Abstracts of Interest
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Abstract: 1905.10025
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Title:A counter-top search for macroscopic dark matter
(Submitted on 24 May 2019)
Abstract: A number of dark matter candidates have been discussed that are macroscopic, of approximately nuclear density, and scatter ordinary matter essentially elastically with approximately their geometric cross-section. A wide range of mass and geometric cross-section is still unprobed for these "macros." Macros passing through rock would melt the material in cylinders surrounding their long nearly straight trajectories. Once cooled, the resolidified rock would be easily distinguishable from its surroundings. We discuss how, by visually examining ordinary slabs of rock such as are widely available commercially for kitchen countertops, one could probe an interesting segment of the open macro parameter space.
Abstract: 1905.10111
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Title:Search for correlations of high-energy neutrinos and ultra-high-energy cosmic rays
(Submitted on 24 May 2019)
Abstract: The IceCube Neutrino Observatory has recently found compelling evidence for a particular blazar producing high-energy neutrinos and $\mathrm{PeV}$ cosmic rays, however the sources of cosmic rays above several $\mathrm{EeV}$ remain unidentified. It is believed that the same environments that accelerate ultra-high-energy cosmic rays (UHECRs) also produce high-energy neutrinos via hadronic interactions of lower-energy cosmic rays. Two out of three joint analyses of the IceCube Neutrino Observatory, the Pierre Auger Observatory and the Telescope Array yielded hints for a possible directional correlation of high-energy neutrinos and UHECRs. These hints however became less significant with more data. Recently, an improved analysis with an approach complementary to the other analyses has been developed. This analysis searches for neutrino point sources in the vicinity of UHECRs with search windows estimated from deflections by galactic magnetic fields. We present this new analysis method for searching common hadronic sources, additionally including neutrino data measured by ANTARES in order to increase the sensitivity to possible correlations in the Southern Hemisphere.
Abstract: 1905.10127
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Title:Penetrating component in cosmic rays
(Submitted on 24 May 2019)
Abstract: We present a study of the high energy spectra of hadrons in cores of extensive air showers. These data were obtained for the first time in the hybrid {\it HADRON} experiment (Tien-Shan) by means of a large X-ray emulsion chamber combined with the shower array. In the local energy interval 3--100 PeV an increase in the energy of hadrons was found, which means the appearance of a penetrating component. This component in our experiment was observed in the atmosphere that indicates the presence of a penetrating strongly interacting component in primary cosmic rays. Along with that, it is worth emphasising that the region where this component is observed coincides with the region of the so-called knee in the spectrum of cosmic rays. On this basis, a new hypothesis of knee formation can be put forward.
Abstract: 1905.10175
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Title:A novel method for component separation of extended sources in X-ray astronomy
(Submitted on 24 May 2019)
Abstract: In high-energy astronomy, spectro-imaging instruments such as X-ray detectors allow investigation of the spatial and spectral properties of extended sources including galaxy clusters, galaxies, diffuse interstellar medium, supernova remnants and pulsar wind nebulae. In these sources, each physical component possesses a different spatial and spectral signature, but the components are entangled. Extracting the intrinsic spatial and spectral information of the individual components from this data is a challenging task. Current analysis methods do not fully exploit the 2D-1D (x,y,E) nature of the data, as the spatial and spectral information are considered separately. Here we investigate the application of a Blind Source Separation algorithm that jointly exploits the spectral and spatial signatures of each component in order to disentangle them. We explore the capabilities of a new BSS method (General Morphological Component Analysis; GMCA), initially developed to extract an image of the Cosmic Microwave Background from Planck data, in an X-ray context. The performance of GMCA on X-ray data is tested using Monte-Carlo simulations of supernova remnant toy models, designed to represent typical science cases. We find that GMCA is able to separate highly entangled components in X-ray data even in high contrast scenarios, and can extract with high accuracy the spectrum and map of each physical component. A modification is proposed to improve the spectral fidelity in the case of strongly overlapping spatial components, and we investigate a resampling method to derive realistic uncertainties associated to the results of the algorithm. Applying the modified algorithm to the deep Chandra observations of Cassiopeia A, we are able to produce detailed maps of the synchrotron emission at low energies (0.6-2.2 keV), and of the red/blue shifted distributions of a number of elements including Si and Fe K.
Abstract: 1905.10198
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Title:Implications of a transition in the dark energy equation of state for the $H_0$ and $σ_8$ tensions
(Submitted on 24 May 2019)
Abstract: We explore the implications of a rapid appearance of dark energy between the redshifts ($z$) of one and two on the expansion rate and growth of perturbations. Using both Gaussian process regression and a parameteric model, we show that this is the preferred solution to the current set of low-redshift ($z<3$) distance measurements if $H_0=73~\rm km\,s^{-1}\,Mpc^{-1}$ to within 1\% and the high-redshift expansion history is unchanged from the $\Lambda$CDM inference by the Planck satellite. Dark energy was effectively non-existent around $z=2$, but its density is close to the $\Lambda$CDM model value today, with an equation of state greater than $-1$ at $z<0.5$. If sources of clustering other than matter are negligible, we show that this expansion history leads to slower growth of perturbations at $z<1$, compared to $\Lambda$CDM, that is measurable by upcoming surveys and can alleviate the $\sigma_8$ tension between the Planck CMB temperature and low-redshift probes of the large-scale structure.
Abstract: 1905.10216
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Title:Debiasing Cosmic Gravitational Wave Sirens
(Submitted on 24 May 2019)
Abstract: Accurate estimation of the Hubble constant, and other cosmological parameters, from distances measured by cosmic gravitational wave sirens requires sufficient allowance for the dark energy evolution. We demonstrate how model independent statistical methods, specifically Gaussian process regression, can remove bias in the reconstruction of $H(z)$, and can be combined model independently with supernova distances. This allows stringent tests of both $H_0$ and $\Lambda$CDM, and can detect unrecognized systematics. We also quantify the redshift systematic control necessary for the use of dark sirens, showing that it must approach spectroscopic precision to avoid significant bias.
Abstract: 1905.10266
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Title:Monte Carlo modelling of particle acceleration in collisionless shocks with an effective mean electric field
(Submitted on 24 May 2019)
Abstract: Relativistic particle acceleration in collisionless shocks of supernova remnants is accompanied by magnetic field amplification from cosmic ray (CR) driven plasma instabilities. Bell's fast CR-current instability is predicted to produce turbulence with a non-zero mean electric field in the shock precursor. We present a Monte Carlo model of Fermi shock acceleration explicitly taking into account an effective mean upstream electric field. Our model is nonlinear and includes the backreaction effects of efficient Fermi acceleration on the shock structure.
Abstract: 1905.10207
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Title:C$ν$B detection through angular correlations in inverse $β$-decay
(Submitted on 24 May 2019)
Abstract: Neutrino capture on beta-decaying nuclei is currently the only known potentially viable method of detection of cosmic background neutrinos. It is based on the idea of separation of the spectra of electrons or positrons produced in captures of relic neutrinos on unstable nuclei from those from the usual $\beta$-decay and requires very high energy resolution of the detector, comparable to the neutrino mass. In this paper we suggest an alternative method of discrimination between neutrino capture and $\beta$-decay, based on periodic variations of angular correlations in inverse beta decay transitions induced by relic neutrino capture. The time variations are expected to arise due to the peculiar motion of the Sun with respect to the C$\nu$B rest frame and the rotation of the Earth about its axis and can be observed in experiments with both polarized and unpolarized nuclear targets. The main advantage of the suggested method is that it does not depend crucially on the energy resolution of detection of the produced $\beta$-particles and can be operative even if this resolution exceeds the largest neutrino mass.
Abstract: 1905.09612
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Title:Improving the muon track reconstruction of IceCube and IceCube-Gen2
(Submitted on 23 May 2019)
Abstract: IceCube is a cubic-kilometer Cherenkov telescope operating at the South Pole. Its goal is to detect astrophysical neutrinos and identify their sources. High-energy muon neutrinos are identified through the secondary muons produced via charge current interactions with the ice. The present best-performing directional reconstruction of the muon track is a maximum likelihood method which uses the arrival time distribution of Cherenkov photons registered by the experiment's photomultipliers. Known systematic shortcomings of this method are to assume continuous energy loss along the muon track, and to neglect photomultiplier-related effects such as prepulses and afterpulses. This work discusses an improvement of about 20% to the muon angular resolution of IceCube and its planned extension, IceCube-Gen2. In the reconstruction scheme presented here, the expected arrival time distribution is now parametrized by a predetermined stochastic muon energy loss pattern. The inclusion of pre- and afterpulses modelling in the PDF has also been studied, but no noticeable improvement was found, in particular in comparison to the modification of the energy loss profile.
Abstract: 1905.09288
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Title:$\bar T$: A New Cosmological Parameter?
(Submitted on 22 May 2019)
Abstract: The background photon temperature $\bar T$ is one of the fundamental cosmological parameters. Despite its significance, $\bar T$ has never been allowed to vary in the data analysis, owing to the precise measurement of the comic microwave background (CMB) temperature by COBE FIRAS. However, even in future CMB experiments, $\bar T$ will remain unknown due to the unknown monopole contribution $\Theta_0$ at our position to the observed (angle-averaged) temperature $\langle T\rangle^{\rm obs}$. By fixing $\bar T\equiv\langle T\rangle^{\rm obs}$, the standard analysis underestimates the error bars on cosmological parameters, and the best-fit parameters obtained in the analysis are biased in proportion to the unknown amplitude of $\Theta_0$. Using the Fisher formalism, we find that these systematic errors are smaller than the error bars from the $Planck$ satellite. However, with $\bar T\equiv\langle T\rangle^{\rm obs}$, these systematic errors will always be present and irreducible, and future cosmological surveys might misinterpret the measurements.
Abstract: 1905.09110
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Title:Nested sampling on non-trivial geometries
(Submitted on 22 May 2019 (v1), last revised 23 May 2019 (this version, v2))
Abstract: Metropolis nested sampling evolves a Markov chain from a current livepoint and accepts new points along the chain according to a version of the Metropolis acceptance ratio modified to satisfy the likelihood constraint, characteristic of nested sampling algorithms. The geometric nested sampling algorithm we present here is a based on the Metropolis method, but treats parameters as though they represent points on certain geometric objects, namely circles, tori and spheres. For parameters which represent points on a circle or torus, the trial distribution is `wrapped' around the domain of the posterior distribution such that samples cannot be rejected automatically when evaluating the Metropolis ratio due to being outside the sampling domain. Furthermore, this enhances the mobility of the sampler. For parameters which represent coordinates on the surface of a sphere, the algorithm transforms the parameters into a Cartesian coordinate system before sampling which again makes sure no samples are automatically rejected, and provides a physically intutive way of the sampling the parameter space. \\ We apply the geometric nested sampler to two types of toy model which include circular, toroidal and spherical parameters. We find that the geometric nested sampler generally outperforms \textsc{MultiNest} in both cases. \\ %We also apply the algorithm to a gravitational wave detection model which includes circular and spherical parameters, and find that the geometric nested sampler and \textsc{MultiNest} appear to perform equally well as one another. Our implementation of the algorithm can be found at \url{this https URL}.
Abstract: 1905.08810
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Title:The Thermal History of Composite Dark Matter
(Submitted on 21 May 2019)
Abstract: We study the thermodynamic history of composite Dark Matter models. We start with classifying the models by means of the symmetries partially protecting the composite Dark Matter decays and constrain their lifetimes. For each model, we determine the impact of number-changing and number-conserving operators on its thermal history. We also develop the analytic formalism to calculate the asymptotic abundance of stable relics. We show how the relative strength between number-changing and number-conserving interactions together with the dark plasma lifetime affect the thermal fate of the various composite models. Additionally, we discover that the final dark relic density of composite particles can be diluted due to an entropy increase stemming from dark plasma decay. Finally, we confront the models with experimental bounds. We find that indirect detection experiments are most promising in testing this large class of models.
Abstract: 1905.08679
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Full Text: [ PostScript, PDF]
Title:Implications of gamma-ray and neutrino observations on source models of ultrahigh energy cosmic rays
(Submitted on 21 May 2019)
Abstract: The origin and nature of the ultrahigh energy cosmic rays (UHECRs) are still unknown. However, great progress has been achieved in past years due to the observations performed by the Pierre Auger Observatory and Telescope Array. Above $10^{18}$ eV the observed energy spectrum presents two features: a hardening of the slope at about $10^{18.7}$ eV, which is known as the ankle and a suppression at approximately $10^{19.6}$ eV. The composition inferred from the experimental data, interpreted by using the current high energy hadronic interaction models, seems to be light below the ankle, showing a trend to heavier nuclei for increasing values of the primary energy. Current high energy hadronic interaction models, updated by using Large Hadron Collider data, are still subject to large systematic uncertainties, which makes difficult the interpretation of the experimental data in terms of composition. On the other hand, it is very well known that gamma rays and neutrinos are produced by UHECRs during propagation from their sources, as a consequence of their interactions with the radiation field present in the universe. The flux at Earth of these secondary particles depends on the source models of UHECRs including the chemical composition at injection. Therefore, both gamma-ray and neutrino observations can be used to constrain source models of UHECRs, including the composition in a way which is independent of the high energy hadronic interaction models. In this article I will review recent results obtained by using the latest gamma-ray and neutrino observations.