Decays Of Top Partners Research Articles

Exotic decays of top partners: Mind the search gap

Many standard model extensions, including composite Goldstone Higgs models, predict vector-like fermionic top-partners at the TeV scale. The intensive search programmes by ATLAS and CMS focus on decays into a 3rd generation quark and an electroweak boson (W,Z,h). However, underlying models of partial compositeness contain additional states that give rise to exotic top partner decays. We consider a well-motivated scenario in which a charge-2/3 top-partner decays into a pseudo-scalar, T→ta, with a→gg or bb¯ dominating below the tt¯ threshold. We show that the constraints on the top partner mass from QCD pair production are substantially weakened, still allowing a top partner mass as light as 400 GeV.

Exotic decays of top partners with charge 5/3: bounds and opportunities

Exotic decays of top partners in new bosons are the norm in realistic models of a composite Higgs. We focus on the custodial charge-5/3 partner, which normally decays exclusively into tW+. The new channels include a colour-sextet, X5/3→ overline{b} π6, as well as singly and doubly charged scalars, X5/3→ t\U0001d719+, b\U0001d719++. We use existing same-sign lepton searches to show that the new final states are constrained at the same level as the standard one. At the same time, exotic final states also offer opportunities for improvement: examples include a hard photon in X5/3→ t\U0001d719+→ tW+γ decays, and top-rich channels which arises in several exotic X5/3 decays.

Common exotic decays of top partners

Many Standard Model extensions which address the hierarchy problem contain Dirac-fermion partners of the top quark at the TeV scale. We discuss several classes of composite Higgs models with top partners which have underlying descriptions in terms of a fundamental gauge-fermion dynamics. All of these models contain additional BSM states which are pseudo-Nambu-Goldstone bosons and are thus likely to be lighter than the top partners. We first discuss constraints from direct production and decay of these new states at the LHC and show that they can be light without conflicting with current searches. The pNGBs couple to top partners and thus provide new decay channels for them. We identify the novel top partner decays which occur most commonly in underlying models, provide effective Lagrangians and benchmarks, and discuss opportunities for future searches.

Common exotic decays of top partners

Many standard model extensions that address the hierarchy problem contain Dirac-fermion partners of the top quark, which are typically expected around the TeV scale. Searches for these vector-like quarks mostly focus on their decay into electroweak gauge bosons and Higgs plus a standard model quark. In this article, backed by models of composite Higgs, we propose a set of simplified scenarios, with effective Lagrangians and benchmarks, that include more exotic decay channels, which modify the search strategies and affect the bounds. Analysing several classes of underlying models, we show that exotic decays are the norm and commonly appear with significant rates. All these models contain light new scalars that couple to top partners with charge 5/3, 2/3, and −1/3.

Experimental considerations motivated by the diphoton excess at the LHC

We consider the immediate or near-term experimental opportunities offered by some scenarios that could explain the new diphoton excess at the LHC. If the excess is due to a new particle $X_s$ at 750 GeV, additional new particles are required, providing further signals. If connected with naturalness, the $X_s$ may be produced in top partner decays. Then a $t'\bar t'$ signal, with $t'\to t X_s$ and $X_s\to gg$ dominantly, might be discovered by reinterpreting 13 TeV SUSY searches in multijet events with low MET and/or a lepton. If $X_s$ is a bound state of quirks, the signal events may be accompanied by an unusual number of soft tracks or soft jets. Other resonances including dilepton and photon+jet as well as dijet may lie at or above this mass, and signatures of hidden glueballs might also be observable. If the "photons" in the excess are actually long-lived particles decaying to photon pairs or to electron pairs, there are opportunities for detecting overlapping photons and/or unusual patterns of apparent photon-conversions in either $X_s$ or 125 GeV Higgs decays. There is also the possibility of events with a hard "photon" recoiling against a narrow isolated HCAL-only "jet", which, after the jet's energy is corrected for its electromagnetic origin, would show a peak at 750 GeV.

Odd top partners at the LHC

LHC searches for fermionic top partners $T$ focus on three decay topologies: $T\to bW$, $T\to tZ$, and $T\to th$. However, top partners may carry new conserved quantum numbers that forbid these decays. The simplest possibility is a conserved parity, under which the top partner is odd and all SM states are even. In this case, decays of top partners may involve new particle-odd scalars, leading to signal topologies more commonly associated with supersymmetry, either with or without R-parity conservation. We study a simplified model in which this possibility is realized, and estimate the bounds on the top partner mass in this model implied by LHC searches for supersymmetry. We find that the bounds can be significantly weaker than in the conventional top partner decay scenario. For example, if the new parity is exact, a 500 GeV top partner is allowed as long as the lightest parity-odd scalar mass is between 325 and 500 GeV. The lower allowed top partner mass reduces the need for fine-tuning in the Higgs mass parameter, compared to the conventional decay scenario. We also present an explicit model, the Oddest Little Higgs, which exhibits this phenomenology.

Exotic top partners and Little Higgs

Little Higgs models often give rise to top partners beyond the minimal ones necessary for the cancellation of quadratic divergences. We review how this occurs and discuss the phenomenology of these exotic states. We emphasize the possible importance of new pseudo-Nambu-Goldstone bosons in top partner decays. Indeed, cascade decays of exotic top partners may be the best way to discover these new bosons. We illustrate these points with a new Little Higgs construction based on an SO(10)/SO(5)^2 coset structure, which fills a gap in the model building literature. These observations motivate new search strategies for top partners at the LHC, including for final states with b-jets and a large multiplicity of electroweak bosons.

Top partner probes of extended Higgs sectors

Natural theories of the weak scale often include fermionic partners of the top quark. If the electroweak symmetry breaking sector contains scalars beyond a single Higgs doublet, then top partners can have sizable branching ratios to these extended Higgs sector states. In fact, top partner decays may provide the most promising discovery mode for such scalars, especially given the large backgrounds to direct and associated production. In this paper, we present a search strategy for top partner decays to a charged Higgs boson and a bottom quark, focusing on the case where the charged Higgs dominantly decays to third-generation quarks to yield a multi-b final state. We also discuss ways to extend this search to exotic neutral scalars decaying to bottom quark pairs.

Multipleb-jets reveal top superpartners and the 125 GeV Higgs

We demonstrate that in supersymmetry (SUSY) with relatively light top superpartners, h -> bb can be a very promising channel to discover the SM-like Higgs resonance at the Large Hadron Collider (LHC), although in general contexts it is thought to be challenging because of its huge QCD background. In this scenario, the SM-like Higgs boson is mainly produced via cascade decays initiated by pair-produced stop or sbottom squarks. The good sensitivity to h -> bb owes a great deal to the application of multiple (>= 4) b-jet tagging in removing the QCD background, and color-flow variables for reconstructing the Higgs resonance. We show in two benchmark points that a SM-like Higgs resonance can be discovered at the 14 TeV LHC (with a signal-to-background ratio as high as 0.35), with ~< 40/fb of data. Potentially, this strategy can be also applied to non-SUSY theories with cascade decays of top partners for the SM-like Higgs search, such as little Higgs, composite Higgs, and Randall-Sundrum models.