arXiv subjects

Kairui Zhang

Publications and source records attributed to Kairui Zhang.

25 records · Page 2Linked to original sources

arXiv2024

Decoding the gaugino code, naturally, at high-lumi LHC

Natural supersymmetry with light higgsinos is most likely to emerge from the string landscape since the volume of scan parameter space shrinks to tiny volumes for electroweak unnatural models. Rather general arguments favor a landscape selection of soft SUSY breaking terms tilted to large values, but tempered by the atomic principle: that the derived value of the weak scale in each pocket universe lie not too far from its measured value in our universe. But that leaves (at least) three different paradigms for gaugino masses in natural SUSY models: unified (as in nonuniversal Higgs models), anomaly-mediation form (as in natural AMSB) and mirage mediation form (with comparable moduli- and anomaly-mediated contributions). We perform landscape scans for each of these, and show they populate different, but overlapping, positions in m(\ell\bar{\ell}) and m(wino) space. The first of these may be directly measurable at high-lumi LHC via the soft opposite-sign dilepton plus jets plus MET signature arising from higgsino pair production while the second of these could be extracted from direct wino pair production leading to same-sign diboson production.

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arXiv2024

Stau pairs from natural SUSY at high luminosity LHC

Natural supersymmetry (SUSY) with light higgsinos is perhaps the most plausible of all weak scale SUSY models while a variety of motivations point to (right) tau sleptons as the lightest of all the sleptons. We examine a SUSY model line with rather light right-staus embedded within natural SUSY. For light stau_1 of a few hundred GeV, then the decays stau_1 -> τ\tchi_{1,2}^0 and ν_τ\tchi_1^- occur at comparable rates where the (higgsino-like) \tchi_1^\pm and \tchi_2^0 release only small visible energy: in this case, the expected τ^+τ^- +\eslt signature is diminished from usual expectations due to the presence of the nearly invisible decay mode \ttau_1 -> ν_τ\tchi_1^-. However, once m_{\ttau_1}> ~m(bino), then decays to binos such as \ttau_1 -> τ\tchi_3^0 open up where \tchi_3^0 decays to higgsinos plus W^\pm, Z^0 and h at comparable rates. For these heavier staus, then stau pair production gives rise to diboson+\eslt events which may contain 0, 1 or 2 additional hard τleptons. From these considerations, we examine the potential for future discovery of tau-slepton pair production at high-luminosity LHC. While we do not find a 5σHL-LHC discovery reach for 3000 fb^{-1}, we do find a 95\% CL exclusion reach, ranging between m_{\ttau_1}:100-450 GeV for m_{\tchi_1^0}~ 100 GeV. This latter reach disappears for m_{\tchi_1^0}>~ 200 GeV.

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arXiv2024

Winos from natural SUSY at the high luminosity LHC

In natural supersymmetric models defined by no worse than a part in thirty electroweak fine-tuning, winos and binos are generically expected to be much heavier than higgsinos. Moreover, the splitting between the higgsinos is expected to be small, so that the visible decay products of the heavier higgsinos are soft, rendering the higgsinos quasi-invisible at the LHC. Within the natural SUSY framwork, heavy electroweak gauginos decay to W, Z or h bosons plus higgsinos in the ratio ~2:1:1, respectively. This is in sharp contrast to models with a bino-like lightest superpartner and very heavy higgsinos, where the charged (neutral) wino essentially always decays to a W (h) boson and an invisible bino. Wino pair production at the LHC, in natural SUSY, thus leads to VV, Vh and hh+MET final states (V=W, Z) where, for TeV scale winos, the vector bosons and h daughters are considerably boosted. We identify eight different channels arising from the leptonic and hadronic decays of the vector bosons and the decay h-> b\bar{b}, each of which offers an avenue for wino discovery at the high luminosity LHC (HL-LHC). By combining the signal in all eight channels we find, assuming \sqrt{s}=14 TeV and an integrated luminosity of 3000 fb^{-1}, that the discovery reach for winos extends to m(wino)~1.1~TeV, while the 95% CL exclusion range extends to a wino mass of almost 1.4~TeV. We also identify ``higgsino specific channels'' which could serve to provide 3σevidence that winos lighter than 1.2~TeV decay to light higgsinos rather than to a bino-like LSP, should a wino signal appear at the HL-LHC.

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arXiv2023

Top squarks from the landscape at high luminosity LHC

Supersymmetric models with low electroweak finetuning are expected to be more prevalent on the string landscape than finetuned models. We assume a fertile patch of landscape vacua containing the minimal supersymmetric standard model (MSSM) as low energy/weak scale effective field theory (LE-EFT). Then, a statistical pull by the landscape to large soft terms is balanced by the requirement of a derived value of the weak scale which is not too far from its measured value in our universe. Such models are characterized by light higgsinos in the few hundred GeV range whilst top squarks are in the 1-2.5 TeV range with large trilinear soft terms which helps to push m_h~ 125 GeV. Other sparticles are generally beyond current LHC reach and the BR(b -> sγ) branching fraction is nearly equal to its SM value. The light top-squarks decay comparably via \tst_1 -> b\tchi_1^+ and \tst_1 -> t\tchi_{1,2}^0 yielding mixed final states of b\bar{b}+MET, t\bar{b}/\ \bar{t}b + MET and t\bar{t}+ MET. We evaluate prospects for top squark discovery at high-luminosity (HL) LHC for the well-motivated case of natural SUSY from the landscape. We find for HL-LHC a 5σreach out to m_{\tst_1}~ 1.7 TeV and a 95\% CL exclusion reach to m_{\tst_1} ~ 2 TeV. These reaches cover {\it most} (but not all) of the allowed stringy natural parameter space!

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arXiv2023

Prospects for charged Higgs bosons in natural SUSY models at the high-luminosity LHC

We continue our examination of prospects for discovery of heavy Higgs bosons of natural SUSY (natSUSY) models at the high luminosity LHC (HL-LHC), this time focussing on charged Higgs bosons. In natSUSY, higgsinos are expected at the few hundred GeV scale whilst electroweak gauginos inhabit the TeV scale and the heavy Higgs bosons, H, A and H^\pm could range up tens of TeV without jeopardizing naturalness. For TeV-scale heavy SUSY Higgs bosons H, A and H^\pm, as currently required by LHC searches, SUSY decays into gaugino plus higgsino can dominate H^\pm decays provided these decays are kinematically accessible. The visible decay products of higgsinos are soft making them largely invisible, whilst the gauginos decay to W, Z or h plus missing transverse energy (MET). Charged Higgs bosons are dominantly produced at LHC14 via the parton subprocess, gb-> H^\pm t. In this paper, we examine the viability of observing signtures from H^\pm -> τν, H^\pm -> tb and H^\pm -> W, Z, h + MET events produced in association with a top quark at the HL-LHC over large Standard Model (SM) backgrounds from (mainly) t\bar{t}, t\bar{t}V and t\bar{t}h production (where V=W, Z). We find that the greatest reach is found via the SM H^\pm(-> τν) +t channel with a subdominant contribution from the H^\pm(-> tb) +t channel. Unlike for neutral Higgs searches, the SUSY decay modes appear to be unimportant for H^\pm searches at the HL-LHC. We delineate regions of the m_A vs. \tanβplane, mostly around m_A \sim 1-2 TeV, where signals from charged Higgs bosons would serve to confirm signals of a heavy, neutral Higgs boson at the 5σlevel or, alternatively, to exclude heavy Higgs bosons at the 95% confidence level at the high luminosity LHC.

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arXiv2022

Detecting heavy neutral SUSY Higgs bosons decaying to sparticles at the high-luminosity LHC

In supersymmetry (SUSY) models with low electroweak naturalness (natSUSY), which have been suggested to be the most likely version of SUSY to emerge from the string landscape, higgsinos are expected at the few hundred GeV scale whilst electroweak gauginos inhabit the TeV scale. For TeV-scale heavy neutral SUSY Higgs bosons H and A, as currently required by LHC searches, then the dominant decay modes of H, A are into gaugino plus higgsino provided these decays are kinematically open. The light higgsinos decay to soft particles so are largely invisible whilst the gauginos decay to W, Z or h plus missing transverse energy (MET). Thus, we examine the viability of H,A-> W+MET, Z+MET and h+MET signatures at the high luminosity LHC (HL-LHC) in light of large Standard Model (SM) backgrounds from (mainly) t\bar{t}, VV and Vh production (where V=W, Z). We also examine whether these signal channels can be enhanced over backgrounds by requiring the presence of an additional soft lepton from the decays of the light higgsinos. We find significant regions in the vicinity of m_A~ 1-2 TeV of the m_A vs. \tanβplane which can be probed at the high luminosity LHC using these dominant signatures by HL-LHC at 5σand at the 95% confidence level (CL).

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arXiv2022

Prospects for heavy neutral SUSY Higgs scalars in the hMSSM and natural SUSY at LHC upgrades

We examine production and decay of heavy neutral SUSY Higgs bosons pp-> H,\ A -> τ\barτ within the hMSSM and compare against a perhaps more plausible natural supersymmetry scenario dubbed m_h^{125}({\rm nat}) which allows for a natural explanation for m_{weak}\simeq m_{W,Z,h}\sim 100 GeV while maintaining m_h\simeq 125 GeV. We evaluate signal against various Standard Model backgrounds from γ,Z ->τ\barτ, t\bar{t} and vector boson pair production VV. We combine the transverse mass method for back-to-back (BtB) taus along with the ditau mass peak m_{ττ} method for acollinear taus as our signal channels. This technique ultimately gives a boost to the signal significance over the standard technique of using just the BtB signal channel. We evaluate both the 95% CL exclusion and 5σdiscovery reach in the m_A vs. \tanβplane for present LHC with 139 fb^{-1}, Run 3 with 300 fb^{-1} and high luminosity LHC (HL-LHC) with 3000 fb^{-1} of integrated luminosity. For \tanβ=10, the exclusion limits range up to m_A\sim 1, 1.1 and 1.4 TeV, respectively. These may be compared to the range of m_A values gleaned from a statistical analysis of the string landscape wherein m_A can range up to ~8 TeV.

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