74 research outputs found
Hadronisation Models and Colour Reconnection
Enhanced production of hadrons with -quark content has been observed in
collisions at LHC, and earlier in collisions of heavy nuclei. We review
the string hadronisation formalism and correc- tions from rope hadronisation
and colour reconnection, corrections that takes place in such dense
environments, and are able to correctly describe data. Since such corrections
are very sensitive to the modelling of transverse proton structure, we
investigate two such models, and compare to final states. Finally we describe
how such corrections can also give a possible explanation to collective
phenomena observed in small systems.Comment: Conference proceedings DIS2016. 6 pages, 3 figure
Rope Hadronization and Strange Particle Production
Rope Hadronization is a model extending the Lund string hadronization model
to describe environments with many overlapping strings, such as high
multiplicity pp collisions or collisions. Including effects of Rope
Hadronization drastically improves description of strange/non-strange hadron
ratios as function of event multiplicity in all systems from to .
Implementation of Rope Hadronization in the MC event generators DIPSY and
Pythia8 is discussed, as well as future prospects for jet studies and studies
of small systems.Comment: Conference proceedings for Strangeness in Quark Matter 201
Soft modifications to jet fragmentation in high energy proton-proton collisions
The discovery of collectivity in proton-proton collisions, is one of the most
puzzling outcomes from the two first runs at LHC, as it points to the
possibility of creation of a Quark-Gluon Plasma, earlier believed to only be
created in heavy ion collisions. One key observation from heavy ion collisions
is still not observed in proton-proton, namely jet-quenching. In this letter it
is shown how a model capable of describing soft collective features of
proton-proton collisions, also predicts modifications to jet fragmentation
properties. With this starting point, several new observables suited for the
present and future hunt for jet quenching in small collision systems are
proposed.Comment: 7 pages, 7 figures, 1 table, updated to the published versio
Diffractive and non-diffractive wounded nucleons and final states in pA collisions
We review the state-of-the-art of Glauber-inspired models for estimating the
distribution of the number of participating nucleons in pA and AA collisions.
We argue that there is room for improvement in these models when it comes to
the treatment of diffractive excitation processes, and present a new simple
Glauber-like model where these processes are better taken into account. We also
suggest a new way of using the number of participating, or wounded, nucleons to
extrapolate event characteristics from pp collisions, and hence get an estimate
of basic hadronic final-state properties in pA collisions, which may be used to
extract possible nuclear effects. The new method is inspired by the Fritiof
model, but based on the full, semi-hard multiparton interaction model of Pythia
8.Comment: 50 pages, 13 figure
Dipole evolution: perspectives for collectivity and gamma*A collisions
The transverse, spatial structure of protons is an area revealing fundamental
properties of matter, and provides key input for deeper understanding of
emerging collective phenomena in high energy collisions of protons, as well as
collisions of heavy ions. In this paper eccentricities and eccentricity
fluctuations are predicted using the dipole formulation of BFKL evolution.
Furthermore, first steps are taken towards generation of fully exclusive final
states of A collisions, by assessing the importance of colour
fluctuations in the initial state. Such steps are crucial for the preparation
of event generators for a future electron-ion collider. Due to the connection
between an impact parameter picture of the proton structure, and cross sections
of ep and pp collisions, the model parameters can be fully determined by fits
to such quantities, leaving results as real predictions of the model.Comment: 54 pages, 20 figures, replacement corresponding to published versio
Hadronic Rescattering in pA and AA Collisions
In a recent article we presented a model for hadronic rescattering, and some
results were shown for pp collisions at LHC energies. In order to extend the
studies to pA and AA collisions, the Angantyr model for heavy-ion collisions is
taken as the starting point. Both these models are implemented within the
general-purpose Monte Carlo event generator Pythia, which makes the matching
reasonably straightforward, and allows for detailed studies of the full
space--time evolution. The rescattering rate is significantly higher than in
pp, especially for central AA collisions, where the typical primary hadron
rescatters several times. We study the impact of rescattering on a number of
distributions, such as pT and eta spectra, and the space--time evolution of the
whole collision process. Notably rescattering is shown to give a significant
contribution to elliptic flow in XeXe and PbPb, and to give a nontrivial impact
on charm production.Comment: 38 pages, 16 figure
Effects of Overlapping Strings in pp Collisions
In models for hadron collisions based on string hadronization, the strings
are usually treated as independent, allowing no interaction between the
confined colour fields. In studies of nucleus collisions it has been suggested
that strings close in space can fuse to form "colour ropes". Such ropes are
expected to give more strange particles and baryons, which also has been
suggested as a signal for plasma formation. Overlapping strings can also be
expected in pp collisions, where usually no phase transition is expected. In
particular at the high LHC energies the expected density of strings is quite
high. To investigate possible effects of rope formation, we present a model in
which strings are allowed to combine into higher multiplets, giving rise to
increased production of baryons and strangeness, or recombine into singlet
structures and vanish. Also a crude model for strings recombining into junction
structures is considered, again giving rise to increased baryon production. The
models are implemented in the DIPSY MC event generator, using PYTHIA 8 for
hadronization, and comparison to pp minimum bias data, reveals improvement in
the description of identified particle spectra.Comment: 48 pages, 22 figure
The dynamic hadronization of charm quarks in heavy-ion collisions
The PYTHIA8/ANGANTYR model for heavy ion collisions was recently updated with
a mechanism for \textit{global colour reconnection}. The colour reconnection
model used is QCD colour algebra inspired and enhances baryon production due to
the formation of string junctions. In this paper, we present updates to the
junction formation and string fragmentation mechanisms, connected to heavy
quark fragmentation. This allows for the simulation of heavy quark
fragmentation, using junction formation, in heavy ion collisions. The framework
is validated for proton collisions, and we show results for charm baryon
production in proton-lead collisions.Comment: 20 pages, 10 figure
The Angantyr model for Heavy-Ion Collisions in PYTHIA8
We present a new model for building up complete exclusive hadronic final
states in high energy nucleus collisions. It is a direct extrapolation of high
energy pp collisions (as described by PYTHIA), and thus bridges a large part of
the existing gap between heavy ion and high energy physics phenomenology. The
model is inspired by the old Fritiof model and the notion of wounded nucleons.
Two essential features are the treatment of multi-parton interactions and
diffractive excitation in each NN sub-collision. Diffractive excitation is
related to fluctuations in the nucleon partonic sub-structure, and fluctuations
in both projectile and target are here included for the first time. The model
is able to give a good description of general final-state properties such as
multiplicity and transverse momentum distributions, both in pA and AA
collisions. The model can therefore serve as a baseline for understanding the
non-collective background to observables sensitive to collective behaviour. As
PYTHIA does not include a mechanism to reproduce the collective effects seen in
pp collisions, such effects are also not reproduced by the present version of
Angantyr. Effects of high string density, shown to be able to reproduce e.g.
higher strangeness ratios and the ridge in pp, will be added in future studiesComment: 57 pages, 26 figures. Updated corresponding to published versio
Setting the string shoving picture in a new frame
Based on the recent success of the \angantyr model in describing multiplicity
distributions of the hadronic final state in high energy heavy ion collisions,
we investigate how far one can go with a such a string-based scenario to
describe also flow effects measured in such collisions. For this purpose we
improve our previous so-called \textit{shoving} model, where strings that are
close in space--time tend to repel each other in a way that could generate
anisotropic flow, and we find that this model can indeed generate such flows in
\AA\ collisions. The flow generated is not quite enough to reproduce
measurements, but we identify some short-comings in the presented
implementation of the model that, when fixed, could plausibly give a more
realistic amount of flow.Comment: 42 pages, 34 figures, submission to JHE
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