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328
result(s) for
"Hooper, Dan"
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Cosmology with a very light Lμ − Lτ gauge boson
A
bstract
In this paper, we explore in detail the cosmological implications of an abelian
L
μ
−
L
τ
gauge extension of the Standard Model featuring a light and weakly coupled
Z
′. Such a scenario is motivated by the longstanding ∼ 4
σ
discrepancy between the measured and predicted values of the muon’s anomalous magnetic moment, (
g
− 2)
μ
, as well as the tension between late and early time determinations of the Hubble constant. If sufficiently light, the
Z
′ population will decay to neutrinos, increasing the overall energy density of radiation and altering the expansion history of the early universe. We identify two distinct regions of parameter space in this model in which the Hubble tension can be significantly relaxed. The first of these is the previously identified region in which a ∼ 10 − 20 MeV
Z
′ reaches equilibrium in the early universe and then decays, heating the neutrino population and delaying the process of neutrino decoupling. For a coupling of
g
μ
−
τ
≃ (3 − 8) × 10
−4
, such a particle can also explain the observed (
g
− 2)
μ
anomaly. In the second region, the
Z
′ is very light (
m
Z
′
∼ 1eV to MeV) and very weakly coupled (
g
μ
−
τ
∼ 10
−13
to 10
−9
). In this case, the
Z
′ population is produced through freeze-in, and decays to neutrinos after neutrino decoupling. Across large regions of parameter space, we predict a contribution to the energy density of radiation that can appreciably relax the reported Hubble tension, Δ
N
eff
≃ 0
.
2.
Journal Article
Dark radiation and superheavy dark matter from black hole domination
A
bstract
If even a relatively small number of black holes were created in the early universe, they will constitute an increasingly large fraction of the total energy density as space expands. It is thus well-motivated to consider scenarios in which the early universe included an era in which primordial black holes dominated the total energy density. Within this context, we consider Hawking radiation as a mechanism to produce both dark radiation and dark matter. If the early universe included a black hole dominated era, we find that Hawking radiation will produce dark radiation at a level Δ
N
eff
∼ 0
.
03 − 0
.
2 for each light and decoupled species of spin 0, 1/2, or 1. This range is well suited to relax the tension between late and early-time Hubble determinations, and is within the reach of upcoming CMB experiments. The dark matter could also originate as Hawking radiation in a black hole dominated early universe, although such dark matter candidates must be very heavy (
m
DM
≳ 10
11
GeV) if they are to avoid exceeding the measured abundance.
Journal Article
A systematic study of hidden sector dark matter: application to the gamma-ray and antiproton excesses
A
bstract
In hidden sector models, dark matter does not directly couple to the particle content of the Standard Model, strongly suppressing rates at direct detection experiments, while still allowing for large signals from annihilation. In this paper, we conduct an extensive study of hidden sector dark matter, covering a wide range of dark matter spins, mediator spins, interaction diagrams, and annihilation final states, in each case determining whether the annihilations are
s
-wave (thus enabling efficient annihilation in the universe today). We then go on to consider a variety of portal interactions that allow the hidden sector annihilation products to decay into the Standard Model. We broadly classify constraints from relic density requirements and dwarf spheroidal galaxy observations. In the scenario that the hidden sector was in equilibrium with the Standard Model in the early universe, we place a lower bound on the portal coupling, as well as on the dark matter’s elastic scattering cross section with nuclei. We apply our hidden sector results to the observed Galactic Center gamma-ray excess and the cosmic-ray antiproton excess. We find that both of these excesses can be simultaneously explained by a variety of hidden sector models, without any tension with constraints from observations of dwarf spheroidal galaxies.
Journal Article
Cosmology with A Very Light L$_{\\mu}$ − L$_{\\tau}$ Gauge Boson
In this paper, we explore in detail the cosmological implications of an abelian L$_{μ}$ − L$_{τ}$ gauge extension of the Standard Model featuring a light and weakly coupled Z′. Such a scenario is motivated by the longstanding ∼ 4σ discrepancy between the measured and predicted values of the muon’s anomalous magnetic moment, (g − 2)$_{μ}$, as well as the tension between late and early time determinations of the Hubble constant. If sufficiently light, the Z′ population will decay to neutrinos, increasing the overall energy density of radiation and altering the expansion history of the early universe. We identify two distinct regions of parameter space in this model in which the Hubble tension can be significantly relaxed. The first of these is the previously identified region in which a ∼ 10 − 20 MeV Z′ reaches equilibrium in the early universe and then decays, heating the neutrino population and delaying the process of neutrino decoupling. For a coupling of g$_{μ − }_{τ}$ ≃ (3 − 8) × 10$^{−4}$, such a particle can also explain the observed (g − 2)$_{μ}$ anomaly. In the second region, the Z′ is very light ( $ {m}_{Z^{\\prime }} $ ∼ 1eV to MeV) and very weakly coupled (g$_{μ − }_{τ}$ ∼ 10$^{−13}$ to 10$^{−9}$). In this case, the Z′ population is produced through freeze-in, and decays to neutrinos after neutrino decoupling. Across large regions of parameter space, we predict a contribution to the energy density of radiation that can appreciably relax the reported Hubble tension, ΔN$_{eff}$ ≃ 0.2.
Journal Article
Maverick dark matter at colliders
Assuming that dark matter is a weakly interacting massive particle (WIMP) species
X
produced in the early Universe as a cold thermal relic, we study the collider signal of
pp
or
+ jets and its distinguishability from standard-model background processes associated with jets and missing energy. We assume that the WIMP is the sole particle related to dark matter within reach of the LHC — a “maverick” particle — and that it couples to quarks through a higher dimensional contact interaction. We simulate the WIMP final-state signal
+ jets and dominant standard-model (SM) background processes and find that the dark-matter production process results in higher energies for the colored final state partons than do the standard-model background processes. As a consequence, the detectable signature of maverick dark matter is an excess over standard-model expectations of events consisting of large missing transverse energy, together with large leading jet transverse momentum and scalar sum of the transverse momenta of the jets. Existing Tevatron data and forthcoming LHC data can constrain (or discover!) maverick dark matter.
Journal Article
Many birds, one stone
A new theory for dark matter has the power to explain several experimental results simultaneously, even those seemingly at odds with each other.
Journal Article
Dark matter: Many birds, one stone
A new theory for dark matter has the power to explain several experimental results simultaneously, even those seemingly at odds with each other. [PUBLICATION ABSTRACT]
Journal Article
What Makes A Discovery
In this contribution to the proceedings of the 182nd Nobel Symposium, I reflect on the concept of \"discovery\" as it is used by physicists and astronomers. In particular, I comment on how the scientific community distinguishes discoveries from propositions that are supported only by lesser forms of evidence, emphasizing the social nature of this process and remarking on the subjective factors that go into making such judgements. I advocate for an approach that is intentionally Bayesian in nature, in which individuals are encouraged to evaluate and publicly state their priors and to update them systematically. I close by applying these practices to the case example of the Galactic Center Gamma-Ray Excess.
Paper
The 511 keV Excess and Primordial Black Holes
An excess of 511 keV photons has been detected from the central region of the Milky Way. It has been suggested that the positrons responsible for this signal could be produced through the Hawking evaporation of primordial black holes. After evaluating the constraints from INTEGRAL, COMPTEL, and Voyager 1, we find that black holes in mass range of \\(\\sim(1-4)\\times10^{16}\\) g could potentially produce this signal if they make up a small fraction of the total dark matter density. Proposed MeV-scale gamma-ray telescopes such as AMEGO or e-ASTROGAM should be able to test this class of scenarios by measuring the diffuse gamma ray emission from the Milky Way's inner halo.
Paper