In collaboration with J. Bellm, J. Gaunt, A. Kulesza, L. Lönnblad and T. Sjöstrand.
Recently in CMS, we have measured the azimuthal separation between the two leading jets in multi-jet events when these are approximately back-to-back. We have also measured the more exclusive and complementary azimuthal separation between the leading jets when the presence of at least an extra much softer jet is required in the event. We have observed a tension between the data and the theory predictions from parton shower event generators as well as from improved predictions using higher order matrix elements.
The MCnet short-term project aimed at implementing modifications to the conventional string fragmentation framework in the Pythia event generator. In the current form of the fragmentation, strings are stretched between quark-antiquark pairs. As a pair moves apart the string may break up by producing a new pair of a quark and antiquark which receive opposite and compensating kicks in transverse momentum according to a Gaussian distribution, independently of their flavour.
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.
Within this project we developed a scheme for the generation of central exclusive final states (pp → p+X+p) in the Pythia 8 program.
Colour reconnection (CR) is expected to occur at a significant rate at the LHC, and to have a non-negligible impact on event properties. The reason is that the high density of particle production points to a high rate of multiparton interactions which, in combination with parton showers, gives a large number of perturbatively defined partons emerging from typical proton-proton collisions. These partons give many force fields - strings or clusters - that are stretched between matching colour and anticolour charges.
The objectives of the LHC physics program include tests of the Standard Model, searches/studies of the Higgs boson(s), and searches for physics beyond the Standard Model. In all three areas polarization of gauge bosons and heavy fermions such as the tau and top quark will play an important role, and yet currently Pythia does not include spin correlation effects except in a few cases. Because of the importance of Pythia and the need for spin handling, it is critical that spin correlation effects are incorporated into Pythia as soon as possible.
For the older Pythia 6.4 code the Q-PYTHIA routines have added modifications in the context of jet production in heavy-ion collisions. Medium effects are introduced through an additive term in the splitting functions, computed in the multiple-soft scattering approximation. The observable effects of this modification are studied for different quantities such as fragmentation functions and the hump-backed plateau, in terms of evolution variables: virtuality, transverse momentum and angle.
Up until this point the modelling of diffraction in Pythia has been primitive. This is a problem for the ALICE collaboration, since typically efficiencies and uncertainties are evaluated by comparing Pythia and Phojet, and these two give appreciably different predictions for diffractive topologies. An improved diffractive description in Pythia would give the ALICE experiment a reduction of the systematic uncertainty on trigger efficiencies and an improved understanding of the underlying event.
Seyi Latunde-Dada is a PhD student from Cambridge UK working on Herwig++, on a three-month MCnet studentship in Lund.
Devdatta Majumder is a PhD student from Tata Institute of Fundamental Research, Mumbai, India working on CMS, on a three-month MCnet studentship in Lund.
Martijn Gosselink is a PhD student from NIKHEF, Amsterdfam, The Netherlands working on Atlas, on a four-month MCnet studentship in Lund.
Florian Bechtel is a PhD student from the University of Hamburg, on the CMS experiment, on a four-month MCnet studentship in Lund. He attended the 2007 MCnet school in Durham.