arXiv subjects

S. Wagner

Publications and source records attributed to S. Wagner.

At least 37 records · Page 2Linked to original sources

arXiv2022

Neutrino Physics with an Opaque Detector

In 1956 Reines & Cowan discovered the neutrino using a liquid scintillator detector. The neutrinos interacted with the scintillator, producing light that propagated across transparent volumes to surrounding photo-sensors. This approach has remained one of the most widespread and successful neutrino detection technologies used since. This article introduces a concept that breaks with the conventional paradigm of transparency by confining and collecting light near its creation point with an opaque scintillator and a dense array of optical fibres. This technique, called LiquidO, can provide high-resolution imaging to enable efficient identification of individual particles event-by-event. A natural affinity for adding dopants at high concentrations is provided by the use of an opaque medium. With these and other capabilities, the potential of our detector concept to unlock opportunities in neutrino physics is presented here, alongside the results of the first experimental validation.

physics.ins-det↗

arXiv2021

Southern African Large Telescope Spectroscopy of BL Lacs for the CTA project

In the last two decades, very-high-energy gamma-ray astronomy has reached maturity: over 200 sources have been detected, both Galactic and extragalactic, by ground-based experiments. At present, Active Galactic Nuclei (AGN) make up about 40% of the more than 200 sources detected at very high energies with ground-based telescopes, the majority of which are blazars, i.e. their jets are closely aligned with the line of sight to Earth and three quarters of which are classified as high-frequency peaked BL Lac objects. One challenge to studies of the cosmological evolution of BL Lacs is the difficulty of obtaining redshifts from their nearly featureless, continuum- dominated spectra. It is expected that a significant fraction of the AGN to be detected with the future Cherenkov Telescope Array (CTA) observatory will have no spectroscopic redshifts, compromising the reliability of BL Lac population studies, particularly of their cosmic evolution. We started an effort in 2019 to measure the redshifts of a large fraction of the AGN that are likely to be detected with CTA, using the Southern African Large Telescope (SALT). In this contribution, we present two results from an on-going SALT program focused on the determination of BL Lac object redshifts that will be relevant for the CTA observatory.

astro-ph.HE↗

arXiv2021

Search for Signatures of Sterile Neutrinos with Double Chooz

We present a search for signatures of neutrino mixing of electron anti-neutrinos with additional hypothetical sterile neutrino flavors using the Double Chooz experiment. The search is based on data from 5 years of operation of Double Chooz, including 2 years in the two-detector configuration. The analysis is based on a profile likelihood, i.e.\ comparing the data to the model prediction of disappearance in a data-to-data comparison of the two respective detectors. The analysis is optimized for a model of three active and one sterile neutrino. It is sensitive in the typical mass range $5 \cdot 10^{-3} $ eV$^2 \lesssim Δm^2_{41} \lesssim 3\cdot 10^{-1} $ eV$^2$ for mixing angles down to $\sin^2 2θ_{14} \gtrsim 0.02$. No significant disappearance additionally to the conventional disappearance related to $θ_{13} $ is observed and correspondingly exclusion bounds on the sterile mixing parameter $θ_{14} $ as function of $ Δm^2_{41} $ are obtained.

hep-ex↗

arXiv2021

Optical spectroscopy of Blazars for the Cherenkov Telescope Array

Context. Blazars are the most numerous class of High Energy (HE; E about 50 MeV - few 100 GeV) and Very High Energy (VHE; E about 100 GeV - 10 TeV) gamma-ray emitters. As of today, a measured spectroscopic redshift is available for only about 50% of gamma-ray BL Lacs, mainly due to the difficulty of measuring reliable redshifts from their nearly featureless, continuum-dominated optical spectra. The knowledge of the redshift is fundamental for understanding the emission from blazars, for population studies and also for indirect studies of the extragalactic background light and searches for Lorentz invariance violation and axion-like particles using blazars. Aims. This paper is the first of a series of papers which aim to measure the redshift of a sample of blazars likely to be detected with the upcoming Cherenkov Telescope Array (CTA), a ground based gamma-ray observatory. Methods. Monte Carlo simulations were performed to select those hard spectrum gamma-ray blazars detected with the Fermi-LAT telescope still lacking redshift measurements but likely to be detected by CTA in 30 hours of observing time or less. Optical observing campaigns involving deep imaging and spectroscopic observations were organised to efficiently constrain their redshifts. We performed deep medium to high resolution spectroscopy of nineteen blazar optical counterparts using the ESI spectrograph at Keck, the RSS spectrograph at the SALT telescope, and the EFOSC2 spectrograph at the ESO NTT. We searched systematically for spectral features and, when possible, we estimated the contribution of the host galaxy to the total flux. Results. We measured eleven firm spectroscopic redshifts with values ranging from 0.1116 to 0.482. one tentative redshift, three redshift lower limits including one at z > 0.449 and another at z > 0.868. There were four objects found to have featureless spectra.

astro-ph.HE↗

arXiv2021

Single $π^0$ Production Off Neutrons Bound in Deuteron with Linearly Polarized Photons

The quasifree $\overrightarrowγ d\toπ^0n(p)$ photon beam asymmetry, $Σ$, has been measured at photon energies, $E_γ$, from 390 to 610 MeV, corresponding to center of mass energy from 1.271 to 1.424 GeV, for the first time. The data were collected in the A2 hall of the MAMI electron beam facility with the Crystal Ball and TAPS calorimeters covering pion center-of-mass angles from 49 to 148$^\circ$. In this kinematic region, polarization observables are sensitive to contributions from the $Δ(1232)$ and $N(1440)$ resonances. The extracted values of $Σ$ have been compared to predictions based on partial-wave analyses (PWAs) of the existing pion photoproduction database. Our comparison includes the SAID, MAID, and Bonn-Gatchina analyses; while a revised SAID fit, including the new $Σ$ measurements, has also been performed. In addition, isospin symmetry is examined as a way to predict $π^0n$ photoproduction observables, based on fits to published data in the channels $π^0p$, $π^+n$, and $π^-p$.

nucl-ex↗

arXiv2020

Reactor Rate Modulation oscillation analysis with two detectors in Double Chooz

A $θ_{13}$ oscillation analysis based on the observed antineutrino rates at the Double Chooz far and near detectors for different reactor power conditions is presented. This approach provides a so far unique simultaneous determination of $θ_{13}$ and the total background rates without relying on any assumptions on the specific background contributions. The analysis comprises 865 days of data collected in both detectors with at least one reactor in operation. The oscillation results are enhanced by the use of 24.06 days (12.74 days) of reactor-off data in the far (near) detector. The analysis considers the \nue interactions up to a visible energy of 8.5 MeV, using the events at higher energies to build a cosmogenic background model considering fast-neutrons interactions and $^{9}$Li decays. The background-model-independent determination of the mixing angle yields sin$^2(2θ_{13})=0.094\pm0.017$, being the best-fit total background rates fully consistent with the cosmogenic background model. A second oscillation analysis is also performed constraining the total background rates to the cosmogenic background estimates. While the central value is not significantly modified due to the consistency between the reactor-off data and the background estimates, the addition of the background model reduces the uncertainty on $θ_{13}$ to 0.015. Along with the oscillation results, the normalization of the anti-neutrino rate is measured with a precision of 0.86\%, reducing the 1.43\% uncertainty associated to the expectation.

hep-ex↗

arXiv2020

Spin polarizabilities of the proton by measurement of Compton double-polarization observables

The Compton double-polarization observable $Σ_{2z}$ has been measured for the first time in the $Δ(1232)$ resonance region using a circularly polarized photon beam incident on a longitudinally polarized target at the Mainz Microtron. This paper reports these results, together with the model-dependent extraction of four proton spin polarizabilities from fits to additional asymmetry data using dispersion relation and chiral perturbation theory calculations, with the former resulting in: $γ_{E1E1} = -3.18 \pm 0.52$, $γ_{M1M1} = 2.98 \pm 0.43$, $γ_{E1M2} = -0.44 \pm 0.67$ and $γ_{M1E2} = 1.58 \pm 0.43$, in units of $10^{-4}~\mathrm{fm}^{4}$.

nucl-ex↗

arXiv2020

Helicity-dependent cross sections for the photoproduction of $π^0$ pairs from nucleons

The double-polarization observable $E$ and helicity-dependent cross sections $σ_{1/2}$, $σ_{3/2}$ have been measured for the photoproduction of $π^0$ pairs off quasi-free protons and neutrons at the Mainz MAMI accelerator with the Crystal Ball/TAPS setup. A circularly polarized photon beam was produced by bremsstrahlung from longitudinally polarized electrons and impinged on a longitudinally polarized deuterated butanol target. The reaction products were detected with an almost $4π$ covering calorimeter. The results reveal for the first time the helicity- and isospin-dependent structure of the $γN\rightarrow Nπ^0π^0$ reaction. They are compared to predictions from reaction models in view of nucleon resonance contributions and also to a refit of one model that predicted results for the proton and for the neutron target. The comparison of the prediction and the refit demonstrate the large impact of the new data.

nucl-ex↗

arXiv2020

Measurement of the beam-helicity asymmetry in photoproduction of $π^{0}η$ pairs on carbon, aluminum, and lead

The beam-helicity asymmetry was measured, for the first time, in photoproduction of $π^{0}η$ pairs on carbon, aluminum, and lead, with the A2 experimental setup at MAMI. The results are compared to an earlier measurement on a free proton and to the corresponding theoretical calculations. The Mainz model is used to predict the beam-helicity asymmetry for the nuclear targets. The present results indicate that the photoproduction mechanism for $π^{0}η$ pairs on nuclei is similar to photoproduction on a free nucleon. This process is dominated by the $D_{33}$ partial wave with the $ηΔ(1232)$ intermediate state.

nucl-ex↗

arXiv2019

Signatures of the $d^*(2380)$ hexaquark in d($γ$,$p\vec{n}$)

We report a measurement of the spin polarisation of the recoiling neutron in deuterium photodisintegration, utilising a new large acceptance polarimeter within the Crystal Ball at MAMI. The measured photon energy range of 300~--~700~MeV provides the first measurement of recoil neutron polarisation at photon energies where the quark substructure of the deuteron plays a role, thereby providing important new constraints on photodisintegration mechanisms. A very high neutron polarisation in a narrow structure centred around $E_γ\sim$~570~MeV is observed, which is inconsistent with current theoretical predictions employing nucleon resonance degrees of freedom. A Legendre polynomial decomposition suggests this behaviour could be related to the excitation of the $d^*(2380)$ hexaquark.

nucl-ex↗

arXiv2019

Cross Section for $γn \to π^0 n$ measured at Mainz/A2

The $γn \to π^0 n$ differential cross section evaluated for 27 energy bins span the photon-energy range 290-813 MeV (W = 1.195-1.553 GeV) and the pion c.m. polar production angles, ranging from 18 deg to 162 deg, making use of model-dependent nuclear corrections to extract pi0 production data on the neutron from measurements on the deuteron target. Additionally, the total photoabsorption cross section was measured. The tagged photon beam produced by the 883-MeV electron beam of the Mainz Microtron MAMI was used for the 0-meson production. Our accumulation of 3.6 x 10^6 $γn \to π^0 n$ events allowed a detailed study of the reaction dynamics. Our data are in reasonable agreement with previous A2 measurements and extend them to lower energies. The data are compared to predictions of previous SAID, MAID, and BnGa partial-wave analyses and to the latest SAID fit MA19 that included our data. Selected photon decay amplitudes $N^* \to γn$ at the resonance poles are determined for the first time.

nucl-ex↗

arXiv2018

Deuteron photodisintegration by polarized photons in the region of the $d^*(2380)$

We report the first large-acceptance measurement of the beam-spin asymmetry for deuteron photodisintegration ($\vecγ d\to pn$) in the photon energy range $400<E_γ<630$~MeV. The measurement provides important new constraints on the mechanisms of photodisintegration above the delta resonance and on the photocoupling of the recently discovered $d^*(2380)$ hexaquark.

nucl-ex↗

arXiv2018

Experimental study of the $γp\rightarrow K^0Σ^+$, $γn\rightarrow K^0Λ$, and $γn\rightarrow K^0 Σ^0$ reactions at the Mainz Microtron

This work measured $dσ/dΩ$ for neutral kaon photoproduction reactions from threshold up to a c.m.\ energy of 1855MeV, focussing specifically on the $γp\rightarrow K^0Σ^+$, $γn\rightarrow K^0Λ$, and $γn\rightarrow K^0 Σ^0$ reactions. Our results for $γn\rightarrow K^0 Σ^0$ are the first-ever measurements for that reaction. These data will provide insight into the properties of $N^*$ resonances and, in particular, will lead to an improved knowledge about those states that couple only weakly to the $πN$ channel. Integrated cross sections were extracted by fitting the differential cross sections for each reaction as a series of Legendre polynomials and our results are compared with prior experimental results and theoretical predictions.

nucl-ex↗

arXiv2018

Yields and production rates of cosmogenic $^9$Li and $^8$He measured with the Double Chooz near and far detectors

The yields and production rates of the radioisotopes $^9$Li and $^8$He created by cosmic muon spallation on $^{12}$C, have been measured by the two detectors of the Double Chooz experiment. The identical detectors are located at separate sites and depths, which means they are subject to different muon spectra. The near (far) detector has an overburden of $\sim$120 m.w.e. ($\sim$300 m.w.e.) corresponding to a mean muon energy of $32.1\pm2.0\,\mathrm{GeV}$ ($63.7\pm5.5\,\mathrm{GeV}$). Comparing the data to a detailed simulation of the $^9$Li and $^8$He decays, the contribution of the $^8$He radioisotope at both detectors is found to be compatible with zero. The observed $^9$Li yields in the near and far detectors are $5.51\pm0.51$ and $7.90\pm0.51$, respectively, in units of $10^{-8}μ^{-1} \mathrm{g^{-1} cm^{2} }$. The shallow overburdens of the near and far detectors give a unique insight when combined with measurements by KamLAND and Borexino to give the first multi--experiment, data driven relationship between the $^9$Li yield and the mean muon energy according to the power law $Y = Y_0( / 1\,\mathrm{GeV})^{\overlineα}$, giving $\overlineα=0.72\pm0.06$ and $Y_0=(0.43\pm0.11)\times 10^{-8}μ^{-1} \mathrm{g^{-1} cm^{2}}$. This relationship gives future liquid scintillator based experiments the ability to predict their cosmogenic $^9$Li background rates.

hep-ex↗

arXiv2018

First measurement of helicity-dependent cross sections in pi0-eta photoproduction from quasi-free nucleons

The helicity-dependent cross sections for the photoproduction of $π^0η$ pairs have been measured for the first time. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball - TAPS calorimeter. The experiment used a polarized deuterated butanol target and a circularly polarized photon beam. This arrangement allowed the $σ_{1/2}$ (photon and target spin antiparallel) and $σ_{3/2}$ (parallel spins) components to be measured for quasi-free production of $π^0η$ pairs off protons and neutrons. The main finding is that the two helicity components contribute identically, within uncertainties, for both participant protons and neutrons. The absolute couplings for protons and neutrons are also identical. This means that nucleon resonances contributing to this reaction in the investigated energy range have almost equal electromagnetic helicity couplings, $A_{1/2}^{n,p}$ and $A_{3/2}^{n,p}$. Identical couplings for protons and neutrons are typical for $Δ$ resonances and identical $A_{1/2}$ and $A_{3/2}$ components are only possible for $J\geq 3/2$ states, which constrains possible contributions of nucleon resonances.

nucl-ex↗

arXiv2018

A hybrid organic/inorgaic scintillator for high performance measurements

New experimental research programs in the field of neutrino physics are calling for new detectors with large masses, high energy resolution and good background rejection capabilities. This paper presents a novel hybrid organic/inorganic scintillator, which is able to improve on all three aspects simultaneously. This scintillator consists of microscopic grains of inorganic crystals suspended in an organic scintillating carrier medium. Due to multiple scattering off the crystals, this scintillator appears opaque over longer distances and is intended for use in specialized detectors. Thanks to the crystal phase it can natively incorporate a large variety of elements in large quantities, so that a sufficiently large detector can reach elemental loadings on the ton- or multiton scale. At the same time, this composition can produce very high light outputs and provides additional particle identification capabilities. This scintillator concept is expected to provide significant advantages for future neutrino experiments, like searches of neutrinoless double beta experiments and reactor antineutrino physics.

physics.ins-det↗

arXiv2018

Measurement of the decay $η^{\prime}\toπ^{0}π^{0}η$ at MAMI

An experimental study of the $η'\to π^0π^0η\to 6γ$ decay has been conducted with the best up-to-date statistical accuracy, by measuring $η'$ mesons produced in the $γp \to η' p$ reaction with the A2 tagged-photon facility at the Mainz Microtron, MAMI. The results obtained for the standard parametrization of the $η'\to π^0π^0η$ matrix element are consistent with the most recent results for $η'\toππη$ decays, but have smaller uncertainties. The available statistics and experimental resolution allowed, for the first time, an observation of a structure below the $π^+π^-$ mass threshold, the magnitude and sign of which, checked within the framework of the nonrelativistic effective-field theory, demonstrated good agreement with the cusp that was predicted based on the $ππ$ scattering length combination, $a_0-a_2$, extracted from $K \to 3π$ decays.

hep-ex↗

arXiv2018

High-statistics measurement of the eta->3pi^0 decay at the Mainz Microtron

The largest, at the moment, statistics of 7x10^6 eta->3pi^0 decays, based on 6.2x10^7 eta mesons produced in the gamma p -> eta p reaction, has been accumulated by the A2 Collaboration at the Mainz Microtron, MAMI. It allowed a detailed study of the eta->3pi^0 dynamics beyond its conventional parametrization with just the quadratic slope parameter alpha and enabled, for the first time, a measurement of the second-order term and a better understanding of the cusp structure in the neutral decay. The present data are also compared to recent theoretical calculations that predict a nonlinear dependence along the quadratic distance from the Dalitz-plot center.

hep-ex↗