Time evolution of light nuclei cumulants and ratios with a first-order phase transition in the UrQMD transport model

The UrQMD model with a density dependent equation of state, including a first-order phase transition, is used to study the time dependence of baryon number and proton number susceptibilities up to third order in heavy ion reactions of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mi>E</a:mi><a:mi>lab</a:mi></a:msub><a:mo>=</a:mo><a:mn>2</a:mn><a:mo>–</a:mo><a:mn>3</a:mn><a:mspace width="0.16em"/><a:mi>A</a:mi><a:mspace width="0.16em"/><a:mi>GeV</a:mi></a:mrow></a:math>. A significant deviation from the Gaussian fluctuations of the baryon number fluctuation in coordinate space is observed. The proton number fluctuations are always suppressed as they constitute only a small fraction of the total baryon number during the dense phase of the collision. It is found that the only measurable, but small, signal would be an enhancement of the third order (or higher) proton cumulant in a finite rapidity window <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mi mathvariant="normal">Δ</d:mi><d:mi>y</d:mi></d:mrow></d:math> that is larger than one unit of rapidity. In addition, it is found that the coordinate fluctuations will lead to an enhancement of cluster production due to the correlations in coordinate space. However, this enhancement is small and mainly occurs during the dense part of the collision before the system actually freezes out.