Wouter J. Waalewijn

Jet mass spectra in Higgs boson plus one jet at next-to-next-to-leading logarithmic order

The invariant mass of a jet is a benchmark variable describing the structure of jets at the LHC. We calculate the jet mass spectrum for Higgs plus one jet at the LHC at next-to-next-to-leading logarithmic (NNLL) order using a factorization formula. At this order, the cross section becomes sensitive to perturbation theory at the soft m2jet/p jet T scale. Our calculation is exclusive and uses the 1-jettiness global event shape to implement a veto on additional jets. The dominant dependence on the jet veto is removed by normalizing the spectrum, leaving residual dependence from non-global logarithms depending on the ratio of the jet mass and jet veto variables. For our exclusive jet cross section these non-global logarithms are parametrically smaller than in the inclusive case, allowing us to obtain a complete NNLL result. Results for the dependence of the jet mass spectrum on the kinematics, jet algorithm, and jet size R are given. Using individual partonic channels we illustrate the difference between the jet mass spectra for quark and gluon jets. We also study the effect of hadronization and underlying event on the jet mass in Pythia. To highlight the similarity of inclusive and exclusive jet mass spectra, a comparison to LHC data is presented.

QCD analysis of double parton scattering: Spin and color correlations, interference effects, and evolution

We derive a factorization formula for the double Drell-Yan cross section in terms of double parton distribution functions (dPDFs). Diparton flavor, spin and color correlations and parton-exchange interference terms contribute, even for unpolarized beams. Soft radiation effects are nontrivial for the color correlation and interference contributions, and are described by non-perturbative soft functions. We provide a field-theoretic definition of the quark dPDFs and study some of their basic properties, including discrete symmetries and their interpretation in a non-relativistic quark model. We calculate the renormalization group evolution of the quark dPDFs and of the soft functions. The evolution receives contributions from both ultraviolet and rapidity divergences. We find that color correlation and interference effects are Sudakov suppressed, greatly reducing the number of dPDFs needed to describe double parton scattering at high energy experiments.

Gaining (mutual) information about quark/gluon discrimination

A bstractDiscriminating quark jets from gluon jets is an important but challenging problem in jet substructure. In this paper, we use the concept of mutual information to illuminate the physics of quark/gluon tagging. Ideal quark/gluon separation requires only one bit of truth information, so even if two discriminant variables are largely uncorrelated, they can still share the same “truth overlap”. Mutual information can be used to diagnose such situations, and thus determine which discriminant variables are redundant and which can be combined to improve performance. Using both parton showers and analytic resummation, we study a two-parameter family of generalized angularities, which includes familiar infrared and collinear (IRC) safe observables like thrust and broadening, as well as IRC unsafe variants like pTD and hadron multiplicity. At leading-logarithmic (LL) order, the bulk of these variables exhibit Casimir scaling, such that their truth overlap is a universal function of the color factor ratio CA/CF. Only at next-to-leading-logarithmic (NLL) order can one see a difference in quark/gluon performance. For the IRC safe angularities, we show that the quark/gluon performance can be improved by combining angularities with complementary angular exponents. Interestingly, LL order, NLL order, Pythia 8, and Herwig++ all exhibit similar correlations between observables, but there are significant differences in the predicted quark/gluon discrimination power. For the IRC unsafe angularities, we show that the mutual information can be calculated analytically with the help of a nonperturbative “weighted-energy function”, providing evidence for the complementarity of safe and unsafe observables for quark/gluon discrimination.

Resummation of double-differential cross sections and fully-unintegrated parton distribution functions

A bstractLHC measurements involve cuts on several observables, but resummed calculations are mostly restricted to single variables. We show how the resummation of a class of double-differential measurements can be achieved through an extension of Soft-Collinear Effective Theory (SCET). A prototypical application is pp → Z + 0 jets, where the jet veto is imposed through the beam thrust event shape T

Soft function for exclusive N-jet production at hadron colliders

\mathcal{T}

Parton Fragmentation within an Identified Jet at NNLL

, and the transverse momentum pT of the Z boson is measured. A standard SCET analysis suffices for pT∼mZ1/2T1/2

Calculating the charge of a jet

{p}_T\sim {m}_Z^{1/2}{\mathcal{T}}^{1/2}