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James M. Cline

Publications and source records attributed to James M. Cline.

At least 55 records · Page 3Linked to original sources
arXiv2023

The Flavor of QCD Axion Dark Matter

We argue that demanding a consistent cosmological history, including the absence of domain walls and strongly interacting relics at the Peccei-Quinn scale, singles out two concrete realizations of hadronic QCD axions as viable dark matter models. These realizations generally feature flavor-violating axion couplings to Standard Model quarks that are unsuppressed at low energies. As a consequence, experiments looking for flavor-violating hadronic processes involving the axion can be sensitive probes of QCD axion dark matter models. In particular, we show that the NA62 and KOTO experiments could detect the $K\rightarrowπ+ a$ decay for axions consistent with the observed dark matter abundance via the post-inflationary misalignment mechanism.

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arXiv2023

Nonabelian Kinetic Mixing in a Confining Phase

Dark matter from a hidden sector with SU($N$) gauge symmetry can have a nonabelian kinetic mixing portal with the standard model. The dark photon becomes massive in the confining phase without the need for spontaneous symmetry breaking. Depending on the particle content of the dark sector, there can be two or more composite vectors that get kinetic mixing through a heavy mediator particle $X$. This provides a model of composite dark photons giving a portal for direct detection of dark baryons. Avoiding exotic charged relics requires additional couplings allowing $X$ to decay to dark quarks and standard model fields, leading to further portals between the dark matter and the standard model. We comprehensively study the constraints on such models from colliders, rare decays, direct detection, and big bang nucleosynthesis.

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arXiv2023

NGC 1068 constraints on neutrino-dark matter scattering

The IceCube collaboration has observed the first steady-state point source of high-energy neutrinos, coming from the active galaxy NGC 1068. If neutrinos interacted strongly enough with dark matter, the emitted neutrinos would have been impeded by the dense spike of dark matter surrounding the supermassive black hole at the galactic center, which powers the emission. We derive a stringent upper limit on the scattering cross section between neutrinos and dark matter based on the observed events and theoretical models of the dark matter spike. The bound can be stronger than that obtained by the single IceCube neutrino event from the blazar TXS 0506+056 for some spike models.

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arXiv2023

Distortion of neutrino oscillations by dark photon dark matter

A weakly coupled and light dark photon coupling to lepton charges $L_μ-L_τ$ is an intriguing dark matter candidate whose coherent oscillations alter the dispersion relations of leptons. We study how this effect modifies the dynamics of neutrino flavor conversions, focusing on long baseline and solar oscillations. We analyze data from the T2K, SNO, and Super-Kamiokande experiments in order to obtain world-leading limits on the dark photon gauge coupling for masses below $\sim 10^{-11}\,\mathrm{eV}$. Degeneracies between shifts in the neutrino mass-squared differences and mixing angles and the new physics effect significantly relax the current constrains on the neutrino vacuum oscillation parameters.

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arXiv2023

Blazar constraints on neutrino-dark matter scattering

Neutrino emission in coincidence with gamma rays has been observed from the blazar TXS 0506+056 by the IceCube telescope. Neutrinos from the blazar had to pass through a dense spike of dark matter (DM) surrounding the central black hole. The observation of such a neutrino implies new upper bounds on the neutrino-DM scattering cross section as a function of DM mass. The constraint is stronger than existing ones for a range of DM masses, if the cross section rises linearly with energy. For constant cross sections, competitive bounds are also possible, depending on details of the DM spike.

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arXiv2022

PeV-scale leptogenesis, gravity waves and black holes from a SUSY-breaking phase transition

Supersymmetry is a highly motivated theoretical framework, whose scale of breaking may be at PeV energies, to explain null searches at the Large Hadron Collider. SUSY breaking through a first order phase transition may have occurred in the early universe, leading to potential gravitational wave signals. Constructing a realistic model for gauge-mediated supersymmetry breaking, we show that such a transition can also induce masses for heavy right-handed neutrinos and sneutrinos, whose CP-violating decays give leptogenesis at the PeV scale, and a novel mechanism of neutrino mass generation at one loop. For the same models we predict the possible gravity wave signals, and we study the possibility of production of primordial black holes during the phase transition.

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arXiv2022

Self-interacting dark baryons

Using results from lattice QCD, it is possible to quantitatively design models of dark baryons leading to velocity-dependent self-interaction cross sections that match the values needed for solving small-scale structure problems of standard cold dark matter. However it is not obvious that the main dark matter component in such models will be nucleons rather than large nuclei, or dark pions or atoms, whose scattering properties would be different. We first identify the parameters of a dark SU(3) sector analogous to QCD -- the confinement scale $Λ$ and pion mass $m_π$ -- needed to reproduce desired self-interaction cross sections. Then we show that these values can generically be compatible with the absence of a sufficiently stable deuteron bound state, and hence leading to no heavier nuclei, thus establishing the consistency of the scenario for self-interacting dark nucleons. The range of dark photon masses needed to avoid dominant pion or atomic dark matter is determined, as well as allowed values for the kinetic mixing parameter. The dark proton might be detected directly in future searches, by dark photon exchange.

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arXiv2022

Constraints on Heavy Neutral Leptons interacting with a singlet scalar

Heavy neutral leptons (HNLs) are an attractive minimal extension of the Standard Model, as is a singlet scalar $s$ mixing with the Higgs boson. If both are present, it is natural for HNLs to interact with $s$. For a light singlet,the decay $N\to sν$ can dominate over weak HNL decays. We reinterpret existing constraints on HNL mixing from the DELPHI, CHARM and Belle experiments for 0.5-100 GeV mass HNLs, taking into account the new decay channel. Although the constraints are typically weakened, in some cases they can become stronger, due to observable $s\to\ell^+\ell^-$ decays in the detectors. The method presented here could be used to recast constraints from other (older) experiments without resorting to computationally expensive Monte Carlo simulations. In addition, we update and correct some errors in the analysis of the original constraints, in the absence of the singlet.

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arXiv2022

First principles determination of bubble wall velocity

The terminal wall velocity of a first-order phase transition bubble can be calculated from a set of fluid equations describing the scalar fields and the plasma's state. We rederive these equations from the energy-momentum tensor conservation and the Boltzmann equation, without linearizing in the background temperature and fluid velocity. The resulting equations have a finite solution for any wall velocity. We propose a spectral method to integrate the Boltzmann equation, which is simple, efficient and accurate. As an example, we apply this new methodology to the singlet scalar extension of the standard model. We find that all solutions are naturally categorized as deflagrations ($v_w\sim c_s$) or ultrarelativistic detonations ($γ_w\gtrsim10$). Furthermore, the contributions from out-of-equilibrium effects are, most of the time, subdominant. Finally, we use these results to propose several approximation schemes with increasing levels of complexity and accuracy. They can be used to considerably simplify the methodology while correctly describing the qualitative behavior of the bubble wall.

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arXiv2022

Sterile neutrino production at small mixing in the early universe

Sterile neutrinos can be produced in the early universe via interactions with their active counterparts. For small active-sterile mixing angles, thermal equilibrium with the standard model plasma is not reached and sterile neutrinos are only produced via flavor oscillations. We study in detail this regime, taking into account matter potentials and decoherence effects caused by elastic scatterings with the plasma. We find that resonant oscillations occurring at temperatures $T\lesssim 10\,\mathrm{GeV}$ lead to a significant enhancement of the sterile neutrino production rate. Taking this into account, we improve constraints on the active-sterile mixing from Big Bang nucleosynthesis and the cosmic microwave background, excluding mixing angles down to $θ_s\sim 10^{-10}-10^{-16}$ for sterile neutrino masses in the $10\,\mathrm{MeV}$ to $10\,\mathrm{GeV}$ range. We observe that if sterile neutrinos predominantly decay into metastable hidden sector particles, this process provides a novel dark matter production mechanism, consistent with the sterile neutrino origin of light neutrino masses via the seesaw mechanism.

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arXiv2022

Theories and Experiments for Testable Baryogenesis Mechanisms: A Snowmass White Paper

The baryon asymmetry of the Universe is one of the central motivations to expect physics beyond the Standard Model. In this Snowmass white paper, we review the challenges and opportunities in testing some of the central paradigms that predict physics at scales low enough to expect new experimental data in the next decade. Focusing on theoretical ideas and some of their experimental implications, in particular, we discuss neutron-antineutron transformations, flavor observables, next generation colliders, future neutron facilities, gravitational waves, searches for permanent electric dipole moments, $0νββ$ decay and some future large underground experiments as methods to test post-sphaleron baryogenesis, electroweak baryogenesis, mesogenesis and low scale leptogenesis. Finally, we comment on the cases where high scale physics can be probed through some of these same mechanisms.

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arXiv2022

Sterile neutrino dark matter catalyzed by a very light dark photon

Sterile neutrinos ($ν_s$) that mix with active neutrinos ($ν_a$) are interesting dark matter candidates with a rich cosmological and astrophysical phenomenology. In their simplest incarnation, their production is severely constrained by a combination of structure formation observations and X-ray searches. We show that if active neutrinos couple to an oscillating condensate of a very light $L_μ-L_τ$ gauge field, resonant $ν_a$-$ν_s$ oscillations can occur in the early universe, consistent with $ν_s$ constituting all of the dark matter, while respecting X-ray constraints on $ν_s\toν_aγ$ decays. Interesting deviations from standard solar and atmospheric neutrino oscillations can persist to the present.

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arXiv2021

Dark atoms and composite dark matter

I selectively review the theoretical properties and observational limits pertaining to dark atoms, as well as composite dark matter candidates bound by a confining gauge interaction: dark glueballs, glueballinos, mesons and baryons. Emphasis is given to cosmological, direct and indirect detection constraints.

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arXiv2021

Gauging lepton flavor SU(3) for the muon $g-2$

Gauging a specific difference of lepton numbers such as $L_μ-L_τ$ is a popular model-building option, which gives rise to economical explanations for the muon anomalous magnetic moment. However, this choice of gauge group seems rather arbitrary, and additional physics is required to reproduce the observed neutrino masses and mixings. We address these shortcomings by embedding $L_μ-L_τ$ in the vectorial SU(3) gauge symmetry of lepton flavor. The vacuum expectation values (VEVs) of scalar fields in the fundamental, six-dimensional and adjoint representations allow for phenomenologically viable lepton and gauge boson masses. The octet scalar gives rise to charged lepton masses, and together with the triplet scalar generates masses for all the leptophilic gauge bosons except for the $L_μ-L_τ$ one. The latter gets its smaller mass from the sextet VEVs, which also generate the neutrino masses, and are determined up to an overall scaling by the observed masses and mixings. The model predicts three heavy neutral leptons at the GeV-TeV scale as well as vectorlike charged lepton partners; it requires the mass of the lightest active neutrino to exceed $10^{-4}\,$eV, and it naturally provides a resolution of the Cabibbo angle anomaly.

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arXiv2021

Electroweak baryogenesis from light fermion sources: a critical study

Electroweak baryogenesis (EWBG) is sourced by nonstandard $CP$-violating interactions of the Higgs boson with fermions, usually taken to be the top quark, enhanced by its large Yukawa coupling. Numerous papers have studied EWBG sourced by lighter fermions, including the tau lepton and off-diagonal quark mass terms. We critically reassess the viability of EWBG in these scenarios, comparing the predictions based on the semiclassical (WKB) formalism for the source term to those from the VEV insertion approximation (VIA), using updated values for the collision terms, and clarifying discrepancies in the definition of the weak sphaleron rate. The VIA systematically predicts a baryon asymmetry that is orders of magnitude larger than the WKB formalism. We trace this to the differing shapes of the $CP$-violating source terms in the two formalisms, showing that the additional spatial derivative in the WKB source term causes large cancellations when it is integrated over the bubble wall profile. An important exception is a source term from $c$-$t$ quark mixing, where the WKB prediction also allows for a realistically large baryon asymmetry. In contrast, the analogous $b$-$s$ mixing source is found to be orders of magnitude too small.

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arXiv2021

TASI Lectures on Early Universe Cosmology: Inflation, Baryogenesis and Dark Matter

These lectures, presented at TASI 2018, provide a concise introduction to inflation, baryogenesis, and aspects of dark matter not covered by the other lectures. The emphasis for inflation is an intuitive understanding and techniques for constraining inflationary models. For baryogenesis we focus on two examples, leptogenesis and electroweak baryogenesis, with attention to singlet-assisted two-step phase transitions. Concerning dark matter, we review different classes of models distinguished by their mechanisms for obtaining the observed relic density, including thermal freeze-out, asymmetric dark matter, freeze-in, SIMP dark matter, the misalignment mechanism for ultralight scalars and axions, and production of primordial black holes during inflation. Problem sets are provided.

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arXiv2021

Composite strongly interacting dark matter

It has been suggested that cold dark matter (CDM) has difficulties in explaining tentative evidence for noncuspy halo profiles in small galaxies, and the low velocity dispersions observed in the largest Milky Way satellites ("too big to fail" problem). Strongly self-interacting dark matter has been noted as a robust solution to these problems. The elastic cross sections required are much larger than predicted by generic CDM models, but could naturally be of the right size if dark matter is composite. We explore in a general way the constraints on models where strongly interacting CDM is in the form of dark "atoms" or "molecules," or bound states of a confining gauge interaction ("hadrons"). These constraints include considerations of relic density, direct detection, big bang nucleosynthesis, the cosmic microwave background, and LHC data.

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