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Michael Spannowsky

Publications and source records attributed to Michael Spannowsky.

At least 181 records · Page 10Linked to original sources

Higgsplosion: Solving the Hierarchy Problem via rapid decays of heavy states into multiple Higgs bosons

We introduce and discuss two inter-related mechanisms operative in the electroweak sector of the Standard Model at high energies. Higgsplosion, the first mechanism, occurs at some critical energy in the 25 to 10^3 TeV range, and leads to an exponentially growing decay rate of highly energetic particles into multiple Higgs bosons. We argue that this a well-controlled non-perturbative phenomenon in the Higgs-sector which involves the final state Higgs multiplicities n in the regime n λ>> 1 where λis the Higgs self-coupling. If this mechanism is realised in nature, the cross-sections for producing ultra-high multiplicities of Higgs bosons are likely to become observable and even dominant in this energy range. At the same time, however, the apparent exponential growth of these cross-sections at even higher energies will be tamed and automatically cut-off by a related Higgspersion mechanism. As a result, and in contrast to previous studies, multi-Higgs production does not violate perturbative unitarity. Building on this approach, we then argue that the effects of Higgsplosion alter quantum corrections from very heavy states to the Higgs boson mass. Above a certain energy, which is much smaller than their masses, these states would rapidly decay into multiple Higgs bosons. The heavy states become unrealised as they decay much faster than they are formed. The loop integrals contributing to the Higgs mass will be cut off not by the masses of the heavy states, but by the characteristic loop momenta where their decay widths become comparable to their masses. Hence, the cut-off scale would be many orders of magnitude lower than the heavy mass scales themselves, thus suppressing their quantum corrections to the Higgs boson mass.

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Neutrino Jets from High-Mass $W_R$ Gauge Bosons in TeV-Scale Left-Right Symmetric Models

We re-examine the discovery potential at hadron colliders of high-mass right-handed (RH) gauge bosons $W_R$ - an inherent ingredient of Left-Right Symmetric Models (LRSM). We focus on the regime where the $W_R$ is very heavy compared to the heavy Majorana neutrino $N$, and investigate an alternative signature for $W_R \rightarrow N$ decays. The produced neutrinos are highly boosted in this mass regime. Subsequently, their decays via off-shell $W_R$ bosons to jets, i.e., $N \rightarrow \ell^\pm j j$ are highly collimated, forming a single neutrino jet $(j_N)$. The final-state collider signature is then $\ell^\pm j_N$, instead of the widely studied $\ell^\pm\ell^\pm jj$. Present search strategies are not sensitive to this hierarchical mass regime due to the breakdown of the collider signature definition. We take into account QCD corrections beyond next-to-leading order (NLO) that are important for high-mass Drell-Yan processes at the 13 TeV Large Hadron Collider (LHC). For the first time, we evaluate $W_R$ production at NLO with threshold resummation at next-to-next-to-leading logarithm (NNLL) matched to the threshold-improved parton distributions. With these improvements, we find that a ${W_R}$ of mass $M_{W_R} = 3~(4)~[5]$ TeV and mass ratio of $(m_N/M_{W_R})<0.1$ can be discovered with a $5-6 σ$ statistical significance at 13 TeV after ${10~(100)~[2000]~\text{fb}^{-1}}$ of data. Extending the analysis to the hypothetical 100 TeV Very Large Hadron Collider (VLHC), $5σ$ can be obtained for $W_R$ masses up to $M_{W_R}=15~(30)$ with approximately $100~\text{fb}^{-1}~(10~\text{ab}^{-1})$.

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Type-II Seesaw Model and Multilepton Signatures at Hadron Colliders

We investigate multilepton signatures, arising from the decays of doubly charged and singly charged Higgs bosons in the Type-II Seesaw model. Depending on the vacuum expectation value of the triplet $v_Δ$, the doubly and singly charged Higgs bosons can decay into a large variety of multi-lepton final states. We explore all possible decay modes corresponding to different regimes of $v_Δ$, that generate distinguishing four and five leptonic signatures. We focus on the 13 TeV Large Hadron Collider (LHC) and further extend the study to a very high energy proton-proton collider (VLHC) with a center-of-mass energy of 100 TeV. We find that a doubly charged Higgs boson of masses around 375 GeV can be discovered at immediate LHC runs. A heavier mass of 630 GeV can instead be discovered at the high-luminosity run of the LHC or at the VLHC with 30 $\rm{fb}^{-1}$.

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Quark-Gluon tagging with Shower Deconstruction: Unearthing dark matter and Higgs couplings

The separation of quark and gluon initiated jets can be an important way to improve the sensitivity in searches for new physics or in measurements of Higgs boson properties. We present a simplified version of the shower deconstruction approach as a novel observable for quark-gluon tagging. Assuming topocluster-like objects as input, we compare our observable with energy correlation functions and find a favorable performance for a large variety of jet definitions. When applied to dark matter searches in mono-jet final states limitations from small signal-to-background ratios can be overcome. We also show that quark-gluon tagging is an alternative way of separating weak boson from gluon-fusion production in the process p + p -> H + jet + jet + X

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Invisible Decays in Higgs Pair Production

Observation of Higgs pair production is an important long term objective of the LHC physics program as it will shed light on the scalar potential of the Higgs field and the nature of electroweak symmetry breaking. While numerous studies have examined the impact of new physics on di-Higgs production, little attention has been given to the well-motivated possibility of exotic Higgs decays in this channel. Here we investigate the consequences of exotic invisible Higgs decays in di-Higgs production. We outline a search sensitive to such invisible decays in the $b\bar b+{\not \!\! E}_T$ channel. We demonstrate that probing invisible branching ratios of order 10$\%$ during the LHC's high-luminosity run will be challenging, but in resonance enhanced di-Higgs production, this final state can become crucial to establish the existence of physics beyond the Standard Model at collider energies. We also briefly discuss the outlook for other exotic Higgs decay modes and the potential to observe such exotic decays in the di-Higgs channel.

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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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Finding top quarks with shower deconstruction

We develop a new method for tagging jets produced by hadronically decaying top quarks. The method is an application of shower deconstruction, a maximum information approach that was previously applied to identifying jets produced by Higgs bosons that decay to bottom quarks. We tag an observed jet as a top jet based on a cut on a calculated variable that is an approximation to the ratio of the likelihood that a top jet would have the structure of the observed jet to the likelihood that a non-top QCD jet would have this structure. We find that the shower deconstruction based tagger can perform better in discriminating boosted top quark jets from QCD jets than other publicly available tagging algorithms.

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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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Hearing the signals of dark sectors with gravitational wave detectors

Motivated by aLIGO's recent discovery of gravitational waves we discuss signatures of new physics that could be seen at ground and space-based interferometers. We show that a first order phase transition in a dark sector would lead to a detectable gravitational wave signal at future experiments, if the phase transition has occurred at temperatures few orders of magnitude higher than the electroweak scale. The source of gravitational waves in this case is associated with the dynamics of expanding and colliding bubbles in the early universe. At the same time we point out that topological defects, such as dark sector domain walls, may generate a detectable signal already at aLIGO. Both -- bubble and domain wall -- scenarios are sourced by semi-classical configurations of a dark new physics sector. In the first case the gravitational wave signal originates from bubble wall collisions and subsequent turbulence in hot plasma in the early universe, while the second case corresponds to domain walls passing through the interferometer at present and is not related to gravitational waves. We find that aLIGO at its current sensitivity can detect smoking-gun signatures from domain wall interactions, while future proposed experiments including the fifth phase of aLIGO at design sensitivity can probe dark sector phase transitions.

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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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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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Measuring rare and exclusive Higgs boson decays into light resonances

We evaluate the LHC's potential of observing Higgs boson decays into light elementary or composite resonances through their hadronic decay channels. We focus on the Higgs boson production processes with the largest cross sections, $pp\to h$ and $pp\to h+\mathrm{jet}$, with subsequent decays $h \to ZA$ or $h\to Z\,η_c$, and comment on the production process $pp\to hZ$. By exploiting track-based jet substructure observables and extrapolating to $3000~\mathrm{fb}^{-1}$ we find ${\cal BR}(h \to ZA) \simeq {\cal BR}(h \to Z η_c) \lesssim 0.02$ at 95% CL. We interpret this limit in terms of the 2HDM Type 1. We find that searches for $h\to ZA$ are complementary to existing measurements and can constrain large parts of the currently allowed parameter space.

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Searching for supersymmetry scalelessly

In this paper we propose a scale invariant search strategy for hadronic top or bottom plus missing energy final states. We present a method which shows flat efficiencies and background rejection factors over broad ranges of parameters and masses. The resulting search can be easily recast into a limit on alternative models. We show the strength of the method in a natural SUSY setup where stop and sbottom squarks are pair produced and decay into hadronically decaying top quarks or bottom quarks and higgsinos.

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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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The Lepton Flavour Violating Higgs Decays at the HL-LHC and the ILC

Run-I results from the CMS collaboration show an excess of events in the decay $h \to μτ_e$ with a local significances of 2.4$σ$. This could be the first hint of flavour violation in the Higgs sector. We summarise the bounds on the flavour violating Yukawa couplings from direct searches, low energy measurements and projected future experiments. We discuss the sensitivity of upcoming HL-LHC runs and future lepton colliders in measuring lepton-flavour violating couplings using an effective field theory framework. For the HL-LHC we find limits on BR($h\to μτ$) and BR$(h\to e τ) \lesssim \mathcal{O}(0.5)\%$ and on BR($h \to e μ) \lesssim \mathcal{O}(0.02)\%$. For an ILC with center-of-mass energy of 1 TeV we expect BR($h \to e τ$) and BR($h\to μτ$) to be measurable down to $\mathcal{O}(0.2)\%$.

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Determining the Quantum Numbers of Simplified Models in $t\bar{t}X$ production at the LHC

Simplified models provide an avenue for characterising and exploring New Physics for large classes of UV theories. In this article we study the ability of the LHC to probe the spin and parity quantum numbers of a new light resonance $X$ which couples predominantly to the third generation quarks in a variety of simplified models through the $t\bar t X$ channel. After evaluating the LHC discovery potential for $X$, we suggest several kinematic variables sensitive to the spin and CP properties of the new resonance. We show how an analysis exploiting differential distributions in the semi-leptonic channel can discriminate among various possibilities. We find that the potential to discriminate a scalar from a pseudoscalar or (axial) vector to be particularly promising.

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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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Cornering diphoton resonance models at the LHC

We explore the ability of the high luminosity LHC to test models which can explain the 750 GeV diphoton excess. We focus on a wide class of models where a 750 GeV singlet scalar couples to Standard Model gauge bosons and quarks, as well as dark matter. Including both gluon and photon fusion production mechanisms, we show that LHC searches in channels correlated with the diphoton signal will be able to probe wide classes of diphoton models with $\mathcal{L} \sim 3000\, \text{fb}^{-1}$ of data. Furthermore, models in which the scalar is a portal to the dark sector can be cornered with as little as $\mathcal{L} \sim 30\, \text{fb}^{-1}$.

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