Showing papers by "Neal Weiner published in 2009"

A theory of dark matter

Abstract: � > 1GeV 1 . The long range allows a Sommerfeld enhancement to boost the annihilation cross section as required, without altering the weak-scale annihilation cross section during dark matter freeze-out in the early universe. If the dark matter annihilates into the new force carrier φ, its low mass can make hadronic modes kinematically inaccessible, forcing decays dominantly into leptons. If the force carrier is a non-Abelian gauge boson, the dark matter is part of a multiplet of states, and splittings between these states are naturally generated with size αm� � MeV, leading to the eXciting dark matter (XDM) scenario previously proposed to explain the positron annihilation in the galactic center observed by the INTEGRAL satellite; the light boson invoked by XDM to mediate a large inelastic scattering cross section is identified with the φ here. Somewhat smaller splittings would also be expected, providing a natural source for the parameters of the inelastic dark matter (iDM) explanation for the DAMA annual modulation signal. Since the Sommerfeld enhancement is most significant at low velocities, early dark matter halos at redshift � 10 potentially produce observable effects on the ionization history of the universe. Because of the enhanced cross section, detection of substructure is more probable than with a conventional WIMP. Moreover, the low velocity dispersion of dwarf galaxies and Milky Way subhalos can increase the substructure annihilation signal by an additional order of magnitude or more.

Inelastic Dark Matter in Light of DAMA/LIBRA

Abstract: Inelastic dark matter, in which weakly interacting massive particle (WIMP)-nucleus scatterings occur through a transition to an excited WIMP state {approx}100 keV above the ground state, provides a compelling explanation of the DAMA annual modulation signal. We demonstrate that the relative sensitivities of various dark matter direct detection experiments are modified such that the DAMA annual modulation signal can be reconciled with the absence of a reported signal at CDMS-Soudan, XENON10, ZEPLIN, CRESST, and KIMS for inelastic WIMPs with masses O(100 GeV). We review the status of these experiments, and make predictions for upcoming ones. In particular, we note that inelastic dark matter leads to highly suppressed signals at low energy, with most events typically occurring between 20 and 45 keV (unquenched) at xenon and iodine experiments, and generally no events at low ({approx}10 keV) energies. Suppressing the background in this high-energy region is essential to testing this scenario. The recent CRESST data suggest seven observed tungsten events, which is consistent with expectations from this model. If the tungsten signal persists at future CRESST runs, it would provide compelling evidence for inelastic dark matter, while its absence should exclude it.

High energy positrons from annihilating dark matter

Abstract: We also consider a range of diffusion parameters consistent with current cosmic ray data. We find that a significant upturn in the positron fraction above 10 GeV is compatible with a wide range of dark matter annihilation modes, although very large annihilation cross sections and/or boost factors arising from inhomogeneities in the local dark matter distribution are required to produce the observed intensity of the signal. We comment on constraints from gamma rays, synchrotron emission, and cosmic ray antiproton measurements. PACS numbers: 95.35.+d; 98.70.Sa; 96.50.S; 95.55.Vj FERMILAB-PUB-08-347-A Dark matter in the form of a thermal relic is an appealing explanation for the approximately 85% of the matter density of the universe not composed of baryons. In addition to being a natural extension of the big bang cosmology, the candidates which naturally give the appropriate relic abundance have annihilation cross sections of the order of the electroweak scale, a natural scale for new particles in theoretical frameworks which provide a solution to the hierarchy problem. If they exist, such thermal relics are expected to be annihilating in the halo today, generating potentially observable fluxes of high energy particles, including gamma rays, electrons, positrons, and antiprotons. To this end, a number of cosmic ray and gamma ray experiments [1, 2, 3, 4, 5, 6] have considered the search for dark matter annihilation products to be an important aspect of their science mission. Of particular interest is the satellite-based cosmic ray experiment, PAMELA [7, 8]. With its large acceptance (21.5 cm 2 sr) and excellent particle identification, PAMELA is anticipated to measure the spectra of cosmic ray protons, antiprotons, electrons and positrons up to energies of 700 GeV, 190 GeV, 2 TeV and 270 GeV, respectively.

Case for a 700+GeV WIMP: Cosmic ray spectra from PAMELA, Fermi, and ATIC

Abstract: Multiple lines of evidence indicate an anomalous injection of high-energy e{sup +}e{sup -} in the galactic halo. The recent e{sup +} fraction spectrum from the payload for antimatter matter exploration and light-nuclei astrophysics (PAMELA) shows a sharp rise up to 100 GeV. The Fermi gamma-ray space telescope has found a significant hardening of the e{sup +}e{sup -} cosmic-ray spectrum above 100 GeV, with a break, confirmed by HESS at around 1 TeV. The advanced thin ionization calorimeter (ATIC) has also detected a similar excess, falling back to the expected spectrum at 1 TeV and above. Excess microwaves towards the galactic center in the WMAP data are consistent with hard synchrotron radiation from a population of 10-100 GeV e{sup +}e{sup -} (the WMAP 'Haze'). We argue that dark matter annihilations can provide a consistent explanation of all of these data, focusing on dominantly leptonic modes, either directly or through a new light boson. Normalizing the signal to the highest energy evidence (Fermi and HESS), we find that similar cross sections provide good fits to PAMELA and the Haze, and that both the required cross section and annihilation modes are achievable in models with Sommerfeld-enhanced annihilation. These models naturally predict significant productionmore » of gamma rays in the galactic center via a variety of mechanisms. Most notably, there is a robust inverse-Compton scattered (ICS) gamma-ray signal arising from the energetic electrons and positrons, detectable at Fermi/GLAST energies, which should provide smoking gun evidence for this production.« less

The PAMELA Positron Excess from Annihilations into a Light Boson

Abstract: Recently published results from the PAMELA experiment have shown conclusive evidence for an excess of positrons at high ( ~ 10–100 GeV) energies, confirming earlier indications from HEAT and AMS-01. Such a signal is generally expected from dark matter annihilations. However, the hard positron spectrum and large amplitude are difficult to achieve in most conventional WIMP models. The absence of any associated excess in anti-protons is highly constraining on models with hadronic annihilation modes. We revisit an earlier proposal, wherein the dark matter annihilates into a new light (GeV) boson , which is kinematically constrained to go to hard leptonic states, without anti-protons or π0's. We find this provides a very good fit to the data. The light boson naturally provides a mechanism by which large cross sections can be achieved through the Sommerfeld enhancement, as was recently proposed. Depending on the mass of the WIMP, the rise may continue above 300 GeV, the extent of PAMELA's ability to discriminate between electrons and positrons.

High Energy Positrons and the WMAP Haze from Exciting Dark Matter

Abstract: We consider the signals of positrons and electrons from ``exciting'' dark matter annihilation. Because of the light (${m}_{\ensuremath{\phi}}\ensuremath{\lesssim}1\text{ }\text{ }\mathrm{GeV}$) force carrier $\ensuremath{\phi}$ into which the dark matter states can annihilate, the electrons and positrons are generally very boosted, yielding a hard spectrum, in addition to the low energy positrons needed for INTEGRAL observations of the Galactic center. We consider the relevance of this scenario for HEAT, PAMELA, and the WMAP ``haze,'' focusing on light (${m}_{\ensuremath{\phi}}\ensuremath{\lesssim}2{m}_{\ensuremath{\pi}}$) $\ensuremath{\phi}$ bosons, and find that significant signals can be found for all three, although significant signals generally require high dark matter densities. We find that measurements of the positron fraction are generally insensitive to the halo model, but do suffer significant astrophysical uncertainties. We discuss the implications for upcoming PAMELA results.

Using the energy spectrum measured by DAMA/LIBRA to probe light dark matter

Abstract: A weakly interacting massive particle (WIMP) weighing only a few GeV has been invoked as an explanation for the signal from the DAMA/LIBRA experiment We show that the data from DAMA/LIBRA are now powerful enough to strongly constrain the properties of any putative WIMP Accounting for the detailed recoil spectrum, a light WIMP with a Maxwellian velocity distribution and a spin-independent interaction cannot account for the data Even neglecting the spectrum, significant parameter space is excluded by a limit that can be derived from the DAMA unmodulated signal at low energies Appreciable modification to the astrophysics or particle physics can open light mass windows

Inelastic dark matter and DAMA/LIBRA: An experimentum crucis

Abstract: The DAMA/LIBRA Collaboration has detected an annual modulation of the recoil rate in NaI crystals with the phase expected for weakly interacting massive particle (WIMP) scattering events. This signal is dramatically inconsistent with upper limits from other experiments for elastically scattering weak-scale WIMPs. However, the results are compatible for the case of inelastic dark matter (iDM). The iDM theory, as implemented by Tucker-Smith and Weiner, constrains the WIMP to a tight contour in ${\ensuremath{\sigma}}_{n}\ensuremath{-}\ensuremath{\delta}$ space, where $\ensuremath{\delta}$ is the mass difference between the ground state and excited WIMPs. An urgent priority in direct detection is to test this scenario. The crucial test of the iDM explanation of DAMA---an experimentum crucis---is an experiment with directional sensitivity, which can measure the daily modulation in direction. Because the contrast can be 100%, it is a sharper test than the much smaller annual modulation in the rate. We estimate the significance of such an experiment as a function of the WIMP mass, cross section, background rate, and other parameters. The proposed experiment severely constrains the DAMA/iDM scenario even with modest exposure ($\ensuremath{\sim}1000\text{ }\text{ }\mathrm{kg}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{day}$) on gaseous xenon.

PAMELA, DAMA, INTEGRAL and Signatures of Metastable Excited WIMPs

Abstract: Models of dark matter with ~ GeV scale force mediators provide attractive explanations of many high energy anomalies, including PAMELA, ATIC, and the WMAP haze. At the same time, by exploiting the ~ MeV scale excited states that are automatically present in such theories, these models naturally explain the DAMA/LIBRA and INTEGRAL signals through the inelastic dark matter (iDM) and exciting dark matter (XDM) scenarios, respectively. Interestingly, with only weak kinetic mixing to hypercharge to mediate decays, the lifetime of excited states with delta < 2 m_e is longer than the age of the universe. The fractional relic abundance of these excited states depends on the temperature of kinetic decoupling, but can be appreciable. There could easily be other mechanisms for rapid decay, but the consequences of such long-lived states are intriguing. We find that CDMS constrains the fractional relic population of ~100 keV states to be <~ 10^-2, for a 1 TeV WIMP with sigma_n = 10^-40 cm^2. Upcoming searches at CDMS, as well as xenon, silicon, and argon targets, can push this limit significantly lower. We also consider the possibility that the DAMA excitation occurs from a metastable state into the XDM state, which decays via e+e- emission, which allows lighter states to explain the INTEGRAL signal due to the small kinetic energies required. Such models yield dramatic signals from down-scattering, with spectra peaking at high energies, sometimes as high as ~1 MeV, well outside the usual search windows. Such signals would be visible at future Ar and Si experiments, and may be visible at Ge and Xe experiments. We also consider other XDM models involving ~ 500 keV metastable states, and find they can allow lighter WIMPs to explain INTEGRAL as well.

PAMELA, DAMA, INTEGRAL and signatures of metastable excited WIMPs

Abstract: Models of dark matter with ~ GeV scale force mediators provide attractive explanations of many high energy anomalies, including PAMELA, ATIC, and the WMAP haze. At the same time, by exploiting the ~ MeV scale excited states that are automatically present in such theories, these models naturally explain the DAMA/LIBRA and INTEGRAL signals through the inelastic dark matter (iDM) and exciting dark matter (XDM) scenarios, respectively. Interestingly, with only weak kinetic mixing to hypercharge to mediate decays, the lifetime of excited states with ? < 2melectron is longer than the age of the universe. The fractional relic abundance of these excited states depends on the temperature of kinetic decoupling, but can be appreciable. There could easily be other mechanisms for rapid decay, but the consequences of such long-lived states are intriguing. We find that CDMS constrains the fractional relic population of ~ 100 keV states to be 10?2, for a 1 TeV WIMP with ?n = 10?40 cm2. Upcoming searches at CDMS, as well as xenon, silicon, and argon targets, can push this limit significantly lower. We also consider the possibility that the DAMA excitation occurs from a metastable state into the XDM state, which decays via e+e? emission, which allows lighter states to explain the INTEGRAL signal due to the small kinetic energies required. Such models yield dramatic signals from down-scattering, with spectra peaking at high energies, sometimes as high as ~ 1 MeV, well outside the usual search windows. Such signals would be visible at future Ar and Si experiments, and may be visible at Ge and Xe experiments, although ?-rays associated with nuclear excitations would complicate the signal for these heavier targets. We also consider other XDM models involving ~ 500 keV metastable states, and find they can allow lighter WIMPs to explain INTEGRAL as well.

Momentum Dependent Dark Matter Scattering

Abstract: It is usually assumed that WIMPs interact through spin-independent and spin-dependent interactions. Interactions which carry additional powers of the momentum transfer, q^2, are assumed to be too small to be relevant. In theories with new particles at the ~ GeV scale, however, these q^2-dependent interactions can be large, and, in some cases dominate over the standard interactions. This leads to new phenomenology in direct detection experiments. Recoil spectra peak at non-zero energies, and the relative strengths of different experiments can be significantly altered. We present a simple parameterization for models of this type which captures much of the interesting phenomenology and allows a comparison between experiments. As an application, we find that dark matter with momentum dependent interactions coupling to the spin of the proton can reconcile the DAMA annual modulation result with other experiments.

MiXDM: Cosmic Ray Signals from Multiple States of Dark Matter

Abstract: Recent data from cosmic ray experiments such as PAMELA, Fermi, ATIC and PPB-BETS all suggest the need for a new primary source of electrons and positrons at high (>~100 GeV) energies. Many proposals have been put forth to explain these data, usually relying on a single particle to annihilate or decay to produce e+e-. In this paper, we consider models with multiple species of WIMPs with significantly different masses. We show if such dark matter candidates chi_i annihilate into light bosons, they naturally produce equal annihilation rates, even as the available numbers of pairs for annihilation n_chi_i^2 differ by orders of magnitude. We argue that a consequence of these models can be to add additional signal naturally at lower (~100 GeV) versus higher (~ TeV) energies, changing the expected spectrum and even adding bumps at lower energies, which may alleviate some of the tension in the required annihilation rates between PAMELA and Fermi. These spectral changes may yield observable consequences in the microwave Haze signal observed at the upcoming Planck satellite. Such a model can connect to other observable signals such as DAMA and INTEGRAL by having the lighter (heavier) state be a pseudo-Dirac fermion with splitting 100 keV (1 MeV). We show that variations in the halo velocity dispersion can alleviate constraints from final state radiation in the galactic center and galactic ridge. If the lighter WIMP has a large self-interaction cross section, the light-WIMP halo might collapse, dramatically altering expectations for direct and indirect detection signatures.

Dark Matter Direct Detection with Non-Maxwellian Velocity Structure

Abstract: The velocity distribution function of dark matter particles is expected to show significant departures from a Maxwell-Boltzmann distribution. This can have profound effects on the predicted dark matter - nucleon scattering rates in direct detection experiments, especially for dark matter models in which the scattering is sensitive to the high velocity tail of the distribution, such as inelastic dark matter (iDM) or light (few GeV) dark matter (LDM), and for experiments that require high energy recoil events, such as many directionally sensitive experiments. Here we determine the velocity distribution functions from two of the highest resolution numerical simulations of Galactic dark matter structure (Via Lactea II and GHALO), and study the effects for these scenarios. For directional detection, we find that the observed departures from Maxwell-Boltzmann increase the contrast of the signal and change the typical direction of incoming DM particles. For iDM, the expected signals at direct detection experiments are changed dramatically: the annual modulation can be enhanced by more than a factor two, and the relative rates of DAMA compared to CDMS can change by an order of magnitude, while those compared to CRESST can change by a factor of two. The spectrum of the signal can also change dramatically, with many features arising due to substructure. For LDM the spectral effects are smaller, but changes do arise that improve the compatibility with existing experiments. We find that the phase of the modulation can depend upon energy, which would help discriminate against background should it be found.

Cosmic ray positrons from annihilations into a new, heavy lepton

Abstract: Recent results from the PAMELA experiment indicate an excess in the positron spectrum above 10 GeV, but antiproton data are consistent with the expected astrophysical backgrounds. We propose a scenario that reproduces these features. Dark matter annihilates through channels involving a new heavy vectorlike lepton which then decays by mixing with standard model leptons. If charged, this heavy lepton might be produced at the LHC, and could lead to multilepton final states or to long-lived charged tracks. Large neutrino detectors such as ANTARES or IceCube might be sensitive to a monochromatic neutrino line. This scenario may be simply embedded in various models, including an extension to the next-to-minimal supersymmetric standard model.

The Fermi gamma-ray spectrum of the inner galaxy: Implications for annihilating dark matter

Abstract: Recently, a preliminary spectrum from the Fermi Gamma-ray Space Telescope has been presented for the inner galaxy (-30 < l < 30, -5 < b < 5), as well as the galactic center (-1 < l < 1, -1< b < 1). We consider the implications of these data for dark matter annihilation models, especially models capable of producing the cosmic-ray excesses previously observed by PAMELA and Fermi. These local cosmic-ray excesses, when extrapolated to the inner galaxy, imply inverse Compton scattering (ICS) gamma-ray signals largely consistent with the preliminary Fermi gamma-ray spectrum. For specific halos and models, particularly those with prompt photons, the data have begun to constrain the allowed parameter space. Although significant modeling and background uncertainties remain, we explore how large a signal is permitted by the current data. Based upon this, we make predictions for what signal could be present in other regions of the sky where dark matter signals may be easier to isolate from the astrophysical backgrounds.

The Fermi Haze: A Gamma-Ray Counterpart to the Microwave Haze

Abstract: The Fermi Gamma-Ray Space Telescope reveals a diffuse inverse Compton signal in the inner Galaxy with a similar spatial morphology to the microwave haze observed by WMAP, supporting the synchrotron interpretation of the microwave signal. Using spatial templates, we regress out pi0 gammas, as well as IC and bremsstrahlung components associated with known soft-synchrotron counterparts. We find a significant gamma-ray excess towards the Galactic center with a spectrum that is significantly harder than other sky components and is most consistent with IC from a hard population of electrons. The morphology and spectrum are consistent with it being the IC counterpart to the electrons which generate the microwave haze seen at WMAP frequencies. In addition, the implied electron spectrum is hard; electrons accelerated in supernova shocks in the disk which then diffuse a few kpc to the haze region would have a softer spectrum. We describe the full sky Fermi maps used in this analysis and make them available for download.

Neutrino Mass, Sneutrino Dark Matter and Signals of Lepton Flavor Violation in the MRSSM

Abstract: We study the phenomenology of mixed-sneutrino dark matter in the Minimal R-Symmetric Supersymmetric Standard Model (MRSSM). Mixed sneutrinos fit naturally within the MRSSM, as the smallness (or absence) of neutrino Yukawa couplings singles out sneutrino A-terms as the only ones not automatically forbidden by R-symmetry. We perform a study of randomly generated sneutrino mass matrices and find that (i) the measured value of $\Omega_{DM}$ is well within the range of typical values obtained for the relic abundance of the lightest sneutrino, (ii) with small lepton-number-violating mass terms $m_{nn}^{2} {\tilde n} {\tilde n}$ for the right-handed sneutrinos, random matrices satisfying the $\Omega_{DM}$ constraint have a decent probability of satisfying direct detection constraints, and much of the remaining parameter space will be probed by upcoming experiments, (iii) the $m_{nn}^{2} {\tilde n} {\tilde n}$ terms radiatively generate appropriately small Majorana neutrino masses, with neutrino oscillation data favoring a mostly sterile lightest sneutrino with a dominantly mu/tau-flavored active component, and (iv) a sneutrino LSP with a significant mu component can lead to striking signals of e-mu flavor violation in dilepton invariant-mass distributions at the LHC.

The Dark Side of the Electroweak Phase Transition

Abstract: Recent data from cosmic ray experiments may be explained by a new GeV scale of physics. In addition the fine-tuning of supersymmetric models may be alleviated by new O(GeV) states into which the Higgs boson could decay. The presence of these new, light states can affect early universe cosmology. We explore the consequences of a light (~ GeV) scalar on the electroweak phase transition. We find that trilinear interactions between the light state and the Higgs can allow a first order electroweak phase transition and a Higgs mass consistent with experimental bounds, which may allow electroweak baryogenesis to explain the cosmological baryon asymmetry. We show, within the context of a specific supersymmetric model, how the physics responsible for the first order phase transition may also be responsible for the recent cosmic ray excesses of PAMELA, FERMI etc. We consider the production of gravity waves from this transition and the possible detectability at LISA and BBO.