Monday, July 27, 2020

What happens if gluons meet?

We have published a new paper on how gluons interact, which is described by the strong force. In fact, how exactly one gluon interacts by being absorbed or emitted by another one. There can be interactions with more of them. These are much more complicated to determine, and so we concentrate on this simplest one.

You may ask yourself, how we cannot yet know this, and still do stuff like calculate the mass of a hadron? And not even bother to do more than this simplest process? Because for the proton we need to now what gluons do, right? Well, not exactly. When we want to calculate the properties of a proton we need to know only how they do so in a particular way of averaging. We do not need to resolve the full details. But if we really want to understand how they interact in detail, this is not enough. And this is crucial, if we want to be able to build up not only the proton, but any particle or thing we want to measure. Being able to do a particular averaging good enough is not sufficient to do all of them as well.

In fact, this way of gluons interacting is the simplest way they can interact. Because of this, we know already quite a bit of it, if the gluons are very energetic. But we know less about how they interact, if they have little energy or travel over very long distances. And there a surprise arose some years back. It was raised in a much older work by myself and other people. It indicated that the gluons undergo a drastic change when they start to traverse distances of the order of the size of a proton or even further (inside bigger hadrons, because of confinement). It appears that at distances of the order of a proton diameter they stop interacting. But they become much stronger interacting again at even longer distances. This is, of course, a very interesting insight what happens, in a sense, at the boundary of a proton.

We used simulations for this back then. But we very limited at this time, because of the available computing power. This was aggravated, because at this time, I was working as a postdoc in Brazil. Which, as a disadvantaged country, does have bright minds, but much less resources than I have nowadays in Austria. At any rate, the result nonetheless got people excited, and there were a lot of follow-up works since then. Still, while most results supported the indications, it is not yet possible to give a fully satisfactory answer.

In our latest work, we picked up the idea of looking at the behavior in a world with one direction less. This saves a lot of computing time. And we did not yet had a final answer there either. This we provided now. There is a clear answer, confirming the behavior described above: First getting weaker, until the interaction vanishes at roughly a (flat) proton across, and then becoming quickly much stronger.

Still, doing the same in our world was too expensive. But we did a trick. Having the results from the fewer dimensions, we knew what to anticipate. So we used this information to test our world for consistency. And this checked out surprisingly well. In fact, we could even predict how much more computing time would be needed for a final confirmation also for our world. Could be done in the next few years. So hang around just a little longer for the final answer.

And, perhaps, we can then also do more complicated interactions. But this is a really tedious business. So you need patience and a long-term perspective.

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