Light Higgsinos Research Articles

Naturalness, dark matter, and the muon anomalous magnetic moment in supersymmetric extensions of the standard model with a pseudo-Dirac gluino

We study the naturalness, dark matter, and muon anomalous magnetic moment in the Supersymmetric Standard Models (SSMs) with a pseudo-Dirac gluino (PDGSSMs) from hybrid F- and D-term supersymmetry (SUSY) breakings. To obtain the observed dark matter relic density and explain the muon anomalous magnetic moment, we find that the low energy fine-tuning measures are larger than about 30 due to strong constraints from the LUX and PANDAX experiments. Thus, to study the natural PDGSSMs, we consider multi-component dark matter and then the relic density of the lightest supersymmetric particle (LSP) neutralino is smaller than the correct value. We classify our models into six kinds: (i) Case A is a general case, which has small low energy fine-tuning measure and can explain the anomalous magnetic moment of the muon; (ii) Case B with the LSP neutralino and light stau coannihilation; (iii) Case C with Higgs funnel; (iv) Case D with Higgsino LSP; (v) Case E with light stau coannihilation and Higgsino LSP; (vi) Case F with Higgs funnel and Higgsino LSP. We study these Cases in details, and show that our models can be natural and consistent with the LUX and PANDAX experiments, as well as explain the muon anomalous magnetic moment. In particular, all these cases except the stau coannihilation can even have low energy fine-tuning measures around 10.

Reach of the high-energy LHC for gluinos and top squarks in SUSY models with light Higgsinos

We examine the top squark (stop) and gluino reach of the proposed 33 TeV energy upgrade of the Large Hadron Collider (LHC33) in the Minimal Supersymmetric Standard Model (MSSM) with light higgsinos and relatively heavy electroweak gauginos. In our analysis, we assume that stops decay to higgsinos via ${\tilde t}_1 \to t {\tilde{Z}}_1$, $\tilde{t}_1 \to t\tilde{Z}_2$ and $\tilde{t}$$_1 \to b\tilde{W}_1$ with branching fractions in the ratio 1:1:2 (expected if the decay occurs dominantly via the superpotential Yukawa coupling) while gluinos decay via $\tilde{g}\to t\tilde{t}_1$ or via three-body decays to third generation quarks plus higgsinos. These decay patterns are motivated by models of natural supersymmetry where higgsinos are expected to be close in mass to $m_Z$, but gluinos may be as heavy as $5 - 6$ TeV and stops may have masses up to $\sim 3 $ TeV. We devise cuts to optimize the signals from stop and gluino pair production at LHC33. We find that experiments at LHC33 should be able to discover stops with $> 5\sigma$ significance if $m_{\tilde{t}_1} < 2.3 \ (2.8) \ [3.2]$ TeV for an integrated luminosity of 0.3 (1)[3] ab$^{-1}$. The corresponding reach for gluinos extends to 5 (5.5) [6] TeV. These results imply that experiments at LHC33 should be able to discover at least one of the stop or gluino pair signals even with an integrated luminosity of 0.3 ab$^{-1}$ for natural SUSY models with no worse than 3% electroweak fine-tuning, and quite likely both gluinos and stops for an integrated luminosity of 3 ab$^{-1}$.

An indirect probe of the higgsino world at the CEPC

The higgsino world is one of the popular natural SUSY scenarios, in which |mu | ll M_{mathrm{gauginos}} ll M_{mathrm{scalars}}. As such, searching for light degenerate higgsinos becomes an essential task of testing naturalness in supersymmetry (SUSY). In this work, we study the indirect effects of light higgsinos in the process e^+e^- rightarrow W^+W^- at future Higgs factories, such as the Circular Electron Positron Collider (CEPC) in China. We find that the higgsino mass parameter mu lesssim 210 , hbox {GeV} favored by naturalness can be covered if the accuracy of the measurement of W^+W^- production can reach 0.1% at 240 GeV CEPC.

Analysis of the TeV-scale mirage mediation with heavy superparticles

We discuss effective models derived from a supersymmetric model whose mediation mechanism of supersymmetry (SUSY) breaking is namely mirage mediation. In this model, light higgsino mass, that is required by the natural realization of the electroweak scale, is achieved by the unification of the soft SUSY breaking parameters at the low scale. Besides, we find that extra Higgs fields are also possibly light in some cases. Then, the effective model is a two Higgs doublet model (2HDM) with higgsinos, and it is distinguishable with namely type-II 2HDM which is widely discussed. In this paper, we study the mass spectrum of SUSY particles and the extra Higgs fields, and summarize the phenomenology in the effective model. We survey the current experimental bounds from the LHC and the dark matter experiments as well as the flavor physics. Then, we point out the expected mass scale of the SUSY particles and reveal the future prospects for the direct and indirect searches. We also discuss the difference between our effective model and the 2HDM in the bottom-up approach.

Sneutrino DM in the NMSSM with inverse seesaw mechanism

In supersymmetric theories like the Next-to-Minimal Supersymmetric Standard Model (NMSSM), the lightest neutralino with bino or singlino as its dominant component is customarily taken as dark matter (DM) candidate. Since light Higgsinos favored by naturalness can strength the couplings of the DM and thus enhance the DM-nucleon scattering rate, the tension between naturalness and DM direct detection results becomes more and more acute with the improved experimental sensitivity. In this work, we extend the NMSSM by inverse seesaw mechanism to generate neutrino mass, and show that in certain parameter space the lightest sneutrino may act as a viable DM candidate, i.e. it can annihilate by multi-channels to get correct relic density and meanwhile satisfy all experimental constraints. The most striking feature of the extension is that the DM-nucleon scattering rate can be naturally below its current experimental bounds regardless of the higgsino mass, and hence it alleviates the tension between naturalness and DM experiments. Other interesting features include that the Higgs phenomenology becomes much richer than that of the original NMSSM due to the relaxed constraints from DM physics and also due to the presence of extra neutrinos, and that the signatures of sparticles at colliders are quite different from those with neutralino as DM candidate.

Study of dark matter physics in non-universal gaugino mass scenario

We study dark matter physics in the Minimal Supersymmetric Standard Model with non-universal gaugino masses at the unification scale. In this scenario, the specific ratio of wino and gluino masses realizes the electro-weak scale naturally and achieves 125 GeV Higgs boson mass. Then, relatively light higgsino is predicted and the lightest neutral particle, that is dominantly given by the neutral component of higgsino, is a good dark matter candidate. The direct detection of the dark matter is sensitive to not only a higgsino mass but also gaugino masses significantly. The upcoming XENON1T experiment excludes the parameter region where bino or gluino is lighter than about 2.5 TeV if the higgsino and the gaugino mass parameters have same signs. We see that the direct detection of dark matter gives stronger bound than the direct search at the LHC experiment when higgsino gives sizable contribution to the dark matter abundance.

Cornering natural SUSY at LHC Run II and beyond

We derive the latest constraints on various simplified models of natural SUSY with light higgsinos, stops and gluinos, using a detailed and comprehensive reinterpretation of the most recent 13 TeV ATLAS and CMS searches with ∼ 15 fb−1 of data. We discuss the implications of these constraints for fine-tuning of the electroweak scale. While the most “vanilla” version of SUSY (the MSSM with R-parity and flavor-degenerate sfermions) with 10% fine-tuning is ruled out by the current constraints, models with decoupled valence squarks or reduced missing energy can still be fully natural. However, in all of these models, the mediation scale must be extremely low (<100 TeV). We conclude by considering the prospects for the high-luminosity LHC era, where we expect the current limits on particle masses to improve by up to ∼ 1 TeV, and discuss further model-building directions for natural SUSY that are motivated by this work.

Gluino reach and mass extraction at the LHC in radiatively-driven natural SUSY

Radiatively-driven natural SUSY (RNS) models enjoy electroweak naturalness at the 10% level while respecting LHC sparticle and Higgs mass constraints. Gluino and top-squark masses can range up to several TeV (with other squarks even heavier) but a set of light Higgsinos are required with mass not too far above m_hsim 125 GeV. Within the RNS framework, gluinos dominantly decay via tilde{g}rightarrow ttilde{t}_1^{*}, bar{t}tilde{t}_1 rightarrow tbar{t}widetilde{Z}_{1,2} or tbar{b}widetilde{W}_1^-+c.c., where the decay products of the higgsino-like widetilde{W}_1 and widetilde{Z}_2 are very soft. Gluino pair production is, therefore, signaled by events with up to four hard b-jets and large not !!{E_T}. We devise a set of cuts to isolate a relatively pure gluino sample at the (high-luminosity) LHC and show that in the RNS model with very heavy squarks, the gluino signal will be accessible for m_{tilde{g}} < 2400 (2800) GeV for an integrated luminosity of 300 (3000) fb^{-1}. We also show that the measurement of the rate of gluino events in the clean sample mentioned above allows for a determination of m_{tilde{g}} with a statistical precision of 2–5% (depending on the integrated luminosity and the gluino mass) over the range of gluino masses where a 5sigma discovery is possible at the LHC.

Light Higgsinos, heavy gluino, and b−τ quasi-Yukawa unification: Prospects for finding the gluino at the LHC

A wide variety of unified models predict asymptotic relations at $M_{GUT}$ between the b quark and $\tau$ lepton Yukawa couplings. Within the framework of supersymmetric SU(4) $\times$ SU(2)$_L \times$ SU(2)$_R$, we explore regions of the parameter space that are compatible with b-$\tau$ quasi-Yukawa unification and the higgsinos being the lightest supersymmetric particles ($\lesssim$ 1 TeV). Among the colored sparticles, the stop weighs more than 1.5 TeV or so, whereas the squarks of the first two families are signifcantly heavier, approaching 10 TeV in some cases. The gluino mass is estimated to lie in the 2-4 TeV range which raises the important question: Will the LHC find the gluino?

Superparticle phenomenology from the natural mini-landscape

The methodology of the heterotic mini-landscape attempts to zero in on phenomenologically viable corners of the string landscape where the effective low energy theory is the Minimal Supersymmetric Standard Model with localized grand unification. The gaugino mass pattern is that of mirage-mediation. The magnitudes of various SM Yukawa couplings point to a picture where scalar soft SUSY breaking terms are related to the geography of fields in the compactified dimensions. Higgs fields and third generation scalars extend to the bulk and occur in split multiplets with TeV scale soft masses. First and second generation scalars, localized at orbifold fixed points or tori with enhanced symmetry, occur in complete GUT multiplets and have much larger masses. This picture can be matched onto the parameter space of generalized mirage mediation. Naturalness considerations, the requirement of the observed electroweak symmetry breaking pattern, and LHC bounds on mg together limit the gravitino mass to the m3/2 ∼ 5-60 TeV range. The mirage unification scale is bounded from below with the limit depending on the ratio of squark to gravitino masses. We show that while natural SUSY in this realization may escape detection even at the high luminosity LHC, the high energy LHC with sqrt{s}=33 TeV could unequivocally confirm or exclude this scenario. It should be possible to detect the expected light higgsinos at the ILC if these are kinematically accessible, and possibly also discriminate the expected compression of gaugino masses in the natural mini-landscape picture from the mass pattern expected in models with gaugino mass unification. The thermal WIMP signal should be accessible via direct detection searches at the multi-ton noble liquid detectors such as XENONnT or LZ.

Light higgsino for gauge coupling unification

We explore gauge coupling unification and dark matter in high scale supersymmetry where the scale of supersymmetry breaking is much above the weak scale. The gauge couplings unify as precisely as in low energy supersymmetry if the higgsinos, whose mass does not break supersymmetry, are much lighter than those obtaining masses from supersymmetry breaking. The dark matter of the universe can then be explained by the neutral higgsino or the gravitino. High scale supersymmetry with light higgsinos requires a large Higgs mixing parameter for electroweak symmetry breaking to take place. It is thus naturally realized in models where superparticle masses are generated at loop level while the Higgs mixing parameter is induced at tree level, like in anomaly and gauge mediation of supersymmetry breaking.

Phenomenological profile of top squarks from natural supersymmetry at the LHC

In supersymmetric models with radiatively-driven naturalness and light higgsinos, the top squarks may lie in the 0.5- 3TeV range and thus only a fraction of natural parameter space is accessible to LHC searches. We outline the range of top squark and lightest SUSY particle masses preferred by electroweak naturalness in the standard parameter space plane. We note that the branching fraction for b-> s\gamma decay favors top squarks much heavier than 500 GeV. Such a range of top-squark mass values is in contrast to previous expectations where m(stop)<500 GeV had been considered natural. In radiative natural SUSY, top squarks decay roughly equally via t1-> bW1 and Z_{1,2} where W1 and Z_{1,2} are higgsino-like electroweak-inos. Thus, top squark pair production should yield all of t\bar{t}+\eslt, t\bar{b}+\eslt, b\bar{t}+\eslt and b\bar{b}+\eslt signatures at comparable rates. We propose that future LHC top squark searches take place within a semi-simplified model which corresponds more closely to expectations from theory.

Tagging a monotop signature in natural SUSY

We study the feasibility of probing a region of Natural Supersymmetry where the stop and higgsino masses are compressed. Although this region is most effectively searched for in the mono-jet channel, this signature is present in many other non-supersymmetric frameworks. Therefore, another channel that carries orthogonal information is required to confirm the existence of the light stop and higgsinos. We show that a supersymmetric version of the $t \bar t H$ process, $pp \to t \tilde t_1 \tilde \chi^0_{1(2)}$, can have observably large rate when both the stop and higgsinos are significantly light, and it leads to a distinctive mono-top signature in the compressed mass region. We demonstrate that the hadronic channel of the mono-top signature can effectively discriminate the signal from backgrounds by tagging a hadronic top-jet. We show that the hadronic channel of mono-top signature offers a significant improvement over the leptonic channel and the sensitivity reaches $m_{\tilde t_1} \simeq 420$ GeV at the 13 TeV LHC with 3 ab$^{-1}$ luminosity.

Surviving scenario of stop decays for ATLASℓ+jets+E̸Tmisssearch

Recently ATLAS reported a $3.3\sigma$ excess in the stop search with $\ell+jets+E_T^{miss}$ channel. We try to interpret the signal by a light stop pair production in the MSSM. We find: (1) simple models where stop decays into a higgsino or a bino are not favored. (2) an extension of them can explain the data at $2\sigma$ level without conflicting with the other search channels. A surviving possibility includes a light stop and a light higgsino, which is expected in a natural SUSY scenario.

Light higgsino dark matter from non-thermal cosmology

We study the scenario of higgsino dark matter in the context of a non-standard cosmology with a period of matter domination prior to Big Bang nucleosynthesis. Matter domination changes the dark matter relic abundance if it ends via reheating to a temperature below the higgsino thermal freeze-out temperature. We perform a model independent analysis of the higgsino dark matter production in such scenario. We show that light higgsino-type dark matter is possible for reheating temperatures close to 1 GeV. We study the impact of dark matter indirect detection and collider physics in this context. We show that Fermi-LAT data rule out non-thermal higgsinos with masses below 300 GeV. Future indirect dark matter searches from Fermi-LAT and CTA will be able to cover essentially the full parameter space. Contrary to the thermal case, collider signals from a 100 TeV collider could fully test the non-thermal higgsino scenario. In the second part of the paper we discuss the motivation of such non-thermal cosmology from the perspective of string theory with late-time decaying moduli for both KKLT and LVS moduli stabilisation mechanisms. We finally describe the impact of embedding higgsino dark matter in these scenarios.

Strong constraints of LUX-2016 results on the natural NMSSM

Given the fact that the relatively light Higgsino mass $\mu$ favored in natural supersymmetry usually results in a sizable scattering cross section between the neutralino dark matter and the nucleon, we study the impact of the recently updated direct detection bounds from LUX experiment, including both Spin Independent (SI) and Spin Dependent (SD) measurements, on the parameter space of natural Next-to-Minimal Supersymmetric Standard Model (nNMSSM). Different from the common impression that the SI bound is stronger than the SD one, we find that the SD bound is complementary to the SI bound and in some cases much more powerful than the latter in limiting the nNMSSM scenarios. After considering the LUX results, nNMSSM is severely limited, e.g. for the peculiar scenarios of the NMSSM where the next-to-lightest CP-even Higgs corresponds to the $125 {\rm GeV}$ Higgs boson discovered at the LHC, the samples obtained in our random scan are excluded by more than $85\%$. By contrast, the monojet search at the LHC Run-I can not exclude any sample of nNMSSM. We also investigate the current status of nNMSSM and conclude that, although the parameter points with low fine tuning are still attainable, they are distributed in some isolated parameter islands which are difficult to get. Future dark matter direct search experiments such as XENON-1T will provide a better test of nNMSSM.

Multichannel assault on natural supersymmetry at the high luminosity LHC

Recent clarifications of naturalness in supersymmetry robustly require the presence of four light higgsinos with mass ~100-300 GeV while gluinos and (top)-squarks may lie in the multi-TeV range, possibly out of LHC reach. We project the high luminosity (300-3000 fb^{-1}) reach of LHC14 via gluino cascade decays and via same-sign diboson production. We compare these to the reach for neutralino pair production \tz_1\tz_2 followed by \tz_2\to\tz_1\ell^+\ell^- decay to soft dileptons which recoil against a hard jet. It appears that 3000 fb^{-1} is just about sufficient integrated luminosity to probe naturalness with up to 3\% fine-tuning at the 5-sigma level, thus either discovering natural supersymmetry or else ruling it out.

The Higgs mass and natural supersymmetric spectrum from the landscape

In supersymmetric models where the superpotential mu term is generated with mu<< m_{soft} (e.g. from radiative Peccei-Quinn symmetry breaking or compactified string models with sequestration and stabilized moduli), and where the string landscape 1. favors soft supersymmetry (SUSY) breaking terms as large as possible and 2. where the anthropic condition that electroweak symmetry is properly broken with a weak scale m_{W,Z,h}~100 GeV ({\it i.e.} not too weak of weak interactions), then these combined landscape/anthropic requirements act as an attractor pulling the soft SUSY breaking terms towards values required by models with radiatively-driven naturalness: near the line of criticality where electroweak symmetry is barely broken and the Higgs mass is ~125 GeV. The pull on the soft terms serves to ameliorate the SUSY flavor and CP problems. The resulting sparticle mass spectrum may barely be accessible at high-luminosity LHC while the required light higgsinos should be visible at a linear e^+e^- collider with \sqrt{s}>2m(higgsino).

Light Higgsino as the tail of theμ−Bμsolution

Gauge mediation predicts 10 TeV or heavier squarks because such a heavy stop is required to explain the observed 125 GeV Higgs boson mass without a large trilinear soft mass term in the minimal supersymmetric standard model. Although such a high scale cannot be searched by the LHC directly, gauge mediation also predicts a hierarchy $\mu^2 \ll B_\mu$ by a simple and naive solution of the $\mu$ problem. We point out that this simple and naive way of generating the $\mu$ ($B_\mu$) term works in the case of 10 TeV or heavier squarks with a slight breaking of a GUT relation among messenger B-terms (or supersymmetric mass terms). Furthermore, the upper bound on the Higgsino mass is obtained from the observed Higgs boson mass, perturbativity of a relevant coupling, and conditions avoiding tachyonic sneutrinos and stop. It turns out that the light Higgsino of O(100) GeV is a promising signal of gauge mediation.

On the MSSM Higgsino mass and fine tuning

It is often argued that low fine tuning in the MSSM necessarily requires a rather light Higgsino. In this note we show that this need not be the case when a more complete set of soft SUSY breaking mass terms are included. In particular an Higgsino mass term, that correlates the $\mu-$term contribution with the soft SUSY-breaking Higgsino masses, significantly reduces the fine tuning even for Higgsinos in the TeV mass range where its relic abundance means it can make up all the dark matter.