arXiv subjects

James M. Cline

Publications and source records attributed to James M. Cline.

At least 73 records · Page 4Linked to original sources

arXiv2021

Asymmetric reheating from a symmetric inflationary potential

We explore a model of two-field inflation with nonminimal kinetic terms in which two identical matter sectors decoupled from each other may reheat to different temperatures while preserving the symmetry of the Lagrangian. This scenario is motivated by mirror dark matter models in which the temperature of the mirror sector is constrained to be $T'\lesssim0.5 T$ by big bang nucleosynthesis and the cosmic microwave background. For a given class of nonminimal kinematic terms, we find that the symmetric field trajectory $X=Y$ is a repeller solution, such that any randomly-occurring asymmetry in the initial conditions is amplified by many orders of magnitude during inflation, far beyond what canonical power-law models can achieve. Isocurvature fluctuations are strongly suppressed in this model, but a $\mathcal O(0.03$--$0.07$) tensor-to-scalar ratio could be observed in the near future. The range of potential parameters compatible with {\it Planck} constraints is shown to be much larger than in corresponding single-field models. This occurs through a mechanism for lowering the spectral index that we dub CTHC: curved trajectory at horizon crossing.

astro-ph.CO↗

arXiv2021

Late-Time Dark Matter Oscillations and the Core-Cusp Problem

The core-cusp problem persists as an unresolved tension between the predictions of $Λ$CDM cosmology and observations of dark matter (DM) profiles in dwarf spheroidal and other galaxies. We present a novel scenario for converting cusps into cores through reactivation of DM annihilation in galaxies at late times. This can happen in asymmetric DM models when there is a very small DM-number violating mass term that causes oscillations between DM and its antiparticle. Using analytic methods as well as gravitational N-body simulations, we show that this mechanism can robustly eliminate cusps from galactic DM profiles for light fermionic DM of mass $m_χ\sim (0.1-1)$ GeV and a lighter mediator into which the DM can annihilate. We identify regions of parameter space where annihilation of DM particles is more efficient than elastic scattering at reducing the inner density of the DM profile. Dark matter annihilation is therefore a qualitatively distinct alternative to the mechanism of elastic self-interacting dark matter for addressing the cusp-core problem.

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arXiv2021

Fluid equations for fast-moving electroweak bubble walls

The cosmological electroweak phase transition can be strongly first order in extended particle physics models. To accurately predict the speed and shape of the bubble walls during such a transition, Boltzmann equations for the CP-even fluid perturbations must be solved. We point out that the equations usually adopted lead to unphysical behavior of the perturbations, for walls traveling close to or above the speed of sound in the plasma. This is an artifact that can be overcome by more carefully truncating the full Boltzmann equation. We present an improved set of fluid equations, suitable for studying the dynamics of both subsonic and supersonic walls, of interest for gravitational wave production and electroweak baryogenesis.

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arXiv2021

Baryogenesis and gravity waves from a UV-completed electroweak phase transition

We study gravity wave production and baryogenesis at the electroweak phase transition, in a real singlet scalar extension of the Standard Model, including vector-like top partners to generate the CP violation needed for electroweak baryogenesis (EWBG). The singlet makes the phase transition strongly first-order through its coupling to the Higgs boson, and it spontaneously breaks CP invariance through a dimension-5 contribution to the top quark mass term, generated by integrating out the heavy top quark partners. We improve on previous studies by incorporating updated transport equations, compatible with large bubble wall velocities. The wall speed and thickness are computed directly from the microphysical parameters rather than treating them as free parameters, allowing for a first-principles computation of the baryon asymmetry. The size of the CP-violating dimension-5 operator needed for EWBG is constrained by collider, electroweak precision, and renormalization group running constraints. We identify regions of parameter space that can produce the observed baryon asymmetry or observable gravitational (GW) wave signals. Contrary to standard lore, we find that for strong deflagrations, the efficiencies of large baryon asymmetry production and strong GW signals can be positively correlated. However we find the overall likelihood of observably large GW signals to be smaller than estimated in previous studies. In particular, only detonation-type transitions are predicted to produce observably large gravitational waves.

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arXiv2021

Wall speed and shape in singlet-assisted strong electroweak phase transitions

Models with singlet fields coupling to the Higgs can enable a strongly first order electroweak phase transition, of interest for baryogenesis and gravity waves. We improve on previous attempts to self-consistently solve for the bubble wall properties -- wall speed $v_w$ and shape -- in a highly predictive class of models with $Z_2$-symmetric singlet potentials. A new algorithm is implemented to determine $v_w$ and the wall profiles throughout the singlet parameter space in the case of subsonic walls, focusing on models with strong enough phase transitions to satisfy the sphaleron washout constraint for electroweak baryogenesis. We find speeds as low as $v_w \cong 0.1$ in our scan over parameter space, and the singlet must be relatively light to have a subsonic wall, $m_s \lesssim 135$ GeV.

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arXiv2020

Advanced Concepts in Quantum Field Theory

These lectures comprise the second half of a quantum field theory course given in 2004. Topics include the path integral, loop expansion, Feynman rules, various regularization methods, renormalization, running couplings and the renormalization group, fixed points and asymptotic freedom, effective action, Coleman-Weinberg effective potential, fermions, the axial anomaly, QED, gauge fixing, nonabelian gauge theories, unitarity, optical theorem, Slavnov-Taylor identities, beta function of Yang-Mills theory, a heuristic derivation of asymptotic freedom, instantons in SU(N) gauge theory, theta vacua and the strong CP problem. 23 homework problems are included.

hep-th↗

arXiv2020

Feynman Lectures on the Strong Interactions

These twenty-two lectures, with exercises, comprise the extent of what was meant to be a full-year graduate-level course on the strong interactions and QCD, given at Caltech in 1987-88. The course was cut short by the illness that led to Feynman's death. Several of the lectures were finalized in collaboration with Feynman for an anticipated monograph based on the course. The others, while retaining Feynman's idiosyncrasies, are revised similarly to those he was able to check. His distinctive approach and manner of presentation are manifest throughout. Near the end he suggests a novel, nonperturbative formulation of quantum field theory in $D$ dimensions. Supplementary material is provided in appendices and ancillary files, including verbatim transcriptions of three lectures and the corresponding audiotaped recordings.

hep-ph↗

arXiv2020

A little theory of everything, with heavy neutral leptons

Recently a new model of "Affleck-Dine inflation" was presented, that produces the baryon asymmetry from a complex inflaton carrying baryon number, while being consistent with constraints from the cosmic microwave background. We adapt this model such that the inflaton carries lepton number, and communicates the lepton asymmetry to the standard model baryons via quasi-Dirac heavy neutral leptons (HNLs) and sphalerons. One of these HNLs, with mass $\lesssim 4.5\,$GeV, can be (partially) asymmetric dark matter (DM), whose asymmetry is determined by that of the baryons. Its stability is directly related to the vanishing of the lightest neutrino mass. Neutrino masses are generated by integrating out heavy sterile neutrinos whose mass is above the inflation scale. The model provides an economical origin for all of the major ingredients missing from the standard model: inflation, baryogenesis, neutrino masses, and dark matter. The HNLs can be probed in fixed-target experiments like SHiP, possibly manifesting $N$-$\bar N$ oscillations. A light singlet scalar, needed for depleting the DM symmetric component, can be discovered in beam dump experiments and searches for rare decays, possibly explaining anomalous events recently observed by the KOTO collaboration. The DM HNL is strongly constrained by direct searches, and could have a cosmologically interesting self-interaction cross section.

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arXiv2020

Constraining galactic structures of mirror dark matter

The simplest model of mirror sector dark matter maintains exact mirror symmetry, but has a baryon abundance $Ω_{b'} = βΩ_b$ and a suppressed temperature $T' = x T$ in the mirror sector; hence it depends only on two parameters, $β,x$. For sufficiently small $x$, early cosmological observables may not constrain mirror baryons from constituting all of the dark matter despite their strong self-interactions, depending on the unknown details of structure formation in the hidden sector. Here we close this loophole by simulating mirror structure formation, mapping out the allowed regions of parameter space using cosmological and astronomical data. We find that the Milky Way disk surface density and bulge mass constrain $Ω_{b'}\lesssim 0.3 Ω_{b}$ at the highest $T'$ allowed by BBN and CMB ($T'=0.5 T$), or $Ω_{b'}\lesssim 0.8 Ω_{b}$ at lower values of $T'$. We also briefly discuss the realization of the necessary temperature asymmetry between the SM and the mirror sector in our model with unbroken mirror symmetry.

astro-ph.CO↗

arXiv2020

Electroweak baryogenesis at high wall velocities

It is widely believed that electroweak baryogenesis should be suppressed in strong phase transitions with fast-moving bubble walls, but this effect has never been quantitatively studied. We rederive fluid equations describing transport of particle asymmetries near the bubble wall without making the small-wall-velocity approximation. We show that the suppression of the baryon asymmetry is a smooth function of the wall speed and that there is no special behavior when crossing the sound speed barrier. Electroweak baryogenesis can thus be efficient also with strong detonations, generically associated with models with observably large gravitational waves. We also make a systematic and critical comparison of our improved transport equations to another one commonly used in the literature, based on the VEV-insertion formalism.

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arXiv2019

Viable secret neutrino interactions with ultralight dark matter

Several anomalies in neutrino oscillation experiments point to the existence of a $\sim 1\,$eV sterile neutrino $ν_s$ mixing with $ν_e$ at the level of $U_{e4}\cong 0.1$, but such a neutrino is strongly disfavored by constraints on additional light degrees of freedom ($δN_{\rm eff}$) and total neutrino mass ($\sum_νm_ν$) from cosmology. "Secret neutrino interactions" that have been invoked to suppress the cosmological production of $ν_s$ typically falter, but recently it was pointed out that $ν_s$ could get a large mass in the early universe by coupling to ultralight dark matter $ϕ$, which can robustly suppress its production. The model has essentially two free parameters: $m_ϕ$, and $m_{s,0}$, the mass of the sterile neutrino at early times, enhanced by its coupling to $ϕ$. I determine the parameter regions allowed by limits on $δN_{\rm eff}$ and $\sum_νm_ν$ from the cosmic microwave background and big bang nucleosynthesis, using a simplified yet accurate treatment of neutrino oscillations in the early universe. This mechanism could have an important impact on laboratory experiments that suggest oscillations with sterile neutrinos.

hep-ph↗

arXiv2019

Affleck-Dine inflation

The Affleck-Dine mechanism in its simplest form provides baryogenesis from the out-of-equilibrium evolution of a complex scalar field with a simple renormalizable potential. We show that such a model, supplemented by nonminimal coupling to gravity, can also provide inflation, consistent with Planck constraints, simultaneously with the generation of the baryon asymmetry. The predictions of the model include significant tensor-to-scalar ratio and possibly baryon isocurvature fluctuations. The reheating temperature is calculable, making the model fully predictive. We require color triplet scalars for reheating and transfering the primordial baryon asymmetry to quarks; these could be observable at colliders. They can also be probed at higher scales by searches for quark compositeness in dijet angular distributions, and flavor-changing neutral current effects.

hep-ph↗

arXiv2019

Pseudo-Goldstone dark matter confronts cosmic ray and collider anomalies

Persistent excesses in the spectra of gamma rays from the galactic center and cosmic ray antiprotons can be explained by dark matter of mass $50\sim 65$\,GeV annihilating into $b$ quarks, but this is typically hard to reconcile with direct detection constraints. We resolve this tension using a simple class of models, where dark matter is a pseudo-Nambu-Goldstone boson, having naturally momentum-suppressed couplings to nuclei. Exploring the parameter space of the model, we find that it can explain the cosmic ray anomalies while remaining compatible with constraints from the relic abundance and annihilation in dwarf spheroidal galaxies. In certain regions of parameter space, the Higgs-dark matter coupling can help stabilize the Higgs potential up to the Planck scale. The scalar partner of the dark matter is an extra Higgs boson, that can explain a tentative diphoton excess observed by CMS, and an excess of $b\bar b$ signal from LEP, if its mass is $\sim 96$\,GeV and the model is extended to include a heavy scalar quark. This extended model predicts a monochromatic gamma-ray line near 64 GeV, at a level close to current experimental sensitivity, from dark matter annihilations in the galaxy.

hep-ph↗

arXiv2019

Baryogenesis from neutron-dark matter oscillations

It was recently suggested that dark matter consists of ~GeV particles that carry baryon number and mix with the neutron. We demonstrate that this could allow for resonant dark matter-neutron oscillations in the early universe, at finite temperature, leading to low-scale baryogenesis starting from a primordial dark matter asymmetry. In this scenario, the asymmetry transfer happens around 30 MeV, just before big bang nucleosynthesis. We illustrate the idea using a model with a dark U(1)' gauge interaction, which has recently been suggested as a way of addressing the neutron lifetime anomaly. The asymmetric dark matter component of this model is both strongly self-interacting and leads to a suppression of matter density perturbations at small scales, allowing to mitigate the small-scale problems of cold dark matter cosmology. Future CMB experiments will be able to consistently probe, or firmly exclude, this scenario.

hep-ph↗

arXiv2019

Early formation of supermassive black holes via dark matter self-interactions

The existence of supermassive black holes at high redshifts ($z\sim7$) is difficult to accommodate in standard astrophysical scenarios. It has been shown that dark matter models with a subdominant self-interacting component are able to produce early seeds for supermassive black holes through the gravothermal catastrophe. Previous studies used a fluid equation approach, requiring some limiting assumptions. Here we reconsider the problem using $N$-body gravitational simulations starting from the formation of the initial dark matter halo. We consider both elastic and dissipative scattering, and elucidate the interplay between the dark matter microphysics and subsequent accretion of the black hole needed to match the properties of observed high redshift supermassive black holes. We find a region of parameter space in which a small component of self-interacting dark matter can produce the observed high redshift supermassive black holes.

astro-ph.CO↗

arXiv2019

Can the ANITA anomalous events be due to new physics?

The ANITA collaboration has observed two ultra-high-energy upgoing air shower events that cannot originate from Standard Model neutrinos that have traversed the Earth. Several beyond-the-standard-model physics scenarios have been proposed as explanations for these events. In this paper we present some general arguments making it challenging for new physics to explain the events. One exceptional class of models that could work is pointed out, in which metastable dark matter decays to a highly boosted lighter dark matter particle, that can interact in the Earth to produce the observed events.

hep-ph↗

arXiv2018

Dark decay of the neutron

New decay channels for the neutron into dark matter plus other particles have been suggested for explaining a long-standing discrepancy between the neutron lifetime measured from trapped neutrons versus those decaying in flight. Many such scenarios are already ruled out by their effects on neutron stars, and the decay into dark matter plus photon has been experimentally excluded. Here we explore the decay into a dark Dirac fermion $χ$ and a dark photon $A'$, which can be consistent with all constraints if $χ$ is a subdominant component of the dark matter. Neutron star constraints are evaded if the dark photon mass to coupling ratio is $m_{A'}/g' \lesssim (45-60)\,$MeV, depending upon the nuclear equation of state. $g'$ and the kinetic mixing between U(1)$'$ and electromagnetism are tightly constrained by direct and indirect dark matter detection, supernova constraints, and cosmological limits.

hep-ph↗

arXiv2018

$R({K^{(*)}})$ from dark matter exchange

Hints of lepton flavor violation have been observed by LHCb in the rate of the decay $B\to Kμ^+μ^-$ relative to that of $B\to K e^+e^-$. This can be explained by new scalars and fermions which couple to standard model particles and contribute to these processes at loop level. We explore a simple model of this kind, in which one of the new fermions is a dark matter candidate, while the other is a heavy vector-like quark and the scalar is an inert Higgs doublet. We explore the constraints on this model from flavor observables, dark matter direct detection, and LHC run II searches, and find that, while currently viable, this scenario will be directly tested by future results from all three probes.

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