Reweighting parton showers

We report on the possibility of reweighting parton-shower Monte Carlo predictions for scale variations in the parton-shower algorithm. The method is based on a generalization of the Sudakov veto algorithm. We demonstrate the feasibility of this approach using example physical distributions. Implementations are available for both of the parton-shower modules in the Herwig 7 event generator.

Figure 1

Distribution of $1-T$ at the parton level for $e^{+}{e}^{-}\to q\overline{q}$ with $\sqrt{s}=91.2\mathrm{GeV}$ using the angular-ordered parton shower. The upper frame shows the effect of varying the scale while the lower shows the ratio of the up and down scale variations calculated using reweighting with respect to those obtained running the event generator with the relevant scale.

Figure 2

Distribution of $1-T$ at the parton level for $e^{+}{e}^{-}\to q\overline{q}$ with $\sqrt{s}=91.2\mathrm{GeV}$ using the dipole shower. The upper frame shows the effect of varying the scale while the lower shows the ratio of the up and down scale variations calculated using reweighting with respect to those obtained running the event generator with the relevant scale.

Figure 3

The transverse momentum of the Higgs boson in the process $gg\to h^0$ at the parton level with $\sqrt{s}=13\mathrm{TeV}$ using the angular-ordered parton shower. The upper frame shows the effect of varying the scale while the lower shows the ratio of the up and down scale variations calculated using reweighting with respect to those obtained running the event generator with the relevant scale.

Figure 4

The transverse momentum of the Higgs boson in the process $gg\to h^0$ at the parton level with $\sqrt{s}=13\mathrm{TeV}$ using the dipole parton shower. The upper frame shows the effect of varying the scale while the lower shows the ratio of the up and down scale variations calculated using reweighting with respect to those obtained running the event generator with the relevant scale.

Figure 5

Distributions of the weights used to determine the “up” and “down” scale variations for the dipole shower in $gg\to h^0$ events at 13 TeV for different values of the detuning parameter. The improvement by using $\lambda >1$ is clearly observed.