The proton structure exhibits different properties depending on how it is probed.
when gently probed by electrons, the proton acts like 3 quarks (SLAC experiments)
when strongly probed by electrons, the proton appears to be a sea of short-lived quark-antiquark pairs and gluons
(HERA experiments)
even higher-energy collisions, the proton appears as a cloud made up of gluons
There are new type of experiments trying to understand the mechanical properties of the proton such as the distribution of internal pressures and shear forces inside the proton.
Findings of the new experiments shed light on the quark confinement as well. Quarks experience a very strong (more than four-ton) confinement force when we try to separate them, but they appear to move freely when they are very close to each other. This brings to mind the metaphor of elastic bands. When we stretch the elastic band, it wants to return to its original shape. Similarly, the proton confinement force increases sharply as we separate the quarks from each other. This behavior is the opposite of the behavior displayed by the electric force or the gravitational force where the force gets weaker as the distance between the objects increases.
New experimental findings [1] [2] indicate that the “elastic band” metaphor is too simplistic. The most interesting new finding is that the core region of the proton twists in the opposite direction compared to the outer layers of the proton. This observation will lead to a better understanding of the quark confinement.
“Nature, of course, does not allow us to separate just one quark from the proton because of a property of quarks called ‘color.’ There are three colors that mix quarks in the proton to make it appear colorless from the outside, a requirement for its existence in space. Trying to pull a colored quark out of the proton will produce a colorless quark/anti-quark pair, a meson, using the energy you put in to attempt to separate the quark, leaving a colorless proton (or neutron) behind. So, the 4-tons is an illustration of the strength of the force that is intrinsic in the proton.” – Volker Burkert (Jefferson Lab)
“Physicists have learned a tremendous amount about the proton over the last 70 years by repeatedly hitting it with electrons. They know that its electric charge extends roughly 0.8 femtometers, or quadrillionths of a meter, from its center. They know that incoming electrons tend to glance off one of three quarks — elementary particles with fractions of charge — that buzz about inside it. They have also observed the deeply strange consequence of quantum theory where, in more forceful collisions, electrons appear to encounter a frothy sea made up of far more quarks as well as gluons, the carriers of the so-called strong force, which glues the quarks together.” – Charlie Wood (Quanta Magazine)
References
[1] V. D. Burkert, L. Elouadrhiri, F. X. Girod, C. Lorcé, P. Schweitzer, and P. E. Shanahan,
“Colloquium: Gravitational form factors of the proton”
Rev. Mod. Phys. 95, 041002 – Published 22 December 2023
[2] Jefferson Lab, GRAVITY HELPS SHOW STRONG FORCE STRENGTH IN THE PROTON
Suresh Emre
Renaissance Universal 