Poisson Fluctuations Research Articles

Neutrino roof: Single-scatter cross section ceilings in dark matter direct detection experiments

We identify the maximum cross sections probed by single-scatter weakly interacting massive particle (WIMP) searches in dark matter direct detection. Owing to Poisson fluctuations in scatter multiplicity, these ceilings scale logarithmically with mass for heavy dark matter and often lie in regions probed by multiscatter searches. Using a generalized formula for single-scatter event rates we recast WIMP searches by the quintal-to-tonne scale detectors XENON1T, XENONnT, LZ, PANDAX-II, PANDAX-4T, DarkSide-50, and DEAP-3600 to obtain ceilings and floors up to a few 1017 GeV mass and 10−22 cm2 per nucleus cross section. We do this for coherent, geometric, isospin-violating xenophobic and argophobic spin-independent scattering, and neutron-only and proton-only spin-dependent scattering. Future large-exposure detectors would register an almost irreducible background of atmospheric neutrinos that would determine a dark matter sensitivity ceiling that we call the “neutrino roof,” in analogy with the well-studied “neutrino floor.” Accounting for this background, we estimate the reaches of the 10–100 tonne scale DarkSide-20k, DARWIN/XLZD, PANDAX-xT, and Argo, which would probe many decades of unconstrained parameter space up to the Planck mass, as well as of 103–104 tonne scale noble liquid detectors that have been proposed in synergy with neutrino experiments. Published by the American Physical Society 2024

Measuring the circular polarization of gravitational waves with pulsar timing arrays

The circular polarization of the stochastic gravitational wave background (SGWB) is a key observable for characterizing the origin of the signal detected by Pulsar Timing Array (PTA) collaborations. Both the astrophysical and the cosmological SGWB can have a sizeable amount of circular polarization, due to Poisson fluctuations in the source properties for the former, and to parity violating processes in the early universe for the latter. Its measurement is challenging since PTA are blind to the circular polarization monopole, forcing us to turn to anisotropies for detection. We study the sensitivity of current and future PTA datasets to circular polarization anisotropies, focusing on realistic modelling of intrinsic and kinematic anisotropies for astrophysical and cosmological scenarios respectively. Our results indicate that the expected level of circular polarization for the astrophysical SGWB should be within the reach of near future datasets, while for cosmological SGWB circular polarization is a viable target for more advanced SKA-type experiments. Published by the American Physical Society 2024

Constraints on the Origins of the Galactic Neutrino Flux Detected by IceCube

Galactic and extragalactic objects in the Universe are sources of high-energy neutrinos that may contribute to the astrophysical neutrino signal seen by IceCube. Recently, a study done using cascade-like events seen by IceCube reported neutrino emission from the Galactic plane with >4σ significance. In this work, we put a lower limit on the number of Galactic sources required to explain this emission. To achieve this, we use a simulation package created to simulate point sources in the Galaxy along with the neutrino and gamma-ray flux emissions originating from them. Along with using past IceCube discovery potential curves, we also account for Eddington bias effects due to Poisson fluctuations in the number of detected neutrino events. We present a toy Monte Carlo simulation to show that there must be at least eight sources, each with luminosity less than 1.6 × 1035 erg s−1, responsible for the Galactic neutrino emission. Our results constrain the number of individual point-like emission regions, which apply both to discrete astrophysical sources and to individual points of diffuse emission.

Circular Polarization of the Astrophysical Gravitational Wave Background.

The circular polarization of gravitational waves is a powerful observable to test parity violation in gravity and to distinguish between the primordial or the astrophysical origin of the stochastic background. This property comes from the expected unpolarized nature of the homogeneous and isotropic astrophysical background, contrary to some specific cosmological sources that can produce a polarized background. However, in this work we show that there is also a non-negligible amount of circular polarization in the astrophysical background, generated by Poisson fluctuations in the number of unresolved sources, which is present in the third-generation interferometers with signal-to-noise ratio larger than 2. We also explain in which cases the gravitational wave maps can be cleaned from this extra source of noise, exploiting the frequency and the angular dependence, in order to search for signals from the early Universe. Future studies about the detection of polarized cosmological backgrounds with ground- and space-based interferometers should account for the presence of such a foreground contribution.

Dark matter from primordial black holes would hold charge

We explore the possibility that primordial black holes (PBHs) contain electric charge down to the present day. We find that PBHs should hold a non-zero net charge at their formation, due to either Poisson fluctuations at horizon crossing or high-energy particle collisions. Although initial charge configurations are subject to fast discharge processes through particle accretion or quantum particle emission, we show that maximally rotating PBHs could produce magnetic fields able to shield them from discharge. Moreover, given that electrons are the lightest and fastest charge carriers, we show that the plasma within virialised dark matter haloes can endow PBHs with net negative charge. We report charge-to-mass ratios between 10-31 C/kg and 10-15 C/kg for PBHs within the mass windows that allow them to constitute the entirety of the dark matter in the Universe.

Hidden biases in flux-resolved X-ray spectroscopy

ABSTRACT Flux-resolved X-ray spectroscopy is widely adopted to investigate the spectral variation of a target between various flux levels. In many cases, it is done through horizontally splitting a single light curve into multiple flux levels with certain count-rate threshold(s). In this work, we point out there are two hidden biases in this approach that could affect the spectral analyses under particular circumstances. The first is that when Poisson fluctuations of the source counts in light curve bins are non-negligible compared with the intrinsic variation, this approach would overestimate (underestimate) the intrinsic average flux level of the high (low) state. The second bias is that when the Poisson fluctuations of the background count rate is non-negligible, the background spectrum of the high (low) state would be underestimated (overestimated), thus yielding biased spectral fitting parameters. We take NuSTAR spectra, for example, to illustrate the effects of the biases, and particularly how the measurements of the coronal temperature in active galactic nuclei would be biased. We present a toy method to assess the significance of such biases and approaches to correct for them when necessary.

Scalar induced gravitational waves from primordial black hole Poisson fluctuations in f(R) gravity

The gravitational potential of a gas of initially randomly distributed primordial black holes (PBH) can induce a stochastic gravitational-wave (GW) background through second-order gravitational effects. This GW background can be abundantly generated in a cosmic era dominated by ultralight primordial black holes, with masses m PBH < 109g. In this work, we consider f(R) gravity as the underlying gravitational theory and we study its effect at the level of the gravitational potential of Poisson distributed primordial black holes. After a general analysis, we focus on the R 2 gravity model. In particular, by requiring that the scalar induced GWs (SIGWs) are not overproduced, we find an upper bound on the abundance of PBHs at formation time ΩPBH,f as a function of their mass, namely that ΩPBH,f < 5.5 × 10-5 (109g/m PBH)1/4, which is 45% tighter than the respective upper bound in general relativity. Afterwards, by considering R 2 gravity as an illustrative case study of an f(R) gravity model, we also set upper bound constraints on its mass parameter M. These mass parameter constraints, however, should not be regarded as physical given the fact that the Cosmic Microwave Background (CMB) constraints on R 2 gravity are quite tight. Finally, we conclude that the portal of SIGWs associated to PBH Poisson fluctuations can act as a novel complementary probe to constrain alternative gravity theories.

A probabilistic method of background removal for high energy astrophysics data

ABSTRACT We present a new statistical method for constructing background subtracted measurements from event list data gathered by X-ray and gamma-ray observatories. This method was initially developed specifically to construct images that account for the high background fraction and low overall count rates observed in survey data from the Mikhail Pavlinsky ART-XC telescope aboard the Spektrum Röntgen Gamma (SRG) mission, although the mathematical underpinnings are valid for data taken with other imaging missions and analysis applications. This method fully accounts for the expected Poisson fluctuations in both the sky photon and non-X-ray background count rates in a manner that does not result in unphysical negative counts. We derive the formulae for arbitrary confidence intervals for the source counts and show that our new measurement converges exactly to the standard background subtraction calculation in the high signal limit. Utilizing these results, we discuss several variants of images designed to optimize different science goals for both pointed and slewing telescopes. Using realistic simulated data of a galaxy cluster as observed by ART-XC, we show that our method provides a more significant and robust detection of the cluster emission as compared to a standard background subtraction. We also demonstrate its advantages using real observations of a point source from the ART-XC telescope. These calculations may have widespread applications for a number of source classes observed with high energy telescopes.

Collision Fluctuations of Lucky Droplets with Superdroplets

Abstract It was previously shown that the superdroplet algorithm for modeling the collision–coalescence process can faithfully represent mean droplet growth in turbulent clouds. An open question is how accurately the superdroplet algorithm accounts for fluctuations in the collisional aggregation process. Such fluctuations are particularly important in dilute suspensions. Even in the absence of turbulence, Poisson fluctuations of collision times in dilute suspensions may result in substantial variations in the growth process, resulting in a broad distribution of growth times to reach a certain droplet size. We quantify the accuracy of the superdroplet algorithm in describing the fluctuating growth history of a larger droplet that settles under the effect of gravity in a quiescent fluid and collides with a dilute suspension of smaller droplets that were initially randomly distributed in space (“lucky droplet model”). We assess the effect of fluctuations upon the growth history of the lucky droplet and compute the distribution of cumulative collision times. The latter is shown to be sensitive enough to detect the subtle increase of fluctuations associated with collisions between multiple lucky droplets. The superdroplet algorithm incorporates fluctuations in two distinct ways: through the random spatial distribution of superdroplets and through the Monte Carlo collision algorithm involved. Using specifically designed numerical experiments, we show that both on their own give an accurate representation of fluctuations. We conclude that the superdroplet algorithm can faithfully represent fluctuations in the coagulation of droplets driven by gravity.

Shot noise and scatter in the star formation efficiency as a source of 21-cm fluctuations

ABSTRACT The 21-cm signal from cosmic dawn and the epoch of reionization probes the characteristics of the high redshift galaxy population. Many of the astrophysical properties of galaxies at high redshifts are currently unconstrained due to the lack of observations. This creates a vast space of possible astrophysical scenarios where the 21-cm signal needs to be modeled in order to plan for, and eventually fit, future observations. This is done with fast numerical methods which make simplifying approximations for the underlying physical processes. In this work, we quantify the effect of Poisson fluctuations and scatter in the star formation efficiency; while Poisson fluctuations are included in some works and not in others, scatter in the star formation efficiency is usually neglected, and all galaxies of a given mass are assumed to have the same properties. We show that both features can have a significant effect on the 21-cm power spectrum, most importantly in scenarios where the signal is dominated by massive galaxies. Scatter in the star formation efficiency does not simply enhance the effect of Poisson fluctuations; for example, we show that the power spectrum shape at cosmic dawn has a feature corresponding to the width of the galaxy brightness distribution. We also discuss some of the consequences for 21-cm imaging, and the signature of reduced correlation between the density and radiation fields.

CMB and 21-cm bounds on early structure formation boosted by primordial black hole entropy fluctuations

The dark matter (DM) can consist of the primordial black holes (PBHs) in addition to the conventional weakly interacting massive particles (WIMPs). The Poisson fluctuations of the PBH number density produce the isocurvature perturbations which can dominate the matter power spectrum at small scales and enhance the early structure formation. We study how the WIMP annihilation from those early formed structures can affect the CMB (in particular the E-mode polarization anisotropies and $y$-type spectral distortions) and global 21cm signals. Our studies would be of particular interest for the light (sub-GeV) WIMP scenarios which have been less explored compared with the mixed DM scenarios consisting of PBHs and heavy ($\gtrsim 1$ GeV) WIMPs. For instance, for the self-annihilating DM mass $m_{\chi}=1$ MeV and the thermally averaged annihilation cross section $\langle \sigma v \rangle \sim 10^{-30} \rm cm^3/s$, the latest Planck CMB data requires the PBH fraction with respect to the whole DM to be at most ${\cal O}(10^{-3})$ for the sub-solar mass PBHs and an even tighter bound (by a factor $\sim 5$) can be obtained from the global 21-cm measurements.

Exact results on Poisson noise, Poisson flights, and Poisson fluctuations

We study non-Markovian stochastic differential equations with additive noise characterized by a Poisson point process with arbitrary pulse shapes and exponentially distributed intensities. Specifically, analytic results concerning transitions between different correlation regimes and the long-time asymptotic probability distribution functions are shown to be controlled by the shape of the pulses and dissipative parameter as time progresses. This program is motivated by the study of stochastic partial differential equations perturbed by space Poisson disorder and becomes the main focus of applications of the present exact functional approach.

Power spectrum of density fluctuations, halo abundances, and clustering with primordial black holes

ABSTRACT We study the effect of dark matter (DM) being encapsulated in primordial black holes (PBHs) on the power spectrum of density fluctuations P(k); we also look at its effect on the abundance of haloes and their clustering. We allow the growth of Poisson fluctuations since matter and radiation equality and study both monochromatic and extended PBH mass distributions. We present updated monochromatic black hole mass constraints by demanding $\lt 10{{\ \rm per\ cent}}$ deviations from the Λ cold dark matter (CDM) power spectrum at a scale of k = 1 h Mpc−1. Our results show that PBHs with masses $\gt 10^4\, h^{-1}\, \mathrm{M}_\odot$ are excluded from conforming all of the DM in the Universe. We also apply this condition to our extended Press–Schechter (PS) mass functions, and find that the Poisson power is scale dependent even before applying evolution. We find that characteristic masses $M^*\le 10^2 \, \mathrm{ h}^{-1}\, \mathrm{M}_\odot$ are allowed, leaving only two characteristic PBH mass windows of PS mass functions when combining with previous constraints, at $M^*\sim 10^2\, h^{-1}\, \mathrm{M}_\odot$ and $\sim \!10^{-8}\, h^{-1}\, \mathrm{M}_\odot$ where all of the DM can be in PBHs. The resulting DM halo mass functions within these windows are similar to those resulting from CDM made of fundamental particles. However, as soon as the parameters produce unrealistic P(k), the resulting halo mass functions and their bias as a function of halo mass deviate strongly from the behaviour measured in the real Universe.

S2D2: Small-scale Significant substructure DBSCAN Detection

Context.The spatial and dynamical structure of star-forming regions can offer insights into stellar formation patterns. The amount of data from current and upcoming surveys calls for robust and objective procedures for detecting structures in order to statistically analyse the various regions and compare them.Aims.We aim to provide the community with a tool capable of detecting, above random expectations, the small-scale significant structure in star-forming regions that could serve as an imprint of the stellar formation process. The tool makes use of the one-point correlation function to determine an appropriate length scale forϵand uses nearest-neighbour statistics to determine a minimum number of pointsNminfor the DBSCAN algorithm in the neighbourhood ofϵ.Methods.We implemented the procedure and applied it to synthetic star-forming regions of different nature and characteristics to obtain its applicability range. We also applied the method to observed star-forming regions to demonstrate its performance in realistic circumstances and to analyse its results.Results.The procedure successfully detects significant small-scale substructures in heterogeneous regions, fulfilling the goals it was designed for and providing very reliable structures. The analysis of regions close to complete spatial randomness (Q ∈ [0.7, 0.87]) shows that even when some structure is present and recovered, it is hardly distinguishable from spurious detection in homogeneous regions due to projection effects. Thus, any interpretation should be done with care. For concentrated regions, we detect a main structure surrounded by smaller ones, corresponding to the core plus some Poisson fluctuations around it. We argue that these structures do not correspond to the small compact regions we are looking for. In some realistic cases, a more complete hierarchical, multi-scale analysis would be needed to capture the complexity of the region.Conclusions.We carried out implementations of our procedure and devised a catalogue of the Nested Elementary STructures (NESTs) detected as a result in four star-forming regions (Taurus, IC 348, Upper Scorpius, and Carina). This catalogue is being made publicly available to the community. Implementations of the 3D versionsof the procedure, as well as up to 6D versions, including proper movements, are in progress and will be provided in a future work.

Size distribution of virus laden droplets from expiratory ejecta of infected subjects

For rebooting economic activities in the ongoing COVID-19 pandemic scenario, it is important to pay detailed attention to infection transfer mechanisms during interaction of people in enclosed environments. Utmost concern is the possibility of aerosol mediated infection transfer, which is largely governed by the size distributions of virus laden droplets, termed as virusols in this work, ejected from humans. We expand on the well-known theory of Poisson fluctuations which acts as statistical barrier against formation of virusols. Analysis suggests that for viral loads < 2 × 105 RNA copies/mL, often corresponding to mild-to-moderate cases of COVID-19, droplets of diameter < 20 µm at the time of emission (equivalent to ~ 10 µm desiccated residue diameter) are unlikely to be of consequence in carrying infections. Cut-off diameters below which droplets will be practically free of contamination, are presented as a function of viral loading. The median diameters of virus laden polydisperse droplet distributions will be 1.5 to 20 times higher depending upon the geometric standard deviation. The studies have implications to risk assessment as well as residence time estimates of airborne infections in indoor environments. Additionally, it will be also helpful for performance evaluation of sanitization and control technologies to mitigate infection risks in workplaces.

Small-scale primordial fluctuations in the 21 cm Dark Ages signal

ABSTRACT Primordial black hole production in the mass range $10\!-\!10^4 \, {\rm M_\odot}$ is motivated respectively by interpretations of the LIGO/Virgo observations of binary black hole mergers and by their ability to seed intermediate black holes that would account for the presence of supermassive black holes at very high redshift. Their existence would imply a boost in the primordial power spectrum if they were produced by overdensities reentering the horizon and collapsing after single-field inflation. This, together with their associated Poisson fluctuations would cause a boost in the matter power spectrum on small scales. The extra power could become potentially observable in the 21 cm power spectrum on scales around $k\sim 0.1\!-\!50\, {\rm Mpc^{-1}}$ with the new generation of filled low-frequency interferometers. We explicitly include the contribution from primordial fluctuations in our prediction of the 21 cm signal that has been previously neglected, by constructing primordial power spectra motivated by single-field models of inflation that would produce extra power on small scales. We find that depending on the mass and abundance of primordial black holes, it is important to include this contribution from the primordial fluctuations, so as not to underestimate the 21 cm signal. Evidently our predictions of detectability, which lack any modelling of foregrounds, are unrealistic, but we hope that they will motivate improved cleaning algorithms that can enable us to access this intriguing corner of primordial black hole-motivated parameter space.

Noise from undetected sources in Dark Energy Survey images

ABSTRACT For ground-based optical imaging with current CCD technology, the Poisson fluctuations in source and sky background photon arrivals dominate the noise budget and are readily estimated. Another component of noise, however, is the signal from the undetected population of stars and galaxies. Using injection of artifical galaxies into images, we demonstrate that the measured variance of galaxy moments (used for weak gravitational lensing measurements) in Dark Energy Survey (DES) images is significantly in excess of the Poisson predictions, by up to 30 per cent, and that the background sky levels are overestimated by current software. By cross-correlating distinct images of ‘empty’ sky regions, we establish that there is a significant image noise contribution from undetected static sources (US), which, on average, are mildly resolved at DES resolution. Treating these US as a stationary noise source, we compute a correction to the moment covariance matrix expected from Poisson noise. The corrected covariance matrix matches the moment variances measured on the injected DES images to within 5 per cent. Thus, we have an empirical method to statistically account for US in weak lensing measurements, rather than requiring extremely deep sky simulations. We also find that local sky determinations can remove most of the bias in flux measurements, at a small penalty in additional, but quantifiable, noise.

A Search for Lensed Gamma-Ray Bursts in 11 yr of Observations by Fermi GBM

Abstract Macrolensing of gamma-ray bursts (GRBs) is expected to manifest as a GRB recurring with the same light curve and spectrum as a previous one, but with a different flux and a slightly offset position. Identifying such lensed GRBs may give important information about the lenses, the cosmology, and the GRBs themselves. Here we present a search for lensed GRBs among ∼2700 GRBs observed by the Fermi Gamma-ray Burst Monitor during 11 yr of operations. To identify lensed GRBs, we perform initial cuts on position, time-averaged spectral properties, and relative duration. We then use the cross-correlation function to assess the similarity of light curves, and finally we analyze the time-resolved spectra of the most promising candidates. We find no convincing lens candidates. The most similar pairs are single-pulsed GRBs with relatively few time bins for the spectral analysis. This is best explained by similarities within the GRB population rather than lensing. However, the null result does not rule out the presence of macrolensed GRBs in the sample. In particular, we find that observational uncertainties and Poisson fluctuations can lead to significant differences within a pair of lensed GRBs.

Global correlation matrix spectra of the surface temperature of the oceans from random matrix theory to Poisson fluctuations

In this work we use the random matrix theory (RMT) to correctly describe the behavior of spectral statistical properties of the sea surface temperature of oceans. This oceanographic variable plays an important role in the global climate system. The data were obtained from National Oceanic and Atmospheric Administration (NOAA) and delimited for the period 1982 to 2016. The results show that oceanographic systems presented specific β values that can be used to classify each ocean according to its correlation behavior. The nearest-neighbors spacing of correlation matrix for north, central and south of the three oceans get close to a RMT distribution. However, the regions delimited in the Antarctic pole exhibited the distribution of the nearest-neighbors spacing well described by the Poisson model, which shows a statistical change of RMT to Poisson fluctuations.

Poisson and Gaussian fluctuations for the components of the f -vector of high-dimensional random simplicial complexes

Poisson and Gaussian fluctuations for the components of the f -vector of high-dimensional random simplicial complexes