Developing new jet algorithms Optimising their parameters for LHC physics – Jets4LHC

In collider physics, the final-state QCD partons, quarks and gluons tend to develop into collimated showers usually referred to as "jets". Being able to identify these showers as a single object, rather than seeing particles individually (like e.g. a high-energy muon), is mandatory if one wants to access the primary QCD partons created in the collisions. In other words, jets are the fundamental QCD object when it comes to final-state analysis, as they are a direct view of the initial process in the collisions.

In practice, it is not precise enough to define jets as "collimated bunches of particles"; one has recourse to a "jet definition", an algorithm and its parameters that outputs a set of jets from a set of particles, or calorimeter towers, given as an input. In all modern colliders, including the LHC that just started its operation at CERN, all the events are passed through these jet definitions.

The study of these important objects occupies a large community since about thirty years. Some fundamental issues have been addressed recently, like the question of the infrared-and-collinear safety that lead to the introduction of the anti-kt algorithm, which I have contributed to, and which is now the default algorithm for both the ATLAS and CMS collaborations.

With the LHC spanning a large range of scales and processes, there is a need to dispose of the best possible jet definitions so we can fully benefit of its capabilities. It is therefore important to develop new jet definitions, new tools to handle them, new techniques to have a deeper view of the events and extract the richest possible information from the data accumulated at the LHC. These are the objectives of our project.

These objectives apply to a wide range of processes, both within and beyond the standard model. We have structured the first 2 tasks of this project according to that logic: we shall start by considering dijet processes, simple and flexible enough to be a perfect development framework, then we shall consider extensions to multi-jet cases including many processes of interest for new-physics searches at the LHC. Then, since jets are widely used for both phenomenological and experimental analysis, our third task consists in providing computer code making the product of our research available to the community.

Even though jets are used in all types of analysis, including noticeably searches for new physics, they remain QCD objects by definition. Our work is thus directly related to the theory of strong interactions. And the main line of thought we shall adopt is to provide an analytic understanding from QCD first principles, a step that is often bypassed by the extensive use of Monte Carlo simulations. While the latter are often required, we strongly believe that a deeper analytic understanding would directly translate in more general results with a better predictive power.

To reach our objectives, we envision two main directions of research. Hadronic collisions are known to produce a substantial hadronic background noise, the Underlying Event, polluting the jet reconstruction. Our first direction is to develop jet algorithms with limited sensitivity to this background, e.g. using sub-jets techniques. That has the direct consequence of increasing the signal-to-background ratio, hence the discovery potential.

Our second direction is to find ways to constrain the free parameters of the jet definitions, like the common opening-angle parameter R, from the main properties of the events (like the hard scales or the underlying-event scales). This would avoid their empirical determination and improve the flexibility of jet finding.

To summarise, our project aims at improving final-state analysis at the LHC, for new physics searches, but also for jet definitions in the challenging heavy-ion collisions.