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Exploration of possible signals beyond special relativity using high-energy astroparticle physics
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Abstract
To unify the standard model of particle physics and general relativity, we may require a quantum description of gravity, which will change our notion of spacetime at very high energies. In this dissertation we explore possible traces of new physics beyond special relativity, using the propagation of high energy astroparticles. For this purpose, the two ways of going beyond Lorentz invariance are presented, a breaking of the Lorentz invariance (Lorentz invariance violation or LIV or its deformation (doubly special relativity or DSR), emphasizing their conceptual and phenomenological differences. For the study of LIV, the work focuses on the prediction of modifications in the expected neutrino flux on Earth, both from astrophysical and cosmogenic origin (from the interaction of cosmic rays with the background radiation during their propagation). For the study of DSR we focus instead on the search for anomalies in the time of flight of massless particles (time delays) and on the study of the expected flux of gamma rays on Earth. The results obtained show the possibility of using astroparticle observations as a window to quantum gravity phenomenology, at energies attainable at present and/or in the very near future.
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We present a combined fit of a simple astrophysical model of Ultra-high-energy cosmic-ray sources to both the energy spectrum and mass composition data measured by the Pierre Auger Observatory. The astrophysical model we adopted consists of identical sources uniformly distributed in a comoving volume, where nuclei are accelerated with a rigidity-dependent mechanism. The fit has been performed at first for energies above 5 EeV, i.e., the region of the all-particle spectrum above the so-called “ankle” feature, and has then been extended to lower energies in order to include this feature. The fit results and their astrophysical implications in both cases are discussed.
In this work we present the interpretation of the energy spectrum and mass composition data as measured by the Pierre Auger Collaboration above eV. We use an astrophysical model with two extragalactic source populations to model the hardening of the cosmic-ray flux at around eV (the so-called "ankle" feature) as a transition between these two components. We find our data to be well reproduced if sources above the ankle emit a mixed composition with a hard spectrum and a low rigidity cutoff. The component below the ankle is required to have a very soft spectrum and a mix of protons and intermediate-mass nuclei. The origin of this intermediate-mass component is not well constrained and it could originate from either Galactic or extragalactic sources. To the aim of evaluating our capability to constrain astrophysical models, we discuss the impact on the fit results of the main experimental systematic uncertainties and of the assumptions about quantities affecting the air shower development as well as the propagation and redshift distribution of injected ultra-high-energy cosmic rays (UHECRs).
The landscape of high- and ultra-high-energy astrophysics has changed in the last decade, largely due to the inflow of data collected by large-scale cosmic-ray, gamma-ray, and neutrino observatories. At the dawn of the multimessenger era, the interpretation of these observations within a consistent framework is important to elucidate the open questions in this field. CRPropa 3.2 is a Monte Carlo code for simulating the propagation of high-energy particles in the Universe. This version represents a major leap forward, significantly expanding the simulation framework and opening up the possibility for many more astrophysical applications. This includes, among others: efficient simulation of high-energy particles in diffusion-dominated domains, self-consistent and fast modelling of electromagnetic cascades with an extended set of channels for photon production, and studies of cosmic-ray diffusion tensors based on updated coherent and turbulent magnetic-field models. Furthermore, several technical updates and improvements are introduced with the new version, such as: enhanced interpolation, targeted emission of sources, and a new propagation algorithm (Boris push). The detailed description of all novel features is accompanied by a discussion and a selected number of example applications.
Ultra-high-energy physics is about to enter a new era thanks to the impressive results of experiments such as the Large High Altitude Air Shower Observatory, detecting photons of up to 1.4×1015 eV (PeV scale). These new results could be used to test deviations with respect to special relativity. While this has been already explored within the approach of Lorentz Invariance Violation theories, in this work we consider, for the first time, modifications due to a relativistic deformed kinematics (which appear in Doubly Special Relativity, or DSR, theories). In particular, we study the mean free path of very high-energy photons due to electron-positron pair creation when interacting with low-energy photons of the cosmic microwave background. Depending on the energy scale of the relativistic deformed kinematics, present (or near future) experiments can be sensitive enough to be able to identify deviations from special relativity.
We review the peculiarities that make neutrinos very special cosmic messengers in high-energy astrophysics, and, in particular, to provide possible indications of deviations from special relativity, as it is suggested theoretically by quantum gravity models. In this respect, we examine the effects that one could expect in the production, propagation, and detection of neutrinos, not only in the well-studied scenario of Lorentz Invariance Violation, but also in models which maintain, but deform, the relativity principle, such as those considered in the framework of Doubly Special Relativity. We discuss the challenges and the promising future prospects offered by this phenomenological window to physics beyond special relativity.
The search for a theory that unifies general relativity and quantum theory has focused attention on models of physics at the Planck scale. One possible consequence of models such as string theory may be that Lorentz invariance is not an exact symmetry of nature. We discuss here some possible experimental and observational tests of Lorentz invariance involving neutrino physics and astrophysics.
Cosmic rays represent one of the most important energy transformation processes of the universe. They bring information about the surrounding universe, our galaxy, and very probably also the extragalactic space, at least at the highest observed energies. More than one century after their discovery, we have no definitive models yet about the origin, acceleration and propagation processes of the radiation. The main reason is that there are still significant discrepancies among the results obtained by different experiments located at ground level, probably due to unknown systematic uncertainties affecting the measurements. In this document, we will focus on the detection of galactic cosmic rays from ground with air shower arrays up to 1018 eV. The aim of this paper is to discuss the conflicting results in the 1015 eV energy range and the perspectives to clarify the origin of the so-called `knee’ in the all-particle energy spectrum, crucial to give a solid basis for models up to the end of the cosmic ray spectrum. We will provide elements useful to understand the basic techniques used in reconstructing primary particle characteristics (energy, mass, and arrival direction) from the ground, and to show why indirect measurements are difficult and results are still conflicting.
The Glashow resonance describes the resonant formation of a W− boson during the interaction of a high-energy electron antineutrino with an electron1, peaking at an antineutrino energy of 6.3 petaelectronvolts (PeV) in the rest frame of the electron. Whereas this energy scale is out of reach for currently operating and future planned particle accelerators, natural astrophysical phenomena are expected to produce antineutrinos with energies beyond the PeV scale. Here we report the detection by the IceCube neutrino observatory of a cascade of high-energy particles (a particle shower) consistent with being created at the Glashow resonance. A shower with an energy of 6.05 ± 0.72 PeV (determined from Cherenkov radiation in the Antarctic Ice Sheet) was measured. Features consistent with the production of secondary muons in the particle shower indicate the hadronic decay of a resonant W− boson, confirm that the source is astrophysical and provide improved directional localization. The evidence of the Glashow resonance suggests the presence of electron antineutrinos in the astrophysical flux, while also providing further validation of the standard model of particle physics. Its unique signature indicates a method of distinguishing neutrinos from antineutrinos, thus providing a way to identify astronomical accelerators that produce neutrinos via hadronuclear or photohadronic interactions, with or without strong magnetic fields. As such, knowledge of both the flavour (that is, electron, muon or tau neutrinos) and charge (neutrino or antineutrino) will facilitate the advancement of neutrino astronomy. A particle shower detected by the IceCube Neutrino Observatory at the very high energy of the Glashow resonance demonstrates its potential for the study of high-energy particle physics and astrophysics.
We show how a deformed composition law of four-momenta can be used to define, at the classical level, a modified notion of spacetime for a system of two particles through the crossing of worldlines in particle interactions. We present a derivation of a generic relativistic isotropic deformed kinematics and discuss the complementarity and relations with other derivations based on κ-Poincaré Hopf algebra or on the geometry of a maximally symmetric momentum space.
This work explores the possibility of resorting to neutrino phenomenology to detect evidence of new physics, caused by the residual signals of the supposed quantum structure of spacetime. In particular, this work investigates the effects on neutrino oscillations and mass hierarchy detection, predicted by models that violate Lorentz invariance, preserving the spacetime isotropy and homogeneity. Neutrino physics is the ideal environment where conducting the search for new “exotic” physics, since the oscillation phenomenon is not included in the original formulation of the minimal Standard Model (SM) of particles. The confirmed observation of the neutrino oscillation phenomenon is, therefore, the first example of physics beyond the SM and can indicate the necessity to resort to new theoretical models. In this work, the hypothesis that the supposed Lorentz Invariance Violation (LIV) perturbations can influence the oscillation pattern is investigated. LIV theories are indeed constructed assuming modified kinematics, caused by the interaction of massive particles with the spacetime background. This means that the dispersion relations are modified, so it appears natural to search for effects caused by LIV in physical phenomena governed by masses, as in the case of neutrino oscillations. In addition, the neutrino oscillation phenomenon is interesting since there are three different mass eigenstates and in a LIV scenario, which preserves isotropy, at least two different species of particle must interact.
The observation of cosmic neutrinos up to 2 PeV is used to put bounds on the energy scale of Lorentz invariance violation through the loss of energy due to the production of e + e - pairs in the propagation of superluminal neutrinos. A model to study this effect, which allows us to understand qualitatively the results of numerical simulations, is presented.
Neutrinos with energies above 1017 eV are detectable with the Surface Detector Array of the Pierre Auger Observatory. The identification is efficiently performed for neutrinos of all flavors interacting in the atmosphere at large zenith angles, as well as for Earth-skimming τ neutrinos with nearly tangential trajectories relative to the Earth. No neutrino candidates were found in ~ 14.7 years of data taken up to 31 August 2018. This leads to restrictive upper bounds on their flux. The 90% C.L. single-flavor limit to the diffuse flux of ultra-high-energy neutrinos with an Eν−2 spectrum in the energy range 1.0 × 1017 eV –2.5 × 1019 eV is E2 dNν/dEν < 4.4 × 10−9 GeV cm−2 s−1 sr−1, placing strong constraints on several models of neutrino production at EeV energies and on the properties of the sources of ultra-high-energy cosmic rays.
We investigate the relativistic properties under boost transformations of the κ-Poincaré model with multiple causally connected interactions, both at the level of its formulation in momentum space only and when it is endowed with a full phase space construction, provided by the relative locality framework. Previous studies focusing on the momentum space picture showed that in the presence of just one interaction vertex the model is relativistic, provided that the boost parameter acting on each given particle receives a “backreaction” from the momenta of the other particles that participate in the interaction. Here we show that in the presence of multiple causally connected vertices the model is relativistic if the boost parameter acting on each given particle receives a backreaction from the total momentum of all the particles that are causally connected, even those that do not directly enter the vertex. The relative locality framework constructs spacetime by defining a set of dual coordinates to the momentum of each particle and interaction coordinates as Lagrange multipliers that enforce momentum conservation at interaction events. We show that the picture is Lorentz invariant if one uses an appropriate “total boost” to act on the particles’ worldlines and on the interaction coordinates. The picture we develop also allows for a reinterpretation of the backreaction as the manifestation of the total boost action. Our findings provide the basis to consistently define distant relatively boosted observers in the relative locality framework.
The blazar Mrk 501 (z = 0.034) was observed at very-high-energy (VHE, E ≳ 100 GeV) gamma-ray wavelengths during a bright flare on the night of 2014 June 23-24 (MJD 56832) with the H.E.S.S. phase-II array of Cherenkov telescopes. Data taken that night by H.E.S.S. at large zenith angle reveal an exceptional number of gamma-ray photons at multi-TeV energies, with rapid flux variability and an energy coverage extending significantly up to 20 TeV. This data set is used to constrain Lorentz invariance violation (LIV) using two independent channels: a temporal approach considers the possibility of an energy dependence in the arrival time of gamma-rays, whereas a spectral approach considers the possibility of modifications to the interaction of VHE gamma-rays with extragalactic background light (EBL) photons. The non-detection of energy-dependent time delays and the non-observation of deviations between the measured spectrum and that of a supposed power-law intrinsic spectrum with standard EBL attenuation are used independently to derive strong constraints on the energy scale of LIV (E QG) in the subluminal scenario for linear and quadratic perturbations in the dispersion relation of photons. For the case of linear perturbations, the 95% confidence level limits obtained are E QG,1 > 3.6 ×10¹⁷ GeV using the temporal approach and E QG,1 > 2.6 ×10¹⁹ GeV using the spectral approach. For the case of quadratic perturbations, the limits obtained are E QG,2 > 8.5 ×10¹⁰ GeV using the temporal approach and E QG,2 > 7.8 ×10¹¹ GeV using the spectral approach. © 2019. The American Astronomical Society. All rights reserved.
In this paper, we consider the impact of Lorentz Invariance Violation (LIV) on the opacity of the Universe
to VHE-gamma rays, compared to the effect of local under-densities (voids) of the Extragalactic Background Light, and on the
Compton scattering process. Both subluminal and superluminal modifications of the photon dispersion relation are considered.
In the subluminal case, LIV effects may result in a significant reduction of the opacity for
photons with energies ~TeV. However, the effect is not expected to be sufficient to explain the
apparent spectral hardening of several observed VHE -ray sources in the energy range from 100 GeV -- a few TeV,
even when including effects of plausible inhomogeneities in the cosmic structure. Superluminal modifications of the
photon dispersion relation lead to a further enhancement of the EBL opacity. We consider, for the first
time, the influence of LIV on the Compton scattering process. We find that this effect becomes relevant only for photons
at ultra-high energies, ~PeV. In the case of a superluminal modification of the photon dispersion relation,
both the kinematic recoil effect and the Klein-Nishina suppression of the cross section are reduced. However, we argue
that the effect is unlikely to be of astrophysical significance.
Spontaneous breaking of Lorentz symmetry at energies on the order of the Planck energy or lower is predicted by many quantum gravity theories, implying non-trivial dispersion relations for the photon in vacuum. Consequently, gamma-rays of different energies, emitted simultaneously from astrophysical sources, could accumulate measurable differences in their time of flight until they reach the Earth. Such tests have been carried out in the past using fast variations of gamma-ray flux from pulsars, and more recently from active galactic nuclei and gamma-ray bursts. We present new constraints studying the gamma-ray emission of the galactic Crab Pulsar, recently observed up to TeV energies by the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) collaboration. A profile likelihood analysis of pulsar events reconstructed for energies above 400 GeV finds no significant variation in arrival time as their energy increases. Ninety-five percent CL limits are obtained on the effective Lorentz invariance violating energy scale at the level of () for a linear, and () for a quadratic scenario, for the subluminal and the superluminal cases, respectively. A substantial part of this study is dedicated to calibration of the test statistic, with respect to bias and coverage properties. Moreover, the limits take into account systematic uncertainties, which are found to worsen the statistical limits by about 36%–42%. Our constraints would have been much more stringent if the intrinsic pulse shape of the pulsar between 200 GeV and 400 GeV was understood in sufficient detail and allowed inclusion of events well below 400 GeV.
We discuss some of the tests of Lorentz symmetry made possible by astrophysical observations of ultrahigh energy cosmic rays, gamma-rays, and neutrinos. These are among the most sensitive tests of Lorentz symmetry violation because they are the highest energy phenomena known to man.
Most of the recent research on extragalactic -ray propagation focused on the study of the absorption process ("absorption-only model"). Starting from a possible anomaly at very high energies (VHE, E >100 GeV), we briefly review several existing deviations from this model. The exotic interpretation of the VHE anomaly is not supported by the recent works. On the other hand, the process of intergalactic electromagnetic cascade development naturally explains these effects. We discuss phenomenology of intergalactic cascades and the main spectral signatures of the electromagnetic cascade model. We also briefly consider the hadronic cascade model; it also may explain the data, but requires low strength of magnetic field around the source of primary protons or nuclei.
We discuss the effect of hypothetical violation of Lorentz invariance at high energies on the formation of atmospheric showers by very-high-energy gamma rays. In the scenario where Lorentz invariance violation leads to a decrease of the photon velocity with energy the formation of the showers is suppressed compared to the Lorentz invariant case. Absence of such suppression in the high-energy part of spectrum of the Crab nebula measured independently by HEGRA and H.E.S.S. collaborations is used to set lower bounds on the energy scale of Lorentz invariance violation. These bounds are competitive with the strongest existing constraints obtained from timing of variable astrophysical sources and the absorption of TeV photons on the extragalactic background light. They will be further improved by the next generation of multi-TeV gamma-ray observatories.
This review covers the measurements related to the extragalactic background light intensity from γ-rays to radio in the electromagnetic spectrum over 20 decades in wavelength. The cosmic microwave background (CMB) remains the best measured spectrum with an accuracy better than 1%. The measurements related to the cosmic optical background (COB), centred at 1μm, are impacted by the large zodiacal light associated with interplanetary dust in the inner Solar System. The best measurements of COB come from an indirect technique involving γ-ray spectra of bright blazars with an absorption feature resulting from pair-production off of COB photons. The cosmic infrared background (CIB) peaking at around 100μm established an energetically important background with an intensity comparable to the optical background. This discovery paved the way for large aperture far-infrared and sub-millimetre observations resulting in the discovery of dusty, starbursting galaxies. Their role in galaxy formation and evolution remains an active area of research in modern-day astrophysics. The extreme UV(EUV) background remains mostly unexplored and will be a challenge to measure due to the high Galactic background and absorption of extragalactic photons by the intergalactic medium at these EUV/soft X-ray energies. We also summarize our understanding of the spatial anisotropies and angular power spectra of intensity fluctuations. We motivate a precise direct measurement of the COB between 0.1 and 5μm using a small aperture telescope observing either from the outer Solar System, at distances of 5AU or more, or out of the ecliptic plane. Other future applications include improving our understanding of the background at TeV energies and spectral distortions of CMB and CIB.
Cosmic opacity for very high-energy gamma rays ( TeV) due to the
interaction with the extragalactic background light can be strongly reduced
because of possible Lorentz-violating terms in the particle dispersion
relations expected, e.g., in several versions of quantum gravity theories. We
discuss the possibility to use very high energy observations of blazars to
detect anomalies of the cosmic opacity induced by LIV, considering in
particular the possibility to use -- besides the bright and close-by BL Lac Mkn
501 -- extreme BL Lac objects. We derive the modified expression for the
optical depth of rays considering also the redshift dependence and we
apply it to derive the expected high-energy spectrum above 10 TeV of Mkn 501 in
high and low state and the extreme BL Lac 1ES 0229+200. We find that, besides
the nearby and well studied BL Lac Mkn 501 -- especially in high state --,
suitable targets are extreme BL Lac objects, characterized by quite hard TeV
intrinsic spectra likely extending at the energies relevant to detect LIV
features.
We use an updated version of SimProp, a Monte Carlo simulation scheme for the
propagation of ultra-high energy cosmic rays, to compute cosmogenic neutrino
fluxes expected on Earth in various scenarios. These fluxes are compared with
the newly detected IceCube events at PeV energies and with recent experimental
limits at EeV energies of the Pierre Auger Observatory. This comparison allows
us to draw some interesting conclusions about the source models for ultra-high
energy cosmic rays. We will show how the available experimental observations
are almost at the level of constraining such models, mainly in terms of the
injected chemical composition and cosmological evolution of sources. The
results presented here will also be important in the evaluation of the
discovery capabilities of the future planned ultra-high energy cosmic ray and
neutrino observatories.
The existence of the cosmic neutrino background (CνB) is a fundamental prediction of the standard Big Bang cosmology. Although current cosmological probes provide indirect observational evidence, the direct detection of the CνB in a laboratory experiment is a great challenge to the present experimental techniques. We discuss the future prospects for the direct detection of the CνB, with the emphasis on the method of captures on beta-decaying nuclei and the PTOLEMY project. Other possibilities using the electron-capture (EC) decaying nuclei, the annihilation of extremely high-energy cosmic neutrinos (EHECνs) at the Z-resonance, and the atomic de-excitation method are also discussed in this review (talk given at the International Conference on Massive Neutrinos, Singapore, 9–13 February 2015).
High energy cosmic neutrino observations provide a sensitive test of Lorentz
invariance violation, which may be a consequence of quantum gravity theories.
We consider a class of non-renormalizable, Lorentz invariance violating
operators that arise in an effective field theory description of Lorentz
invariance violation in the neutrino sector inspired by Planck-scale physics
and quantum gravity models. We assume a conservative generic scenario for the
redshift distribution of extragalactic neutrino sources and employ Monte Carlo
techniques to describe superluminal neutrino propagation, treating
kinematically allowed energy losses of superluminal neutrinos caused by both
vacuum pair emission and neutrino splitting. We consider EFTs with both
non-renormalizable CPT-odd and non-renormalizable CPT-even operator dominance.
We then compare the spectra derived using our Monte Carlo calculations in both
cases with the spectrum observed by IceCube in order to determine the
implications of our results regarding Planck-scale physics. We find that if the
drop off in the neutrino flux above ~2 PeV is caused by Planck scale physics,
rather than by a limiting energy in the source emission, a potentially
significant pileup effect would be produced just below the drop off energy in
the case of CPT-even operator dominance. However, such a clear drop off effect
would not be observed if the CPT-odd, CPT-violating term dominates.
A search for high-energy neutrinos interacting within the IceCube detector between 2010 and 2012 provided the first evidence for a high-energy neutrino flux of extraterrestrial origin. Results from an analysis using the same methods with a third year (2012–2013) of data from the complete IceCube detector are consistent with the previously reported astrophysical flux in the 100 TeV–PeV range at the level of 10−8 GeV cm−2 s−1 sr−1 per flavor and reject a purely atmospheric explanation for the combined three-year data at 5.7σ. The data are consistent with expectations for equal fluxes of all three neutrino flavors and with isotropic arrival directions, suggesting either numerous or spatially extended sources. The three-year data set, with a live time of 988 days, contains a total of 37 neutrino candidate events with deposited energies ranging from 30 to 2000 TeV. The 2000-TeV event is the highest-energy neutrino interaction ever observed.
The observation of ultrahigh energy (UHE) neutrinos has become a priority in
experimental astroparticle physics. UHE neutrinos can be detected with a
variety of techniques. In particular, neutrinos can interact in the atmosphere
(downward-going neutrinos) or in the Earth crust (Earth-skimming neutrinos),
producing air showers that can be observed with arrays of detectors at the
ground. With the Surface Detector Array of the Pierre Auger Observatory we can
detect these types of cascades. The distinguishing signature for neutrino
events is the presence of very inclined showers produced close to the ground
(i.e. after having traversed a large amount of atmosphere). In this work we
review the procedure and criteria established to search for UHE neutrinos in
the data collected with the ground array of the Pierre Auger Observatory. This
includes Earth-skimming as well as downward-going neutrinos. No neutrino
candidates have been found, which allows us to place competitive limits to the
diffuse flux of UHE neutrinos in the EeV range and above.
Over the last decade it has been found that nonlinear laws of composition of
momenta are predicted by some alternative approaches to "real" 4D quantum
gravity, and by all formulations of dimensionally-reduced (3D) quantum gravity
coupled to matter. The possible relevance for rather different quantum-gravity
models has motivated several studies, but this interest is being tempered by
concerns that a nonlinear law of addition of momenta might inevitably produce a
pathological description of the total momentum of a macroscopic body. I here
show that such concerns are unjustified, finding that they are rooted in
failure to appreciate the differences between two roles for laws composition of
momentum in physics. Previous results relied exclusively on the role of a law
of momentum composition in the description of spacetime locality. However, the
notion of total momentum of a multi-particle system is not a manifestation of
locality, but rather reflects translational invariance. By working within an
illustrative example of quantum spacetime I show explicitly that spacetime
locality is indeed reflected in a nonlinear law of composition of momenta, but
translational invariance still results in an undeformed linear law of addition
of momenta building up the total momentum of a multi-particle system.
The IceCube observation of cosmic neutrinos with TeV, most of
which are likely of extragalactic origin, allows one to severely constrain
Lorentz invariance violation (LIV) in the neutrino sector, allowing for the
possible existence of superluminal neutrinos. The subsequent neutrino energy
loss by vacuum pair emission (VPE) is strongly dependent on the
strength of LIV. In this paper we explore the physics and cosmology of
superluminal neutrino propagation. We consider a conservative scenario for the
redshift distribution of neutrino sources. Then by propagating a generic
neutrino spectrum, using Monte Carlo techniques to take account of energy
losses from both VPE and redshifting, we obtain the best present constraints on
LIV parameters involving neutrinos. We find that . Taking ,
we then obtain an upper limit on the superluminal velocity fraction for
neutrinos alone of . Interestingly, by taking , we obtain a cutoff in the predicted neutrino
spectrum above 2 PeV that is consistent with the lack of observed neutrinos at
those energies, and particularly at the Glashow resonance energy of 6.3 PeV.
Thus, such a cutoff could be the result of neutrinos being slightly
superluminal, with being .
The arrival of TeV-energy photons from distant galaxies is expected to be
affected by their QED interaction with intergalactic radiation fields through
electron-positron pair production. In theories where high-energy photons
violate Lorentz symmetry, the kinematics of the process is altered and the cross-section suppressed.
Consequently, one would expect more of the highest-energy photons to arrive if
QED is modified by Lorentz violation than if it is not. We estimate the
sensitivity of Cherenkov Telescope Array (CTA) to changes in the -ray
horizon of the Universe due to Lorentz violation, and find that it should be
competitive with other leading constraints.
Accurate measurement of neutrino energies is essential to many of the scientific goals of large-volume neutrino telescopes. The fundamental observable in such detectors is the Cherenkov light produced by the transit through a medium of charged particles created in neutrino interactions. The amount of light emitted is proportional to the deposited energy, which is approximately equal to the neutrino energy for νe and νμ charged-current interactions and can be used to set a lower bound on neutrino energies and to measure neutrino spectra statistically in other channels. Here we describe methods and performance of reconstructing charged-particle energies and topologies from the observed Cherenkov light yield, including techniques to measure the energies of uncontained muon tracks, achieving average uncertainties in electromagnetic-equivalent deposited energy of ∼15% above 10 TeV.
The observation of two PeV-scale neutrino events reported by Ice Cube can, in
principle, allow one to place constraints on Lorentz invariance violation (LIV)
in the neutrino sector. These constraints depend on independent constraints on
LIV in the electron sector. Using the results of Cohen and Glashow I derive a
constraint for {\it the difference} between putative superluminal neutrino and
electron velocities of in units where ,
indicating that the observed PeV neutrinos could not have reached Earth from
most extragalactic sources. I then give a new constraint on the superluminal
electron velocity. This is derived using data obtained from {\it Fermi}
observations of the synchrotron radiation component of the exceptional flare
that occurred in the Crab Nebula in September, 2010. The observed 1 GeV
-rays from this superflare were produced by electrons of energy up to
PeV, indicating the non-occurrence of vacuum \'{C}erenkov radiation
by PeV electrons. This implies a new, strong constraint on
superluminal electron velocities . One
then derives a constraint on the superluminal neutrino velocity {\it alone} of
, almost ten orders of magnitude
better than the time-of-flight constraint from the SN1987A neutrino burst.
However, if the electrons are subluminal, the constraint on obtained from the observation of 80 GeV
-rays from the Crab Nebula implies a weaker constraint on superluminal
neutrino velocity of .
We analyze the MeV/GeV emission from four bright Gamma-Ray Bursts (GRBs)
observed by the Fermi-Large Area Telescope to produce robust, stringent
constraints on a dependence of the speed of light in vacuo on the photon energy
(vacuum dispersion), a form of Lorentz invariance violation (LIV) allowed by
some Quantum Gravity (QG) theories. First, we use three different and
complementary techniques to constrain the total degree of dispersion observed
in the data. Additionally, using a maximally conservative set of assumptions on
possible source-intrinsic spectral-evolution effects, we constrain any vacuum
dispersion solely attributed to LIV. We then derive limits on the "QG energy
scale" (the energy scale that LIV-inducing QG effects become important, E_QG)
and the coefficients of the Standard Model Extension. For the subluminal case
(where high energy photons propagate more slowly than lower energy photons) and
without taking into account any source-intrinsic dispersion, our most stringent
limits (at 95% CL) are obtained from GRB090510 and are E_{QG,1}>7.6 times the
Planck energy (E_Pl) and E_{QG,2}>1.3 x 10^11 GeV for linear and quadratic
leading order LIV-induced vacuum dispersion, respectively. These limits improve
the latest constraints by Fermi and H.E.S.S. by a factor of ~2. Our results
disfavor any class of models requiring E_{QG,1} \lesssim E_Pl.
It is well known in the literature that the momentum space associated to the κ-Poincaré algebra is described by the Lie group AN(3). In this letter we show that the full κ-Poincaré Hopf algebra structure can be obtained from rather straightforward group-theoretic manipulations starting from the Iwasawa decomposition of the SO(1,4) group.
Astrophysical neutrinos are excellent probes of astroparticle physics and high-energy physics. With energies far beyond solar, supernovae, atmospheric, and accelerator neutrinos, high-energy and ultra-high-energy neutrinos probe fundamental physics from the TeV scale to the EeV scale and beyond. They are sensitive to physics both within and beyond the Standard Model through their production mechanisms and in their propagation over cosmological distances. They carry unique information about their extreme non-thermal sources by giving insight into regions that are opaque to electromagnetic radiation. This white paper describes the opportunities astrophysical neutrino observations offer for astrophysics and high-energy physics, today and in coming years.
The IceCube Neutrino Observatory has established the existence of a high-energy all-sky neutrino flux of astrophysical origin. This discovery was made using events interacting within a fiducial region of the detector surrounded by an active veto and with reconstructed energy above 60 TeV, commonly known as the high-energy starting event sample (HESE). We revisit the analysis of the HESE sample with an additional 4.5 years of data, newer glacial ice models, and improved systematics treatment. This paper describes the sample in detail, reports on the latest astrophysical neutrino flux measurements, and presents a source search for astrophysical neutrinos. We give the compatibility of these observations with specific isotropic flux models proposed in the literature as well as generic power-law-like scenarios. Assuming νe:νμ:ντ=1:1:1, and an equal flux of neutrinos and antineutrinos, we find that the astrophysical neutrino spectrum is compatible with an unbroken power law, with a preferred spectral index of 2.87−0.19+0.20 for the 68% confidence interval.
The spectral lags of gamma-ray bursts (GRBs) have been viewed as the most promising probes of the possible violations of Lorentz invariance (LIV). However, these constraints usually depend on the assumption of the unknown intrinsic time lag in different energy bands and the use of a single highest-energy photon. A new approach to test the LIV effects has been proposed by directly fitting the spectral-lag behavior of a GRB with a well-defined transition from positive lags to negative lags. This method simultaneously provides a reasonable formulation of the intrinsic time lag and robust lower limits on the quantum-gravity energy scales ( E QG ). In this work, we perform a global fitting to the spectral-lag data of GRB 190114C by considering the possible LIV effects based on a Bayesian approach. We then derive limits on E QG and the coefficients of the standard model extension. The Bayes factor output in our analysis shows very strong evidence for the spectral-lag transition in GRB 190114C. Our constraints on a variety of isotropic and anisotropic coefficients for LIV are somewhat weaker than existing bounds, but they can be viewed as comparatively robust and have the promise to complement existing LIV constraints. The observations of GRBs with higher-energy emissions and higher temporal resolutions will contribute to a better formulation of the intrinsic time lag and more rigorous LIV constraints in the dispersive photon sector.
The dominant neutrino fluxes at Earth from different sources are reviewed and the grand unified neutrino spectrum ranging from meV to PeV energies is presented. For each energy band and source, both theoretical expectations and experimental data are discussed. This compact review serves as a reference to those interested in neutrino astronomy, fundamental particle physics, dark-matter detection, high-energy astrophysics, geophysics, and other related topics.
Lorentz invariance (LI) has a central role in science and its violation (LIV) at some high-energy scale has been related to possible solutions for several of the most intriguing puzzles in nature such as dark matter, dark energy, cosmic rays generation in extreme astrophysical objects and quantum gravity. We report on a search for LIV signal based on the propagation of gamma rays from astrophysical sources to Earth. An innovative data analysis is presented which allowed us to extract unprecedented information from the most updated data set composed of 111 energy spectra of 38 different sources measured by current gamma-ray observatories. No LIV signal was found, and we show that the data are best described by LI assumption. We derived limits for the LIV energy scale at least 3 times better than the ones currently available in the literature for subluminal signatures of LIV in high-energy gamma rays.
The cosmic background (CB) radiation, encompassing the sum of emission from all sources outside our own Milky Way galaxy across the entire electromagnetic spectrum, is a fundamental phenomenon in observational cosmology. Many experiments have been conceived to measure it (or its constituents) since the extragalactic Universe was first discovered; in addition to estimating the bulk (cosmic monopole) spectrum, directional variations have also been detected over a wide range of wavelengths. Here we gather the most recent of these measurements and discuss the current status of our understanding of the CB from radio to -ray energies. Using available data in the literature we piece together the sky-averaged intensity spectrum, and discuss the emission processes responsible for what is observed. We examine the effect of perturbations to the continuum spectrum from atomic and molecular line processes and comment on the detectability of these signals. We also discuss how one could in principle obtain a complete census of the CB by measuring the full spectrum of each spherical harmonic expansion coefficient. This set of spectra of multipole moments effectively encodes the entire statistical history of nuclear, atomic and molecular processes in the Universe.
We have carried out a detailed study to understand the observed energy spectrum and composition of cosmic rays with energies up to ~10^18 eV. Our study shows that a single Galactic component with subsequent energy cut-offs in the individual spectra of different elements, optimised to explain the observed spectra below ~10^14 eV and the knee in the all-particle spectrum, cannot explain the observed all-particle spectrum above ~2x10^16 eV. We discuss two approaches for a second component of Galactic cosmic rays -- re-acceleration at a Galactic wind termination shock, and supernova explosions of Wolf-Rayet stars, and show that the latter scenario can explain almost all observed features in the all-particle spectrum and the composition up to ~10^18 eV, when combined with a canonical extra-galactic spectrum expected from strong radio galaxies or a source population with similar cosmological evolution. In this two-component Galactic model, the knee at ~ 3x10^15 eV and the second knee at ~10^17 eV in the all-particle spectrum are due to the cut-offs in the first and second components, respectively. We also discuss several variations of the extra-galactic component, from a minimal contribution to scenarios with a significant component below the ankle (at ~4x10^18 eV), and find that extra-galactic contributions in excess of regular source evolution are neither indicated nor in conflict with the existing data. Our main result is that the second Galactic component predicts a composition of Galactic cosmic rays at and above the second knee that largely consists of helium or a mixture of helium and CNO nuclei, with a weak or essentially vanishing iron fraction, in contrast to most common assumptions. This prediction is in agreement with new measurements from LOFAR and the Pierre Auger Observatory which indicate a strong light component and a rather low iron fraction between ~10^17 and 10^18 eV.
The hypothesis of an unstable charged boson to mediate muon decay radically affects the cross section for the process \overline{\nu{}}+e\rightarrow{}\overline{\nu{}}+{\mu{}}^{-{}} near the energy at which the intermediary may be produced. If the boson is assumed to have K-meson mass, the resonance occurs at an incident antineutrino energy of {}2\ifmmode\times\else\texttimes\fi{} ev. The flux of energetic antineutrinos produced in association with cosmic-ray muons will then produce two muon counts per day per square meter of detector, independently of the depth and the orientation at which the experiment is performed.
Attenuation of high-energy gamma-rays by pair production with
ultraviolet, optical and infrared (IR) extragalactic background light
(EBL) photons provides a link between the history of galaxy formation
and high-energy astrophysics. We present results from our latest
semi-analytic models (SAMs), which employ the main ingredients thought
to be important to galaxy formation and evolution, as well as an
improved model for reprocessing of starlight by dust to mid- and far-IR
wavelengths. These SAMs are based upon a Λ cold dark matter
hierarchical structural formation scenario, and are successful in
reproducing a large variety of observational constraints such as number
counts, luminosity and mass functions and colour bimodality. Our
fiducial model is based upon a Wilkinson Microwave Anisotropy Probe
5-year cosmology, and treats dust emission using empirical templates.
This model predicts a background flux considerably lower than optical
and near-IR measurements that rely on subtraction of zodiacal and
galactic foregrounds, and near the lower bounds set by number counts of
resolvable sources at a large number of wavelengths. We also show the
results of varying cosmological parameters and dust attenuation model
used in our SAM. For each EBL prediction, we show how the optical depth
due to electron-positron pair production is affected by redshift and
gamma-ray energy, and the effect of gamma-ray absorption on the spectra
of a variety of extragalactic sources. We conclude with a discussion of
the implications of our work, comparisons to other models and key
measurements of the EBL and a discussion of how the burgeoning science
of gamma-ray astronomy will continue to help constrain cosmology. The
low EBL flux predicted by our fiducial model suggests an optimistic
future for further studies of distant gamma-ray sources.
This review deals with the development of concepts on the structure and evolution of the Universe. Two revolutions in astronomy will be considered, such as he transition from the geocentric to heliocentric model, and from the static Universe to the nonstationary expanding Universe, including the early inflation phase. Natural sciences disciplines are emasculated from school educational programs. A wave of militant obscurantism has engulfed television, radio and other mass media. Human astronomical practice described in written accounts is related to historic astronomy. The ancient Indian natural philosophers have proposed another interesting idea, assuming at some invisible universal medium, which they called 'prana', possesses properties of executing self-motion and being in the state of a 'tension'.
Astronomy is now performed over the entire range of the electromagnetic spectrum, from radio to γ-ray energies. From the so-called “New Astronomies”, which are performed outside the optical window, we learned during the second half of the 20th century that each spectral range provides specific information which cannot be obtained by other means. Gamma radiation represents the most energetic part of the electromagnetic spectrum (see Fig. 1.1). Therefore it is natural that it provides information about the most energetic processes and phenomena in the Universe.
The recombination of free electrons and free positrons and its connection with the Compton effect have been treated by Dirac before the experimental discovery of the positron. In the present note are given analogous calculations for the production of positron electron pairs as a result of the collision of two light quanta. The angular distribution of the ejected pairs is calculated for different polarizations, and formulas are given for the angular distribution of photons due to recombination. The results are applied to the collision of high energy photons of cosmic radiation with the temperature radiation of interstellar space. The effect on the absorption of such quanta is found to be negligibly small.