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Torbjörn Sjöstrand

Publications and source records attributed to Torbjörn Sjöstrand.

At least 37 records · Page 2Linked to original sources
arXiv2018

Collective Effects: the viewpoint of HEP MC codes

Collective effects are observed in high-multiplicity pp events, similar to the signals traditionally attributed to the formation of a Quark Gluon Plasma in heavy ion collisions. In core--corona models it is assumed that a partial plasma formation is indeed possible also in pp, but here the focus is on several recent models that attempt to explain pp data without invoking plasma formation. These attempts are partly successful, but there is still not a unified framework.

hep-ph↗
arXiv2018

Models of Total, Elastic and Diffractive Cross Sections

The LHC has brought much new information on total, elastic and diffractive cross sections, which is not always in agreement with extrapolations from lower energies. The default framework in the Pythia event generator is one case in point. In this article we study and implement two recent models, as more realistic alternatives. Both describe total and elastic cross sections, whereas one also includes single diffraction. Noting some issues at high energies, a variant of the latter is proposed, and extended also to double and central diffraction. Further, the experimental definition of diffraction is based on the presence of rapidity gaps, which however also could be caused by colour reconnection in nondiffractive events, a phenomenon that is studied in the context of a specific model. Throughout comparisons with LHC and other data are presented.

hep-ph↗
arXiv2018

Some Dipole Shower Studies

Parton showers have become a standard component in the description of high-energy collisions. Nowadays most final-state ones are of the dipole character, wherein a pair of partons branches into three, with energy and momentum preserved inside this subsystem. For initial-state showers a dipole picture is also possible and commonly used, but the older global-recoil strategy remains a valid alternative, wherein larger groups of partons share the energy--momentum preservation task. In this article we introduce and implement a dipole picture also for initial-state radiation in PYTHIA, and compare with the existing global-recoil one, and with data. For the case of Deeply Inelastic Scattering we can directly compare with matrix element expressions and show that the dipole picture gives a very good description over the whole phase space, at least for the first branching.

hep-ph↗
arXiv2018

Hard Diffraction with Dynamic Gap Survival

We present a new framework for the modelling of hard diffraction in pp and ppbar collisions. It starts from the the approach pioneered by Ingelman and Schlein, wherein the single diffractive cross section is factorized into a Pomeron flux and a Pomeron PDF. To this it adds a dynamically calculated rapidity gap survival factor, derived from the modelling of multiparton interactions. This factor is not relevant for diffraction in ep collisions, giving non-universality between HERA and Tevatron diffractive event rates. The model has been implemented in Pythia 8 and provides a complete description of the hadronic state associated with any hard single diffractive process. Comparisons with ppbar and pp data reveal improvement in the description of single diffractive events.

hep-ph↗
arXiv2017

The Development of MPI Modelling in PYTHIA

Many of the basic ideas in multiparton interaction (MPI) phenomenology were first developed in the context of the PYTHIA event generator, and MPIs have been central in its modelling of both minimum-bias and underlying-event physics in one unified framework. This chapter traces the evolution towards an increasingly sophisticated description of MPIs in PYTHIA, including topics such as the ordering of MPIs, the regularization of the divergent QCD cross section, the impact-parameter picture, colour reconnection, multiparton PDFs and beam remnants, interleaved and intertwined evolution, and diffraction.

hep-ph↗
arXiv2017

Thermodynamical String Fragmentation

The observation of heavy-ion-like behaviour in pp collisions at the LHC suggests that more physics mechanisms are at play than traditionally assumed. The introduction e.g. of quark-gluon plasma or colour rope formation can describe several of the observations, but as of yet there is no established paradigm. In this article we study a few possible modifications to the Pythia event generator, which describes a wealth of data but fails for a number of recent observations. Firstly, we present a new model for generating the transverse momentum of hadrons during the string fragmentation process, inspired by thermodynamics, where heavier hadrons naturally are suppressed in rate but obtain a higher average transverse momentum. Secondly, close-packing of strings is taken into account by making the temperature or string tension environment-dependent. Thirdly, a simple model for hadron rescattering is added. The effect of these modifications is studied, individually and taken together, and compared with data mainly from the LHC. While some improvements can be noted, it turns out to be nontrivial to obtain effects as big as required, and further work is called for.

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arXiv2016

Status and developments of event generators

Event generators play a crucial role in the exploration of LHC physics. This presentation summarizes news and plans for the three general-purpose pp generators HERWIG, PYTHIA and SHERPA, as well as briefer notes on a few other generators. Common themes, such as the matching and merging between matrix elements and parton showers, are highlighted. Other topics include a historical introduction, from the Lund perspective, and comments on the role of MCnet.

hep-ph↗
arXiv2015

Colour Reconnection at Future $\mathrm{e}^\mathbf{+}\mathrm{e}^\mathbf{-}$ Colliders

The effects of colour reconnection (CR) at $\mathrm{e}^+\mathrm{e}^-$ colliders are revisited, with focus on recently developed CR models. The new models are compared with the LEP2 measurements for $\mathrm{e}^+\mathrm{e}^- \to \mathrm{W}^+\mathrm{W}^- \to\mathrm{q}_1 \overline{\mathrm{q}}_2 \mathrm{q}_3 \overline{\mathrm{q}}_4$ and found to lie within their limits. Prospects for constraints from new high-luminosity $\mathrm{e}^+\mathrm{e}^-$ colliders are discussed. The novel arena of CR in Higgs decays is introduced, and illustrated by shifts in angular correlations that would be used to set limits on a potential $CP$-odd admixture of the 125 GeV Higgs state.

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arXiv2014

An Introduction to PYTHIA 8.2

The PYTHIA program is a standard tool for the generation of events in high-energy collisions, comprising a coherent set of physics models for the evolution from a few-body hard process to a complex multiparticle final state. It contains a library of hard processes, models for initial- and final-state parton showers, matching and merging methods between hard processes and parton showers, multiparton interactions, beam remnants, string fragmentation and particle decays. It also has a set of utilities and several interfaces to external programs. PYTHIA 8.2 is the second main release after the complete rewrite from Fortran to C++, and now has reached such a maturity that it offers a complete replacement for most applications, notably for LHC physics studies. The many new features should allow an improved description of data.

hep-ph↗
arXiv2014

Effects of color reconnection on $t\bar{t}$ final states at the LHC

The modeling of color reconnection has become one of the dominant sources of systematic uncertainty in the top mass determination at hadron colliders. The uncertainty on the top mass due to color reconnection is conventionally estimated by taking the difference in the predictions of a model with and a model without color reconnection. We show that this procedure underestimates the uncertainty when applied to the existing models in {\sc Pythia}~8. We introduce two new classes of color reconnection models, each containing several variants, which encompass a variety of scenarios that could be realized in nature and we study how they affect the reconstruction of the top mass. After tuning the new models to existing LHC data, the remaining spread of predictions is used to derive a more realistic uncertainty for the top mass, which is found to be around 500 MeV. We also propose how future LHC measurements with $t\bar{t}$ events can be used to further constrain these models and reduce the associated modeling uncertainty.

hep-ph↗
arXiv2014

Weak Gauge Boson Radiation in Parton Showers

The emission of W and Z gauge boson is included in a traditional QCD + QED shower. The unitarity of the shower algorithm links the real radiation of the weak gauge bosons to the negative weak virtual corrections. The shower evolution process leads to a competition between QCD, QED and weak radiation, and allows for W and Z boson production inside jets. Various effects on LHC physics are studied, both at low and high transverse momenta, and effects at higher-energy hadron colliders are outlined.

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arXiv2011

Multiparton Interactions with an x-dependent Proton Size

Theoretical arguments, supported by other indirect evidence, suggest that the wave function of high-x partons should be narrower than that of low-x ones. In this article, we present a modification to the variable impact parameter framework of Pythia 8 to model this effect. In particular, a Gaussian hadronic matter profile is introduced, with a width dependent on the x value of the constituent being probed. Results are compared against the default single- and double-Gaussian profiles, as well as an intermediate overlap function.

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arXiv2010

Interleaved Parton Showers and Tuning Prospects

General-purpose Monte Carlo event generators have become important tools in particle physics, allowing the simulation of exclusive hadronic final states. In this article we examine the Pythia 8 generator, in particular focusing on its parton-shower algorithms. Some relevant new additions to the code are introduced, that should allow for a better description of data. We also implement and compare with 2 to 3 real-emission QCD matrix elements, to check how well the shower algorithm fills the phase space away from the soft and collinear regions. A tuning of the generator to Tevatron data is performed for two PDF sets and the impact of first new LHC data is examined.

hep-ph↗
arXiv2010

A Comparison of new MC-adapted Parton Densities

A selection of the latest and most frequently used parton distribution functions (PDFs) is incorporated in Pythia8, including the Monte Carlo-adapted PDFs from the MSTW and CTEQ collaborations. This article examines the differences in PDFs as well as the effect they have on results of simulations and compare with data collected by the CDF experiment. Monte Carlo-adapted PDFs do a better job than leading- and next-to-leading order PDFs for many observables, but there is room for further improvements.

hep-ph↗
arXiv2010

Improved Parton Showers at Large Transverse Momenta

Several methods to improve the parton-shower description of hard processes by an injection of matrix-element-based information have been presented over the years. In this article we study (re)weighting schemes for the first/hardest emission. One objective is to provide a consistent matching of the POWHEG next-to-leading order generator to the Pythia shower algorithms. Another is to correct the default behaviour of these showers at large transverse momenta, based on a comparison with real-emission matrix elements.

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arXiv2009

Monte Carlo Tools

Given the current landscape in experimental high-energy physics, these lectures are focused on applications of event generators for hadron colliders like the Tevatron and LHC. Section 2 contains a first overview of the physics picture and the generator landscape. Thereafter section 3 describes the usage of matrix elements, section 4 the important topics of initial- and final-state showers, and section 5 how showers can be matched to different hard processes. The issue of multiparton interactions and their role in mimimum-bias and underlying-event physics is introduced in section 6, followed by some comments on hadronization in section 7. The article concludes with an outlook on the ongoing generator-development work in section 8.

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arXiv2009

Multiparton Interactions and Rescattering

The concept of multiple partonic interactions in hadronic events is vital for the understanding of both minimum-bias and underlying-event physics. The area is rather little studied, however, and current models offer a far from complete coverage, even of the effects we know ought to be there. In this article we address one such topic, namely that of rescattering, where an already scattered parton is allowed to take part in another subsequent scattering. A framework for rescattering is introduced for the Pythia 8 event generator and fully integrated with normal multiparton interactions and initial- and final-state radiation. Using this model, the effects on event structure are studied, and distributions are shown both for minimum-bias and jet events.

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