Astrophysical Uncertainties Research Articles

Particle Physics on Ice: Constraints on Neutrino Interactions Far above the Weak Scale

Ultrahigh energy cosmic rays and neutrinos probe energies far above the weak scale. Their usefulness might appear to be limited by astrophysical uncertainties; however, by simultaneously considering up- and down-going events, one may disentangle particle physics from astrophysics. We show that present data from the AMANDA experiment in the South Pole ice already imply an upper bound on neutrino cross sections at energy scales that will likely never be probed at man-made accelerators. The existing data also place an upper limit on the neutrino flux valid for any neutrino cross section. In the future, similar analyses of IceCube data will constrain neutrino properties and fluxes at the theta(10%) level.

Spectra of neutrinos from dark matter annihilations

We study the fluxes of neutrinos from annihilations of dark matter particles in the Sun and the Earth. We give the spectra of all neutrino flavors for the main known annihilation channels: ν ν ¯ , b b ¯ , τ τ ¯ , c c ¯ , light quarks, ZZ, W + W − . We present the appropriate formalism for computing the combined effect of oscillations, absorptions, ν τ -regeneration. Total rates are modified by an O ( 0.1 – 10 ) factor, comparable to astrophysical uncertainties, that instead negligibly affect the spectra. We then calculate different signal topologies in neutrino telescopes: through-going muons, contained muons, showers, and study their capabilities to discriminate a dark matter signal from backgrounds. We finally discuss how measuring the neutrino spectra can allow to reconstruct the fundamental properties of the dark matter: its mass and its annihilation branching ratios.

Neutrinos as astrophysical probes

The aim of these notes is to provide a brief review of the topic of neutrino astronomy and in particular of neutrinos from core collapse supernovae. They are addressed to a curious reader, beginning to work in a multidisciplinary area that involves experimental neutrino physics, astrophysics, nuclear physics and particle physics phenomenology. After an introduction to the methods and goals of neutrinos astronomy, we focus on core collapse supernovae, as (one of) the most promising astrophysical source of neutrinos. The first part is organized almost as a tale, the last part is a bit more technical. We discuss the impact of flavor oscillations on the supernova neutrino signal (=the change of perspective due to recent achievements) and consider one specific example of signal in detail. This shows that effects of oscillations are important, but astrophysical uncertainties should be thought as an essential systematics for a correct interpretation of future experimental data. Three appendices corroborate the text with further details and some basics on flavor oscillations; but no attempt of a complete bibliographical survey is done (in practice, we selected a few references that we believe are useful for a `modern' introduction to the subject. We suggest the use of public databases for papers as SPIRES or NASA/ESO or for experiments as SPIRES or PaNAGIC to get a more complete information).

The gravitational wave background from cosmological compact binaries

We use a population synthesis approach to characterize, as a function of cosmic time, the extragalactic close binary population descended from stars of low to intermediate initial mass. The unresolved gravitational wave (GW) background due to these systems is calculated for the 0.1‐10 mHz frequency band of the planned Laser Interferometer Space Antenna (LISA). This background is found to be dominated by emission from close white dwarf‐white dwarf (WD‐ WD) pairs. The spectral shape can be understood in terms of some simple analytic arguments. To quantify the astrophysical uncertainties, we construct a range of evolutionary models which produce populations consistent with Galactic observations of close WD‐WD binaries. The models differ in binary evolution prescriptions as well as initial parameter distributions and cosmic star formation histories. We compare the resulting background spectra, the shapes of which are found to be insensitive to the model chosen, and different to those found recently by Schneider et al. From this set of models, we constrain the amplitude of the extragalactic

Angular dependence of neutrino flux inKM3detectors in low scale gravity models

Cubic kilometer neutrino telescopes are capable of probing fundamental questions of ultra-high energy neutrino interactions. There is currently great interest in neutrino interactions caused by low-scale, extra dimension models. Above 1 PeV the cross section in low scale gravity models rises well above the total Standard Model cross section. We assess the observability of this effect in the 1 PeV - 100 PeV energy range of kilometer-scale detectors with several new points of emphasis that hinge on enhanced neutral current cross sections. A major point is the importance of ``feed-down'' regeneration of upward neutrino flux, driven by new-physics neutral current interactions in the flux evolution equations. Feed-down is far from negligible, and it is essential to include its effect. We then find that the angular distribution of events has high discriminating value in separating models. In particular the ``up-to-down'' ratio between upward and downward-moving neutrino fluxes is a practical diagnostic tool which can discriminate between models in the near future. The slope of the angular distribution, in the region of maximum detected flux, is also substantially different in low-scale gravity and the Standard Model. These observables are only weakly dependent on astrophysical flux uncertainties. We conclude that angular distributions can reveal a breakdown of the Standard Model and probe the new physics beyond, as soon as data become available.

The high energy cosmic ray spectrum from relic particle decay

It has been speculated that the recently detected ultra-high energy cosmic rays may originate from the decays of relic particles with mass of order 10 12 GeV clustered in the halo of our Galaxy. This hypothesis can be tested through forthcoming measurements of the spectra of both high energy cosmic nucleons and neutrinos, which are determined in this model by the physics of QCD fragmentation, with no astrophysical uncertainties. We evolve fragmentation spectra measured at LEP energies up to the scale of the decaying particle mass by numerical solution of the DGLAP equations. This enables incorporation of the effects of supersymmetry on the development of the cascade and we also allow for decays into many-particle states. The calculated spectral shape agrees well with present cosmic ray data beyond the Greisen–Zatsepin–Kuzmin energy.

Probing the supersymmetric parameter space by weakly interacting massive particle direct detection

We discuss to what extent the present experiments of direct search for weakly interacting massive particles (WIMPs), when interpreted in terms of relic neutralinos, probe interesting regions of the supersymmetric parameter space, which are also being progressively explored at accelerators. Our analysis is performed in a number of different supersymmetric schemes. We derive the relevant neutralino cosmological properties, locally and on the average in the universe. We prove that part of the supersymmetric (SUSY) configurations probed by current WIMP experiments entails relic neutralinos of cosmological interest. The main astrophysical and particle physics uncertainties, relevant for a proper comparison between theory and experimental data, are stressed and taken into account.

Cosmic Antiprotons as a Probe for Supersymmetric Dark Matter?

The flux of cosmic ray antiprotons from neutralino annihilations in the galactic halo is computed for a large sample of models in the MSSM (the Minimal Supersymmetric extension of the Standard Model). We also revisit the problem of estimating the background of low-energy cosmic ray induced secondary antiprotons, taking into account their subsequent interactions (and energy loss) and the presence of nuclei in the interstellar matter. We consider a two-zone diffusion model, with and without a galactic wind. We find that, given the uncertainties in the background predictions, there is no need for a primary (exotic) component to explain present data. However, allowing for a signal by playing with the uncertainties in the background estimate, we discuss the characteristic features of the supersymmetric models which give a satisfactory description of the data. We point out that in some cases the optimal kinetic energy to search for a signal from supersymmetric dark matter is above several GeV, rather than the traditional sub-GeV region. The large astrophysical uncertainties involved do not, one the other hand, allow the exclusion of any of the MSSM models we consider, on the basis of data. We present besides numerical results also convenient parameterizations of the antiproton yields of all `basic' two-body final states. We also give examples of the yield and differential energy spectrum for a set of supersymmetric models with high rates. We also remark that it is difficult to put a limit on the antiproton lifetime from present measurements, since the injection of antiprotons from neutralino annihilation can compensate the loss from decay.

Establishing aνμ,τComponent in the Solar Neutrino Flux

We point out that the recoil electron kinetic energy spectra in the nu-e elastic scattering are different for incident nu_{e} or nu_{mu,tau}, and hence one can in principle establish the existence of the nu_{mu,tau} component in the solar neutrino flux by fitting the shape of the spectrum. This would be a new model-independent test of the solar neutrino oscillation in a single experiment, free from astrophysical and nuclear physics uncertainties. For the ^7Be neutrinos, it is possible to determine the nu_{mu,tau} component at BOREXINO or KamLAND, if the background is sufficiently low. Note that this effect is different from the distortion in the incident neutrino energy spectrum, which has been discussed in the literature.

Multi-GeV photons from electron–dark-matter scattering near active galactic nuclei

Active galactic nuclei (AGN) may emit highly collimated and intense jets of relativistic electrons which upscatter ambient photons. These electrons can also scatter off the cold dark matter halo of the galaxy to produce high energy photons which have a more isotropic signature than the upscattered photons from QED processes. We propose to look for these high energy photons coming from AGN as a method to detect dark matter. As a primary example we work out the expected signal from electrons scattering off the lightest supersymmetric partner into a photon plus selectron. Using the optimistic side of astrophysical uncertainties, we still find the signal from M87 or Centaurus A, two close-by AGN, smaller than the sensitivities expected of the currently proposed photon detectors. However, long running photon detectors and future detectors of higher sensitivity might be able to distinguish a signal from AGN sources. In order to have confidence that new physics sources are discernible, we also emphasize the importance of multiwavelength studies of AGN with varying jet axis orientation to the Earth.

Big-bang nucleosynthesis limit to the number of neutrino species

Concern about the systematic uncertainty in the {sup 4}He abundance as well as the chemical evolution of {sup 3}He leads us to reexamine this important limit. It is shown that with conservative assumptions no more than the equivalent four massless neutrino species are allowed. Even with the most extreme estimates of the astrophysical uncertainties a meaningful limit still exists, less than five massless neutrino species, and illustrates the robustness of this argument. We show that a definitive measurement of the deuterium abundance in high-redshift hydrogen clouds can sharpen the limit. {copyright} {ital 1997} {ital The American Physical Society}

Resonant spin-flavor conversion of supernova neutrinos and deformation of the electron antineutrino spectrum.

The neutrino spin-flavor conversion of \bar\nu_e and \nu_\mu which is induced by the interaction of the Majorana neutrino magnetic moment and magnetic fields in the collapse-driven supernova is investigated in detail. We calculate the conversion probability by using the latest precollapse models of Woosley and Weaver (1995), and also those of Nomono and Hashimoto (1988), changing the stellar mass and metallicity in order to estimate the effect of the astrophysical uncertainties. Contour maps of the conversion probability are given for all the models as a function of neutrino mass squared difference and the neutrino magnetic moment times magnetic fields. It is shown that in the solar metallicity models some observational effects are expected with \Delta m^2 = 10^{-5}--10^{-1} [eV^2] and \mu_\nu >~ 10^{-12} (10^9 G / B_0) [\mu_B], where B_0 is the strength of the magnetic fields at the surface of the iron core. We also find that although the dependence on the stellar models or stellar mass is not so large, the metallicity of precollapse stars has considerable effects on this conversion. Such effects may be seen in a supernova in the Large or Small Magellanic Clouds, and should be taken into account when one considers an upper bound on \mu_\nu from the SN1987A data.

EFFECTS OF NEUTRINO OSCILLATION ON THE SUPERNOVA RELIC NEUTRINO BACKGROUND

We investigate to what extent the oscillation or conversion of neutrinos enhances the expected event rate of the supernova relic neutrino background (SRN) at the SuperKamiokande detector (SK). The SRN [Formula: see text] can be almost completely exchanged with vμ-like neutrinos by the MSW oscillation under the inverse mass hierarchy with Δm2~ 10−8–105[eV2], or by the magnetic moment of Majorana neutrinos with μv≳10−12μB and Δm2~10−4–10° [eV2]. In the standard calculation of the SRN flux, the event rate of the SRN [Formula: see text] at the SK in the observable energy range of 15–40 MeV can be enhanced from 1.2 yr−1 to 2.4 yr−1 if all [Formula: see text] are exchanged with vμ-like neutrinos. The enhancement is prominent especially in the high energy range (≳ 25 MeV). In the astrophysically optimistic calculation, the event rate becomes as high as 9.4 yr−1. Because the theoretical upper bound of the SRN events without oscillation is about 5 yr−1 taking into account the various astrophysical uncertainties, we might have to resort to the neutrino oscillation if more than 5 events in a year, as well as a significantly harder spectrum, were observed in the SK.

Earth effect in the MSW analysis of the solar neutrino experiments.

We consider the Earth effect in the MSW analysis of the Homestake, Kamiokande, GALLEX, and SAGE solar neutrino experiments. Using the time-averaged data and assuming two-flavor oscillations, the large-angle region of the combined fit extends to much smaller angles (to $\sin^22\theta \simeq 0.1$) than when the Earth effect is ignored. However, the additional constraint from the Kamiokande II day-night data excludes most of the parameter space sensitive to the Earth effect independent of astrophysical uncertainties, and leaves only a small large-angle region close to maximal mixing at 90\% C.L. The nonadiabatic solution remains unaffected by the Earth effect and is still preferred. Both theoretical and experimental uncertainties are included in the analysis.

Implications of combined solar-neutrino observations and their theoretical uncertainties.

Constraints on the core temperature (T_c) of the Sun and on neutrino- oscillation parameters are obtained from the existing solar neutrino data, including the recent GALLEX and Kamiokande III results. (1) A purely astrophysical solution to the solar neutrino problem is strongly disfavored by the data: the best fit in a cooler Sun model requires an 8% reduction in T_c, but the chi-sqaured test rejects this hypothesis at 99.99% C.L., suggesting new neutrino physics. (2) Assuming the Standard Solar Model (SSM) and MSW oscillations, the MSW parameters are constrained to two small regions: one in the non-adiabatic region and the other in the large-mixing region. The non-adiabatic solution gives a considerably better fit. For nu_e oscillations into sterile neutrinos, the allowed region (90%) is constrained to non- adiabatic oscillations. As long as the SSM is assumed, the neutrino mixing angles are at least four times larger, or considerably smaller, than the corresponding quark mixing angles. (3) Allowing both MSW oscillations and a non-standard core temperature, a) the experiments determine the core temperature at the 5% level, yielding a value consistent with the SSM prediction. b) When T_c is used as a free parameter, the allowed MSW region is broadened: a cooler Sun (T_c=0.95) allows mass and mixing implied by the SUSY SO(10) GUT, while a warmer Sun (T_c=1.05) allows parameter space suggested by intermediate-scale SO(10) GUTs. Superstring-inspired models are consistent with all solutions. (4) From the narrowed parameter space, we predict the neutrino spectral shape which should be observed in SNO. Throughout the calculation we use the latest Bahcall-Pinsonneault SSM, and include nuclear and astrophysical uncertainties in a simplified, but physically transparent way.

Monte Carlo exploration of Mikheyev-Smirnov-Wolfenstein solutions to the solar neutrino problem.

The paper explores the impact of astrophysical uncertainties on the Mikheyev-Smirnov-Wolfenstein (MSW) solution by calculating the allowed MSW solutions for 1000 different solar models with a Monte Carlo selection of solar model input parameters, assuming a full three-family MSW mixing. Applications are made to the chlorine, gallium, Kamiokande, and Borexino experiments. The initial GALLEX result limits the mixing parameters to the upper diagonal and the vertical regions of the MSW triangle. The expected event rates in the Borexino experiment are also calculated, assuming the MSW solutions implied by GALLEX.

An update on the solar neutrino problem and the three-family MSW effect allowing for astrophysical uncertainties

Three-neutrino MSW mixing is considered, assuming m 2 2 ⪢ m 2 2 ⪢ m 2 1 as expected from theoretical considerations, and the corresponding allowed mixing parameter space is calculated from the 37Cl and Kamiokande II experiments. The expected depletion for the 71Ga experiment is also calculated. The possible variations induced by a range of astrophysical uncertainty is explored by varying the 8B neutrino flux and redoing the MSW mixing calculation with such variations, including the consistency with the possible atmospheric neutrino mixing observations.

Distinctive positron feature from particle dark-matter annihilations in the galactic halo.

If the dark matter in our galactic halo consists of weakly interacting massive particles (WIMP's) heavier than the W± boson which have a significant annihilation branch into W± and Z0 pairs, e.g., a Higgsino-like neutralino, a very distinctive feature in the cosmic-ray positron spectrum arises from W+ and Z0 decays. Because of inherent astrophysical uncertainties such a signal is by no means guaranteed even if heavy WIMP's do comprise the galactic halo. However, the positron signature is virtually a smoking gun for particle dark matter in the halo and thus worthy of note.

Implications for dark matter particles of searches for energetic solar neutrinos

We investigate whether recent searches for energetic solar neutrinos give new constraints on candidate dark-matter particles such as photinos γ and higgsinos h̃, in view of the astrophysical and particle physics uncertainties. Previous quark model estimates of the elastic cross sections responsible for γ or h̃ capture in the sun can be improved using hyperon decay data, but are thrown into doubt by recent European Muon Collaboration data on the spin-dependent proton structure function, which reduce the estimate of σ( γ p ) by a factor up to four. We emphasize the need to include charm and bottom quark fragmentation effects when calculating the flux of neutrinos from dark matter annihilation events, which may reduce the observable flux by a factor ∼ 2. Even the most stringent upper limits from the Fréjus experiment still allow a local γ or h̃ density considerably above the range of 0.20–0.43 GeV/cm 3 favoured by astrophysics, and no range of m γ or m h ̃ can yet be excluded.

Possibility of detecting heavy neutral fermions in the Galaxy.

It is shown that heavy neutral fermions in the galactic halo could produce numerous detectable low-energy events in a ``thermal'' neutrino detector if the heavy-fermionndashnucleon vector coupling is comparable in strength to the vector weak interaction. The conditions under which a detectable event rate could arise for fermions with purely axial-vector couplings are also discussed. In a silicon detector heavy-fermion events would be concentrated at low energies, and could be distinguished from solar or supernova neutrino events, which are expected to have a flatter energy spectrum. A measurement of the energy spectrum of heavy-fermion events could lead to a determination of the fermion mass, subject to astrophysical uncertainties concerning the velocity distribution of halo particles.