Like a Fat Globule in Milk: Why Heavy Quarks Rule Inside Mesons express
A drop of cream in a glass of milk. Shake it, and the fat doesn't spread evenly: it clumps into denser blobs that move differently from the rest of the liquid. Quarks inside mesons, tiny particles that live for mere fractions of a second, behave in much the same way.
A meson consists of a quark and an antiquark, and they are rarely equals. In a kaon, for instance, the strange quark is noticeably heavier than the light up quark. Calculations show that the heavy partner takes almost two-thirds of the particle's total momentum, leaving only crumbs for the light one. As energy increases, this imbalance only grows, like a fatty droplet racing ever faster ahead of the watery base.
The authors traced how this picture changes at higher energies: those that will be accessible at the upcoming Electron-Ion Collider (EIC) and in the COMPASS++/AMBER experiment. They calculated the cross sections for muon pair production (the Drell–Yan process) for kaons on carbon, tungsten, and aluminum targets. Such predictions are like a map for future experimentalists: they show where to look for signals and how to distinguish one type of meson from another.
Knowing who is "in charge" inside a meson helps us understand how quarks stick together at all. It's a step toward deciphering the strong interaction — the most powerful, yet the most capricious, of the four fundamental forces of nature.
Original abstract
as written by the authors arXiv:2605.17389
In this work, we investigate the constituent parton distribution functions (PDFs) of the kaon and heavy pseudoscalar mesons within the light-cone quark model. Starting from the initial scale quark and antiquark PDFs, obtained by evaluating the quark-quark correlation functions for individual mesons, we perform quantum chromodynamics (QCD) evolution to determine the partonic structure at higher energy scales. The QCD evolution has been carried out through the next-to-leading order (NLO) Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) equations. We further compute the average longitudinal momentum fractions carried by the individual constituents both at the model scale and evolved scale. In addition, the NLO structure functions for the kaon are predicted at higher energy scales relevant for the upcoming Electron-Ion Collider (EIC). For the COMPASS++/AMBER experiment, we further present detailed predictions for the NLO Drell-Yan cross sections corresponding to both isospin states of the kaon, employing Carbon, Tungsten, and Aluminum as nuclear targets. We demonstrate the dominance of the heavy constituents over the lighter constituents within their heavy mesons in terms of their possessed momentum fractions.
If you could weigh a single up quark, you'd find it's nearly 600 times lighter than a proton, like a feather compared to a brick. But inside particles, these "feathers" never fly solo.