arXiv subjects

Christoph Englert

Publications and source records attributed to Christoph Englert.

At least 109 records · Page 6Linked to original sources

arXiv2016

Same-sign W pair production in composite Higgs models

Non-minimal composite Higgs scenarios can contain exotic Higgs states which, if getting observed at the Large Hadron Collider, will help to constrain the underlying UV structure of the strong dynamics. Doubly charged Higgs bosons are well-motivated scalar degrees of freedom in this context. Their phenomenology in typical composite scenarios can differ from well-established Higgs triplet extensions of the SM. Related search strategies are not necessarily adapted to such a scenario as a consequence. In this paper we discuss the sensitivity reach to doubly charged Higgs bosons with decays into pairs of same-sign $W$ bosons. While production cross sections are small, we show that significant constraints on $H^{\pm\pm}\to W^{\pm}W^{\pm}$ can be obtained, providing a new opportunity to constrain the potential composite structure of the TeV scale up to $m_{H^{\pm\pm}}\simeq 800$~GeV.

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arXiv2016

Towards resolving strongly-interacting dark sectors at colliders

Dark sectors with strong interactions have received considerable interest. Assuming the existence of a minimally coupled dark sector which runs to strong interactions in the infrared, we address the question whether the scaling behavior of this dark sector can be observed in missing energy signatures at present and future hadron colliders. We compare these findings to the concrete case of self-interacting dark matter and demonstrate that the energy dependence of high-momentum transfer final states can in principle be used to gain information about the UV structure of hidden sectors at future hadron colliders, subject to large improvements in systematic uncertainties, which could complement proof-of-principle lattice investigations. We also comment on the case of dark Abelian $U(1)$ theories.

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arXiv2016

Higgs coupling measurements at the LHC

Due to the absence of tantalising hints for new physics during the LHC's run 1, the extension of the Higgs sector by dimension six operators will provide the new phenomenological standard for searches of non-resonant extensions of the Standard Model. Using all dominant and subdominant Higgs production mechanisms at the LHC, we compute the constraints on Higgs physics-relevant dimension six operators in a global and correlated fit. We show in how far these constraints can be improved by new Higgs channels becoming accessible at higher energy and luminosity, both through inclusive cross sections as well as through highly sensitive differential distributions. This allows us to discuss the sensitivity to new effects in the Higgs sector that can be reached at the LHC if direct hints for physics beyond the SM remain elusive and the impact of these constraints on well-motivated BSM scenarios.

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arXiv2016

New physics and signal-background interference in associated $pp\to HZ$ production

We re-investigate electroweak signal-background interference in associated Higgs production via gluon fusion in the presence of new physics in the top-Higgs sector. Considering the full final state $pp \to b \bar b \ell^+\ell^-$ ($\ell=e,μ$), we discuss how new physics in the top-Higgs sector that enhances the $ZZ$ component can leave footprints in the $HZ$ limit setting. In passing we investigate the phenomenology of a class of new physics interactions that can be genuinely studied in this process.

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arXiv2016

S-Channel Dark Matter Simplified Models and Unitarity

The ultraviolet structure of $s$-channel mediator dark matter simplified models at hadron colliders is considered. In terms of commonly studied $s$-channel mediator simplified models it is argued that at arbitrarily high energies the perturbative description of dark matter production in high energy scattering at hadron colliders will break down in a number of cases. This is analogous to the well documented breakdown of an EFT description of dark matter collider production. With this in mind, to diagnose whether or not the use of simplified models at the LHC is valid, perturbative unitarity of the scattering amplitude in the processes relevant to LHC dark matter searches is studied. The results are as one would expect: at the LHC and future proton colliders the simplified model descriptions of dark matter production are in general valid. As a result of the general discussion, a simple new class of previously unconsidered `Fermiophobic Scalar' simplified models is proposed, in which a scalar mediator couples to electroweak vector bosons. The Fermiophobic simplified model is well motivated and exhibits interesting collider and direct detection phenomenology.

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arXiv2016

LHC Signatures Of Scalar Dark Energy

Scalar dark energy fields that couple to the Standard Model can give rise to observable signatures at the LHC. In this work we show that $t\bar t+$missing energy and mono-jet searches are suitable probes in the limit where the dark energy scalar is stable on collider distances. We discuss the prospects of distinguishing the dark energy character of new physics signals from dark matter signatures and the possibility of probing the self-interactions of the dark energy sector.

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arXiv2016

Measuring the Higgs-bottom coupling in weak boson fusion

We study Higgs production through weak boson fusion with subsequent decay to bottom quarks. By combining jet substructure techniques and matrix element methods in different limits we motivate this channel as a probe of the bottom-Yukawa interactions in the boosted regime. In particular we ameliorate the "no-go" results of cut-and-count analyses in this channel. After applying a data-driven reconstruction approach we find that the Higgs-bottom coupling can be limited to $0.82 < y_b/y_b^{\text{SM}} <1.14$ with $600~\mathrm{fb}^{-1}$.

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arXiv2015

A global fit of top quark effective theory to data

In this paper we present a global fit of beyond the Standard Model (BSM) dimension six operators relevant to the top quark sector to currently available data. Experimental measurements include parton-level top-pair and single top production from the LHC and the Tevatron. Higher order QCD corrections are modelled using differential and global K-factors, and we use novel fast-fitting techniques developed in the context of Monte Carlo event generator tuning to perform the fit. This allows us to provide new, fully correlated and model-independent bounds on new physics effects in the top sector from the most current direct hadron-collider measurements in light of the involved theoretical and experimental systematics. As a by-product, our analysis constitutes a proof-of-principle that fast fitting of theory to data is possible in the top quark sector, and paves the way for a more detailed analysis including top quark decays, detector corrections and precision observables.

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arXiv2015

Unitarity-controlled resonances after Higgs discovery

If the recently discovered Higgs boson's couplings deviate from the Standard Model expectation, we may anticipate new resonant physics in the weak boson fusion channels resulting from high scale unitarity sum rules of longitudinal gauge boson scattering. Motivated by excesses in analyses of multi-leptons+missing energy+jets final states during run 1, we perform a phenomenological investigation of these channels at the LHC bounded by current Higgs coupling constraints. Such an approach constrains the prospects to observe such new physics at the LHC as a function of very few and generic parameters and allows the investigation of the strong requirement of probability conservation in the electroweak sector to high energies. Our analysis is directly relevant for the 2 TeV excess reported recently by the CMS and ATLAS collaborations.

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arXiv2015

$hhjj$ production at the LHC

The search for di-Higgs production at the LHC in order to set limits on Higgs trilinear coupling and constraints on new physics is one of the main motivations for the LHC high luminosity phase. Recent experimental analyses suggest that such analyses will only be successful if information from a range of channels is included. We therefore investigate di-Higgs production in association with two hadronic jets and give a detailed discussion of both the gluon- and weak boson fusion contributions, with a particular emphasis on the phenomenology with modified Higgs trilinear and quartic gauge couplings. We perform a detailed investigation of the full hadronic final state and find that $hhjj$ production should add sensitivity to a di-Higgs search combination at the HL-LHC with 3 ab$^{-1}$. Since the WBF and GF contributions are sensitive to different sources of physics beyond the Standard Model, we devise search strategies to disentangle and isolate these production modes. While gluon fusion remains non-negligible in WBF-type selections, sizeable new physics contributions to the latter can still be constrained. As an example of the latter point we investigate the sensitivity that can be obtained for a measurement of the quartic Higgs-gauge boson couplings.

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arXiv2015

Disformal dark energy at colliders

Disformally coupled, light scalar fields arise in many of the theories of dark energy and modified gravity that attempt to explain the accelerated expansion of the universe. They have proved difficult to constrain with precision tests of gravity because they do not give rise to fifth forces around static non-relativistic sources. However, because the scalar field couples derivatively to standard model matter, measurements at high energy particle colliders offer an effective way to constrain and potentially detect a disformally coupled scalar field. Here we derive new constraints on the strength of the disformal coupling from LHC run 1 data and provide a forecast for the improvement of these constraints from run 2. We additionally comment on the running of disformal and standard model couplings in this scenario under the renormalisation group flow.

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arXiv2015

Off-Shell Higgs Coupling Measurements in BSM scenarios

Proposals of measuring the off-shell Higgs contributions and first measurements at the LHC have electrified the Higgs phenomenology community for two reasons: Firstly, probing interactions at high invariant masses and momentum transfers is intrinsically sensitive to new physics beyond the Standard Model, irrespective of a resonant or non-resonant character of a particular BSM scenario. Secondly, under specific assumptions a class of models exists for which the off-shell coupling measurement together with a measurement of the on-shell signal strength can be re-interpreted in terms of a bound on the total Higgs boson width. In this paper, we provide a first step towards a classification of the models for which a total width measurement is viable and we discuss examples of BSM models for which the off-shell coupling measurement can be important in either constraining or even discovering new physics in the upcoming LHC runs. Specifically, we discuss the quantitative impact of the presence of dimension six operators on the (de)correlation of Higgs on- and off-shell regions keeping track of all interference effects. We furthermore investigate off-shell measurements in a wider context of new (non-)resonant physics in Higgs portal scenarios and the MSSM.

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arXiv2015

Combining LEP and LHC to bound the Higgs Width

The correlation of on- and off-shell Higgs boson production at the LHC in $gg \to h^* \to ZZ$ to bound the Higgs width, under specific model-dependent assumptions, has recently received a lot of attention. As off-shell cross section measurements in this channel suffer from a small signal yield, large backgrounds, and theoretical uncertainties, we propose an alternative complementary constraint which is only possible through the combination of LEP and LHC measurements. Previous precision electroweak measurements at LEP allow for the determination of indirect constraints on Higgs couplings to vector bosons by considering one-loop processes involving virtual Higgs exchange. As the Higgs is off-shell in these diagrams we venture that LEP can be interpreted as an off-shell `Higgs Factory'. By combining these LEP constraints with current LHC 8 TeV Higgs measurements a stronger limit on the Higgs width can be achieved than with LHC data alone for models with rescaled Higgs couplings. Looking to the future, avoiding ambiguities arising due to new physics modifications of the $hGG$ coupling, a theoretically more robust constraint can be achieved by correlating LEP measurements with WBF Higgs production followed by Higgs decays to $WW$ and $ZZ$. This method for indirectly constraining the Higgs width is very effective for specific BSM scenarios and is highly complementary to other proposed methods. The limits we obtain particularly highlight the power of a concrete LEP+LHC combination, not only limited to Higgs width measurements.

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arXiv2015

Higgs Self-Coupling Measurements at a 100 TeV Hadron Collider

An important physics goal of a possible next-generation high-energy hadron collider will be precision characterisation of the Higgs sector and electroweak symmetry breaking. A crucial part of understanding the nature of electroweak symmetry breaking is measuring the Higgs self-interactions. We study dihiggs production in proton-proton collisions at 100 TeV centre of mass energy in order to estimate the sensitivity such a machine would have to variations in the trilinear Higgs coupling around the Standard Model expectation. We focus on the two b-jets plus diphotons final state, including possible enhancements in sensitivity by exploiting dihiggs recoils against a hard jet. We find that it should be possible to measure the trilinear self-coupling with 40% accuracy given 3/ab and 12% with 30/ab of data.

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arXiv2015

On-shell interference effects in Higgs final states

Top quark loops in Higgs production via gluon fusion at large invariant final state masses can induce important interference effects in searches for additional Higgs bosons as predicted in, e.g., Higgs portal scenarios and the MSSM when the heavy scalar is broad or the final state resolution is poor. Currently, the limit setting as performed by both ATLAS and CMS is based on injecting a heavy Higgs-like signal neglecting interference effects. In this paper, we perform a study of such "on-shell" interference effects in $pp\to ZZ$ and find that they lead to a $\lesssim{\cal{O}}(30%)$ width scheme-dependent modification of the signal strength. Including the continuum contributions to obtain e.g. the full $pp\to ZZ \to 4\ell$ final state, this modification is reduced to the 10% level in the considered intermediate mass range.

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arXiv2014

Effective Theories and Measurements at Colliders

If the LHC run 2 will not provide conclusive hints for new resonant Physics beyond the Standard Model, dedicated and consistent search strategies at high momentum transfers will become the focus of searches for anticipated deviations from the Standard Model expectation. We discuss the phenomenological importance of QCD and electroweak corrections in bounding higher dimensional operators when analysing energy-dependent differential distributions. In particular, we study the impact of RGE-induced operator running and mixing effects on measurements performed in the context of an Effective Field Theory extension of the SM. Furthermore we outline a general analysis strategy which allows a RGE-improved formulation of constraints free of theoretical shortcomings that can arise when differential distributions start to probe the new interaction scale. We compare the numerical importance of such a programme against the standard analysis approach which is widely pursued at present.

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arXiv2014

Gluon-initiated associated production boosts Higgs physics

Analyses of boosted Higgs bosons from associated production comprise some of the main search channels for the Higgs boson at the LHC. The gluon-initiated $gg\to hZ$ subprocess has largely been ignored in phenomenological analyses of boosted associated production although this contribution is sizable as the $p_T$ spectrum for this process is maximised in the boosted regime due to the top quark loop threshold. In this paper, we discuss this contribution to boosted $pp\to hZ$ analyses in detail. We find there are previously overlooked modifications of Standard Model Higgs rates at the LHC which depend on the $p_T$ cuts applied and can be significant. There are also important consequences for physics beyond the Standard Model as the $gg\to hZ$ process introduces significant dependence on the magnitude and sign of the Higgs-top quark coupling $c_t$, which is overlooked if it is assumed that associated production depends only on the Higgs-$Z$ boson coupling as $c_V^2$. This new dependence on $c_t$ impacts interpretations of Higgs rates in the contexts of Supersymmetry, Two Higgs Doublet Models, and general scenarios with modified couplings. We suggest that these effects be included in current and future LHC boosted Higgs analyses.

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arXiv2014

Di-Higgs phenomenology: The forgotten channel

Searches for multi-Higgs final states allow to constrain parameters of the SM (or extensions thereof) that directly relate to the mechanism of electroweak symmetry breaking. Multi-Higgs production cross sections, however, are small and the phenomenologically accessible final states are challenging to isolate in the busy multi-jet hadron collider environment of the LHC run 2. This makes the necessity to extend the list of potentially observable production mechanisms obvious. Most of the phenomenological analyses in the past have focused on $gg\to hh+jets$; in this paper we study $pp\to t\bar t hh$ at LHC run 2 and find that this channel for $h\to b\bar b$ and semi-leptonic and hadronic top decays has the potential to provide an additional handle to constrain the Higgs trilinear coupling in a global fit at the end of run 2.

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