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Register a new userAccessing the core of naturalness, nearly degenerate higgsinos, at the LHC
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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.
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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.
In supersymmetric models with light higgsinos (which are motivated by electroweak naturalness arguments), the direct production of higgsino pairs may be difficult to search for at LHC due to the low visible energy release from their decays. However, the wino pair production reaction tw_2^pmtz_4to (W^pmtz_{1,2})+(W^pmtw_1^mp) also occurs at substantial rates and leads to final states including equally opposite-sign (OS) and same-sign (SS) diboson production. We propose a novel search channel for LHC14 based on the SS diboson plus missing E_T final state which contains only modest jet activity. Assuming gaugino mass unification, and an integrated luminosity > 100 fb^{-1}, this search channel provides a reach for SUSY well beyond that from usual gluino pair production.
We study the implications of a large degree of compositeness for the light generation quarks in composite pseudo-Nambu-Goldstone-boson Higgs models. We focus in particular on viable scenarios where the right-handed up-type quarks have a sizable mixing with the strong dynamics. For concreteness we assume the latter to be characterized by an SO(5)/SO(4) symmetry with fermionic resonances in the SO(4) singlet and fourplet representations. Singlet partners dominantly decay to a Higgs boson and jets. As no dedicated searches are currently looking for these final states, singlet partners can still be rather light. Conversely, some fourplet partners dominantly decay to an electroweak gauge boson and a jet, a signature which has been analyzed at the LHC. To constrain the parameter space of this scenario we have reinterpreted various LHC analyses. In the limit of first two generation degeneracy, as in minimal flavor violation or U(2)-symmetric flavor models, fourplet partners need to be relatively heavy, with masses above 1.8 TeV, or the level of compositeness needs to be rather small. The situation is rather different in models that deviate from the first two generation degeneracy paradigm, as the charm parton distribution functions are suppressed relative to the up quark ones. The right-handed charm quark can be composite and its partners being as light as 600 GeV, while the right-handed up quark needs either to be mostly elementary or to have its partners as heavy as 2 TeV. Models with fully composite singlet fermions are also analyzed, leading to similar conclusions. Finally, we consider the case where both the fourplet and the singlet states are present. In this case the bounds could be significantly weaken due to a combination of smaller production rates and the opening of new channels including cascade processes.
Mirage mediation realized in the KKLT flux compactification can naturally suppress the up-type Higgs soft mass at low energy scales, and consequently it can reduce the degree of electroweak fine-tuning up to a loop factor. Interestingly, this feature holds even in high-scale supersymmetry as long as the gauge coupling unification is achieved for light Higgsinos below TeV. Under the experimental constraints on the observed Higgs boson, it turns out that mirage mediation can exhibit low electroweak fine-tuning better than a few percent for stops between about 2 and 6 TeV, i.e., at the same level as in the weak scale supersymmetry, if the Higgsinos are around or below a few hundred GeV.
Light states associated with the hierarchy problem affect the Higgs LHC production and decays. We illustrate this within the MSSM and two simple extensions applying the latest bounds from LHC Higgs searches. Large deviations in the Higgs properties are expected in a natural SUSY spectrum. The discovery of a non-Standard-Model Higgs may signal the presence of light stops accessible at the LHC. Conversely, the more the Higgs is Standard-Model-like, the more tuned the theory becomes. Taking the ratio of different Higgs decay channels at the LHC cancels the leading QCD uncertainties and potentially improves the accuracy in Higgs coupling measurements to the percent level. This may lead to the possibility of doing precision Higgs physics at the LHC. Finally, we entertain the possibility that the ATLAS excess around 125 GeV persists with a Higgs production cross-section that is enhanced compared to the SM. This increase can only be accommodated in extensions of the MSSM and it may suggest that stops lie below 400 GeV, likely within reach of next years LHC run.
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