arXiv subjects

Davison E. Soper

Publications and source records attributed to Davison E. Soper.

At least 37 records · Page 2Linked to original sources

arXiv2015

Effects of subleading color in a parton shower

Parton shower Monte Carlo event generators in which the shower evolves from hard splittings to soft splittings generally use the leading color (LC) approximation, which is the leading term in an expansion in powers of $1/N_\Lc^2$, where $N_\Lc = 3$ is the number of colors. In the parton shower event generator \textsc{Deductor}, we have introduced a more general approximation, the LC+ approximation, that includes some of the color suppressed contributions. In this paper, we explore the differences in results between the LC approximation and the LC+ approximation. Numerical comparisons suggest that, for simple observables, the LC approximation is quite accurate. We also find evidence that for gap-between-jets cross sections neither the LC approximation nor the LC+ approximation is adequate.

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arXiv2014

Scattering of Dark Particles with Light Mediators

We present a treatment of the high energy scattering of dark Dirac fermions from nuclei, mediated by the exchange of a light vector boson. The dark fermions are produced by proton-nucleus interactions in a fixed target and, after traversing shielding that screens out strongly interacting products, appear similarly to neutrino neutral current scattering in a detector. Using the Fermilab experiment E613 as an example, we place limits on a secluded dark matter scenario. Visible scattering in the detector includes both the familiar regime of large momentum transfer to the nucleus ($Q^2$) described by deeply inelastic scattering, as well as small $Q^2$ kinematics described by the exchanged vector mediator fluctuating into a quark-antiquark pair whose interaction with the nucleus is described by a saturation model. We find that the improved description of the low $Q^2$ scattering leads to important corrections, resulting in more robust constraints in a regime where a description entirely in terms of deeply inelastic scattering cannot be trusted.

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arXiv2014

Finding physics signals with event deconstruction

We develop a matrix element based reconstruction method called event deconstruction. The method uses information from the hard matrix element and a parton shower to assign probabilities to whether a final state was initiated by a signal or background process. We apply this method in the signal process of a Z' decaying to boosted top quarks in an all hadronic final state and discuss envisioned improvements of the method. We find that event deconstruction can considerably improve on existing reconstruction techniques.

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arXiv2014

A parton shower based on factorization of the quantum density matrix

We present first results from a new parton shower event generator, Deductor. Anticipating a need for an improved treatment of parton color and spin, the structure of the generator is based on the quantum density matrix in color and spin space. So far, Deductor implements only a standard spin-averaged treatment of spin in parton splittings. Although Deductor implements an improved treatment of color, in this paper we present results in the standard leading color approximation so that we can compare to the generator Pythia. The algorithms used incorporate a virtuality based shower ordering parameter and massive initial state bottom and charm quarks.

hep-ph↗

arXiv2014

Ordering variable for parton showers

The parton splittings in a parton shower are ordered according to an ordering variable, for example the transverse momentum of the daughter partons relative to the direction of the mother, the virtuality of the splitting, or the angle between the daughter partons. We analyze the choice of the ordering variable and conclude that one particular choice has the advantage of factoring softer splittings from harder splittings graph by graph in a physical gauge.

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arXiv2014

Parton distribution functions in the context of parton showers

When the initial state evolution of a parton shower is organized according to the standard "backward evolution'' prescription, ratios of parton distribution functions appear in the splitting probabilities. The shower thus organized evolves from a hard scale to a soft cutoff scale. At the end of the shower, one expects that only the parton distributions at the soft scale should affect the results. The other effects of the parton distributions should have cancelled. This means that the kernels for parton evolution should be related to the shower splitting functions. If the initial state partons can have non-zero masses, this requires that the evolution kernels cannot be the usual MSbar kernels. We work out what the parton evolution kernels should be to match the shower evolution contained in the parton shower event generator Deductor, in which the b and c quarks have non-zero masses.

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arXiv2012

MEKS: a program for computation of inclusive jet cross sections at hadron colliders

EKS is a numerical program that predicts differential cross sections for production of single-inclusive hadronic jets and jet pairs at next-to-leading order (NLO) accuracy in a perturbative QCD calculation. We describe MEKS 1.0, an upgraded EKS program with increased numerical precision, suitable for comparisons to the latest experimental data from the Large Hadron Collider and Tevatron. The program integrates the regularized patron-level matrix elements over the kinematical phase space for production of two and three partons using the VEGAS algorithm. It stores the generated weighted events in finely binned two-dimensional histograms for fast offline analysis. A user interface allows one to customize computation of inclusive jet observables. Results of a benchmark comparison of the MEKS program and the commonly used FastNLO program are also documented.

hep-ph↗

arXiv2012

Through a glass, darkly: theory summary

This is a summary of the theoretical contributions to the QCD session of the 47th Rencontre de Moriond, including some perspectives on the implications of the reported experimental results on the status of our theoretical understanding.

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arXiv2012

Parton shower evolution with subleading color

Parton shower Monte Carlo event generators in which the shower evolves from hard splittings to soft splittings generally use the leading color approximation, which is the leading term in an expansion in powers of $1/N_c^2$, where $N_c = 3$ is the number of colors. We introduce a more general approximation, the LC+ approximation, that includes some of the color suppressed contributions. There is a cost: each generated event comes with a weight. There is a benefit: at each splitting the leading soft$\times$collinear singularity and the leading collinear singularity are treated exactly with respect to color. In addition, an LC+ shower can start from a state of the color density matrix in which the bra state color and the ket state color do not match.

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arXiv2011

Finding physics signals with shower deconstruction

We introduce shower deconstruction, a method to look for new physics in a hadronic environment. The method aims to be a full information approach using small jets. It assigns to each event a number chi that is an estimate of the ratio of the probability for a signal process to produce that event to the probability for a background process to produce that event. The analytic functions we derive to calculate these probabilities mimic what full event generators like Pythia or Herwig do and can be depicted in a diagrammatic way. As an example, we apply this method to a boosted Higgs boson produced in association with a Z-boson and show that this method can be useful to discriminate this signal from the Z+jets background.

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arXiv2010

Combining subjet algorithms to enhance ZH detection at the LHC

The signal for a highly boosted heavy resonance competing against a background of light parton jets at the LHC can be enhanced by analyzing subjets in the "fat" jet that possibly contains the heavy resonance. Three methods for doing this are known as filtering, pruning, and trimming. We study the possibility of combining these methods using a relative likelihood approach. We find that, because the methods are not the same, one achieves an enhanced statistical power by combining them. We illustrate the possibilities first with a simple problem of combining trimming and pruning to enhance the signal for finding a boosted top quark. We then study the more difficult problem of disentangling from the background the signal for the production of a Higgs boson in association with a Z-boson. For this problem, we combine filtering, trimming, and pruning.

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arXiv2010

Correlated theoretical uncertainties for the one-jet inclusive cross section

We discuss the correlated systematic theoretical uncertainties that may be ascribed to the next-to-leading order QCD theory used to predict the one-jet inclusive cross section in hadron collisions. We estimate the magnitude of these errors as functions of the jet transverse momentum and rapidity. The total theoretical error is decomposed into a set of functions of transverse momentum and rapidity that give a model for statistically independent contributions to the error. This representation can be used to include the systematic theoretical errors in fits to the experimental data.

hep-ph↗

arXiv2009

On the transverse momentum in Z-boson production in a virtuality ordered parton shower

Cross sections for physical processes that involve very different momentum scales in the same process will involve large logarithms of the ratio of the momentum scales when calculated in perturbation theory. One goal of calculations using parton showers is to sum these large logarithms. We ask whether this goal is achieved for the transverse momentum distribution of a Z-boson produced in hadron-hadron collisions when the shower is organized with higher virtuality parton splittings coming first, followed successively by lower virtuality parton splittings. We find that the virtuality ordered shower works well in reproducing the known QCD result.

hep-ph↗

arXiv2009

Final state dipole showers and the DGLAP equation

We study a parton shower description, based on a dipole picture, of the final state in electron-positron annihilation. In such a shower, the distribution function describing the inclusive probability to find a quark with a given energy depends on the shower evolution time. Starting from the exclusive evolution equation for the shower, we derive an equation for the evolution of the inclusive quark energy distribution in the limit of strong ordering in shower evolution time of the successive parton splittings. We find that, as expected, this is the DGLAP equation. This paper is a response to a recent paper of Dokshitzer and Marchesini that raised troubling issues about whether a dipole based shower could give the DGLAP equation for the quark energy distribution.

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arXiv2008

Direct numerical integration of one-loop Feynman diagrams for N-photon amplitudes

One approach to the calculation of cross sections for infrared-safe observables in high energy collisions at next-to-leading order is to perform all of the integrations, including the virtual loop integration, by Monte Carlo numerical integration. In a previous paper, two of us have shown how one can perform such a virtual loop integration numerically after first introducing a Feynman parameter representation. In this paper, we perform the integration directly, without introducing Feynman parameters, after suitably deforming the integration contour. Our example is the N-photon scattering amplitude with a massless electron loop. We report results for N = 6 and N = 8.

hep-ph↗

arXiv2008

Structure of parton showers including quantum interference

It is useful to describe a leading order parton shower as the solution of a linear equation that specifies how the state of the partons evolves. This description involves an essential approximation of a strong ordering of virtualities as the shower progresses from a hard interaction to softer interactions. If this is to be the only approximation, then the partons should carry color and spin and quantum interference graphs should be included. We explain how the evolution equation for this kind of a shower can be formulated. We discuss briefly our efforts to implement this evolution equation numerically.

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arXiv2008

Parton showers with quantum interference: leading color, with spin

We have previously described a mathematical formulation for a parton shower based on the approximation of strongly ordered virtualities of successive parton splittings. Quantum interference, including interference among different color and spin states, was included. A practical numerical implementation strategy was left unspecified. In a subsequent paper, we showed that if we add the further approximations of taking only the leading color limit and averaging over spins, we obtain a shower evolution that can be implemented as a Markov process. In this paper, we outline a strategy for including the correlations induced by parton spins.

hep-ph↗

arXiv2008

Parton showers with quantum interference: leading color, spin averaged

We have previously described a mathematical formulation for a parton shower based on the approximation of strongly ordered virtualities of successive parton splittings. Quantum interference, including interference among different color and spin states, is included. In this paper, we add the further approximations of taking only the leading color limit and averaging over spins, as is common in parton shower Monte Carlo event generators. Soft gluon interference effects remain with this approximation. We find that the leading color, spin averaged shower in our formalism is similar to that in other shower formulations. We discuss some of the differences.

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