Probing the Hardest Branching within Jets in Heavy-Ion Collisions

Heavy ion collisions present exciting opportunities to study the effects of quantum coherence in the formation of subatomic particle showers. We report on the first calculation of the momentum sharing and angular separation distributions between the leading subjets inside a reconstructed jet in such collisions. These observables are directly sensitive to the hardest branching within jets and can probe the early stage of the jet formation. We find that the leading-order medium-induced splitting functions, here obtained in the framework of soft-collinear effective theory with Glauber gluon interactions, capture the essential many-body physics, which is different from proton-proton reactions. Qualitative and in most cases quantitative agreement between theory and preliminary CMS measurements suggests that hard parton branching in strongly interacting matter can be dramatically modified. We also propose a new measurement that will illuminate its angular structure.

Figure 1

Comparison of theoretical calculations and preliminary CMS data for the ratio of momentum sharing distributions of inclusive anti-$k_T$ $R=0.4$ jets in central $\mathrm{Pb}+\mathrm{Pb}$ and $p+p$ collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. Jets are soft dropped with $\beta =0$, $z_{\mathrm{cut}}=0.1$, and $\mathrm{\Delta }{R}_{12}>0.1$. The bands correspond to the theoretical uncertainty estimated by varying the jet-medium coupling ($g=2.0\pm 0.2$). The purple (red) bands correspond to calculations with (without) the implementation of collisional energy loss. Upper panel: modification for jets with $140<p_T<160\mathrm{GeV}$ and $|\eta |<1.3$. The exploratory calculation with $g=2.5$ is also shown. Lower panel: modification for jets with $250<p_T<300\mathrm{GeV}$ and $|\eta |<1.3$.

Figure 2

Comparison of theoretical calculations and preliminary CMS data for the momentum sharing modification of inclusive jets in proton-proton and lead-lead collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. Anti-$k_T$ $R=0.4$ jets with $140<p_T<160\mathrm{GeV}$ and $|\eta |<1.3$ in midperipheral collisions are considered and the same soft-drop parameters as in Fig. 1 are used to groom the jets.

Figure 3

Theoretical calculations for the momentum sharing distribution ratio of inclusive jets in central lead-lead and proton-proton collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. Jets are soft dropped with $\beta =0$, $z_{\mathrm{cut}}=0.1$, and $\mathrm{\Delta }{R}_{12}>0.2$. We study its jet $p_T$ dependence and provide results for $60<p_T<80\mathrm{GeV}$ (red band) and $250<p_T<300\mathrm{GeV}$ (blue band).

Figure 4

Theoretical calculations for the groomed jet radius modification of inclusive jets in proton-proton and central lead-lead collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. The soft drop parameters $\beta =0$, $z_{\mathrm{cut}}=0.1$, and $\mathrm{\Delta }{R}_{12}>0.1$ are used. Shown are results for four $p_T$ bins with $60<p_T<80\mathrm{GeV}$ (red band), $100<p_T<120\mathrm{GeV}$ (green band), $140<p_T<160\mathrm{GeV}$ (blue band), and $250<p_T<300\mathrm{GeV}$ (purple band).