A model comparison of Υ production in Au+Au collisions at RHIC favors thermal decay as the better explanation for suppression across the states studied. In the calculation, continuous thermal decay described the measured behavior of both the ground state, Υ(1S), and the excited state, Υ(2S), while a sudden color-screening picture could not describe both at once.
The study is a transport-model test of the two mechanisms. It solves a transport equation driven only by suppression rates and compares the resulting nuclear modification factors with RHIC measurements.
How the comparison was set up
The modeled system was Υ production in Au+Au collisions at RHIC energy. The calculation followed five states separately: Υ(1S), Υ(1P), Υ(2S), Υ(2P) and Υ(3S). Each state had its own transport equation, and final yields included both direct production and feed-down from excited states.
The surrounding medium was modeled with MUSIC hydrodynamics and a lattice-QCD-based equation of state. The setup used a critical crossover temperature of approximately 160 MeV, a maximum medium temperature of 391 MeV and an initial proper time of 0.6 fm/c.
At RHIC energy, the model assumed that Υ regeneration was negligible, so the collision term contained only suppression.
The screening picture struggled to fit both states
In the color-screening scenario, a state was assumed to dissociate suddenly when the medium temperature exceeded its melting temperature. Those melting temperatures were fitted directly to experimental RAA(Npart) data with the Nelder-Mead simplex algorithm. The RAA(pT) dependence was treated as a model prediction rather than an independently fitted result.
One version of the screening calculation, using V=U, described the centrality and transverse-momentum dependence of Υ(2S) reasonably well, but the remaining cases deviated significantly. Neither the U nor F choice simultaneously described Υ(1S) and Υ(2S) suppression.
The reported melting-temperature-to-critical-temperature ratios also depended on how the heavy-quark potential was defined. With the free energy used for the potential, the ratios for Υ(1S), Υ(1P), Υ(2S), Υ(2P) and Υ(3S) were 3.00, 1.12, 1.08, 1.00 and less than 1. Using the internal energy instead, the corresponding ratios were greater than 4.00, 1.76, 1.60, 1.19 and 1.17.
A continuous-loss model matched the measured trends
The alternative model treated suppression as thermal decay, with temperature-dependent decay widths. It incorporated lattice-QCD decay widths obtained from smooth-cut and simple-cut Lorentzian parametrizations. For the fits, each state's width was assumed to vary linearly with temperature, with a fitted slope for each state.
The best-fit thermal widths described both the centrality and transverse-momentum dependence measured for Υ(1S) and Υ(2S). In the authors' overall comparison, an appropriate thermal-decay width also described all the measured Υ states, giving that scenario the stronger combined account of the data.
What the result does not settle
The calculation could not determine the thermal width of Υ(3S) from the stated observables. Feed-down from Υ(3S) to Υ(1S) was reported as only 0.9%, leaving the measured Υ(1S) and Υ(2S) modification factors insensitive to the Υ(3S) yield.
The supplied analysis reports no numerical fit uncertainty for the best-fit thermal widths. Because only RAA(Npart) was fitted, RAA(pT) was treated as a model prediction, so its agreement with the measurements was not an independent fit.
The conclusion concerns the modeled RHIC Au+Au system under the assumption of negligible regeneration. Additional observables would be needed to constrain the Υ(3S) thermal width, which the stated measurements could not determine.
The manuscript is an arXiv submission in the hep-ph category. The work lists support from Yantai university grant 2226001 and NSFC Grant 12575149.
Paper data and sources
Original title: Color screening versus thermal decay as the mechanism of $Υ$ suppression in high energy nuclear collisions
Authors: Yida Yang, Baoyi Chen, Jiaxing Zhao, Pengfei Zhuang
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-28
DOI: Not available
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