arXiv subjects

Christoph Englert

Publications and source records attributed to Christoph Englert.

At least 91 records · Page 5Linked to original sources

arXiv2019

Machine Learning Uncertainties with Adversarial Neural Networks

Machine Learning is a powerful tool to reveal and exploit correlations in a multi-dimensional parameter space. Making predictions from such correlations is a highly non-trivial task, in particular when the details of the underlying dynamics of a theoretical model are not fully understood. Using adversarial networks, we include a priori known sources of systematic and theoretical uncertainties during the training. This paves the way to a more reliable event classification on an event-by-event basis, as well as novel approaches to perform parameter fits of particle physics data. We demonstrate the benefits of the method explicitly in an example considering effective field theory extensions of Higgs boson production in association with jets.

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arXiv2018

Showcasing HH production: Benchmarks for the (HL-)LHC

Current projections suggest that the LHC will have only limited sensitivity to di-Higgs production in the Standard Model (SM), possibly even after the completion of its high luminosity phase. Multi-Higgs final states play a fundamental role in many extensions of the SM as they are intrinsically sensitive to modifications of the Higgs sector. Therefore, any new observation in multi-Higgs final states could be linked to a range of beyond the SM (BSM) phenomena that are not sufficiently addressed by the SM. Extensions of the Higgs sector typically lead to new phenomenological signatures in multi-Higgs final states that are vastly different from the SM expectation. In this work, we provide a range of signature-driven benchmark points for resonant and non-resonant BSM di-Higgs production that motivate non-SM kinematic correlations and multi-fermion discovery channels. Relying on theoretically well-motivated assumptions, special attention is devoted to the particular case where the presence of new physics will dominantly manifest itself in multi-Higgs final states.

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arXiv2018

Probing Electroweak Precision Physics via boosted Higgs-strahlung at the LHC

We study the process $pp \to Z(\ell^+ \ell^-)h(b\bar b)$ in the Standard Model Effective Field Theory (SMEFT) at high energies using subjet techniques to reconstruct the Higgs boson. We show that at high energies this process probes four directions in the dimension 6 EFT space, namely the operators that contribute to the four contact interactions, $hZ_μ\bar{f}γ^μf$, where $f=u_L, u_R,d_L$ and $d_R$. These four directions are, however, already constrained by the $Z$-pole and diboson measurements at LEP. We show that by utilising the energy growth of this process in the SMEFT and the accuracy that can be achieved by using subjet techniques at the High Luminosity LHC, one can obtain bounds on these operators that are an order of magnitude better than existing LEP bounds.

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arXiv2018

Particle Physics with Gravitational Wave Detector Technology

Gravitational wave detector technology provides high-precision measurement apparatuses that, if combined with a modulated particle source, have the potential to measure and constrain particle interactions in a novel way, by measuring the pressure caused by scattering particle beams off the mirror material. Such a measurement does not rely on tagging a final state. This strategy has the potential to allow us to explore novel ways to constrain the presence of new interactions beyond the Standard Model of Particle Physics and provide additional constraints to poorly understood cross sections in the non-perturbative regime of QCD and Nuclear Physics, which are limiting factors of dark matter and neutrino physics searches. Beyond high-energy physics, if technically feasible, the proposed method to measure nucleon-nucleon interactions can lead to practical applications in material and medical sciences.

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arXiv2018

Constraining top quark effective theory in the LHC Run II era

We perform an up-to-date global fit of top quark effective theory to experimental data from the Tevatron, and from LHC Runs I and II. Experimental data includes total cross-sections up to 13 TeV, as well as differential distributions, for both single top and pair production. We also include the top quark width, charge asymmetries, and polarisation information from top decay products. We present bounds on the coefficients of dimension six operators, and examine the interplay between inclusive and differential measurements, and Tevatron / LHC data. All results are currently in good agreement with the Standard Model.

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arXiv2018

Anomalous neutral gauge boson interactions and simplified models

Trilinear $Z$ boson interactions are sensitive probes both of new sources of $CP$ violation in physics Beyond the Standard Model and of new particle thresholds. Measurements of trilinear $Z$ interactions are typically interpreted in the frameworks of anomalous couplings and effective field theory, both of which require care in interpretation. To obtain a quantitative picture of the power of these measurements when interpreted in a TeV-scale context, we investigate the anatomy of $ZZZ$ interactions and consider two minimal and perturbative simplified models which induce such interactions through new scalar and fermion loops at the weak scale, focusing on $ZZ$ and vector boson fusion-induced $Zjj$ production at the LHC and $ZZ$ production at a future $e^+ e^-$ collider. We show that both threshold and non-threshold effects often are small compared to the sensitivity of the LHC, while the increased sensitivity of a future lepton collider should allow us to constrain such scenarios through associated electroweak precision effects complementary to direct searches at hadron colliders.

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arXiv2018

$hh+\text{jet}$ production at 100 TeV

Higgs pair production is a crucial phenomenological process in deciphering the nature of the TeV scale and the mechanism underlying electroweak symmetry breaking. At the Large Hadron Collider, this process is statistically limited. Pushing the energy frontier beyond the LHC's reach will create new opportunities to exploit the rich phenomenology at higher centre-of-mass energies and luminosities. In this work, we perform a comparative analysis of the $hh+\text{jet}$ channel at a future 100 TeV hadron collider. We focus on the $hh\to b\bar b b\bar b$ and $hh \to b\bar b τ^+τ^-$ channels and employ a range of analysis techniques to estimate the sensitivity potential that can be gained by including this jet-associated Higgs pair production to the list of sensitive collider processes in such an environment. In particular, we observe that $hh \to b\bar b τ^+τ^-$ in the boosted regime exhibits a large sensitivity to the Higgs boson self-coupling and the Higgs self-coupling could be constrained at the 8\% level in this channel alone.

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arXiv2017

Jet-associated resonance spectroscopy

We present a model-independent study aimed at characterizing the nature of possible resonances in the jet-photon or jet-$Z$ final state at hadron colliders. Such resonances are expected in many models of compositeness and would be a clear indication of new physics. At leading order, in the narrow width approximation, the matrix elements are parameterized by just a few constants describing the coupling of the various helicities to the resonance. We present the full structure of such amplitudes up to spin two and use them to simulate relevant kinematic distributions that could serve to constrain the coupling structure. This also generalizes the signal generation strategy that is currently pursued by ATLAS and CMS to the most general case in the considered channels. While the determination of the P/CP properties of the interaction seems to be out of reach within this framework, there is a wealth of information to be gained about the spin of the resonance and the relative couplings of the helicities.

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arXiv2017

Higgs characterisation in the presence of theoretical uncertainties and invisible decays

While the Higgs characterisation programme is well underway, direct signs for new physics beyond the Standard Model remain elusive. Performing a fit of fully differential Higgs production cross sections at the LHC to a subset of Higgs-relevant effective operators, we discuss the extent to which theoretical uncertainties can limit the sensitivity in such a new physics search programme. Extending the dimension-6 Higgs Effective Field Theory framework by introducing new light degrees of freedom that can contribute to an invisible (or undetectable) Higgs decay width $h \to ϕϕ$, we show how differential coupling fits can disentangle effects from non-Standard Model couplings and an invisible decay width, as present in many new physics scenarios, such as Higgs-portal dark matter. Including the so-called off-shell measurement that has been advocated as a sensitive determination of the Higgs width in the $κ$ framework, we show explicitly that this method does not provide complementary sensitivity for scale-separated new physics $Λ\gg m_h \gg m_ϕ$, which is favoured in beyond the Standard Model scenarios that relate astrophysics and collider phenomenology in light of non-observation of new physics during run 1 of the LHC.

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arXiv2017

Top quark electroweak couplings at future lepton colliders

We perform a comparative study of the reach of future $e^+e^-$ collider options for the scale of non-resonant new physics effects in the top quark sector, phrased in the language of higher-dimensional operators. Our focus is on the electroweak top quark pair production process $e^+e^- \to Z^*/γ\to t\bar t $, and we study benchmark scenarios at the ILC and CLIC. We find that both are able to constrain mass scales up to the few TeV range in the most sensitive cases, improving by orders of magnitude on the forecasted capabilities of the LHC. We discuss the role played by observables such as forward-backward asymmetries, and making use of different beam polarisation settings, and highlight the possibility of lifting a degeneracy in the allowed parameter space by combining top observables with precision $Z$-pole measurements from LEP1.

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arXiv2017

A UV Complete Compositeness Scenario: LHC Constraints Meet The Lattice

We investigate a recently proposed UV-complete composite Higgs scenario in the light of the first LHC runs. The model is based on a $SU(4)$ gauge group with global flavour symmetry breaking $SU(5) \to SO(5)$, giving rise to pseudo Nambu-Goldstone bosons in addition to the Higgs doublet. This includes a real and a complex electroweak triplet with exotic electric charges. Including these, as well as constraints on other exotic states, we show that LHC measurements are not yet sensitive enough to significantly constrain the model's low energy constants. The Higgs potential is described by two parameters which are on the one hand constrained by the LHC measurement of the Higgs mass and Higgs decay channels and on the other hand can be computed from correlation functions in the UV-complete theory. Hence to exclude the model at least one constant needs to be determined and to validate the Higgs potential both constants need to be reproduced by the UV-theory. Due to its UV-formulation, a certain number of low energy constants can be computed from first principle numerical simulations of the theory formulated on a lattice, which can help in establishing the validity of this model. We assess the potential impact of lattice calculations for phenomenological studies, as a preliminary step towards Monte Carlo simulations.

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arXiv2017

Giving top quark effective operators a boost

We investigate the prospects to systematically improve generic effective field theory-based searches for new physics in the top sector during LHC run 2 as well as the high luminosity phase. In particular, we assess the benefits of high momentum transfer final states on top EFT-fit as a function of systematic uncertainties in comparison with sensitivity expected from fully-resolved analyses focusing on $t\bar t$ production. We find that constraints are typically driven by fully-resolved selections, while boosted top quarks can serve to break degeneracies in the global fit. This demystifies and clarifies the importance of high momentum transfer final states for global fits to new interactions in the top sector from direct measurements.

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arXiv2017

Constraining new resonant physics with top spin polarisation information

We provide a comprehensive analysis of the power of including top quark-polarisation information to kinematically challenging $t\bar t$ resonance searches, for which ATLAS and CMS start losing sensitivity. Following the general modeling and analysis strategies pursued by the experiments, we analyse the semi-leptonic and the di-lepton $t\bar t$ channels and show that including polarisation information can lead to large improvements in the limit setting procedures with large data sets. This will allow us to set limits for parameter choices where sensitivity from $m(t\bar t)$ is not sufficient. This highlights the importance of spin observables as part of a more comprehensive set of observables to gain sensitivity to BSM resonance searches.

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arXiv2017

Phenomenology of a Composite Higgs Model

Several UV complete models of physics beyond the Standard Model are currently under scrutiny, their low-energy dynamics being compared with the experimental data from the LHC. Lattice simulations can play a role in these studies by providing a first principles computations of the low-energy constants that describe this low-energy dynamics. In this work, we study in detail a specific model recently proposed by Ferretti, and discuss the potential impact of lattice calculations.

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arXiv2017

VBS ${\rm W}^\pm {\rm W}^\pm {\rm H}$ production at the HL-LHC and a 100 TeV pp-collider

${\rm W}^\pm {\rm W}^\pm {\rm H}$ production at hadron colliders through vector boson scattering is a so far unconsidered process, which leads to a clean signature of two same-sign charged leptons and two widely separated jets. This process is sensitive to the ${\rm HHH}$ and ${\rm WWHH}$ couplings and any deviation of these couplings from their SM predictions serves as direct evidence of new physics beyond the SM. In this paper we perform a Monte Carlo study of this process for the $\sqrt{s}=14$ TeV LHC and a $100$ TeV pp-collider, and provide projections of the constraints on the triple-Higgs and ${\rm WWHH}$ quartic couplings for these environments. In particular, we consider the impact of pileup on the expected sensitivity in this channel. Our analysis demonstrates that although the sensitivity to the ${\rm HHH}$ coupling is rather low, the ${\rm WWHH}$ coupling can be constrained in this channel within $\sim 100\%$ and $\sim 20\%$ at 95\% confidence level around the SM prediction at the HL-LHC and a 100 TeV pp-collider, respectively.

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arXiv2017

Getting Stuck: Using Monosignatures to Test Highly Ionizing Particles

In this paper we argue that monojet and monophoton searches can be a sensitive test of very highly ionizing particles such as particles with charges $\gtrsim 150e$ and more generally particles that do not reach the outer parts of the detector. 8 TeV monojet data from the CMS experiment excludes such objects with masses in the range $\lesssim 650~{\text{GeV}}$ and charges $\gtrsim 100e$. This nicely complements searches for highly ionizing objects at ALICE, ATLAS, CMS and LHCb. Expected improvements in these channels will extend the sensitivity range to $m\lesssim 750~{\text{GeV}}$. This search strategy can directly be generalized to other particles that strongly interact with the detector material, such as e.g. magnetic monopoles.

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arXiv2016

Results from TopFitter

We discuss a global fit of top quark BSM couplings, phrased in the model-independent language of higher-dimensional effective operators, to the currently available data from the LHC and Tevatron. We examine the interplay between inclusive and differential measurements, and the complementarity of LHC and Tevatron results. We conclude with a discussion of projections for improvement over LHC Run II.

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arXiv2016

Perturbative Higgs CP violation, unitarity and phenomenology

Perturbative probability conservation provides a strong constraint on the presence of new interactions of the Higgs boson. In this work we consider CP violating Higgs interactions in conjunction with unitarity constraints in the gauge-Higgs and fermion-Higgs sectors. Injecting signal strength measurements of the recently discovered Higgs boson allows us to make concrete and correlated predictions of how CP-violation in the Higgs sector can be directly constrained through collider searches for either characteristic new states or tell-tale enhancements in multi-Higgs processes.

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