A proton can't be seen as a simple ball. If you hit it with a photon, it can absorb energy and jump into an excited state, like a bell that starts ringing at a certain frequency after being struck. These states are called nucleon resonances, and they live very briefly before decaying into other particles.
In the new work, physicists studied the reaction in which a photon (a particle of light) collides with a proton and produces an η' meson (a particle made of a quark and an antiquark) along with a proton. They measured the so-called photon beam asymmetry — the difference in the number of events for different polarizations (directions of spin) of the photon. For the first time, they obtained data at photon energies above 1.84 GeV. In addition, they obtained the most precise differential cross sections to date (that is, how often particles fly out at different angles) especially at very backward angles, when the η' flies almost backward relative to the photon's direction.
These data allowed the scattering amplitudes, mathematical quantities describing how particles interact, to be refined. The analysis showed that the coupling of the η' and proton system to the N(2250) resonance may be stronger than previously thought. N(2250) is an excited state of the proton with a mass of about 2250 MeV (roughly 2.4 times heavier than a normal proton). A stronger coupling means this resonance plays a more important role in the production of η' mesons.
To understand how small these effects are, imagine: the photon energy of 1.84 GeV is roughly the energy needed to lift 1 milligram to a height of 1 nanometer. And the reaction cross section, the probability of the collision, is about 1 microbarn, which is a billion times smaller than the cross-sectional area of an atom. Such measurements require enormous statistics and precise detectors.
The N(2250) resonance is one of the heaviest known excited states of the proton, with a mass almost 2.4 times the proton's rest mass.
In the series "Star Trek," "resonant frequencies" are often mentioned for destabilizing objects — in real physics, particle resonances help us look inside matter.