arXiv subjects

Michael Spannowsky

Publications and source records attributed to Michael Spannowsky.

At least 145 records · Page 8Linked to original sources

arXiv2019

Revisiting the $t\bar{t}hh$ channel at the FCC-hh

The exploration of the scalar sector of the Standard Model is at the core of current and future science programs at collider experiments, with increasing focus on the self-interaction of the Higgs boson. This important parameter of the Higgs sector can be measured in various channels, among the production of a Higgs boson associated with a top-quark pair, $\bar{t}thh$. In this paper we study this channel and its potential to measure or constrain the self-coupling and possible new physics contributions at a future 100 TeV proton-proton collider. Analysing this highly complex final state adds to the sensitivity for enhanced self-coupling interactions, and we argue that a measurement of this process is a necessity to constrain blind directions in the multi-dimensional parameter space of well-motivated new physics scenarios.

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arXiv2019

Adversarially-trained autoencoders for robust unsupervised new physics searches

Machine learning techniques in particle physics are most powerful when they are trained directly on data, to avoid sensitivity to theoretical uncertainties or an underlying bias on the expected signal. To be able to train on data in searches for new physics, anomaly detection methods are imperative, which can be realised by an autoencoder acting as an unsupervised classifier. The last source of uncertainties affecting the classifier are then experimental uncertainties in the reconstruction of the final-state objects. To mitigate their effect on the classifier and to allow for a realistic assessment of the method, we propose to combine the autoencoder with an adversarial neural network to remove its sensitivity to the smearing of the final-state objects. We quantify its effect and show that one can achieve a robust anomaly detection in resonance-induced $t\bar{t}$ final states.

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arXiv2019

Towards a generic implementation of matrix-element maximisation as a classifier in particle physics

The so-called matrix-element method (MEM) has long been used successfully as a classification tool in particle physics searches. In the presence of invisible final state particles, the traditional MEM typically assigns probabilities to an event -- based on whether it is more signal or background-like -- through a phase space integration over all degrees of freedom of the invisible particles in the process(es). One inherent shortcoming of the traditional MEM is that the phase space integration can be slow, and therefore impractical for high multiplicity final states and/or large data sets. The recent alternative of matrix-element maximisation has recently been introduced to circumvent this problem, since maximising a highly-dimensional function can be a far more CPU-efficient task than that of integration. In this work, matrix-element maximisation is applied to the process of fully-leptonic top associated Higgs production, where the Higgs boson decays to two $b$-quarks. A variety of optimisation algorithms are tested in terms of their performance and speed, and it is explicitly found that the maximisation technique is far more CPU-efficient than the traditional MEM at the cost of a slight reduction in performance. An interesting consequence of using matrix-element maximisation is that the result of the procedure gives an estimate of the four-momenta for the invisible particles in the event. As a result, the idea of using these estimates as input information for more complicated tools is discussed with potential prospects for future developments of the method.

hep-ph↗

arXiv2019

Novel $B$-decay signatures of light scalars at high energy facilities

We study the phenomenology of light scalars of masses $m_1$ and $m_2$ coupling to heavy flavour-violating vector bosons of mass $m_V$. For $m_{1,2}\lesssim $ few GeV, this scenario triggers the rare $B$ meson decays $B_s^0\to 3μ^+ 3μ^-$, $B^0\to 3μ^+ 3μ^-$, $B^+\to K^+ 3μ^+ 3μ^-$ and $B_s^0\to K^{0*} 3μ^+ 3μ^-$; the last two being the most important ones for $m_1\sim m_2$. None of these signals has been studied experimentally; therefore we propose analyses to test these channels at the LHCb. We demonstrate that the reach of this facility extends to branching ratios as small as $6.0\times 10^{-9}$, $1.6\times 10^{-9}$, $5.9\times 10^{-9}$ and $1.8\times 10^{-8}$ for the aforementioned channels, respectively. For $m_{1,2}\gg \mathcal{O}(1)$ GeV, we show that slightly modified versions of current multilepton and multitau searches at the LHC can probe wide regions of the parameter space of this scenario. Altogether, the potential of the searches we propose outperform other constraints such as those from meson mixing.

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arXiv2019

Solving differential equations with neural networks: Applications to the calculation of cosmological phase transitions

Starting from the observation that artificial neural networks are uniquely suited to solving optimisation problems, and most physics problems can be cast as an optimisation task, we introduce a novel way of finding a numerical solution to wide classes of differential equations. We find our approach to be very flexible and stable without relying on trial solutions, and applicable to ordinary, partial and coupled differential equations. We apply our method to the calculation of tunnelling profiles for cosmological phase transitions, which is a problem of relevance for baryogenesis and stochastic gravitational wave spectra. Comparing our solutions with publicly available codes which use numerical methods optimised for the calculation of tunnelling profiles, we find our approach to provide at least as accurate results as these dedicated differential equation solvers, and for some parameter choices even more accurate and reliable solutions. In particular, we compare the neural network approach with two publicly available profile solvers, \texttt{CosmoTransitions} and \texttt{BubbleProfiler}, and give explicit examples where the neural network approach finds the correct solution while dedicated solvers do not. We point out that this approach of using artificial neural networks to solve equations is viable for any problem that can be cast into the form $\mathcal{F}(\vec{x})=0$, and is thus applicable to various other problems in perturbative and non-perturbative quantum field theory.

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arXiv2019

Approaching robust EFT limits for CP-violation in the Higgs sector

Constraining CP-violating interactions in effective field theory (EFT) of dimension six faces two challenges. Firstly, degeneracies in the multi-dimensional space of Wilson coefficients have to be lifted. Secondly, quadratic contributions of CP-odd dimension six operators are difficult to disentangle from squared contributions of CP-even dimension six operators and from linear contributions of dimension eight operators. Both of these problems are present when new sources of CP-violation are present in the interactions between the Higgs boson and heavy strongly-interacting fermions. We show that degeneracies in the Wilson coefficients can be removed by combining measurements of Higgs-plus-two-jet production via gluon fusion with measurements of top-pair associated Higgs production. In addition, we demonstrate that the sensitivity of the analysis can be improved by exploiting the top-quark threshold in the gluon fusion process. Finally, we substantiate a perturbative argument about the validity of EFT by comparing the quadratic and linear contributions from CP-odd dimension six operators and use this to show explicitly that high statistics measurements at future colliders enable the extraction of perturbatively robust constraints on the associated Wilson coefficients.

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arXiv2019

A fresh look at ALP searches in fixed target experiments

A significant number of high power proton beams are available or will go online in the near future. This provides exciting opportunities for new fixed target experiments and the search for new physics in particular. In this note we will survey these beams and consider their potential to discover new physics in the form of axion-like particles, identifying promising locations and set ups. To achieve this, we present a significantly improved calculation of the production of axion-like particles in the coherent scattering of protons on nuclei, valid for lower ALP masses and/or beam energies. We also provide a new publicly available tool for this process: the Alpaca Monte Carlo generator. This will impact ongoing and planned searches based on this process.

hep-ph↗

arXiv2019

Precision measurements for the Higgsploding Standard Model

Higgsplosion is the mechanism that leads to exponentially growing decay rates of highly energetic particles into states with very high numbers of relatively soft Higgs bosons. In this paper we study quantum effects in the presence of Higgsplosion. First, we provide a non-perturbative definition of Higgsplosion as a resolved short-distance singularity of quantum propagators at distances shorter than the inverse Higgsplosion energy scale, $E_*$. We then consider quantum effects arising from loops in perturbation theory with these propagators on internal lines. When the loop momenta exceed the Higgsplosion scale $E_*$, the theory dynamics deviates from what is expected in the standard QFT settings without Higgsplosion. The UV divergences are automatically regulated by the Higgsplosion scale, leading to the change of slopes for the running couplings at the RG scales $μ> E_*$. Thus, the theory becomes asymptotically safe. Further, we find that the finite parts are also modified and receive power-suppressed corrections in $1/E_*^2$. We use these results to compute a set of precision observables for the Higgsploding Standard Model. These and other precision observables could provide experimental evidence and tests for the existence of Higgsplosion in particle physics.

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arXiv2019

Constraining four-fermion operators using rare top decays

New physics can manifest itself by an appreciable increase of the decay rate of top quarks in rare flavour-changing final states. Exploiting the large top quark production rate at the LHC, we bound four-fermion operators contributing to non-resonant $t\rightarrow \ell^+\ell^- j$ using different signal regions of the latest LHC searches for $t\rightarrow Zj$. We also provide prospects for the high-luminosity LHC to test these as well as four-fermion operators contributing to $t\rightarrow b\overline{b}j$, based on improved analysis strategies of existing searches. We single out all weakly-coupled ultraviolet completions inducing such contact interactions at tree level and translate the previous bounds to the parameter space of specific complete models. Being above the TeV, LHC bounds from rare top decays improve over those from flavour physics, electroweak precision data and other LHC searches in several cases.

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arXiv2019

Angles on CP-violation in Higgs boson interactions

CP-violation in the Higgs sector remains a possible source of the baryon asymmetry of the universe. Recent differential measurements of signed angular distributions in Higgs boson production provide a general experimental probe of the CP structure of Higgs boson interactions. We interpret these measurements using the Standard Model Effective Field Theory and show that they do not distinguish the various CP-violating operators that couple the Higgs and gauge fields. However, the constraints can be sharpened by measuring additional CP-sensitive observables and exploiting phase-space-dependent effects. Using these observables, we demonstrate that perturbatively meaningful constraints on CP-violating operators can be obtained at the LHC with luminosities of ${\cal{O}}$(100/fb). Our results provide a roadmap to a global Higgs boson coupling analysis that includes CP-violating effects.

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arXiv2019

Gravitational wave and collider probes of a triplet Higgs sector with a low cutoff

We study the scalar triplet extension of the standard model with a low cutoff, preventing large corrections to the quadratic masses that would otherwise worsen the hierarchy problem. We explore the reach of LISA to test the parameter space region of the scalar potential (not yet excluded by Higgs to diphoton measurements) in which the electroweak phase transition is strongly first-order and produces sizeable gravitational waves. We also demonstrate that the collider phenomenology of the model is drastically different from its renormalizable counterpart. We study the reach of the LHC in ongoing searches and project bounds for the HL-LHC. Likewise, we develop a dedicated analysis to test the key but still unexplored signature of pair-production of charged scalars decaying to third-generation quarks: $pp\rightarrow t\overline{b} (\overline{t}b), b\overline{b}$. These results apply straightforwardly to other extensions of the Higgs sector such as the 2HDM/MSSM.

hep-ph↗

arXiv2019

Searching new physics in rare $B$-meson decays into multiple muons

New heavy vector bosons and light scalars are predicted in a plethora of models of new physics. In particular, in new strongly interacting sectors they play the role of the $ρ$ and $π$ mesons in QCD. We show that some of their interactions, for example those required for the explanation of the $B$ anomalies and the $g-2$ of the muon, can be only probed in $B$ meson decays. We highlight new golden channels not yet studied experimentally, including $B^+ \rightarrow K^+(D^+)μ^+μ^-μ^+μ^-$ and $B^0\rightarrow K^{*0}μ^+μ^-μ^+μ^-$. Relying on generator level simulations for data taking with the LHCb detector, we determine the reach of this facility to the aforementioned processes. We show that branching ratios as small as $9\times 10^{-12}$ ($3.2\times 10^{-10}$) and $2.7\times 10^{-11}$ can be tested at the $95\, \%$ CL respectively.

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arXiv2019

HYTREES: Combining Matrix Elements and Parton Shower for Hypothesis Testing

We present a new way of performing hypothesis tests on scattering data, by means of a perturbatively calculable classifier. This classifier exploits the "history tree" of how the measured data point might have evolved out of any simpler (reconstructed) points along classical paths, while explicitly keeping quantum-mechanical interference effects by copiously employing complete leading-order matrix elements. This approach extends the standard Matrix Element Method to an arbitrary number of final state objects and to exclusive final states where reconstructed objects can be collinear or soft. We have implemented this method into the standalone package HYTREES and have applied it to Higgs boson production in association with two jets, with subsequent decay into photons. HYTREES allows to construct an optimal classifier to discriminate this process from large Standard Model backgrounds. It further allows to find the most sensitive kinematic regions that contribute to the classification.

hep-ph↗

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

Fifth forces, Higgs portals and broken scale invariance

We study the relationship between the strength of fifth forces and the origin of scale breaking in the Standard Model (SM) of particle physics. We start with a light scalar field that is conformally coupled to a toy SM matter sector through a Weyl rescaling of the metric. After appropriately normalizing the fields, the conformally coupled scalar only interacts directly with the would-be Higgs field through kinetic-mixing and Higgs-portal terms. Thus, for the first time, we describe the equivalence of conformally coupled scalar-tensor modifications of gravity and Higgs-portal theories, and we find that the usual tree-level fifth forces only emerge if there is mass mixing between the conformally coupled scalar and the Higgs field. The strength of the fifth force, mediated by the light scalar, then depends on whether the mass of the Higgs arises from an explicit symmetry-breaking term or a spontaneous mechanism of scale breaking. Solar System tests of gravity and the non-observation of fifth forces therefore have the potential to provide information about the structure of the Higgs sector and the origin of its symmetry breaking, setting an upper bound on the magnitude of any explicit scale-breaking terms. These results demonstrate the phenomenological importance (both for cosmology and high-energy physics) of considering how scalar-tensor modifications of gravity are embedded within extensions of the SM.

hep-th↗

arXiv2018

Consistency of Higgsplosion in Localizable QFT

We show that large n-particle production rates derived in the semiclassical Higgsplosion limit of scalar field theoretical models with spontaneous symmetry breaking, are consistent with general principles of localizable quantum field theory. The strict localizability criterium of Jaffe defines quantum fields as operator-valued distributions acting on test functions that are localized in finite regions of space-time. The requirement of finite support of test functions in space-time ensures the causality property of QFT. The corresponding localizable fields need not be tempered distributions, and they fit well into the framework of local quantum field theory.

hep-ph↗

arXiv2018

Reconstructing the invisible with matrix elements

We propose a fully flexible method to perform an hypothesis test between signal and background based on the Matrix Element Method in the presence of multiple invisible particles. The proposed method performs a mapping of the measured final state onto its minimal hypersurface of degrees of freedom for a given process and then maximises the matrix element on this hypersurface separately for signal and background. To show how performant the method is in separating signal from background, we apply it to the prominent partly invisible decay of a Higgs boson into a muon-antimuon pair and two muon-neutrinos via two W bosons.

hep-ph↗

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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