Search Results (16 total)

A search for TeV-PeV muon neutrinos from unresolved sources was performed on AMANDA-II data collected between 2000 and 2003 with an equivalent live time of 807 days. This diffuse analysis sought to find an extraterrestrial neutrino flux from sources with nonthermal components. The signal is expected to have a harder spectrum than the atmospheric muon and neutrino backgrounds. Since no excess of events was seen in the data over the expected background, an upper limit of E²Φ_90%C.L.<7.4×10^-8  GeV cm^-2 s^-1 sr^-1 is placed on the diffuse flux of muon neutrinos with a Φ∝E^-2 spectrum in the energy range 16 TeV to 2.5 PeV. This is currently the most sensitive Φ∝E^-2 diffuse astrophysical neutrino limit. We also set upper limits for astrophysical and prompt neutrino models, all of which have spectra different from Φ∝E^-2.

The IceCube neutrino detector is a cubic kilometer TeV to PeV neutrino detector under construction at the geographic South Pole. The dominant population of neutrinos detected in IceCube is due to meson decay in cosmic-ray air showers. These atmospheric neutrinos are relatively well understood and serve as a calibration and verification tool for the new detector. In 2006, the detector was approximately 10% completed, and we report on data acquired from the detector in this configuration. We observe an atmospheric neutrino signal consistent with expectations, demonstrating that the IceCube detector is capable of identifying neutrino events. In the first 137.4 days of live time, 234 neutrino candidates were selected with an expectation of 211±76.1(syst)±14.5(stat) events from atmospheric neutrinos.

We report the results of a five-year survey of the northern sky to search for point sources of high energy neutrinos. The search was performed on the data collected with the AMANDA-II neutrino telescope in the years 2000 to 2004, with a live time of 1001 days. The sample of selected events consists of 4282 upward going muon tracks with high reconstruction quality and an energy larger than about 100 GeV. We found no indication of point sources of neutrinos and set 90% confidence level flux upper limits for an all-sky search and also for a catalog of 32 selected sources. For the all-sky search, our average (over declination and right ascension) experimentally observed upper limit Φ⁰=(E / 1  TeV)^γ·dΦ / dE to a point source flux of muon and tau neutrino (detected as muons arising from taus) is Φ_{νμ+ν̅ μ}⁰+Φ_{ντ+ν̅ τ}⁰=11.1×  10^-11  TeV^-1 cm^-2 s^-1, in the energy range between 1.6 TeV and 2.5 PeV for a flavor ratio Φ_{νμ+ν̅ μ}⁰/Φ_{ντ+ν̅ τ}⁰=1 and assuming a spectral index γ=2. It should be noticed that this is the first time we set upper limits to the flux of muon and tau neutrinos. In previous papers we provided muon neutrino upper limits only neglecting the sensitivity to a signal from tau neutrinos, which improves the limits by 10% to 16%. The value of the average upper limit presented in this work corresponds to twice the limit on the muon neutrino flux Φ_{νμ+ν̅ μ}⁰=5.5×10^-11  TeV^-1 cm^-2 s^-1. A stacking analysis for preselected active galactic nuclei and a search based on the angular separation of the events were also performed. We report the most stringent flux upper limits to date, including the results of a detailed assessment of systematic uncertainties.

On 27 December 2004, a giant γ flare from the Soft Gamma-Ray Repeater 1806-20 saturated many satellite gamma-ray detectors, being the brightest transient event ever observed in the Galaxy. AMANDA-II was used to search for down-going muons indicative of high-energy gammas and/or neutrinos from this object. The data revealed no significant signal, so upper limits (at 90% C.L.) on the normalization constant were set: 0.05(0.5)  TeV^-1 m^-2 s^-1 for γ=-1.47 (-2) in the gamma flux and 0.4(6.1)  TeV^-1 m^-2 s^-1 for γ=-1.47 (-2) in the high-energy neutrino flux.

The results of a search for point sources of high energy neutrinos in the northern hemisphere using data collected by AMANDA-II in the years 2000, 2001, and 2002 are presented. In particular, a comparison with the single-year result previously published shows that the sensitivity was improved by a factor of 2.2. The muon neutrino flux upper limits on selected candidate sources, corresponding to an E_ν^-2 neutrino energy spectrum, are included. Sky grids were used to search for possible excesses above the background of cosmic ray induced atmospheric neutrinos. This search reveals no statistically significant excess for the three years considered.

We present the results of a search for point sources of high-energy neutrinos in the northern hemisphere using AMANDA-II data collected in the year 2000. Included are flux limits on several active-galactic-nuclei blazars, microquasars, magnetars, and other candidate neutrino sources. A search for excesses above a random background of cosmic-ray-induced atmospheric neutrinos and misreconstructed downgoing cosmic-ray muons reveals no statistically significant neutrino point sources. We show that AMANDA-II has achieved the sensitivity required to probe known TeV γ-ray sources such as the blazar Markarian 501 in its 1997 flaring state at a level where neutrino and γ-ray fluxes are equal.

Data from the AMANDA-B10 detector taken during the austral winter of 1997 have been searched for a diffuse flux of high energy extraterrestrial muon neutrinos. This search yielded no excess events above those expected from background atmospheric neutrinos, leading to upper limits on the extraterrestrial neutrino flux measured at the earth. For an assumed E^-2 spectrum, a 90% classical confidence level upper limit has been placed at a level E²Φ(E)=8.4×10^-7   cm^-2 s^-1 sr^-1 GeV (for a predominant neutrino energy range 6–1000 TeV), which is the most restrictive bound placed by any neutrino detector. Some specific predicted model spectra are excluded. Interpreting these limits in terms of the flux from a cosmological distributions of sources requires the incorporation of neutrino oscillations, typically weakening the limits by a factor of 2.

The elastic scattering of protons on ⁷Be has been measured in the energy region from 1–3.3 MeV via the thick-target technique. The data conclusively demonstrate the existence of a 2^- state at an excitation energy of approximately 3.5 MeV in ⁸B, and rule out a predicted 1⁺ state near 1.4 MeV. The relevance of these results for the ⁷Be (p,γ) reaction, of interest in solar neutrino physics, is discussed.

The angular momentum distribution of the compound nucleus is a fundamental characteristic of the reaction dynamics and can provide insight into reactions involving neutron- or proton-rich projectiles. Specifically, following the fusion of ⁶He with ²⁰⁹Bi (at center-of-mass energies of 18 to 27 MeV), ²¹²At is formed by the evaporation of three neutrons from the compound nucleus. The decay process leaves the residual ²¹²At in either the ground state (J^π=1^-, T_1/2=314 ms) or a metastable state (J^π=9^-, T_1/2=119 ms). The ratio of the number of residual ^212mAt to the total number of ²¹²At residual nuclei is sensitive to the original momentum distribution of the compound nucleus. The measured isomer ratio is consistent with that predicted by standard models. This agreement is observed even at the lower energies where the measured three neutron evaporation cross section is greatly enhanced compared to model calculations. While the inclusion of coupling to the neutron-transfer channels improves the agreement with the observed cross-section data somewhat, the predicted isomer ratio then diverges from the measured ratio.

Reaction products from the interaction of ⁶He with ²⁰⁹Bi have been measured at energies near the Coulomb barrier. A ⁴He group of remarkable intensity, which dominates the total reaction cross section, has been observed. The angular distribution of the group suggests that it results primarily from a direct nuclear process. It is likely that this transfer and/or breakup channel is the doorway state that accounts for the previously observed large sub-barrier fusion enhancement in this system.

This report details a greatly improved measurement of the four-neutron evaporation cross section following the fusion of ⁶He+²⁰⁹Bi for center-of-mass energies between 23.5 and 30.7 MeV. The results, for energies above the Coulomb barrier, are interpreted within the context of the standard statistical model.

The fusion of ⁶He with a ²⁰⁹Bi target has been studied at energies near to and below the Coulomb barrier. Despite the weak binding of the valence neutrons in ⁶He, little evidence is found for suppression of fusion due to projectile breakup. Instead, a large enhancement of sub-barrier fusion is observed. It is suggested that this enhancement may arise from coupling to positive Q value neutron transfer channels, resulting in “neutron flow” between the projectile and the target.

We report results of a survey of microwave transition frequencies between S,P, andD singlet and triplet Rydberg states of calcium in the range n=22–55 . The accuracy of the measurements is approximately one part in 10⁶. The data have been analyzed using generalized energy expansions for the quantum defects, with the exception of the 4snd ¹D₂ series, for which numerical values of the quantum defects are presented.

We report results of an experimental study of the evolution of small ensembles of atoms in a microwave cavity. The experiment employs an atomic beam of calcium Rydberg atoms and a split superconducting cavity operating at 35 GHz. At the ambient temperature of 2 K, the mean blackbody photon number is 0.8. Selective field ionization allows us to monitor simultaneously the populations of the initial and final states. The time evolution of the atomic system is probed by ‘‘Stark switching,’’ i.e., shifting the atomic resonance away from the cavity resonance by applying an electric field. The collective oscillations of energy between ensembles of atoms and a cavity with Q>10⁷ are studied for one to several hundred atoms. The results agree well with theory for ∼50 or more atoms, but some discrepancies are observed for smaller numbers of atoms. Possible experimental sources of these discrepancies are discussed.

We report an experimental study of one- and two-photon Rabi oscillations and other coherent effects in microwave transitions between Rydberg states of calcium. Selective field ionization allows us to observe the time evolution of both the initial and final states with high discrimination. The experiments include measurements of the dependence of the one- and two-photon oscillation frequencies on microwave power and detuning from resonance. We have used the off-resonant behavior of the two-photon oscillation frequency to measure the ac Stark shift. In a magnetic field we have also observed the free evolution of the magnetic sublevels and the effect of Zeeman splitting on the one-photon Rabi oscillations. The theory for these experiments is presented; our results are in excellent agreement.