Cosmological Signatures of a Mirror Twin Higgs

Transcription

1 Cosmological Signatures of a Mirror Twin Higgs Zackaria Chacko University of Maryland, College Park Curtin, Geller & Tsai

2 Introduction

3 The Twin Higgs framework is a promising approach to the naturalness problem of the Standard Model (SM). In Mirror Twin Higgs models, the SM is extended to include a complete mirror ( twin ) copy of the SM, with its own particle content and gauge groups. The SM and its twin counterpart are related by a discrete Z 2 twin symmetry. SM A Z 2 SM B The mirror particles are completely neutral under the SM strong, weak and electromagnetic forces. Only feel gravity.

4 In Mirror Twin Higgs models, the one loop quadratic divergences that contribute to the Higgs mass are cancelled by twin sector states that carry no charge under the SM gauge groups. Discovery of these states at LHC is therefore difficult. May explain null results.

5 The SM and twin SM primarily interact through the Higgs portal. This interaction is needed for cancellation of quadratic divergences. After electroweak symmetry breaking, SM Higgs and twin Higgs mix. Higgs couplings to SM states are suppressed by the mixing. Higgs now has (mixing suppressed) couplings to twin states. A soft breaking of the Z 2 symmetry ensures that v B, the VEV of the twin Higgs, is greater than v A, the VEV of the SM Higgs. The mixing angle ~ v A /v B. Higgs measurements constrain v A /v B 1/3. Twin fermions are heavier than SM fermions by a factor of v B /v A. Naturalness requires v A /v B 1/5. (Twin top should not be too heavy.)

6 The Higgs portal interaction has implications for cosmology. Interactions mediated by the Higgs keep the SM and twin sectors in thermal equilibrium until temperatures of order a few GeV. Then the twin photon and twin neutrinos contribute significantly to the energy density in radiation at the time of BBN and CMB. Leads to a contribution to effective number of neutrinos = 5.7. The 2σ bound from CMB on dark radiation is given by The simplest Mirror Twin Higgs model is excluded!

7 Two distinct approaches to this problem have been proposed. Introduce hard breaking of Z 2 to alter the decoupling temperature and the number of degrees of freedom in the twin sector at a given temperature. Farina; Barbieri, Hall & Harigaya; Csaki, Kuflik & Lombardo Introduce new dynamics that preferentially heats up the SM sector after the two sectors have decoupled. May not require further Z 2 breaking. ZC, Craig, Fox & Harnik; Craig, Koren & Trott We will focus on the second approach, and assume no further breaking of Z 2. Then the light degrees of freedom at CMB include the twin photon plus the 3 (massless) twin neutrinos. Treat as a free parameter. If there is a baryon asymmetry in the mirror sector, the bath will also contain twin baryons and electrons. Treat the asymmetry as a non-zero free parameter.

8 In the cosmological framework, mirror baryons, electrons, photons and neutrinos lead to distinctive signals that can potentially distinguish this class of models. The mirror particles affect the dynamics of the visible sector through gravity. mirror particles protons gravity electrons photons

9 What are the distinctive cosmological signals associated with this scenario? Twin photons and twin neutrinos constitute distinct forms of dark radiation that have different effects on the CMB, and can be distinguished. The twin neutrinos free stream, suppressing inhomegeneities. The twin photons scatter of dark baryons. Do not free stream till late. Fraction of dark radiation that free streams is fixed by the model. A prediction! The twin baryons constitute an acoustic subcomponent of dark matter. Baryon acoustic oscillations in the twin sector lead to a characteristic suppression of large scale structure. The twin baryons in our galaxy may have cooled to form a dark disc in some regions of parameter space.

10 Parametrizing Mirror Cosmology

11 To describe Mirror Twin Higgs cosmology we need 3 (additional) parameters, ΔN eff represents the energy density in dark radiation, expressed in terms of the effective number of neutrinos. The ratio of Higgs VEVs v B /v A fixes the masses of the mirror particles. The parameter r all represents the fractional contribution of mirror matter to the total energy density in dark matter. Given r all the fractional contributions of mirror hydrogen and helium to the dark matter density are determined by twin Big Bang nucleosynthesis (TBBN). The signals are sensitive to the relative fractions of hydrogen and helium.

12 Twin Big Bang Nucleosynthesis The cosmological signals are sensitive to the relative abundances of mirror hydrogen and helium. This depends on the neutron fraction at freeze out. To determine the neutron fraction at freeze out, we must know the masses of the mirror proton and neutron, and the temperature of the twin sector. Given v B /v A we can determine the mass of a mirror nucleon, From lattice QCD data we can determine the mass splitting between the mirror proton and neutron as a function of v B /v A,

13 The temperature in the mirror sector is related in a simple way to ΔN eff, We can calculate the neutron fraction at freeze out by solving the appropriate Boltzmann equation, Neutron decays after freeze out are neglected, since lattice QCD data indicates that binding energy of mirror deuterium is relatively large compared to the SM. The relative abundances of hydrogen and helium depend on v B /v A and ΔN eff, but are not sensitive to r all.

14 The fraction of helium is about 75% by weight, as compared to just 25% in the visible sector! The effects of mirror helium on cosmology cannot be neglected.

15 Recombination The cosmological signals depend sensitively on the time when mirror hydrogen and helium recombine. Hydrogen recombination determines when twin photons start to freestream. Also sets the time when twin Baryon Acoustic oscillations (TBAO) cease. We can determine the time at which mirror hydrogen recombines as a function of v B /v A, ΔN eff and r all by solving a Boltzmann equation. The fraction of free electrons evolves with time as Peebles correction forward rate detailed balance backward rate

16 Since the mirror sector is colder than the visible sector, and the atomic binding energies larger, recombination occurs earlier! We can obtain an estimate of the time at which helium recombines by using detailed balance.

17 CMB Signals of a Mirror Twin Higgs

18 How is CMB sensitive to dark radiation? Consider ΛCDM parameters. ρ m ρ b ρ Λ A s n s τ r The amplitude of a CMB mode is very sensitive to the fraction of energy density in matter when it crosses the horizon. In the presence of dark radiation, the ΛCDM fit will increase ρ m to keep the time of matter-radiation equality fixed. But then Hubble expansion is larger! Time to last scattering reduced. The fit will also increase ρ Λ to keep the angular size of the last scattering surface fixed. The primary CMB sensitivity to dark radiation is because the amount of time for diffusion damping during the era of acoustic oscillations is affected.

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20 At a subdominant level, the CMB signals of dark radiation also depends on whether it free streams (like neutrinos), or scatters with a short mean free path (like a fluid). While the twin neutrinos free stream, the twin photons are prevented from free streaming by Compton scattering off twin electrons. At later times, after recombination happens in the twin sector, the twin photons also free stream. Since the twin electron is heavier, this happens during the CMB epoch, when the SM temperature is of order an ev.

21 The size of these subleading effects depends on the free streaming fraction, f υ. This is defined as the total energy in free streaming radiation expressed as a fraction of the total energy in radiation. In the limit that is small, free streaming dark radiation and scattering dark radiation contribute to υ f with opposite sign! Their effects on the CMB are different!

22 The amplitudes of the CMB modes depend on f υ. Peebles Hu & Sugiyama The locations of the CMB peaks also depend on f υ. For higher l, Bashinsky & Seljak Free Streaming DR Scattering DR The sign of the effect is different in the two cases! Distinguishable!

23 The Mirror Twin Higgs predicts the ratio How well can current and future CMB experiments distinguish this? The current 2σ bounds on and stand at 0.5 and 0.6 respectively. The sensitivity is expected to improve by an order of magnitude in CMB-S4. Baumann, Green, Meyers & Wallisch Brust, Cui & Sigurdson Planck 2015 CMB-S4

24 Signals in Large Scale Structure

25 The interactions of twin baryons with twin photons at early times suppresses the growth of density perturbations in the twin sector. e γ e p γ e γ e p The size of these effects is determined by Γ, the rate of momentum transfer between twin photons and twin baryons, Since Γ > H at early times, these effects are large and suppress the growth of structure in the twin sector till recombination occurs (at around an ev).

26 Consider a mode that enters the horizon well before recombination. Acoustic oscillations in the twin sector suppress growth of structure relative to ΛCDM. horizon entry acoustic oscillations Modes which enter after twin recombination are relatively unaffected.

27 Consider first the case with just mirror hydrogen. For modes that enter before twin recombination, the matter power spectrum has an overall suppression, In addition, there is an oscillatory feature in the power spectrum with frequency set by the size of the sound horizon at the time of twin recombination!

28 When both hydrogen and helium are present, for modes that enter prior to the recombination of helium there is an overall suppression, (1 r all ) 2. The oscillatory feature is still present, but is now a superposition of the oscillations of hydrogen with those of the dominant helium component. Since helium recombines earlier, the period of oscillations is now different!

29 For a given ΔN eff, it does not appear possible to reproduce all the features of the matter power spectrum with just a single species of atom, even in linear regime. Since hydrogen recombines later, its oscillations determine the power spectrum at lower k. At higher k, the effects of helium begin to dominate. A highly distinctive feature of the MTH framework! Weak lensing, CMB lensing and eventually 21 cm line measurements will test this.

30 Future measurements will probe the matter power spectrum at the few percent level. In the absence of a signal, this can be used to constrain r all. If no signal is observed, r all will be bounded at the few percent level.

31 Mirror Matter Distribution in the Galaxy

32 Most of the visible matter in our galaxy has collapsed into a disc. Is the mirror matter in our galaxy in the form of a halo or a disc? When halo formation occurs at z ~ 10, the mirror atoms fall into gravitational wells. Their collisions result in a shock wave that heats up the mirror sector. If the resulting temperature is high enough, the mirror atoms will be ionized! The ionized mirror particles lose energy in collisions with background mirror photons, and by emitting radiation after colliding with each other. If the time scale for this energy loss is less than the age of the universe, the ionized halo will collapse into a disc!

33 There are two processes that play a role in energy loss. Compton scattering off background mirror photons. Bremsstrahlung radiation arising from mirror electron-proton collisions. Compton cooling rate is not very sensitive to the mirror particle distribution. However, the bremsstrahlung cooling rate is sensitive to the mirror particle distribution, since it depends on collisions of particles in the halo. In the case of Mirror Twin Higgs, bremsstrahlung cooling is dominant. To calculate the cooling rate, need to make an assumption about the distribution of mirror particles in the halo during the cooling process. assume a uniform distribution of mirror particles assume cooling happens in the core region of an NFW halo

34 In the case of the Mirror Twin Higgs, the result of the analysis is very sensitive to assumptions about the initial distribution of matter in the halo! The model lives close to the boundary in parameter space that divides the disc region from the halo region. Neutral halo, ionized halo and ionized disc may all be possible in the region of parameter space preferred by naturalness!

35 Conclusions

36 The Mirror Twin Higgs framework leads to characteristic cosmological signals. Mirror photons and mirror neutrinos constitute distinct forms of dark radiation that have different effects on the CMB, and can be distinguished. While mirror neutrinos free stream, the mirror photons scatter off dark baryons. Fraction of dark radiation that free streams is a prediction of the Mirror Twin Higgs framework that can potentially be tested in future CMB experiments. The mirror baryons constitute an acoustic subcomponent of dark matter. Baryon acoustic oscillations of mirror hydrogen and helium leave a distinctive imprint on large scale structure that can be probed in future experiments. These two effects together can potentially help address two large scale anomalies, the H 0 problem and σ 8 problem. The mirror baryons in a galaxy might cool to form a double disc in some regions of parameter space.