arXiv subjects

Dorival Goncalves

Publications and source records attributed to Dorival Goncalves.

31 records · Page 2Linked to original sources

arXiv2015

Higgs-Strahlung: Merging the NLO Drell-Yan and Loop-Induced 0+1 jet Multiplicities

We analyse the production of a Higgs boson in association with a Z boson at hadron colliders in the Standard Model and some simple extensions. We show how multi-jet merging algorithms at leading and next-to-leading order for the loop-induced gluon fusion and the Drell-Yan like quark-induced processes, respectively, improve the descriptions for various differential distributions, in particular those that involve the production of additional jets. The phenomenological studies focus on two relevant channels of Higgs boson decays, namely $H\rightarrow invisible$ and $H\rightarrow b\bar{b}$. We find sizable and phenomenologically relevant corrections to the transverse momentum and invariant mass distributions for the Higgs boson candidate. Thanks to the large destructive interference for the top Yukawa terms, this process is very sensitive to the magnitude and sign of a possible non-standard top-Higgs coupling. We analyse the impact of this anomalous interaction on distributions and estimate constraints from LHC Run II.

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arXiv2015

The Higgs Legacy of the LHC Run I

Based on Run I data we present a comprehensive analysis of Higgs couplings. For the first time this SFitter analysis includes independent tests of the Higgs-gluon and top Yukawa couplings, Higgs decays to invisible particles, and off-shell Higgs measurements. The observed Higgs boson is fully consistent with the Standard Model, both in terms of coupling modifications and effective field theory. Based only on Higgs total rates the results using both approaches are essentially equivalent, with the exception of strong correlations in the parameter space induced by effective operators. These correlations can be controlled through additional experimental input, namely kinematic distributions. Including kinematic distributions the typical Run I reach for weakly interacting new physics now reaches 300 to 500 GeV.

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arXiv2014

Scalar Simplified Models for Dark Matter

We introduce a set of minimal simplified models for dark matter interactions with the Standard Model, connecting the two sectors via either a scalar or pseudoscalar particle. These models have a wider regime of validity for dark matter searches at the LHC than the effective field theory approach, while still allowing straightforward comparison to results from non-collider dark matter detection experiments. Such models also motivate dark matter searches in multiple correlated channels. In this paper, we constrain scalar and pseudoscalar simplified models with direct and indirect detection experiments, as well as from existing LHC searches with missing energy plus tops, bottoms, or jets, using the exact loop-induced coupling with gluons. This calculation significantly affects key differential cross sections at the LHC, and must be properly included. We make connections with the Higgs sector, and conclude with a discussion of future searches at the LHC.

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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

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

Non-standard top substructure

The top quark, being the heaviest particle of the Standard Model, is a prime candidate of where physics beyond the SM might currently hide before our eyes. There are many natural extensions of the SM that rely on top compositeness, and the top quark could follow the paradigm of revealing a substructure when it is probed at high enough momentum transfers. Observing high $p_T$ top final states naturally drives us towards boosted hadronic analyses that can be tackled efficiently with jet substructure techniques. In this paper we analyse the prospects of constraining exemplary non-standard QCD top interactions in this kinematical regime. We correctly include QCD modifications to additional gluon emission off the boosted top quark and keep track of the modified top tagging efficiencies. We conclude that non-standard top QCD interactions can be formidably constrained at the LHC 14 TeV. Experimental systematic uncertainties are a major obstacle of the described measurement. Unless significantly improved for the 14 TeV run, they will saturate the direct sensitivity to non-resonant BSM top physics at luminosities of around 100/fb.

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arXiv2013

The shape of spins

After the discovery of a Higgs-like particle at the LHC, the determination of its spin quantum numbers across different channels will be the next step in arriving at a more precise understanding of the new state and its role in electroweak symmetry breaking. Event shape observables have been shown to provide extremely sensitive observables for the discrimination of the scalar Higgs boson's CP quantum numbers as a consequence of the different radiation patterns of Higgs production via gluon fusion vs. weak boson fusion in the $pp\to X+2j$ selection. We show that a similar strategy serves to constrain the spin quantum numbers of the discovered particle as a function of the involved couplings. We also discuss the prospects of applying a similar strategy to future discoveries of Higgs-like particles.

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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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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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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

Constraining the Unhiggs with LHC data

Recent measurements by the ATLAS and CMS experiments have excluded the Standard Model Higgs boson in the high mass region, even if it is produced with a significantly smaller cross section than expected. The bounds are dominated by the non-observation of a signal in the clean gold-plated mode $h\to ZZ\to 4\ell$ and, hence, are directly related to the special role of the Higgs in electroweak symmetry breaking. A smaller cross section in comparison to the Standard Model is expected if the Higgs is realized as an unparticle in the Unhiggs scenario. With the LHC probing $σ/σ^{SM}<1$, we can therefore reinterpret the $h\to ZZ\to 4\ell$ exclusion limits as bounds on the Unhiggs' scaling dimension. Throughout the high Higgs mass range, where we expect a large signal in the presence of the Standard Model Higgs for the 2011 ATLAS and CMS data sets, the observed limits translate into mild bounds on the Unhiggs scaling dimension in the high mass region.

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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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