Multistep strongly first order phase transitions from new fermions at the TeV scale

In spite of the vast literature on the subject of first order electroweak phase transitions (EWPTs), which can provide the necessary conditions for generating the baryon asymmetry in the Universe, fermion-induced EWPTs still remain a rather uncharted territory. In this paper, we consider a simple fermionic extension of the Standard Model involving one $SU(2{)}_L$ doublet and two $SU(2{)}_L$ singlet vectorlike leptons, strongly coupled to the Higgs scalar and with masses close to the TeV scale. We show how such a simple scenario can give rise to a nontrivial thermal history of the Universe, involving strongly first order multistep phase transitions occurring at temperatures close to the electroweak scale. Finally, we investigate the distinct gravitational wave (GW) signatures of these phase transitions at future space-based GW detectors, such as LISA, DECIGO, and BBO, and briefly discuss the possible LHC signatures of the vectorlike leptons.

Figure 1

Thermal evolution of the potential, with $V(\varphi ,T)$ (in units of $m_Z^4$) plotted as a function of the background field value $\varphi$ for six temperature values: $T=700$, 500, 190, 174, 110, and 95 GeV. In these plots, the scalar and Daisy contributions have been taken into account when calculating the effective potential.

Figure 2

Strength of the earlier phase transition ${\xi }_2$ plotted against the strength of the late phase transition ${\xi }_1$.

Figure 3

Correlation between the strength of the later phase transition $\xi$ and the mass in GeV of the lighter neutral (left) and charged (right) VLL eigenstate.

Figure 4

Scatter plot of the strength of the later phase transition, $\xi$, vs the $h\to \gamma \gamma$ signal strength ${\mu }_{\gamma \gamma }$.

Figure 5

GW spectrum $h^2{\mathrm{\Omega }}_{\mathrm{GW}}$ for the earlier (dashed black line) and later (solid black line) SFOPTs, as a function of the frequency $f$ in hertz, for the benchmark point BM1 described in Table 1. The colored regions correspond to the sensitivities of several future GW experiments: the four possible LISA configurations, C1–C4, [2], and the DECIGO [3] and BBO [4] experiments. The sensitivity curves for the latter two have been taken from Ref. [71].

Figure 6

The same as Fig. 5, but for the benchmark point BM2 described in Table 1.

Figure 7

The same as Fig. 5, but for the benchmark point BM3 described in Table 1.