An official website of the United States government
Abstract The recent analysis from the SH0ES collaboration has confirmed the existence of a Hubble tension between measurements at high redshift ( z > 1000) and at low redshift ( z < 1) at the 5 σ level with the low redshift measurement giving a higher value. In this work we propose a particle physics model that can help alleviate the Hubble tension via an out-of-equilibrium hidden sector coupled to the visible sector. The particles that populate the dark sector consist of a dark fermion, which acts as dark matter, a dark photon, a massive scalar and a massless pseudo-scalar. Assuming no initial population of particles in the dark sector, feeble couplings between the visible and the hidden sectors via kinetic mixing populate the dark sector even though the number densities of hidden sector particles never reach their equilibrium distribution and the two sectors remain at different temperatures. A cosmologically consistent analysis is presented where a correlated evolution of the visible and the hidden sectors with coupled Boltzmann equations involving two temperatures, one for the visible sector and the other for the hidden sector, is carried out. The relic density of the dark matter constituted of dark fermions is computed in this two-temperature formalism. As a consequence, BBN predictions are upheld with a minimal contribution to Δ N eff . However, the out-of-equilibrium decay of the massive scalar to the massless pseudo-scalar close to the recombination time causes an increase in Δ N eff that can help weaken the Hubble tension.
more »
« less
A cosmologically consistent millicharged dark matter solution to the EDGES anomaly of possible string theory origin
A bstract Analysis of EDGES data shows an absorption signal of the redshifted 21-cm line of atomic hydrogen at z ∼ 17 which is stronger than expected from the standard ΛCDM model. The absorption signal interpreted as brightness temperature T 21 of the 21-cm line gives an amplitude of $$ -{500}_{-500}^{+200} $$ − 500 − 500 + 200 mK at 99% C.L. which is a 3.8 σ deviation from what the standard ΛCDM cosmology gives. We present a particle physics model for the baryon cooling where a fraction of the dark matter resides in the hidden sector with a U(1) gauge symmetry and a Stueckelberg mechanism operates mixing the visible and the hidden sectors with the hidden sector consisting of dark Dirac fermions and dark photons. The Stueckelberg mass mixing mechanism automatically generates a millicharge for the hidden sector dark fermions providing a theoretical basis for using millicharged dark matter to produce the desired cooling of baryons seen by EDGES by scattering from millicharged dark matter. We compute the relic density of the millicharged dark matter by solving a set of coupled equations for the dark fermion and dark photon yields and for the temperature ratio of the hidden sector and the visible sector heat baths. For the analysis of baryon cooling, we analyze the evolution equations for the temperatures of baryons and millicharged dark matter as a function of the redshift. We exhibit regions of the parameter space which allow consistency with the EDGES data. We note that the Stueckelberg mechanism arises naturally in strings and the existence of a millicharge would point to its string origin.
more »
« less
- Award ID(s):
- 1913328
- PAR ID:
- 10332176
- Date Published:
- Journal Name:
- Journal of High Energy Physics
- Volume:
- 2021
- Issue:
- 12
- ISSN:
- 1029-8479
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
A variety of supergravity and string models involve hidden sectors where the hidden sectors may couple feebly with the visible sectors via a variety of portals. While the coupling of the hidden sector to the visible sector is feeble its coupling to the inflaton is largely unknown. It could couple feebly or with the same strength as the visible sector which would result in either a cold or a hot hidden sector at the end of reheating. These two possibilities could lead to significantly different outcomes for observables. We investigate the thermal evolution of the two sectors in a cosmologically consistent hidden sector dark matter model where the hidden sector and the visible sector are thermally coupled. Within this framework we analyze several phenomena to illustrate their dependence on the initial conditions. These include the allowed parameter space of models, dark matter relic density, proton-dark matter cross section, effective massless neutrino species at BBN time, self-interacting dark matter cross-section, where self-interaction occurs via exchange of dark photon, and Sommerfeld enhancement. Finally fits to the velocity dependence of dark matter cross sections from galaxy scales to the scale of galaxy clusters is given. The analysis indicates significant effects of the initial conditions on the observables listed above. The analysis is carried out within the framework where dark matter is constituted of dark fermions and the mediation between the visible and the hidden sector occurs via the exchange of dark photons. The techniques discussed here may have applications for a wider class of hidden sector models using different mediations between the visible and the hidden sectors to explore the impact of Big Bang initial conditions on observable physics.more » « less
-
null (Ed.)A bstract An analysis of sub-MeV dark photon as dark matter is given which is achieved with two hidden sectors, one of which interacts directly with the visible sector while the second has only indirect coupling with the visible sector. The formalism for the evolution of three bath temperatures for the visible sector and the two hidden sectors is developed and utilized in solution of Boltzmann equations coupling the three sectors. We present exclusion plots where the sub-MeV dark photon can be dark matter. The analysis can be extended to a multi-temperature universe with multiple hidden sectors and multiple heat baths.more » « less
-
A bstract Hidden sectors are ubiquitous in supergravity theories, in strings and in branes. Well motivated models such as the Stueckelberg hidden sector model could provide a candidate for dark matter. In such models, the hidden sector communicates with the visible sector via the exchange of a dark photon (dark Z ′) while dark matter is constituted of Dirac fermions in the hidden sector. Using data from collider searches and precision measurements of SM processes as well as the most recent limits from dark matter direct and indirect detection experiments, we perform a comprehensive scan over a wide range of the Z ′ mass and set exclusion bounds on the parameter space from sub-GeV to several TeV. We then discuss the discovery potential of an $$ \mathcal{O} $$ O (TeV) scale Z ′ at HL-LHC and the ability of future forward detectors to probe very weakly interacting sub-GeV Z ′ bosons. Our analysis shows that the parameter space in which a Z ′ can decay to hidden sector dark matter is severely constrained whereas limits become much weaker for a Z ′ with no dark decays. The analysis also favors a self-thermalized dark sector which is necessary to satisfy the dark matter relic density.more » « less
-
A bstract An analysis of a tower of hidden sectors coupled to each other, with one of these hidden sectors coupled to the visible sector, is given and the implications of such couplings on physics in the visible sector are investigated. Thus the analysis considers n number of hidden sectors where the visible sector couples only to hidden sector 1, while the latter couples also to hidden sector 2, and the hidden sector 2 couples to hidden sector 3 and so on. A set of successively feeble couplings of the hidden sectors to the visible sector are generated in such a set up. In general each of these sectors live in a different heat bath. We develop a closed form set of coupled Boltzmann equations for the correlated evolution of the temperatures and number densities of each of the heat baths. We then apply the formalism to a simplified model with scalar portals between the different sectors. Predictions related to dark matter direct detection experiments and future CMB probes of dark radiation are made.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1007/JHEP12(2021)148
