In the era of precision physics measurements at the LHC, efficient and exhaustive estimations of theoretical uncertainties play an increasingly crucial role. In the context of Monte Carlo (MC) event ...generators, the estimation of such uncertainties traditionally requires independent MC runs for each variation, for a linear increase in total run time. In this work, we report on an automated evaluation of the dominant (renormalization-scale and nonsingular) perturbative uncertainties in the pythia 8 event generator, with only a modest computational overhead. Each generated event is accompanied by a vector of alternative weights (one for each uncertainty variation), with each set separately preserving the total cross section. Explicit scale-compensating terms can be included, reflecting known coefficients of higher-order splitting terms and reducing the effect of the variations. The formalism also allows for the enhancement of rare partonic splittings, such as g→bb¯ and q→qγ, to obtain weighted samples enriched in these splittings while preserving the correct physical Sudakov factors.
Tuning PYTHIA 8.1: the Monash 2013 tune Skands, P.; Carrazza, S.; Rojo, J.
The European physical journal. C, Particles and fields,
08/2014, Letnik:
74, Številka:
8
Journal Article
Recenzirano
Odprti dostop
We present an updated set of parameters for the PYTHIA 8 event generator. We reevaluate the constraints imposed by LEP and SLD on hadronization, in particular with regard to heavy-quark fragmentation ...and strangeness production. For hadron collisions, we combine the updated fragmentation parameters with the new NNPDF2.3 LO PDF set. We use minimum-bias, Drell–Yan, and underlying-event data from the LHC to constrain the initial-state-radiation and multi-parton-interaction parameters, combined with data from SPS and the Tevatron to constrain the energy scaling. Several distributions show significant improvements with respect to the current defaults, for both
e
e
and
p
p
collisions, though we emphasize that interesting discrepancies remain in particular for strange particles and baryons. The updated parameters are available as an option starting from PYTHIA 8.185, by setting Tune:ee = 7 and Tune:pp = 14.
Celotno besedilo
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
VINCIA for hadron colliders Fischer, N.; Prestel, S.; Ritzmann, M. ...
The European physical journal. C, Particles and fields,
2016/11, Letnik:
76, Številka:
11
Journal Article
Recenzirano
Odprti dostop
We present the first public implementation of antenna-based QCD initial- and final-state showers. The shower kernels are
2
→
3
antenna functions, which capture not only the collinear dynamics but ...also the leading soft (coherent) singularities of QCD matrix elements. We define the evolution measure to be inversely proportional to the leading poles, hence gluon emissions are evolved in a
p
⊥
measure inversely proportional to the eikonal, while processes that only contain a single pole (e.g.,
g
→
q
q
¯
) are evolved in virtuality. Non-ordered emissions are allowed, suppressed by an additional power of
1
/
Q
2
. Recoils and kinematics are governed by exact on-shell
2
→
3
phase-space factorisations. This first implementation is limited to massless QCD partons and colourless resonances. Tree-level matrix-element corrections are included for QCD up to
O
(
α
s
4
)
(4 jets), and for Drell–Yan and Higgs production up to
O
(
α
s
3
)
(
V
/
H
+ 3 jets). The resulting algorithm has been made publicly available in
Vincia
2.0.
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Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We propose a sophisticated framework for high-energy hadronic collisions, wherein different QCD physics processes are interleaved in a common sequence of falling transverse-momentum values. Thereby ...phase-space competition is introduced between multiple parton-parton interactions and initial-state radiation. As a first step we develop new transverse-momentum-ordered showers for initial- and final-state radiation, which should be of use also beyond the scope of the current article. These showers are then applied in the context of multiple interactions, and a few tests of the new model are presented. The article concludes with an outlook on further aspects, such as the possibility of a shower branching giving partons participating in two different interactions.
We define a new set of observables to probe the structure of the underlying event in hadron collisions. We use the conventional definition of the “transverse region” in jet events and, for a fixed ...window in jet
p
⊥
, propose to measure several discriminating quantities as a function of the level of activity in the transverse region. The measurement of these observables in LHC data would reveal whether, e.g., the properties of “low-UE” events are compatible with equivalent measurements in
e
+
e
-
collisions (jet universality), and whether the scaling behaviour towards “high-UE” events exhibits properties of non-trivial soft-QCD dynamics, such as colour reconnections or other collective phenomena. We illustrate at
s
=
13
TeV
that significant discriminatory power is obtained in comparisons between MC models with varying treatments of collective effects, including
Pythia 8
,
epos
, and
Dipsy
.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We present a new, universally applicable toy model of colour reconnections in hadronic final states. The model is based on hadronising strings and has one free parameter. We next present an ...implementation of this model in the PYTHIA event generator and provide several parameter sets (‘tunes’), constrained by fits to Tevatron minimum-bias data. Finally, we consider the sensitivity of a simplified top mass analysis to these effects, in exclusive semi-leptonic top events at the Tevatron. A first attempt at isolating the genuine non-perturbative effects gives an estimate of order δmtop ∼ ±0.5 GeV from non-perturbative uncertainties, and a further δmtop ∼ ±1 GeV from shower effects.
Beauty and charm quarks are ideal probes of pertubative Quantum Chromodymanics in proton–proton collisions, owing to their large masses. In this paper the role of multi-parton interactions in the ...production of doubly-heavy hadrons is studied using simulation samples generated with
Pythia
, a Monte Carlo event generator. Comparisons are made to the stand-alone generators
BcVegPy
and
GenXicc
. New methods of speeding up
Pythia
simulations for events containing heavy quarks are described, enabling the production of large samples with multiple heavy-quark pairs. We show that significantly higher production rates of doubly-heavy hadrons are predicted in models that allow heavy quarks originating from different parton–parton interactions (within the same hadron–hadron collision) to combine to form such hadrons. Quantitative predictions are sensitive to the modelling of colour reconnections. We suggest a set of experimental measurements capable of differentiating these additional contributions.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
We consider the probability for a colour-singlet
pair to emit a gluon, in strongly and smoothly ordered antenna showers. We expand to second order in
α
s
and compare to the second-order QCD ...matrix elements for
Z
→ 3 jets, neglecting terms suppressed by
. We give a prescription that corrects the shower to the matrix-element result at this order for both soft and hard emissions, thereby explicitly reducing its dependence on evolution- and renormalization-scale choices. We confirm that the choice of
p
⊥
for both of these scales absorbs all logarithms through
, and contrast this with various alternatives. We include these corrections in the vincia shower generator and study the impact on LEP event-shape and fragmentation observables. An uncertainty estimate is provided for each event, in the form of a vector of alternative weights.
We review the theoretical motivations and experimental status of searches for stable massive particles (SMPs) which could be sufficiently long-lived as to be directly detected at collider ...experiments. The discovery of such particles would address a number of important questions in modern physics including the origin and composition of dark matter in the universe and the unification of the fundamental forces. This review describes the techniques used in SMP-searches at collider experiments and the limits so far obtained on the production of SMPs which possess various colour, electric and magnetic charge quantum numbers. We also describe theoretical scenarios which predict SMPs along with the phenomenology needed to model their production at colliders and interactions with matter. In addition, the interplay between collider searches and open questions in cosmology such as dark matter composition is addressed.
The
mcplots.cern.ch
web site (
mcplots
) provides a simple online repository of plots made with high-energy-physics event generators, comparing them to a wide variety of experimental data. The ...repository is based on the
hepdata
online database of experimental results and on the
rivet
Monte Carlo analysis tool. The repository is continually updated and relies on computing power donated by volunteers, via the
lhc@home 2.0
platform.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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