400 research outputs found
Modelling Bose-Einstein correlations at LEP 2
We present new algorithms for simulating Bose-Einstein correlations among
final-state bosons in an event generator. The algorithms are all based on
introducing Bose-Einstein correlations as a shift of final-state momenta among
identical bosons, and differ only in the way energy and momentum conservation
is ensured. The benefits and shortcomings of this approach, that may be viewed
as a local reweighting strategy, is compared to the ones of recently proposed
algorithms involving global event reweighting.
We use the new algorithms to improve on our previous study of the effects of
Bose-Einstein correlations on the W mass measurement at LEP 2. The intrinsic
uncertainty could be as high as 100 MeV but is probably reduced to the order of
30 MeV with realistic experimental reconstruction procedures.Comment: 1+34 pages, LaTeX2e, 10 eps figures attache
Reconnecting coloured dipoles
Reconnections among the produced colour dipoles in the Dipole Cascade Model are discussed, and an implementation in theAriadne program is presented. The reconnections are shown to have only small effects on event shapes at LEP 1, on theW mass determination at LEP 2 and on rapidity gaps in deep inelastic scattering at HERA
Correcting the Colour-Dipole Cascade Model with Fixed Order Matrix Elements
An algorithm is presented in which the Colour-Dipole Cascade Model as
implemented in the Ariadne program is corrected to match the fixed order
tree-level matrix elements for e+e- -> n jets. The result is a full parton
level generator for e+e- annihilation where the generated states are correct on
tree-level to fixed order in alpha_S and to all orders with modified leading
logarithmic (MLLA) accuracy. In this paper, matrix elements are used up to
second order in alpha_S, but the scheme is applicable also for higher orders.
An improvement to also include exact virtual corrections to fixed order is
suggested and the possibility to extend the scheme to hadronic collisions is
discussed
Estimating the Parameters of Bose-Einstein Correlations from the Two-Particle Correlation Function in Multihadronic Final States
To estimate the strength of the Bose-Einstein correlations and the radius of
the hadronization region in multiparticle production, the two-particle
correlation functions for identical pairs is adjusted to a parametric
function describing the enhancement at small momentum differences. This is
usually done by means of a binned uncorrelated least squares fit. This article
demonstrates that this procedure underestimates the statistical errors. A
recipe is given to construct from the data the covariance matrix To estimate
the strength of the Bose-Einstein correlations and the radius of the
hadronization region in multiparticle production, the two-particle correlation
functions for identical pairs is adjusted to a parametric function
describing the enhancement at small momentum differences. This is usually done
by means of a binned uncorrelated least squares fit. This article demonstrates
that this procedure underestimates the statistical errors. A recipe is given to
construct from the data the covariance matrix between the bins of the histogram
of the two-particle correlation function.Comment: 9 pages, 4 figure
Bose-Einstein effects and W mass determinations
In e+ef- → W+Wf- → q1q2q3q4 events at LEP 2, the two W decay vertices are much closer to each other than typical hadronization distances. Therefore the Bose-Einstein effects, associated with the production of identical bosons (mainly pions), may provide a 'cross-talk' between the W+ and the Wf- decay products. If so, the observable W masses are likely to be affected. We develop algorithms for the inclusion of Bose-Einstein effects in multi-hadronic events. In this way we can study potential uncertainties in the W mass determination. In some scenarios the effects are significant, so that this source of uncertainty cannot be neglected
Outstanding problems in the phenomenology of hard diffractive scattering
This paper is a summary of the discussion within the Diffractive and Low-x
Physics Working Group at the 1999 Durham Collider Workshop of the
interpretation of the Tevatron and HERA measurements of inclusive hard
diffraction.Comment: 5 pages, 1 figure. Talks and discussions from the UK Phenomenology
Workshop on Collider Physics, Durham, September 199
Matching Tree-Level Matrix Elements with Interleaved Showers
We present an implementation of the so-called CKKW-L merging scheme for
combining multi-jet tree-level matrix elements with parton showers. The
implementation uses the transverse-momentum-ordered shower with interleaved
multiple interactions as implemented in PYTHIA8. We validate our procedure
using e+e--annihilation into jets and vector boson production in hadronic
collisions, with special attention to details in the algorithm which are
formally sub-leading in character, but may have visible effects in some
observables. We find substantial merging scale dependencies induced by the
enforced rapidity ordering in the default PYTHIA8 shower. If this rapidity
ordering is removed the merging scale dependence is almost negligible. We then
also find that the shower does a surprisingly good job of describing the
hardness of multi-jet events, as long as the hardest couple of jets are given
by the matrix elements. The effects of using interleaved multiple interactions
as compared to more simplistic ways of adding underlying-event effects in
vector boson production are shown to be negligible except in a few sensitive
observables. To illustrate the generality of our implementation, we also give
some example results from di-boson production and pure QCD jet production in
hadronic collisions.Comment: 44 pages, 23 figures, as published in JHEP, including all changes
recommended by the refere
Uncertainties on Central Exclusive Scalar Luminosities from the unintegrated gluon distributions
In a previous report we used the Linked Dipole Chain model unintegrated gluon
densities to investigate the uncertainties in the predictions for central
exclusive production of scalars at hadron colliders. Here we expand this
investigation by also looking at other parameterizations of the unintegrated
gluon density, and look in more detail on the behavior of these at small k_T.
We confirm our conclusions that the luminosity function for central exclusive
production is very sensitive to this behavior. However, we also conclude that
the available densities based on the CCFM and LDC evolutions are not
constrained enough to give reliable predictions even for inclusive Higgs
production at the LHC
String Effects on Fermi--Dirac Correlation Measurements
We investigate some recent measurements of Fermi--Dirac correlations by the
LEP collaborations indicating surprisingly small source radii for the
production of baryons in -annihilation at the peak. In the
hadronization models there are besides the Fermi--Dirac correlation effect also
a strong dynamical (anti-)correlation. We demonstrate that the extraction of
the pure FD effect is highly dependent on a realistic Monte Carlo event
generator, both for separation of those dynamical correlations which are not
related to Fermi--Dirac statistics, and for corrections of the data and
background subtractions. Although the model can be tuned to well reproduce
single particle distributions, there are large model-uncertainties when it
comes to correlations between identical baryons. We therefore, unfortunately,
have to conclude that it is at present not possible to make any firm conclusion
about the source radii relevant for baryon production at LEP
Selection and reconstruction of the top quarks in the all-hadronic decays at a Linear Collider
A method of reconstruction of the top quarks produced in the process E+E- ->
t\bar{t} -> 6 jets at a Linear Collider (LC) is proposed. The approach does not
involve a kinematic fit, as well as assumptions on the invariant masses of the
dijets originating from the decays of W bosons and, therefore, the method is
expected to be less sensitive to theoretical and experimental uncertainties on
the top-mass measurement than traditional reconstruction methods. For the first
time, the reconstruction of the top quarks was investigated using the full LC
detector simulation after taking into account the background arising from QCD
multi-jet production.Comment: 22 pages, including 13 figures and 3 table
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