The inside of a proton is under a lot of pressure. The particle’s centre withstands a billion billion billion times the pressure found at the bottom of the Mariana Trench.
and his colleagues at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia, made the first measurement of the intense conditions inside a proton. And they had to use a bit of trickery to do so.
Protons are made up of three fundamental particles called quarks, which are held together by a force originating from other particles called gluons. To probe this minuscule material, Burkert and his team fired an electron beam at a proton. The electron carried with it a packet of energy that behaves like a photon, a particle of light, which it passed off to one of the quarks.
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When the electron bounced off one quark, the entire proton recoiled in response, and the quark emitted another high-energy photon.
Measuring how the electron, the proton, and the photon are moving at the end of the experiment – including their momenta and the angles at which they come away from the collision – let the researchers create a 3D map of the quarks inside the proton.
Faking gravity
But that map doesn’t directly tell us about the forces within the proton. To measure that, we’d need a theoretical particle called a graviton – the carrier of gravity – but we haven’t found any yet. That’s where the trick comes in.
Information from two photons can be combined to essentially mimic what a graviton would tell us. The two photons here – the one absorbed at the beginning of the experiment, and the one emitted at the end – probed this system as if they were a single graviton, Burkert says.
This loophole gets us the same information without the need for a direct gravitational probe. “Now we can really probe the heart of the proton,” says collaborator .
The researchers found intense pressures of about 1035 pascals – that’s 10 times the pressure inside a neutron star, which is the densest known object in the universe. That density is a result of extreme pressures. But protons withstand even more.
The quarks at a proton’s centre are pressed tightly together and straining outward to escape with a huge amount of force. Toward the outer edges of the proton, the team also found a confining pressure, likely created by gluons, that holds back the quarks. Good thing, or the proton would explode.
Nature
Read more: Latest measurement of a proton’s mass has got physicists puzzled
