The Lund jet plane: visualizing jet showers using the jets-within-the-jets

by Cristian Baldenegro (Massachusetts Inst. of Technology (US))

Monday 19 Oct 2026, 14:00 → 15:00 Europe/Zurich

4/2-037 - TH meeting room (CERN)

How do quarks and gluons evolve into the hadrons we observe in our detectors? Understanding this evolution means connecting the basic rules of quark and gluon branchings to the strongly coupled physics of hadron formation. Jets, collimated showers of particles produced abundantly at the LHC, provide a way of studying this process across several scales.

In this seminar, I will discuss one way of understanding jet showers in a modular fashion via the “Lund jet plane,” a two-dimensional representation of jet phase space constructed using the “jets-within-the-jets.” I will start with lessons from LHC measurements of the primary Lund jet plane, constructed by undoing the jet clustering tree and following the harder branch at each step. The jet-averaged emission density gives us access to different building blocks of the shower: the running of the strong coupling αs at high momentum scales, hadronization effects at low momentum scales, and the dead-cone effect for heavy quarks.

I will then discuss new observables I have been developing, including secondary Lund jet planes, which offer a way to isolate high-purity gluon-jet samples and address the long-standing challenge of studying gluon-initiated showers. Finally, I will argue that the Lund jet plane provides a common framework for studying hidden-heavy-flavor jets (containing $c\bar{c}$ or $b\bar{b}$ bound states) and double-open-heavy-flavor jets (containing $c\bar{c}$ or $b\bar{b}$ pairs), with gluon-initiated showers serving as their natural “massless” baseline. These comparisons could offer a new jet substructure perspective on the mechanisms governing heavy-flavor bound-state formation.

QCD_seminar.pdf