Viable twin cosmology from neutrino mixing

Abstract

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 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 O(1 − 20) 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 Z2-breaking Higgs mass term needed to produce the hierarchy between the symmetry breaking scales f and v. INTRODUCTIONTwin Higgs (TH) models provide an elegant solution to the hierarchy problem without introducing new states that are charged under the SM gauge symmetries [1]. Instead, a mirror sector with its own SU(3)×SU(2)×U(1) gauge symmetry is assumed. The Z 2 symmetry relating the SM and mirror sectors protects the Higgs mass from large radiative corrections, with the twin partners cancelling the SM quadratic divergences at one loop. Other variations of neutral naturalness include [2][3][4][5][6][7][8]. While this idea is very efficient at hiding new physics from the LHC and future colliders, it often leads to tension with cosmological observations due to the appearance of new light relativistic degrees of freedom (DOF), namely the twin photon and twin neutrinos.The standard assumption of TH models is that only the Higgs portal connects the SM and the mirror sectors. This maintains thermal equilibrium between the two sectors down to temperatures of a few GeV, below which the twin sector decouples [9]. At this point the twin and SM sectors have similar energy densities, and the twin photon and neutrinos contribute significantly to the radiation density at late times. In particular, the Mirror Twin Higgs (MTH) model-the scenario where the mirror sector is a full copy of the SM-predicts an exceedingly large contribution to the correction of the total radiation density (usually expressed in terms of ∆N eff , as measured from Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background (CMB)).Recently, several solutions have been proposed for the cosmological problems of the Twin Higgs, including the Fraternal Twin Higgs (FTH) [10], hard Z 2 -breaking in the Yukawa couplings [11], and SM reheating from a light right-handed neutrino [12]. Further cosmological aspects of Twin Higgs models, including dark matter, have been studied in [11][12][13][14][15][16][17][18][19].In this paper we propose that the neutrino portal can also naturally be used to connect the twin and the SM sectors. Mixing between the SM and twin neutrinos appears in many simple implementations of the twin neutrino sector. We show that such mixing can lower the decoupling tem...

Thermal neutrino portal to sub-MeV dark matter

“…where n eq J,S is the equilibrium number density of J, S, respectively [53,73]. These processes are able to maintain kinetic equilibrium between the HS and SM if Γ X eq (J, S ↔ ν ν) H. The ratio,…”

Cosmological signatures of a mirror twin Higgs

“…1 One approach is to admit hard breaking of the discrete Z 2 symmetry in the Yukawa couplings in the twin sector. This allows a large reduction in the number of degrees of freedom in the twin sector at the time when the two sectors decouple, leading to a suppression of ∆N ef f [38][39][40][41]. More radical proposals that produce the same result involve making the mirror sector vector-like [42], or even simply removing from the theory the first two generations of twin fermions, which do not play a role in solving the little hierarchy problem.…”

Collider signals of the Mirror Twin Higgs boson through the hypercharge portal

“…This can be realized without requiring additional breaking of the discrete Z 2 symmetry that relates the two sectors [42,43]. An alternative approach is to introduce hard breaking of the Z 2 into the twin sector Yukawa couplings, thereby altering the spectrum of mirror states [44][45][46][47]. This affects the number of degrees of freedom in the two sectors at the time of decoupling, allowing the cosmological bounds to be satisfied.…”