arXiv subjects

M. Kachelriess

Publications and source records attributed to M. Kachelriess.

At least 91 records · Page 5Linked to original sources

arXiv2006

High energy neutrino yields from astrophysical sources I: Weakly magnetized sources

We calculate the yield of high energy neutrinos produced in astrophysical sources with negligible magnetic fields varying their interaction depth from nearly transparent to opaque. We take into account the scattering of secondaries on background photons as well as the direct production of neutrinos in decays of charm mesons. If multiple scattering of nucleons becomes important, the neutrino spectra from meson and muon decays are strongly modified with respect to transparent sources. Characteristic for neutrino sources containing photons as scattering targets is a strong energy-dependence of the ratio $R^0$ of $ν_μ$ and $ν_e$ fluxes at the sources, ranging from $R^0=ϕ_μ/ϕ_e\sim 0$ below threshold to $R^0\sim 4$ close to the energy where the decay length of charged pions and kaons equals their interaction length on target photons. Above this energy, the neutrino flux is strongly suppressed and depends mainly on charm production.

astro-ph↗

arXiv2006

The Compton-Getting effect on ultra-high energy cosmic rays of cosmological origin

Deviations from isotropy have been a key tool to identify the origin and the primary type of cosmic rays at low energies. We suggest that the Compton-Getting effect can play a similar role at ultra-high energies: If at these energies the cosmic ray flux is dominated by sources at cosmological distances, then the movement of the Sun relative to the cosmic microwave background frame induces a dipole anisotropy at the 0.6% level. The energy dependence and the orientation of this anisotropy provide important information about the transition between galactic and extragalactic cosmic rays, the charge of the cosmic ray primaries, the galactic magnetic field and, at the highest energies, the energy-loss horizon of cosmic rays. A 3-sigma detection of this effect requires around 10^6 events in the considered energy range and is thus challenging but not impossible with present detectors. As a corollary we note that the Compton-Getting effect allows one also to constrain the fraction of the diffuse gamma-ray background emitted by sources at cosmological distance, with promising detection possibilities for the GLAST satellite.

astro-ph↗

arXiv2006

The Galactic magnetic field as spectrograph for ultra-high energy cosmic rays

We study the influence of the regular component of the Galactic magnetic field (GMF) on the arrival directions of ultra-high energy cosmic rays (UHECRs). We find that, if the angular resolution of current experiments has to be fully exploited, deflections in the GMF cannot be neglected even for E=10^20 eV protons, especially for trajectories along the Galactic plane or crossing the Galactic center region. On the other hand, the GMF could be used as a spectrograph to discriminate among different source models and/or primaries of UHECRs, if its structure would be known with sufficient precision. We compare several GMF models introduced in the literature and discuss for the example of the AGASA data set how the significance of small-scale clustering or correlations with given astrophysical sources are affected by the GMF. We point out that the non-uniform exposure to the extragalactic sky induced by the GMF should be taken into account estimating the significance of potential (auto-)correlation signals.

astro-ph↗

arXiv2006

On the status of superheavy dark matter

Superheavy particles are a natural candidate for the dark matter in the universe and our galaxy, because they are produced generically during inflation in cosmologically interesting amounts. The most attractive model for the origin of superheavy dark matter (SHDM) is gravitational production at the end of inflation. The observed cosmological density of dark matter determines the mass of the SHDM particle as $m_X=$(a few)$\:\times 10^{13}$ GeV, promoting it to a natural candidate for the source of the observed ultra-high energy cosmic rays (UHECR). After a review of the theoretical aspects of SHDM, we up-date its predictions for UHECR observations: no GZK cutoff, flat energy spectrum with $dN/dE\approx 1/E^{1.9}$, photon dominance and galactic anisotropy. We analyze the existing data and conclude that SDHM as explanation for the observed UHECRs is at present disfavored but not yet excluded. We calculate the anisotropy relevant for future Auger observations that should be the conclusive test for this model. Finally, we emphasize that negative results of searches for SHDM in UHECR do not disfavor SHDM as a dark matter candidate. Therefore, UHECRs produced by SHDM decays and with the signatures as described should be searched for in the future as subdominant effect.

astro-ph↗

arXiv2006

Clustering of ultra-high energy cosmic ray arrival directions on medium scales

The two-point autocorrelation function of ultra-high energy cosmic ray (UHECR) arrival directions has a broad maximum around 25 degrees, combining the data with energies above $4\times 10^{19}$ eV (in the HiRes energy scale) of the HiRes stereo, AGASA, Yakutsk and SUGAR experiments. This signal is not or only marginally present analyzing events of a single experiment, but becomes significant when data from several experiments are added. Both the energy dependence of the signal and its angular scale might be interpreted as first signatures of the large-scale structure of UHECR sources and of intervening magnetic fields.

astro-ph↗

arXiv2005

Reconciling the ultra-high energy cosmic ray spectrum with Fermi shock acceleration

The energy spectrum of ultra-high energy cosmic rays (UHECR) is usually calculated for sources with identical properties. Assuming that all sources can accelerate UHECR protons to the same extremely high maximal energy E_max > 10^{20} eV and have the steeply falling injection spectrum 1/E^{2.7}, one can reproduce the measured cosmic ray flux above E > 10^{18} eV. We show that relaxing the assumption of identical sources and using a power-law distribution of their maximal energy allows one to explain the observed UHECR spectrum with the injection 1/E^2 predicted by Fermi shock acceleration.

astro-ph↗

arXiv2005

Constraining cosmic superstrings with dilaton emission

Brane inflation predicts the production of cosmic superstrings with tension 10^{-12}<Gμ<10^{-7}. Superstring theory predicts also the existence of a dilaton with a mass that is at most of the order of the gravitino mass. We show that the emission of dilatons imposes severe constraints on the allowed evolution of a cosmic superstring network. In particular, the detection of gravitational wave burst from cosmic superstrings by LIGO is only possible if the typical length of string loops is much smaller than usually assumed.

hep-th↗

arXiv2005

Measuring the 13-mixing angle and the CP phase with neutrino telescopes

The observed excess of high-energy cosmic rays from the Galactic plane in the energy range \sim 10^18 eV may be naturally explained by neutron primaries generated in the photo-dissociation of heavy nuclei. In this scenario, neutrons with lower energy decay before reaching the Earth and produce a detectable flux in a 1 km^3 neutrino telescope. The initial flavor composition of these neutrinos, ϕ(\barν_e):ϕ(\barν_μ):ϕ(\barν_τ)=1:0:0, offers the opportunity to perform a combined \barν_μ/\barν_τappearance and \barν_e disappearance experiment. The observable ratio ϕ(\barν_μ)/ϕ(\barν_e+\barν_τ) of fluxes arriving on Earth depends appreciably on the 13-mixing angle θ_13 and the leptonic CP phase δ_CP, opening thus a new experimental avenue to measure these two quantities.

hep-ph↗

arXiv2005

Ultra-high energy cosmic rays from a finite number of point sources

We have calculated the probability that the clustering of arrival directions of ultra-high energy cosmic rays (UHECRs) is consistent with a finite number of uniformly distributed proton sources. The case of a continuous source distribution is reached only for an unrealisticly high source density, $n_s\gg 10^{-2}/Mpc^3$. Even for densities as large as $n_s=10^{-3}/Mpc^3$, less than half of the observed cluster are on average by chance. For the best-fit value $n_s=(1-3)\times 10^{-5}/Mpc^3$ derived from the AGASA data, the probability that at least one observed cluster is from a true point source is larger than 99.97%, while on average almost all observed clusters are true. The best-fit value found is comparable to the density of AGNs and consistent with the recent HiRes stereo data. In this scenario, the Pierre Auger Observatory will not only establish the clustering of UHECRs but also determine the density of UHECR sources within a factor of a few after one year of data taking.

astro-ph↗

arXiv2005

Gravitational radiation from rotating monopole-string systems

We study the gravitational radiation from a rotating monopole-antimonopole pair connected by a string. While at not too high frequencies the emitted gravitational spectrum is described asymptotically by $P_n\propto n^{-1}$, the spectrum is exponentially suppressed in the high-frequency limit, $P_n\propto \exp(-n/n_{\rm cr})$. Below $n_{\rm cr}$, the emitted spectrum of gravitational waves is very similar to the case of an oscillating monopole pair connected by a string, and we argue therefore that the spectrum found holds approximately for any moving monopole-string system. As application, we discuss the stochastic gravitational wave background generated by monopole-antimonopole pairs connected by strings in the early Universe and gravitational wave bursts emitted at present by monopole-string networks. We confirm that advanced gravitational wave detectors have the potential to detect a signal for string tensions as small as $Gμ\sim 10^{-13}$.

astro-ph↗

arXiv2005

Exploiting the neutronization burst of a galactic supernova

One of the robust features found in simulations of core-collapse supernovae (SNe) is the prompt neutronization burst, i.e. the first $\sim 25$ milliseconds after bounce when the SN emits with very high luminosity mainly $ν_e$ neutrinos. We examine the dependence of this burst on variations in the input of current SN models and find that recent improvements of the electron capture rates as well as uncertainties in the nuclear equation of state or a variation of the progenitor mass have only little effect on the signature of the neutronization peak in a megaton water Cherenkov detector for different neutrino mixing schemes. We show that exploiting the time-structure of the neutronization peak allows one to identify the case of a normal mass hierarchy and large 13-mixing angle $θ_{13}$, where the peak is absent. The robustness of the predicted total event number in the neutronization burst makes a measurement of the distance to the SN feasible with a precision of about 5%, even in the likely case that the SN is optically obscured.

astro-ph↗

arXiv2004

Identifying the neutrino mass hierarchy with supernova neutrinos

We review how a high-statistics observation of the neutrino signal from a future galactic core-collapse supernova (SN) may be used to discriminate between different neutrino mixing scenarios. Most SN neutrinos are emitted in the accretion and cooling phase, during which the flavor-dependent differences of the emitted neutrino spectra are small and rather uncertain. Therefore the discrimination between neutrino mixing scenarios using these neutrinos should rely on observables independent of the SN neutrino spectra. We discuss two complementary methods that allow for the positive identification of the mass hierarchy without knowledge of the emitted neutrino fluxes, provided that the 13-mixing angle is large, $\sin^2θ_{13}\gg 10^{-5}$. These two approaches are the observation of modulations in the neutrino spectra by Earth matter effects or by the passage of shock waves through the SN envelope. If the value of the 13-mixing angle is unknown, using additionally the information encoded in the prompt neutronization $ν_e$ burst--a robust feature found in all modern SN simulations--can be sufficient to fix both the neutrino hierarchy and to decide whether $θ_{13}$ is ``small'' or ``large.''

hep-ph↗

arXiv2004

Neutrino signatures of supernova shock and reverse shock propagation

A few seconds after bounce in a core-collapse supernova, the shock wave passes the density region corresponding to resonant neutrino oscillations with the ``atmospheric'' neutrino mass difference. The transient violation of the adiabaticity condition manifests itself in an observable modulation of the neutrino signal from a future galactic supernova. In addition to the shock wave propagation effects that were previously studied, a reverse shock forms when the supersonically expanding neutrino-driven wind collides with the slower earlier supernova ejecta. This implies that for some period the neutrinos pass two subsequent density discontinuities, giving rise to a ``double dip'' feature in the average neutrino energy as a function of time. We study this effect both analytically and numerically and find that it allows one to trace the positions of the forward and reverse shocks. We show that the energy dependent neutrino conversion probabilities allow one to detect oscillations even if the energy spectra of different neutrino flavors are the same as long as the fluxes differ. These features are observable in the \barν_e signal for an inverted and in the ν_e signal for a normal neutrino mass hierarchy, provided the 13-mixing angle is ``large'' (sin^2θ_{13}\gg 10^{-5}).

astro-ph↗

arXiv2004

Ultra High Energy Cosmic Rays Spectra in Top-Down models

In this work we present a detailed computation of the spectra of UHECR in the top-down scenario. We compare the spectra of hadrons obtained by two different methods in QCD and supersymmetric (SUSY) QCD with large primary energies $\sqrt{s}$ up to $10^{16}$ GeV. The two methods discussed are a Monte Carlo (MC) simulation and the evolution of the hadron fragmentation functions as described by the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) equations. The hadron spectra obtained by the two methods agree fairly well in the interesting energy range $10^{-5}M_{X}<E<0.3M_{X}$ ($M_X$ is the energy scale of the process $M_{X}\ge 10^{12}$ GeV). We have also computed the spectra of photons, neutrinos and nucleons obtaining a good agreement with other published results. The consistency of the spectra computed by different methods allows us to consider the spectral shape as a signature of the production model for UHECR, such as the decay of super heavy relic particles or topological defects.

astro-ph↗

arXiv2004

Status of particle physics solutions to the UHECR puzzle

The status of solutions to the ultra-high energy cosmic ray puzzle that involve particle physics beyond the standard model is reviewed. Signatures and experimental constraints are discussed for most proposals like the Z burst model and topological defects (both allowed only as subdominant contribution), supermassive dark matter (no positive evidence from its key signatures galactic anisotropy and photon dominance), strongly interacting neutrinos or new primaries (no viable models known), and violation of Lorentz invariance (possible).

hep-ph↗

arXiv2004

Fragmentation functions in SUSY QCD and UHECR spectra produced in top-down models

We present results from two different methods for the calculation of hadron spectra in QCD and SUSY QCD with large primary energies $\sqrt{s}$ up to $10^{16}$ GeV. The two methods considered are a Monte Carlo (MC) simulation and the evolution of fragmentation functions described by the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) equations. We find that the pion, nucleon and all-hadron spectra calculated with the two methods agree well. The MC simulation is performed with new hadronization functions (in comparison with our previous work), motivated by low-energy ($\sqrt s < M_Z$) data and DGLAP. The hadron spectra calculated with both sets of hadronization functions agree well, which indicates that our method for calculating the hadronization function works successfully. The small difference in the calculated hadron spectra characterizes the uncertainties of this method. We calculate also the spectra of photons, neutrinos and nucleons and compare them with other published results. The agreement is good for all $x$ from $\sim 10^{-5}$ up to $x\leq 0.3$. The consistency of the spectra calculated by different methods allows to consider the spectral shape as a signature of models with decays or annihilations of superheavy particles, such as topological defects or superheavy DM. The UHECR spectra from these sources are calculated.

hep-ph↗

arXiv2003

Signatures of supernova neutrino oscillations in the Earth mantle and core

The Earth matter effects on supernova (SN) neutrinos can be identified at a single detector through peaks in the Fourier transform of their ``inverse energy'' spectrum. The positions of these peaks are independent of the SN models and therefore the peaks can be used as a robust signature of the Earth matter effects, which in turn can distinguish between different neutrino mixing scenarios. Whereas only one genuine peak is observable when the neutrinos traverse only the Earth mantle, traversing also the core gives rise to multiple peaks. We calculate the strengths and positions of these peaks analytically and explore their features at a large scintillation detector as well as at a megaton water Cherenkov detector through Monte Carlo simulations. We propose a simple algorithm to identify the peaks in the actual data and quantify the chances of a peak identification as a function of the location of the SN in the sky.

hep-ph↗

arXiv2003

Supernova pointing with low- and high-energy neutrino detectors

A future galactic SN can be located several hours before the optical explosion through the MeV-neutrino burst, exploiting the directionality of $ν$-$e$-scattering in a water Cherenkov detector such as Super-Kamiokande. We study the statistical efficiency of different methods for extracting the SN direction and identify a simple approach that is nearly optimal, yet independent of the exact SN neutrino spectra. We use this method to quantify the increase in the pointing accuracy by the addition of gadolinium to water, which tags neutrons from the inverse beta decay background. We also study the dependence of the pointing accuracy on neutrino mixing scenarios and initial spectra. We find that in the ``worst case'' scenario the pointing accuracy is $8^\circ$ at 95% C.L. in the absence of tagging, which improves to $3^\circ$ with a tagging efficiency of 95%. At a megaton detector, this accuracy can be as good as $0.6^\circ$. A TeV-neutrino burst is also expected to be emitted contemporaneously with the SN optical explosion, which may locate the SN to within a few tenths of a degree at a future km$^2$ high-energy neutrino telescope. If the SN is not seen in the electromagnetic spectrum, locating it in the sky through neutrinos is crucial for identifying the Earth matter effects on SN neutrino oscillations.

hep-ph↗