arXiv subjects

James M. Cline

Publications and source records attributed to James M. Cline.

At least 127 records · Page 7Linked to original sources

arXiv2012

Millicharged Atomic Dark Matter

We present a simplified version of the atomic dark matter scenario, in which charged dark constituents are bound into atoms analogous to hydrogen by a massless hidden sector U(1) gauge interaction. Previous studies have assumed that interactions between the dark sector and the standard model are mediated by a second, massive Z' gauge boson, but here we consider the case where only a massless gamma' kinetically mixes with the standard model hypercharge and thereby mediates direct detection. This is therefore the simplest atomic dark matter model that has direct interactions with the standard model, arising from the small electric charge for the dark constituents induced by the kinetic mixing. We map out the parameter space that is consistent with cosmological constraints and direct searches, assuming that some unspecified mechanism creates the asymmetry that gives the right abundance, since the dark matter cannot be a thermal relic in this scenario. In the special case where the dark "electron" and "proton" are degenerate in mass, inelastic hyperfine transitions can explain the CoGeNT excess events. In the more general case, elastic transitions dominate, and can be close to current direct detection limits over a wide range of masses.

hep-ph↗

arXiv2012

Interacting dark matter contribution to the Galactic 511 keV gamma ray emission: constraining the morphology with INTEGRAL/SPI observations

We compare the full-sky morphology of the 511 keV gamma ray excess measured by the INTEGRAL/SPI experiment to predictions of models based on dark matter (DM) scatterings that produce low-energy positrons: either MeV-scale DM that annihilates directly into e+e- pairs, or heavy DM that inelastically scatters into an excited state (XDM) followed by decay into e+e- and the ground state.By direct comparison to the data, we find that such explanations are consistent with dark matter halo profiles predicted by numerical many-body simulations for a Milky Way-like galaxy. Our results favor an Einasto profile over the cuspier NFW distribution and exclude decaying dark matter scenarios whose predicted spatial distribution is too broad. We obtain a good fit to the shape of the signal using six fewer degrees of freedom than previous empirical fits to the 511 keV data. We find that the ratio of flux at Earth from the galactic bulge to that of the disk is between 1.9 and 2.4, taking into account that 73% of the disk contribution may be attributed to the beta decay of radioactive 26Al.

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arXiv2011

Light dark matter versus astrophysical constraints

Hints of direct dark matter detection coming from the DAMA, CoGeNT experiments point toward light dark matter with isospin-violating and possibly inelastic couplings. However an array of astrophysical constraints are rapidly closing the window on light dark matter. We point out that if the relic density is determined by annihilation into invisible states, these constraints can be evaded. As an example we present a model of quasi-Dirac dark matter, interacting via two U(1) gauge bosons, one of which couples to baryon number and the other which kinetically mixes with the photon. Annihilation is primarily into "dark neutrinos" that do not mix with the SM, but which could provide a small component of dark radiation. The model could soon be tested by several experiments searching for such light gauge bosons, and we predict that both could be detected. The model also requires a fourth generation of quarks, whose existence might increase the production cross section of Higgs bosons at the Tevatron and LHC.

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arXiv2011

Warped Radion Inflation

We show that the radion in a warped geometry bounded by two branes can have a potential suitable for inflation. Our construction is based upon a solution known in string theory as the linear dilaton, in which the back-reaction from a bulk scalar Φis exactly accounted for. The radion, stabilized by Φ, is much heavier than the TeV scale and its couplings to the standard model are much more suppressed than in the usual Randall-Sundrum solution. We present a new formalism for obtaining approximate time-dependent solutions, based on perturbing the exact solution to the coupled Einstein and scalar field equations in the bulk. It allows the radion potential to be computed directly in terms of the brane potentials for Φ. We show that simple exponential potentials on the branes can lead to a 4D radion potential with a flattened hilltop form, yielding inflation with a spectral index of typically n_s=0.96 and no higher than 0.99. With more complicated brane potentials, the descent from the hilltop can be a linear potential, giving a tensor-to-scalar ratio as large as r=0.07 with n_s=0.974. The couplings of the radion to the standard model particles are dictated by general covariance, so the details of reheating are explicitly calculable, leading to a reheat temperature of at least 10^7 GeV. The quantum corrections to the inflaton potential from its couplings to matter are also calculable and are shown to be small, so that the prediction for the shape of the potential is under theoretical control, even with superPlanckian field excursions.

hep-th↗

arXiv2011

Electroweak Baryogenesis in Two Higgs Doublet Models and B meson anomalies

Motivated by 3.9 sigma evidence of a CP-violating phase beyond the standard model in the like-sign dimuon asymmetry reported by DO, we examine the potential for two Higgs doublet models (2HDMs) to achieve successful electroweak baryogenesis (EWBG) while explaining the dimuon anomaly. Our emphasis is on the minimal flavour violating 2HDM, but our numerical scans of model parameter space include type I and type II models as special cases. We incorporate relevant particle physics constraints, including electroweak precision data, b to s gamma, the neutron electric dipole moment, R_b, and perturbative coupling bounds to constrain the model. Surprisingly, we find that a large enough baryon asymmetry is only consistently achieved in a small subset of parameter space in 2HDMs, regardless of trying to simultaneously account for any B physics anomaly. There is some tension between simultaneous explanation of the dimuon anomaly and baryogenesis, but using a Markov chain Monte Carlo we find several models within 1 sigma of the central values. We point out shortcomings with previous studies that reached different conclusions. The restricted parameter space that allows for EWBG makes this scenario highly predictive for collider searches. We discuss the most promising signatures to pursue at the LHC for EWBG-compatible models.

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arXiv2011

Minimal hidden sector models for CoGeNT/DAMA events

Motivated by recent attempts to reconcile hints of direct dark matter detection by the CoGeNT and DAMA experiments, we construct simple particle physics models that can accommodate the constraints. We point out challenges for building reasonable models and identify the most promising scenarios for getting isospin violation and inelasticity, as indicated by some phenomenological studies. If inelastic scattering is demanded, we need two new light gauge bosons, one of which kinetically mixes with the standard model hypercharge and has mass < 2 GeV, and another which couples to baryon number and has mass 6.8 +/- 0.2 GeV. Their interference gives the desired amount of isospin violation. The dark matter is nearly Dirac, but with small Majorana masses induced by spontaneous symmetry breaking, so that the gauge boson couplings become exactly off-diagonal in the mass basis, and the small mass splitting needed for inelasticity is simultaneously produced. If only elastic scattering is demanded, then an alternative model, with interference between the kinetically mixed gauge boson and a hidden sector scalar Higgs, is adequate to give the required isospin violation. In both cases, the light kinetically mixed gauge boson is in the range of interest for currently running fixed target experiments.

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arXiv2011

Metastable dark matter mechanisms for INTEGRAL 511 keV $γ$ rays and DAMA/CoGeNT events

We explore dark matter mechanisms that can simultaneously explain the galactic 511 keV gamma rays observed by INTEGRAL/SPI, the DAMA/LIBRA annual modulation, and the excess of low-recoil dark matter candidates observed by CoGeNT. It requires three nearly degenerate states of dark matter in the 4-7 GeV mass range, with splittings respectively of order an MeV and a few keV. The top two states have the small mass gap and transitions between them, either exothermic or endothermic, can account for direct detections. Decays from one of the top states to the ground state produce low-energy positrons in the galaxy whose associated 511 keV gamma rays are seen by INTEGRAL. This decay can happen spontaneously, if the excited state is metastable (longer-lived than the age of the universe), or it can be triggered by inelastic scattering of the metastable states into the shorter-lived ones. We focus on a simple model where the DM is a triplet of an SU(2) hidden sector gauge symmetry, broken at the scale of a few GeV, giving masses of order \lsim 1 GeV to the dark gauge bosons, which mix kinetically with the standard model hypercharge. The purely decaying scenario can give the observed angular dependence of the 511 keV signal with no positron diffusion, while the inelastic scattering mechanism requires transport of the positrons over distances \sim 1 kpc before annihilating. We note that an x-ray line of several keV in energy, due to single-photon decays involving the top DM states, could provide an additional component to the diffuse x-ray background. The model is testable by proposed low-energy fixed target experiments.

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arXiv2011

Chain Inflation Reconsidered

We investigate density perturbations in the chain inflation scenario, where the inflaton undergoes successive tunneling transitions along one field direction. First we show that when the bubble walls associated with such a phase transition meet, they induce the next phase transition and continue to propagate as phase interfaces. Then we present an analytical calculation of the density fluctuations and an estimate of non-Gaussianities such as f_{NL}, which we find to be small (of order 1). To get the right amplitude for the power spectrum, there have to be 10^8 phase transitions per Hubble time, a significant model building challenge. We find that working models of chain inflation must be rather strongly coupled, and thus have a very limited range of validity as effective field theories. We discuss generalizations to the multiple field case, the curvaton scenario, and noncanonical kinetic terms.

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arXiv2010

Excited Dark Matter versus PAMELA/Fermi

Excitation of multicomponent dark matter in the galactic center has been proposed as the source of low-energy positrons that produce the excess 511 keV gamma rays that have been observed by INTEGRAL. Such models have also been promoted to explain excess high-energy electrons/positrons observed by the PAMELA, Fermi/LAT and H.E.S.S. experiments. We investigate whether one model can simultaneously fit all three anomalies, in addition to further constraints from inverse Compton scattering by the high-energy leptons. We find models that fit both the 511 keV and PAMELA excesses at dark matter masses M < 400 GeV, but not the Fermi lepton excess. The conflict arises because a more cuspy DM halo profile is needed to match the observed 511 keV signal than is compatible with inverse Compton constraints at larger DM masses.

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arXiv2010

Can multistate dark matter annihilation explain the high-energy cosmic ray lepton anomalies?

Multistate dark matter (DM) models with small mass splittings and couplings to light hidden sector bosons have been proposed as an explanation for the PAMELA/Fermi/H.E.S.S. high-energy lepton excesses. We investigate this proposal over a wide range of DM density profiles, in the framework of concrete models with doublet or triplet dark matter and a hidden SU(2) gauge sector that mixes with standard model hypercharge. The gauge coupling is bounded from below by the DM relic density, and the Sommerfeld enhancement factor is explicitly computable for given values of the DM and gauge boson masses M, mu and the (largest) dark matter mass splitting delta M_{12}. Sommerfeld enhancement is stronger at the galactic center than near the Sun because of the radial dependence of the DM velocity profile, which strengthens the inverse Compton (IC) gamma ray constraints relative to usual assumptions. We find that the PAMELA/Fermi/H.E.S.S. lepton excesses are marginally compatible with the model predictions, and with CMB and Fermi gamma ray constraints, for M ~ 800 GeV, mu ~ 200 MeV, and a dark matter profile with noncuspy Einasto parameters alpha > 0.20, r_s ~ 30 kpc. We also find that the annihilating DM must provide only a subdominant (< 0.4) component of the total DM mass density, since otherwise the boost factor due to Sommerfeld enhancement is too large.

hep-ph↗

arXiv2010

Leptons from Dark Matter Annihilation in Milky Way Subhalos

Numerical simulations of dark matter collapse and structure formation show that in addition to a large halo surrounding the baryonic component of our galaxy, there also exists a significant number of subhalos that extend hundreds of kiloparsecs beyond the edge of the observable Milky Way. We find that for dark matter (DM) annihilation models, galactic subhalos can significantly modify the spectrum of electrons and positrons as measured at our galactic position. Using data from the recent Via Lactea II simulation we include the subhalo contribution of electrons and positrons as boundary source terms for simulations of high energy cosmic ray propagation with a modified version of the publicly available GALPROP code. Focusing on the DM DM -> 4e annihilation channel, we show that including subhalos leads to a better fit to both the Fermi and PAMELA data. The best fit gives a dark matter particle mass of 1.2 TeV, for boost factors of 90 in the main halo and 1950-3800 in the subhalos (depending on assumptions about the background), in contrast to the 0.85 TeV mass that gives the best fit in the main halo-only scenario. These fits suggest that at least a third of the observed electron cosmic rays from DM annihilation could come from subhalos, opening up the possibility of a relaxation of recent stringent constraints from inverse Compton gamma rays originating from the high-energy leptons.

astro-ph.CO↗

arXiv2010

Exciting dark matter in the galactic center

We reconsider the proposal of excited dark matter (DM) as an explanation for excess 511 keV gamma rays from positrons in the galactic center. We quantitatively compute the cross section for DM annihilation to nearby excited states, mediated by exchange of a new light gauge boson with off-diagonal couplings to the DM states. In models where both excited states must be heavy enough to decay into e^+ e^- and the ground state, the predicted rate of positron production is never large enough to agree with observations, unless one makes extreme assumptions about the local circular velocity in the Milky Way, or alternatively if there exists a metastable population of DM states which can be excited through a mass gap of less than 650 keV, before decaying into electrons and positrons.

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arXiv2009

Testing for Features in the Primordial Power Spectrum

Well-known causality arguments show that events occurring during or at the end of inflation, associated with reheating or preheating, could contribute a blue component to the spectrum of primordial curvature perturbations, with the dependence k^3. We explore the possibility that they could be observably large in CMB, LSS, and Lyman-alpha data. We find that a k^3 component with a cutoff at some maximum k can modestly improve the fits (Delta chi^2=2.0, 5.4) of the low multipoles (l ~ 10 - 50) or the second peak (l ~ 540) of the CMB angular spectrum when the three-year WMAP data are used. Moreover, the results from WMAP are consistent with the CBI, ACBAR, 2dFGRS, and SDSS data when they are included in the analysis. Including the SDSS galaxy clustering power spectrum, we find weak positive evidence for the k^3 component at the level of Delta chi' = 2.4, with the caveat that the nonlinear evolution of the power spectrum may not be properly treated in the presence of the k^3 distortion. To investigate the high-k regime, we use the Lyman-alpha forest data (LUQAS, Croft et al., and SDSS Lyman-alpha); here we find evidence at the level Delta chi^2' = 3.8. Considering that there are two additional free parameters in the model, the above results do not give a strong evidence for features; however, they show that surprisingly large bumps are not ruled out. We give constraints on the ratio between the k^3 component and the nearly scale-invariant component, r_3 < 1.5, over the range of wave numbers 0.0023/Mpc < k < 8.2/Mpc. We also discuss theoretical models which could lead to the k^3 effect, including ordinary hybrid inflation and double D-term inflation models. We show that the well-motivated k^3 component is also a good representative of the generic spikelike feature in the primordial perturbation power spectrum.

astro-ph↗

arXiv2009

A new twist on excited dark matter: implications for INTEGRAL, PAMELA/ATIC/PPB-BETS, DAMA

We show that the 511 keV gamma ray excess observed by INTEGRAL/SPI can be more robustly explained by exciting dark matter (DM) at the center of the galaxy, if there is a peculiar spectrum of DM states chi_0, chi_1 and chi_2, with masses M_0 ~ 500 GeV, M_1 <~ M_0 + 2 m_e, and M_2 = M_1 + delta M >~ M_0 + 2 m_e. The small mass splitting delta M should be <~ 100 keV. In addition, we require at least two new gauge bosons (preferably three), with masses ~100 MeV. With this spectrum, chi_1 is stable, but can be excited to chi_2 by low-velocity DM scatterings near the galactic center, which are Sommerfeld-enhanced by two of the 100 MeV gauge boson exchanges. The excited state chi_2 decays to chi_0 and nonrelativistic e+e-, mediated by the third gauge boson, which mixes with the photon and Z. Although such a small 100 keV splitting has been independently proposed for explaining the DAMA annual modulation through the inelastic DM mechanism, the need for stability of chi_1 (and hence seqestering it from the Standard Model) implies that our scenario cannot account for the DAMA signal. It can however address the PAMELA/ATIC positron excess via DM annihilation in the galaxy, and it offers the possibility of a sharper feature in the ATIC spectrum relative to previously proposed models. The data are consistent with three new gauge bosons, whose couplings fit naturally into a broken SU(2) gauge theory where the DM is a triplet of the SU(2). We propose a simple model in which the SU(2) is broken by new Higgs triplet and 5-plet VEV's, giving rise to the right spectrum of DM, and mixing of one of the new gauge bosons with the photon and Z boson. A coupling of the DM to a heavy Z' may also be necessary to get the right relic density and PAMELA/ATIC signals.

hep-ph↗

arXiv2009

Nonabelian dark matter: models and constraints

Numerous experimental anomalies hint at the existence of a dark matter (DM) multiplet chi_i with small mass splittings. We survey the simplest such models which arise from DM in the low representations of a new SU(2) gauge symmetry, whose gauge bosons have a small mass mu < 1 GeV. We identify preferred parameters M_chi ~ 1 TeV, mu ~ 100 MeV, alpha_g ~ 0.04 and the chi chi -> 4e annihilation channel, for explaining PAMELA, Fermi, and INTEGRAL/SPI lepton excesses, while remaining consistent with constraints from relic density, diffuse gamma rays and the CMB. This consistency is strengthened if DM annihilations occur mainly in subhalos, while excitations (relevant to the excited DM proposal to explain the 511 keV excess) occur in the galactic center (GC), due to higher velocity dispersions in the GC, induced by baryons. We derive new constraints and predictions which are generic to these models. Notably, decays of excited DM states chi' -> chi gamma arise at one loop and could provide a new signal for INTEGRAL/SPI; big bang nucleosynthesis (BBN) constraints on the density of dark SU(2) gauge bosons imply a lower bound on the mixing parameter epsilon between the SU(2) gauge bosons and photon. These considerations rule out the possibility of the gauge bosons that decay into e^+e^- being long-lived. We study in detail models of doublet, triplet and quintuplet DM, showing that both normal and inverted mass hierarchies can occur, with mass splittings that can be parametrically smaller, e.g., O(100) keV, than the generic MeV scale of splittings. A systematic treatment of Z_2 symmetry which insures the stability of the intermediate DM state is given for cases with inverted mass hierarchy, of interest for boosting the 511 keV signal from the excited dark matter mechanism.

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arXiv2009

Warped Kaluza-Klein Dark Matter

Warped compactifications of type IIB string theory contain natural dark matter candidates: Kaluza-Klein modes along approximate isometry directions of long warped throats. These isometries are broken by the full compactification, including moduli stabilization; we present a thorough survey of Kaluza-Klein mode decay rates into light supergravity modes and Standard Model particles. We find that these dark matter candidates typically have lifetimes longer than the age of the universe. Interestingly, some choices for embedding the Standard Model in the compactification lead to decay rates large enough to be observed, so this dark matter sector may provide constraints on the parameter space of the compactification.

hep-th↗

arXiv2009

Dynamical Fine Tuning in Brane Inflation

We investigate a novel mechanism of dynamical tuning of a flat potential in the open string landscape within the context of warped brane-antibrane inflation in type IIB string theory. Because of competing effects between interactions with the moduli stabilizing D7-branes in the warped throat and anti-D3-branes at the tip, a stack of branes gives rise to a local minimum of the potential, holding the branes high up in the throat. As branes successively tunnel out of the local minimum to the bottom of the throat the potential barrier becomes lower and is eventually replaced by a flat inflection point, around which the remaining branes easily inflate. This dynamical flattening of the inflaton potential reduces the need to fine tune the potential by hand, and also leads to successful inflation for a larger range of inflaton initial conditions, due to trapping in the local minimum.

hep-th↗

arXiv2009

Electroweak Phase Transition and LHC Signatures in the Singlet Majoron Model

We reconsider the strength of the electroweak phase transition in the singlet Majoron extension of the Standard Model, with a low (~TeV) scale of the singlet VEV. A strongly first order phase transition, of interest for electroweak baryogenesis, is found in sizeable regions of the parameter space, especially when the cross-coupling lambda_{hs}|S|^2|H|^2 between the singlet and the doublet Higgs is significant. Large Majorana Yukawa couplings of the singlet neutrinos, y_i S nu_i^c nu_i, are also important for strengthening the transition. We incorporate the LEP and Tevatron constraints on the Higgs masses, and electroweak precision constraints, in our search for allowed parameters; successful examples include singlet masses ranging from 5 GeV to several TeV. Models with a strong phase transition typically predict a nonstandard Higgs with mass in the range 113 GeV < m_H < 200 GeV and production cross sections reduced by mixing with the singlet, with cos^2(theta) significantly less than 1. We also find examples where the singlet is light and the decay H -> SS can modify the Higgs branching ratios relative to Standard Model expectations.

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