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Lian-Tao Wang

Publications and source records attributed to Lian-Tao Wang.

At least 109 records · Page 6Linked to original sources
arXiv2015

Precision Natural SUSY at CEPC, FCC-ee, and ILC

Testing the idea of naturalness is and will continue to be one of the most important goals of high energy physics experiments. It will play a central role in the physics program of future colliders. In this paper, we present projections of the reach of natural SUSY at future lepton colliders: CEPC, FCC-ee and ILC. We focus on the observables which give the strongest reach, the electroweak precision observables (for left-handed stops), and Higgs to gluon and photon decay rates (for both left- and right-handed stops). There is a "blind spot" when the stop mixing parameter Xt is approximately equal to the average stop mass. We argue that in natural scenarios, bounds on the heavy Higgs bosons from tree-level mixing effects that modify the bottom Yukawa coupling together with bounds from b to s gamma play a complementary role in probing the blind spot region. For specific natural SUSY scenarios such as folded SUSY in which the top partners do not carry Standard Model color charges, electroweak precision observables could be the most sensitive probe. In all the scenarios discussed in this paper, the combined set of precision measurements will probe down to a few percent in fine-tuning.

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arXiv2015

Pseudoscalar Portal Dark Matter

A fermion dark matter candidate with a relic abundance set by annihilation through a pseudoscalar can evade constraints from direct detection experiments. We present simplified models that realize this fact by coupling a fermion dark sector to a two-Higgs doublet model. These models are generalizations of mixed bino-Higgsino dark matter in the MSSM, with more freedom in the couplings and scalar spectra. Annihilation near a pseudoscalar resonance allows a significant amount of parameter space for thermal relic dark matter compared to singlet-doublet dark matter, in which the fermions couple only to the SM Higgs doublet. In a general two-Higgs doublet model, there is also freedom for the pseudoscalar to be relatively light and it is possible to obtain thermal relic dark matter candidates even below 100 GeV. In particular, we find ample room to obtain dark matter with mass around 50 GeV and fitting the Galactic Center excess in gamma-rays. This region of parameter space can be probed by LHC searches for heavy pseudoscalars or electroweakinos, and possibly by other new collider signals.

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arXiv2015

Neutralinos in Vector Boson Fusion at High Energy Colliders

Discovering dark matter at high energy colliders continues to be a compelling and well-motivated possibility. Weakly interacting massive particles are a particularly interesting class in which the dark matter particles interact with the standard model weak gauge bosons. Neutralinos are a prototypical example that arise in supersymmetric models. In the limit where all other superpartners are decoupled, it is known that for relic density motivated masses, the rates for neutralinos are too small to be discovered at the Large Hadron Collider (LHC), but that they may be large enough for a 100 TeV collider to observe. In this work we perform a careful study in the vector boson fusion channel for pure winos and pure higgsinos. We find that given a systematic uncertainty of 1% (5%), with 3000 fb$^{-1}$, the LHC is sensitive to winos of 240 GeV (125 GeV) and higgsinos of 125 GeV (55 GeV). A future 100 TeV collider would be sensitive to winos of 1.1 TeV (750 GeV) and higgsinos of 530 GeV (180 GeV) with a 1% (5%) uncertainty, also with 3000 fb$^{-1}$.

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arXiv2015

The Twin Higgs mechanism and Composite Higgs

We combine the Twin Higgs mechanism with the paradigm of Composite Higgs models. In this class of models the Higgs is a pseudo-Nambu-Goldstone boson from a strongly coupled sector near the TeV scale, and it is additionally protected by a discrete symmetry due to the twin mechanism. We discuss the model building issues associated with this setup and quantify the tuning needed to achieve the correct electroweak vacuum and the Higgs mass. In contrast to standard Composite Higgs models, the lightest resonance associated with the top sector is the uncolored mirror top, while the colored top partners can be made parameterically heavier without extra tuning. In some cases, the vector resonances are predicted to lie in the multi-TeV range. We present models where the resonances - both fermions and vectors - being heavier alleviates the pressure on naturalness coming from direct searches demonstrating that theories with low tuning may survive constraints from the Large Hadron Collider.

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arXiv2015

Radiative Return Capabilities of a High-Energy, High-Luminosity $e^+e^-$ Collider

An electron-positron collider operating at a center-of-mass energy $E_{CM}$ can collect events at all lower energies through initial-state radiation (ISR or radiative return). We explore the capabilities for radiative return studies by a proposed high-luminosity collider at $E_{CM}$ = 250 or 90 GeV, to fill in gaps left by lower-energy colliders such as PEP, PETRA, TRISTAN, and LEP. These capabilities are compared with those of the lower-energy $e^+e^-$ colliders as well as hadron colliders such as the Tevatron and the CERN Large Hadron Collider (LHC). Some examples of accessible questions in dark photon searches and heavy flavor spectroscopy are given.

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arXiv2015

Supersymmetric Sub-Electroweak Scale Dark Matter, the Galactic Center Gamma-ray Excess, and Exotic Decays of the 125 GeV Higgs Boson

We continue our exploration of the nearly Peccei-Quinn symmetric limit shared by common singlet extensions of the Minimal Supersymmetric Standard Model. This limit has been established as a viable framework for studying sub-electroweak scale dark matter phenomenology and has interesting and direct connections to new exotic Higgs decay physics. We present analytic calculations to motivate the important phenomenological features mentioned above. We also discuss benchmark points in this model framework that accommodate the observed Galactic Center gamma ray excess. We emphasize connections between phenomenology of dark matter direct detection and indirect detection, and new exotic decay channels for the 125 GeV Higgs boson. We conclude by identifying two benchmark modes of exotic Higgs decays for $h \to τ^+ τ^- \mbox{${\not\! E}_{\rm T}$}$ and $h \to b \bar{b} \mbox{${\not\! E}_{\rm T}$}$ final states and estimate their sensitivity prospects at the LHC.

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arXiv2014

Prospects for Electroweakino Discovery at a 100 TeV Hadron Collider

We investigate the prospects of discovering split Supersymmetry at a future 100 TeV proton-proton collider through the direct production of electroweakino next-to-lightest-supersymmetric-particles (NLSPs). We focus on signatures with multi-lepton and missing energy: $3\ell$, opposite-sign dileptons and same-sign dileptons. We perform a comprehensive study of different electroweakino spectra. A 100 TeV collider with 3000/fb data is expected to exclude Higgsino thermal dark matter candidates with $m_{\rm{LSP}}\sim 1 $ TeV if Wino NLSPs are lighter than about 3.2 TeV. The $3\ell$ search usually offers the highest mass reach, which varies in the range of (2-4) TeV depending on scenarios. In particular, scenarios with light Higgsinos have generically simplified parameter dependences. We also demonstrate that, at a 100 TeV collider, lepton collimation becomes a crucial issue for NLSPs heavier than about 2.5 TeV. We finally compare our results with the discovery prospects of gluino pair productions and deduce which SUSY breaking model can be discovered first by electroweakino searches.

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arXiv2014

Jet Cleansing: Pileup Removal at High Luminosity

One of the greatest impediments to extracting useful information from high luminosity hadron-collider data is radiation from secondary collisions (i.e. pileup) which can overlap with that of the primary interaction. In this paper we introduce a simple jet-substructure technique termed cleansing which can consistently correct for large amounts of pileup in an observable independent way. Cleansing works at the subjet level, combining tracker and calorimeter-based data to reconstruct the pileup-free primary interaction. The technique can be used on its own, with various degrees of sophistication, or in concert with jet grooming. We apply cleansing to both kinematic and jet shape reconstruction, finding in all cases a marked improvement over previous methods both in the correlation of the cleansed data with uncontaminated results and in measures like S/rt(B). Cleansing should improve the sensitivity of new-physics searches at high luminosity and could also aid in the comparison of precision QCD calculations to collider data.

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arXiv2014

Neutralino Dark Matter at 14 and 100 TeV

In recent years the search for dark matter has intensified with competitive bounds coming from collider searches, direct detection, and indirect detection. Collider searches at the Large Hadron Collider (LHC) lack the necessary center-of-mass energy to probe TeV-scale dark matter. It is TeV-scale dark matter, however, that remains viable for many models of supersymmetry. In this paper, we study the reach of a 100 TeV proton-proton collider for neutralino dark matter and compare to 14 TeV LHC projections. We employ a supersymmetric simplified model approach and present reach estimates from monojet searches, soft lepton searches, and disappearing track searches. The searches are applied to pure neutralino spectra, compressed neutralino spectra, and coannihilating spectra. We find a factor of 4-5 improvement in mass reach in going from 14 TeV to 100 TeV. More specifically, we find that given a 1% systematic uncertainty, a 100 TeV collider could exclude winos up to 1.4 TeV and higgsinos up to 850 GeV in the monojet channel. Coannihilation scenarios with gluinos can be excluded with neutralino masses of 6.2 TeV, with stops at 2.8 TeV, and with squarks at 4.0 TeV. Using a soft lepton search, compressed spectra with a chargino-neutralino splitting of $Δm = 20 - 30$ GeV can exclude neutralinos at $\sim$ 1 TeV. Given a sufficiently long chargino lifetime, the disappearing track search is very effective and we extrapolate current experimental bounds to estimate that a $\sim$ 2 TeV wino could be discovered and a $\sim$ 3 TeV wino could be excluded.

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arXiv2014

The Fermionic Dark Matter Higgs Portal: an effective field theory approach

We consider fermionic (Dirac or Majorana) cold thermal relic dark-matter coupling to standard-model particles through the effective dimension-5 Higgs portal operators $Λ^{-1} \ \mathcal{O}_{\text{DM}} \cdot H^\dagger H$, where $\mathcal{O}_{\text{DM}}$ is an admixture of scalar $\barχχ$ and pseudoscalar $\barχiγ_5 χ$ DM operators. Utilizing the relic abundance requirement to fix the couplings, we consider direct detection and invisible Higgs width constraints, and map out the remaining allowed parameter space of dark-matter mass and the admixture of scalar and pseudoscalar couplings. We emphasize a subtlety which has not previously been carefully studied in the context of the EFT approach, in which an effect arising due to electroweak symmetry breaking can cause a naïvely pure pseudoscalar coupling to induce a scalar coupling at higher order, which has important implications for direct detection bounds. We provide some comments on indirect detection bounds and collider searches.

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arXiv2014

Supersymmetric Exotic Decays of the 125 GeV Higgs Boson

We reveal a set of novel decay topologies for the 125 GeV Higgs boson in supersymmetry which are initiated by its decay into a pair of neutralinos, and discuss their collider search strategies. This category of exotic Higgs decays are characterized by the collider signature: visible objects + $\mbox{${\not\! E}_{\rm T}$}$, with $\mbox{${\not\! E}_{\rm T}$}$ dominantly arising from escaping dark matter particles. Their benchmark arises naturally in the Peccei-Quinn symmetry limit of the MSSM singlet-extensions, which is typified by the co-existence of three light particles: singlet-like scalar $h_1$ and pseudoscalar $a_1$, and singlino-like neutralino $χ_1$, all with masses of $\lesssim 10$ GeV, and the generically suppression of the exotic decays of the 125 GeV Higgs boson $h_2\to h_1 h_1$, $a_1a_1$ and $χ_1χ_1$, however. As an illustration, we study the decay topology: $h_2 \to χ_1 χ_2$, where the bino-like $χ_2$ decays to $h_1 χ_1$ or $a_1 χ_1$, and $h_1/a_1 \to f\bar f$, with $f\bar f = μ^+μ^-$, $b\bar b$. In the di-muon case ($m_{h_1/a_1} \sim 1$ GeV), a statistical sensitivity of $\frac{S}{\sqrt{B}} > 6 σ$ can be achieved easily at the 8 TeV LHC, assuming $\frac{σ(pp \rightarrow W h_2)}{σ(pp \rightarrow W h_{\rm SM})} {\rm Br}(h_2 \to μ^+μ^- χ_1 χ_1)=0.1$. In the $b\bar b$ case ($m_{h_1/a_1} \sim 45$ GeV), 600 fb$^{-1}$ data at the 14 TeV LHC can lead to a statistical sensitivity of $\frac{S}{\sqrt{B}} > 5 σ$, assuming $\frac{σ(pp \rightarrow Z h_2)}{σ(pp \rightarrow Z h_{\rm SM})} {\rm Br}(h_2 \to b\bar b χ_1 χ_1)=0.5$. These exotic decays open a new avenue for exploring new physics couplings with the 125 GeV Higgs boson at colliders.

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arXiv2014

Dark matter with $t$-channel mediator: a simple step beyond contact interaction

Effective contact operators provide the simplest parameterization of dark matter searches at colliders. However, light mediator can significantly change the sensitivity and search strategies. Considering simple models of mediators is an important next-step for collider searches. In this paper, we consider the case of a $t$-channel mediator. Its presence opens up new contributions to the monojet$+\displaystyle{\not{E}}_T$ searches and can change the reach significantly. We also study the complementarity between searches for processes of monojet$+\displaystyle{\not{E}}_T$ and direct pair production of the mediators. Mediator pair production also gives important contribution to a CMS-like monojet$+\displaystyle{\not{E}}_T$ search where a second hard jet is allowed.There is a large region of parameter space in which the monojet$+\displaystyle{\not{E}}_T$ search provides the stronger limit. Assuming the relic abundance of the dark matter is thermally produced within the framework of this model, we find that in the Dirac fermion dark matter case, there is no region in the parameter space that satisfies the combined constraint of monojet$+\displaystyle{\not{E}}_T$ search and direct detection; whereas in the Majorana fermion dark matter case, the mass of dark matter must be larger than about 100 GeV. If the relic abundance requirement is not assumed, the discovery of the $t$-channel mediator predicts additional new physics.

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arXiv2014

Mono-Higgs Detection of Dark Matter at the LHC

Motivated by the recent discovery of the Higgs boson, we investigate the possibility that a missing energy plus Higgs final state is the dominant signal channel for dark matter at the LHC. We consider examples of higher-dimension operators where a Higgs and dark matter pair are produced through an off-shell Z or photon, finding potential sensitivity at the LHC to cutoff scales of around a few hundred GeV. We generalize this production mechanism to a simplified model by introducing a Z' as well as a second Higgs doublet, where the pseudoscalar couples to dark matter. Resonant production of the Z' which decays to a Higgs plus invisible particles gives rise to a potential mono-Higgs signal. This may be observable at the 14 TeV LHC at low tan beta and when the Z' mass is roughly in the range 600 GeV to 1.3 TeV.

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arXiv2013

Constraints on a Very Light Sbottom

We investigate the phenomenological viability of a very light bottom squark, with a mass less than half of the Z boson mass. The decays of the Z and Higgs bosons to light sbottom pairs are, in a fairly model independent manner, strongly constrained by the precision electroweak data and Higgs signal strength measurements, respectively. These constraints are complementary to direct collider searches, which depend in detail on assumptions regarding the superpartner spectrum and decays of the sbottom. In particular, if the lightest sbottom has a mass below about 15 GeV, compatibility with these measurements is possible only in a special region of parameter space in which the couplings of the lightest sbottom to the Z and Higgs are suppressed. In this region, the second sbottom is predicted to be lighter than about 300 GeV and can also be searched for directly at the LHC. We also survey relevant collider searches for canonical scenarios with a bino, gravitino, or singlino LSP in the compressed and stealth kinematic regimes and provide suggestions to cover remaining open regions of parameter space.

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arXiv2013

Non-Decoupling MSSM Higgs Sector and Light Superpartners

In the ``non-decoupling'' region of the Higgs sector in MSSM, the heavier CP-even Higgs boson (H^0) is Standard-Model-like and close to the charged Higgs bosons (H^\pm) in mass, while other neutral Higgs bosons (h^0,A^0) are lighter and near the Z mass. This scenario is consistent with the current Higgs search limits, although the improved sensitivity for a light charged Higgs boson search t -> H^+ b may result in certain degree of tension. We demonstrate that it can pass the stringent flavor constraints, provided there are other light SUSY particles to contribute in the loop induced processes. In turn, the non-decoupling Higgs sector implies the existence of light (left-handed) stop, sbottom and Wino-like gauginos, with mass all below 250 GeV. These light super-partners can still escape the current SUSY searches at the LHC. Dedicated searches for soft decay products should be devised for the LHC experiments to improve the searching sensitivity. The ILC would be able to cover the full spectrum region. The solutions for the viable SUSY parameters result from subtle cancellations and are often missed by the generic multiple dimensional scans, highlighting the importance of theoretical guidance in search for such special cases.

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arXiv2013

Gamma-ray constraints on dark-matter annihilation to electroweak gauge and Higgs bosons

Dark-matter annihilation into electroweak gauge and Higgs bosons results in $γ$-ray emission. We use observational upper limits on the fluxes of both line and continuum $γ$-rays from the Milky Way Galactic Center and from Milky Way dwarf companion galaxies to set exclusion limits on allowed dark-matter masses. (Generally, Galactic Center $γ$-ray line search limits from the Fermi-LAT and the H.E.S.S. experiments are most restrictive.) Our limits apply under the following assumptions: a) the dark matter species is a cold thermal relic with present mass density equal to the measured dark-matter density of the universe; b) dark-matter annihilation to standard-model particles is described in the non-relativistic limit by a single effective operator ${\cal O} \propto J_{DM}\cdot J_{SM}$, where $J_{DM}$ is a standard-model singlet current consisting of dark-matter fields (Dirac fermions or complex scalars), and $J_{SM}$ is a standard-model singlet current consisting of electroweak gauge and Higgs bosons; and c) the dark-matter mass is in the range 5 GeV to 20 TeV. We consider, in turn, the 34 possible operators with mass dimension 8 or lower with non-zero s-wave annihilation channels satisfying the above assumptions. Our limits are presented in a large number of figures, one for each of the 34 possible operators; these limits can be grouped into 13 classes determined by the field content and structure of the operators. We also identify three classes of operators (coupling to the Higgs and $SU(2)_L$ gauge bosons) that can supply a 130 GeV line with the desired strength to fit the putative line signal in Fermi data, while saturating the relic density and satisfying all other indirect constraints we consider.

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arXiv2013

Constraining RS Models by Future Flavor and Collider Measurements: A Snowmass Whitepaper

Randall-Sundrum models are models of quark flavor, because they explain the hierarchies in the quark masses and mixings in terms of order one localization parameters of extra dimensional wavefunctions. The same small numbers which generate the light quark masses suppress contributions to flavor violating tree level amplitudes. In this note we update universal constraints from electroweak precision parameters and demonstrate how future measurements of flavor violation in ultra rare decay channels of Kaons and B mesons will constrain the parameter space of this type of models. We show how collider signatures are correlated with these flavor measurements and compute projected limits for direct searches at the 14 TeV LHC run, a 14 TeV LHC luminosity upgrade, a 33 TeV LHC energy upgrade, and a potential 100 TeV machine. We further discuss the effects of a warped model of leptons in future measurements of lepton flavor violation.

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