Jet Topics: Disentangling Quarks and Gluons at Colliders

We introduce jet topics: a framework to identify underlying classes of jets from collider data. Because of a close mathematical relationship between distributions of observables in jets and emergent themes in sets of documents, we can apply recent techniques in “topic modeling” to extract jet topics from the data with minimal or no input from simulation or theory. As a proof of concept with parton shower samples, we apply jet topics to determine separate quark and gluon jet distributions for constituent multiplicity. We also determine separate quark and gluon rapidity spectra from a mixed $Z$-plus-jet sample. While jet topics are defined directly from hadron-level multidifferential cross sections, one can also predict jet topics from first-principles theoretical calculations, with potential implications for how to define quark and gluon jets beyond leading-logarithmic accuracy. These investigations suggest that jet topics will be useful for extracting underlying jet distributions and fractions in a wide range of contexts at the Large Hadron Collider.

Figure 1

The generation of mixed samples of quark and gluon jets highlighting the correspondence with topic models. Each jet is either a quark or gluon jet sampled according to the underlying quark fraction. The observable is then sampled according to a universal distribution for that jet type. Each mixed-sample observable distribution is then a mixture of the two universal distributions, giving rise to a jet document.

Figure 2

The jet topics method applied to constituent multiplicity starting with $Z+\text{jet}$ (pink) and dijet (purple) distributions from Pythia 8.226. There is good agreement between the two extracted jet topics (orange and green) and pure $Z+\mathrm{quark}$ and $Z+\mathrm{gluon}$ distributions (red and blue).

Figure 3

Cross sections for jet topics (orange and green) using topic fractions extracted from the $Z+\text{jet}$ sample across ten rapidity bins. The extracted topic cross sections closely track the underlying $Z+\text{quark}$ and $Z+\text{gluon}$ cross sections (red and blue).

Figure 4

The jet topics method applied to jet mass up to 35 GeV (up to the black dashed line). The gray curve is the corrected quark topic using Pythia to determine $\kappa (q|g)$ extrapolated beyond 35 GeV by letting jet topic 1 go negative. There is good agreement between the $\kappa$-corrected quark topic (gray) and the pure $Z+\text{quark}$ distribution (red).