the AB audience is, and select the language and explanations accordingly.

Very best of luck to you. AB is lucky to have you aruond.
-- answer removed --
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Dictionaries in particle physics have a tendency to be a bit like Alice tumbling down the rabbit hole, and also everything is defined in terms of everything else, so it can be messy. I did define a baryon, I thought, at least.

Still, here goes (a useful glossary for any future posts, and please do use this as a basis for googling the complicated ideas that I haven't adequately explained in a single sentence).

-- Quark: six particles that (so far as we know) are the most basic building blocks of all matter.
-- Hadron: any particle made out of some number of quarks.
-- Baryon: any particle made of exactly three quarks
-- Meson: any particle made of exactly one quark plus one antiquark.
-- Lepton: the other type of fundamental particle, including electrons and neutrinos.

The six quarks (in ascending order of mass) are up, down, strange, charm, bottom, and top. All of these are tiny, but compare to the proton mass, then their respective masses are about 0.005, 0.01, 0.1, 1.5, 5, and 200. Nobody knows why the top quark is so much bigger than anything else.

A few other useful terms,

1. "Charge" refers to electric Charge, as in normal electricity. This is linked with magnetism to form the electromagnetic force.
2. Mass = how much "stuff" there is. Still not well-understood, although linked among other things to the Higgs boson.
3. Strong force - the force that binds quarks to form hadrons, and binds protons and neutrons together to form atoms.
4. Weak force: the force that drives radioactive beta decay. Extremely short-ranged, which is lucky because otherwise everything would decay all the time.

Forces in particle physics are "carried" by force particles:

1. Photon -- carries the electromagnetic force.
2. Gluon -- carries the strong force. For technical reasons, there are actually eight different gluons, but they are all basically the same, and usually we just say "the gluon".
3. W-plus, W-minus and Z bosons -- carriers of the weak force. On the mass scale defined earlier, these weight around 90 times as much as a proton, which accounts for the short range of the weak force.
4. Higgs boson -- accounts for the masses of the fundamental particles.

In the above, "boson" can be treated as shorthand for "force particle", but if you search this term you'll find another, more accurate, meaning that requires a whole lecture course to explore on its own and I can't be bothered right now.
Asimov.. Explains...

mabrouk baby
my physics master has an M Sc in acoustics ( which he cdnt explain anyway) and so I missed out on all this

so far quite straightforward
@21.51 made the graphic I posted so much clearer for me.
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Did it really though? :P
Already way beyond my intellectual pay grade. All the same, I'll be watching and waiting for you to slip up and spell hadron wrong.

The pressure's on!
Well ,done Jim ( are you sure that your real first name isn't Sheldon ).
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I mean, if it were easy to understand then there'd be no need to study this stuff for a decade.
Prof. Brian Cox would love this ;o)

All too much for my simple brain, but I still find it fascinating. Looking forward to your next blog :o)
Prof. Brian Cox would be pulling out a bag of jelly beans for those of us that are a bit dim ;-)
I see quarks - till I blink ;)
jim couple of questions, I am familiar with the ground you have covered so far but one of the questions that I always ask myself is how are these things discovered. eg How did Rutherford realise that the atom is almost entirely empty space?

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Turning Ab Into My Personal Blog, #1: So What Is Particle Physics Anyway?

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