Mass depletion: A new parameter for quantitative jet modification

We propose an extension to classify jet modification in heavy-ion collisions by including the jet mass along with its energy. The mass of a jet, as measured by jet reconstruction algorithms, is constrained by the jet's virtuality, which in turn has a considerable effect on such observables as the fragmentation function and jet shape observables. The leading parton, propagating through a dense medium, experiences substantial virtuality (or mass) depletion along with energy loss. Meaningful comparisons between surviving jets and jets produced in $p\text{−}p$ collisions require mass depletion to be taken into account. Using a vacuum event generator, we show the close relationship between the actual jet mass and that after applying a jet reconstruction algorithm. Using an in-medium event generator, we demonstrate the clear difference between the mass of a surviving parton exiting a dense medium and a parton with a similar energy formed in a hard scattering event. Effects of this difference on jet observables are discussed.

Figure 1

The correlation between an underestimate of the jet mass [Eq. (4)] and the reconstructed mass using the anti-$k_T$ algorithm, with an $R=0.7$, in fastjet. Jets with energies between 90 and 110 GeV, generated in vacuum using the pythia generator. The black points represent the mean $M_{\text{parton}}$ in a given $M_{\text{Jet}}^{PL}$ bin. See text for details.

Figure 2

Pythia8 fragmentation function: for an $R=0.7$ jet reconstructed with the anti-$k_T$ algorithm, with a $p_T^{\text{Jet}}=70$–80 GeV/c for different jet mass $\left(M_{\text{Jet}}\right)$ selections. The comparison is limited to the leading jet (jet with highest energy) obtained after applying the anti-$k_T$ algorithm; this is referred to as “Leading Anti-$k_T$” in the legend.

Figure 3

Virtuality distribution of the leading parton at $L=0\mathrm{fm}$ (parton that originates the shower), from matter, compared with the results from jetset. Virtuality distribution of the leading parton at $L=2$ fm (red dot-dot-dashed line) and 4 fm (green dashed line). The leading parton is seen to drop virtuality swiftly with distance.

Figure 4

The mean energy and mass of the leading parton in a quark jet as a function of length in four different scenarios. See text for details.