arXiv subjects

Tilman Plehn

Publications and source records attributed to Tilman Plehn.

At least 163 records · Page 9Linked to original sources

arXiv2014

Mass Effects in the Higgs-Gluon Coupling: Boosted vs Off-Shell Production

In the upcoming LHC run we will be able to probe the structure ofthe loop--induced Higgs--gluon coupling through kinematics. First, we establish state-of-the-art simulations with up to two jets to next-to-leading order including top mass effects. They allow us to search for deviations from the low-energy limits in boosted Higgs production. In addition, the size of the top mass effects suggests that they should generally be included in Higgs studies at the LHC. Next, we show how off-shell Higgs production with a decay to four leptons is sensitive to the same top mass effects. We compare the potential of both methods based on the same top--Higgs Lagrangian. Finally, we comment on related model assumptions required for a Higgs width measurement.

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arXiv2014

Polarized WW Scattering on the Higgs Pole

The Higgs discovery has given us the Higgs-gauge sector as a new handle to search for physics beyond the Standard Model. This includes physics scenarios originally linked to massive gauge boson scattering at high energies. We investigate how one can separately probe the Higgs couplings to the longitudinal and transverse parts of the massive gauge bosons away from this high-energy limit. Deviations from the Standard Model could originate from higher-dimensional operators, compositeness, or even more fundamentally from a violation of gauge invariance. The signature we propose is the tagging jet kinematics in weak boson scattering for scattering energies close to the Higgs resonance. During the upcoming LHC run at 13 TeV we will be able to test these couplings at the 20% level.

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arXiv2014

Automated third generation squark production to next-to-leading order

If light--flavor squarks and gluinos are indeed heavy and chargino pair production is plagued with overwhelming backgrounds, pair production and associated production of stops and charginos will become the key signatures in the upcoming LHC runs. We present fully automated next-to-leading order predictions for heavy flavor squark production at the LHC, including stop-chargino associated production, based on MadGolem. We compute the total and differential NLO rates for a variety of MSSM scenarios with a light third generation. We focus on theoretical uncertainties on differential rates, including a comparison to multi-jet merging and the impact of bottom parton densities for stop-chargino associated production. We find that for the associated production channel the rates can reach tens of femtobarns, but the scale dependence does not provide a conservative estimate of the theoretical uncertainties.

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arXiv2014

Resolving the Higgs-Gluon Coupling with Jets

In the Standard Model the Higgs coupling to gluons is almost entirely induced by top quark loops. We derive the logarithmic structure of Higgs production in association with two jets. Just like in the one-jet case the transverse momentum distributions exhibit logarithms of the top quark mass and can be used to test the nature of the loop-induced Higgs coupling to gluons. Using Higgs decays to W bosons and to tau leptons we show how the corresponding analyses hugely benefit from the second jet in the relevant signal rate as well as in the background rejection.

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arXiv2014

Constraining Supersymmetry using the relic density and the Higgs boson

Recent measurements by Planck, LHC experiments, and Xenon100 have significant impact on supersymmetric models and their parameters. We first illustrate the constraints in the mSUGRA plane and then perform a detailed analysis of the general MSSM with 13 free parameters. Using SFitter, Bayesian and Profile Likelihood approaches are applied and their results compared. The allowed structures in the parameter spaces are largely defined by different mechanisms of dark matter annihilation in combination with the light Higgs mass prediction. In mSUGRA the pseudoscalar Higgs funnel and stau co-annihilation processes are still avoiding experimental pressure. In the MSSM stau co-annihilation, the light Higgs funnel, a mixed bino--higgsino region including the heavy Higgs funnel, and a large higgsino region predict the correct relic density. Volume effects and changes in the model parameters impact the extracted mSUGRA and MSSM parameter regions in the Bayesian analysis.

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arXiv2014

Stop on Top

The most natural supersymmetric solution to the hierarchy problem prefers the scalar top partner to be close in mass to the top quark. Experimental searches exclude top squarks across a wide range of masses, but a gap remains when the difference between the masses of the stop and the lightest supersymmetric particle is close to the top mass. We propose to search for stops in this regime by exploiting the azimuthal angular correlation of forward tagging jets in (s)top pair production. As shown in earlier work, this correlation is sensitive to the spin of the heavy states, allowing one to distinguish between top and stop pair production. Here, we demonstrate that this angular information can give a statistically significant stop pair production signal in the upcoming LHC run. While the appropriate simulation including parton showering and detector simulation requires some care, we find stable predictions for the angular correlation using multi-jet merging.

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arXiv2014

Lectures on LHC Physics

With the discovery of the Higgs boson the LHC experiments have closed the most important gap in our understanding of fundamental interactions. We now know that the interactions between elementary particles can be described by quantum field theory, more specifically by a renormalizable gauge theory. This theory is valid to arbitrarily high energy scales and do not require an ultraviolet completion. In these notes I cover three aspects to help understand LHC results in the Higgs sector and in searches for physics beyond the Standard Model: many facets of Higgs physics, QCD as it is relevant for LHC measurements, and standard phenomenological background knowledge. The lectures should put young graduate students into a position to really follow advanced writeups and first research papers. In that sense they can serve as a starting point for a research project in LHC physics. With this new, significantly expanded version I am confident that also some more senior colleagues will find them useful and interesting.

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arXiv2014

Benchmarking an Even Better HEPTopTagger

Top taggers are established analysis tools to reconstruct boosted hadronically decaying top quarks for example in searches for heavy resonances. We first present a dedicated study of signal efficiency versus background rejection, allowing for an improved choice of working points. Next, we determine to what degree our mass drop selection can be improved by systematically including angular correlations between the decay subjets. Finally, we extend the reach of the top tagger to transverse momenta below the top mass. This momentum range will be crucial in searches for the associated production of a Higgs boson with top quarks.

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arXiv2013

Understanding Jet Scaling and Jet Vetos in Higgs Searches

Jet counting and jet vetos are crucial analysis tools for many LHC searches. We can understand their properties from the distribution of the exclusive number of jets. LHC processes tend to show either a distinct staircase scaling or a Poisson scaling, depending on kinematic cuts. We illustrate our approach in a detailed study of jets in weak boson fusion Higgs production.

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arXiv2013

Improving Higgs plus Jets analyses through Fox--Wolfram Moments

It is well known that understanding the structure of jet radiation can significantly improve Higgs analyses. Using Fox--Wolfram moments we systematically study the geometric patterns of additional jets in weak boson fusion Higgs production with a decay to photons. First, we find a significant improvement with respect to the standard analysis based on an analysis of the tagging jet correlations. In addition, we show that replacing a jet veto by a Fox-Wolfram moment analysis of the extra jet radiation almost doubles the signal-to-background ratio. Finally, we show that this improvement can also be achieved based on a modified definition of the Fox--Wolfram moments which avoids introducing a new physical scale below the factorization scale. This modification can reduce the impact of theory uncertainties on the Higgs rate and couplings measurements.

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arXiv2013

MadMax, or Where Boosted Significances Come From

In an era of increasingly advanced experimental analysis techniques it is crucial to understand which phase space regions contribute a signal extraction from backgrounds. Based on the Neyman-Pearson lemma we compute the maximum significance for a signal extraction as an integral over phase space regions. We then study to what degree boosted Higgs strategies benefit ZH and ttH searches and which transverse momenta of the Higgs are most promising. We find that Higgs and top taggers are the appropriate tools, but would profit from a targeted optimization towards smaller transverse momenta. MadMax is available as an add-on to Madgraph5.

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arXiv2013

Buckets of Higgs and Tops

We show that associated production of a Higgs with a top pair can be observed in purely hadronic decays. Reconstructing the top quarks in the form of jet buckets allows us to control QCD backgrounds as well as signal combinatorics. The background can be measured from side bands in the reconstructed Higgs mass. We back up our claims with a detailed study of the QCD event simulation, both for the signal and for the backgrounds.

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arXiv2013

Measuring extended Higgs sectors as a consistent free couplings model

Extended Higgs sectors appear in many models for physics beyond the Standard Model. Current Higgs measurements at the LHC are starting to significantly constrain them. We study their Higgs coupling patterns at tree level as well as including quantum corrections. Our benchmarks include a dark singlet-doublet extension and several two-doublet setups. Using SFitter we translate the current Higgs coupling measurements for one light Higgs state into their respective parameter spaces. Finally, we show how two-Higgs-doublet models can serve as a consistent ultraviolet completion of an assumed single Standard-Model-like Higgs boson with free couplings.

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arXiv2013

Looking for leptogluons

We present search results based on next-to-leading order predictions for the pair production of color-adjoint leptons at the LHC. Quantum effects are sizable, dominated by pure QCD corrections, and sensitive to threshold effects. We illustrate the stabilization of scale dependences and confirm an excellent agreement between fixed-order and multi-jet predictions for representative distributions. Finally, we examine the trademark collider signatures of leptogluon pairs. Based on the CMS leptoquark search we derive a mass bound of 1.2-1.3 TeV for charged leptogluons, significantly improving the constraints available in the literature.

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arXiv2013

Buckets of Tops

Reconstructing hadronically decaying top quarks is a key challenge at the LHC, affecting a long list of Higgs analyses and new physics searches. We propose a new method of collecting jets in buckets, corresponding to top quarks and initial state radiation. This method is particularly well suited for moderate transverse momenta of the top quark, closing the gap between top taggers and traditional top reconstruction. Applying it to searches for supersymmetric top squarks we illustrate the power of buckets.

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arXiv2013

Measuring Higgs Couplings at a Linear Collider

Higgs couplings can be measured at a linear collider with high precision. We estimate the uncertainties of such measurements, including theoretical errors. Based on these results we show an extrapolation for a combined analysis at a linear collider and a high-luminosity LHC.

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arXiv2012

Fox-Wolfram Moments in Higgs Physics

Geometric correlations between jets as part of hard processes or in addition to hard processes are key ingredients to many LHC analyses. Fox--Wolfram moments systematically describe these correlations in terms of spherical harmonics. These moments, either computed from the tagging jets or from all jets in each event, can significantly improve Higgs searches in weak boson fusion. Applications of Fox--Wolfram moments in LHC analyses obviously surpass jets as analysis objects as well as Higgs searches in terms of analyses.

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arXiv2012

Automated Squark and Gluino Production to Next-to-Leading Order

We present completely general next-to-leading order predictions for squark and gluino production at the LHC, based on the fully automated MadGolem tool. Without any assumptions on the mass spectrum we predict production rates and examine the structure of the massless and massive quantum corrections. This allows us to quantify theory uncertainties induced by the spectrum assumptions commonly made. Going beyond total rates we compare general fixed-order distributions to resummed predictions from jet merging. As part of this comprehensive study we present the MadGolem treatment of ultraviolet, infrared and on-shell divergences.

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