List of previous students

Nicola Orlando
Faten Hariri
Emma Kuwertz
Spyridon Argyropoulo
Sabrina Sacerdoti
Simone Amoroso
Jesper Roy Christiansen
Nathan Hartland
Christian Roehr
Benjamin Watt
Philip Ilten
Nishita Desai
Sercan Sen
Miroslav Myska
Sudha Ahuja
Holger Schulz
Avi Gershan
Aleksander Kusina
Magdalena Slawinska
Flavia Dias
Kenneth Wraight
Irais Bautista Guzman
Sparsh Navin
Paolo Francavilla
Riccardo Di Sipio
Seyi Latunde-Dada
Devdatta Majumder
Martijn Gosselink
Christopher Bignamini
Marek Schönherr
Michal Deak
Noam Hod
Florian Bechtel
Jonathan Ferland
Manuel Bähr
Alexander Flossdorf
Piergiulio Lenzi

HEJ is a parton-level Monte Carlo generator designed to calculate cross sections for multi-jet processes at the LHC in the limit where the jets are widely separated in rapidity. The HEJ formalism provides an all-order description of wide angle radiation, which becomes logarithmically enhanced in this limit. In the HEJ treatment, the jets which arise will only contain a small number partons, that is, they will be largely unpopulated. This is not necessarily a problem if the observables of interest are sufficiently inclusive, however, studies of jet vetoes and gap fractions for example (which are of relevance to Higgs couplings measurements) are known to be sensitive to both wide angle radiation described by HEJ, and the soft-collinear radiation described by the parton shower.

It is not sufficient to simply shower the final state from HEJ however, because soft singularities are already included in HEJ to all orders, thus doing so would be to double count an infinite tower of terms. Instead it is necessary to modify the splitting kernel of the parton shower. Furthermore, as the parton shower is ordered, certain states produced by HEJ have a configuration which would unphysically restrict the parton shower evolution. This may be amended by reweighting on an event to event basis with Sudakov factors, or the probability the shower would have produced that event, in a similar way to the treatment in the CKKW-L merging algorithm. These elements form the core of a new merging algorithm which was implemented for Pythia as part of a short-term (3 month) studentship at Lund.