53 research outputs found
RGIsearch: A C++ program for the determination of Renormalization Group Invariants
RGIsearch is a C++ program that searches for invariants of a user-defined set
of renormalization group equations. Based on the general shape of the
-functions of quantum field theories, RGIsearch searches for several
types of invariants that require different methods. Additionally, it supports
the computation of invariants up to two-loop level. A manual for the program is
given, including the settings and set-up of the program, as well as a test
case
Final-state QED Multipole Radiation in Antenna Parton Showers
We present a formalism for a fully coherent QED parton shower. The complete
multipole structure of photonic radiation is incorporated in a single branching
kernel. The regular on-shell 2 to 3 kinematic picture is kept intact by
dividing the radiative phase space into sectors, allowing for a definition of
the ordering variable that is similar to QCD antenna showers. A modified
version of the Sudakov veto algorithm is discussed that increases performance
at the cost of the introduction of weighted events. Due to the absence of a
soft singularity, the formalism for photon splitting is very similar to the QCD
analogon of gluon splitting. However, since no color structure is available to
guide the selection of a spectator, a weighted selection procedure from all
available spectators is introduced.Comment: 33 pages, 12 figures. Added subsection 4.3 and some comments and
references per reviewer request. Version accepted by JHE
Competing Sudakov Veto Algorithms
We present a way to analyze the distribution produced by a Monte Carlo
algorithm. We perform these analyses on several versions of the Sudakov veto
algorithm, adding a cutoff, a second variable and competition between emission
channels. The analysis allows us to prove that multiple, seemingly different
competition algorithms, including those that are currently implemented in most
parton showers, lead to the same result. Finally, we test their performance and
show that there are significantly faster alternatives to the commonly used
algorithms.Comment: 16 pages, 1 figur
Renormalization group invariants in supersymmetric theories: one- and two-loop results
We stress the potential usefulness of renormalization group invariants.
Especially particular combinations thereof could for instance be used as probes
into patterns of supersymmetry breaking in the MSSM at inaccessibly high
energies. We search for these renormalization group invariants in two
systematic ways: on the one hand by making use of symmetry arguments and on the
other by means of a completely automated exhaustive search through a large
class of candidate invariants. At the one-loop level, we find all known
invariants for the MSSM and in fact several more, and extend our results to the
more constrained pMSSM and dMSSM, leading to even more invariants. Extending
our search to the two-loop level we find that the number of invariants is
considerably reduced
Collinear Electroweak Radiation in Antenna Parton Showers
We present a first implementation of collinear electroweak radiation in the
Vincia parton shower. Due to the chiral nature of the electroweak theory,
explicit spin dependence in the shower algorithm is required. We thus use the
spinor-helicity formalism to compute helicity-dependent branching kernels,
taking special care to deal with the gauge relics that may appear in
computation that involve longitudinal polarizations of the massive electroweak
vector bosons. These kernels are used to construct a shower algorithm that
includes all possible collinear final-state electroweak branchings, including
those induced by the Yang-Mills triple vector boson coupling and all Higgs
couplings, as well as vector boson emissions from the initial state. We
incorporate a treatment of features particular to the electroweak theory, such
as the effects of bosonic interference and recoiler effects, as well as a
preliminary description of the overlap between electroweak branchings and
resonance decays. Some qualifying results on electroweak branching spectra at
high energies, as well as effects on LHC physics are presented. Possible future
improvements are discussed, including treatment of soft and spin effects, as
well as issues unique to the electroweak sector.Comment: 23 pages, 7 figure
Phase Space Sampling and Inference from Weighted Events with Autoregressive Flows
We explore the use of autoregressive flows, a type of generative model with
tractable likelihood, as a means of efficient generation of physical particle
collider events. The usual maximum likelihood loss function is supplemented by
an event weight, allowing for inference from event samples with variable, and
even negative event weights. To illustrate the efficacy of the model, we
perform experiments with leading-order top pair production events at an
electron collider with importance sampling weights, and with
next-to-leading-order top pair production events at the LHC that involve
negative weights.Comment: 26 pages, 7 figure
Multipole Photon Radiation in the Vincia Parton Shower
We present algorithms that interleave photon radiation from the final state
and the initial state with the QCD evolution in the antenna-based Vincia parton
shower. One of the algorithms incorporates the complete soft and collinear
structure associated with photon emission, but may be computationally
expensive, while the other approximates the soft structure at a lower cost.
Radiation from fermions and W bosons is included, and a strategy for photon
radiation off leptons below the hadronization scale is set up. We show results
of the application of the shower algorithms to Drell-Yan and W+W- production at
the LHC, showing the impact of the inclusion of the full soft structure and
treatment of radiation off W bosons.Comment: 9 pages, 3 figure
Interleaved resonance decays and electroweak radiation in the Vincia parton shower
We propose a framework for high-energy interactions in which resonance decays and electroweak branching processes are interleaved with the QCD evolution in a single common sequence of decreasing resolution scales. The interleaved treatment of resonance decays allows for a new treatment of finite-width effects in parton showers. At scales above their offshellness (i.e., typically Q > γ ), resonances participate explicitly as incoming and outgoing states in branching processes, while they are effectively "integrated out" of the description at lower scales. We implement this formalism, together with a full set of antenna functions for branching processes involving electroweak (W=Z=H) bosons in the Vincia shower module in Pythia 8.3, and study some of the consequences
Soft spin correlations in final-state parton showers
We introduce a simple procedure that resolves the long-standing question of
how to account for single-logarithmic spin-correlation effects in parton
showers not just in the collinear limit, but also in the soft wide-angle limit,
at leading colour. We discuss its implementation in the context of the
PanScales family of parton showers, where it complements our earlier treatment
of the purely collinear spin correlations. Comparisons to fixed-order matrix
elements help validate our approach up to third order in the strong coupling,
and an appendix demonstrates the small size of residual subleading-colour
effects. To help probe wide-angle soft spin correlation effects, we introduce a
new declustering-based non-global spin-sensitive observable, the first of its
kind. Our showers provide a reference for its single-logarithmic resummation.
The work in this paper represents the last step required for final-state
massless showers to satisfy the broad PanScales next-to-leading logarithmic
accuracy goals.Comment: 26 pages, 12 figure
Matching and event-shape NNDL accuracy in parton showers
To explore the interplay of NLO matching and next-to-leading logarithmic (NLL) parton showers, we consider the simplest case of γ* and Higgs-boson decays to qq¯¯and gg respectively. Not only should shower NLL accuracy be retained across observables after matching, but for global event-shape observables and the two-jet rate, matching can augment the shower in such a way that it additionally achieves next-to-next-to-double-logarithmic (NNDL) accuracy, a first step on the route towards general NNLL. As a proof-of-concept exploration of this question, we consider direct application of multiplicative matrix-element corrections, as well as simple implementations of MC@NLO and POWHEG-style matching. We find that the first two straightforwardly bring NNDL accuracy, and that this can also be achieved with POWHEG, although particular care is needed in the handover between POWHEG and the shower. Our study involves both analytic and numerical components and we also touch on some phenomenological considerations
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