Researchers have built a new event-by-event model called WAGASHI that shows a large number of quark-antiquark pairs can be produced non-perturbatively in the first moments after two heavy ions collide. Those quarks are created by the strong color fields of the “glasma,” the very early, far-from-equilibrium state that precedes the quark-gluon plasma (QGP). The study finds that the number of quarks produced in this stage can be comparable to the number of hadrons seen at the end of the collision.

WAGASHI combines three pieces: a classical evolution of the glasma fields, a transport model that moves colored particles through those fields, and a non-perturbative production mechanism known as the Schwinger effect. The Schwinger pair production is implemented using the locally constant field approximation (LCFA) together with an Abelian projection of the non‑Abelian gauge fields. The glasma fields are evolved on a real-time, boost-invariant two-plus-one dimensional lattice, and the produced quarks are followed with Wong equations for colored particle motion plus a spin-precession equation.

On average, the model produces roughly four hundred quarks and antiquarks per unit of spacetime rapidity in central lead–lead collisions by a time of order 0.5 femtoseconds after the collision. In much smaller oxygen–oxygen collisions the yield is about fifteen quarks. The authors report that the strange-quark yield is only slightly lower than for up or down quarks, remaining about 90% of the up-quark yield at early times. The yield scaling between large and small nuclei follows a simple geometric expectation proportional to the mass number to the four-thirds power.

The newly produced quarks start with very low transverse momentum because of Landau-level quantization in the strong color magnetic fields. As they move through the spatially varying glasma, they gain momentum through the color analogue of the Lorentz force and undergo a random walk in momentum space. This momentum broadening shifts quarks from very soft modes toward higher transverse momentum. At low transverse momentum (below about 1 GeV) the spectrum looks roughly thermal with an effective temperature near 150 MeV, but the full spectrum deviates from a thermal form and is better described by a Tsallis-like shape.