Physicists calculate the proton pressure distribution for the first time

For the first time, MIT physicists have calculated the proton pressure distribution and discovered that the particle contained a highly pressurized core that, at its most intense point, generates higher pressures than those found at inside a neutron star.

This nucleus emerges from the center of the proton, while the surrounding region grows inward. (Imagine a baseball that is trying to expand inside a football falling down.) The opposing pressures act to stabilize the overall structure of the proton.

The results of physicists, published today in Letters of physical examination, represent the first time that scientists calculate the pressure distribution of a proton taking into account the contributions of quarks and gluons, fundamental and subatomic constituents of the proton.

"Pressure is a fundamental aspect of the proton on which we know very little at the moment," said lead author Phiala Shanahan, an assistant professor of physics at MIT. "We have now found that the quarks and gluons in the center of the proton generate a significant external pressure and that at the edges there is a confining pressure, which allows us to paint a complete picture of the proton structure. "

Shanahan conducted the study with co-author William Detmold, an associate professor of physics at MIT.

Remarkable quarks

In May 2018, physicists at the Thomas Jefferson National Acceleration Center of the US Department of Energy announced that they had measured for the first time the proton pressure distribution, at the same time. Using a beam of electrons that they shot at a hydrogen target. The electrons interacted with the quarks inside the protons of the target. Physicists then determined the distribution of pressure throughout the proton, depending on how the electrons dispersed from the target. Their results showed a center of high pressure in the proton which, at its highest pressure point, measured about 1035 pascals, or 10 times the pressure inside a neutron star.

However, Shanahan says that their image of the proton pressure was incomplete.

"They found a pretty remarkable result," says Shanahan. "But this result was subject to a number of important assumptions that were necessary because of our incomplete understanding."

Specifically, the researchers based their pressure estimates on the interactions of the quarks of a proton, but not of its gluons. Protons consist of quarks and gluons, which constantly interact dynamically and fluctuating within the proton. The Jefferson Lab team was unable to determine the quark contributions with its detector, which, according to Shanahan, leaves out much of the pressure of a proton.

"Over the last 60 years, we have gained a fairly good understanding of the role of quarks in the proton structure," she said. "But the structure of the gluon is much, much more difficult to understand because it is notoriously difficult to measure or calculate."

A gluon shift

Instead of measuring the pressure of a proton with the help of particle accelerators, Shanahan and Detmold sought to include the role of gluons by using supercomputers to compute interactions between quarks and particles. gluons that contribute to the pressure of a proton.

"Inside a proton, a bubbling quantum vacuum of pairs of quarks and antiquarks, as well as gluons, appear and disappear," says Shanahan. "Our calculations include all these dynamic fluctuations."

To do this, the team used a physics technique known as lattice QCD, for quantum chromodynamics, which is a set of equations describing the strong force, one of the three fundamental forces of the standard model of particle physics. (The other two are the weak force and the electromagnetic force.) The strong force is what binds quarks and gluons to form a proton.

Network QCD calculations use a four-dimensional grid, or network, of points to represent the three dimensions of space and one of time. The researchers calculated the pressure inside the proton using the equations of quantum chromodynamics defined on the lattice.

"It's extremely computationally demanding, so we're using the most powerful supercomputers in the world to do these calculations," says Shanahan.

The team spent about 18 months performing various quark and gluon configurations in several supercomputers, then determining the average pressure at each point in the center of the proton, right up to its edge.

Compared to the Jefferson Lab results, Shanahan and Detmold found that by including the gluon contribution, the pressure distribution in the proton had changed considerably.

"We have examined gluon's contribution to pressure distribution for the first time, and we can really see that, compared with previous results, the peak has become stronger and the pressure distribution is expanding further. from the center of the proton, "Shanahan said.

In other words, it appears that the highest pressure in the proton is about 1035 pascals, 10 times that of a neutron star, which is similar to what the Jefferson Lab researchers reported. The surrounding low-pressure region extends further than previously expected.

Confirming these new calculations will require much more powerful detectors, such as the Electron-Ion collider, a proposed accelerator that physicists are looking to use to probe the internal structures of protons and neutrons, in more detail than ever, including gluons. .

"We are in the early days of quantitatively understanding the role of gluons in a proton," says Shanahan. "By combining the experimentally measured quark contribution with our new calculation of the gluon part, we get the first complete table of proton pressure, a prediction that can be tested by the new collider over the next 10 years."

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