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Light nuclei elliptic flow at mid-rapidity in $$\sqrt{s_\text {NN}}$$ = 3.0–3.9 GeV Au+Au collisions using coalescence model
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DOI：10.1007/s41365-026-02038-6.png)
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Abstract
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The collective flow of light nuclei is an important probe for understanding their production mechanisms in heavy-ion collisions. The STAR collaboration has reported that the atomic mass-number (A) scaling of light nuclei elliptic flow $$v_2$$ breaks down at $$\sqrt{s_\text {NN}}= 3.0-3.9$$ GeV. The observations reveal that, while protons maintain negative $$v_2$$ values at mid-rapidity for both 3.0 and 3.2 GeV, light nuclei $$v_2$$ exhibit a sign change from negative at 3.0 GeV to positive at 3.2 GeV. In this study, we investigate the $$v_2$$ of protons and deuterons in mid-central Au+Au collisions at $$\sqrt{s_\text {NN}}=$$ 3.0, 3.2, 3.5 and 3.9 GeV using the JAM2 microscopic transport model. Deuterons are formed via nucleon coalescence, with the spatial distance $$\Delta R$$ and momentum difference $$\Delta P$$ between constituent nucleons serving as the coalescence criteria. Our calculations reproduce the sign change in deuteron $$v_2$$ at 3.2 GeV. We observe a pronounced dependence of the nucleon coalescence probability on the azimuthal angle relative to the reaction plane. This effect is primarily driven by the transverse momentum dependence of the mean spatial $$\langle \Delta R\rangle$$ and momentum $$\langle \Delta P\rangle$$ separations between nucleon pairs, which vary with the nucleon azimuthal angle. Moreover, our analysis indicates that the stiffness of the nuclear equation of state plays a crucial role in determining whether the sign change in deuteron $$v_2$$ occurs near $$\sqrt{s_\text {NN}}=3.2$$ GeV.
Keywords:
Heavy-ion collision
Elliptic flow
Nucleon coalescence
High baryon density
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