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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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Very much appreciated Jim.
Best wishes.
Do you think Jim, that the recent "discoveries regarding dark matter will influence the research into Particle Physics? It also strikes me that the smaller the "particle" being identified, the larger the Universe is subsequently acknowledged to be using that information. All very theological if we are not careful. Dark Matter Lives.
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They're bound to, to an extent, but I'm not sure that Particle Physics will have much to say about Dark Matter as long as the detectors here on Earth aren't finding signs of it. That may well change in the next decade or so, but for now Dark Matter is probably something that we will know exists without understanding its nature. Which, to be fair, is far more exciting.

jim have you seen the recent dark matter map? It is detectable but only by measuring the bending of light as we know that it does distort space time, ie had mass and thus gravity.
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I did see it although didn't look into it as deeply as perhaps I should have (although to be fair the news broke whilst I was otherwise engaged, and I've been a bit more tuned into the muon g-2 results etc).

Also, while granted DM is detectable through its strong gravitational pull, I was thinking more of what's termed "direct detection", ie when a DM particle bumps into something on Earth and gives off enough energy to be noticed. Likewise, if DM interacts with normal matter in any other way than just gravity, then it might be produced at the LHC and be detected there in one way or another. So far as I know, neither of these searches has seen anything interesting yet.
here's one for you jim, as we know dark matter is approx 27% of the universe over 5 times the amount of what we know as matter. If there is life living in the dark matter would they have detected the missing 5% of their universe and if they did would they also have called it dark matter?
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Couldn't possibly begin to answer that. An interesting thought for sure, but it's in the realm of pure speculation at this point.
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I should perhaps add that I thought it was a interesting idea! It's just not one I have anything useful to say about.

Also I realise I haven't answered the spin stuff yet, I was going to try and say something about it over the weekend.

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