95 research outputs found
Colour Reconnection from Soft Gluon Evolution
We consider soft gluon evolution at the amplitude level to expose the
structure of colour reconnection from a perturbative point of view. Considering
the cluster hadronization model and an universal Ansatz for the soft anomalous
dimension we find strong support for geometric models considered earlier. We
also show how reconnection into baryonic systems arises, and how larger cluster
systems evolve. Our results provide the dynamic basis for a new class of colour
reconnection models for cluster hadronization.Comment: 24 pages, 15 figure
Baryon production from cluster hadronization
We present an extension to the colour reconnection model in the Monte-Carlo
event generator Herwig to account for the production of baryons and compare it
to a series of observables for soft physics. The new model is able to improve
the description of charged-particle mutliplicities and hadron flavour
observables in pp collisionsComment: 13 pages; v3: updated to match journal versio
Parton branching at amplitude level
We present an algorithm that evolves hard processes at the amplitude level by
dressing them iteratively with (massless) quarks and gluons. The algorithm
interleaves collinear emissions with soft emissions and includes
Coulomb/Glauber exchanges. It includes all orders in , is spin
dependent and is able to accommodate kinematic recoils. Although it is
specified at leading logarithmic accuracy, the framework should be sufficient
to go beyond. Coulomb exchanges make the factorisation of collinear and soft
emissions highly non-trivial. In the absence of Coulomb exchanges, we show how
factorisation works out and how a partial factorisation is manifest in the
presence of Coulomb exchanges. Finally, we illustrate the use of the algorithm
by deriving DGLAP evolution and computing the resummed thrust, hemisphere jet
mass and gaps-between-jets distributions in .Comment: 54 pages, minor changes in version
Revisiting Radiation Patterns in Collisions
We propose four simple event-shape variables for semi-inclusive -jet events. The observables and cuts are designed to be especially sensitive
to subleading aspects of the event structure, and allow to test the reliability
of phenomenological QCD models in greater detail. Three of them, ,
, and , focus on soft emissions off three-jet topologies
with a small opening angle, for which coherence effects beyond the leading QCD
dipole pattern are expected to be enhanced. A complementary variable,
, measures the ratio of the hemisphere masses in 4-jet events with
a compressed scale hierarchy (Durham ), for which subleading
splitting effects are expected to be enhanced. We consider several
different parton-shower models, spanning both conventional and dipole/antenna
ones, all tuned to the same reference data, and show that a
measurement of the proposed observables would allow for additional significant
discriminating power between the models.Comment: 27 pages, 10 figure
Baryon production from cluster hadronisation
We present an extension to the colour reconnection model in the Monte Carlo event generator Herwig to account for the production of baryons and compare it to a series of observables for soft physics. The new model is able to improve the description of charged-particle multiplicities and hadron flavour observables in pp collisions
Color matrix element corrections for parton showers
We investigate the effects of keeping the full color structure for parton
emissions in parton showers for both LEP and LHC. This is done within the
Herwig 7 dipole shower, and includes gluon emission, gluon splitting, initial
state branching processes, as well as hadronization. The subleading Nc terms
are included as color matrix element corrections to the splitting kernels by
evolving an amplitude-level density operator and correcting the radiation
pattern for each parton multiplicity, up to a fixed number of full color
emissions, after which a standard leading color shower takes over. Our results
are compared to data for a wide range of LEP and LHC observables and show that
the subleading Nc corrections tend to be small for most observables probing
hard, perturbative dynamics, for both LEP and LHC. However, for some of these
observables they exceed 10%. On soft physics we find signs of significantly
larger effects.Comment: 25 pages, 10 figure
On the Cutoff Dependence of the Quark Mass Parameter in Angular Ordered Parton Showers
We show that the presence of an infrared cutoff in the parton shower
(PS) evolution for massive quarks implies that the generator quark mass
corresponds to a -dependent short-distance mass scheme and is therefore
not the pole mass. Our analysis considers an angular ordered parton shower
based on the coherent branching formalism for quasi-collinear stable heavy
quarks and splitting functions at next-to-leading logarithmic (NLL) order, and
it is based on the analysis of the peak of hemisphere jet mass distributions.
We show that NLL shower evolution is sufficient to describe the peak jet mass
at full next-to-leading order (NLO). We determine the relation of this
short-distance mass to the pole mass at NLO. We also show that the shower cut
affects soft radiation in a universal way for massless and
quasi-collinear massive quark production. The basis of our analysis is (i) an
analytic solution of the PS evolution based on the coherent branching
formalism, (ii) an implementation of the infrared cut of the angular
ordered shower into factorized analytic calculations in the framework of
Soft-Collinear-Effective-Theory (SCET) and (iii) the dependence of the peak of
the jet mass distribution on the shower cut. Numerical comparisons to
simulations with the HERWIG 7 event generator confirm our findings. Our
analysis provides an important step towards a full understanding concerning the
interpretation of top quark mass measurements based on direct reconstruction.Comment: 110 pages, 23 figures; v2: improved version, references updated,
typos eliminate
- âŠ