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Physicists make first 3D measurements of proton’s internal structure

High-energy experiments reveal the balanced pressure distribution behind the particle’s stability.

Reporter Lizzie Gibney talks to Latifa Elouadrhiri about her team’s latest research on the proton’s interior1.

Transcript

Interviewer: Lizzie Gibney

Here at Nature, we talk a fair amount about dark matter. But what about boring old ordinary matter? It may account for just 15% of material in the Universe, but it's a pretty important part. After all, it makes up everything we can see, from stars to us. And even though it’s visible, it still hides plenty of mysteries. The bulk of the matter in the universe is made up of protons. But what makes up the proton? That's where things get a little fuzzy. Protons are far too small to see under a microscope – around 100,000 times smaller than an atom. So instead, physicists study protons by pinging high-energy electrons off them. These experiments show that each proton must consist of more fundamental particles: three quarks, which are held together by the strong nuclear force. But scientists haven’t known much about how the quarks are arranged in 3D or anything about the proton's mechanical properties. Only now are physicists developing techniques that allow them to probe inside the proton: the particle that's crucial to anything being here at all. To hear how, I spoke to physicist Latifa Elouadrhiri, who explained how techniques to study the proton have evolved.

Interviewee: Latifa Elouadrhiri

Prior to the 90s, the only thing we could study is one-dimensional structure of the proton. And in the 90s there were developments of new formalism that enabled us to connect electromagnetic processes to do three-dimensional structure of the proton. Let me just make simple analogy – so what we have we been doing prior to the 90s and 00s, is like we want to study the heart, and we are studying it through electrography, which is the process of just recording electrical activity of the heart that give us one-dimensional structure that tells us lots about the heart, but not everything. Now with the heart, we have the medical 3D imaging technology that now allow the doctors to learn more in non-invasive manner, the structure of the heart. And this is what we want to do with the new generation of experiments.

Interviewer: Lizzie Gibney

And how do you go about doing those experiments then?

Interviewee: Latifa Elouadrhiri

We are firing high-energy electrons at our protons with very precise measurements. But in order to understand the structure, we want to be able to understand the energy and the momentum that is transferred to the quark so that you acquire both the developments in the theory to interpret results, but also developments in the technology to perform the measurements.

Interviewer: Lizzie Gibney

So, new kinds of theory can connect how electrons bounce of the protons with what’s actually going on inside the proton, things like the forces on the quarks?

Interviewee: Latifa Elouadrhiri

Exactly, and it’s only able to do this interpretation if from the experiment, we have measured all the necessary observables.

Interviewer: Lizzie Gibney

So, what do you actually do then? You fire your electron at the proton, and what happens?

Interviewee: Latifa Elouadrhiri

So, we fire a high-energy electron because by increasing the energy, the wavelength is smaller so we can see deeper into the object. We measure the light that is emitted by the quark, together with the scattered electron, then we also measure the proton. So, we leave the proton intact, we measure it, we measure the produced photon, the light, and the scattered electron. We need to detect all these particles in the final state in order to use the formalism and understand the structure, and that was, this is what was not possible in earlier experiments with electron scattering.

Interviewer: Lizzie Gibney

And what is it then, what did you see, what did you discover about the structure inside the proton?

Interviewee: Latifa Elouadrhiri

So, this is our, the first measurement of the pressure distribution experienced by the quark inside the proton. So, what we found is that there is this extreme outward pressure, but if there was only this pressure in the centre, the proton would explode. But there is another pressure that is going in the other direction, that balances this pressure at the centre that makes the proton stable.

Interviewer: Lizzie Gibney

Which is something we’re very grateful for.

Interviewee: Latifa Elouadrhiri

Yes!

Interviewer: Lizzie Gibney

And can you give me an idea of the scale of the forces that we are talking about, or the pressure at the heart of the proton?

Interviewee: Latifa Elouadrhiri

So, the pressure that we measured is 10 to 35 pascal. This is 10 times larger than, for example, the pressure inside the neutron star.

Interviewer: Lizzie Gibney

Wow, and that’s pretty much the densest matter that we know of, right?

Interviewee: Latifa Elouadrhiri

Exactly.

Interviewer: Lizzie Gibney

And does that match with what theorists predicted?

Interviewee: Latifa Elouadrhiri

That matches some of them. There was a model that were theoretical prediction before this experiment, but this is the first observation, yeah.

Interviewer: Lizzie Gibney

And are you going to be able to use this technique to find out anything else about what’s going on inside a proton?

Interviewee: Latifa Elouadrhiri

This measurement now is just the beginning of a new field of research. So, the paper we published is related to the pressure distributions inside the proton, but next will be to calculate forces, and then move on and understand the 3D imaging of the proton, the spatial distribution of the quark inside the proton, and also the motion of the quark inside the proton.

Interviewer: Lizzie Gibney

Gosh, so the proton might not be so much of a mystery anymore.

Interviewee: Latifa Elouadrhiri

That’s the beginning of solving the mysteries.

Interviewer: Benjamin Thomspon

That was Latifa Elouadrhiri who’s based at the Jefferson Lab in the United States, speaking with reporter Lizzie Gibney. You can find her paper over at nature.com/nature.

doi: 10.1038/d41586-018-05186-x
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References

  1. 1.

    Burkert,V. D., Elouadrhiri, L. & Girod, F. X. Nature 557, 396–399 (2018).

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