LHC Phenomenology of an Extended Standard Model with a Real Scalar Singlet

TL;DR

The paper analyzes a minimal extension of the Standard Model that adds a real scalar singlet (the xSM) to the Higgs sector, exploring two primary scenarios: mixing with the SM Higgs leading to two Higgs mass eigenstates with altered couplings and possible H2→H1H1 decays, and a Z2-symmetric stable singlet providing a dark matter candidate via the Higgs portal. It assesses electroweak precision constraints, LEP bounds, and LHC discovery prospects for the Higgs sector, showing that mixing can relieve tensions and still yield detectable signals, while a DM singlet both fixes relic abundance and constrains Higgs phenomenology through potential invisible decays and direct detection signatures. The work provides benchmark points illustrating viable regions and demonstrates how collider signals, relic density, and direct detection are interconnected through the Higgs-singlet couplings. Overall, the xSM offers testable implications for EWSB and dark matter, with distinct LHC and direct-detection signatures depending on whether the singlet mixes with the Higgs or is stable.

Abstract

Gauge singlet extensions of the Standard Model (SM) scalar sector may help remedy its theoretical and phenomenological shortcomings while solving outstanding problems in cosmology. Depending on the symmetries of the scalar potential, such extensions may provide a viable candidate for the observed relic density of cold dark matter or a strong first order electroweak phase transition needed for electroweak baryogenesis. Using the simplest extension of the SM scalar sector with one real singlet field, we analyze the generic implications of a singlet-extended scalar sector for Higgs boson phenomenology at the Large Hadron Collider (LHC). We consider two broad scenarios: one in which the neutral SM Higgs and singlet mix and the other in which no mixing occurs and the singlet can be a dark matter particle. For the first scenario, we analyze constraints from electroweak precision observables and their implications for LHC Higgs phenomenology. For models in which the singlet is stable, we determine the conditions under which it can yield the observed relic density, compute the cross sections for direct detection in recoil experiments, and discuss the corresponding signatures at the LHC.

Figures (11)

• Figure 1: Reduction factor of the Higgs boson signal with respect to the SM. The LEP SM Higgs mass bound for $\xi_i^2=1$ is marked as the vertical red line. All of the points indicated are consistent with the LEP bounds. The right (left) panel does (not) include constraints from EWPO as discussed below.
• Figure 2: Feynman diagrams of gauge boson propagators that are affected by Higgs bosons.
• Figure 3: Discovery potential (by traditional modes) of two Higgs bosons at CMS with 30 fb$^{-1}$ of data vs. the Higgs boson masses. Shown are the points where both, one or no Higgs bosons are discoverable at the LHC. Consistency with EWPO restricts the range of the lightest Higgs boson, making discovery of at least one Higgs boson likely. Exceptions include the case of a light SM-like Higgs boson that dominantly decays to a light singlet-like Higgs. The lightest Higgs can be very light due to the large singlet composition, making its coupling to SM fields weak.
• Figure 4: Discovery potential of two Higgs bosons at CMS with 30 fb$^{-1}$ of data for masses and the signal reduction factor, $\xi^2$. The left (right) panels show the mass and $\xi^2$ value for the light (heavy) Higgs boson while the bottom (top) panels do (do not) apply the constraints from EWPO. Shown are the points where both, one or no Higgs bosons are discoverable at the LHC through the traditional modes. Consistency with EWPO restricts the range of the lightest Higgs boson, making discovery of at least one Higgs boson likely. Exceptions include the case of a light SM-like Higgs boson that dominantly decays to a light singlet-like Higgs.
• Figure 5: Minimum value of $\xi^2$ that can be probed at CMS with 30 fb$^{-1}$ of data in discovering a Higgs boson with $5\sigma$ significance. We consider the modes $H\to ZZ\to 4\ell$ and $H\to WW\to \ell \nu jj$. Over most of the range $M_H \gtrsim 150$ GeV, the Higgs boson can be discovered and its mixing can be shown to significantly deviate from the SM value. Points shown that are above the SM coupling line imply that an enhanced Higgs coupling is required to yield a $5\sigma$ signal. The $ZZ$ mode is weakened at $m_H=170$ GeV since the Higgs decay becomes dominated by $H\to WW$.