Selecting specific initial configurations using spectator neutrons in U + U collisions

We present a method to select events with a specific initial configuration, namely, the body-tip, in heavy-ion collisions using deformed uranium nuclei. We propose to use asymmetry in spectator neutron numbers to filter out these body-tip events from the unbiased configurations in U + U collisions. We have used a variable $S_{\eta }$ to differentiate between the body-tip and unbiased configurations. We have calculated the second-order azimuthal anisotropy, namely, elliptic flow $v_2$, for this body-tip configuration in the framework of a transport model and found it to be consistently lower compared to that in unbiased configurations as we expected. The purity of selecting such events in a real experiment is also discussed.

Figure 1

(a) Body-tip configuration in the laboratory frame of reference. The impact parameter is along the $x$ axis. (b) The cross sectional view of the overlap region (shown by mesh) for a central $\left(b=0\right)$ body-tip collision.

Figure 2

(a) Spectator neutron distribution for 0–5% central events in body-tip configuration of U + U collisions at $\sqrt{s_{NN}}=193$ GeV. (b) Energy deposited by spectator neutrons in ZDC for the same events of body-tip configuration as in (a). (c) Energy deposited by spectator neutrons in ZDC for all configurations in U + U collisions. The dotted lines show the selection range for body-tip events. (d) Spectator neutron energy in ZDC after selecting events using the dotted lines in (c).

Figure 3

$S_{\eta }$ [see Eq. (3)] for minimum bias, selected body-tip events from minimum bias, and pure body-tip events.

Figure 4

(a) $v_2\left\{pp\right\}$ for 0–5% central U + U collisions at $\sqrt{s_{NN}}=193$ GeV without ZDC cut (open markers) and with ZDC cut (solid markers). (b) $v_2\left\{\text{EP}\right\}$ for the same events without ZDC cut (open markers) and with ZDC cut (solid markers). Solid line in (a) and (b) corresponds to $v_{2\mathrm{pp}}$ and $v_{\mathrm{EP}}$ for pure body-tip events. (c) Distribution of the angle of the major axis with the beam axis for body $\left({\theta }_b\right)$ and for (d) tip $\left({\theta }_t\right)$ oriented uranium nuclei for different slope parameters. For pure body-tip events, $(\pi /2-0.005)<{\theta }_b<(\pi /2+0.005)$ and $0<{\theta }_t<0.05$ (radian).