Light Top Partners for a Light Composite Higgs

TL;DR

The paper establishes a robust, model-independent link between a light composite Higgs and the presence of light top partners in SO(5)/SO(4) composite Higgs frameworks with partial compositeness. Using Discrete Composite Higgs Models (DCHM_3 and DCHM_2), it shows that reducing the Higgs mass requires reducing elementary–composite mixings, which in turn lowers top-partner masses, yielding an approximately linear m_H–m_light correlation. Quantitative analyses indicate that a Higgs around 115–130 GeV typically implies at least one top partner below ~1 TeV, making such states accessible to the LHC, with current 7 TeV data already constraining the exotic X_{5/3} state and ṼT in certain parameter regions. The results hold across the calculable 3-site model and its simpler 2-site limit and have implications for 14 TeV LHC searches and broader composite-Higgs constructions.

Abstract

Anomalously light fermionic partners of the top quark often appear in explicit constructions, such as the 5d holographic models, where the Higgs is a light composite pseudo Nambu-Goldstone boson and its potential is generated radiatively by top quark loops. We show that this is due to a structural correlation among the mass of the partners and the one of the Higgs boson. Because of this correlation, the presence of light partners could be essential to obtain a realistic Higgs mass. We quantitatively confirm this generic prediction, which applies to a broad class of composite Higgs models, by studying the simplest calculable framework with a composite Higgs, the Discrete Composite Higgs Model. In this setup we show analytically that the requirement of a light enough Higgs strongly constraints the fermionic spectrum and makes the light partners appear. The light top partners thus provide the most promising manifestation of the composite Higgs scenario at the LHC. Conversely, the lack of observation of these states can put strong restrictions on the parameter space of the model. A simple analysis of the 7-TeV LHC searches presently available already gives some non-trivial constraint. The strongest bound comes from the exclusion of the 5/3-charged partner. Even if no dedicated LHC search exists for this particle, a bound of 611 GeV is derived by adapting the CMS search of bottom-like states in same-sign dileptons.

Figures (11)

• Figure 1: Schematic structure of the three-site DCHM.
• Figure 2: Scatter plots of the masses of the lightest $T$ and $\widetilde{T}$ resonances for $\xi = 0.2$ (left panel) and $\xi = 0.1$ (right panel) in the three-site DCHM model. The black dots denote the points for which $115\ {\rm GeV} \leq m_H \leq 130\ {\rm GeV}$, while the gray dots have $m_H > 130\ {\rm GeV}$. The scans have been obtained by varying all the composite sector masses in the range $[-8 f, 8f]$ and keeping the top mass fixed at the value $m_t = 150\ {\rm GeV}$. The area between the solid red lines represents the range obtained by applying the result in eq. (\ref{['eq:mHmt']}) for $115\ {\rm GeV} \leq m_H \leq 130\ {\rm GeV}$. The dashed blue line corresponds to the estimate of the lower bound on $m_{T-}$ given in eq. (\ref{['eq:mTlow']}).
• Figure 3: Scatter plots of the masses of the lightest exotic state of charge $5/3$ and of the lightest $\widetilde{T}$ resonance for $\xi = 0.2$ (left panel) and $\xi = 0.1$ (right panel) in the three-site DCHM model. The black dots denote the points for which $115\ {\rm GeV} \leq m_H \leq 130\ {\rm GeV}$, while the gray dots have $m_H > 130\ {\rm GeV}$. The scans have been obtained by varying all the composite sector masses in the range $[-8 f, 8f]$ and keeping the top mass fixed at the value $m_t = 150\ {\rm GeV}$.
• Figure 4: Schematic structure of the two-site DCHM.
• Figure 5: Scatter plots of the masses of the $T$ and $\widetilde{T}$ resonances for $\xi = 0.2$ (left panel) and $\xi = 0.1$ (right panel) in the two-site DCHM model. The black dots denote the points for which $115\ {\rm GeV} \leq m_H \leq 130\ {\rm GeV}$, while the gray dots have $m_H > 130\ {\rm GeV}$. The scans have been obtained by varying all the composite sector masses in the range $[-8 f_\pi, 8 f_\pi]$ and keeping the top mass fixed at the value $m_t = 150\ {\rm GeV}$. The area between the solid red lines represents the range obtained by applying the result in eq. (\ref{['eq:mHmt_2-site']}) for $115\ {\rm GeV} \leq m_H \leq 130\ {\rm GeV}$.