QCD-Electroweak First-Order Phase Transition in a Supercooled Universe

If the electroweak sector of the standard model is described by classically conformal dynamics, the early Universe evolution can be substantially altered. It is already known that—contrarily to the standard model case—a first-order electroweak phase transition may occur. Here we show that, depending on the model parameters, a dramatically different scenario may happen: A first-order, six massless quark QCD phase transition occurs first, which then triggers the electroweak symmetry breaking. We derive the necessary conditions for this dynamics to occur, using the specific example of the classically conformal $\mathcal{B}\text{−}\mathcal{L}$ model. In particular, relatively light weakly coupled particles are predicted, with implications for collider searches. This scenario is also potentially rich in cosmological consequences, such as renewed possibilities for electroweak baryogenesis, altered dark matter production, and gravitational wave production, as we briefly comment upon.

Figure 1

Contour plot of the percolation temperature $T_p$ (black lines) as a function of $g$ and $m_{Z^{\prime }}$. The horizontal color bands show the temperature $T_{\mathrm{GH}}\equiv \mathcal{H}/2\pi$.

Figure 2

Possible trajectories of the scalar fields ($\varphi$, $h$) in the early Universe. All start from the origin (0,0).

Figure 3

Schematic cosmological histories for the parameter region $m_{Z^{\prime }}\sim \mathrm{TeV}$, assuming $m_N=0$ and $v_{\mathrm{QCD}}/T_n^{\mathrm{QCD}}=3$. The horizontal thin dashed lines are the boundaries between (I) and (II) for $v_{\mathrm{QCD}}/T_n^{\mathrm{QCD}}=1.5$, 2, and 2.5. Below the lower edge, $B>0$ is violated. For reference, we plot the large electron positron bound [35] (red line) and some LHC bounds from Fig. 6 in Ref. [36] (magenta line).

Figure 4

The GW power spectrum for $\beta /\mathcal{H}=10$, 100, 1000. We chose $T_{\mathrm{i}}=10\mathrm{GeV}$ and $\zeta =0.6$, 0.3, 0.15. The sensitivity of three configurations foreseen for the space mission LISA [50] are also shown.