Decoding the flow evolution in Au+Au reactions at $1.23 A$ GeV using hadron flow correlations and dileptons

Understand

We investigate the development of the directed, $v_1$, and elliptic flow, $v_2$, in heavy ion collisions in mid-central Au+Au reactions at $E_\mathrm{lab}=1.23 A$ GeV.

• We demonstrate that the elliptic flow of hot and dense matter is initially positive ($v_2>0$) due to the early pressure gradient.
• This positive $v_2$ transfers its momentum to the spectators, which leads to the creation of the directed flow $v_1$.
• In turn, the spectator shadowing of the in-plane expansion leads to a preferred decoupling of hadrons in the out-of-plane direction and results in a negative $v_2$ for the observable final state hadrons.