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We present a new strategy to uncover light, quasi-degenerate Higgsinos, a likely ingredient in a natural supersymmetric model. Our strategy focuses on Higgsinos with inter-state splittings of O(5-50) GeV that are produced in association with a hard, initial state jet and decay via off-shell gauge bosons to two or more leptons and missing energy, $pp to j + text{MET}, + 2^+, ell$. The additional jet is used for triggering, allowing us to significantly loosen the lepton requirements and gain sensitivity to small inter-Higgsino splittings. Focusing on the two-lepton signal, we find the seemingly large backgrounds from diboson plus jet, $bar tt$ and $Z/gamma^* + j$ can be reduced with careful cuts, and that fake backgrounds appear minor. For Higgsino masses $m_{chi}$ just above the current LEP II bound ($mu simeq 110,$) GeV we find the significance can be as high as 3 sigma at the LHC using the existing 20 fb$^{-1}$ of 8 TeV data. Extrapolating to LHC at 14 TeV with 100 fb$^{-1}$ data, and as one example $M_1 = M_2 = 500$ GeV, we find 5 sigma evidence for $m_{chi} lesssim, 140,$ GeV and 2 sigma evidence for $m_{chi} lesssim, 200,$ GeV . We also present a reinterpretation of ATLAS/CMS monojet bounds in terms of degenerate Higgsino ($delta m_{chi} ll 5,$) GeV plus jet production. We find the current monojet bounds on $m_{chi}$ are no better than the chargino bounds from LEP II.
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The presence of two light higgsinos nearly degenerate in mass is one of the important characteristics of suspersymmetric models meeting the naturalness criteria. Probing such higgsinos at the LHC is very challenging, in particular when the mass-splitting between them is less than 5 GeV. In this study, we analyze such a degenerate higgsino scenario by exploiting the high collinearity between the two muons which originate from the decay of the heavier higgsino into the lighter one and which are accompanied by a high-$p_T$ QCD jet. Using our method, we can achieve a statistical significance $sim 2.9,sigma$ as well as a $S/B sim 17%$ with an integrated luminosity of 3000 fb$^{-1}$ at the 14 TeV LHC, for the pair production of higgsinos with masses 124 GeV and 120 GeV. A good sensitivity can be achieved even for a smaller mass-splitting when the higgsinos are lighter.
We discuss a novel signature of dark matter production at the LHC resulting from the emission of an additional Higgs boson in the dark sector. The presence of such a dark Higgs boson is motivated simultaneously by the need to generate the masses of the particles in the dark sector and the possibility to relax constraints from the dark matter relic abundance by opening up a new annihilation channel. If the dark Higgs boson decays into Standard Model states via a small mixing with the Standard Model Higgs boson, one obtains characteristic large-radius jets in association with missing transverse momentum that can be used to efficiently discriminate signal from backgrounds. We present the sensitivities achievable in LHC searches for dark Higgs bosons with already collected data and demonstrate that such searches can probe large regions of parameter space that are inaccessible to conventional mono-jet or di-jet searches.
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The LHC search strategies for leptoquarks that couple dominantly to a top quark are different than for the ones that couple mostly to the light quarks. We consider charge $1/3$ ($phi_1$) and $5/3$ ($phi_5$) scalar leptoquarks that can decay to a top quark and a charged lepton ($tell$) giving rise to a resonance system of a boosted top and a high-$p_{rm T}$ lepton. We introduce simple phenomenological models suitable for bottom-up studies and explicitly map them to all possible scalar leptoquark models within the Buchm{u}ller-R{u}ckl-Wyler classifications that can have the desired decays. We study pair and single productions of these leptoquarks. Contrary to the common perception, we find that the single production of top-philic leptoquarks $phi = {phi_1,phi_5}$ in association with a lepton and jets could be significant for order one $phi tell$ coupling in certain scenarios. We propose a strategy of selecting events with at least one hadronic-top and two high-$p_{rm T}$ same flavour opposite sign leptons. This captures events from both pair and single productions. Our strategy can significantly enhance the LHC discovery potential especially in the high-mass region where single productions become more prominent. Our estimation shows that a scalar leptoquark as heavy as $sim1.7$ TeV can be discovered at the $14$ TeV LHC with 3 ab$^{-1}$ of integrated luminosity in the $tellell+X$ channel for $100%$ branching ratio in the $phito tell $ decay mode. However, in some scenarios, the discovery reach can increase beyond $2$ TeV even though the branching ratio comes down to about $50%$.
New physics close to the electroweak scale is well motivated by a number of theoretical arguments. However, colliders, most notably the Large Hadron Collider (LHC), have failed to deliver evidence for physics beyond the Standard Model. One possibility for how new electroweak-scale particles could have evaded detection so far is if they carry only electroweak charge, i.e. are color neutral. Future $e^+e^-$ colliders are prime tools to study such new physics. Here, we investigate the sensitivity of $e^+e^-$ colliders to scalar partners of the charged leptons, known as sleptons in supersymmetric extensions of the Standard Model. In order to allow such scalar lepton partners to decay, we consider models with an additional neutral fermion, which in supersymmetric models corresponds to a neutralino. We demonstrate that future $e^+e^-$ colliders would be able to probe most of the kinematically accessible parameter space, i.e. where the mass of the scalar lepton partner is less than half of the colliders center-of-mass energy, with only a few days of data. Besides constraining more general models, this would allow to probe some well motivated dark matter scenarios in the Minimal Supersymmetric Standard Model, in particular the incredible bulk and stau co-annihilation scenarios.
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