Search Results (48 total)

We present the first detailed measurement of incoherent photoproduction of neutral pions to a discrete state of a residual nucleus. The ¹²C(γ,π⁰)¹²C_4.4  MeV^* reaction has been studied with the Glasgow photon tagger at MAMI employing a new technique which uses the large solid angle Crystal Ball detector both as a π⁰ spectrometer and to detect decay photons from the excited residual nucleus. The technique has potential applications to a broad range of future nuclear measurements with the Crystal Ball and similar detector systems elsewhere. Such data are sensitive to the propagation of the Δ in the nuclear medium and will give the first information on matter transition form factors from measurements with an electromagnetic probe. The incoherent cross sections are compared to two theoretical predictions including a Δ-hole model.

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.

A measurement of the helicity dependence of the total inclusive photoabsorption cross section on the deuteron was carried out at MAMI (Mainz) in the energy range 200<E_γ<800  MeV. The experiment used a 4π detection system, a circularly polarized tagged photon beam and a frozen-spin target which provided longitudinally polarized deuterons. The contribution to the Gerasimov-Drell-Hearn sum rule for the deuteron determined from the data is 407±20(stat)±24(syst)  μb for 200<E_γ<800  MeV.

The helicity dependence of the γp→nπ⁺ reaction has been measured for the first time in the photon energy range from 450 to 790 MeV. The experiment, performed at the Mainz Microtron (MAMI), used a 4π-detector system, a circularly polarized, tagged photon beam, and a longitudinally polarized frozen-spin target. Although these polarized data are mainly sensitive to mechanisms involving the D₁₃(1520) resonance, a cusp structure at the η production threshold can be clearly observed. These data are significantly different from the predictions of the existing multipole analyses and are used to determine the helicity amplitudes of the D₁₃(1520) and S₁₁(1535) resonances with improved accuracy.

Helicity-dependent total photoabsorption cross sections on the deuteron have been measured for the first time at ELSA (Bonn) in the photon energy range from 815 to 1825 MeV. Circularly polarized tagged photons impinging on a longitudinally polarized LiD target have been used together with a highly efficient 4π detector system. The data around 1 GeV are not compatible with predictions from existing multipole analyses. From the measured energy range an experimental contribution to the GDH integral on the neutron of [33.9±5.5(stat)±4.5(syst)] μb is extracted.

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.

A Reply to the Comment by C. Hanhart.

For the first time we checked the fundamental Gerasimov-Drell-Hearn (GDH) sum rule for the proton experimentally in the photon energy range from 0.2–2.9 GeV with the tagged photon facilities at MAMI (Mainz) and ELSA (Bonn). New data of the doubly polarized total cross section difference are presented in the energy range from 1.6 to 2.9 GeV. The contribution to the GDH integral from 0.2–2.9 GeV yields [254±5(stat)±12(syst)]  μb with negative contributions in the Regge regime at photon energies above 2.1 GeV. This trend supports the validity of the GDH sum rule.

The photoproduction of η-mesic ³He has been investigated using the TAPS calorimeter at the Mainz Microtron accelerator facility MAMI. The total inclusive cross section for the reaction γ³He→ηX has been measured for photon energies from threshold to 820 MeV. The total and angular differential coherent η cross sections have been extracted up to energies of 745 MeV. A resonancelike structure just above the η production threshold with an isotropic angular distribution suggests the existence of a resonant quasibound state. This is supported by studies of a competing decay channel of such a quasibound η-mesic nucleus into π⁰pX. A binding energy of (-4.4±4.2)  MeV and a width of (25.6±6.1)  MeV is deduced for the quasibound η-mesic state in ³He.

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.

To verify the fundamental Gerasimov-Drell-Hearn (GDH) sum rule for the first time experimentally, we measured the helicity dependent total photoabsorption cross section with circularly polarized real photons and longitudinally polarized nucleons in the photon energy range 0.68–1.82 GeV with the tagged photon facility at ELSA. The experiment was carried out with a 4π detection system, a circularly polarized tagged photon beam, and a frozen spin polarized proton target. The contribution to the GDH sum rule in this photon energy range is [49.9±2.4(stat)±2.2(syst)]   μb.

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.

We report on a search for electromagnetic and/or hadronic showers (cascades) induced by a diffuse flux of neutrinos with energies between 5 TeV and 300 TeV from extraterrestrial sources. Cascades may be produced by matter interactions of all flavors of neutrinos, and contained cascades have better energy resolution and afford better background rejection than throughgoing ν_μ-induced muons. Data taken in 1997 with the AMANDA detector were searched for events with a high-energy cascadelike signature. The observed events are consistent with expected backgrounds from atmospheric neutrinos and catastrophic energy losses from atmospheric muons. Effective volumes for all flavors of neutrinos, which allow the calculation of limits for any neutrino flux model, are presented. The limit on cascades from a diffuse flux of ν_e+ν_μ+ν_τ+ν̅ _e+ν̅ _μ+ν̅ _τ is E²(dΦ/dE)<9.8×10^-6 GeV cm^-2 s^-1 sr^-1, assuming a neutrino flavor flux ratio of 1:1:1 at the detector. The limit on cascades from a diffuse flux of ν_e+ν̅ _e is E²(dΦ/dE)<6.5×10^-6 GeV cm^-2 s^-1 sr^-1, independent of the assumed neutrino flavor flux ratio.

The reaction γp→π⁰γ^′p has been measured with the TAPS calorimeter at the Mainz Microtron accelerator facility MAMI for energies between sqrt[s]=1221–1331  MeV. The cross section’s differential in angle and energy have been determined for the photon γ^′ in three bins of the excitation energy. This reaction channel provides access to the magnetic dipole moment of the Δ⁺(1232) resonance and, for the first time, a value of μ_Δ⁺=[2.7_-1.3^+1.0(stat)±1.5(syst)±3(theor)]μ_N has been extracted.

Differential cross sections of the reactions (γ,π⁰π⁰) and (γ,π⁰π^+/-) have been measured for several nuclei (¹H,¹²C, and ^natPb) at an incident-photon energy of E_γ=400–460  MeV at the tagged-photon facility at MAMI-B using the TAPS spectrometer. A significant nuclear-mass dependence of the ππ invariant-mass distribution is found in the π⁰π⁰ channel. This dependence is not observed in the π⁰π^+/- channel and is consistent with an in-medium modification of the ππ interaction in the I=J=0 channel. The data are compared to π-induced measurements and to calculations within a chiral-unitary approach.

A search for nearly vertical up-going muon-neutrinos from neutralino annihilations in the center of the Earth has been performed with the AMANDA-B10 neutrino detector. The data collected in 130.1 days of live time in 1997, ∼10⁹ events, have been analyzed for this search. No excess over the expected atmospheric neutrino background has been observed. An upper limit at 90% confidence level has been obtained on the annihilation rate of neutralinos in the center of the Earth, as well as the corresponding muon flux limit, both as a function of the neutralino mass in the range 100 GeV–5000 GeV.

The Antarctic muon and neutrino detector array (AMANDA) began collecting data with ten strings in 1997. Results from the first year of operation are presented. Neutrinos coming through the Earth from the Northern Hemisphere are identified by secondary muons moving upward through the array. Cosmic rays in the atmosphere generate a background of downward moving muons, which are about 10⁶ times more abundant than the upward moving muons. Over 130 days of exposure, we observed a total of about 300 neutrino events. In the same period, a background of 1.05×10⁹ cosmic ray muon events was recorded. The observed neutrino flux is consistent with atmospheric neutrino predictions. Monte Carlo simulations indicate that 90% of these events lie in the energy range 66 GeV to 3.4 TeV. The observation of atmospheric neutrinos consistent with expectations establishes AMANDA-B10 as a working neutrino telescope.

Measurements of the ⁴He(γ,π⁺n)³H and ⁴He(γ,π⁺p)nnn reactions in quasifree π production kinematic regimes have been performed using unpolarized and linearly polarized tagged photons in conjunction with large solid angle π and n detectors at the MAMI-B accelerator, Mainz. Differential cross sections are presented for photon energies spanning the Δ(1232) excitation region. These data combined with previously presented results for the ¹²C(γ,π⁺n)¹¹B reaction provide an extensive data set for future investigations of Δ-medium effects in nuclei. The ⁴He(γ,π⁺n)³H cross sections are significantly smaller than theoretical predictions based on the free proton amplitude for Δ excitation with a crude correction for final state interactions, and exhibit different trends, which could possibly be a consequence of Δ-medium effects. From a comparison with published ¹²C(γ,π⁺p)¹¹Be data, it is concluded that the ⁴He(γ,π⁺p)nnn reaction mechanisms are probably two-step processes, although the possibility that part or all of the cross section arises from interactions with preexisting Δ⁺⁺ particles cannot be ruled out.

The helicity dependence of the γ→p→→pπ⁰ reaction has been measured for the first time in the photon-energy range from 550 to 790 MeV. The experiment, performed at the Mainz microtron MAMI, used a 4π-detector system, a circularly polarized, tagged photon beam, and a longitudinally polarized frozen-spin target. These data are predominantly sensitive to the D₁₃(1520) resonance and are used to determine its helicity amplitudes.

Measuring Compton scattered photons and recoil neutrons in coincidence, quasifree Compton scattering by the neutron has been investigated at MAMI (Mainz) at θ_γ^lab  =  136° in an energy range from 200 to 400 MeV. From the data a polarizability difference of α_n-β_n  =  9.8±3.6(stat)_-1.1^+2.1(syst)±2.2(model) in units of 10^-4 fm³ has been determined. In combination with the polarizability sum α_n+β_n  =  15.2±0.5 deduced from photoabsorption data, the first precise results for the neutron electric and magnetic polarizabilities, α_n  =  12.5±1.8(stat)_-0.6^+1.1(syst)±1.1(model) and β_n  =  2.7∓1.8(stat)_-1.1^+0.6(syst)∓1.1(model), are obtained.