Viable twin cosmology from neutrino mixing

Twin Higgs models solve the little hierarchy problem without introducing new colored particles; however, they are often in tension with measurements of the radiation density at late times. Here we explore viable cosmological histories for twin Higgs models. In particular, we show that mixing between the Standard Model (SM) and twin neutrinos can thermalize the two sectors below the twin QCD phase transition, significantly reducing the twin sector’s contribution to the radiation density. The requisite twin neutrino masses of $\mathcal{O}(1–20)\mathrm{GeV}$ and mixing angle with SM neutrinos of ${10}^{-3}-{10}^{-5}$ can be probed in a variety of current and planned experiments. We further find that these parameters can be naturally accessed in a warped UV completion, where the neutrino sector can also generate the $Z_2$-breaking Higgs mass term needed to produce the hierarchy between the symmetry breaking scales $f$ and $v$.

Figure 1

Contribution to $\mathrm{\Delta }{N}_{\mathrm{eff}}$ with heavy twin neutrinos ($m_{\tilde{\nu }}=10\mathrm{GeV}$) and massless twin photon, when varying ${\mathrm{\Lambda }}^{\prime }/\mathrm{\Lambda }$. Each band corresponds to varying $f/v$ from 3 (top of band) to 7 (bottom of band). The smallest contribution is obtained when the two sectors decouple between their QCD phase transitions. The dashed bounded regions correspond to one- and two-generation twin Higgs models. The dotted lines correspond to the MTH cases where the twin electron or twin electron and muon are heavy. The Planck constraint of $N_{\mathrm{eff}}=3.15\pm 0.23$ [25] is also shown.

Figure 2

Diagrams (in the interaction basis) responsible for thermalizing the SM and twin sector. Mass insertions correspond to an insertion of a mixing between a SM and twin neutrino.

Figure 3

Contours of the cosmologically preferred region $0.3\mathrm{GeV}<T_d<1\mathrm{GeV}$ as a function of the twin neutrino mass and mixing, as well as current and projected bounds on sterile neutrinos. The shaded purple region shows the 95% C.L. limits from DELPHI on sterile neutrinos produced from $Z$ decays at LEP [28]. We also show projected reaches from displaced searches (see next section) as dashed curves. Projections for SHiP [29] (green curve), DUNE [30] (yellow curve), and FCC-ee [31, 32] (red curve) are taken from [33]. The LHC reach at $\sqrt{s}=13\mathrm{TeV}$ with $300{\mathrm{fb}}^{-1}$, with searches for lepton jets (blue curve) and trileptons (brown curve) are also shown [34]. The exclusion region and projection curves are valid only if the twin neutrino cannot decay into light twin particles, for instance, in the FTH (one-generation) scenario.

Figure 4

An illustration of the structure of the neutrino sector in the warped UV completion.