We explore the possibility of discovering the mirror baryons and electrons of the Mirror Twin Higgs model in direct detection experiments, in a scenario in which these particles constitute a subcomponent of the observed DM. We consider a framework in
which the mirror fermions are sub-nano-charged, as a consequence of kinetic mixing between the photon and its mirror counterpart. We consider both nuclear recoil and electron recoil experiments. The event rates depend on the fraction of mirror DM that is ionized, and also on its distribution in the galaxy. Since mirror DM is dissipative, at the location of the Earth it may be in the form of a halo or may have collapsed into a disk, depending on the cooling rate. For a given mirror DM abundance we determine the expected event rates in direct detection experiments for the limiting cases of an ionized halo, an ionized disk, an atomic halo and an atomic disk. We find that by taking advantage of the complementarity of the different experiments, it may be possible to establish not just the multi-component nature of mirror dark matter, but also its distribution in the galaxy. In addition, a study of the recoil energies may be able to determine the masses and charges of the constituents of the mirror sector. By showing that the mass and charge of mirror helium are integer multiples of those of mirror hydrogen, these experiments have the potential to distinguish the mirror nature of the theory. We also carefully consider mirror plasma screening effects, showing that the capture of mirror dark matter particles in the Earth has at most a modest effect on direct detection signals.
We consider a class of models in which the neutrinos acquire Majorana masses through mixing with singlet neutrinos that emerge as composite states of a strongly coupled hidden sector. In this framework, the light neutrinos are partially composite par
ticles that obtain their masses through the inverse seesaw mechanism. We focus on the scenario in which the strong dynamics is approximately conformal in the ultraviolet, and the compositeness scale lies at or below the weak scale. The small parameters in the Lagrangian necessary to realize the observed neutrino masses can naturally arise as a consequence of the scaling dimensions of operators in the conformal field theory. We show that this class of models has interesting implications for a wide variety of experiments, including colliders and beam dumps, searches for lepton flavor violation and neutrinoless double beta decay, and cosmological observations. At colliders and beam dumps, this scenario can give rise to striking signals involving multiple displaced vertices. The exchange of hidden sector states can lead to observable rates for flavor violating processes such as $mu rightarrow e gamma$ and $mu rightarrow e$ conversion. If the compositeness scale lies at or below a hundred MeV, the rate for neutrinoless double beta decay is suppressed by form factors and may be reduced by an order of magnitude or more. The late decays of relic singlet neutrinos can give rise to spectral distortions in the cosmic microwave background that are large enough to be observed in future experiments.
We explore the cosmological signals of theories in which the neutrinos decay into invisible dark radiation after becoming non-relativistic. We show that in this scenario, near-future large scale structure measurements from the Euclid satellite, when
combined with cosmic microwave background data from Planck, may allow an independent determination of both the lifetime of the neutrinos and the sum of their masses. These parameters can be independently determined because the Euclid data will cover a range of redshifts, allowing the growth of structure over time to be tracked. If neutrinos are stable on cosmological timescales, these observations can improve the lower limit on the neutrino lifetime by seven orders of magnitude, from $mathcal{O}(10)$ years to $2times 10^8$ years ($95%$ C.L.), without significantly affecting the measurement of neutrino mass. On the other hand, if neutrinos decay after becoming non-relativistic but on timescales less than $mathcal{O}(100)$ million years, these observations may allow, not just the first measurement of the sum of neutrino masses, but also the determination of the neutrino lifetime from cosmology.
At present, the strongest upper limit on $sum m_{ u}$, the sum of neutrino masses, is from cosmological measurements. However, this bound assumes that the neutrinos are stable on cosmological timescales, and is not valid if the neutrino lifetime is l
ess than the age of the universe. In this paper, we explore the cosmological signals of theories in which the neutrinos decay into invisible dark radiation on timescales of order the age of the universe, and determine the bound on the sum of neutrino masses in this scenario. We focus on the case in which the neutrinos decay after becoming non-relativistic. We derive the Boltzmann equations that govern the cosmological evolution of density perturbations in the case of unstable neutrinos, and solve them numerically to determine the effects on the matter power spectrum and lensing of the cosmic microwave background. We find that the results admit a simple analytic understanding. We then use these results to perform a Monte Carlo analysis based on the current data to determine the limit on the sum of neutrino masses as a function of the neutrino lifetime. We show that in the case of decaying neutrinos, values of $sum m_{ u}$ as large as 0.9 eV are still allowed by the data. Our results have important implications for laboratory experiments that have been designed to detect neutrino masses, such as KATRIN and KamLAND-ZEN.
We consider the collider signals arising from kinetic mixing between the hypercharge gauge boson of the Standard Model and its twin counterpart in the Mirror Twin Higgs model, in the framework in which the twin photon is massive. Through the mixing,
the Standard Model fermions acquire charges under the mirror photon and the mirror Z boson. We determine the current experimental bounds on this scenario, and show that the mixing can be large enough to discover both the twin photon and the twin Z at the LHC, or at a future 100 TeV hadron collider, with dilepton resonances being a particularly conspicuous signal. We show that, in simple models, measuring the masses of both the mirror photon and mirror Z, along with the corresponding event rates in the dilepton channel, overdetermines the system, and can be used to test these theories.
We consider theories in which the generation of neutrino masses is associated with the breaking of an approximate global lepton number symmetry. In such a scenario the spectrum of light states includes the Majoron, the pseudo-Nambu Goldstone boson as
sociated with the breaking of the global symmetry. For a broad range of parameters, the Majoron decays to neutrinos at late times, after the cosmic neutrinos have decoupled from the thermal bath, resulting in a secondary contribution to the cosmic neutrino background. We determine the current bounds on this scenario, and explore the possibility of directly detecting this secondary cosmic neutrino background in experiments based on neutrino capture on nuclei. For Majoron masses in the eV range or below, the neutrino flux from these decays can be comparable to that from the primary cosmic neutrino background, making it a promising target for direct detection experiments. The neutrinos from Majoron decay are redshifted by the cosmic expansion, and exhibit a characteristic energy spectrum that depends on both the Majoron mass and its lifetime. For Majoron lifetimes of order the age of the universe or larger, there is also a monochromatic contribution to the neutrino flux from Majoron decays in the Milky Way that can be comparable to the diffuse extragalactic flux. We find that for Majoron masses in the eV range, direct detection experiments based on neutrino capture on tritium, such as PTOLEMY, will be sensitive to this scenario with 100 gram-years of data. In the event of a signal, the galactic and extragalactic components can be distinguished on the basis of their distinct energy distributions, and also by using directional information obtained by polarizing the target nuclei.
