arXiv subjects

Christoph Englert

Publications and source records attributed to Christoph Englert.

At least 127 records · Page 7Linked to original sources

arXiv2014

Limitations and Opportunities of Off-Shell Coupling Measurements

Indirect constraints on the total Higgs width $Γ_h$ from correlating Higgs signal strengths with cross section measurements in the off-shell region for $p(g)p(g)\to 4\ell$ production have received considerable attention recently, and the CMS collaboration have published a first measurement. We revisit this analysis from a new physics and unitarity constraints perspective and conclude that limits on $Γ_h$ obtained in this fashion are not reliable unless we make model-specific assumptions, which cannot be justified at the current stage of the LHC programme. Relaxing the $Γ_h$ interpretation, we discuss the merits of high invariant mass cross section measurements in the context of Higgs CP analyses, higher dimensional operator testing, and resolved new physics in the light of electroweak precision constraints beyond effective theory limitations. Furthermore, we show that a rather model-independent LHC constraint can be obtained from adapting the $gg\to 4\ell$ analysis to the weak boson fusion channels at lower statistical yield.

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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

Bounding the top Yukawa with Higgs-associated single-top production

After the discovery of the 125 GeV scalar boson with gauge properties similar to the Standard Model Higgs, the search for beyond the SM interactions will focus on studying the discovered particles' coupling properties more precisely and shedding light on the relation of fermion masses with the electroweak vacuum. The large mass of the top quark and the SM-predicted order one top Yukawa coupling is a natural candidate for BSM physics, though experimentally challenging to constrain. In this paper, we argue that investigating angular correlations in $pp\to tHj$ production provides an excellent handle to constrain the top Yukawa coupling $y_t$ via direct measurements, even when we focus on rare exclusive final states. We perform a hadron-level analysis and show that we may expect to constrain $y_t\gtrsim 0.5\, y_t^{\text{SM}}$ at 95%-99% confidence level at the high luminosity LHC using semi-leptonic top decays and $H\to γγ$ alone, by employing a two-channel measurement approach.

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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

Further on up the road: $hhjj$ production at the LHC

A measurement of the $hh+2j$ channel at the LHC would be particularly thrilling for electroweak physics. It is not only the leading process which is sensitive to the $W^+W^- hh$ and $ZZhh$ interactions, but also provides a potentially clear window to study the electroweak symmetry-breaking sector by probing Higgs-Goldstone interactions through the weak boson fusion component of the scattering process. Until now, a phenomenologically complete analysis of this channel has been missing. This is mostly due to the high complexity of the involved one-loop gluon fusion contribution and the fact that a reliable estimate thereof cannot be obtained through simplified calculations in the $m_t\to \infty$ limit. In particular, the extraction of the Higgs trilinear coupling from this final state rests on a delicate $m_t$-dependent interference pattern which is not captured in an effective field theory approach. In this paper, we report on the LHC's potential to access di-Higgs production in association with two jets in a fully-showered hadron-level analysis. Our study includes the finite top and bottom mass dependencies for the gluon fusion contribution. On the basis of these results, we also comment on the potential sensitivity to the trilinear Higgs and $VV^\dagger hh$ ($V=W^\pm,Z$) couplings that can be expected from measurements of this final state.

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arXiv2013

Di-Higgs final states augMT2ed -- selecting $hh$ events at the high luminosity LHC

Higgs boson self-interactions can be investigated via di-Higgs ($pp\to hh+X$) production at the LHC. With a small ${\cal{O}}(30)$ fb Standard Model production cross section, and a large $t\bar t$ background, this measurement has been considered challenging, even at a luminosity-upgraded LHC. We demonstrate that by using simple kinematic bounding variables, of the sort already employed in existing LHC searches, the dominant $t\bar t$ background can be largely eliminated. Simulations of the signal and the dominant background demonstrate the prospect for measurement of the di-Higgs production cross section at the 30% level using 3/ab of integrated luminosity at a high-luminosity LHC. This corresponds to a Higgs self-coupling determination with 60% accuracy in the $b\bar b τ^+τ^-$ mode, with potential for further improvements from e.g. subjet technologies and from additional di-Higgs decay channels.

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arXiv2013

Pinning down Higgs triplets at the LHC

Extensions of the Standard Model Higgs sector involving weak isotriplet scalars are not only benchmark candidates to reconcile observed anomalies of the recently discovered Higgs-like particle, but also exhibit a vast parameter space, for which the lightest Higgs' phenomenology turns out to be very similar to the Standard Model one. A generic prediction of this model class is the appearance of exotic doubly charged scalar particles. In this paper we adapt existing dilepton+missing energy+jets measurements in the context of SUSY searches to the dominant decay mode $H^{\pm\pm}\to W^\pm W^\pm$ and find that the LHC already starts probing the model's parameter space. A simple modification towards signatures typical of weak boson fusion searches allows us to formulate even tighter constraints with the 7 TeV LHC data set. A corresponding analysis of this channel performed at 14 TeV center of mass energy will constrain the model over the entire parameter space and facilitate potential $H^{\pm\pm}\to W^\pm W^\pm$ discoveries.

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arXiv2013

Di-boson and Tri-boson production at the LHC

The status of di-boson and tri-boson production is shortly review. Using the VBFNLO and the LOOPSIM package, approximated results at NNLO QCD are given for WZ production. Results for waa + jet at NLO QCD are also shown.

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arXiv2013

Triplet Higgs boson collider phenomenology after the LHC

ATLAS and CMS have discovered a Standard Model Higgs-like particle. One of the main discovery channels is the Higgs decay to two photons, which, at the moment, seems to be considerably enhanced over the Standard Model expectation. Models with additional charged matter coupling to the Higgs sector can enhance or decrease the Higgs to two photons branching ratio. We take this as motivation to confront the so-called Georgi-Machacek model of Higgs triplets with the results of recent searches for a SM Higgs boson performed at the LHC. We also investigate the model in regions of the allowed parameter space relevant for a SM-like phenomenology. The Georgi-Machacek model avoids tree-level issues of the T parameter, while offering a vastly modified Higgs phenomenology compared to the Standard Model. This comes at the price of introducing another fine-tuning problem related to electroweak precision measurements. We investigate the collider phenomenology of the Georgi-Machacek model in the light of existing collider constraints beyond any effective field theory approximation and contextualize our findings with electroweak precision constraints.

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arXiv2013

A New Probe of Naturalness

Any new scalar fields that perturbatively solve the hierarchy problem by stabilizing the Higgs mass also generate new contributions to the Higgs field-strength renormalization, irrespective of their gauge representation. These new contributions are physical and their magnitude can be inferred from the requirement of quadratic divergence cancellation, hence they are directly related to the resolution of the hierarchy problem. Upon canonically normalizing the Higgs field these new contributions lead to modifications of Higgs couplings which are typically great enough that the hierarchy problem and the concept of electroweak naturalness can be probed thoroughly within a precision Higgs program. Specifically, at a Linear Collider this can be achieved through precision measurements of the Higgs associated production cross-section. This would lead to indirect constraints on perturbative solutions to the hierarchy problem in the broadest sense, even if the relevant new fields are gauge singlets.

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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

Constraining the Intrinsic Structure of Top-Quarks

The basic structure of top-quarks as spin-1/2 particles is characterized by the radius $R_t$ and the intrinsic magnetic dipole moment $κ_t$, both individually associated with gauge interactions. They are predicted to be zero in pointlike theories as the Standard Model. We derive upper limits of these parameters in the color sector from cross sections measured at Tevatron and LHC in top-pair production $p{\bar{p}}/pp \to t{\bar{t}}$, and we predict improved limits expected from LHC in the future, especially for analyses exploiting boosted top final states. An additional method for measuring the intrinsic parameters is based on $t \bar{t} + jet$ final states.

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arXiv2013

Modified Higgs Sectors and NLO Associated Production

Many beyond the Standard Model (BSM) scenarios involve Higgs couplings to additional electroweak fields. It is well established that these new fields may modify Higgs gamma-gamma and gamma-Z decays at one-loop. However, one unexplored aspect of such scenarios is that by electroweak symmetry one should also expect modifications to the Higgs Z-Z coupling at one-loop and, more generally, modifications to Higgs production and decay channels beyond tree-level. In this paper we investigate the full BSM modified electroweak corrections to associated Higgs production at both the LHC and a future lepton collider in two simple SM extensions. From both inclusive and differential NLO associated production cross sections we find BSM-NLO corrections can be as large as O(>10%) when compared to the SM expectation, consistent with other precision electroweak measurements, even in scenarios where modifications to the Higgs diphoton rate are not significant. At the LHC such corrections are comparable to the involved QCD uncertainties. At a lepton collider the Higgs associated production cross section can be measured to high accuracy (O(1%) independent of uncertainties in total width and other couplings), and such a deviation could be easily observed even if the new states remain beyond kinematic reach. This should be compared to the expected accuracy for a model-independent determination of the Higgs diphoton coupling at a lepton collider, which is O(15%). This work demonstrates that precision measurements of the Higgs associated production cross section constitute a powerful probe of modified Higgs sectors and will be valuable for indirectly exploring BSM scenarios.

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arXiv2013

Emergence of the Electroweak Scale through the Higgs Portal

Having discovered a candidate for the final piece of the Standard Model, the Higgs boson, the question remains why its vacuum expectation value and its mass are so much smaller than the Planck scale (or any other high scale of new physics). One elegant solution was provided by Coleman and Weinberg, where all mass scales are generated from dimensionless coupling constants via dimensional transmutation. However, the original Coleman-Weinberg scenario predicts a Higgs mass which is too light; it is parametrically suppressed compared to the mass of the vectors bosons, and hence is much lighter than the observed value. In this paper we argue that a mass scale, generated via the Coleman-Weinberg mechanism in a hidden sector and then transmitted to the Standard Model through a Higgs portal, can naturally explain the smallness of the electroweak scale compared to the UV cutoff scale, and at the same time be consistent with the observed value. We analyse the phenomenology of such a model in the context of present and future colliders and low energy measurements.

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arXiv2012

Partially (in)visible Higgs decays at the LHC

Both Atlas and CMS have reported a discovery of a Standard Model-like Higgs boson $H$ of mass around 125 GeV. Consistency with the Standard Model implies the non-observation of non-SM like decay modes of the newly discovered particle. Sensitivity to such decay modes, especially when they involve partially invisible final states is currently beyond scrutiny of the LHC. We systematically study such decay channels in the form of $H\rightarrow AA\rightarrow jets+missing energy$, with $A$ a light scalar or scalar, and analyze to what extent these exotic branching fractions can be constrained by direct measurements at the LHC. While the analysis is challenging, constraints as good as ${BR}\lesssim 10%$ can be obtained.

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arXiv2012

Polishing a shiny Higgs with matrix elements

After the recent discovery of a Standard Model Higgs boson-like particle at the LHC, the question of its couplings to known and unknown matter is eminent. In this letter, we present a method that allows for an enhancement in S/B(irreducible) of the order of 100% in $pp\to (h\to γγ) jj$ for a center of mass energy of 8 and 14 TeV. This is achieved by applying the matrix element method. We discuss the implications of detector resolution effects and various approximations of the involved event simulation and reconstruction. The matrix element method provides a reliable, stable, and efficient handle to separate signal from background, and the gluon and weak boson fusion components involved in this process. Employing this method, a more precise Higgs boson coupling extraction can be obtained, and our results are of immediate relevance for current searches.

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arXiv2012

New Physics in LHC Higgs boson pair production

Multi-Higgs production provides a phenomenologically clear window to the electroweak symmetry breaking sector. We perform a comprehensive and comparative analysis of new electroweak physics effects in di-Higgs and di-Higgs+jet production. In particular, we discuss resonant di-Higgs phenomenology, which arises in the Higgs portal model and in the MSSM at small tan beta, and non-resonant new physics contributions to di-Higgs production in models where the newly discovered Higgs candidate is interpreted as a pseudo-Nambu-Goldstone boson. We show that, for all these scenarios, a measurement of the di-Higgs and di-Higgs+jet final states provides an accessible and elaborate handle to understand electroweak symmetry breaking in great detail.

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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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