28 March, 2012

1) Asymptotic Darkness

• On Asymptotic Darkness in Horava-Lifshitz GRavity
  abstract:  Ho\v{r}ava-Lifshitz gravity is shown to exhibit a peculiar behavior, when scattering amplitudes are considered. At low energies it seems to classicalize i.e. the effective size of the interaction grows as a function of the $s$-parameter, with BHs forming part of the spectrum; but when the probing energy is increased such that higher order operators become important, this behavior changes and the classicalon recedes to a new configuration where ordinary quantum regimes take over. Eventually, the theory behaves as a usual field theory that allows probing arbitrarily small distances. In particular, the classical potential created by a point-like source is finite everywhere exhibiting a Vainshtein alike screening behavior. The transition from one behavior to the other is carefully described in a particular case.

2) Asymmetric Dark Matter

• Closing in on asymmetric dark matter I: Model independent limits for interaction with quarks 
  abstract:  It is argued that experimental constraints on theories of asymmetric dark matter (ADM) almost certainly require that the DM be part of a richer hidden sector of interacting states of comparable mass or lighter. A general requisite of models of ADM is that the vast majority of the symmetric component of the DM number density must be removed in order to explain the observed relationship $\Omega_B\sim\Omega_{DM}$ via the DM asymmetry. Demanding the efficient annihilation of the symmetric component leads to a tension with experimental limits if the annihilation is directly to Standard Model (SM) degrees of freedom. A comprehensive effective operator analysis of the model independent constraints on ADM from direct detection experiments and LHC monojet searches is presented. Notably, the limits obtained essentially exclude models of ADM with mass 1GeV$\lesssim m_{DM} \lesssim$ 100GeV annihilating to SM quarks via heavy mediator states. This motivates the study of portal interactions between the dark and SM sectors mediated by light states. Resonances and threshold effects involving the new light states are shown to be important for determining the exclusion limits.

21 March, 2012

1) The neutrino velocity

• Measurement of the neutrino velocity with the ICARUS detector at the CNGS beam
  abstract:  The CERN-SPS accelerator has been briefly operated in a new, lower intensity neutrino mode with ~10^12 p.o.t. /pulse and with a beam structure made of four LHC-like extractions, each with a narrow width of ~3 ns, separated by 524 ns. This very tightly bunched beam structure represents a substantial progress with respect to the ordinary operation of the CNGS beam, since it allows a very accurate time-of-flight measurement of neutrinos from CERN to LNGS on an event-to-event basis. The ICARUS T600 detector has collected 7 beam-associated events, consistent with the CNGS delivered neutrino flux of 2.2 10^16 p.o.t. and in agreement with the well known characteristics of neutrino events in the LAr-TPC. The time of flight difference between the speed of light and the arriving neutrino LAr-TPC events has been analysed. The result is compatible with the simultaneous arrival of all events with equal speed, the one of light. This is in a striking difference with the reported result of OPERA [1] that claimed that high energy neutrinos from CERN should arrive at LNGS about 60 ns earlier than expected from luminal speed.

2) Limits on Dark Matter annihilation from dwarf galaxies

• Extracting limits on Dark Matter annihilation from dwarf Spheroidal galaxies at gamma-rays 
  abstract:  Dwarf spheroidal galaxies compose one of the most dark matter dominated classes of objects, making them a set of targets to search for signals of dark matter annihilation. The recent developments in gamma-ray astronomy, most importantly the launch of the Fermi-LAT instrument, have brought those targets into attention. Yet, no clear excess of gamma-rays has been confirmed from these targets, resulting in some of the tightest limits on dark matter annihilation from indirect searches. In extracting limits from dwarf spheroidal galaxies, it is of great importance, to properly take into account all relevant uncertainties, which include the dark matter distribution properties of the dwarf spheroidals, and the uncertainties on the underlying background. We revisit the limits on dark matter annihilation, from gamma-rays studying a set of close-by dwarf spheroidal galaxies, for which, we have good understanding of the uncertainties in the dark matter distribution. For those targets, we perform and compare results for alternative methods in extracting the background gamma-ray flux; which provides a method to discriminate among the dark matter annihilation targets, those that can give robust constraints. We finally present our tightest limits on dark matter annihilation, coming only from the targets that ensure accurate understanding of both the gamma-ray background and the dark matter distribution uncertainties.

14 March, 2012

1) Dark Matter after freeze-out

• Changes in dark matter properties after freeze-out
  abstract:  The properties of the dark matter that determine its thermal relic abundance can be very different from the dark matter properties today. We investigate this possibility by coupling a dark matter sector to a scalar that undergoes a phase transition after the dark matter freezes out. If the value of Omega_DM h^2 calculated from parameters measured at colliders and by direct and indirect detection experiments does not match the astrophysically observed value, a novel cosmology of this type could provide the explanation. This mechanism also has the potential to account for the “boost factor” required to explain the PAMELA data.

2) Spatial variation of the fine structure constant

• Spatial variation in the fine-structure constant — new results from VLT/UVES
  abstract:  We present a new analysis of a large sample of quasar absorption-line spectra obtained using UVES (the Ultraviolet and Visual Echelle Spectrograph) on the VLT (Very Large Telescope) in Chile. In the VLT sample (154 absorbers), we find evidence that alpha increases with increasing cosmological distance from Earth. However, as previously shown, the Keck sample (141 absorbers) provided evidence for a smaller alpha in the distant absorption clouds. Upon combining the samples an apparent variation of alpha across the sky emerges which is well represented by an angular dipole model pointing in the direction RA=(17.3 +/- 1.0) hr, dec. = (-61 +/- 10) deg, with amplitude (0.97 +0.22/-0.20) x 10^(-5). The dipole model is required at the 4.1 sigma statistical significance level over a simple monopole model where alpha is the same across the sky (but possibly different to the current laboratory value). The data sets reveal a number of remarkable consistencies: various data cuts are consistent and there is consistency in the overlap region of the Keck and VLT samples. Assuming a dipole-only (i.e. no-monopole) model whose amplitude grows proportionally with `lookback-time distance’ (r=ct, where t is the lookback time), the amplitude is (1.1 +/- 0.2) x 10^(-6) GLyr^(-1) and the model is significant at the 4.2 sigma confidence level over the null model [Delta alpha]/alpha = 0). We apply robustness checks and demonstrate that the dipole effect does not originate from a small subset of the absorbers or spectra. We present an analysis of systematic effects, and are unable to identify any single systematic effect which can emulate the observed variation in alpha.

7 March, 2012

1) Neutrinos: Dirac or Majorana?

• Probing Majorana neutrino in rare decays
  abstract:  We study the rare decays of charged $\pi$ mesons, $\pi^+ \to e^+ e^+ \mu^- \bar\nu_\mu $ and $\pi^+ \to e^+ \mu^- e^+ \nu_e $ induced by a sterile neutrino $N$ with a mass in the range $m_\mu < m_N < m_\pi$. The first process violates Lepton Number by two units and so occurs only if $N$ is Majorana, while the second process conserves Lepton Number and occurs irrespective of the Majorana or Dirac character of $N$. We study a way to distinguish the Majorana vs. Dirac character of $N$ in these processes using the muon spectrum. We also find that the branching ratios could be at the reach of high luminosity experiments like Project X at FNAL or any proposed neutrino (or muon) factories worldwide.

2) Solar Neutrinos as an explanation for DAMA?

• Elastic scattering signals of solar neutrinos with enhanced baryonic currents 
  abstract:  The coupling of the baryonic current to new neutrino states \nu_b with strength in excess of the weak interactions is a viable extension of the Standard Model. We analyze the signature of \nu_b appearance in the solar neutrino flux that gives rise to an elastic scattering signal in dark matter direct detection and in solar neutrino experiments. This paper lays out an in-depth study of \nu_b detection prospects for current and future underground rare event searches. We scrutinize the model as a possible explanation for the reported anomalies from DAMA, CoGeNT, and CRESST-II and confront it with constraints from other null experiments.

29 February, 2012

1) Composite Inflation

• Composite inflation setup and Glueball inflation
  abstract:  We explore the paradigm according to which inflation is driven by a four-dimensional strongly coupled dynamics coupled non-minimally to gravity. We start by introducing the general setup, both in the metric and Palatini formulation, for generic models of composite inflation. We then analyze the relevant example where the inflaton is identified with the glueball field of a pure Yang-Mills theory. We introduce the dilatonic-like glueball action which is obtained by requiring saturation of the underlying Yang-Mills trace anomaly at the effective action level. We couple the resulting action non-minimally to gravity. We demonstrate that it is possible to achieve successful inflation with the confining scale of the underlying Yang-Mills theory naturally of the order of the grand unified energy scale. We also argue that within the metric formulation models of composite inflation lead to a more consistent picture than within the Palatini one. Finally we show that, in the metric formulation, the model nicely respects tree-level unitarity for the scattering of the inflaton field all the way to the Planck scale.

15 February, 2012

1) Dark Matter Phenomenology

• Impact of semi-annihilations on dark matter phenomenology – an example of Z_N symmetric scalar dark matter.
  abstract:  We study the impact of semi-annihilations x_i x_j <-> x_k X, where x_i is any dark matter and X is any standard model particle, on dark matter phenomenology. We formulate minimal scalar dark matter models with an extra doublet and a complex singlet that predict non-trivial dark matter phenomenology with semi-annihilation processes for different discrete Abelian symmetries Z_N, N>2. We implement two such example models with Z_3 and Z_4 symmetry in micrOMEGAs and work out their phenomenology. We show that both semi-annihilations and annihilations involving only particles from two different dark matter sectors significantly modify the dark matter relic abundance in this type of models. We also study the possibility of dark matter direct detection in XENON100 in those models.

8 February, 2012

1) Masses of a pair of semi-invisibly decaying particles

• A novel technique for measuring masses of a pair of semi-invisibly decaying particles
  abstract:  Motivated by evidence for the existence of dark matter, many new physics models predict the pair production of new particles, followed by the decays into two invisible particles, leading to a momentum imbalance in the visible system. For the cases where all four components of the vector sum of the two `missing’ momenta are measured from the momentum imbalance, we present analytic solutions of the final state system in terms of measureable momenta, with the mass shell constraints taken into account. We then introduce new variables which allow the masses involved in the new physics process, including that of the dark matter particles, to be extracted. These are compared with a selection of variables in the literature, and possible applications at lepton and hadron colliders are discussed.

2) Pair-production anomaly

• Indications for a pair-production anomaly from the propagation of VHE gamma-rays
  abstract:  In the recent years, the number of detected very high energy (VHE: E > 100 GeV) gamma-ray sources has increased rapidly. The sources have been observed at redshifts up to z = 0.536 without strong indications for the presence of absorption features in the energy spectra. Absorption is however expected due to pair-production processes of the propagating photons with the photon bath in intergalactic space. Even though this photon density is not well known, lower limits can be firmly set by the resolved emission from galaxy counts. Using this guaranteed background light, we investigate the behaviour of the energy spectra in the transition region from the optically thin to the optically thick regime. Among the sample of 50 energy spectra, 7 spectra cover the the range from optical depth $\tau < 1$ to $\tau > 2$. For these sources, the transition to $\tau > 2$ takes place at widely different energies ranging from 0.4 TeV to 21 TeV. Consistently, in all of these sources, an upturn of the absorption-corrected spectrum is visible at this transition with a combined significance of 4.2 standard deviations. Given the broad range of energies and redshifts covered by the sample, source-intrinsic features are unlikely to explain the observed effect. Systematic effects related to observations have been investigated and found to be not sufficient to account for the observed effect. The pair-production process seems to be suppressed in a similar way as expected in the extension of the standard model by a light (<neV) pseudoscalar (axion-like) particle.

18 January, 2012

1) Variable sources in gamma-ray 

• Variable gamma-ray sky at 1 GeV
  abstract:  We search for the long-term variability of the \gamma-ray sky in the energy range E > 1 GeV with 168 weeks of Fermi-LAT data. We perform a full sky blind search for regions with variable flux looking for deviations from uniformity. We bin the sky into 12288 bins using Healpix package and use Kolmogorov-Smirnov test to compare weekly photon counts in each bin with a constant flux hypothesis. The weekly exposure of Fermi-LAT for each bin is calculated with the Fermi-LAT tools. We consider flux variations in the bin significant if statistical probability of uniformity is less than 4e-6, which corresponds to 0.05 false detections in the whole set. We identified 117 variable sources, variability of 27 of which has not been reported before. Among the sources with previously unidentified variability there are 25 AGNs belonging to blazar class (11 BL Lacs and 14 FSRQs), one AGN of uncertain type and one pulsar PSR J0633+1746 (Geminga). The observed long term flux variability of Geminga has a statistical significance of 5.1\sigma.

11 January, 2012

1) Gamma-ray Emissions and Dark Matter

• Evidence for extended gamma-ray emission from galaxy clusters
  abstract:  We report evidence for extended gamma-ray emission from the Virgo, Fornax and Coma clusters based on a maximum-likelihood analysis of the 3-year Fermi-LAT data. For all three clusters, excess emission is observed within three degrees of the center, peaking at the GeV scale. This emission cannot be accounted for by known Fermi sources or by the galactic and extragalactic backgrounds. If interpreted as annihilation emission from supersymmetric dark matter (DM) particles, the data prefer models with a particle mass in the range 20-60 GeV annihilating into the b-bbar channel, or 2-10 GeV and >1 TeV annihilating into mu-mu final states. Our results are consistent with those obtained by Hooper and Linden from a recent analysis of Fermi-LAT data in the region of the Galactic Centre. An extended DM annihilation profile dominated by emission from substructures is preferred over a simple point source model. The significance of DM detection is 4.4 sigma in Virgo and lower in the other two clusters. We also consider the possibility that the excess emission arises from cosmic ray (CR) induced gamma-rays, and infer a CR level within a factor of three of that expected from analytical models. However, the significance of a CR component is lower than the significance of a DM component, and there is no need for such a CR component in the presence of a DM component in the preferred DM mass range. We also set flux and cross-section upper limits for DM annihilation into the b-bbar and mu-mu channels in all three clusters.
• The Empirical case for 10 Gev Dark Matter
  abstract: In this article, I summarize and discuss the body of evidence which has accumulated in favor of dark matter in the form of approximately 10 GeV particles. This evidence includes the spectrum and angular distribution of gamma rays from the Galactic Center, the synchrotron emission from the Milky Way’s radio filaments, the diffuse synchrotron emission from the Inner Galaxy (the “WMAP Haze”) and low-energy signals from the direct detection experiments DAMA/LIBRA, CoGeNT and CRESST-II. This collection of observations can be explained by a relatively light dark matter particle with an annihilation cross section consistent with that predicted for a simple thermal relic (sigma v ~ 10^-26 cm^3/s) and with a distribution in the halo of the Milky Way consistent with that predicted from simulations. Astrophysical explanations for the gamma ray and synchrotron signals, in contrast, have not been successful in accommodating these observations. Similarly, the phase of the annual modulation observed by DAMA/LIBRA (and now supported by CoGeNT) is inconsistent with all known or postulated modulating backgrounds, but are in good agreement with expectations for dark matter scattering. This scenario is consistent with all existing indirect and collider constraints, as well as the constraints placed by CDMS. Consistency with xenon-based experiments can be achieved if the response of liquid xenon to very low-energy nuclear recoils is somewhat suppressed relative to previous evaluations, or if the dark matter possesses different couplings to protons and neutrons.

21 December, 2011

1) Galactic Radio Emission and WIMPs

• Cosmological Radio Emission induced by WIMP Dark Matter
  abstract:  We present a detailed analysis of the radio synchrotron emission induced by WIMP dark matter annihilations and decays in extragalactic halos. We compute intensity, angular correlation, and source counts and discuss the impact on the expected signals of dark matter clustering, as well as of other astrophysical uncertainties as magnetic fields and spatial diffusion. Bounds on dark matter microscopic properties are then derived, and, depending on the specific set of assumptions, they are competitive with constraints from other indirect dark matter searches. At GHz frequencies, dark matter sources can become a significant fraction of the total number of sources with brightness below the microJansky level. We show that, at this level of fluxes (which are within the reach of the next-generation radio surveys), properties of the faint edge of differential source counts, as well as angular correlation data, can become an important probe for WIMPs.
• Galactic Synchroton emission from WIMPs at radio frequencies
  abstract: Dark matter annihilations in the Galactic halo inject relativistic electrons and positrons which in turn generate a synchrotron radiation when interacting with the galactic magnetic field. We calculate the synchrotron flux for various dark matter annihilation channels, masses, and astrophysical assumptions in the low-frequency range and compare our results with radio surveys from 22 MHz to 1420 MHz. We find that current observations are able to constrain particle dark matter with “thermal” annihilation cross-sections, i.e. (\sigma v) = 3 x 10^-26 cm^3/s, and masses M_DM < 10 GeV. We discuss the dependence of these bounds on the astrophysical assumptions, namely galactic dark matter distribution, cosmic rays propagation parameters, and structure of the galactic magnetic field. Prospects for detection in future radio surveys are outlined.
• A dark matter interpretation for the ARCADE excess?
  abstract: The ARCADE 2 Collaboration has recently measured an isotropic radio emission which is significantly brighter than the expected contributions from known extra-galactic sources. The simplest explanation of such excess involves a “new” population of unresolved sources which become the most numerous at very low (observationally unreached) brightness. We investigate this scenario in terms of synchrotron radiation induced by WIMP annihilations or decays in extragalactic halos. Intriguingly, for light-mass WIMPs with thermal annihilation cross-section, and fairly conservative clustering assumptions, the level of expected radio emission matches the ARCADE observations.

2) Asymptotically Safe gravity and the Higgs mass

• Asymptotic safety of gravity and the Higgs boson mass
  abstract: There are indications that gravity is asymptotically safe. The Standard Model (SM) plus gravity could be valid up to arbitrarily high energies. Supposing that this is indeed the case and assuming that there are no intermediate energy scales between the Fermi and Planck scales we address the question of whether the mass of the Higgs boson $m_H$ can be predicted. For a positive gravity induced anomalous dimension $A_\lambda>0$ the running of the quartic scalar self interaction $\lambda$ at scales beyond the Planck mass is determined by a fixed point at zero. This results in $m_H=m_{\rm min}=126$ GeV, with only a few GeV uncertainty. This prediction is independent of the details of the short distance running and holds for a wide class of extensions of the SM as well. For $A_\lambda <0$ one finds $m_H$ in the interval $m_{\rm min}< m_H < m_{\rm max}\simeq 174$ GeV, now sensitive to $A_\lambda$ and other properties of the short distance running. The case $A_\lambda>0$ is favored by explicit computations existing in the literature.