Top Partners Research Articles

Precise determination of the Higgs mass in supersymmetric models with vectorlike tops and the impact on naturalness in minimal GMSB

We present a precise analysis of the Higgs mass corrections stemming from vectorlike top partners in supersymmetric models. We reduce the theoretical uncertainty compared to previous studies in the following aspects: (i) including the one-loop threshold corrections to SM gauge and Yukawa couplings due to the presence of the new states to obtain the $$ \overline{\mathrm{DR}} $$ parameters entering all loop calculations, (ii) including the full momentum dependence at one-loop, and (iii) including all two-loop corrections but the ones involving g1 and g2. We find that the additional threshold corrections are very important and can give the largest effect on the Higgs mass. However, we identify also parameter regions where the new two-loop effects can be more important than the ones of the MSSM and change the Higgs mass prediction by up to 10 GeV. This is for instance the case in the low tan β, small MA regime. We use these results to calculate the electroweak fine-tuning of an UV complete variant of this model. For this purpose, we add a complete 10 and $$ \overline{10} $$ of SU(5) to the MSSM particle content. We embed this model in minimal Gauge Mediated Supersymmetry Breaking and calculate the electroweak fine-tuning with respect to all important parameters. It turns out that the limit on the gluino mass becomes more important for the fine-tuning than the Higgs mass measurements which is easy to satisfy in this setup.

Anarchic Yukawas and top partial compositeness: the flavour of a successful marriage

The top quark can be naturally singled out from other fermions in the Standard Model due to its large mass, of the order of the electroweak scale. We follow this reasoning in models of pseudo Nambu Goldstone Boson composite Higgs, which may derive from an underlying confining dynamics. We consider a new class of flavour models, where the top quark obtains its mass via partial compositeness, while the lighter fermions acquire their masses by a deformation of the dynamics generated at a high flavour scale. One interesting feature of such scenario is that it can avoid all the flavour constraints without the need of flavour symmetries, since the flavour scale can be pushed high enough. We show that both flavour conserving and violating constraints can be satisfied with top partial compositeness without invoking any flavour symmetry for the up-type sector, in the case of the minimal SO(5)/SO(4) coset with top partners in the four-plet and singlet of SO(4). In the down-type sector, some degree of alignment is required if all down-type quarks are elementary. We show that taking the bottom quark partially composite provides a dynamical explanation for the hierarchy causing this alignment. We present explicit realisations of this mechanism which do not require to include additional bottom partner fields. Finally, these conclusions are generalised to scenarios with non-minimal cosets and top partners in larger representations.

Higgs boson mass and high-luminosity LHC probes of supersymmetry with vectorlike top quark

We consider an extension of the MSSM with an added vectorlike top partner. Our aim is to revisit to what extent such an extension can raise the Higgs boson mass through radiative corrections and help ameliorate the MSSM hierarchy problem, and to specify what experimental probes at the LHC will find or exclude this possibility during the high-luminosity phase. Direct detection, precision electroweak and precision Higgs analyses are all commissioned to this end. To achieve the $ 125 \, \textrm{GeV} \ $ Higgs boson mass, we find that superpartner masses can be reduced by a factor of more than three in this scenario compared to the MSSM without the extra vectorlike top quark, and that during the high-luminosity phase of the LHC precision Higgs analysis is expected to become the most powerful experimental probe of the scenario.

Anatomy of coannihilation with a scalar top partner

We investigate a simplified model of dark matter where a Majorana fermion $\chi$ coannihilates with a colored scalar top partner $\tilde{t}$. We explore the cosmological history, with particular emphasis on the most relevant low-energy parameters: the mass splitting between the dark matter and the coannihilator, and the Yukawa coupling $y_\chi$ that connects these fields to the Standard Model top quarks. We also allow a free quartic coupling $\lambda_h$ between a pair of Higgs bosons and $\tilde{t}$ pairs. We pay special attention to the case where the values take on those expected where $\tilde{t}$ corresponds to the superpartner of the right-handed top, and $\chi$ is a bino. Direct detection, indirect detection, and colliders are complementary probes of this simple model.

Twin Higgs mechanism and a composite Higgs boson

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.

A naturally light Higgs without light top partners

We demonstrate that the inclusion of a realistic lepton sector can relax significantly the upper bound on top partner masses in minimal composite Higgs models, induced by the lightness of the Higgs boson. To that extend, we present a comprehensive survey of the impact of different realizations of the fermion sectors on the Higgs potential, with a special emphasis on the role of the leptons. The non-negligible compositeness of the $\tau_R$ in a general class of models that address the flavor structure of the lepton sector and the smallness of the corresponding FCNCs, can have a significant effect on the potential. We find that, with the $\tau_R$ in the symmetric representation of $SO(5)$, an increase in the maximally allowed mass of the lightest top partner of $\gtrsim 1$ TeV is possible for minimal quark setups like the MCHM$_{5,10}$, without increasing the tuning. A light Higgs boson $m_H \sim(100-200)$ GeV is a natural prediction of such models, which thus provide a new setup that can evade ultra-light top partners without ad-hoc tuning in the Higgs mass. Moreover, we advocate a more minimal realization of the lepton sector than generally used in the literature, which still can avoid light partners due to its contributions to the Higgs mass in a different and very natural way, triggered by the seesaw mechanism. This allows to construct the most economical $SO(5)/SO(4)$ composite Higgs models possible. Using both a transparent 4D approach, as well as presenting numerical results in the 5D holographic description, we demonstrate that, including leptons, minimality and naturalness do not imply light partners. Leptonic effects, not considered before, could hence be crucial for the viability of composite models.

Heavy color-octet particles at the LHC

Many new-physics models, especially those with a color-triplet top-quark partner, contain a heavy color-octet state. The “naturalness” argument for a light Higgs boson requires that the color-octet state be not much heavier than a TeV, and thus it can be pair-produced with large cross sections at high-energy hadron colliders. It may decay preferentially to a top quark plus a top partner, which subsequently decays to a top quark plus a color-singlet state. This singlet can serve as a WIMP dark-matter candidate. Such decay chains lead to a spectacular signal of four top quarks plus missing energy. We pursue a general categorization of the color-octet states and their decay products according to their spin and gauge quantum numbers. We review the current bounds on the new states at the LHC and study the expected discovery reach at the 8-TeV and 14-TeV runs. We also present the production rates at a future 100-TeV hadron collider, where the cross sections will be many orders of magnitude greater than at the 14-TeV LHC. Furthermore, we explore the extent to which one can determine the color octet’s mass, spin, and chiral couplings. Finally, we propose a test to determine whether the fermionic color octet is a Majorana particle.

The dynamics of Composite Higgses

The nature of the 125 GeV Higgs recently discovered at CERN has not been established yet. One intriguing possibility is that it may arise as a light composite state from a confining dynamics at the TeV scale. I will review the recent progress in understanding the dynamics that may be behind this mechanism, focusing on what we can learn by knowing its details. The masses of the spin-1 resonances can in fact be extracted from lattice calculations thus providing an estimate of the mass scale of new states. Furthermore, a lot can be learned about the physics of eventual top partners.

Natural supersymmetry without light Higgsinos

We present a mechanism that allows a large Higgsino mass without large fine-tuning. The Higgs is a pseudo Nambu-Goldstone boson (PNGB) of the global symmetry breaking pattern $SO(5) \to SO(4)$. Because of the PNGB nature of the light Higgs, the $SO(5)$ invariant Higgsino mass does not directly contribute to the Higgs mass. Large couplings in the Higgs sector that spontaneously breaks $SO(5)$ minimize the tuning, and are also motivated by the requirements of generating a sufficiently large Higgs quartic coupling and of maintaining a natural approximate global $SO(5)$ symmetry. When these conditions are imposed, theories of this type predict heavy Higgsinos. This construction differs from composite Higgs models in that no new particles are introduced to form complete $SO(5)$ multiplets involving the top quark---the stop is the only top partner. Compatibility with Higgs coupling measurements requires cancelations among contributions to the Higgs mass squared parameter at the 10% level. An important implication of this construction is that the compressed region of stop and sbottom searches can still be natural.

Usefulness of effective field theory for boosted Higgs production

The $\text{Higgs}+\text{jet}$ channel at the LHC is sensitive to the effects of new physics both in the total rate and in the transverse momentum distribution at high ${p}_{T}$. We examine the production process using an effective field theory (EFT) language and discussing the possibility of determining the nature of the underlying high-scale physics from boosted Higgs production. The effects of heavy color triplet scalars and top partner fermions with TeV scale masses are considered as examples and Higgs-gluon couplings of dimension five and dimension seven are included in the EFT. As a byproduct of our study, we examine the region of validity of the EFT. Dimension-seven contributions in realistic new physics models give effects in the high ${p}_{T}$ tail of the Higgs signal which are so tiny that they are likely to be unobservable.

Colorless top partners, a 125 GeV Higgs boson, and the limits on naturalness

Theories of physics beyond the Standard Model that address the hierarchy problem generally involve top partners, new particles that cancel the quadratic divergences associated with the Yukawa coupling of the Higgs to the top quark. With extensions of the Standard Model that involve new colored particles coming under strain from collider searches, scenarios in which the top partners carry no charge under the strong interactions have become increasingly compelling. Although elusive for direct searches, these theories predict modified couplings of the Higgs boson to the Standard Model particles. This results in corrections to the Higgs production and decay rates that can be detected at the Large Hadron Collider (LHC) provided the top partners are sufficiently light, and the theory correspondingly natural. In this paper we consider three theories that address the little hierarchy problem and involve colorless top partners, specifically the Mirror Twin Higgs, Folded Supersymmetry, and the Quirky Little Higgs. For each model we investigate the current and future bounds on the top partners, and the corresponding limits on naturalness, that can be obtained from the Higgs program at the LHC. We conclude that the LHC will not be able to strongly disfavor naturalness, with mild tuning at the level of about one part in ten remaining allowed even with 3000 fb$^{-1}$ of data at 14 TeV.

Sneaky light stop

A light supersymmetric top quark partner (stop) with a mass nearly degenerate with that of the standard model (SM) top quark can evade direct searches. The precise measurement of SM top properties such as the cross-section has been suggested to give a handle for this ‘stealth stop’ scenario. We present an estimate of the potential impact a light stop may have on top quark mass measurements. The results indicate that certain light stop models may induce a bias of up to a few GeV, and that this effect can hide the shift in, and hence sensitivity from, cross-section measurements. Due to the different initial states, the size of the bias is slightly different between the LHC and the Tevatron. The studies make some simplifying assumptions for the top quark measurement technique, and are based on truth-level samples.

Radiative Corrections to e+e− → Zh at Future Higgs Factory in the Minimal Dilaton Model

The minimal dilaton model (MDM) extends the Standard Model by one singlet scalar called dilaton and one top quark partner called t′. In this work we investigate the t′-induced radiative correction to the Higgs-strahlung production process e+e− → Zh at future Higgs factory. We first present the analytical calculations in detail and show how to handle the ultraviolet divergence. Then we calculate the correction numerically by considering the constraints from precision electroweak data. We find that, for sinθL = 0.2 and mt′ = 1200 GeV, the correction is 0.26% and 2.1% for , 1 TeV respectively, and a larger value can be achieved as sin θL increases.

Discovery potential for T ′ → tZ in the trilepton channel at the LHC

The LHC discovery potential of heavy top partners decaying into a top quark and a $Z$ boson is studied in the trilepton channel at $\sqrt{s}=13$ TeV. The clean multilepton final state allows to strongly reduce the background contaminations and to reconstruct the $T'$ mass. We show that a simple cut-and-count analysis probes the parameter space of a simplified model as efficiently as a dedicated multivariate analysis. The trilepton signature finally turns out to be as sensitive in the low $T'$ mass region as the complementary channel with a fully hadronic top quark, and more sensitive in the large mass domain. The reinterpretation in terms of the top-$Z$-up anomalous coupling is shown.

Friendship, reciprocation, and interchange in an individual-based model

Reciprocation and interchange of grooming and support may emerge as a consequence of the socio-spatial structure of the group through which individuals interact with certain partners more frequently than with others. This is shown in a computational model of grouping, fighting, and grooming, called Groofiworld. In this case, no specific mechanism of exchange is needed, such as described in calculated reciprocity or emotional bookkeeping. One of the drawbacks of this model, GroofiWorld, however, is that it lacks social bonding, a factor that may play an important role in real societies of primates. To investigate the effect of social bonding on exchange relations, in the present study, we add ‘social bonding’ to the model ‘GrooFiWorld.’ In the new model, called ‘FriendsWorld,’ social bonds or ‘friends’ are defined as the top 25 % grooming partners and individuals are given a tendency to follow their friends. Note that they do not intend to reciprocate or interchange social services with friends. Results show that this mechanism of ‘follow-your-friends,’ not only increases social interactions among top grooming partners, but also strengthens the patterns of reciprocation and interchange. Our findings suggest that, in real primates, reciprocation and interchange may emerge as a side-effect of the social–spatial structure of the group and subsequently be strengthened by social bonding as represented in FriendsWorld. We give predictions that distinguish between the mechanism of ‘follow-your-friends’ and emotional bookkeeping.

The production and decay of the top partner T in the left–right twin Higgs model at the ILC and CLIC

The left–right twin Higgs model (LRTHM) predicts the existence of the top partner T. In this work, we make a systematic investigation for the single and pair production of this top partner T through the processes: e+e−→tT¯+Tt¯ and TT¯, the neutral scalar (the SM-like Higgs boson h or neutral pseudoscalar boson ϕ0) associate productions e+e−→tT¯h+Tt¯h, TT¯h, tT¯ϕ0+Tt¯ϕ0 and TT¯ϕ0. From the numerical evaluations for the production cross sections and relevant phenomenological analysis we find that (a) the production rates of these processes, in the reasonable parameter space, can reach the level of several or tens of femtobarns; (b) for some cases, the peak value of the resonance production cross section can be enhanced significantly and reaches to the level of pb; (c) the subsequent decay of T→ϕ+b→tb¯b may generate typical phenomenological features rather different from the signals from other new physics models beyond the standard model (SM); and (d) since the relevant SM background is generally not large, some signals of the top partner T predicted by the LRTHM may be detectable in the future ILC and CLIC experiments.

The elusive gluon

We study the phenomenology of vector resonances in the context of natural composite Higgs models. A mild hierarchy between the fermionic partners and the vector resonances can be expected in these models based on the following arguments. Both direct and indirect (electroweak and flavor precision) constraints on fermionic partners are milder than the ones on spin one resonances. Also the naturalness pressure coming from the top partners is stronger than that induced by the gauge partners. This observation implies that the search strategy for vector resonances at the LHC needs to be modified. In particular, we point out the importance of heavy gluon decays (or other vector resonances) to top partner pairs that were overlooked in previous experimental searches at the LHC. These searches focused on simplified benchmark models in which the only new particle beyond the Standard Model was the heavy gluon. It turns out that, when kinematically allowed, such heavy-heavy decays make the heavy gluon elusive, and the bounds on its mass can be up to 2TeV milder than in the simpler models considered so far for the LHC14. We discuss the origin of this difference and prospects for dedicated searches.

Top Partner Discovery in the T → tZ channel at the LHC

In this paper we study the discovery potential of the LHC run II for heavy vector-like top quarks in the decay channel to a top and a $Z$ boson. Despite the usually smaller branching ratio compared to charged-current decays, this channel is rather clean and allows for a complete mass reconstruction of the heavy top. The latter is achieved in the leptonic decay channel of the $Z$ boson and in the fully hadronic top channel using boosted jet and jet substructure techniques. To be as model-independent as possible, a simplified model approach with only two free parameters has been applied. The results are presented in terms of parameter space regions for $3\sigma$ evidence or $5\sigma$ discovery for such new states in that channel.

Phenomenological aspects of supersymmetric composite Higgs models

We study previously introduced models of pseudo--Nambu-Goldstone boson Higgs from linearly realized minimal coset SO(5)/SO(4) and matter in the fundamental in supersymmetric theories. Partial compositeness is at work for top and electroweak gauge fields. New states potentially relevant for LHC signatures are identified and we show how to reinterpret existing experimental results as exclusion bounds. The lightest colored particles, with a mass below the TeV, are fermionic and scalars top partners. We outline a viable mechanism originating masses of other standard model quarks: they result from the generation of dimension five operators in a nonminimal flavor violating context. We study the impact of such operators on flavor processes and we show how experimental bounds are satisfied.

Fully hadronic decays of a singly produced vectorlike top partner at the LHC

Single production of vectorlike top partners is becoming a major focus of new searches, as the increasing mass limits on vectorlike quarks obtained by the ATLAS and CMS collaborations are such that single production is becoming competitive with respect to pair production. Typically searches focus on decays containing leptons in the final state in order to have less background from the Standard Model processes. However the current centre of mass energies available in the latest LHC runs limit the searches to low masses. Fully hadronic final states may be an alternative option for discovery, as they allow a larger number of signal events if backgrounds can be kept under control. We study the fully hadronic decay of a singly produced vectorlike top partner and we show a strategy to extract the signal over the background, considering as a benchmark the $20\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of data collected at 8 TeV with the run I of the LHC.