Dr. Melissa Van Beekveld

Abstract:

We introduce a class of collider observables, named Lund-Tree Shapes (LTS), defined from declustering trees originating from the Lund jet plane representation of the QCD radiation pattern in multi-jet scattering processes. At the differential level, they are continuous, global variables akin classical event shapes and n → n + 1 jet-resolution parameters, which probe the geometry and hierarchical structure of the radiation in an event. At the integrated, cumulative level, they naturally define n jet rates, providing a jet-multiplicity-based characterisation of multi-jet final states. In addition, the versatile definition of the LTS can be exploited to scan systematically the QCD radiation pattern in scattering events and jets. From a theoretical viewpoint, such observables feature a simple all-order structure and are free of non-global logarithmic corrections. Their definition applies to scattering processes with any number of resolved jets in the final state, as well as to groomed jets. They are thus usable as resolution variables in the context of higher-order calculations via phase-space slicing, matching fixed-order calculations to parton showers, and testing the logarithmic accuracy of shower algorithms. As an initial application, we derive next-to-next-to-leading-logarithmic accurate predictions for processes with two QCD legs at ee, pp and ep colliders, and matched predictions to next-to-next-to-leading order for the LHC, discussing aspects of collider phenomenology.

Abstract:

The elliptic and triangular flow of charged particles are measured using two-particle angular correlations in pPb collisions in the pseudorapidity range 2.0 <|η|< 4.8.The data sample was collected by the LHCb experiment in 2016 at a centre-of-mass energyper nucleon pair of √sNN = 8.16 TeV, containing in total approximately 1.5 billion collisionevents. Non-flow contributions are obtained in low-multiplicity collisions and subtracted toextract the flow harmonics. The results are presented as a function of event multiplicity andhadron transverse momentum. Comparisons with a full (3+1)D dynamic model indicate thatit overestimates the measured elliptic flow. A comparison between the forward and backwardregions reveals no significant differences in flow parameters, suggesting that final-state effectsmay dominate over initial-state effects in the origin of flow in small systems

Abstract:

We present methods to achieve NLL+NLO accurate parton showering for processes with two coloured legs: neutral- and charged-current Drell-Yan, and Higgs production in pp collisions, as well as DIS and e+e− to jets. The methods include adaptations of existing approaches, as well as a new NLO matching scheme, ESME, that is positive-definite by construction. Our implementations of the methods within the PanScales framework yield highly competitive NLO event generation speeds. We validate the fixed-order and combined resummation accuracy with tests in the limit of small QCD coupling and briefly touch on phenomenological comparisons to standard NLO results and to Drell-Yan data. The progress reported here is an essential step towards showers with logarithmic accuracy beyond NLL for processes with incoming hadrons.

Abstract:

We formulate a collinear partonic shower algorithm that achieves next-to-single-logarithmic (NSL, αsnLn−1) accuracy for collinear-sensitive non-singlet fragmentation observables. This entails the development of an algorithm for nesting triple-collinear splitting functions. It also involves the inclusion of the one-loop double-collinear corrections, through a z-dependent NLO-accurate effective 1 → 2 branching probability, using a formula that can be applied more generally also to future full showers with 1 → 3 splitting kernels. The specific NLO branching probability is calculated in two ways, one based on slicing, the other using a subtraction approach based on recent analytical calculations. We close with demonstrations of the shower’s accuracy for non-singlet partonic fragmentation functions and the energy spectrum of small-R quark jets. This work represents an important conceptual step towards general NNLL accuracy in parton showers.

Abstract:

We study the collinear fragmentation of highly energetic jets defined with a small jet radius. In particular, we investigate how the corresponding fragmentation functions differ from their hadronic counterpart defined in the common MS¯ scheme. We find that the anomalous dimensions governing the perturbative evolution of the two fragmentation functions differ starting at the two loop order. We compute for the first time the new anomalous dimensions at two loops and confirm our predictions by comparing the inclusive small-R jet spectrum against a fixed order perturbative calculation at O(αs2). To investigate the dependence of the anomalous dimension on the kinematic cutoff variable, we study the fragmentation functions of Cambridge jets defined with a transverse momentum cutoff as opposed to an angular cutoff R. We further study the evolution of the small-R fragmentation function with an alternative cutoff scale, proportional to zR, representing the maximum possible transverse momentum of emissions within a jet. In these cases we find that the two-loop anomalous dimensions coincide with the MS¯ DGLAP ones, highlighting a correspondence between the MS¯ scheme and a transverse-momentum cutoff.