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Andre Sopczak
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After the discovery of a Higgs boson, the measurements of its properties are at the forefront of research. The determination of the associated production of a Higgs boson and a pair of top quarks is of particular importance as the ttH Yukawa coupling is large and thus an excellent probe for physics beyond the Standard Model (SM). For the complete LHC Run-1 dataset, the combined ATLAS and CMS signal strength (defined as the ratio of measured cross-section to the SM prediction) was $mu=2.3^{+0.7}_{-0.6}$, which indicated a mild excess with respect to the SM expectation. The results of the ttH search with multilepton and diphoton signatures are presented for the first LHC Run-2 data (13 fb$^{-1}$ at 13 TeV) recorded by the ATLAS experiment: $mu=2.5^{+1.3}_{-1.1}$ for the multilepton analyses and $mu=-0.3^{+1.2}_{-1.0}$ for the diphoton analyses. Both measurements are in agreement with the SM expectation.
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We study the effects of top-Higgs anomalous coupling in the production of a pair of Higgs boson via gluon fusion at the Large Hadron Collider (LHC). The introduction of anomalous $ttH$ coupling can alter the hadronic double Higgs boson cross section and can lead to characteristic changes in certain kinematic distributions. We perform a global analysis based on available LHC data on the Higgs to constrain the parameters of $ttH$ anomalous coupling. Possible overlap of the predictions due to anomalous $ttH$ coupling with those due to anomalous trilinear Higgs coupling is also studied. We briefly discuss the effect of the anomalous $ttH$ coupling on the $HZ$ production via gluon fusion which is one of the main backgrounds in the $HH to gammagamma b {bar b}$ channel.
The signatures of dark matter at the LHC commonly involve, in simplified scenarios, the production of a single particle plus large missing energy, from the undetected dark matter. However, in $Z$-portal scenarios anomaly cancellation requires the presence of extra dark leptons in the dark sector. We investigate the signatures of the minimal scenarios of this kind, which involve cascade decays of the extra $Z$ boson into the dark leptons, identifying a four-lepton signal as the most promising one. We estimate the sensitivity to this signal at the LHC, the high-luminosity LHC upgrade, a possible high-energy upgrade, as well as a future circular collider. For $Z$ couplings compatible with current dijet constraints the multilepton signals can reach the $5sigma$ level already at Run 2 of the LHC. At future colliders, couplings two orders of magnitude smaller than the electroweak coupling can be probed with $5sigma$ sensitivity.
We first build a minimal model of vector-like quarks where the dominant Higgs boson production process at LHC -- the gluon fusion -- can be significantly suppressed, being motivated by the recent stringent constraints from the search for direct Higgs production over a wide Higgs mass range. Within this model, compatible with the present experimental constraints on direct Higgs searches, we demonstrate that the Higgs ($h$) production via a heavy vector-like top-partner ($t_2$) decay, $pp to t_2 bar t_2$, $t_2to t h$, allows to discover a Higgs boson at the LHC and measure its mass, through the decay channels $hto gammagamma$ or $hto ZZ$. We also comment on the recent hint in LHC data from a possible $sim 125$ GeV Higgs scalar, in the presence of heavy vector-like top quarks.
The ATLAS collaboration has recently reported a 2.6 sigma excess in the search for a heavy resonance decaying into a pair of weak gauge bosons. Only fully hadronic final states are being looked for in the analysis. If the observed excess really originates from the gauge bosons decays, other decay modes of the gauge bosons would inevitably leave a trace on other exotic searches. In this paper, we propose the use of the Z boson decay into a pair of neutrinos to test the excess. This decay leads to a very large missing energy and can be probed with conventional dark matter searches at the LHC. We discuss the current constraints from the dark matter searches and the prospects. We find that optimizing these searches may give a very robust probe of the resonance, even with the currently available data of the 8 TeV LHC.
The PADME experiment is searching for the Dark Photon $A$ in the $e^{+}e^{-} to gamma A$ process, assuming a $A$ decay into invisible particles. In extended Dark Sector models, a Dark Higgs $h$ can be produced alongside $A$ in the process $e^{+}e^{-} to h A$. If the $h$ mass is greater than twice the $A$ mass the final state will be composed by three $e^{+}e^{-}$ pairs. Such extremely rare process is explorable by the PADME experiment, which could get a first measure and impose limits on models of physics beyond the Standard Model.
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