arXiv subjects

Tilman Plehn

Publications and source records attributed to Tilman Plehn.

At least 181 records · Page 10Linked to original sources

arXiv2012

MadGolem: automating NLO calculations for New Physics

With the LHC close to complete its 8 TeV run, the experimental searches have already started to probe the vast beyond-the-standard Model scenery. Providing next-to-leading order (NLO) predictions for the major new physics discovery channels is therefore a most pressing request to particle phenomenologists these days. MadGolem is a new computational tool that automates NLO calculations of generic 2->2 new physics processes in the MadGraph/Golem framework. In this contribution we concisely describe the structure and performance of the code, with particular focus on the generation of the renormalized one-loop amplitudes and the automatized subtraction of infrared and on-shell divergences. We briefly survey the many dedicated tests of all these aspects and outline some applications to LHC phenomenology.

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arXiv2012

Higgs Couplings after the Discovery

Following the ATLAS and CMS analyses presented around ICHEP 2012 we determine the individual Higgs couplings. The new data allow us to specifically test the effective coupling to photons. We find no significant deviation from the Standard Model in any of the Higgs couplings.

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arXiv2012

Scaling Patterns for QCD Jets

Jet emission at hadron colliders follows simple scaling patterns. Based on perturbative QCD we derive Poisson and staircase scaling for final state as well as initial state radiation. Parton density effects enhance staircase scaling at low multiplicities. We propose experimental tests of our theoretical findings in Z+jets and QCD gap jets production based on minor additions to current LHC analyses.

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arXiv2012

Tagging single Tops

Top taggers which identify and reconstruct boosted top quarks have been established as novel tools for a multitude of LHC analyses. We show how single top production in association with a light-flavor or bottom jet can be observed in the boosted phase space regime. The full top reconstruction as part of the tagging algorithm allows us to define a distinctive kinematic angle which clearly separates different single top production processes.

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arXiv2012

Supersymmetry production cross sections in pp collisions at sqrt{s} = 7 TeV

This document emerged from work that started in January 2012 as a joint effort by the ATLAS, CMS and LPCC supersymmetry (SUSY) working groups to compile state-of-the-art cross section predictions for SUSY particle production at the LHC. We present cross sections for various SUSY processes in pp collisions at $\sqrt{s} =7$ TeV, including an estimate of the theoretical uncertainty due to scale variation and the parton distribution functions. Further results for higher LHC centre-of-mass energies will be collected at https://twiki.cern.ch/twiki/bin/view/LHCPhysics/SUSYCrossSections. For squark and gluino production, which dominate the inclusive SUSY cross section, we employ calculations which include the resummation of soft gluon emission at next-to-leading logarithmic accuracy, matched to next-to-leading order (NLO) SUSY-QCD. In all other cases we rely on NLO SUSY-QCD predictions.

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arXiv2012

From Jet Counting to Jet Vetoes

The properties of multi-jet events impact many LHC analysis. The exclusive number of jets at hadron colliders can be described in terms of two simple patterns: staircase scaling and Poisson scaling. In photon plus jets production we can interpolate between the two patterns using simple kinematic cuts. The associated theoretical errors are well under control. Understanding such exclusive jet multiplicities significantly impacts Higgs searches and searches for supersymmetry at the LHC.

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arXiv2012

Measuring Higgs Couplings from LHC Data

Following recent ATLAS and CMS publications we interpret the results of their Higgs searches in terms of Standard Model operators. For a Higgs mass of 125 GeV we determine several Higgs couplings from 2011 data and extrapolate the results towards different scenarios of LHC running. Even though our analysis is limited by low statistics we already derive meaningful constraints on modified Higgs sectors.

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arXiv2012

Stop searches in 2012

For this year's 8TeV run of the LHC we lay out different strategies to search for scalar top pairs. We show results for the hadronic and for the semi-leptonic channels based on hadronic top tagging. For the di-lepton channel we illustrate the impact of transverse mass variables. Each of our signal-to-background ratios ranges around unity for a stop mass around 400GeV. The combined signal significances show that dedicated stop searches are becoming sensitive over a non-negligible part of parameter space.

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arXiv2012

Sgluon Pair Production to Next-to-Leading Order

Scalar color octets are generic signals for new physics at LHC energies. We examine their pair production at the LHC to next-to-leading order QCD. This computation serves as another test of the fully automized MadGolem framework. We find large NLO production rates and sizeable quantum effects which depend on the sgluon mass. The shift in the sgluon distributions is mild and in good agreement with a multi-jet merging calculation.

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arXiv2012

Establishing Jet Scaling Patterns with a Photon

Staircase and Poisson scaling are two typical patterns we observe for the exclusive number of jets at high energy hadron colliders. We examine these scaling properties for photon plus jets production at the LHC and find that this channel is well suited to study these features. We illustrate and discuss when to expect each of the two patterns, how to induce a transition through kinematic cuts, and how photons are related to heavy gauge bosons. Measurements of photon+jets production is therefore providing valuable information on exclusive jet scaling, which is going to help to eventually understand the theoretical origin of exclusive jet scaling properties in more detail.

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arXiv2012

LHC: Standard Higgs and Hidden Higgs

Interpretations of Higgs searches critically involve production cross sections and decay probabilities for different analysis channels. Mixing effects can reduce production rates, while invisible decays can reduce decay probabilities. Both effects can transparently be quantified in Higgs systems where a visible Higgs boson is mixed with a hidden sector Higgs boson. Recent experimental exclusion bounds can be re-interpreted in this context as a sign for non-standard Higgs properties. Should a light Higgs boson be discovered, then our analysis will quantify how closely it may coincide with the Standard Model.

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arXiv2011

Top Tagging

Top tagging is a recent approach to identifying boosted hadronic top quarks. It avoids reconstructing individual top decay products and instead uses a jet algorithm to reconstruct the entire top decay. Quite generally, geometrically large jets including heavy particles (fat jets) can be analyzed on the level of their subjet constituents. LHC data will soon allow us to establish this new analysis method. We discuss different tagging algorithms, their critical QCD aspects, and currently available experimental results. For the development of taggers and their different applications this review should provide a firm theoretical and algorithmic background.

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arXiv2011

How to Improve Top Tagging

In time for the first tests on LHC data we introduce a set of improvements and tests of purely kinematic top tagging algorithms. First, we show how different jet algorithms can be used for different transverse momentum regimes. Combining pruning and filtering in the reconstruction can enhance the signal over background ratio significantly, while larger jet radii only give minor improvements. Finally, bottom tagging can be added to the top tagger, but at least for the HEPTopTagger does not improve the kinematic selection algorithm.

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arXiv2011

W+jets, Z+jets, multijets and new physics searches

W+jets, Z+jets and QCD multijet production processes at hadron colliders are backgrounds to many searches for physics beyond the Standard Model which involve leptons and missing energy in the final state. We review the current theoretical and experimental status of these processes at the LHC. Furthermore, we discuss several methods that allow for reliable predictions for these processes in the context of new-physics searches.

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arXiv2011

Travels on the squark-gluino mass plane

Soft supersymmetry breaking appears in the weak-scale effective action but is usually generated at higher scales. For these models the structure of the renormalization group evolution down to the electroweak scale leaves only part of the squark-gluino and slepton-gaugino mass planes accessible. Our observations divide these physical mass planes into three wedges: the first can be reached by all models of high-scale breaking; the second can only be populated by models with a low mediation scale; in the third wedge squarks and gluinos would have to be described by an exotic theory. All usual benchmark points reside in the first wedge, even though an LHC discovery in the third wedge would arguably be the most exciting outcome.

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arXiv2011

Exploring the Higgs portal

We study the Higgs portal from the Standard-Model to a hidden sector and examine which elements of the extended theory can be discovered and explored at the LHC. Our model includes two Higgs bosons covering parameter regions where the LHC will be sensitive to two, one or none of the particles at typical discovery luminosities for Standard Model Higgs production.

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arXiv2011

Graviton collider effects in one and more large extra dimensions

Astrophysical bounds severely limit the possibility of observing collider signals of gravity with less than 3 flat extra dimensions. However, small distortions of the compactified space can lift the masses of the lightest graviton excitations, evading astrophysical bounds without affecting collider signals of quantum gravity. Following this procedure we reconsider theories with one large extra dimension. A slight space warping gives a model which is safe in the infrared against astrophysical and observational bounds, and which has the ultraviolet properties of gravity with a single flat extra dimension. We extend collider studies to the case of one extra dimension, pointing out its peculiarities. Finally, for a generic number of extra dimensions, we compare different channels in LHC searches for quantum gravity, introducing an ultraviolet cutoff as an additional parameter besides the Planck mass.

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arXiv2011

W+Jets at CDF: Evidence for Top Quarks

Recently, an anomaly of W+jets events at large invariant masses has been reported by CDF. Many interpretations as physics beyond the Standard Model are being offered. We show how such an invariant mass peak can arise from a slight shift in the relative normalization of the top and WW backgrounds.

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