Effects of jet algorithms from higher order QCD in W±W^\pm mass determinations at LEP2

We analyse the impact of systematic effects due to the scale dependence of QCD corrections in combination with the use of different jet clustering algorithms in the measurement of the $W^\pm$ mass in the fully hadronic decay mode of $W^+W^-$ pairs produced at LEP2. We consider higher order contributions induced by both virtual and real gluon radiation onto the electroweak CC03 and CC11 channels through ${\cal O}(\alpha_{s})$ at the parton level. We find that the associated uncertainties can be of order 100 MeV, thus competitive with those possibly arising in the non-perturbative regime and indeed above the current experimental estimates.Comment: 16 pages, latex, 4 figures, version to appear in EPJdirect

Vector Boson scattering at the LHC: counting experiments for unitarized models in a full six fermion approach

Unitarization models describe phenomenologically the high energy behaviour of a strongly interacting symmetry breaking sector. In this work, predictions of some unitarized models in vector boson scattering at LHC are studied and compared with analogous studies in Equivalent Vector Boson Approximation and previous results for the benchmark no-Higgs scenario. To perform such studies, unitarized model amplitudes have been implemented in the PHANTOM Monte Carlo in a complete calculation with six fermions in the final state.Comment: version accepted for publication on JHE

PHASE - An event generator for six fermion physics at the LHC

PHASE is a Monte Carlo event generator, under construction, for all Standard Model processes with six fermions in the final state at the LHC. It employs the full set of tree level Feynman diagrams, taking into account fermion masses for b quarks. The program can generate unweighted events for any subset of all six fermion final states in a single run, by making use of dedicated pre-samples. An interface to hadronization is provided.Comment: 4 pages, Latex, 1 figure. Talk given by E. Accomando at the IX International Workshop on Advanced Computing and Analysis Techniques in Physics Research (ACAT03), KEK, Tsukuba, December 1-5, 200

Hysteresis of Backflow Imprinted in Collimated Jets

We report two different types of backflow from jets by performing 2D special relativistic hydrodynamical simulations. One is anti-parallel and quasi-straight to the main jet (quasi-straight backflow), and the other is bent path of the backflow (bent backflow). We find that the former appears when the head advance speed is comparable to or higher than the local sound speed at the hotspot while the latter appears when the head advance speed is slower than the sound speed bat the hotspot. Bent backflow collides with the unshocked jet and laterally squeezes the jet. At the same time, a pair of new oblique shocks are formed at the tip of the jet and new bent fast backflows are generated via these oblique shocks. The hysteresis of backflow collisions is thus imprinted in the jet as a node and anti-node structure. This process also promotes broadening of the jet cross sectional area and it also causes a decrease in the head advance velocity. This hydrodynamic process may be tested by observations of compact young jets.Comment: 9 pages, 5 figures, accepted for publication in ApJ

Perturbative rates and colour rearrangement effects in four-jet events at LEP2

An important issue in the direct reconstruction method of determining the W mass from q\bar q Q\bar Q events at LEP2 concerns the impact of the relatively unknown QCD interconnection effects. It has been suggested that a study of `short string' states, in which colour singlet states are formed from q \bar Q and Q\bar q pairs with small phase--space separation, could shed important light on this issue. We show that such configurations can also be generated by conventional background e^+e^-\ar 4~parton processes, in particular QCD q \bar q g g and q \bar q Q \bar Q and non--resonant electroweak q \bar q Q \bar Q production. We study the colour and kinematic structure of these background contributions, and estimate the event rate to be expected at LEP2. We find that the QCD processes are heavily suppressed, but that non--resonant q \bar q Q \bar Q production may be comparable in rate to the expected `short string' signal from W^+W^- production

Multi-Parton Interactions at the LHC

We review the recent progress in the theoretical description and experimental observation of multiple parton interactions. Subjects covered include experimental measurements of minimum bias interactions and of the underlying event, models of soft physics implemented in Monte Carlo generators, developments in the theoretical description of multiple parton interactions and phenomenological studies of double parton scattering. This article stems from contributions presented at the Helmholtz Alliance workshop on "Multi-Parton Interactions at the LHC", DESY Hamburg, 13-15 September 2010.Comment: 68 page

NNLO corrections to event shapes in e+e−e^+e^- annihilation

We compute the next-to-next-to-leading order (NNLO) QCD corrections to the six most important event shape variables related to three-particle final states in electron-positron annihilation. The corrections are sizeable for all variables, however their magnitude is substantially different for different observables. We observe that the NNLO corrections yield a considerably better agreement between theory and experimental data both in shape and normalisation of the event shape distributions. The renormalisation scale dependence of the theoretical prediction is substantially reduced compared to the previously existing NLO results. Our results will allow a precise determination of the strong coupling constant from event shape data collected at LEP.Comment: 30 pages, LaTeX, numercial results corrected for oversubtraction of large-angle soft radiatio