Overall momentum balance and redistribution of the lost energy in asymmetric dijet events in $2.76A$ TeV Pb-Pb collisions with a multiphase transport model

The overall transverse momentum balance and the redistribution of the lost energy from hard jets for asymmetric dijet events in PbPb collisions at $2.76A$ TeV at the LHC is studied within a multiphase transport (AMPT) model. A detailed analysis is performed for the projected transverse momentum $\langle {p/}_T^{\left|\right|}\rangle$ contributed from the final charged hadrons carrying different transverse momenta and emitted from different angular directions. We find that the transverse momentum projection $\langle {p/}_T^{\left|\right|}\rangle$ in the leading jet direction is mainly contributed by hard hadrons ($p_T>8.0\mathrm{GeV}/c$) in both peripheral and central PbPb collisions, while the opposite direction in central collisions is dominated by soft hadrons ($p_T=0.5–2.0\mathrm{GeV}/c$). The study of in-cone and out-of-cone contributions to $\langle {p/}_T^{\left|\right|}\rangle$ shows that these soft hadrons are mostly emitted at large angles away from the dijet axis. Our AMPT calculation is in qualitative agreement with the CMS measurements and the primary mechanism for the energy transported to large angles in the AMPT model is the elastic scattering at the partonic stage. Future studies including also inelastic processes should be helpful in understanding the overestimation of the magnitudes of in-cone and out-of-cone imbalances from our AMPT calculations, and shed light on different roles played by radiative and collisional processes in the redistribution of the lost energy from hard jets.

Figure 1

Dijet $A_J$ event distributions in 0–10%, 10–20%, 20–30%, 30–50%, and 50–100% PbPb collisions at $2.76A$ TeV from the AMPT model ($\sigma =1.5$ and 3.0 mb), compared to CMS data [32]. The cone size for jet reconstruction is $R=0.5$.

Figure 2

The event-averaged transverse momentum projection $\langle {p/}_T^{\left|\right|}\rangle$ calculated from the AMPT model as a function of $A_J$ for $pp$ collisions, and 50–100%, 30–50%, 10–30%, 0–10% PbPb collisions at $2.76A$ TeV. Each band shows the contribution from charged hadrons with $p_T=0.5$–1 GeV/$c$, 1–2 GeV/$c$, 2–4 GeV/$c$, 4–8 GeV/$c$, $p_T>8.0\mathrm{GeV}/c$. The solid lines at the edges of the bands denote the cumulative contributions from different $p_T$ ranges, and the thick dashed line denotes the cumulative contribution from charged hadrons with $p_T>0.5\mathrm{GeV}/c$. The symbols represent the CMS data [64] for cumulative contributions from different $p_T$ ranges (to be compared to the corresponding lines calculated from the AMPT model). The cone size for jet reconstruction is $R=0.3$. The partonic cross section $\sigma =1.5\mathrm{mb}$.

Figure 3

Same as Fig. 2 but for $\sigma =3.0\mathrm{mb}$.

Figure 4

$\langle {p/}_T^{\left|\right|}\rangle$ as a function of $p_T$ bin in different evolution stages for 0–10% PbPb collisions at $2.76A$ TeV calculated from the AMPT model, where the dijet asymmetry is taken as $A_J>0.15$ (upper) and $A_J>0.25$ (lower), and the partonic cross section is taken to be 0 mb (left) and 1.5 mb (right).

Figure 5

The in-cone (left) and out-of-cone (right) contributions to the event-averaged transverse momentum projection $\langle {p/}_T^{\left|\right|}\rangle$ as a function of $A_J$ in 0–30% (upper) and 50–100% (lower) PbPb collisions at $2.76A$ TeV calculated from the AMPT model ($\sigma =1.5\mathrm{mb}$). The cone size is 0.5.

Figure 6

Same as Fig. 5 but for cone size 0.8. The CMS data are from Ref. [32].

Figure 7

Same as Fig. 5 but for cone size 1.2.