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  • There’s a certain attitude that particle physicists tend to take when it comes to their experiments:

  • Smash first, ask questions later.

  • When it comes to the tiniest particles in the universe, there’s a lot we still don’t know.

  • So physicists study them using particle accelerators, huge machines that get particles -- like protons,

  • or electrons -- moving almost as fast as the speed of light.

  • Then they smash them together, andphysics happens. Very explode-y physics.

  • And sometimes, the researchers can’t predict what’s going to come out of the reaction.

  • Instead, they have to work backward, looking at the data and using math to figure out what

  • the heck just happened.

  • Which is why, in September, physicists from the Technical University of Vienna announced

  • that they think theyve identified a whole new kind of subatomic particleall because

  • they did some new math.

  • The particle’s called a glueball, which is, somewhat surprisingly, a pretty accurate name.

  • It’s a particle of gluons, the sticky particles responsible for keeping an atom’s nucleus

  • from flying apart.

  • Now that is a weird thing because this particle doesn’t contain any matter -- it’s made

  • up of pure force.

  • And if they can confirm the discovery, itll be another big piece of evidence that The

  • Standard Model of Physics -- the current understanding of how physics works -- is in fact right.

  • You might remember the Higgs boson -- the main particle that gives matter its mass -- which

  • physicists confirmed not too long ago using the same working-backward method.

  • They bashed together some particles, then sifted through the data.

  • It can take years to analyze all that information, and new insights can crop up long after the

  • particle-smashing is over.

  • In the case of Higgs, scientists knew exactly what to look for, but had to analyze years

  • worth of data before they could confirm the discovery.

  • With glueballs, theyre not even sure what the thing theyre trying to find looks like

  • -- though they do know that theyre made of stuck-together gluons.

  • Now, normally, gluons are the things that hold quarks together.

  • Quarks join up to form protons and neutrons, the basic components of atoms.

  • So, no atoms without protons, and no protons without quarks.

  • And to stick together, the quarks need gluons.

  • That’s because the protons in an atomic nucleus are all positively charged. If it

  • were up to the electromagnetic force, those charges would repel each other and atoms would never form.

  • The protons can all stay inside the nucleus because they have the strong force keeping them there.

  • The strong force is a fundamental force -- meaning that we describe it separately from other

  • fundamental forces, like gravity or electromagnetism.

  • It’s more than a hundred times stronger than the electromagnetic force and many, many

  • more times stronger than gravity, but it’s mostly important on the scale of an atomic nucleus.

  • Each proton or neutron in the nucleus is made of three different quarks, held together by

  • the strong force.

  • The strong force holds together not only the three quarks that make up the proton, but

  • the quarks in the other protons as well, so that electromagnetic force doesn’t send

  • them flying apart.

  • Every force has a particle to carry it, known, appropriately, as a force carrier. Force carriers

  • have no mass. The one for electromagnetism, for instance, is the photon.

  • The same goes for the strong force, which also has a massless force carrier, which is

  • -- you guessed it -- the gluon.

  • That’s what gluons are, and where their name comes from -- they glue quarks together.

  • And gluons are definitely a thing -- researchers discovered them in 1979 using the PETRA particle

  • accelerator in Germany.

  • But as far back as 1972, physicists were using math to show that gluons might have an odd

  • property: the ability to stick to each other, forming a glueball.

  • 43 years later, we still haven’t been able to prove they exist.

  • But scientists really want to, because glueballs are predicted by the Standard Model -- the

  • best explanation we have for how particles and forces behave.

  • Almost all of our understanding of science is based on the Standard Model -- so, uh,

  • it’s kind of important to make sure that it’s right.

  • And there’s a lot of evidence that it is right -- like when we discovered the Higgs

  • boson, which the Standard Model said we should.

  • Until then, there was a chance that the Standard Model was totally wrong about how mass works,

  • and if it was, we’d have to rethink a LOT of science.

  • So, since the Standard Model tells us that glueballs should exist, researchers are looking

  • for them as another way to confirm that our science isn’t totally off track.

  • And it’s not like glueballs occur in nature. You can’t just walk outside, pull out a

  • powerful microscope, and find one in a tree.

  • But the Standard Model does tell us that there are a few different ways to make them in a

  • particle accelerator, like by smashing together a proton and an antiproton -- which is exactly

  • like a proton, just negatively charged.

  • I mean, don’t try this at home or anything, but it sounds easy, right?

  • Trouble is, there’s a lot of stuff that comes out of these glueball-making reactions,

  • and it’s hard to tell what’s what because we don’t know exactly what were looking for.

  • Things that seem like they could be glueballs have been showing up in particle accelerators for ages.

  • But nothing’s for sure, because we don’t know some of the properties of a glueball

  • -- how much mass it has, for example.

  • The idea of a glueball having mass is strange to think about in general, because -- like

  • we said earlier -- none of the individual gluons that stick together to form the glueball have mass.

  • And when you add together a bunch of things with zero mass, youve got 0 + 0 + 0...that’s 0, right?

  • Wrong! You don’t, because of a weird property of energy -- it’s related to mass, by a

  • little equation you mightve heard of: E = mc^2.

  • And turns out, the energy that’s holding all those gluons together in the glueball?

  • It translates to masswe just don’t know exactly how much.

  • To make matters worse, glueballs aren’t stable. You couldn’t pick up a handful of

  • them. They would flit into existence and right back out a moment later.

  • And when you do create a glueball, it causes quarks -- and their opposites, antiquarks

  • -- to just materialize.

  • And whenever a particle and its opposite interact with each other, they both get annihilated.

  • The blast of energy from a quark and antiquark coming into contact with each other blows

  • your glueball to smithereens.

  • So how do you find a glueball, if you can’t observe one directly?

  • Physicists have to put on their Sherlock Holmes deerstalker hats and deduce its existence

  • from the traces it leaves when it’s destroyed.

  • They look at the data from the experiment, and if any of it looks like it could be a

  • sign of an unidentified particle, they do the math to see if that particle would match

  • what we know about glueballs.

  • Which is why every so often, candidate glueballs turn up in the scientific literature. For

  • the most promising ones, there are usually a dozen or so papers arguing for or against

  • its glueball-hood.

  • Two of these are the catchily named f0(1500) and f0(1710).

  • Weve known about both of them for a while, discovered in various particle accelerators

  • over the years. We just don’t know what they are -- or whether one of them could be a glueball.

  • 1710 is about the right mass to be a glueball -- even though we can’t be sure what mass

  • they should be, we can estimate it, and 1710 fits the description.

  • There’s a problem, though: when this mystery particle decays, it produces a variety of

  • quarks known as strange quarks.

  • From everything we knew about the math of glueballs, we thought that when gluons interact

  • with each other, they’d make all kinds of quarks, with no bias toward the strange ones.

  • But that’s where Anton Rebhan and Frederic Brünner, the physicists from Vienna, come in.

  • They changed the mathematical approach to thinking about glueballs.

  • Ordinary, garden-variety quantum physics tells us that glueballs should exist. But it’s

  • not very good at predicting how they decay in a particle accelerator. That’s why there’s

  • been so much back and forth over which candidates are glueballs.

  • So instead of using the more conventional math, Rebhan and Brünner decided to use a

  • different, more theoretical model -- one that includes gravity with the other forces in

  • its description of glueballs.

  • The mathematical model that they used isn’t universally accepted yet. But it might turn

  • out to be a lot better at predicting glueball decay, because their results were a lot more specific.

  • When they crunched the numbers, they found that it actually kind of made sense for 1710

  • to produce those strange quarks, even if it’s a glueball.

  • But the finding isn’t quite enough to definitely confirm 1710 is a glueball, or to rule out

  • the other candidate, 1500.

  • For that, we need more data -- researchers need to make more of this 1710 particle and

  • see if the pattern of its decay matches what the new math predicts.

  • Luckily, it’s fantastic timing, because other particle accelerator experiments around

  • the world may be able to give us more data very soon.

  • Rebhan and Brünner specifically say that theyre keeping an eye on some experiments

  • going on at CERN, the Swiss lab that includes the Large Hadron Collider, the most powerful

  • particle accelerator in the world.

  • Theyre also waiting for the results of an experiment using an accelerator in Beijing,

  • which should be coming out within the next few months.

  • With these new clues, the first confirmed glueball discovery could be much closer than

  • it’s ever been before, and researchers will be looking.

  • The game, as they say, is afoot.

  • Thanks for watching this episode, and thank you for sharing in the excitement of science

  • with us. One more thing I want to share with you is my friend Derek Muller’s Kickstarter

  • project Snatoms: the magnetic molecular modeling kit. This isn’t a paid endorsement or anything,

  • I just really like these and it’s awesome and I want people to know about them. You

  • can’t construct a glueball with them, but you can make water and all kinds of organic

  • compounds and see the energy of molecular bonds as they happen. Well put a link in

  • the description to the kickstarter. It’s only going on for about another week.

  • Check it out, and have some fun!

There’s a certain attitude that particle physicists tend to take when it comes to their experiments:

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グルーボールのクエスト (The Quest for Glueballs)

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    BH に公開 2021 年 01 月 14 日
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